Assessment of femoral neck fracture risk for a novel proximal epiphyseal hip prosthesis

The impact of proximal femoral morphology on failure strength with a mid-head resection short-stem hip arthroplasty

Mid-head resection short-stem hip arthroplasty is a conservative alternative to conventional total hip replacement and addresses proximal fixation challenges in patients not suitable for hip resurfacing. It is unclear whether proximal femoral morphology impacts the ultimate failure load of mid-head resection implanted femurs, thus the aim of this study was to investigate the effect of native neck-shaft angle (NSA) and coronal implant alignment on proximal femoral strength. In total, 36 synthetic femurs with two different proximal femoral morphologies were utilized in this study. Of them, 18 femurs with a varus NSA of 120° and 18 femurs with a valgus NSA of 135° were each implanted with a mid-head resection prosthesis. Femurs within the two different femoral morphology groups were divided into three equal coronal implant alignment groups: 10° valgus, 10° varus or neutral alignment. Prepared femurs were tested for stiffness and to failure in axial compression. There was no significant difference in stiffness nor failure load between femurs implanted with valgus-, varus- or neutrally aligned implants in femurs with a NSA of 120° (p = 0.396, p = 0.111, respectively). Femurs implanted in valgus orientation were significantly stiffer and failed at significantly higher loads than those implanted in varus alignment in femurs with a NSA of 135° (p = 0.001, p = 0.007, respectively). A mid-head resection short-stem hip arthroplasty seems less sensitive to clinically relevant variations of coronal implant alignment and may be more forgiving upon implantation in some femoral morphologies, however, a relative valgus component alignment is recommended.

To what extent can linear finite element models of human femora predict failure under stance and fall loading configurations?

Proximal femur strength estimates from computed tomography (CT)-based finite element (FE) models are finding clinical application. Published models reached a high in-vitro accuracy, yet many of them rely on nonlinear methodologies or internal best-fitting of parameters. The aim of the present study is to verify to what extent a linear FE modelling procedure, fully based on independently determined parameters, can predict the failure characteristics of the proximal femur in stance and sideways fall loading configurations. Fourteen fresh-frozen cadaver femora were CT-scanned. Seven femora were tested to failure in stance loading conditions, and seven in fall. Fracture was monitored with high-speed videos. Linear FE models were built from CT images according to a procedure already validated in the prediction of strains. An asymmetric maximum principal strain criterion (0.73% tensile, 1.04% compressive limit) was used to define a node-based risk factor (RF). FE-predicted failure load, mode (tensile/compressive) and location were determined from the first node reaching RF=1. FE-predicted and measured failure loads were highly correlated (R(2)=0.89, SEE=814N). In all specimens, FE models correctly identified the failure mode (tensile in stance, compressive in fall) and the femoral region where fracture started (supero-lateral neck aspect). The location of failure onset was accurately predicted in eight specimens. In summary, a simple FE model, adaptable in the future to multiple loads (e.g. including muscles), was highly correlated with experimental failure in two loading conditions on specimens ranging from normal to osteoporotic. Thus, it can be suitable for use in clinical studies.

Accurate in vitro identification of fracture onset in bones: failure mechanism of the proximal human femur

Bone fractures have extensively been investigated, especially for the proximal femur. While failure load can easily be recorded, and the fracture surface is readily accessible, identification of the point of fracture initiation is difficult. Accurate location of fracture initiation is extremely important to understand the multi-scale determinants of bone fracture. In this study, a recently developed technique based on electro-conductive lines was applied to the proximal femoral metaphysis to elucidate the fracture mechanism. Eight cadaveric femurs were prepared with 15-20 electro-conductive lines (crack-grid) covering the proximal region. The crack-grid was connected to a dedicated data-logger that monitored electrical continuity of each line at 700 kHz. High-speed videos (12,000 frames/s, 0.1-0.2 mm pixel size) of the destructive tests were acquired. Most crack-grid-lines failed in a time-span of 0.08-0.50 ms, which was comparable to that identified in the high-speed videos, and consistent with previous video recordings. However, on all specimens 1-3 crack-grid-lines failed significantly earlier (2-200 ms) than the majority of the crack-grid-lines. The first crack-grid-line to fail was always the closest one to the point of fracture initiation identified in the high-speed videos (superior-lateral neck region). Then the crack propagated simultaneously, at comparable velocity on the anterior and posterior sides of the neck. Such a failure pattern has never been observed before, as spatial resolution of the high-speed videos prevented from observing the initial opening of a crack. This mechanism (fracture onset, time-lag, followed by catastrophic failure) can be explained with a transfer of load to the internal trabecular structure caused by the initial fracture of the thin cortical shell. This study proves the suitability of the crack-grid method to investigate bone fractures associated to tensile stress. The crack-grid method enables significantly faster sampling than high-speed cameras. The present findings elucidate some aspects of the failure mechanism of the proximal human femoral metaphysis.

Neck-preserving femoral stems

BACKGROUND

Surgeons undertaking total hip arthroplasty (THA) routinely perform a distal femoral neck resection. It has been argued that retaining the femoral neck during THA can provide mechanical and biological advantages.

PURPOSES

The objectives of this study were to review: (1) the current evidence on the advantages of femoral neck preservation during THA and (2) the clinical and radiological outcome of neck-preserving femoral stems.

METHODS

A search of the English-language literature on neck-preserving THA and on the individual neck-preserving implants was performed using PubMed, Ovid SP and Science Direct.

RESULTS

Studies have indicated that neck preservation offers superior tri-planar implant stability and allows more accurate restoration of the hip geometry and biomechanics. The trend towards tissue sparing surgery has contributed to the development of bone-conserving short-stem implants that offer variable levels of neck preservation. Despite an initial learning curve, these implants have generated promising early clinical results, with low revision rates and high outcome scores. However, radiological evaluation of some neck-preserving implants has detected a characteristic pattern of proximal femoral bone loss with distal cortical hypertrophy. The long-term implications of this finding are not yet known.

CONCLUSIONS

Preserving the femoral neck during THA has biomechanical advantages. However, long-term outcome data are needed on neck-preserving femoral stems to evaluate on-going bone remodelling and assess implant performance and survival.