AU2011226788A1 - Bone Cement and Methods of Use Thereof - Google Patents

Abstract

- 34 A bone cement comprising an acrylic polymer mixture which is formulated to have a relatively high viscosity for a relatively long window, due to distributions 5 of molecular weights and/or sizes of acrylic beads. 21/09/11 jb19474 specificalion,34 MONOMER AND 110 POLYMER COMBINED Mix 112 WORKING WINDOW 116 (OPTIONAL) HARDfENING

Description

AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION INVENTION TITLE: Bone Cement and Methods of Use Thereof The following statement is a full description of this invention, including the best method of performing it known to us:- -2 BONE CEMENT AND METHODS OF USE THEREOF RELATED APPLICATIONS The present application is a further application of Australian Patent 5 Application No. 2007297097, the disclosure of which is incorporated herein by reference. Australian Patent Application 2007297097 claims priority from US Patent Application 60/825,609 filed September 14, 2006, the disclosure of which in incorporated herein by reference. 10 The present application is related to US Patent Application 11/461,072 filed on July 31, 2006 and entitled "Bone Cement and Methods of Use Thereof", which is a Continuation-in-Part of US Application 11/360,251 filed on February 22, 2006, entitled "Methods, Materials and Apparatus for Treating Bone and Other Tissue" and is also a Continuation-in-Part of PCT/1L2005/000812 filed on July 31, 2005. The 15 disclosures of these applications are incorporated herein by reference. The present application is related to PCT Application PCT/1L2006/052612 filed on July 31, 2006 and entitled "Bone Cement and Methods of Use thereof', the disclosure of which is incorporated herein by reference. The present application is related to Israel Application No. 174347 filed on 20 March 16, 2006 and entitled "Bone Cement and Methods of Use thereof', the disclosure of which is incorporated herein by reference. The present application is also related to a series of US Provisional Applications entitled "Methods, Materials and Apparatus for Treating Bone and Other Tissue": 60/765,484 filed on February 2, 2006; 60/762,789 filed on January 25 26, 2006; 60/738,556 filed November 22, 2005; 60/729,505 filed October 25, 2005; 60/720,725 filed on September 28, 2005 and 60/721,094 filed on September 28, 2005. The disclosures of these applications are incorporated herein by reference. The present application is related to PCT Application PCT/IL2006/000239 filed on February 22, 2006; US Provisional Application 60/763,003, entitled 30 "Cannula" filed on January 26, 2006; US Provisional Application No. 60/654,495 entitled "Materials, devices and methods for treating bones", filed February 22, 2005; US 11/194,411 filed August 1, 2005; IL 166017 filed December 28, 2004; 21/09/11 jb19474 specification.2 -3 IL 160987 filed March 21, 2004; US Provisional Application No. 60/654,784 filed on January 31, 2005; US Provisional Application No. 60/592,149 filed on July 30, 2004; PCT Application No. PCT/IL2004/000527 filed on June 17, 2004, Israel Application No. 160987 filed on March 21, 2004, U.S. Provisional Applications: 5 -60/478,841 filed on June 17, 2003; 60/529,612 filed on December 16, 2003; 60/534,377 filed on January 6, 2004 and 60/554,558 filed on March 18, 2004; U.S. Application No. 09/890,172 filed on July 25, 2001; U.S. Application No. 09/890,318 filed on July 25, 2001 and US Application 10/549,409 entitled "Hydraulic Device for the injection of Bone Cement in Percutaneous Vertebroplasty" filed on September 10 14, 2005. The disclosures of all of these applications are incorporated herein by reference. FIELD OF THE INVENTION The present invention relates to bone cement, formulations thereof and methods of use thereof. 15 BACKGROUND OF THE INVENTION It is common to employ cement to repair bones in a variety of clinical scenarios. For example, compression fractures of the vertebrae, which are a common occurrence in older persons, cause pain and/or a shortening (or other distortion) of 20 stature. In a procedure known as vertebroplasty cement is injected into a fractured vertebra. Vertebroplasty stabilizes the fracture and reduces pain, although it does not restore the vertebra and person to their original height. In vertebroplasty the cement is typically injected in a liquid phase so that resistance to injection is not too great. Liquid cement may unintentionally be injected outside of the vertebra and/or may 25 migrate out through cracks in the vertebra. In another procedure, known as kyphoplasty, the fracture is reduced by expanding a device, such as a balloon inside the vertebra and then injecting a fixing material and/or an implant. Kyphoplasty reduces the problem of cement leakage by permitting a lower pressure, to be used for injection of the cement. 30 In general, polymeric cements become more viscous as the polymer chain grows by reacting directly with the double bond of a monomer. Polymerization begins by the "addition mechanism" in which a monomer becomes unstable by 21/09/1 1 jb19474 specification.3 -4 reacting with an initiator, a volatile molecule that is most commonly a radical (molecules that contain a single unpaired electron). Radicals bond with monomers, forming monomer radicals that can attack the double bond of the next monomer to propagate the polymer chain. Because radicals are so transient, initiators are often 5 added in the form of an un-reactive peroxide form which is stable in solution. Radicals are formed when heat or light cleaves the peroxide molecule. For applications in which high temperatures are not practical (such as the use of bone cement in vivo), peroxide is typically cleaved by adding a chemical activator such as N, N-dimethyl-p-toluidine. (Nussbaum DA et al: "The Chemistry of Acrylic Bone 10 Cement and Implication for Clinical Use in Image-guided Therapy", J Vase Interv Radiol (2004); 15:121-126; the content of which is fully incorporated herein by reference). Examples of commercially available viscous bone cements include, but are not limited to, CMW@ Nos. 1, 2 and 3 (DePuy Orthopaedics Inc.; Warsaw, IN, 15 USA) and Simplex TM -P and -RO (Stryker Orthopaedics; Mahwah, NJ, USA). These cements are characterized by a liquid phase after mixing and prior to achieving a viscosity of 500 Pascal-second. In a typical use scenario, these previously available cements are poured, while in a liquid phase, into a delivery device. There have also been attempts to reduce cement leakage by injecting more 20 viscous cement, for example, during the doughing time and the beginning of polymerization. However, the viscous materials, such as hardening PMMA, typically harden very quickly once they reach a high viscosity. This has generally prevented injection of viscous materials in orthopedic procedures. Some bone fixing materials, such as polymethylmethacrylate (PMMA), emit 25 heat and possibly toxic materials while setting. US Patents and Publication 4,969,888, 5,108,404, 6,383,188, 2003/0109883, 2002/0068974, 6,348,055, 6,383,190, 4,494,535, 4,653,489 and 4,653,487, the disclosures of which are incorporated herein by reference describe various tools and methods for treating bone. 30 US Patent Publication 2004/0260303, the disclosure of which is incorporated herein by reference, teaches an apparatus for delivering bone cement into a vertebra. 2 1/09/1 ljb19474 specification,4 -5 Pascual, B., et al., "New Aspects of the Effect of Size and Size Distribution on the Setting Parameters and Mechanical Properties of Acrylic Bone Cements", Biomaterials, 17(5): 509-516 (1996) considers the effect of PMMA bead size on setting parameters of cement. This article is fully incorporated herein by reference. 5 Hernandez, et al., (2005) "Influence of Powder Particle Size Distribution on Complex Viscosity and Other Properties of Acrylic Bone Cement for Vertebroplasty and Kyphoplasty", Wiley International Science DOI:10:1002/jbm.b.30409 (pages 98 103) considers the effect of PMMA bead size distribution on setting parameters of cement. Hernandez suggests that it is advantageous to formulate cement with a liquid 10 phase to facilitate injection. This article is fully incorporated herein by reference. US 5,276,070 to Arroyo discloses use of acrylic polymers with a molecular weight in the range of 0.5 to 1.5 million Daltons in formulation of bone cement. The disclosure of this patent is fully incorporated herein by reference. US 5,336,699 to Cooke discloses use of acrylic polymers with a molecular 15 weight of about one hundred thousand Daltons in formulation of bone cement. The disclosure of this patent is fully incorporated herein by reference. SUMMARY OF THE INVENTION A broad aspect of the invention relates to a bone cement characterized by a rapid transition from separate liquid monomer and powdered polymer components to 20 a single phase characterized by a high viscosity when the components are mixed together with substantially no intervening liquid phase. Optionally, high viscosity indicates 500 Pascal-second or more. Mixing is deemed complete when 95-100% of the polymer beads are wetted by monomer. In an exemplary embodiment of the invention, mixing is complete in within 60, optionally within 45, optionally within 30 25 seconds. In an exemplary embodiment of the invention, the cement is characterized by a working window of several minutes during which the viscosity remains high prior to hardening of the cement. Optionally, viscosity during the working window does not vary to a degree which significantly influences injection parameters. In an 30 exemplary embodiment of the invention, viscosity increases by less than 10% during a sub- window of at least 2 minutes during the working window. Optionally, the viscosity in the working window does not exceed 500, optionally 1,000, optionally 21/09/11,jb19474 specification,5 -6 1,500, optionally 2,000 Pascal-second or lesser or greater or intermediate values. In an exemplary embodiment of the invention, the working window lasts 6, optionally 8, optionally 10, optionally 15 minutes or lesser or greater or intermediate times. Optionally, ambient temperature influences a duration of the working window. In an 5 exemplary embodiment of the invention, the cement can be cooled or heated to influence a length of the working window. An aspect of some embodiments of the invention relates to formulations of bone cement which rely upon two, optionally three or more, sub-populations of polymer beads which are mixed with liquid monomer. 10 According to exemplary embodiments of the invention, sub-populations may be characterized by average molecular weight (MW) and/or physical size and/or geometry, and/or density. In an exemplary embodiment of the invention, size based and MW based sub-populations are defined independently. In an exemplary embodiment of the invention, the sub-populations are selected to produce desired 15 viscosity characterization and/or polymerization kinetics. Optionally, the polymer beads comprise polymethylmethacrylate (PMMA) and/or a PMMA styrene copolymer. Optionally, PMMA is employed in conjunction with a methylmethacrylate (MMA) monomer. Optionally, a high molecular weight sub-population contributes to a rapid 20 transition to a high viscosity with substantially no liquid phase. Optionally, a low molecular weight subpopulation contributes to a longer working window. Optionally, a sub-population with small size contributes to rapid wetting of polymer beads with monomer solution. In an exemplary embodiment of the invention, rapid wetting contributes to a direct transition to a viscous cement with 25 substantially no liquid phase. In some cases a small percentage of beads may not belong to any relevant sub- population. The small percentage may be, for example 1%, 1.5%, 2%, 3%, 4%, 5% or lesser or intermediate or greater percentages. In one exemplary embodiment of the invention, there are at least two sub 30 populations of PMMA polymer beads characterized by molecular weights. For example, a first sub- population comprising 95 to 97% (w/w) of the total PMMA beads can be characterized by an average MW of 270,000-300,000 Dalton; a second 21/09/11,jb]9474 specification,6 -7 sub-population (2-3% w/w) can be characterized by an average MW of 3,500,000 4,000,000 Dalton; and a third sub-population (0-3% w/w) can be characterized by an average MW of 10,000-15,000 Dalton. In an exemplary embodiment of the invention, the polymer beads are 5 characterized by a high surface area per unit weight. Optionally;- the beads have a surface area of 0.5 to 1, optionally 0.5 to 0.8 optionally about 0.66 m 2 /gram -or intermediate or lesser or greater values. Optionally, the high surface area/weight ratio improves wetting properties and/or shortens polymerization times, for example by contributing to polymer monomer contact. 10 In an exemplary embodiment of the invention, a cement characterized by an immediate transition to high viscosity is injected during a working window in a vertebroplasty or kyphoplasty procedure. Optionally, injection is under sufficient pressure to move fractured bone, such as vertebral plates of a collapsed vertebra. Optionally, injection of viscous cement under high pressure contributes to fracture 15 reduction and/or restoration of vertebral height. In an exemplary embodiment of the invention, the material (e.g., bone cement) includes processed bone (from human or animals origin) and/or synthetic bone. Optionally, the cement has osteoconductive and/or osteoinductive behavior. Additional additives as commonly used in bone cement preparation may optionally 20 be added. These additives include, but are not limited to, barium sulfate and benzoyl peroxide. According to some embodiments of the invention, a working window length is determined by an interaction between an immediate effect and a late effect. In an exemplary embodiment of the invention, the immediate effect includes MMA 25 solvation and/or encapsulation of PMMA polymer beads. The immediate effect contributes to a high viscosity of the initial mixture resulting from solvation and/or friction between the beads. The late effect is increasing average polymer MW as the beads dissolve and the polymerization reaction proceeds. This increasing average polymer MW keeps viscosity high throughout the working window. 30 In an exemplary embodiment of the invention, a set of viscosity parameters are used to adjust a cement formulation to produce a cement characterized by a desired working window at a desired viscosity. 21/09/1l jb19474 specification,7 -8 In an exemplary embodiment of the invention, there is provided a bone cement comprising an acrylic polymer mixture, the cement characterized in that it achieves a viscosity of at least 500 Pascal-second within 180 seconds following initiation of mixing of a monomer component and a polymer component and 5 characterized by sufficient biocompatibility to permit in-vivo use. Optionally, the viscosity of the mixture remains between 500 and 2000 Pascal-second for a working window of at least 5 minutes after the initial period. Optionally, the working window is at least 8 minutes long. Optionally, the mixture includes PMMA. Optionally, the mixture includes 10 Barium Sulfate. Optionally, the PMMA is provided as a PMMA/styrene copolymer. Optionally, the PMMA is provided as a population of beads divided into at least two sub-populations, each sub-population characterized by an average molecular weight. 15 Optionally, a largest sub-population of PMMA beads is characterized by an MW of 150,000 Dalton to 300,000 Dalton. Optionally, a largest sub-population of PMMA beads includes 90-98% (w/w) of the beads. Optionally, a high molecular weight sub-population of PMMA beads is 20 characterized by an average MW of at least 3,000,000 Dalton. Optionally, a high molecular weight sub-population of PMMA beads includes 2 to 3% (w/w) of the beads. Optionally, a low molecular weight sub-population of PMMA beads is characterized by an average MW of less than 15,000 Dalton. 25 Optionally, a low molecular weight sub-population of PMMA beads includes 0.75 to 1.5 % (W/W) of the beads. Optionally, the PMMA is provided as a population of beads divided into at least two sub-populations, each sub-population characterized by an average bead diameter. 30 Optionally, at least one bead sub-population characterized by an average diameter is further divided into at least two sub-sub-populations, each sub-sub population characterized by an average molecular weight. 21/09/1l1jb19474 specification.8 -9 Optionally, the PMMA is provided as a population of beads divided into at least three sub-populations, each sub-population characterized by an average bead diameter. Optionally, the cement further includes processed bone and/or synthetic 5 bone. Optionally, the cement is characterized in that the cement achieves a viscosity of at least 500 Pascal-second when 100% of a polymer component is wetted by a monomer component. Optionally, the viscosity is at least 800 Pascal-second. Optionally, the viscosity is at least 1500 Pascal-second. 10 Optionally, the viscosity is achieved within 2 minutes. Optionally, the viscosity is achieved within 1 minute. Optionally, the viscosity is achieved within 45 seconds. In an exemplary embodiment of the invention, there is provided a bone cement comprising: a polymer component; and a monomer component, wherein, contacting the polymer component 15 and the monomer component produces a mixture which attains a viscosity greater than 200 Pascal-second within 1 minute from onset of mixing and remains below 2000 Pascal-second until at least 6 minutes from onset of mixing. Optionally, the polymer component comprises an acrylic polymer. In an exemplary embodiment of the invention, there is provided a 20 particulate mixture formulated for preparation of a bone cement, the mixture comprising: (a) 60 to 80% polymer beads comprising a main sub-population characterized by an MW of 150,000 Dalton to 300,000 Dalton and a high molecular weight sub-population characterized by an MW of 3,000,000 Dalton to 4,000,000 25 Dalton; and (b) 20 to 40% of a material which is non-transparent with respect to X-ray. Optionally, the polymer beads comprise a third subpopulation characterized by an MW of 10,000 Dalton to 15 ,000 Dalton. 30 In an exemplary embodiment of the invention, there is provided a method of making a polymeric bone cement, the method comprising: 21/09/1 1 jb19474 specification,9 - 10 (a) defining a viscosity profile including a rapid transition to a working window characterized by a high viscosity; (b) selecting a polymer component and a monomer component to produce a cement conforming to the viscosity profile; and 5 (c) mixing the polymer component and a monomer component to produce a cement which conforms to the viscosity profile. In an exemplary embodiment of the invention, there is provided a cement kit, comprising: (a) a liquid component including a monomer; and 10 (b) a powder component including polymeric beads, characterized in that said powder component is provided in a substantially non-normal distribution of at least one of molecular weight of the polymeric beads and size of powder particles such that a cement mixed from the kit has both an increased immediate viscosity and an increased working window as compared to a cement having a substantially normal 15 distribution. Optionally, the substantially non-normal distribution is a skewed distribution. Optionally, the substantially non-normal distribution comprises a relatively small component including higher molecular weight beads. 20 Optionally, said component has an average molecular weight of at least a factor of 2 of an average molecular weight of said polymeric beads. Optionally, said factor is at least 3 or is at least 5. Optionally, the substantially non-normal distribution comprises a relatively small component including smaller sized particles. 25 BRIEF DESCRIPTION OF THE FIGURES Exemplary non-limiting embodiments of the invention will be described with reference to the following description of embodiments in conjunction with the figures. Identical structures, elements or parts which appear in more than one figure are generally labeled with a same or similar number in all the figures in which they 30 appear, in which: Fig. I is a flow diagram illustrating an exemplary method 100 of preparation and behavior of exemplary cements according to the present invention; 21/09/1 ijbl9474 spccification,10 -11 Fig. 2 is a graph of viscosity profiles depicting viscosity (Pascal-second) as a function of time (minutes) for an exemplary cement according to the invention and an exemplary prior art cement; Figs. 3 and 4 are graphs indicating viscosity as Newtons of applied force per 5 unit displacement (mm) under defined conditions for exemplary cements according to the invention and illustrate the time window for injection which is both early and long; and Fig. 5 is a graph showing the results of bead size distribution analysis, for a bead formulation in accordance with an exemplary embodiment of the invention. 10 DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS Overview of preparation of exemplary bone cement Fig. 1 is a flow diagram illustrating preparation and behavior of exemplary cements according to some embodiments of the present invention. In an exemplary embodiment of the invention, a liquid monomer and a 15 powdered polymer component of a bone cement are combined 110. Optionally, liquid monomer is poured onto powdered polymer. According to various embodiments of the invention, average polymer molecular weight and/or polymer molecular weight distribution and/or polymer bead size is precisely controlled in order to influence polymerization kinetics and/or 20 cement viscosity. Alternatively or additionally, polymer and/or monomer components may contain ingredients which are not directly involved in the polymerization reaction. In an exemplary embodiment of the invention, the polymer (e.g. an acrylic polymer such as PMMA) beads are divided into two or more sub-populations. 25 Optionally, the sub-populations are defined by molecular weight (MW). In an exemplary embodiment of the invention, the average molecular weight of the acrylic polymer in all the beads is in the range < of about 300,000 to 400,000, optionally about 373,000 Dalton. This average MW for all beads was determined experimentally for a batch of beads which produced cement with a desired 30 polymerization profile. Optionally, the polymer beads are provided as part of an acvrylic polymer mixture, for example a mixture including barium sulfate. 21/09/11 jb19474 specification,l I -12 At 112 the components are mixed until the polymer is wetted by the monomer. Optionally, when wetting is 95 to 100% complete, the mixture has achieved a desired high viscosity, for example 500 Pascal-second or more. Optionally, mixing 112 is complete within 1, 5, 10, 15, 30, 60, 90, 120 or 180 5 seconds. In a modem medical facility, it can be advantageous to shorten the mixing time in order to reduce the demand on physical facilities and/or medical personnel. A savings of even 1 to 2 minutes with respect to previously available alternatives can be significant. In an exemplary embodiment of the invention, mixing 112 is conducted in a mixing apparatus of the type described in co-pending US Application 10 11/428,908, the disclosure of which is fully incorporate herein by reference. After mixing 112 is complete, a working window 114 during which the cement remains viscous but has not fully hardened occurs. According to various exemplary embodiments of the invention, working window 114 may be about 2, 5, 8, 10, 15 or 20 minutes or intermediate or greater times. The duration of the working 15 window may vary with the exact cement formulation and/or ambient conditions (e.g. temperature and/or humidity). Formulation considerations include, but are not limited to polymer MW (average and/or distribution), polymer bead size, concentrations of non-polymerizing ingredient and polymer:monomer ratio. Working window 114, permits a medical practitioner sufficient time to load 20 a high pressure injection device and inject 120 the cement into a desired location. Optionally, an injection needle or cannula is inserted into the body prior to, or concurrent with mixing 112 so that window 114 need only be long enough for loading and injection 120. Exemplary injection systems are disclosed in co-pending US Application 11/360,251 entitled "Methods, materials, and apparatus for treating 25 bone and other tissue" filed February 22, 2006, the disclosure of which is fully incorporated herein by reference. In an exemplary embodiment of the invention, hardening 116 to a hardened condition occurs after working window 114. The cement hardens 116 even if it has not been injected. 30 21/09/11 jb19474 specification, 12 - 13 Advantages with Respect to Relevant Medical Procedures In an exemplary embodiment of the invention, cement with a viscosity profile as described above is useful in vertebral repair, for example in vertebroplasty and/or kyphoplasty procedures. 5 Optionally, use of cement which is viscous at the time of injection reduces the risk of material leakage and/or infiltrates into the intravertebral cancellous bone (interdigitaion) and/or reduces the fracture [see G Baroud et al, Injection biomechanics of bone cements used in vertebroplasty, Bio-Medical Materials and Engineering 00 (2004) 1-18]. Reduced leakage optionally contributes to increased 10 likelihood of a positive clinical outcome. In an exemplary embodiment of the invention, the viscosity of the bone cement is 500, optionally 1,000, optionally 1,500, optionally 2,000 Pascal-second or lesser or greater or intermediate values at the time injection begins, optionally 3, 2 or 1 minutes or lesser or intermediate times after mixing 112 begins. Optionally, the 15 viscosity does not exceed 2,000 Pascal-second during working window 114. In an exemplary embodiment of the invention, this viscosity is achieved substantially as soon as 95-100% of the polymer beads are wetted by monomer. Cement characterized by a high viscosity as described above may optionally be manually manipulated. 20 In an exemplary embodiment of the invention, cement is sufficiently viscous to move surrounding tissue as it is injected. Optionally, moving of the surrounding tissue contributes to fracture reduction and/or restoration of vertebral height. An injected volume of cement may vary, depending upon the type and/or number of orthopedic procedures being performed. The volume injected may be, for 25 example, 2-5 cc for a typical vertebral repair and as high as 8-12 cc or higher for repairs of other types of bones. Other volumes may be appropriate, depending for example, on the volume of space and the desired effect of the injection. In some cases, a large volume of viscous cement is loaded into a delivery device and several vertebrae are repaired in a single medical procedure. Optionally, one or more 30 cannulae or needles are employed to perform multiple procedures. Viscous cements according to exemplary embodiments of the invention may be delivered at a desired flow rate through standard orthopedic cannulae by 21/09/11,jb19474 specification.13 - 14 applying sufficient pressure. Exemplary average injection rates may be in the range of 0.01 to 0.5 ml/sec, optionally about 0.05, about 0.075 or 0.1 ml/sec or lesser or intermediate or greater average flow rates. Optionally, the flow rate varies significantly during an injection period (e.g., pulse injections). Optionally, the flow 5 rate is controlled manually or using electronic or mechanical circuitry. In an exemplary embodiment of the invention, medical personnel view the cement as it is being injected (e.g. via fluoroscopy) and adjust a flow rate and/or delivery volume based upon observed results. Optionally, the flow rate is adjusted and/or controlled to allow a medical practitioner to evaluate progress of the procedure based upon 10 medical images (e.g. fluoroscopy) acquired during the procedure. In an exemplary embodiment of the invention, the cement is sufficiently viscous that advances into the body when pressure is applied above a threshold and ceases to advance when pressure is reduced below a threshold. Optionally, the threshold varies with one or more of cement viscosity, cannula diameter and cannula length. 15 Comparison of exemplary formulations according to some embodiments of the invention to previously available formulations Although PMMA has been widely used in preparation of bone cement, previously available PMMA based cements were typically characterized by a persistent liquid state after mixing of components. 20 In sharp contrast, cements according to some exemplary embodiments of the invention are characterized by essentially no liquid state. Optionally, a direct transition from separate polymer and monomer components to a highly viscous state results from the presence of two or more sub-populations of polymer beads. As a result of formulations based upon bead sub-populations, a viscosity 25 profile of a cement according to an exemplary embodiment of the invention is significantly different from a viscosity profile of a previously available polymer based cement (e.g. PMMA) with a similar average molecular. Because the viscosity profile of previously available PMMA cements is typically characterized by a rapid transition from high viscosity to fully hardened, 30 these cements are typically injected into bone in a liquid phase so that they do not harden during injection. 21/0911 Ijb19474 specification,14 - 15 In sharp contrast, exemplary cements according to the invention remain highly viscous during a long working window 114 before they harden. This long working window permits performance of a medical procedure of several minutes duration and imparts the advantages of the high viscosity material to the procedure. 5 It should be noted-that while specific examples are described, it is often the case that the formulation will be varied to achieve particular desired mechanical properties. For example, different diagnoses may suggest different material viscosities which may, in turn lead to adjustment of one or more of MW (average and/or distribution), bead size and bead surface area. 10 In an exemplary embodiment of the invention, the cement is mixed 112 and reaches high viscosity outside the body. Optionally the materials are mixed under vacuum or ventilated. In this manner, some materials with potentially hazardous by products can be safely mixed and then used in the body. In an exemplary embodiment of the invention, the cement is formulated so 15 that its mechanical properties match the bone in which it will be injected/implanted. In an exemplary embodiment of the invention, the cement is formulated to mechanically match healthy or osteoporotic trabecular (cancellous) bone. Optionally, the mechanical properties of the bone are measured during access, for example, based on a resistance to advance or using sensors provided through a cannula or by 20 taking samples, or based on x-ray densitometry measurements. In an exemplary embodiment of the invention, strength of the cement varies as a function of one or more of a size of the high MW sub-population and/or a relationship between bead size and bead MW. In general, PMMA is stronger and has a higher Young modulus than 25 trabecular bone. For example, healthy Trabecular bone can have a strength of between 1.5-8.0 mega Pascal and a Young modulus of 60-500 mega Pascal. Cortical bone, for example, has strength values of 65-160 mega Pascal and Young modulus of 12-40 giga Pascal. PMMA typically has values about half of Cortical bone (70-120 mega Pascal strength). 30 Fig. 2 is a plot of viscosity as a function of time for an exemplary bone cement according to the present invention. The figure is not drawn to scale and is provided to illustrate the principles of exemplary embodiments of the invention. The 21/09/1 Ijb19474 specification, 15 - 16 end of a mixing process is denoted as time 0. Mixing is deemed to end when 95 100% of acrylic polymer beads have been wetted with monomer. The graph illustrates an exemplary bone cement which enters a high viscosity plastic phase upon mixing so that it has substantially no liquid phase. 5 Fig. 2 illustrates that once a high viscosity is achieved, the viscosity remains relatively stable for 2, optionally 5, optionally 8 minutes or more. In an exemplary embodiment of the invention, this interval of stable viscosity provides a working window 114 (indicated here as At,) for performance of a medical procedure. In an exemplary embodiment of the invention, stable viscosity means that the 10 viscosity of the cement changes by less than 200 Pascal-second during a window of at least 2 minutes optionally at least 4 minutes after mixing is complete. Optionally, the window begins 1, 2, 3, 4 or 5 minutes after mixing begins or lesser or intermediate times. In an exemplary embodiment of the invention, the viscosity of the cement remains below 1500, optionally 2000 Pascal-second for at least 4, 15 optionally at least 6, optionally at least 8, optionally at least 10 minutes or intermediate or greater times from onset of mixing. For purposes of comparison, the graph illustrates that an exemplary prior art cement reaches a viscosity comparable to that achieved by an exemplary cement according to the invention at time zero at a time of approximately 10.5 minutes post 20 mixing and is completely set by about 15.5 minutes (At 2 ). A working window 114 during which viscosity is between 400 and 2000 Pascal-second for an exemplary cement according to some embodiments of the invention (At,) is both longer and earlier than a comparable window for an exemplary prior art cement (At 2 ). Optionally, (At,) begins substantially as soon as 25 mixing is complete. Exemplary cement formulations According to various exemplary embodiments of the invention, changes in the ratios between a powdered polymer component and a liquid monomer component can effect the duration of working window 114 and/or a viscosity of the cement 30 during that window. Optionally, these ratios are adjusted to achieve desired results. 21/09/1 1Jb19474 specification,16 - 17 In an exemplary embodiment of the invention, the powdered polymer component contains PMMA (69.3% w/w); Barium sulfate (30.07% w/w) and Benzoyl peroxide (0.54% w/w). In an exemplary embodiment of the invention, the liquid monomer 5 component contains MMA (98.5% v/v); N, N-dimethyl-p-toluidine (DMPT) (1.5% v/v) and Hydroquinone (20 ppm). In a first exemplary embodiment of the invention, 20 + 0.3 grams of polymer powder and 9 1 0.3 grams of liquid monomer are combined (weight ratio of ~2.2:1). 10 In a second exemplary embodiment of the invention, 20 + 0.3 grams of polymer powder and 8 ± 0.3 grams of liquid are combined (weight ratio of 2.5:1). Under same weight ratio of second exemplary embodiment (2.5:1), a third exemplary embodiment may include a combination of 22.5 ± 0.3 grams of polymer powder and 9 + 0.3 grams of liquid. 15 In general, increasing the weight ratio of polymer to monomer produces a cement which reaches a higher viscosity in less time. However, there is a limit beyond which there is not sufficient monomer to wet all of the polymer beads. Optionally the powdered polymer component may vary in composition and contain PMMA (67-77%, optionally 67.5-71.5% w/w); Barium sulfate (25-35%; 20 optionally 28-32% w/w) and Benzoyl peroxide (0.4 -0.6 % w/w) and still behave substantially as the powder component recipe set forth above. Optionally the liquid monomer component may vary in composition and contain Hydroquinone (1-30 ppm; optionally 20-25 ppm) and still behave substantially as the liquid component recipe set forth above. 25 Viscosity measurements over time for exemplary cements In order to evaluate the viscosity profile of different exemplary batches of cement according to some embodiments of the invention, a bulk of pre-mixed bone cement is placed inside a Stainless Steel injector body. Krause et al. described a method for calculating viscosity in terms of applied force. ("The viscosity of acrylic 30 bone cements", Journal of Biomedical Materials Research, (1982): 16:219-243). This article is fully incorporated herein by reference. 21/09/11 jb19474 specification, 17 - 18 In the experimental apparatus an inner diameter of the injector body is approximately 18 mm. A distal cylindrical outlet has an inner diameter of approximately 3 mm and a length of more than 4 mm. This configuration simulates a connection to standard bone cement delivery cannula/bone access needle. A piston 5 applies force (F), thus causing the bone cement to flow through the outlet. The piston is set to move with constant velocity of approximately 3 mm/min. As a result, piston deflection is indicative of elapsed time. The experimental procedure serves as a kind of capillary extrusion rheometer. The rheometer measures the pressure difference from an end to end of the 10 capillary tube. The device is made of an 18 mm cylindrical reservoir and a piston. The distal end of the reservoir consist of 4 mm long 3 mm diameter hole. This procedure employs a small diameter needle and high pressure. Assuming steady flow, isothermal conditions and incompressibility of the tested material, the viscous force resisting the motion of the fluid in the capillary is equal to the applied force 15 acting on the piston measured by a load cell and friction. Results are presented as force vs. displacement. As displacement rate was constant and set to 3 mm/min, the shear rate was constant as well. In order to measure the time elapses from test beginning, the displacement rate is divided by 3 (jog speed). Fig. 3 indicates a viscosity profile of a first exemplary batch of cement 20 according to the invention as force (Newtons) vs. displacement (mm). The cement used in this experiment included a liquid component and a powder component as described above in "Exemplary cement formulations". In this test (Average temperature: 22.3*C; Relative Humidity: app. 48%) the cement was mixed for 30-60 seconds, then manipulated by hand and placed inside 25 the injector. Force was applied via the piston approximately 150 seconds after end of mixing, and measurements of force and piston deflection were taken. At a time of 2.5 minutes after mixing (0 mm deflection) the force applied was higher than 30 N. At a time of 6.5 minutes after mixing (12 mm deflection) the force applied 30 was about 150 N. At a time of 7.5 minutes after mixing (15 mm deflection) the force applied was higher than 200 N. 21/09/11 jbl9474specification,18 - 19 At a time of 8.5 minutes after mixing (18 mm deflection) the force applied was higher than 500 N. At a time of 9.17 minutes after mixing (20 mm deflection) the force applied was higher than 1300 N. 5 Fig. 4 indicates a viscosity profile of an additional exemplary batch of cement according to the invention as force (Newtons) vs. displacement (mm). The cement in this test was prepared according to the same formula described for the experiment of Fig. 3. In this test (Average 21.l*C; Relative Humidity: app. 43%) the cement was mixed for approximately 45 seconds, then manipulated by hand and 10 placed inside the injector. Force was applied via piston approximately 150 seconds after end of mixing, and measurements of force and piston deflection were taken. At a time of 2.25 minutes after mixing (0 mm deflection) the force applied was higher than 30 N. At a time of 8.25 minutes after mixing (18 mm deflection) the force applied 15 was about 90 N. At a time of 10.3 minutes after mixing (25 mm deflection) the force applied was higher than 150 N. At a time of 11.4 minutes after mixing (28.5 mm deflection) the force applied was higher than 500 N. 20 At a time of 12.25 minutes after mixing (30 mm deflection) the force applied was higher than 800 N. Results shown in Figs. 3 and 4 and summarized hereinabove illustrate that exemplary bone cements according to some embodiments the invention achieve high viscosity in 2.25 minutes or less after mixing is completed. Alternatively or 25 additionally, these cements are characterized by short mixing time (i.e. transition to highly viscous plastic phase in 30 to 60 seconds). The exemplary cements provide a "working window" for injection of 4.5 to 6.3 minutes, optionally longer if more pressure is applied and/or ambient temperatures are lower. These times correspond to delivery volumes of 14.9 and 20.8 ml respectively (vertebroplasty of a single 30 vertebra typically requires about 5 ml of cement). These volumes are sufficient for most vertebral repair procedures. These results comply with the desired 21/09/1 1jb19474 specification,19 -20 characteristics described in Fig. 2. Differences between the two experiments may reflect the influence of temperature and humidity on reaction kinetics. Molecular weight distribution In an exemplary embodiment of the invention, the average molecular weight 5 (MW) is skewed by the presence of one or more small sub-population of beads with a molecular weight which is significantly different from a main sub-population of polymer beads. The one or more small sub-population of beads may have a MW which is significantly higher and/or significantly lower than the average MW. In an exemplary embodiment of the invention, the presence of even a 10 relatively small sub-population of polymer beads with a MW significantly above the average MW causes the cement to achieve a high viscosity in a short time after wetting of polymer beads with monomer solution. Optionally, increasing a size of the high MW sub-population increases the achieved viscosity. Alternatively or additionally, increasing an average MW of the high MW sub-population increases 15 the achieved viscosity and/or decreases the time to reach high viscosity. Optionally, the one or more small sub-population of beads are provided in a formulation in which, the average molecular weight of PMMA in all beads is 80,000, optionally 100,000, optionally 120,000, optionally 140,000, optionally 160,000, optionally 180,000, optionally, 250,000, optionally 325,000, optionally 375.000, 20 optionally 400,000, optionally 500,000 Dal ton or intermediate or lesser or greater values. In another exemplary embodiment of the invention, the average molecular weight of the acrylic polymer in the beads is in the range of about 130,000 to 170,000, optionally about 160,000 Dalton. 25 In an exemplary embodiment of the invention, a main sub-population of PMMA beads has a MW of about 150,000 Dalton to about 500,000 Dalton, optionally about 250,000 Dalton to about 300,000 Dalton, optionally about 275,000 Dalton to about 280,000 Dalton. Optionally, about 90-98% [w/w], optionally about 93% to 98%, optionally about 95% to 97% of the beads belong to the main sub 30 population. In an exemplary embodiment of the invention, a second high MW sub population of PMMA beads has a MW of about 600,000 Dalton, to about 5,000,000 21/09/11 jb19474 specification,20 - 21 Dalton, optionally about 3,000,000 Dalton to about 4,000,000 Dalton, Optionally about 3,500,000 Dalton to about 3,900,000 Dalton. Optionally, approximately 0.25% to 5% [w/w], optionally about 1% to 4%, ; optionally about 2% to 3% of the beads belong to this high MW sub-population. Optionally, this high molecular weight sub 5 population comprises a styrene co-polymer. In an exemplary embodiment of the invention, a higher molecular weight in this sub-population of beads contributes to a high viscosity within 2, optionally within 1, optionally within 0.5 minutes or less of wetting of polymer beads with monomer solution. In an exemplary embodiment of the invention, a third low MW sub 10 population of PMMA beads has a MW in the range of about 1,000 Dalton to about 75,000 Dalton, optionally about 10,000 Dalton to about 15,000 Dalton, optionally about 11,000 Dalton to about 13,000 Dalton. Optionally, approximately 0.5 to 2.0% [w/w], optionally about 1% of the beads belong to this sub-population. Optionally the MW sub-populations are distinct from one another. This can 15 cause gaps between sub-populations with respect to one or more parameters. In an exemplary embodiment of the invention, the sub-populations are represented as distinct peaks in a chromatographic separation process. Optionally, the peaks are separated by a return to baseline. Depending upon the sensitivity of detection, a background level of noise may be present. Optionally, gaps are measured relative to 20 the noise level. Optionally the sub-populations abut one another so that no gaps are apparent. In an exemplary embodiment of the invention, the sub-populations are represented as overlapping peaks in a chromatographic separation process. In this case, there is no return to baseline between the peaks. 25 Experimental analysis of an exemplary batch of cement Sub-populations characterized by an average molecular weight were identified and quantitated using chromatographic techniques known in the art. Exemplary results described herein are based upon GPC analysis. Each peak in the GPC analysis is considered a sub- population. Similar analyses may be conducted 30 using HPLC. Results are summarized in table 1. 21/09/11jb]9474 specification,21 - 22 Table I: MW distribution of polymer beads based upon GPC analysis of a bone cement according to the powdered polymer component described in "Exemplary cement formulations" hereinabove. Fraction % of total PDI' Mw MnI 1 _ 96.5 1.957 278,986 142,547 2 - 2.5 1.048 3,781,414 3,608,941 3 1.0 1.009 12,357 12,245 100.0 2.955 373,046 126,248 5 1 polydispersity index (PDI), is a measure of the distribution of molecular weights in a given polymer sample and is equal to MW/Mn. 2 MW is the weight average molecular weight in Daltons 3 Mn is the number average molecular weight in Daltons 10 Table I illustrates an exemplary embodiment of the invention with three sub- populations of acrylic polymer beads. The main sub-population (fraction 1) of PMMA beads has a molecular weight (MW) of 278,986 Dalton. About 96.5% of the beads belong to this sub 15 population. A second sub-population (fraction 2) of PMMA beads has MW of 3,781,414 Dalton. Approximately 2.5% of the beads belong to this sub-population. A third sub-population of PMMA beads (fraction 3) has an MW of 12,357 Dalton. Approximately 1% of the beads belong to this sub-population. 20 In an exemplary embodiment of the invention, cement comprising these three sub-populations is characterized by a short mixing time and/or achieves a viscosity of 500 to 900 Pascal-second in 0.5 to 3, optionally 0.5 to 1.5 minutes from the beginning of mixing and/or which remains below 2000 Pascal-second for at least 6 to 10 minutes after mixing. A short mixing time followed by a long working 25 window is considered advantageous in orthopedic procedures where operating room availability and medical staff are at a premium. Size Distribution In an exemplary embodiment of the invention, the bone cement is characterized by beads with a size distribution including at least two sub-populations 30 of polymer beads. 21/09/1 Ijb19474 specification,22 - 23 In an exemplary embodiment of the invention, polymer bead diameter is in the range of 10-250 microns, with a mean value of approximately 25, 30, 40, 50, 60 microns, or a lower or a higher or an intermediate diameter. In an exemplary embodiment of the invention, sub- populations of beads are defined by their size. 5 - Optionally, a main sub-population of polymer (e.g. PMMA) beads is characterized by a diameter of about 20 to about 150, optionally about 25 to about 35, optionally an average of about 30 microns. Beads in this main sub-population are optionally far smaller than the smallest beads employed by Hernandez et al. (2005; as cited above). Presence of small beads can contribute to a rapid increase in 10 viscosity after wetting with monomer. Optionally a second sub-population of large polymer beads is characterized by a diameter of about 150 microns or more. Presence of large beads can slow down the polymerization reaction and prevent hardening, contributing to a long working window. 15 Optionally, the remaining beads are characterized by a very small average diameter, for example less than 20, optionally less than 15, optionally about 10 microns or less. Presence of very small beads can facilitate rapid wetting with monomer liquid during mixing and contribute to a fast transition to a viscous state with substantially no liquid phase. Microscopic analysis indicates that the beads are 20 typically spherical or spheroid. Hernandez et al. (2005; as cited above) examined the possibility of adjusting the average polymer bead size by combining two types of beads with average sizes of 118.4 p (Colacry) and 69.7p (Plexigum) together in different ratios. However, Hernandez's goal was a formulation which is "liquid enough to be 25 injected". All formulations described by Hernandez are characterized by an increase in viscosity from 500 Pascal-sec to 2000 Pascal-sec in about two minutes or less (corresponds to window 114). Hernandez does not hint or suggest that there is any necessity or advantage to increasing the size of this window. Microscopic analysis also indicated that the barium sulfate particles are 30 present as elongate amorphous masses with a length of approximately 1 micron. In some cases aggregates of up to 70 microns in size were observed. In some cases, barium sulfate particles and polymer beads aggregated together. Optionally, 21/09/1 ljb19474 specification,23 - 24 aggregates of Barium sulfate and polymer beads can delay wetting of polymer beads by monomer. In an exemplary embodiment of the invention, MMA solvates and/or encapsulates the PMMA polymer beads and the viscosity of the initial mixture is 5 -high due to the solvation and/or friction between the beads. As the beads dissolve viscosity remains high due to polymerization which increases the average polymer MW. The following table II shows an exemplary particle size distribution, for example, one suitable for the cement of Table I, based on an analysis of particles 10 within the ranges of 0.375- 2000 microns: Table II: Particles size distribution of an exemplary powdered component Vol. % 10 25 50 75 90 Max Beads 2.3 25.75 45.07 60.68 76.34 Diameter [microns] 15 Experimental analysis of a second exemplary batch of cement Another example of a cement kit for mixture includes a liquid and a powder, which includes a mass of acrylic polymer beads. This cement kit is formulated as follows: (a) liquid (9.2 gr) 20 (i) Methylmethacrylate (MMA) 98.5% (vol) (ii) N,N-dimethyl-p-toluidine 1.5% (vol) (iii) Hydroquinone 20ppm (vol) (b) powder (20 gr) (i) Polymethylmethacrylate (PMMA) 69.39% (weight) 25 (ii) Barium Sulfate 30.07% (weight) (iii) Benzoyl Peroxide 0.54% (weight) As noted above, in other formulations the amounts may be varied, for example, to achieve specific mechanical (or other) properties, or they may be varied and achieve same mechanical properties. In another variation, medication may be 30 added to the powder and/or liquid phases. Other liquid phases may be used as well, 21/09/11 ,jb19474 specification,24 - 25 for example, as known in the art for PMMA-type cements. The ratios may be varied, for example, as described above. Table III summarizes a molecular weight distribution of the acrylic bead component of this exemplary cement. It is hypothesized that providing a non-normal 5 distribution of molecular weights with a heavier molecular weight component (e.g., by skewing the MW distribution by including relatively higher molecular weight beads) provides an increased immediate viscosity. In an exemplary embodiment of the invention, the higher MW beads are in a relatively small amount (for example, less than 20%, less than 10%, less than 5%) and have a MW of between 500,000 to 10 2,000,000 Dalton, optionally 600,000 to 1,200,000 Dalton (for example as shown in the table below). Table III: MW distribution of polymer beads of a bone cement of the second exemplary batch Range of Molecular Weights [Dalton] % of total 1,000,000-2,000,000 0.38% 500,000-1,000,000 3.6% 250,000-500,000 12.4% 100,000-250,000 36.4% 50,000-100,000 26.6% 25,000-50,000 14.2% 10,000-25,000 5.3% 8,000-10,000 0.5% 5,000-8,000 0.4% 15 In an exemplary embodiment of the invention, the bone cement is characterized by beads with a size distribution including at least two sub-populations of different materials. Optionally, at least two sub-populations include polymer (e.g. PMMA) beads and Barium Sulfate particles. Optionally, the range of particles 20 diameter of the Barium Sulfate is 0.01-15 microns, optionally 0.3 to 3 microns, optionally with an average of about 0.5 microns or lesser or intermediate or greater sizes. In an exemplary embodiment of the invention, polymer bead diameter is in the range of 10-250 microns, optionally 15-150 microns, with a mean value of 25 approximately 25, 30, 40, 50, 60 microns. Lower or a higher or intermediate diameters are possible as well, for example, based on the setting considerations 21/09/11 jb 19474 specificarion,25 -26 described above. In some cases, large particle sizes, for example, particles having diameters exceeding 120 microns (e.g., when the average diameter is on the order of 60 microns) are the result of Barium sulfate primary particle aggregation on PMMA particle beads. 5 An exemplary distribution of bead sizes for the exemplary cement of table III, based on an analysis of particles within the range of 0.04-2000 microns, is described in Table IV: Table IV: Particles size distribution of a second exemplary powdered component of bone cement 10 Vol. % 10 25 50 75 90 Max Beads 2 9 46.5 70.7 90.5 Diameter [microns] Fig. 5 is a graph which visually shows the values of table IV Size and MW are independent variables In an exemplary embodiment of the invention, size based and MW based 15 sub-populations are determined independently. For example, MW may be determined chromatographically and size may be determined by microscopic analysis. As a result, beads classed in a single size sub-population may be classed in two or more MW sub-populations and/or beads classed in a single MW sub population may be classed in two or more size sub- populations. 20 Mechanical viscosity increasing agents In an exemplary embodiment of the invention, the cement includes particles characterized by a large surface which do not participate in the polymerization reaction. The large surface area particles can impart added viscosity to the cement mixture independent of polymerization. Optionally, the added viscosity comes from 25 friction of particles against one another in the cement. Examples of materials which do not participate in the polymerization reaction but increase viscosity include, but are not limited to Zirconium, hardened acrylic polymer, barium sulfate and bone. Optionally, materials which do not participate in the polymerization reaction but increase viscosity can at least partially 21/09/11 jb19474 specification,26 -27 substitute for high MW polymers in influencing a viscosity profile. Desired Polymerization Reaction Kinetics In an exemplary embodiment of the invention, mixture of polymer and monomer produces a high viscosity mixture with substantially no intervening liquid 5 phase within 180, optionally within 120, optionally within 100, optionally within 60, optionally within 30, optionally within 15 seconds or greater or intermediate times from onset of mixing. In an exemplary embodiment of the invention, once a high viscosity is achieved, the viscosity remains stable for 5 minutes, optionally 8 minutes, optionally 10 10 minutes or lesser or intermediate or greater times. Optionally, stable viscosity indicates a change of 10% or less in two minutes and/or a change of 20% or less in 8 minutes. The time during which viscosity is stable provides a working window for performance of a medical procedure. These desired reaction kinetics can be achieved by adjusting one or more of 15 average polymer MW, polymer MW distribution, polymer to monomer ratio and polymer bead size and/or size distribution. General considerations In an exemplary embodiment of the invention, a powdered polymer component and a liquid monomer component are provided as a kit. Optionally, the 20 kit includes instructions for use. Optionally, the instructions for use specify different proportions of powder and liquid for different desired polymerization reaction kinetics. In an exemplary embodiment of the invention, a bone cement kit including at least two, optionally three or more separately packaged sub-populations of beads 25 and a monomer liquid is provided. Optionally, the kit includes a table which provides formulations based on combinations of different amounts of bead sub-populations and monomer to achieve desired properties. It is common practice in formulation of acrylic polymer cements to include an initiator (e.g. benzoyl peroxide; BPO) in the powdered polymer component and/or 30 a chemical activator (e.g. DMPT) into the liquid monomer component. These components can optionally be added to formulations according to exemplary 21/09/11 jb 19474 specification,27 - 28 embodiments of the invention without detracting from the desired properties of the cement. Optionally, an easily oxidized molecule (e.g. hydroquinone) is added to the liquid component to prevent spontaneous polymerization during storage (stabilizer). 5 The hydroquinone can be oxidized during storage. Optionally, cement may be rendered radio-opaque, for example by adding a radio- opaque material such as barium sulfate and/or zirconium compounds and/or bone (e.g. chips or powder) to the powder and/or liquid component. While the above description has focused on the spine, other tissue can be 10 treated as well, for example, compacted tibia plate and other bones with compression fractures and for fixation of implants, for example, hip implants or other bone implants that loosened, or during implantation. Optionally, for tightening an existing implant, a small hole is drilled to a location where there is a void in the bone and material is extruded into the void. 15 It should be noted that while use of the disclosed material as bone cement is described, non-bone tissue may optionally be treated. For example, cartilage or soft tissue in need of tightening may be injected with a high viscosity polymeric mixture. Optionally, the delivered material includes an encapsulated pharmaceutical and is used as a matrix to slowly release the pharmaceutical over time. Optionally, this is 20 used as a means to provide anti-arthritis drugs to a joint, by forming a void and implanting an eluting material near the joint. It should be noted that while use of PMMA has been described, a wide variety of materials can be suitable for use in formulating cements with viscosity characteristics as described above. Optionally, other polymers could be employed by 25 considering polymer molecular weight (average and/or distribution) and/or bead size as described above. Optionally, at least some of the beads include styrene. In an exemplary embodiment of the invention, styrene is added to MMA beads in a volumetric ratio of 5-25%. Optionally, addition of styrene increases creep resistance. According to various embodiments of the invention, a bone cement 30 according to the invention is injected into a bone void as a preventive therapy and/or as a treatment for a fracture, deformity, deficiency or other abnormality. Optionally, the bone is a vertebral body and/or a long bone. In an exemplary embodiment of the 21/09/11 jb19474 specification,28 - 29 invention, the cement is inserted into the medullary canal of a long bone. Optionally, the cement is molded into a rod prior to or during placement into the bone. In an exemplary embodiment of the invention, the rod serves as an intra-medular nail. Exemplary Characterization Tools 5 Molecular weight and polydispersity can be analyzed, for example by Gel permeation chromatography(GPC) system (e.g. Waters 1515 isocratic HPLC pump with a Waters 2410 refractive-index detector and a Rheodyne (Coatati, CA) injection valve with a 20- iL loop (Waters Ma)). Elution of samples with CHCl 3 through a linear Ultrastyragel column (Waters; 500-A pore size) at a flow rate of 1 ml/min 10 provides satisfactory results. It will be appreciated that various tradeoffs may be desirable, for example, between available injection force, viscosity, degree of resistance and forces that can be withstood (e.g. by bone or injection tools). In addition, a multiplicity of various features, both of method and of cement formulation have been described. It should be 15 appreciated that different features may be combined in different ways. In particular, not all the features shown above in a particular embodiment are necessary in every similar exemplary embodiment of the invention. Further, combinations of the above features are also considered to be within the scope of some exemplary embodiments of the invention. In addition, some of the features of the invention described herein 20 may be adapted for use with prior art devices, in accordance with other exemplary embodiments of the invention. Section headers are provided only to assist in navigating the application and should not be construed as necessarily limiting the contents described in a certain section, to that section. Measurements are provided to serve only as exemplary 25 measurements for particular cases, the exact measurements applied will vary depending on the application. When used in the following claims, the terms "'comprises", "comprising", "includes", "including" or the like means "including but not limited to". It will be appreciated by a person skilled in the art that the present invention 30 is not limited by what has thus far been described. Rather, the scope of the present invention is limited only by the following claims. 21/09/I jb19474 spccification,29

Claims (35)

1. A bone cement comprising an acrylic polymer mixture, the cement characterized in that it achieves a.viscosity of at least 500 Pascal-second within 180 5 seconds following initiation of mixing of a monomer component and a polymer component and characterized by sufficient biocompatibility to permit in-vivo use:

2. A bone cement according to claim 1, wherein the viscosity of the mixture remains between 500 and 2000 Pascal-second for a working window of at least 5 minutes after the initial period. 10

3. A bone cement according to claim 2, wherein the working window is at least 8 minutes long.

4. A bone cement according to claim 1, wherein the mixture includes PMMA.

5. A bone cement according to claim I , wherein the mixture includes 15 Barium Sulfate.

6. A bone cement according to claim 4, wherein the PMMA is provided as a PMMA/styrene copolymer.

7. A bone cement according to claim 4, wherein the PMMA is provided as a population of beads divided into at least two sub-populations, each 20 sub-population characterized by an average molecular weight.

8. A bone cement according to claim 7, wherein a largest sub population of PMMA beads is characterized by an MW of 150,000 Dalton to 300,000 Dalton.

9. A bone cement according to claim 7, wherein a largest sub 25 population of PMMA beads includes 90-98% (w/w) of the beads.

10. A bone cement according to claim 7, wherein a high molecular weight sub-population of PMMA beads is characterized by an average MW of at least 3,000,000 Dalton.

11. A bone cement according to claim 7, wherein a high molecular 30 weight sub-population of PMMA beads includes 2 to 3% (w/w) of the beads. 21/09111 jb19474 specification,30 -31

12. A bone cement according to claim 7, wherein a low molecular weight sub-population of PMMA beads is characterized by an average MW of less than 15,000 Dalton.

13. A bone cement according to claim 7, wherein a low molecular 5 weight sub-population of PMMA beads includes 0.75 to 1.5 % (W/W) of the beads.

14. A bone cement according to claim 4, wherein the PMMA is provided as a population of beads divided into at least two sub-populations, each sub-population characterized by an average bead diameter.

15. A bone cement according to claim 14, wherein at least one bead 10 sub-population of characterized by an average diameter is further divided into at least two sub-sub-populations, each sub-sub-population characterized by an average molecular weight.

16. A bone cement according to claim 14, wherein the PMMA is provided as a population of beads divided into at least three sub-populations, each 15 sub-population characterized by an average bead diameter.

17. A bone cement according to claim 1, further comprising processed bone and/or synthetic bone.

18. A bone cement according to claim 1, characterized in that the cement achieves a viscosity of at least 500 Pascal-second when 100% of a polymer 20 component is wetted by a monomer component.

19. A bone cement according to claim 1, wherein the viscosity is at least 800 Pascal-second.

20. A bone cement according to claim 1, wherein the viscosity is at least 1500 Pascal-second. 25

21. A bone cement according to claim 1, wherein the viscosity is achieved within 2 minutes.

22. A bone cement according to claim 1, wherein the viscosity is achieved within 1 minute.

23. A bone cement according to claim 1, wherein the viscosity is 30 achieved within 45 seconds. 21/09/1 jb19474 specification.31 -32

24. A bone cement comprising: a polymer component; and a monomer component; wherein contacting the polymer component and the monomer 5 component produces a mixture which attains a viscosity -greater than 200 Pascal second within 1 minute from onset of mixing and remains below 2000 Pascal-second until at least 6 minutes from onset of mixing.

25. A bone cement according to claim 24, wherein the polymer component comprises an acrylic polymer. 10

26. A particulate mixture formulated for preparation of a bone cement, the mixture comprising: (a) 60 to 80% polymer beads comprising a main sub-population characterized by an MW of 150,000 Dalton to 300,000 Dalton and a high molecular weight sub-population characterized by an MW of 3,000,000 Dalton to 4,000,000 15 Dalton; and (b) 20 to 40% of a material which is non-transparent with respect to X-ray.

27. A mixture according to claim 26, wherein the polymer beads comprise a third subpopulation characterized by an MW of 10,000 Dalton to 15,000 20 Dalton.

28. A method of making a polymeric bone cement, the method comprising: (a) defining a viscosity profile including a rapid transition to a working window characterized by a high viscosity; 25 (b) selecting a polymer component and a monomer component to produce a cement conforming to the viscosity profile; and (c) mixing the polymer component and a monomer component to produce a cement which conforms to the viscosity profile.

29. A cement kit, comprising: 30 (a) a liquid component including a monomer; and (b) a powder component including polymeric beads, characterized in that said powder component is provided in a substantially non-normal distribution 21/09/1 Ijbl9474 specification,32 -33 of at least one of molecular weight of the polymeric beads and size of powder particles such that a cement mixed from the kit has both an increased immediate viscosity and an increased working window as compared to a cement having a substantially normal distribution. 5

30. A cement kit according to claim 29, wherein said substantially non normal distribution is a skewed distribution.

31. A cement kit according to claim 29, wherein said substantially non normal distribution comprises a relatively small component including higher molecular weight beads. 10

32. A cement kit according to claim 31, wherein said component has an average molecular weight of at least a factor of 2 of an average molecular weight of said polymeric beads.

33. A cement kit according to claim 32, wherein said factor is at least 3. 15

34. A cement kit according to claim 32, wherein said factor is at least 5.

35. A cement kit according to claim 29, wherein said substantially non normal distribution comprises a relatively small component including smaller sized particles. 20 21/09/11 jb19474 specification.33