Biomechanical Analysis of the Less Invasive Stabilization System for Mechanically Unstable Fractures of the Distal Femur: Comparison of Titanium Versus Stainless Steel and Bicortical Versus Unicortical Fixation

Abstract

The purpose of this study was to test the effect of (1) titanium versus stainless steel metal and (2) locked unicortical versus bicortical shaft fixation on stiffness of Less Invasive Stabilization System (LISS) constructs in a mechanically unstable nonosteoporotic distal femur fracture model. An AO/OTA 33-A3 fracture was created in 24 sawbone femur specimens that simulated normal bone quality. Testing was performed on three groups: titanium unicortical LISS fixation, stainless steel unicortical LISS fixation, and bicortical stainless steel LISS fixation. Specimens were tested in axial, torsional, and cyclic axial loading in a materials testing machine. The outcome measurement was stiffness in axial and torsional loading, total deformation, and irreversible (plastic) deformation in cyclical axial loading. No difference was found in stiffness in axial loading or plastic and total deformation between all three groups. There was a small decrease in stiffness in torsional loading in the titanium group compared with the stainless steel groups (p < 0.0001) and a small increase in stiffness in torsional loading with bicortical versus unicortical proximal fixation (p = 0.04). Unstable distal femur fracture constructs, created in sawbone specimens, fixed with stainless steel and titanium LISS implants, with or without bicortical locking screws proximally, show similar biomechanical properties under loading. Although there are numerous considerations when selecting implants for these fractures, our results indicate that stiffness is not a significant factor in choosing between the constructs tested.