Physical Examination
The test most commonly used to test for intrinsic contracture is the intrinsic tightness (Bunnell) test. The examiner holds the metacarpophalangeal (MCP) joints in extension while passively flexing the interphalangeal (IP) joints. The IP joints are then passively flexed with the MCP joints held in flexion.
If IP joint flexion is blocked or lessened when the MCP joint is extended in comparison with when it is flexed, the result is positive and there is tightness of the intrinsic muscles. In contrast, if extensor contracture is present at the proximal IP (PIP) joint, then PIP joint flexion is greater with MCP joint extension.
In some circumstances, the intrinsic tightness test may not be reliable in diagnosing intrinsic contracture. IP joint stiffness and capsular or intra-articular adhesions can mask underlying intrinsic muscle tightness. In these situations, IP joint flexion may be decreased regardless of the position of the MCP joint.
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Metacarpal head is uniquely shaped: ovoid in sagittal plane and widening from dorsal to volar dimension. Collateral ligaments are eccentrically mounted dorsal to axis of rotation of metacarpophalangeal joint. This anatomy causes variable degrees of tightness on collateral ligaments based on position of joint via camlike effect. When joint is in extension, collateral ligaments are lax; in flexion, collateral ligaments span greater distance and are tight.
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Proximal interphalangeal (PIP) joint collateral ligaments originate close to axis of rotation, providing smaller change in length with joint position and providing lateral stability. PIP joint ranges in only one plane, and its trochlear shape also adds to its lateral stability.
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Architectural differences exist between volar plates of metacarpophalangeal (MCP) and interphalangeal (IP) joints. MCP volar plate is composed of crisscrossing bands of fibers that collapse like accordion on flexion and expand with extension. IP volar plate is more rigid cartilaginous structure that does not collapse but glides with movement. It is attached to proximal phalanx only by proximal checkrein ligaments; therefore, rigid IP volar plate can slide proximally and distally with joint motion to protect joint.
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With injury, checkreins form at IP joints. Checkreins are collagenous bands connecting lateral sides of proximal volar plate to assembly lines on volar lateral surfaces of phalanx. Assembly lines are two ridges along volar lateral surfaces of phalanx to which are attached volar ligamentous structures (eg, flexor sheath, Cleland and Grayson ligaments, and oblique retinacular ligaments of Landsmeer).
