Trauma Metacarpophalangeal joint

Adolescent male with sporting injury to thumb clinically

Xray shows widening of the ulnar side of thumb MCP joint with no bony injury  and sof tissue haziness in periarticular location-possibly sprain in the  form of collateral ligament injury, sesamoids though atypical in position are not injured. MRI would  help delineate the ligamentous injury and occult bone marrow oedema. Case submitted by Dr MGK Murthy.

Teaching points--

Hinged(condyloid joint) with shallow proximal phalanx base articulation with spherical(thumb rather quadrilateral)metacarpal head

Thumb MCP gets more often injured than relatively stable finger MCP

Factors for stability include soft issue contractility including volar plate, true and accessory collateral ligaments , sagittal band, dorsal capsule, extrinsic and intrinsic tendons

Range permitted usually is 30 deg hypertext, 120 deg flexion, 30 to 40deg mediolateral laxity and small rotation

Volar plate is most susceptible for injury

Usually trauma leads to sprains in the form of  collateral ligament  injury(25 to 40%)

When dislocated, dorsal more common than palmar

Dorsal is described simple if no soft tissue interposed and complex other wise

Two level dislocations are when concomitant Bennett or interphalangeal joint is involved as well

Xray is adequate particularly if additional views (to AP/Lat and Obique ) are added

(a) Brewertons view- place proximal phalanx touching on the cassette after flexing MCP joint to 65 degrees and angulate the beam 15 degress from ulnar to radial side

(b) True lateral of the digit/thumb concerned rather than hand

1. Hands have 5 sesamoids (nodule of calcium within tendons or joint capsule) , with 2 at MCP joint, and 1 at interphalangeal joint of thumb, with  rest at MCP joints of index and small fingers
2. Position of the sesamoids while could help, decide the articulation postion, they usually do not dislocate , however could be fractured, infected, inflamed, or turn neoplastic