Biomechanical Comparison of the Durability of Two Locking Plate Constructs Under Cyclic Loading in Torsion - Two Screws Versus Three Screws Per Fragment

Introduction
The number of locking screws required per fragment during bridging osteosynthesis has not been evaluated in the dog. The purpose of this study was to measure the durability under cyclic torsion of two constructs with either two or three screws per fragment.

Material and methods
A ten hole 3.5 mm stainless steel locking compression plate (LCP®) was fixed 1mm away from a bone surrogate with a fracture gap of 47mm. Two groups of ten constructs were prepared with either 2 or 3 bicortical locking screws per fragment placed at the extremities of the plate and tested in cyclic torsion (range 0 to 12.5°) until failure.

Results
The cycle of failure was significantly lower for the 2-screw constructs (15,600 ± 5,272 cycles) compared with the 3-screw constructs (20,700 ± 5,735 cycles). Stiffness was significantly lower for the 2-screw constructs (23.73 ± 0.87 N.m/rad) than for the 3-screw constructs (29.65 ± 1.89 N.m/rad). Most constructs failed by screw fracture at the junction of the shaft and head. Failure in the remaining constructs was due to screw head unlocking with some of related to incomplete seating of the screw head prior to testing.

Discussion/Conclusion
Omitting the third innermost locking screw during bridging osteosynthesis reduced fatigue life by 25% and construct stiffness by 20%. Fracture of the screws is believed to occur sequentially starting with the innermost screw.

A construct with 2 screws per fragment should be used only in selected patients and when adequate insertion of each screw can be ensured.