Biomechanical comparison of mono- and bicortical screws in an experimentally induced gap fracture.

OBJECTIVES: To compare the bending and torsional mechanical properties of mono- and bicortical locking screws in a canine cadaveric tibial gap ostectomy bridged by a locking compression plate (LCP). METHODS: A 10-hole 3.5 mm LCP was applied medially to the tibia with a gap ostectomy using locking screws in the two proximal and distal plate holes. One tibia of each pair was randomly assigned monocortical screws and the other bicortical screws. Constructs were tested non-destructively in mediolateral and caudocranial four-point bending and torsion, and then to failure in four-point bending. Stiffness, yield and failure variables were compared between screw lengths and load conditions using analysis of variance. RESULTS: Caudocranial and mediolateral four-point bending stiffnesses were not different between screw constructs. Torsional stiffness was greater and neutral zone smaller for bicortical constructs. Constructs were stiffer and stronger in caudocranial bending than in mediolateral bending. In caudocranial bending, bicortical constructs failed by bone fracture and monocortical constructs by screw loosening. CONCLUSION: Bicortical constructs were stiffer than monocortical constructs in torsion but not bending. Bicortical screw constructs failed by bone fracture under the applied loads whereas monocortical screw constructs failed at the bone-screw interface. CLINICAL RELEVANCE: Bicortical screw placement may be a safer clinical alternative than monocortical screw placement for minimally invasive percutaneous osteosynthesis LCP-plated canine tibiae with comminuted diaphyseal fractures.