Biomechanical Properties of the Extensor Mechanism Following Retrograde Intramedullary Nailing for Metacarpal Fractures.

PURPOSE: Retrograde percutaneous placement of headless compression screws or threaded intramedullary nails for metacarpal fracture fixation may injure the extensor mechanism. Alternatively, mini-open, tendon-sparing approaches may be used, with instrumentation placed through the sagittal band or a longitudinal split of the tendon. Our study evaluated extensor mechanism biomechanical properties after using different techniques for retrograde threaded intramedullary nail insertion.

METHODS

Sixty-four cadaveric fingers were divided into a control group and three testing groups based on the following implant drilling/insertion techniques: transtendinous, longitudinal tendon split with repair, and trans-sagittal band. After instrumentation was applied, the extensor mechanisms were dissected and loaded for tensile testing to determine the maximum load to failure (LTF), stiffness, and site of failure. Analyses comparing LTF and stiffness were conducted using a mixed-effects linear regression model, adjusting for metacarpal number and cadaver age. Fisher exact tests assessed differences in the failure scenario.

RESULTS

There were no significant differences in mean (SD) LTF comparing each test group versus control: control 398.4 N (77.0 N), transtendinous 355.0 N (94.7 N), longitudinal split repair 347.9 N (123.4 N), and trans-sagittal band 384.6 N (110.4 N). Load to failure of extensor tendons from the index and long finger metacarpals was significantly higher compared with the extensor tendon associated with the fifth metacarpal. The mean stiffness was 43.2 N/mm (10.3 N/mm), with no difference between the groups. Tensile failure occurred at the implant insertion site in 44% transtendinous, 25% longitudinal split repair, and 13% trans-sagittal band specimens.

CONCLUSIONS

Retrograde intramedullary implant placement directly through the extensor tendon, a longitudinal tendon split, or the sagittal band does not significantly decrease extensor mechanism maximum LTF or stiffness. Implant insertion potentiates a tensile failure site, but only under tension far exceeding physiologic loads.

CLINICAL RELEVANCE: Any method of retrograde insertion for intramedullary metacarpal nails likely has minimal clinical impact on extensor mechanism integrity during early functional motion.