Abstract
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.
Correspondence should be addressed to Mr Carlos Wigderowitz, Honorary Secretary BORS, University Dept of Orthopaedic & Trauma Surgery, Ninewells Hospital & Medical School, Dundee DD1 9SY.
None of the authors have received anything of value from a commercial or other party related directly or indirectly to the subject of the presentation
References
[1] Joyce, TJ Expert Review Medical Devices, 20041(2): 193–204. Google Scholar
[2] Goldfarb, CA & PJ Stern J Bone Joint Surg, 200385A(10): 1869–1878. Google Scholar
[3] Moller, K et al. J Hand Surg, 200530(1): 8–13. Google Scholar
[4] Joyce, TJ et al J Hand Surg, 200328B(1): 86–91. Google Scholar
