The anatomy of the extensor tendons(tendons that extend or straighten your finger) is very complex and the muscles for the main extensor tendons to the fingers are located in the forearm. The extensor tendon to the finger (extensor digitorum) runs on the backward dorsum of the hand, down to the knuckle, or MCP joint. It attaches to a fibrous band called the sagittal bands that wrap around the base of the first finger bone (proximal phalanx). This attachment allows the extensor digitorum to pull the finger up at the MCP joint and extend the finger at the knuckle. The extensor digitorum then travels further to attach to the middle phalanx or middle finger bone. At that point it is called the central slip. Pulling or tension on the central slip straightens or extends the middle joint or PIP joint. Tendinous extensions of the extensor digitorum called the lateral bands extend out from either side of the extensor digitorum and are joined by tendons from the small intrinsic muscles located deep in the hand. The lateral bands run toward and connect at the left bone or distal phalanx. The form the terminal tendon, which is used to extend or straighten the last joint or DIP joint. The extensor tendons at this level are covered by peritenon, a slick layer of tissue which allowed gliding. The extensor tendons in general are thinner than the powerful flexor tendons and glide together as a sheet over the finger.
A laceration or injury to the terminal tendon over the last joint or DIP joint will produce a flexion deformity of the PIP joint note has a mallet finger. In some cases the extensor tendon will retract and place more tension on the central slip located at the middle joint or PIP joint. The lateral bands may also contract and move dorsally to the top of the finger. In patients with elastic ligaments and mobile PIP joints, the PIP joint may become hyperextended or bent backward. this deformity including DIP joint flexion and PIP joint hyperextension is known as a swan-neck deformity due to its appearance. Initially the deformity may be flexible, with little symptoms. Later the patient may have difficulty beginning or initiating flexion at the PIP joint if the finger becomes too hyperextended. There may be catching or locking of the finger in extension. Finally the finger may become stiffened in this position secondary to scarring of the extensor tendon and soft tissue. a swan-neck deformity may also develop in cases of inflammatory arthritis, such as rheumatoid arthritis or lupus, when inflammation from the DIP joint erodes through the terminal tendon.
Initially treatment of an acute swan-neck deformity may be conservative. A splint may be used to keep the DIP joint straight and allow the terminal tendon to heal. Some splints also incorporate the PIP joint, keeping it flexed. These essentially reverse the swan-neck deformity. Splinting is used for approximately 6 to 8 weeks to allow tendon healing. In cases of tendon laceration, the terminal tendon is usually repaired surgically together with pinning the DIP joint straight to protect the repair. The surgery is usually performed as an outpatient with general anesthesia or axillary block anesthesia. A dressing and splint is applied to the DIP joint afterwards. Motion of the PIP joint and MCP joint are usually initiated after the surgery to preserve flexibility. The pin is removed at 4 to 6 weeks, followed by range of motion therapy for the DIP joint.
For cases of chronic swan-neck deformity or in cases where conservator has failed, surgery is usually required. in cases where the joints are stiff, joint mobility must first be regained through occupational therapy or progressive splinting. When joint mobility cannot be regained through therapy or splinting, a capsulotomy or release of scar tissue around the joint may be performed at the time of surgery. After the joints are mobilized, the extensor tendon and lateral bands are freed up(tenolysis) and the terminal tendon is repaired. The PIP joint and/or DIP joint may need to be pinned in the corrected position in order to protect the repair. The volar plate, or ligament on the palm side of the PIP joint, may need to be repaired in order to keep the PIP joint from hyperextending. The central slip may need to be lengthened in order to allow the PIP joint to flex. DIP joint fusion in a functional position is occasionally used in cases where the terminal tendon cannot be repaired. The surgery is usually performed as an outpatient with general anesthesia or axillary block anesthesia. A protective splint is applied afterwards in therapy for MCP joint motion is initiated. The pins are usually removed at 4 to 6 weeks postoperatively , finger range of motion is initiated and a removable protective splint may be used for several weeks. Risks with this condition and surgery are anesthesia related risks, wound healing problems or infection, stiffness of the finger.