GB2341323A

GB2341323A - Prosthesis

Info

Publication number: GB2341323A
Application number: GB9819875A
Authority: GB
Inventors: Christopher Pardoe; Alan M Wilson; James Schumacher
Original assignee: Royal Veterinary College
Current assignee: Royal Veterinary College
Priority date: 1998-09-11
Filing date: 1998-09-11
Publication date: 2000-03-15
Also published as: GB9819875D0

Abstract

A prosthesis is provided which comprises a tensile member 6 having first 16 and second 26 ends, and first 8 and second 12 load distributing members for anchoring said first and second ends. It is suitable for use in equine laryngoplasty.

Description

2341323 Prosthesis The present invention relates to a prosthesis.

one known use of prostheses is in laryngoplasty, a common treatment for horses affected by left recurrent laryngeal neuropathy.

Left recurrent laryngeal neuropathy is a condition which causes decreased motor activity of most of the intrinsic muscles on the left side of the larynx. In particular, the cricoarytenoideus dorsalis muscle which is the primary dilator of the larynx is affected by this condition. Thus, the left arytenoid cartilage and associated soft tissues of affected horses collapse into the airway during exercise causing inspiratory stertor and exercise intolerance.

In order to overcome this problem, the surgical technique of laryngoplasty has been developed, whereby normal airflow may be restored to exercising horses affected with left recurrent laryngeal neuropathy.

In known laryngoplasty techniques, the prosthesis used usually consists of one or two strands of No. 2 or non absorbable suture material such as polyester or nylon. Various techniques have been described but the general intent of all of these is to prevent collapse of the arytenoid cartilage and associated soft tissues into the laryngeal lumen during exercise by placement of a suture between the muscular process of the arytenoid and cricoid cartilages to simulate the action of the contracted cricoarytenoideus dorsalis muscle.

In the known techniques, a hypodermic or suture needle is used to form respective tunnels through the arytenoid and cricoid cartilages and the suture material is passed through the tunnels. The free ends of the suture material are then brought together and tied to form a loop of suture material acting in tension to pull the arytenoid cartilage towards the cricoid cartilage.

However, although the arytenoid cartilage is fixed in abduction at surgery, endoscopic examinations performed after convalescence indicate that, in a substantial proportion of horses, the degree of abduction achieved at surgery is not permanent. This failure to maintain abduction is believed to be due to the prosthesis in the form of a loop gradually cutting through the laryngeal cartilages. In particular, the tightening of the looped sutures during placement is believed to produce a sawing action on the muscular process of the arytenoid cartilage which may precipitate failure.

Efforts to reduce the likelihood of failure have included anchoring the prosthesis with a suture to the adjacent lamina of the thyroid cartilage where the prosthesis enters and exits the muscular process. In addition, a U shaped Teflon (TM) implant has been provided between the caudal border of the cricoid cartilage and the prosthesis to prevent the prosthesis from cutting the caudal border of the cricoid cartilage.

However, these solutions are used in addition to the usual surgical procedure and are thus time-consuming for the surgeon.

Thus, the present invention provides a prosthesis comprising a tensile member having first and second ends, and first and second load distributing members for anchoring said first and second ends.

The provision of load distributing members in use reduces the load applied at any one point on animal parts and thus reduces the likelihood of the tension member cutting through the animal parts. A sawing action created by bringing the ends of the tensile member together for tying can be avoided.

The ends of the tensile member may be held relative to the load distributing members in any convenient way.

For example, they could be welded or otherwise attached thereto. The prothesis is preferably constructed such that during installation the first and second load distributing members can swivel relative to the tensile member. This will allow the load distributing members to adapt their orientation relative to the tensile members, so as to assist positioning of the prosthesis during implantation. Thus, rather than being fixed at 900 to the tensile member, the load distributing members can swivel to an appropriate angle. The tensile member does not have to bend at the join, thereby benefiting load distribution.

Preferably,, the prosthesis comprises a retaining means, e.g. a bead, fixed at said first end of the tensile member for limiting axial displacement of the first load distributing member along the tensile member.

Thus, when the tensile member is pulled taught, the is retaining means will rest against the first load distribution member, thus anchoring the end of the tensile member to an animal part. The retaining means preferably has a curved, e.g. convex surface for engagement against a corresponding curved surface of the first load distributing member, thereby providing a swivel joint.

At the other end of the tensile member, a connection may again be formed between the end of the tensile member and the second load distributing member in any suitable manner. Preferably, the prosthesis comprises a connector which during installation of the prosthesis is positionable at a desired location along the length of the tensile member and is then securable to the tensile member to limit axial displacement of the second load distributing member along the tensile member.

This will allow the tension in the tensile member to be adjusted to the required level before the connection is formed. The second load distributing member can be correctly positioned along the tensile member before the connector is secured. The second load distributing member may be formed with a hole through which the tensile member passes, and which allows the second load distributing member to move along the length of the tensile member into a desired position. Thus, for example, the tensile member may be threaded through a tunnel in an animal part before the second load distributing member is threaded onto the tensile member and moved into position. Once the second load distributing member is secured in position, any excess length of tensile member can be cut away, leaving the second load distributing member anchoring the second end of the tensile member.

In a preferred embodiment, the connector is securable to the tensile member by crimping.

The connector for the second load distributing is member may comprise a curved, e.g. convex, surface for engagement against a corresponding curved surface of the second load distributing member, thereby providing a swivel joint. Preferably, the connector comprises a ball provided on the tensile member for engagement with the second load distributing member.

As described above the load distributing members reduce the tendency of the tensile member to cut through the animal parts. Thus, any shape of member which would effectively distribute the load away from the point at which the tensile member is attached to an animal part could be used, for example, a square or rectangular plate.

Preferably however, the or each load distributing member is of round shape and is formed with a hole for receiving the tensile member therethrough. In preferred arrangements, the hole is of wider diameter than the tensile member, to allow swivelling of the load distributing member on the tensile member.

As will be appreciated, any tensile member capable of sustaining the required loading could be used. Preferably, the tensile member has a breaking force of more than 100ON.

Still more preferably, the tensile member comprises steel cable.

Still more preferably, the tensile member comprises 1.5mm diameter stainless steel cable.

The prosthesis of the invention may be used in any animals, both human and non-human animals. It may be used in various parts of the body, wherever a prosthesis providing a tensile force is required, for example to provide a substitute for a contracted muscle.

When used in an animal, the prosthesis of the invention is preferably arranged with said first and second load distributing members anchored against first and second parts of the animal.

Still more preferably, the tensile member passes is through respective passages formed in the first and second animal parts and the load distributing members abut against respective portions of those animal parts.

Thus, as the tensile member need not be looped around either the first or second part of the animal, the sawing action caused by tightening the sutures of the prior art may be avoided.

Preferably, the prosthesis is for use in equine laryngoplasty.

Thus, one of the first and second animal parts may comprise the cricoid cartilage of an equine larynx and the other of the first and second animal parts may comprise the arytenoid cartilage of an equine larynx.

Preferably, tension in the tensile member fixes the arytenoid cartilage in abduction.

A preferred embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of an equine larynx with a prosthesis installed on one side; Figure 2 is a detail in cross section of each of the ends of the tensile member; and Figure 3 is a schematic view of an equine larynx as - 6 used in experimentation.

Figure 1 shows an equine larynx 2 having a prosthesis 4 implanted therein for treating idiopathic recurrent laryngeal neuropathy.

The prosthesis comprises a steel cable 6 which extends from a first washer 8, anchored in the cricoid cartilage 10 to a second washer 12, anchored in the muscular process of the arytenoid cartilage 14.

The prosthesis before implantation is shown in Figure 2. At a first end 16 of the prosthesis corresponding to the cricoid cartilage end, the cable 6 has a bead 18 formed on the end thereof. The cable passes through a hole 20 at the centre of the washer 8 so that the bead extends beyond the washer. The side of is the washer facing the bead is formed with a hemispherical hollow 22 which corresponds to the form of the bead. Thus, on implantation, when the cable is pulled tight, the bead 18 rests in the corresponding hollow 22 formed in washer 8. Additionally, the opposite face 24 of the washer is flat so that it acts as a load distributing member when the prosthesis has been implanted.

At the end 26 of the prosthesis corresponding to the arytenoid cartilage end, the cable again passes through a hole 28 in the centre of the washer 12. A ball 30 is then provided on the cable outside the washer and is slidable along the cable length.

The washer has a hollow 32 formed in the edge facing the ball which again corresponds substantially to the shape of the ball. The other face 34 of the washer is flat so that the washer may act as a load distributing member.

Finally, a sleeve 36 is placed over the cable outside of the ball 30 and, after implantation, this sleeve may be crimped so as to form a connection between the washer and the cable. Thus the sleeve and the ball together provide a connector between the cable and the 7 washer.

The method of implanting the suture in an equine larynx will now be described with reference to Figure 1:

Firstly, a passage 38 is formed extending from the caudal edge of the cricoid cartilage at the axial notch to a point approximately 10mm rostral to the notch immediately adjacent to the median crest of the cricoid cartilage. Similarly, a passage 40 is formed extending through the centre of the base of the muscular process of the arytenoid cartilage in a caudomedial to rostrolateral direction. The passages are formed using a 16-gauge hypodermic needle and are then expanded to form tunnels using a 14-gauge needle.

Next, the cable is passed through the cricoid cartilage tunnel 38 such that the beaded end and corresponding washer 8 abut against the base of the cricoid cartilage. The cable is then passed through the arytenoid cartilage tunnel such that the end 26 of the cable extends beyond the rostral end of the muscular process of the arytenoid cartilage.

Once the cable extends through both the cricoid and arytenoid cartilage, a washer 12 and a ball 30 are placed onto the cable in that order and slid down the cable until they abut against the arytenoid cartilage.

The cable is then tensioned using a tensioning device. A suitable tensioning device is an Atlas (trade mark) cable tensioner, Model No. 825-210, made by Sofamor Danek of Memphis, Tennessee, which is designed to take a 1mm diameter cable but maybe reamed to a larger diameter, e.g. 1.6 mm to take a 1.5 mm cable.

This device allows the degree of tension applied to the cable (and therefore the force applied to the muscular process) to be measured so that the cable will not be over tightened.

When the required tension has been reached in the cable, the sleeve 36 is tightened against the ball 30 and washer 12 and is then crimped so that the washer is held in place. The curvatures of the bead 18 and the corresponding hollow 22, and of the ball 30 and the corresponding hollow 32, enable the washers to adapt a suitable orientation to the cable, which may not necessarily be 90'.

Exam,ole Exneriment As will be described below, experiments were conducted to compare the function of the prosthesis of a preferred embodiment of the present invention to a standard prior art method. These tests showed that a prosthesis 50 comprising cables 52 and washers 54,56 could withstand significantly higher distractive forces than a laryngoplasty performed using a single polyester suture 58.

Materials and methods Fifteen larynxes, harvested at an abattoir, were stored at -200C, and defrosted at room temperature prior to testing.

Bilateral laryngoplasties were performed on each larynx 60, using a single No.5 (7 metric) polyester suture 58 on one side of the larynx and a single 1.5mm stainless steel cable 52 with a breaking force of more than 1000N, and minimal extensibility at in vivo loads, on the opposite side. The end of the cable was TIG welded to a stainless steel bead, which sat in a central hole in a load- di stribut ing washer 54 (10mm diameter) Selection of the side of the larynx for insertion of each prosthesis was randomized.

Tunnels were formed on each side of the larynx in the cricoid 62 and arytenoid 64 cartilage using a 16gauge hypodermic needle, and the tunnels for receiving the steel cable were enlarged using a 14 gauge needle.

A steel cable with a washer 56 seated at the beaded end was threaded through the tunnel in the muscular process in a rostrolateral to caudomedial direction so that the washer and beaded end of the cable contacted the rostrolateral aspect of the muscular process. Another similar cable with a washer 54 seated at the beaded end was passed through the tunnel in the cricoid cartilage in a caudal to rostral direction so that the washer and beaded end of the cable contacted the ventral surface of the dorsal lamina of the cricoid cartilage. Each cable was passed through a 10mm diameter loop of 12-gauge wire 66 positioned over the dorsum of the cricoid cartilage.

The purpose of the wire loop was to help prevent laryngeal rotation by maintaining orientation of the arms of the prostheses at 1800.

On the contralateral side of the larynx, a single strand of the No.5 polyester suture was threaded through the 16-gauge tunnels in the cricoid cartilage and muscular process. Each arm of the suture was passed in opposite directions through another 10mm diameter wire loop positioned over the dorsum of the cricoid cartilage.

The specimens were then mechanically tested as follows:

The specimens were suspended vertically, rostral end up and the respective suture arms were secured to loops attached to the fixed and moving heads of a servohydraulic materials testing machine. Force and displacement were zeroed, and the sutures distracted to failure at a displacement rate of 20mms-1. Applied force was measured using an in-line piezoelectric load cell, and displacement was measured using a linear voltage displacement transducer attached to the moving head.

Data were logged at 100 samples per second through a 12 bit analogue to a digital card into a personal computer for analysis. Graphs of load versus displacement were generated in a spreadsheet program, and a series of simple formulae used to plot a force-deformation plot and derive values for maximum force, force of first yield, and slope of the linear region of the load- deformation plot. These values were loaded into a second spread sheet. Mean values and standard deviations were calculated for these measurements in two treatment groups. A paired t-test was used to determine if the type of prosthesis had a significant effect on the mechanical properties of the laryngoplasty.

Results Distraction initially caused abduction of the arytenoid cartilage, and the deformation of the larynx and the suture occurred prior to final failure of laryngoplasty. The wire prosthesis was much stiffer than the polyester suture, presumably due to the lower compliance of stainless steel suture and the pulley is arrangement of the polyester suture.

Each laryngoplasty failed eventually by the prosthesis cutting through the muscular process of the arytenoid cartilage. All 15 laryngoplasties performed with suture failed by tearing of the muscular process of the arytenoid cartilage at a mean force of 55.8N ( sd 13.1). Six laryngoplasties performed with the device failed by avulsion of the device from the muscular process at a mean force of 219.6 125.0N. The arytenoid cartilage of the other nine was avulsed from the larynx at a mean force of 206.4 75.3N. Displacement continued until the arytenoid cartilage became restrained by the wire loop, and the device was then torn from the muscular process at a mean force of 357.0 32.0N. The difference in force required to cause failure of the two types of laryngoplasty was significant (p<0.0001).

Determining an exact figure for the stiffness of the two techniques of repair was not possible because a greater length of suture was used in the trial than would be used for surgical repair. Mean slope of the load deformation plot for the two techniques was, however, very different [0.83 0.25N/mm (suture) and 6.40+2.33N/mm (device)] (p<0.0001).

In the study, the washers distributed the force over the cricoid cartilage and muscular process of the arytenoid cartilage. Testing often resulted in avulsion of the entire arytenoid cartilage rather than failure of the muscular process, implying that improving the strength of the device or its ability to be retained within the cartilage was not necessary. Although the wire suture was much stiffer than the polyester suture, the difference in stiffness could not be determined directly because the experimental design involved a much longer length of suture than would be used in surgical repair.

The washer and bead were designed to spread the load over a large area of the muscular process, and the is large diameter of the cable was selected to reduce the likelihood of the suture cutting through the cartilage. The diameter of the 7 metric polyester suture under load was 0.75mm. The length of suture bearing on the muscular process was about Smm yielding a total bearing area of 3.75MM2 and, assuming an even distribution of load, a pressure of 14.9MPa under the suture at failure. By comparison, the metal washer spread the force over a 1Omm-diameter disk with an area of 7 9Mm2, resulting in an equivalent pressure of 0.70MPa assuming that the load was evenly distributed over the washer.

The tensile strength of the implant and its anchorage in the cartilage are both as strong or stronger than the larynx in a single distractiontofailure model. Results obtained in this single distraction-to-failure model are likely to be repeated in a fatigue loading model.

It will be appreciated that the prosthesis and its implantation as described above relate to a preferred embodiment of the invention. Thus, it would be readily apparent to the skilled person that modifications could be made to the prosthesis which would fall within the scope of the invention. For example, the tensile member - 12 could be formed of a material such as nylon rather than steel. In addition, the prosthesis could be used for any relevant applications in human or non-human animal surgery and so should not be considered to be limited to use in equine laryngoplasties.

Claims

Claims:

1. A prosthesis comprising a tensile member having first and second ends, and first and second load distributing members for anchoring said first and second ends.

2. A prosthesis as claimed in claim 1, constructed such that during installation the first and second load distributing members can swivel relative to the tensile member.

3. A prosthesis as claimed in claim 1 or 2, comprising a retaining means fixed at said first end of the tensile is member for limiting axial displacement of the first load distributing member along the tensile member.

4. A prosthesis as claimed in claim 1, 2 or 3, comprising a connector which during installation of the prosthesis is positionable at a desired location along the length of the tensile member and is then securable to the tensile member to limit axial displacement of the second load distributing member along the tensile member.

5. A prosthesis as claimed in claim 4, wherein the connector is securable to the tensile member by crimping.

6. A prosthesis as claimed in claim 4 or 5, wherein the connector comprises a ball provided on the tensile member for engagement with the second load distributing member.

7. A prosthesis as claimed in any preceding claim, wherein the or each load-distributing member is of round shape and is formed with a hole for receiving the - 14 tensile member therethrough.

8. A prosthesis as claimed in any preceding claim, wherein the tensile member has a breaking force of more than 100ON.

9. A prosthesis as claimed in any preceding claim, wherein the tensile member comprises a steel cable.

10. A prosthesis as claimed in any preceding claim, wherein the tensile member comprises a 1.5mm diameter stainless steel cable.

11. A prosthesis as claimed in any preceding claim, wherein the prosthesis is for use in equine laryngoplasty.

12. A prosthesis as claimed in any preceding claim, in an animal and arranged with said first and second load distributing members anchored against first and second parts of the animal.

13. A prosthesis as claimed in claim 12, wherein the tensile member passes through respective passages formed in the first and second animal parts and wherein the load distributing members abut against respective portions of those animal parts.

14. A prosthesis as claimed in claim 12 or 13, wherein one of the first and second animal parts comprises the cricoid cartilage of an equine larynx and the other of the first and second animal parts comprises the arytenoid cartilage of an equine larynx.

15. A prosthesis as claimed in claim 14, wherein tension in the tensile member fixes the arytenoid cartilage in abduction.

- is -

16. A prosthesis substantially as herein described and with reference to the accompanying drawings.