The Effect of Fracture Pattern Stability on Implant Loading in OTA Type 31-A2 Proximal Femur Fractures

Background:Internal fixation of OTA type 31-A2 proximal femoral fractures can be performed with either a sliding hip screw and side plate (SHS-P) or a sliding hip screw and intramedullary nail (SHS-IMN). Controversy exists as to which is the best implant for these types of fractures. The primary aim of this study was to investigate the stability of 31-A2 fractures as a function of loss of medial cortical buttress. The secondary aim was to assess the influence of fracture stability on the different internal fixation constructs.Methods:Simulated simple intertrochanteric fractures were made in 12 cadaver proximal femurs. Six fractures were fixed with an SHS-P and 6 with an SHS-IMN. Both implants were instrumented with a strain gauge at the lag screwnail/plate interface to allow assessment of implant load bearing (ILB). A primary fracture line, in accordance with the 31-A2 OTA classification, was created after which 3 subsequent horizontal osteotomies in 1-cm increments were made across the medial cortex. Compressive loading up to 1050 N was performed after each osteotomy.Results:ILB was presented as percentage of maximal ILB. SHS-P constructs increased their load bearing gradually. For SHS-P constructs, ILB was 8.1% 1.8% in the intact state, increasing to 49.6% +/- 14.0% after the initial intertrochanteric osteotomy (P = 0.0002), 68.7% +/- 15.9% after the first medial osteotomy (P = 0.028), and 80.0% +/- 15.9% after the second medial osteotomy (P = 0.15). After the first-level medial osteotomy, SHS-IMN constructs reached a plateau in which the implant carried the entire load.Conclusions:Type 31-A2 fractures become increasingly unstable with increased medial comminution (or fragment size). SHS-P constructs were more load sharing than SHS-IMN constructs. These findings may help guide the surgeon in choice of implant for a 31-A2 intertrochanteric fracture, leaning toward SHS-IMN for the more unstable fracture patterns.