Orthopaedic polymer-on-polymer bearings

Abstract

The invention is a prosthetic joint, for example, knee joint, including a first joint component which is preferably polyetheretherketone containing barium sulphate and a second joint component which is preferably a polyolefin.

Claims

1. An orthopaedic prosthetic joint comprising a joint couple comprising: (a) a first joint component having a first bearing surface comprising a first material which comprises a first polymer; (b) a second joint component having a second bearing surface comprising a second material which comprises a second polymer; wherein said first and second bearing surfaces are in sliding engagement with one another; wherein said first polymer is harder than said second polymer; wherein said first material includes a polyaryletherketone and barium sulfate wherein the joint is a knee joint; wherein the second material comprises polyethylene cross-linked by irradiation; wherein said first bearing surface includes at least 1 wt % barium sulphate and includes 30 wt % or less of barium sulphate, wherein said polyaryletherketone has a melt viscosity (MV) of at least 0.35 kNsm.sup.−2; and wherein said first joint component consists essentially of polyetheretherketone and barium sulphate; and at leas 80 wt % of said second joint component is made up of said second polymer.

2. An orthopaedic prosthetic joint comprising a joint couple comprising: (a) a first joint component having a first bearing surface made of a first polymer; and (b) a second joint component having a second bearing surface made of a second polymer; wherein said first and second bearing surfaces are in sliding engagement with one another; wherein said first bearing surface is made of a first material which is harder than a second material from which the second joint surface is made; wherein said first material comprise a polyaryletherketone and barium sulphate; wherein the joint is a knee joint; wherein the second material comprises polyethylene cross-linked by irradiation; and wherein said first bearing surface includes at least 1 wt % barium sulphate and includes 30 wt % or less of barium sulphate; wherein said polyaryletherketone has a melt viscosity (MV) of at least kNsm.sup.2, and wherein said first joint component consists essentially of polyetheretherketone and barium sulphate; and at least 80 wt % of said second joint component is made up of said second polymer.

3. A joint according to claim 1, wherein said barium sulphate has a D.sub.10 particle size in the range 0.1 to 1.0 μm.

4. A joint according to claim 1, wherein the ratio of the wt % of barium sulphate to the wt % of said first polymer is greater than 0.04 and is less than 0.4.

5. A joint according to claim 1, wherein said polyaryletherketone has a repeat unit of formula (I) ##STR00002## where t1 and w1 independently represent 0 or 1 and v1 represents 0, 1 or 2.

6. A joint according to claim 1, wherein said polyaryletherketone has a crystallinity of greater than 25%.

7. A joint according to claim 1, wherein said second material comprises at least 60 wt % of a polymeric material selected from polyethylene, a polyurethane and a polyamide.

8. A joint according to claim 2, wherein said barium sulphate has a D.sub.10 particle size in the range 0.1 to 1.0 μm.

9. A joint according to claim 2, wherein the ratio of the wt % of barium sulphate to the wt % of said first polymer is greater than 0.04 and is less than 0.4.

10. A joint according to claim 2, wherein said polyaryletherketone has a repeat unit of formula (I) ##STR00003## where t1 and w1 independently represent 0 or 1 and v1 represents 0, 1 or 2.

11. A joint according to claim 2, wherein said polyaryletherketone has a crystallinity of greater than 25%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings in which:

(2) FIG. 1 shows the Volumetric Wear Rate in units of mm.sup.3/MC.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS AND BEST MODE

(3) Any feature of any aspect of the invention or embodiment described herein may be combined with any other feature of any aspect of an invention or embodiment described herein mutatis mutandis.

(4) Specific embodiments of the invention will now be described by way of example, with reference to FIG. 1 which is a graph showing volumetric wear rates for a series of samples.

(5) The following materials are referred to hereinafter:

(6) PEEK-OPTIMA (Trade Mark)—Long term implantable grade polyetheretherketone with a melt viscosity of approximately 0.45kNsm.sup.−2, obtainable from Invibio Limited, UK;

(7) GUR 1020 E (Trade Mark)—ultra-high molecular weight polyethylene containing Vitamin E obtained from Ticona;

(8) GUR 1020 (Trade Mark)—as previous but not containing Vitamin E;

(9) MOTIS (Trade Mark)—polyetheretherketone containing carbon fibres obtainable from Invibio Ltd, UK;

(10) Barium sulphate—Barium sulphate grade 10175 from Merck.

(11) Pin-on-disc testing was undertaken to assess various combinations of materials as described below.

EXAMPLES 1 to 8

Manufacture of Pins and Discs

(12) Materials to be tested were machined into discs and pins as applicable. As manufactured, the pins had a diameter of 9 mm and length of 12 mm; the articulating end of the pins was machined flat. The discs were 10 mm thick, with the upper articulating surface of diameter 24.8 mm, with a circumferential groove to accommodate an o-ring to seal the chamber onto the disc. Prior to the start of the test, the discs and pins were soaked in deionized water for 56 days to allow for fluid uptake to reach equilibrium. The compositions of the discs and pins are detailed in Table 1.

(13) TABLE-US-00001 TABLE 1 Example No. Composition Test Specimen 1. PEEK-OPTIMA LT-1 with 6 wt % barium Disc sulphate dispersed 2. PEEK-OPTIMA LT-1 with 6 wt % barium Pin sulphate dispersed 3. GUR 1020 E Pin 4. GUR 1020 E Disc 5. PEEK OPTIMA LT-1 Disc 6. MOTIS Disc 7. GUR 1020 Pin 8. PEEK-OPTIMA LT-1 Pin

EXAMPLE 9

Testing Equipment and Method

(14) A T87 multi-station Pin-on-Disc machine (SuperPOD) was used, available from

(15) Phoenix Tricology, UK. The discs were mounted in a test bath on top of a motion module. Water was circulated through the test bath at 37±3° C. A lubricant chamber (volume 15 ml) was mounted around each disc, so that each station had an independent volume of bovine serum lubricant. Each test pin was mounted on a carrier rod, restrained against rotation, with each pin independently loaded by means of a pneumatic piston. Air was supplied to the pistons from a common plenum chamber, resulting in constant air pressure applied to each piston, so each pin had an equal load applied. The load applied to each pin was 128 N, giving a nominal pressure of 2.01 MPa (pin diameter 9 mm).

(16) The lubricant was Thermo Scientific Hyclone Wear Testing Fluid (Bovine serum) with 20 g/l total protein concentration. An antibiotic and fungicide were each added at a concentration of 1.5% to reduce the deterioration of the test fluid due to fungal and bacterial action during each 0.25 MC interval. At the end of each interval, the test fluid for each material combination was collected and frozen.

(17) In the T87 SuperPOD, the pins were stationary and the discs followed an elliptical path without any rotation. The motion module was mounted on X and Y linear bearings and actuated through a double scotch yoke mechanism. The amplitude was 5 mm (stroke 10 mm) in the X direction and the amplitude was 2.5 mm (stroke 5 mm) in the Y direction. This gave a total path length of 24.2 mm per cycle. Throughout the test, the frequency of motion was 1 Hz. The motion of the SuperPOD meant the pins were subjected to full cross shear, but the discs were only subjected to cross shear in the limited zone in the middle of the “worn area” where the wear track crossed itself during the cycle.

(18) For each combination of materials referred to, five specimens were tested in wear stations on the SuperPOD and one as a load soak control. The load soak was maintained under the same load as the wear specimens in an identical solution of bovine serum, but was not subjected to the sliding motion. The discs and pins were randomly assigned to the material pairing and either to a load soak or wear station. The location of the five stations testing each material pair was randomly distributed throughout the 100 stations on SuperPOD.

(19) Every 0.25 MC the wear test was stopped for interval analysis and the test specimens removed for characterisation. At each interval, the lubricant was collected from each of the five wear stations for each material combination and frozen for potential future particle analysis. The pins and discs were cleaned and dried using the procedure described in ASTM F1714 “Standard Guide for Gravimetric Wear Assessment of Prosthetic Hip Designs in Simulator Devices”. The o-rings, chambers and pin holders were subjected to the same cleaning process but were not dried. Photographs were taken of the pins and discs at each interval analysis.

(20) The samples were weighed three times in rotation using a Sartorius balance (Bohemia, N.Y.). During each interval analysis, the load soak was dried and weighed and used to compensate for the fluid uptake of the specimens in the wear stations. The increase in mass of the load soak due to fluid uptake was added to the reduction in mass of each sample to give a corrected wear.

(21) The gravimetric wear loss was converted to a volumetric wear loss using the material densities.

(22) The wear factor was calculated for each material couple and was intended to allow comparisons between wear tests using different loads and sliding distance. The wear factor was derived from the Archard Wear Equation, which is based on the principle that the wear volume is proportional to the product of the sliding distance and real area of contact.

(23) The wear factor k was calculated by:

(24) $k=\frac{V}{\mathrm{WxL}}$
where V is the volumetric wear (mm.sup.3/), W is the normal load (N) and L the sliding distance (m).

(25) The units are mm.sup.3/Nm.

EXAMPLES 10-16

(26) A series of wear couples were tested as detailed in Table 2 and volumetric wear rates established. Results are summarized in FIG. 1. For each wear couple the wear rate for the pin is shown on the left and that for the disc is shown on the right.

(27) TABLE-US-00002 TABLE 2 Example No. Pin Material Disc Material 10. 3 1 11. 7 6 12. 3 6 13. 7 5 14. 3 5 15. 2 4 16. 8 4
The following should be noted from FIG. 1. (a) Example 13 relates to a combination of PEEK and ultra-light molecular weight polyethylene. The example can be compared to Example 11 which shows a combination of carbon fibre filled PEEK bearing against the same polyethylene. The comparison illustrates the effect on the wear of the polyethylene in adding filler into the PEEK-wear on the polyethylene increases significantly. (b) Example 14 may be compared with Example 12 as described in (a) above and the same conclusions drawn. (c) Example 10 includes a disc which comprises PEEK and barium sulphate bearing against a pin which comprises a polyethylene. This may be compared to Example 14 which is the same combination except that the disc comprises pure PEEK with no barium sulphate. In contrast to the effect of filler discussed in (a) and (b), it will be noted that the wear on the polyethylene for both Examples 10 and 14 are similar, meaning the addition of barium sulphate has had surprisingly little effect on wear. (d) Example 15 includes a pin which comprises PEEK and barium sulphate bearing against a disc which comprises polyethylene. This may be compared to Example 16 which is the same combination except that the pin comprises pure PEEK with no barium sulphate. Thus, as described in (c), it should be appreciated that the comparison surprisingly shows addition of barium sulphate has little effect on wear.

(28) This, it should be appreciated that barium sulphate can be added to PEEK for one part of a bearing component, at a level sufficient to give image contrast, with surprisingly little effect on wear of an associated polyethylene bearing component in use.

(29) The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.