The biomechanics of varied proximal locking screw configurations in a synthetic model of proximal third tibial fracture fixation.

OBJECTIVE To determine if 1) angularly stable devices created by compressing ("locking") proximal locking screws to intramedullary nails using end caps or compression screws or 2) increasing the number of proximal screws from two to three increases the stiffness of intramedullary constructs that stabilize proximal third tibia fractures in a nonosteopenic bone model. METHODS Four proximal locking screw configurations were examined in a synthetic composite tibia model with a 2-cm gap simulating a comminuted proximal third tibia fracture with no bony contact: 1) two proximal screws not compressed to the nail; 2) one of two proximal screws compressed to the nail; 3) two proximal screws compressed to the nail; and 4) three proximal screws with only the most proximal screw compressed to the nail. An 11-mm tibial nail with two distal locking screws was used. Stiffness was measured in axial and torsional loading. An analysis of variance was performed to compare results of the screw configurations for each testing mode. RESULTS Compressing two screws to the nail produced 22% to 39% greater (P ≤ 0.01) axial and 16% to 29% greater (P ≤ 0.03) torsional stiffness than securing neither or only one of the screws. Adding a third proximal transverse locking screw increased the axial stiffness by 28% (P = 0.005) and the torsional stiffness by 15% to 28% (P ≤ 0.04) compared with using two oblique proximal screws. CONCLUSIONS "Locking" two proximal locking screws to the nail through compression or adding a third proximal screw increases the axial and torsional stiffness of intramedullary nails used to fix unstable proximal third tibia fractures.