Effect of proximal phalangeal fracture deformity on extensor tendon function

Extensor lag is a common sequella of proximal phalangeal fractures. Proximal phalangeal fractures frequently lead to extensor tendon dysfunction via adhesions and phalanx malunion with angulation and/or shortening. The primary purpose of this study was to quantify the influence of proximal phalangeal angulation and shortening on proximal interphalangeal (PIP) joint extension by examining the effect of isolated proximal phalangeal bone to extensor digitorum communis tendon discrepancy on PIP joint lag. The secondary goal was to determine the reserve of the extensor digitorum communis to overcome the PIP joint lag. A proximal phalangeal bone-extensor tendon discrepancy results in either a PIP lag or a potential lag that must be compensated for by another system, such as by overpull of the extrinsic extensor or by the pull of the intrinsics. Six cadaveric middle fingers were used, proximal phalangeal malunions were simulated, and the extensor lag was measured radiographically. A linear relationship between extensor tendon lengthening and the resulting PIP lag was observed. Similarly, a linear relationship between proximal phalangeal shortening and the lag was observed. For both, the average slope was 12 degrees of lag/mm of bone-tendon discrepancy. For average apex palmar angulations of 16 degrees, 27 degrees, and 46 degrees, PIP lags of 10 degrees, 24 degrees, and 66 degrees, respectively, resulted. The extensor tendon reserve was 2 to 6 mm. The sagittal bands became tight at the limits of the extensor tendon reserve. The results of this study quantify the importance of re-establishing the bone-tendon relationship for proximal phalangeal fractures.