The effects of locking inserts and overtorque on the mechanical properties of a large fragment locking compression plate.

PURPOSE The study was to determine the effect of locking hole inserts and their insertion torque on the fatigue life of a large fragment Locking Compression Plate (LCP) under bending forces. METHODS Fatigue strength of the LCP was examined using cyclic three-point bend testing at 80% yield strength of the construct. Locking hole inserts were used in 2, 4, and 6-hole of a 12-hole plate to simulate three different working lengths. Within each working length, plates were tested without locking inserts serving as the control group. In the experimental groups, inserts were tightened to manufacturer recommendations (4 Nm) and using overtorque (8 Nm). RESULTS Significantly fewer cycles to failure were observed in control groups versus the locking hole insert groups for all working lengths (2-hole: 4 Nm p = 0.003, 8 Nm p = 0.003; 4-hole: 4 Nm p = 0.02, 8 Nm p < 0.001; 6-hole: 4 Nm p = 0.004, 8 Nm p < 0.001). There was a statistically significant increase in fatigue strength when using overtorque in the 4-hole (p = 0.04) and 6-hole (p = 0.01) defect groups. This was not shown in the 2-hole defect group (p = 0.99). CONCLUSIONS By placing locking inserts in the empty locking regions of Combi holes along the working length, the number of cycles to failure was increased. Tightening inserts to twice the recommended insertion torque further increased cycles to failure in longer working length models. A longer fatigue life has the potential to decease the incidence of plate failure especially in the setting of delayed union due to poor intrinsic healing capacity, fractures in the geriatric population, osteoporosis and periprosthetic fractures.