Strain analysis of intra- and extra-medullary implants for fixing the subtrochanteric femoral fractures.

Abstract

Background: Intra or extra medullary fixation for subtrochanteric fractures (STF) is still controversial. And the strain on fracture site and implant have not been reported. The aim of our study is to evaluate the differences in strain at the medial and lateral cortex of the subtrochanteric region with PFNA, 'upside-down' LISS and PFLP fixing different types of subtrochanteric femoral fractures under static loads.

Hypothesis: Our hypothesis was that the PFNA was superior than LISS and PFLP for fixation of STF. However, PFNA also had some weakness.

Methods: Thirty composite femurs with PFNA, LISS or PFLP fixation were divided into three groups. Four models of implant-femur constructs were tested under axial compression load with strain recording by gauges. While finite element analysis (FEA) was performed to simulate the biomechanical test with synthetic bone and implant construct finite element model in order to measure the strain on femur and implant.

Results: In model I, both of the strains recorded by gauge 2 of PFNA and LISS were significantly lower than that of PFLP (PFNA: p = 0.002; LISS: p = 0.038). In model II, both of the strains recorded by gauge 2 (PFNA: p = 0.019; LISS: p = 0.016) and 3 (PFNA: p = 0.016; LISS: p = 0.006) of PFNA and LISS were significantly lower than that of PFLP. In model III, the strain recorded by gauge 2 (p = 0.007) and 3 (p = 0.002) of PFNA were significantly lower than that of LISS. While the strain recorded by gauge 1 (p = 0.028) and 2 (p < 0.0001) of PFNA were significantly lower than that of PFLP. The strain value recorded by four gauges in each femur construct group changed from model I to IV similar to the results of FEA, except for individual gauges.

Discussion: Our study demonstrated that PFNA and reverse LISS fixation were superior to PFLP for stable subtrochanteric fractures and unstable subtrochanteric fractures that have restored medial bone support. For comminuted subtrochanteric fracture, PFNA fixation was slightly superior to reverse LISS and PFLP. However, it was needed to know that the main nail of PFNA had huge compress force on the subtrochanteric medial bone area.

Level of evidence: IV.