Arthroplasty

Arthroplasty of the MP joint is a useful technique in the attempt to maintain thumb length, relieve stress about the CMC joint, and improve positioning of the thumb tip [15]. Arthro-plasty of the MP joint of the thumb is relatively easy to accomplish, because the MP joint anatomy approximates that of a simple hinge [15].

Brannon and Klein reported the first prosthetic MP joint in 1959 [13]. This early prosthesis was a hinged replica of the MP joint, first constructed of stainless steel and then titanium [13]. Many problems were reported with this device. Out of 12 subjects tested, 2 patients developed deformity because of prosthesis migration into the proximal phalanx. In 2 other patients, a screw at the hinge portion became loose and needed to be tightened. These unsatisfactory results and high number of complications sparked the search for better methods of arthroplasty, and since then various materials and techniques have been developed, such as Swanson's silicone arthroplasty and the hinged metal-polyethylene arthroplasty.

The primary indications for arthroplasty are considerable arthridities in which there appears to be significant involvement of the IP or CMC joint [7,14,15]. It is important to note that most rheumatoid patients are able to accommodate to level of normal activities of daily living, even at the presence of gross clinical deformity. Therefore, it is only in the patient who complains of significant persistent pain caused by destruction of the MP joint that arthroplasty should be considered [14].

Swanson silicone arthroplasty

Silicone rubber prostheses were first introduced in the 1960s by Swanson [7,8]. The need for a material such as silicone arose, as Swanson stated, from the observation that metal implants at that time were shown to be primarily ineffective and had minimal acceptance. Moreover, these implants were shown to corrode after some time, and the bone was unable to tolerate the metal [8,13]. The idea for the use of silicone by Swanson in his prosthesis came from the observed success of intramedullary stemmed silicone rubber implants to cushion the end of long bones after lower extremity amputations. This provided a framework for an inert, flexible, and lightweight alternative to the conventional titanium or stainless steel prostheses.

The material chosen by Swanson and co-workers was Silastic, a silicone dioxide elastomer created by the Dow Corning Corp of Midland, Michigan [7,8]. Swanson made this his material of choice because it is inert, stable at high temperature, and it is very durable [8,14]. In the original report [8], it was stated that this material could withstand greater than 50 million repetitions without failure. The feature of the Swanson arthro-plasty that sets it apart from other types of arthroplasties is that this implant is not a total joint replacement, but rather a noncemented spacer between the metacarpal and proximal phalanx (Fig. 1). As a result of this, instability is of

Fig. 1. Original Swason Silastic MP joint spacer. (From Swanson AB. Silicone rubber implants for replacement of arthritic or destroyed joints in the hand. Surg Clin North Am 1968;48(5):1117; with permission.)

concern, and therefore, adequate reconstruction of the collateral ligaments is paramount.

The surgical technique involves a dorsal incision over the MP joint, with dissection carried down through the subcutaneous tissue until the extensor hoods are exposed [8]. The extensor hood is dislocated ulnarly and the fibers are carefully dissected away from the synovium. A thorough synovectomy is performed, and if there is considerable joint surface ablation, the metacarpal head may be osteotomized. The ulnar and radial collateral ligaments are released and then tagged with 2-0 nonabsorbable suture for future attachment [14]. In RA patients, the collateral ligaments often demonstrate some compromise in their integrity; however, any soft tissue around the ligament may be tagged as part of the reconstruction later. Once ligament/soft-tissue tagging is accomplished, the intramedullary canal of the metacarpal and the proximal phalanx are reamed with the aid of a curette or a broach. The implant is subsequently inserted on the proximal end first, and then the distal end is flexed and guided into the proximal phalanx intramedullary canal. The collateral ligaments or any of the surrounding capsular tissue are then reattached to their anatomical position for stability of the joint. Extensor pollicis brevis (EPB) tendon is subsequently sutured to the proximal phalanx (Fig. 2A). The medial and lateral expansions are sutured over the EPB (Fig. 2B), restoring the intrinsic mechanism. Finally, the EPB tendon is pulled distally and sutured in the mid-line over the MP joint (Fig. 2C). Pulling the tendon distally and anchoring its proximal end relaxes its distal insertion on the distal phalanx, thus relieving extension force over the IP joint [17]. Once this is complete, the wound is copiously irrigated and closed.

Following Swanson's original report on his new prosthesis, Swanson and Herndon [17] reviewed 42 rheumatoid thumbs that had undergone arthroplasty with the Swanson spacer. Follow-up was 2 to 6.5 years. Forty-two of the patients (100%) reported positive results, described as a resolution of their pain. The study authors also reported that 6 individuals who received ar-throplasty had previously received arthrodesis in the opposite thumb. Out of these 6 individuals, 5 reported that their arthroplasty thumb felt better than their fused thumb, and the remaining patient said that he could tell no difference between them. More recent studies have also shown great success from this type of arthroplasty. A 1990 study [18] also showed good results from the

Fig. 2. (A) Once implant has been inserted, EPB tendon is reattached to the proximal phalanx. (B) Medial and lateral expansions are sutured over the EPB tendon. (C) Extensor pollicis longus tendon is pulled distally and sutured to the midline over the MP joint. Pulling the tendon distally and tacking it to the extensor mechanism releases pressure on the distal end, thus releasing flexion restraint from distal phalanx. (From Swanson AB, Herndon JH. Flexible (silicone) implant arthroplasty of the metacarpo-phalangeal joint of the thumb. J Bone Joint Surg Am 1977;59(3):364; with permission from the Journal of Bone and Joint Surgery, Inc.)

Fig. 2. (A) Once implant has been inserted, EPB tendon is reattached to the proximal phalanx. (B) Medial and lateral expansions are sutured over the EPB tendon. (C) Extensor pollicis longus tendon is pulled distally and sutured to the midline over the MP joint. Pulling the tendon distally and tacking it to the extensor mechanism releases pressure on the distal end, thus releasing flexion restraint from distal phalanx. (From Swanson AB, Herndon JH. Flexible (silicone) implant arthroplasty of the metacarpo-phalangeal joint of the thumb. J Bone Joint Surg Am 1977;59(3):364; with permission from the Journal of Bone and Joint Surgery, Inc.)

Swanson thumb MP prosthesis. Forty-three thumbs were looked at in rheumatoid patients. The average range of active motion was reported at 25°, with a flexion arc from 15° to 40°. Forty of these patients (93%) had improvements in activities of daily living. Only one thumb required reoperation with arthrodesis. Moreover, the authors report that only 3 patients showed disease progression radiographically—1 at the IP joint, and 2 at the CMC.

Not all investigators report such great outcomes from the Swanson Silastic prosthesis, however. In a 1975 study out of Sweden [19], the investigators decided to study the Swanson prosthesis radio-graphically on a variety of fingers of patients who had RA, in an effort to determine the integrity and the durability of 104 Silastic implants. The investigators found radiographic evidence of prosthesis failure in many asymptomatic patients. They went on to describe three types of failure of the prosthesis: (1) surface damage (2 of 42), (2) cracking or fragmentation of midsection (14 of 104) (Fig. 3), and (3) stem fracture (11 of 104). Overall, 25% of prostheses failed within 1.5 to 5.5 years. As for the causes of failure of the implant, the study authors attributed them to several different reasons: (1) deformation of implant—this was described as a "pinch," which was a sharp, palmarly bend of the implant at the junction of the distal stem and midsection; (2) laceration of the implant surface— because of the nature of the polymer, any laceration of the surface led to a cracking of the device; (3) lipid absorption—body fluids were absorbed by the silicon dioxide, weakening the polymer [19]. From the study results, it was clear that these pros-theses failed at a much higher rate than had been reported previously, and because many of these failures were not clinically evident, further studies needed to take place with long enough follow-up to make these failures clinically apparent.

Two other studies [20,21] have also shown that the Silastic material is prone to fracture, although these clinical studies did not involve the thumb. Bass and colleagues [20] reported an implant failure rate of 45%, whereas Lourie [22] found an implant failure rate of 27.5%. More surprisingly, both studies reported high patient satisfaction

Fig. 3. Radiograph of Swanson prosthesis 12 months postoperatively at MP joints III, IV, and V, showing two different types of implant failure: (1) fragmentation of implant; (2) sharp bone edge on metacarpal bone. m, metacarpal bone; p, proximal phalanx. (From Hagert CG, Eiken O, Ohlsson NM, et al. Metacarpophalangeal joint implants. I. Roentgenographic study on the Silastic finger joint implant, Swanson design. Scand J Plast Re-constr Surg 1975;9(2):152; with permission.)

Fig. 3. Radiograph of Swanson prosthesis 12 months postoperatively at MP joints III, IV, and V, showing two different types of implant failure: (1) fragmentation of implant; (2) sharp bone edge on metacarpal bone. m, metacarpal bone; p, proximal phalanx. (From Hagert CG, Eiken O, Ohlsson NM, et al. Metacarpophalangeal joint implants. I. Roentgenographic study on the Silastic finger joint implant, Swanson design. Scand J Plast Re-constr Surg 1975;9(2):152; with permission.)

despite the high failure rate. Bass and colleagues showed that fracture of the implant was not related to patient satisfaction and 80% would undergo the procedure again [20], whereas Schmidt and colleagues [21] showed relief of pain in 75% of patients.

These studies show that despite the relatively high fracture rate, the Swanson prosthesis works relatively well for patients in terms of pain relief and return to activity. Based on these results, the authors extrapolate that the implant fracture rate in the thumb must also be relatively high, but as the other studies also show, these implant failures may not translate into clinical failures.

Hinged metal-polyethylene arthroplasty

Although shown to be successful, the Swanson arthroplasty was not a recreation of the MP joint. The Swanson arthroplasty was more of an elastic spacer whose flexibility allowed for much play at the joint interface. As an attempt to anatomically recreate the MP joint in the style of Brannon in the 1950s [13], Steffee [23] designed a hinged metal-and-polyethylene device in 1973 that would approximate a total joint reconstruction similar to that performed in the knee and hip. This new procedure was designed to provide more anatomical stability than the Swanson spacer by its hinged design, which approximated the normal MP joint architecture and restricted the amount of dynamic freedom at the joint interface. The polyethylene proximal part was designed with a dorsally displaced stem to bring down the center of rotation of the joint to the inferior portion of the metacarpal head [23]. The distal metal component snap-locks into the proximal component, providing a tight hinge at the joint interface. The volar lip was also designed to block flexion beyond 40° in an effort to prevent flexion deformity if the extensor mechanism subluxes (Fig. 4).

In 1981, Beckenbaugh and Steffee published a report evaluating 42 prosthetic replacements in the thumbs of 38 patients [23]. Of the 42 procedures, 33 were performed for rheumatic disease, 8 for degenerative or traumatic osteoarthritis, and the remaining case was a revision. In this cohort of patients, postoperative range of motion averaged -12° extension to 28° flexion, for an average motion arc of 16°. Pain was reported absent in 100% of patients, and no infections of fractures were reported in the follow-up period. The report authors, however, described the results of this prosthesis as "disappointing" because of the

Fig. 4. Radiograph before (top) and after surgery (bottom) with Steffee prosthesis. Measurements show an increase of 0.4 cm in thumb length. (From Beckenbaugh RB, Steffee AD. Total joint arthroplasty for the metacarpophalangeal joint of the thumb—a preliminary report. Orthopedics 1981;4(3):297; with permission.)

Fig. 4. Radiograph before (top) and after surgery (bottom) with Steffee prosthesis. Measurements show an increase of 0.4 cm in thumb length. (From Beckenbaugh RB, Steffee AD. Total joint arthroplasty for the metacarpophalangeal joint of the thumb—a preliminary report. Orthopedics 1981;4(3):297; with permission.)

very shallow 16° motion arc. Functionally and es-thetically, there was no objective difference between thumbs with fusion and thumbs with this type of arthroplasty. Moreover, Beckenbaugh and Steffee proposed that although the complication rate was minimal, it could be potentially higher than that reported in this study for the surgeon that is not as well-versed with the implant technique [23].

In 1994, a newer metal-polyethylene implant was created by Harris and Dias [24] in efforts to address the relatively low motion arc attained with the Steffee prosthesis. This particular implant was designed similarly to the Steffee total joint implant, made from cobalt chrome molybdenum alloy and ultra-high-density polyethylene. The joint surface was determined from cadaveric specimens and MRI images from several adult patients. Unique to this design were uncemented finned polyethylene plugs, which allowed metacarpal component motion, hopefully giving more mobility about the MP joint.

This implant was reviewed by Harris and Dias in 2003 [24]. In this study, 13 joints in eight patients were replaced and followed over 5 years. From this cohort, the investigators reported no implant failures. Out of the 13 implants, they reported one infection that required revision at 3 months, 2 joints that showed lucency in the area around the phalan-geal component, and 1 joint that showed a 2-mm subsidence of the metacarpal component. Seven of the eight patients were pain-free, and the remaining patient had minor discomfort. Moreover, Harris and Dias reported a joint movement arc improvement from 27° to 60° and improvement on a validated Patient Evaluated Measure (PEM) questionnaire from 77% to 9% [24].

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