Abstract
We report a case of a locked thumb metacarpophalangeal joint secondary to metacarpal head fracture. As fractures of the radial condyle are not readily seen in routine X-rays, other imaging modalities, including CT, should be considered if the patient complains of limited extension after hyperextension injury of the thumb.
CASE REPORT
A 33 year-old-man sustained a dorsal dislocation of the metacarpophalangeal (MCP) joint of the right thumb which was reduced by a local physician. The thumb was put in a thumb spica cast for 3 weeks. The patient was then encouraged to perform range of motion exercises. He presented to our clinic 4 months after injury with the complaints of loss of extension and pain in the joint. Passive and active extension were both +20° on the right, while these measurements on the (normal) left side were −20°. Stress testing in extension revealed mild instability. Although X-ray showed only subtle abnormality of the appearance of the sesamoids, three-dimensional (3D)-CT clearly demonstrated an osteochondral defect of the radial condyle of the metacarpal head and an osteochondral fragment, which had migrated down to the neck of the first metacarpal bone (Fig 1). This finding was confirmed by surgical exploration (Fig 2). The block to extension of the joint was released easily by removing the fragment.
DISCUSSION
According to Aubriot (1981), the head of the first metacarpal differs from that of the other metacarpals. The surface of the palmar aspect speads over the palmar tubercles, which protrude, condyle-like, to correspond to the sesamoids and palmar plate. The radial condyle has greater dorsopalmar height than the ulnar condyle, allowing an element of conjunct rotation in pronation with increasing flexion. Since the joint has little intrinsic stability from the skeletal architecture, stability is primarily achieved by the ligamentous and musculotendinous structures. According to Stern (1999), metacarpal head fracture is unusual because any longitudinally directed force which might produce this fracture is dissipated at the trapeziometacarpal joint (Stern, 1999). Most intraarticular fractures are avulsion fractures from the radial side, ie. radial collateral ligament injuries. Stener (1963) also reported four cases of intraarticular shearing fracture of the radial condyle of the metacarpal head in association with ulnar collateral ligament injury.
Ishizuki et al. (1994) classified hyperextension injuries of the MCP joint of the thumb into: (1) dorsal dislocations of the MCP joint, or volar plate avulsions, (2) locking of the MCP joint in hyperextension and (3) sesamoid bone fracture. Although interposition of tendons or the volar plate can compromise closed reduction, most dorsal dislocations are reducible. Despite the initial triviality of these lesions, persistent symptoms of considerable inconvenience may develop due to hyperextension instability or recurrent subluxation (Jespersen et al., 1998). None of the above-mentioned pathomechanisms could explain our patient’s symptoms and, to the best of our knowledge, no reports in the English language literature have described locking of the MCP joint of the thumb with loss of extension as a result of an intraarticular fracture fragment that had migrated beneath the thenar muscle insertion.
As the radial condyle fracture is not readily seen in routine roentgenography, we believe that other imaging modalities are needed for accurate diagnosis. In this case, 3D-CT clearly delineated the bony injury. Other investigations, such as MRI scan or arthrography, might also have provided significant information. Although these modalities are better in delineating soft tissue abnormalities, CT is less invasive and creates higher-quality bone images at lower cost. Considering that joint locking is often caused by small loose fragments, we believe 3D-CT is the investigation of choice for this condition.