The protective effect of locking screw placement on nonlocking screw extraction torque in an osteoporotic supracondylar femur fracture model.

OBJECTIVE: To examine the impact of number and position of locking screws in the diaphyseal portion of an osteoporotic distal femoral fracture model with hybrid fixation.

METHODS: Four groups containing 5 osteoporotic bone models were used with varying combinations of diaphyseal screw fixation: 4 nonlocking screws (control); 1 locking screw adjacent to the osteotomy and 3 nonlocking screws; 1 locking screw in the most proximal screw hole of the plate and 3 nonlocking screws; and 2 locking screws at opposite ends of the diaphyseal fixation with 2 nonlocking screws in between. Fixation in the distal articular segment was identical in all constructs. Testing was performed for 50,000 cycles at 2 Hz using simultaneous axial compression (700 N) and bidirectional torque (±5 Nm) applied along the long axis of the bone. All screws were inserted with 4 Nm of torque.

RESULTS: The extraction torque for nonlocking screws in those specimens that had a locking screw nearest the osteotomy was significantly greater than those that did not (P = 0.037). In addition, the 10 constructs with a locking screw nearest the osteotomy had no failures compared with 5 of 10 failures in those without a locking screw in this position (P = 0.033).

CONCLUSIONS: The placement of a locking screw adjacent to the osteotomy was more beneficial in protecting against failure and maintaining the extraction torque of neighboring proximal nonlocking screws. No benefit in adjacent screw extraction torque was seen with a locking screw proximal in the diaphysis. Two locking screws at opposite ends of the diaphyseal fixation were not superior to a single locking screw adjacent to the osteotomy in failure rates or screw extraction torque.