REIDER PART 127
Chapter 4_Hand and Wrist 117
Damage to the insertion of the central slip of the extensor digitorum communis tcndon at the dorsal base of the middle phalanx can cause a boutonniere defor-mity, a combination of flexion contracture of the PIP joint and extension deformity of the DIP joint (see Fig. 4-27B). This insertion may rupture acutely owing to a jamming injury of the finger or attenuate graduaUy owing to rheumatoid synovitis. Initially, this damage results in a flexion deformity at the PIP joint when the patient attcmpts to extend the involved finger. Overpull of the extensor tcndon, which is still attached to the dis-tal phalanx, results in hyperextension of the DIP joint.
Figurę 4-24. Capitate ulna syndrome (arrow indicates head of ulna).
With time, the lateral bands of the extensor tendon sub-lux volarly past the center of rotation of the PIP joint, causing the boutonniere deformity to become morę rigid.
A swan neck deformity may follow an untreated mallet injury, follow an untreated PIP joint dislocation, or occur within the context of rheumatoid arthritis (see Fig. 4-27C). It is a combination of flexion deformity of the DIP joint and hyperextension of the PIP joint. The insertion of the extensor digitorum communis tendon at the base of the distal phalanx is avulsed or the volar piąte of the PIP joint is attenuated by rheumatoid synovitis or trauma. This allows the extensor tendon to retract and ovcrpull, resulting in a hyperextension deformity of the PIP joint. Initially, the hyperextension of the PIP joint is reversible by active flexion, but ultimately the joint can become stiff and fixed.
Just as wrist extension crcates a flexor tenodesis and defines an arcade of bent finger position, flexion of the wrist causes all the finger to assume an extended position. A break in the extension is a sign of injury to an extensor tendon.
Nerve Palsies. Two classic deformities that are due to peripheral nerve injuries are benediction hand and claw hand. A severe ulnar nerve palsy produces the ulnar claw hand deformity, in which the littlc finger and the ring finger are claw-shaped (hyperextended at the MCP joints but flexed at the interphalangeal joints) owing to dener-vation of the interossei muscles, hypothcnar musclcs, and ulnar two lumbricals (see Fig. 4-27D). Combincd median and ulnar nerve palsy can produce a claw hand, in which all fingers are claw-shaped owing to overpull of the finger extensors at the MCP joints (see Fig. 4-27E). Notę also
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