Biomechanical Study of Extensor Tendon Lacerations Over the Finger Metacarpophalangeal Joints

Abstract

Purpose: To assess the percentages of extensor tendon lacerations over the finger metacarpophalangeal (MCP) joints that may lead to an MCP joint extensor lag. Methods: We obtained 8t fresh-frozen cadaveric hand specimens. The extensor and flexor tendons were isolated for simulated active extension and passive flexion. The specimens were secured to a testing jig. Sequential full-thickness, transverse cuts were made through the extensor tendons over each MCP joint. The fingers were cycled before and after each incremental tendon cut. Extensor tendon gaps were measured and inclinometer measurements of MCP joint rotation were used to determine MCP joint extensor lags. Results: Incremental cuts of the extensor tendons caused sequential increases in subjacent MCP joint extensor lags in addition to interactive effects on other finger MCP joints. Extensor lags of the index and little finger MCP joints were statistically significant; lacerations extended across 75% or more of the combined widths of the extensor tendon slips. In comparison, extensor lags of the middle and ring finger MCP joints were statistically significant; lacerations involved 90% or more of the widths of the extensor tendons. A statistically significant MCP joint extensor lag in the index and little fingers was also observed when 1 of 2 extensor tendon slips in either finger was completely transected. Conclusions: In this biomechanical study of extensor tendon lacerations over the finger MCP joints, we found that MCP joint extensor lags varied by both the finger involved and the percentage of tendon width damage. Additional work is needed to determine the clinical importance of MCP joint extensor lags of differing degrees. Clinical relevance: Lacerations of extensor tendons overlying the finger MCP joints in clenched-fist to mouth injuries are typically repaired in a delayed fashion. An improved understanding of the relations between extensor tendon slip lacerations and MCP joint extensor lags may provide a basis for treatment algorithms that could potentially reduce the costs of care. Key words: Biomechanics, extensor tendon injury, fight bite, hand