Biomechanical comparison of pull-out force of unicortical versus bicortical screws in proximal phalanges of the hand: a human cadaveric study.

BACKGROUND: Bicortical fixation of proximal phalangeal fractures may damage underlying flexor tendons secondary to drilling and screw protrusion in the dorsal-ventral direction. The aim of this study was to measure and compare the pull-out force of unicortical screws compared to bicortical screws in human cadaveric proximal phalanges to identify optimal configuration for internal fixation.

METHODS: Forty proximal phalanges were harvested. Bicortical and unicortical self tapping 1.7mm screws were inserted into paired phalanges at the distal and proximal metaphysis and at the mid-diaphysis placed in the dorso-ventral direction. Pull-out force of unicortical and bicortical screws were measured and compared.

FINDINGS: Bicortical screw pull-out force is significantly higher than that of unicortical screws in the dorsal-ventral direction. Pull-out strength of unicortical screws at the mid-diaphysis was significantly higher than the pull-out strength of bicortical screws at the proximal metaphysis (181.8N versus 31.5N, P<0.0001).

INTERPRETATION: Diaphyseal fixation is stronger than metaphyseal for both unicortical and bicortical configurations. Unicortical mid-diaphyseal is stronger than bicortical proximal metaphyseal screw pull-out strength. This study provides biomechanical data that may be helpful for individualizing fracture fixation techniques at the proximal phalanx.