A Flexible Implant For Replacement Of Arthritic Or Destroyed Joints In The Hand

Alfred B. Swanson, M.D.

Restoring function to a hand disabled by unstable, stiffened, or dislocated joints continues to be a challenge to those surgeons interested in the care of patients with these afflictions.

The functional efficiency of hand adaptation patterns depends upon muscle balance and the integrity of the skeletal system. Loss of joint function is extremely common in rheumatoid arthritis. Instability of joints, stiffness, and nonfunctional positioning lead to a gradual and disastrous decline in the functional a-bility of the hand.1,2 This loss of hand use may occur early in the lives of these patients and will severely limit them in their quest for economic and personal independence.3,4

During the past ten years advances in surgical techniques have significantly improved the possibilities for restoration of hand function.5,6,7 In the hands of skilled surgeons, synovial resection, restoration of tendon balance, arthrodesis, and resection arthroplasty have been used with reasonable success.8,9,10, 11,12 Attempts to replace joints with metal-hinged implants have met with mixed success but limited acceptance.13 Nevertheless, the concept of joint replacement is excellent, since the restoration of joint motion and stability with provision for early motion has obvious advantages over resection arthroplasty or joint fusion.

The key problem in joint replacement revolves around the development of an ideal implant material. The criteria for the ideal implant for joint replacement are:

  1. Normal joint relationship and stability must be maintained without interference with joint movement.

  2. The design and material must interfere as little as possible with the flexion-extension arc while offering lateral stability.

  3. The material must be biologically inert; also it must not cause bone absorption due to unphysi-ological mechanical stresses.

  4. The implant must consist of a minimum of parts, be durable, be resistant to stress and breakdown, and, preferably, be softer than bone.

  5. The implant should be inexpensive and uncomplicated, and its use should require a minimum of time and special instrumentation.

Joint-Resection Arthroplasty

Joint-resection arthroplasty works well in the hand if the joint space can be kept open and the joint alignment maintained. This procedure frequently requires postoperative fixation with pins and external support. The resultant fixation, if overused, will compromise the expected range of useful motion. A considerable number of these cases gradually stiffen and subluxate. Exceptional results do occur and are related to the proper development of a supportive, fibrous capsule which is organized during the time when a guarded range of motion can be developed. The proper amount of flexion, extension, limitation of lateral movement, and dorsopalmar subluxation is difficult to obtain, but necessary for a good result. One of the most important functions for the implant is to hold the joint in proper alignment internally, while allowing early motion, until this new functionally adapted, fibrous capsule can mature. The implant thus exerts its most important contribution early in the postoperative course.

Silicone Rubber Implants

Experience obtained in the development of an intramedullary stemmed Silicone rubber implant to cushion the long bone in lower-extremity amputations suggested the use of this material in the development of implants for replacement of diseased and destroyed joints. 14

A flexible, heat-molded joint-replacement implant of Silicone rubber (Silastic #373-Dow Corning) has been designed and used for metacarpophalangeal joint arthroplasty in the rheumatoid arthritic hand (Fig. 1 ). The results have been interesting and rewarding. Implants of Silastic also have been used in proximal interphalangeal joint arthroplasties and in the replacement of carpal bones. The intramedullary stemmed, one-piece implant satisfactorily fulfills the specified requirements for joint replacements. It provides a degree of stability, and because of its intrinsic flexibility can be used as a flexible hinge. It is as biologically inert as any implant material known. It is softer than bone and is not as likely as metal implants to stimulate absorption phenomena. It has withstood machine flexion tests of more than 2 1/2 million repetitions. The implant is inexpensive and its use requires a minimum of time and special instrumentation. It is easy to handle and if necessary can be trimmed and shaped to some degree at the time of surgery.

A variety of implants have been designed to meet different size requirements. Two years of experience with these Silastic implants in the hands of 14 patients of all ages has demonstrated that they can be a useful adjunct to the armamentarium of the hand surgeon (Fig. 2A/2B ).

The rehabilitation of these hands in the early postoperative course is extremely important. The use of dynamic bracing, applied two days postoperatively, and a continuing well-organized program to maintain joint motion must be part of the treatment program (Fig. 3 and 4 ).

Alfred B. Swanson, M.D. is the Medical Consultant Area Child Amputee Center Grand Rapids, Michigan

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