Effect of monocortical and bicortical screw numbers on the properties of a locking plate-intramedullary rod configuration. An in vitro study on a canine femoral fracture gap model.

Objectives: To evaluate the effect of varying the number and configuration of locking bicortical and monocortical screws on a plate-rod construct using a mid-diaphyseal femoral ostectomy model.

Methods: Thirty Greyhound femurs were assigned to six groups (A-F). An intramedullary pin was placed in each bone following which a 3.5 mm locking plate was applied with six differing locking screw configurations. Groups A to C had one bicortical screw in the most proximal and distal plate holes and one to three monocortical locking screws in the proximal and distal fragments. Groups D to F had no bicortical screws placed and two to four monocortical locking screws in proximal and distal fragments. Each construct was axially loaded at 4 Hz from a preload of 10 Newtons (N) to 72 N, increasing to 144 N and 216 N, each of 6000 cycles with a further 45,000 cycles at 216 N to simulate a three to six week postoperative convalescence period. Constructs were then loaded to failure.

Results: No construct suffered screw loosening or a significant change in construct stiffness during cyclic loading. There was no significant difference in load to failure of any construct (p = 0.34), however, less variation was seen with monocortical constructs. All constructs failed at greater than 2.5 times physiological load, and failure was by bending of the intramedullary pin and plate rather than screw loosening or pull-out.

Clinical Significance: Axially loaded locking monocortical plate-rod constructs applied to the canine femur may confer no difference biomechanically to those employing locking bicortical screws.