Injections of hyaluronic acid solutions, often known as visco-supplements, into the joints of patients suffering from osteoarthritis are an accepted therapy. In most replacement joints, wear of the biomaterials used in them is a critical concern. For in vitro wear tests of such materials the recommended lubricant is one based on bovine serum. However, unlike synovial fluid, bovine serum does not contain hyaluronic acid. The aim of the work reported here was to take a clinically used hyaluronic acid solution, Ostenil, and to investigate its influence on the wear of two orthopaedic biopolymers

Ultra high molecular weight polyethylene (UHMWPE) and poly tetra fluoro ethylene (PTFE) were tested in turn using a four-station, multi-directional, pin-on-plate wear test rig which had previously been shown to reproduce clinical wear factors for UHMWPE, PTFE and polyacetal. For each biopolymer three lubricants were employed: 33% bovine serum (2 stations); 33% bovine serum + Ostenil (1 station); and distilled water + Ostenil (1 station). Polymeric test pins were subject to a load of 40N and articulated against polished stainless steel plates. Wear factors were determined by dividing the volume lost by the product of the load and the sliding distance (units × 10-6mm3/Nm).

The UHMWPE wear tests ran to 66.3km sliding distance. The addition of Ostenil to dilute bovine serum resulted in a wear factor of 1.4 × 10-6mm3/Nm for UHMWPE. The wear factor was 1.6 × 10-6mm3/Nm when dilute bovine serum alone was used as the lubricant. This shows good agreement with a wear factor of 2.1 × 10-6mm3/Nm reported for failed UHMWPE acetabular cups. PTFE provides an accelerated wear test with clinical validity. In the presence of 33% bovine serum a mean wear factor for PTFE of 40 × 10-6mm3/Nm was measured. The wear factor was 59 × 10-6mm3/Nm for dilute bovine serum plus Ostenil. For explanted PTFE acetabular cups a wear factor of 37 × 10-6mm3/Nm has been calculated. For both polymers wear was least when the lubricant was distilled water plus Ostenil. However a transfer film was found and such films are not clinically valid.

This paper outlines the recent development of an exchange Travelling Fellowship scheme between the British and American Orthopaedic Research Societies.

The first metatarsophalangeal (MTP) joint is the key joint of the foot in terms of function during gait. Various replacement toe joint prostheses are commercially available but unlike other replacement joints such as the hip or knee, few simulator based studies have been conducted to evaluate the performance and reliability of these prostheses. Presented are results obtained using a newly developed and validated multi-station MTP joint test-rig that is able to simulate the natural biomechanics of the toe joint. The developed simulator is a multi-station computer controlled electro-pneumatic metataso-phalangeal joint simulator that applies dynamic loading and motions commensurate with the walking gait cycle. This involves the combination of plantar-dorsi flexion range of 32 degrees, 5 degrees of inversion/eversion and toe-off dynamic loading peaking at up to 820 N. Presented are the validation and in vitro test results of MTP joint simulations carried out on silastic and articulating metal and polymer designs of MTP prostheses. Despite being subjected to a reduced loading regime of 300 N peak force, the silastic prostheses were found to perform poorly in the simulator, ultimately failing due to a combination of fatigue crack growth and joint collapse. This behaviour and failure mode was consistent with that of ex vivo origin silastic MTP prostheses examined and provides confidence in the validity of the simulator.

Introduction: The wear of orthopaedic biopolymers is recognised as a major factor in the failure of total joint replacements. Clinical wear data exists for acetabular cups manufactured from three biopolymers: ultra high molecular weight polyethylene (UHMWPE); poly tetra fluoro ethylene (PTFE); and polyacetal. The aim of this paper was to wear test these biopolymers and compare the results with clinical data.

Method and Materials: The biopolymers were tested using a modified, four-station, pin-on-plate wear rig [1]. In the tests, two of the four stations applied reciprocating motion and two applied multi-directional motion. Biopolymer pins articulated against stainless steel plates under a load of 40N. The lubricant consisted of 25% bovine serum and 75% distilled water. A standardised cleaning and weighing protocol was followed, and the biopolymer wear factors were calculated by dividing the volume lost by the product of the load and the sliding distance.

Discussion and Conclusions: Failed and retrieved UHMWPE acetabular cups have been reported as having a clinical wear factor of 2.1 x 10⁻⁶mm³/Nm [2]. However, UHMWPE cups which have been functioning well until removal at post-mortem have been said to show 45 to 69% less wear than revised UHMWPE cups [3]. Combining these values suggests clinical wear factors for functional UHMWPE in the range of 0.95 to 1.45 x 10⁻⁶mm³/Nm. This range fits well with the value of 1.1 x 10⁻⁶mm³/Nm shown in table 1 for UHMWPE under multi-directional motion. A clinical wear factor of 37 x10⁻⁶mm³/Nm has been calculated for PTFE acetabular cups [4]. When compared with the mean wear factor for PTFE pins under multi-directional motion obtained from the pin-on-plate rig, the match is remarkable. For polyacetal cups a mean volumetric wear of 136mm³/ year has been reported [5] and it has been calculated that explanted hip prostheses averaged 1.54 million cycles/year [2]. In polyacetal acetabular cups of 37mm diameter, an average sliding distance of 25mm/cycle can be calculated [6] and it has been said that an equivalent static load of 1000N applies [7]. Taking these four values permits a clinical wear factor for polyacetal cups of 3.5 x 10⁻⁶mm³/Nm to be calculated. This number compares well with the value of 3.8 x 10⁻⁶mm³/Nm seen for the polyacetal test pins under multi-directional motion. In summary, all three biopolymers subject to multi-directional motion exhibited clinically relevant values of wear.

Metacarpophalangeal (MCP) arthroplasty usually involves the fitting of a silicone spacer, commonly Swanson prosthesis, but more recently the Sutter prosthesis has been introduced.

Four Sutter MCP prostheses, two each sized 30 and 40, were removed from the right hand of a female patient. The patient aged 61 years ate revision, had longstanding rheumatoid arthritis. Using a single station finger stimulator¹ two Sutter size 50 MCP prostheses were tested. This stimulator ran at a speed of 100 cycles per minute. During each of these cycles, which flexed the test prosthesis through a 90° arc of motion, the load across the test prosthesis varied between 10N and 15N after 3000 cycles, the stimulator applied a static ‘pinch’ load and the whole combined load cycle began again. Ringer solution heated at 37°C was used as a lubricant. Clinically, the prostheses had been implanted for 53 months. All four had fractured at the junction of the hinge and distal stem. In the simulator tests the Sutter size 50 prosthesis managed just over 10 million cycles of flexion-extension, including over 3300 ‘pinch’ loads before fracture occurred, at the junction of the distal stem and hinge. The second prosthesis fractured in the same manner after 5.3 million cycles of flexion-extension.

These are the first reported in vitro results of fracture of Sutter prosthesis as well as the first paper to state the site of ex vivo fractures of Sutter prostheses. A computer model described in a recent paper ² indicated that failure of the Sutter prosthesis should occur at the central hinge region. Clearly the in vitro results and the ex vivo experience disagree with the computer model. McArthur and Milner ³ have shown clinically that the Swanson joint appears to be superior to the Sutter implant, a result confirmed elsewhere4. The finger stimulator has previously caused fracture of Swanson pros-thesis in a time and a manner comparable with surgical experience¹. Therefore another correlation with ex vivo results, but testing the Sutter prostheses has further validated the finger simulator.