The laboratory development of a new metacarpophalangeal prosthesis

A test procedure for artificial finger joints

This paper highlights the lack of an agreed testing standard for artificial finger joints. It reviews the anatomy, pathology and biomechanics of finger joints as well as the various designs of finger prostheses and the machines used to test them. While pre-implantation testing should be fundamental, increasing regulation of the biomedical engineering industry will further demand testing of prostheses to pre-agreed standards. Standards relating to the testing of other artificial joints are reviewed before possible parameters for testing finger prostheses are offered.

The design of a finger wear simulator and preliminary results

A dual-cycle finger wear simulator has been designed, manufactured and commissioned. The simulator interspersed dynamic flexion-extension motion under light load with a heavier static 'pinch' load to a test prosthesis immersed in a lubricant heated to 37 degrees C. A validation test was undertaken on a size 2 Swanson prosthesis, leading to prosthesis failure in less than 1 million cycles. A second test was carried out on a Durham metacarpophalangeal prosthesis. After 4.8 million cycles a total wear factor for the joint of 0.60 x 10(-6) mm3/N m was calculated, with no cracks or damage visible. Both test results compare well with earlier tests undertaken on the Stokoe finger wear simulator.

Implant arthroplasty of the hand: retrospective and prospective considerations

Small joint arthroplasty has lagged behind the development of that in large joints because of their small sizes, different shapes, presence within kinetic chains, complex soft tissue investments, presence of adjacent rays, secondary displacement and contracture, and the differing requirements of degenerative and rheumatoid arthritis. Prosthetic development must take into consideration range of motion, stability, tendon moment arms, fixation, ease of implantation, biocompatibility, wear and strength characteristics, and soft tissue reconstruction. The metacarpophalangeal, interphalangeal, and trapeziometacarpal joints each present different problems in the design of kinematic equivalent prostheses. One-piece polymeric designs have advantages in cost, adaptability, and known performance but show degradation of function with time. Total joint designs have the potential of better simulating normal joint function but have shown tendencies to subsidence, loosening, and breakage. The rigidity of hinge joints limits the damping of out-of-plane forces and places greater stress on bone-stem interfaces, whereas global designs have poor constraint features.

Computer model analysis of the Swanson and Sutter metacarpophalangeal joint implants

A representative model which mimics the behaviour of Silastic finger metacarpophalangeal joint implants was constructed using a finite element software package. The modelled implants were moved through a range of flexion, lateral deviation and a combination of both. Pistoning of both implants stems occurred within the modelled medullary cavities. For equivalent flexion angles, the Sutter implant produced a higher stress field than the Swanson implant, and the field was positioned at the central hinge mechanism. In both implants, lateral deviation increased the internal stress concentrations more than when pure flexion was applied. Overall the Swanson style of implant had lower stress magnitudes than the Sutter implant, and it is predicted that the Sutter implant will be more likely to fail than the Swanson. The failure mode for the Sutter implant would be at the central hinge region. The Swanson implant is likely to fail at the central hinge-stem interface regions.

Metacarpophalangeal joint prostheses. A review of the clinical results of past and current designs

The clinical results of past and current hinge, flexible and third generation designs of MP prosthetic joints are reviewed. The hinged prostheses did not achieve acceptable short term clinical results while the silastic and third generation prostheses provided good results with with correction of deformity and adequate range of motion (ROM). These good short term results did, however, get progressively worse with the recurrence of deformities and loss of ROM. It is evident that while most of the existing prostheses can relieve pain and restore appearance, none provide the degree of stability and ROM that is required to restore normal function to the MP joint. The moderate results could be partly due to the stage of the disease at which the surgery is carried out. At present, surgery on patients with rheumatoid arthritis is undertaken at a stage in the disease where the muscles and the ligaments surrounding the joint, and the bone, are generally in a poor condition. Surgery at this stage is really only a salvage procedure.

Metacarpophalangeal joint prostheses: a review of past and current designs

The anatomy and biomechanics of the metacarpophalangeal (MCP) joint are briefly described. Hinge, flexible and surface designs of past and current MCP prosthetic joints are reviewed, outlining their respective advantages and disadvantages. Although existing prostheses can restore cosmetic appearance and relieve pain, none can equal the stability and versatility of the natural joint. Delayed reconstructive surgery may be partly responsible for the mediocre results experienced, since the later the surgery the worse will be the condition of the muscles and ligaments surrounding the joint. These are the structures responsible for strength, movement and stability of the joint. From a mechanical viewpoint it may be desirable to operate at an earlier stage of the disease than is currently indicated, but this is a clinical decision. Some design aspects, namely fixation and wear, require a different approach when designing an MCP prosthesis from that adopted in the case of prosthetic hips and knees.

Alumina ceramic finger implants: a preliminary biomaterial and clinical evaluation

An alumina ceramic hinge-type finger prosthesis, composed of alumina ceramics and high-density polyethylene, is described. Biomaterial evaluations, including flexion-extension tolerance, stretching, twisting, extraction tests, and histologic examinations of affinity for bone, demonstrated that this implant has characteristics superior to those of previous finger implants. Eighteen proximal interphalangeal joints and 13 metacarpophalangeal joints have been replaced with this alumina ceramic finger implant. Follow-up studies (12 to 31 months) have been most encouraging with satisfactory functional recovery and no fracture or dislocation of the implants.