A Biomechanical Evaluation of Orthopaedic Implants for Hip Fractures by Finite Element Analysis and In-Vitro Tests

Abstract

The aim of this study was to test the hypothesis that a reinforced gamma nail for the fixation of subtrochanteric fractures would experience less stress during loading compared with a common gamma nail. The issue of whether the use of the stronger implant would result in more stress shielding in the surrounding bone was also addressed. A finite element analysis (FEA) of a synthetic bone was employed to calculate the stress distribution in implant and bone for two fracture types (AO 31-A3.1 and AO 31-A3.3). The FEA was validated by mechanical tests on six synthetic femurs. To test the hypothesis in vitro, mechanical tests on six pairs of fresh-frozen human femurs were conducted. The femurs were supplied with a common or a reinforced gamma nail in a cross-over study design. Strains were measured on the nail shaft to quantify the loading of the nail. The FEA resulted in 3-51 per cent lower stresses for the reinforced gamma nail. No increase in stress shielding could be observed. In the in-vitro tests, the reinforced gamma nail experienced less strain during loading (p < 0.016). The study demonstrated the benefit of a reinforced gamma nail in subtrochanteric fractures. It experienced less stress but did not result in more stress shielding.

The importance of lag screw position for the stabilization of trochanteric fractures with a sliding hip screw: A subject‐specific finite element study

“…27 We did not include muscle forces. Indeed, a recent study 8 on hip fractures concluded that additional muscle loading, as suggested by Heller et al, 28 had no significant effect on the results. There are discrepancies in the values reported for the peak joint reaction forces acting on the hip.…”

“…Friction coefficients were taken from the literature: 0.46 for bone-bone interactions, 8 0.42 for bone-implant interactions, 9 and 0.2 for implant-implant interactions. 10 The models were subjected to a load of 1,866 N corresponding to 300% body weight (BW), in accordance with values reported for walking and stair climbing.…”

Experimental validation of numerically predicted strain and micromotion in intact and implanted composite hemi-pelvises

“…Instead of real bone, a composite bone specimen has been used that possibly cannot reproduce all in-vivo conditions, precisely. However, composite bone has been successfully used in several biomechanical studies, since inter-specimen variability is small and therefore provides more consistency among specimens than cadaveric bone [29][30][31][32][33][34][35][36][37]47]. The support structure is still not entirely representative of the in-vivo situation, where there are no rigid constraints.…”

Internal fixation of fragility fracturesof the femoral neck

“…Other methods to measure strain in bone include extensometers, fiber optic sensors and non-contact strain measurements such as 3D image correlation photogrammetry (Turner and Burr 1993, Tyson et al 2002, Fresvig et al 2008). Measures of strain on the femoral cortex or the implant itself are not common in the literature dealing with fixation of hip fractures, but examples do exist (Mizrahi et al 1980, Eberle et al 2010.…”

“…In other words, three screws through the femoral neck stiffens the proximal femur on which a fractured, yet fixed femoral head fragment, shows increased elastic rotations when compared to the intact state. There are few publications presenting cortical strain data from femoral neck fracture experiments and such studies have often been performed in order to validate computer models (Eberle et al 2010, Peleg et al 2010). Eberle and colleagues measured cortical strain on the proximal femur in intact cadaver specimens and after implantation of a compression hip screw (CHS) with an antirotation screw (n = 6) or a femoral neck plate (FNP) with three partially threaded screws (n = 6) in unfractured femurs (Eberle et al 2011).…”