Locking Hole Inserts: Effect of Insertion Torque on Fatigue Performance and Insert Loosening in Locking Plates.

Abstract

To evaluate the effect of locking hole inserts (LHIs) and their insertion torque on locking plate fatigue life. Eighteen standard 3.5-mm locking plates were instrumented with LHIs (Smith & Nephew, Memphis, TN) of 1.70 or 3.96 Nm insertion torque, or without LHIs, whereas eleven 4.5-mm locking plates were instrumented with LHIs at 3.96 Nm insertion torque or without LHIs. Plates were cyclically loaded to failure (ie, plate fracture) in four-point bending. Number of cycles to plate failure were measured. The 3.5-mm plates with 1.70 Nm LHI insertion torque had a 52% increase in cycles to failure compared with plates without LHIs (114,300 ± 23,680 vs. 75,487 ± 15,746 cycles; P = 0.01). Increasing insertion torque to 3.96 Nm led to a further increase of 36% in fatigue life (155,177 ± 32,493 cycles; P = 0.02) and a 106% increase compared with plates without LHIs (P = 0.001). The 4.5-mm plates with 3.96 Nm insertion torque had a 48% increase in cycles to failure when compared with plates without LHIs (74,369 ± 10,181 vs. 50,214 ± 5544 cycles; P = 0.001). LHIs significantly extend plate fatigue length, which would be advantageous in the setting of delayed fracture healing. We recommend the use of LHIs in round locking holes over bony gaps whenever possible; however, we recognize that these findings are limited to implants manufactured by Smith & Nephew.