Arthroplasty consists of two components, that is, joint stabilization by ligamentoperiosteal flap [21] and partial excision of excessive portion of phalangeal or metacarpal head on which two thumbs sit.

Two thumbs sit on a single proximal phalangeal head in Wassel type 1 or 2 radial polydactyly, and on a single metacarpal in type 3 or 4. During dissection of minor thumb, distal insertion of collateral ligament should be preserved with adjacent periosteal tissue for later reconstruction. This ligamentoperiosteal flap will be reattached to the base of the phalangeal bone of the remaining main thumb after removal of the minor thumb. The phalangeal or metacarpal head, when the minor thumb is removed, is relatively large for the remaining dominant thumb. This size mismatching between two bones may cause angular deformity and/or bony prominence if it is not corrected. Thus, excessive portion of the head needs to be shaved or removed. Sometimes, a separate facet that articulates with the radial thumb to be deleted is observed. This facet can be used as a guideline to cut the excessive portion. An oscillating saw or osteotome cannot be used for very small phalangeal bones of young children. Their phalangeal bones are soft enough that shaving of articular cartilage and partial ostectomy can be performed by a small rongeur or a surgical blade. Excessive tension of the reconstructed collateral ligament to correct angular deformity at the joint level is not recommended because the deformity is likely to recur and stiffness of the joint may occur. However, angular deformity of less than 10° at the joint level can be corrected by this arthroplasty procedure. A longitudinal Kirschner wire (K-wire) is inserted to protect the reconstructed collateral ligament.

Angulation at interphalangeal joint and reduced nail width are primary issues affecting appearance after the surgery [17]. More than 20° of angular deformity is not acceptable to most patients and parents. This can be corrected by closing wedge osteotomy. Double level osteotomy at proximal phalangeal and metacarpal levels can be indicated to align a severe divergent-convergent Wassel type 4.

Abnormal insertions of flexor pollicis longus (FPL) and/or extensor pollicis longus (EPL) are not uncommon in radial polydactyly, especially in Wassel type 4. The FPL tendon attaches not only at its customary insertion, but also into the extensor by a tendon that passes around the radial aspect of the thumb. This anomalous muscle abducts the thumb instead of flexion, and is called as “pollex abductus” [22, 23]. The abnormal insertion of FPL and/or EPL may cause gradual angular deformity even after successful bony alignment has been achieved by corrective osteotomy. If there are abnormal insertions of FPL and/or EPL tendons, the insertion sites should be realigned to achieve good flexion-extension arc. The abnormal insertion can be completely detached and reattached into the correct position. The tendon can be sutured into the distal phalanx of young children using 4-0 or 5-0 nylon. When the phalangeal bone is too hard to be sutured by nylon suture, a pull-out suture technique can be used. When the distal portion of the tendon is bifid and inserted into both polydactylic thumbs, it usually inserts at ulnar side of the radial thumb, and radial side of the ulnar thumb. If the radial thumb is to be removed, the tendon is detached from the insertion of radial thumb and reattached into the ulnar side of dominant ulnar thumb in a “Y” shape to balance the vector forces (Figs. 19.4 and 19.5). During this procedure, the portion of tendinous insertion into the radial side of the remaining radial thumb needs to be detached to avoid abnormal abduction force.

In Wassel type 4, 5, 6, or 7 polydactyly of the thumb, some of thenar muscles insert into the radial side thumb. In most cases, the radial thumb is removed and the ulnar thumb is reconstructed. The insertion site of thenar muscles on radial thumb should be identified and dissected carefully for later reattachment to the main ulnar thumb.

A 7-month-old girl showed radial polydactyly on right thumb (Fig. 19.6). The nail size, length, and girth of ulnar side thumb of left hand were good enough to perform the “excision of radial thumb and reconstruction” procedure. As the epiphyses of phalanges and metacarpal were not observed in simple radiograph (Fig. 19.7), it was hard to assess the exact Wassel type but easy to decide the surgical plan as “arthroplasty” with or without “tendon realignment.” Corrective osteotomy was not necessary because angular deformity at the IP joint is minimal. A racquet-shaped incision was designed. A zigzag incision has an advantage to prevent possible scar contracture, but this technique is not easy to apply to an infant with a thumb less than an inch in length. During dissection, it was confirmed that base of two distal phalanges was fused to be Wassel type 1. The dissection was deepened to expose distal phalangeal bone of the radial thumb, and a ligamentoperiosteal flap was raised. Distal phalanx of the radial thumb was cut to be removed, and the articular surface of this radial thumb was seen. Articular surface for the radial thumb was cut using a number-15 blade scalpel (Fig. 19.8). The consistency of phalangeal bone in an infant is soft enough to be cut by surgical blade. There was no misalignment of tendons. After the arthroplasty procedure, the articular surface of the proximal phalanx fits that of the ulnar thumb. Before reconstruction of the collateral ligament, a 0.7-mm K-wire was inserted longitudinally to protect it. The ligamentoperiosteal flap was attached to the new insertion site by 5-0 absorbable suture, reconstructing the collateral ligament (Fig. 19.9). The reconstructed thumb looked straight (Fig. 19.10).

A 14-month-old boy showed a divergent-convergent Wassel type 4 radial polydactyly on the right side. The radial thumb was hypoplastic, but the ulnar thumb showed good size and shape (Fig. 19.11). There was 35° of angular deformity at the IP joint of ulnar thumb that needed corrective osteotomy at the proximal phalangeal neck level (Fig. 19.12). Medical photos and simple radiographs suggested strong possibility that arthroplasty of MP joint and tendon realignment of EPL, FPL, and thenar muscles were necessary for proper reconstruction. The proximal phalangeal head of the ulnar thumb was underdeveloped, suggesting a potential recurrence of angular deformity postoperatively. A racquet-shaped incision was designed (Fig. 19.13). The EPL tendon was bifurcated at MP joint level, and inserted into both thumbs (Fig. 19.14). The insertion site of radial EPL slip was detached and sutured to the ulnar side of dominant thumb to balance the extension force. The FPL tendon showed the same pattern (Fig. 19.15). The insertion of the radial FPL slip was detached and tagged with suture for later reattachment into the ulnar side of dominant thumb (Fig. 19.16). The abductor pollicis brevis muscle insertion into the radial thumb was detached from the proximal phalangeal base for later reattachment into the reconstructed thumb. The radial thumb was removed, leaving ligamentoperiosteal flap for later reconstruction of the MP joint. The portion of metacarpal head to be resected was lined (Fig. 19.17), and a ligamentoperiosteal flap was raised and preserved. Excessive articular cartilage and bone was resected by number 15 blade and small osteotomes. The author finds that it is very difficult to do a fine osteotomy with power instruments like an oscillating saw. The ligamentoperiosteal flap and detached abductor pollicis brevis tendon were preserved for later reattachment (Fig. 19.18). The proximal phalanx was dissected subperiosteally for ulnarly based closing wedge osteotomy to correct angular deformity at the IP joint (Fig. 19.19). It is convenient to perform ulnarly based closing wedge osteotomy from a separate ulnar side incision. However, simultaneous medial and lateral incision on the same thumb may jeopardize blood circulation. The MP joint was fixed in a reduced position with a K-wire, and also osteotomy site of proximal phalanx was fixed with an additional K-wire (Fig. 19.20). Finally, the FPL tendon detached from the radial thumb was reattached into the ulnar side of reconstructed distal phalanx. The abductor pollicis brevis tendon detached from radial thumb as well as the previously raised ligamentoperiosteal flap for collateral ligament reconstruction was also reinserted into the base of proximal phalanx (Fig. 19.21). Alignment and appearance of the reconstructed thumb (Fig. 19.22) and the immediate postoperative radiograph (Fig. 19.23) showed good result.