Introduction The clinical use of an all-polymer knee which articulated a polyacetal femoral component against an ultra high molecular weight polyethylene (UHMWPE) tibial component has been reported [1]. A ‘polyacetal group’ of 63 total knee replacements were followed for at least ten years and no instances of femoral component fracture or failure due to wear occurred [1]. Such results are remarkable for an all-polymer pros-thesis in such a heavily loaded joint as the knee. Recently a wear screening device has been described which reproduced in vitro the clinical wear rates reported for three biopolymers which have been employed as the acetabular cup material in hip prostheses [2]. Given this validated rig, the objective of the work reported here was to undertake wear tests of polyacetal against UHMWPE.

Materials and Methods The polyacetal and UHMWPE couples were tested using a modified, four-station, pin-on-plate wear test rig [2]. The modification entailed the addition of rotational motion to the test pins, in addition to the standard reciprocating motion, to give multi-directional motion. In the wear tests, two stations had reciprocation-only and two applied multi-directional motion. Investigating the influence of both types of motion permitted a fuller tribological analysis to be undertaken. Control pins and control plates were included to account for any weight change due to lubricant uptake. A load of 40N was employed and reciprocating and rotating speeds of 1Hz were chosen. The lubricant consisted of 25% bovine calf serum and 75% distilled water, which was heated to 37°C during testing. A standardised cleaning and weighing protocol was followed, and the pins and plates were weighed on a balance sensitive to 0.1mg.

Results and Discussion After an average of 1.4 million cycles of sliding, the mean wear factors were: UHMWPE pins rubbing against polyacetal plates, 1.5 x 10-6mm3/ Nm under reciprocation, and 4.1 x 10-6mm3/Nm under multi-directional motion. For polyacetal pins rubbing against UHMWPE plates they were 0.7 x 10-6mm3/ Nm under reciprocation, and 2.8 x 10-6mm3/Nm under multi-directional motion. As can be seen, the wear factors depended on both the orientation of the material, whether it was a pin or a plate, and the motion it was subjected to. The increase in weight of the polyacetal control components due to lubricant uptake was many times that of the UHMWPE components. For example the UHMWPE control plate showed an increase of 0.2mg compared with 33.4mg for the polyacetal control plate. Using the same wear screening rig, the wear factors for UHMWPE articulating against stainless steel were measured to be 0.1 x10-6mm3/Nm under reciprocating motion and 1.1 x10-6mm3/Nm under multi-directional motion [2]. Though greater than this latter value, the all-polymer wear factors were not excessively high and were less under reciprocation-only. How much multi-directional motion, or cross-shear, it is appropriate to apply to a wear simulation of an artificial knee joint is worth further investigation, as it may be much less than in the hip joint.

Introduction Finger prostheses lack the long-term clinical success associated with hip and knee replacements. The most commonly implanted type of finger prostheses consists of single-piece silicone designs such as the Swanson, the Sutter and the NeuFlex [1]. Such designs act as flexible spacers around which a process of encapsulation can occur. A recent long-term study stated that, at an average of 14 years after surgery, Swanson meta-carpophalangeal (MCP) prostheses showed a fracture rate of 67% compared with 52% for Sutter MCP prostheses [2]. A 2005 paper reported that, at 2 years follow-up, the fracture rates were 13% and 20% respectively for these two designs [3]. Perhaps such high rates could be reduced if a better understanding of the nature of fracture of these implants was attained.

Materials and Methods Twelve Sutter MCP prostheses were obtained from three hands (two dominant) of two women and one man who were aged 56–66 years at time of surgery [4]. They were retrieved at a mean of 42 (range 32–53) months following implantation. All patients had rheumatoid arthritis. Of the twelve explanted prostheses, eleven had fractured, ten completely. These fractured prostheses were visually examined and were then sliced so that, after washing and gold-coating, the two fracture faces of each prosthesis could be examined using a Hitachi S-4700 scanning electron microscope (SEM).

Results and Discussion All of the ten total fractures occurred at the junction of the distal stem and the hinge of the implant. Visual inspection showed that the initial point of fracture was on the dorsal aspect of the prosthesis, indicating that fracture is due to the subluxing forces seen in rheumatoid MCP joints. Also, the fracture began distally and travelled in a slightly proximal direction as well as in the dominant dorsal to palmar direction. For the prostheses removed from a right hand, it appeared that the crack direction was also from ulnar to radial. When all of the fracture faces were examined by SEM, significant variation was seen. Some fracture faces appeared to show surface gouging of the material, which may have been caused by bone after fracture had taken place, therefore indicating that fracture had occurred long before the prostheses were removed. In contrast another fracture face showed what appeared to be a region of gradual abrasion, perhaps caused by osteophytes, next to a relatively smooth zone which could have indicated an area of rapid fracture or tearing. The author is not aware of any similar topographical analysis having been undertaken elsewhere on fractured, ex-vivo silicone MCP prostheses. While the time span between fracture and removal of the implant can never be known precisely, so that the ‘virgin’ fracture face could have been damaged post-fracture, it is still hoped that such ex-vivo analysis can contribute to improved finger prostheses.