Effects of positioning and notching of resurfaced femurs on femoral neck strength: a biomechanical test

Journal of Orthopaedic Surgery, Aug 2009 by Nabavi, A, Yeoh, K M, Shidiac, L, Appleyard, R, Gillies, R M, Turnbull, A

ABSTRACT

Purpose. To assess the effects of positioning and notching of resurfaced femurs on the mechanical strength of third-generation saw bone (TGSB) femurs using an in vitro analogue bone model.

Methods. 30 TGSB femurs were equally divided into 6 resurfaced femur groups (intact, anatomic, varus, valgus, anatomically notched, and valgus notched) for testing the load to failure, stiffness, and total energy.

Results. Compared to the intact femurs, the load to failure in all resurfaced femurs was significantly decreased by 29 to 57%. Among the resurfaced femurs, valgus and anatomic femurs had the highest load to failure, followed by valgus notched, varus, and anatomically notched femurs. Notching weakened the construct by a further 24 to 30%.

Conclusion. To minimise the risk of femoral neck fracture, resurfaced femoral heads should be placed in an anatomic or valgus orientation, and the superior cortex of the femoral neck should remain intact.

Key words: arthroplasty, replacement, hip; femoral neck fractures; mechanics

INTRODUCTION

Hip resurfacing arthroplasty is an alternative to total hip arthroplasty (THA). It preserves proximal femoral bone stock and keeps the medullary canal intact to facilitate a revision THA should the hip resurfacing fail.1 Nonetheless, periprosthetic femoral neck fractures are common complications, ensuing in 1.46% of the 2497 Birmingham hip resurfacing arthroplasties carried out in Australia between 1999 and 2004.2 Femoral neck notching and component malalignment are risk factors, particularly varus malpositioning.3,4 The time to fracture varies from 0 to 56 (mean, 15.4) weeks.2,5 The biomechanical properties of the femoral neck may change over the short term (because of stress shielding) and lead to fracture.6-8

We assessed the effects of hip resurfacing alignment and superior neck notching on the mechanical integrity of third-generation saw bone (TGSB) femurs using an in vitro analogue bone model.

MATERIALS AND METHODS

30 TGSB femurs were equally divided into 6 groups (intact, anatomic, varus, valgus, anatomically notched, and valgus notched). They were tested for the load to failure after hip resurfacing using a 48-mm Biomet ReCap (Biomet, Sydney, Australia). These femurs simulated natural cortical bone and were produced by pressure-injecting short e-glass fibre and epoxy resin around a solid rigid polyurethane foam cancellous core. Under compression, the cortex has a strength of 120 MPa and an elastic modulus of 7600 MPa (compared to 17000 MPa for bone). The cancellous core has a strength of 4.8 MPa and an elastic modulus of 104 MPa.

Standard Biomet ReCap instrumentation was used to prepare the femurs. In the anatomic group, the resurfacing head was placed at 127?. In the varus and valgus groups respectively, the entry point of the alignment guide wire was moved a fixed distance superiorly or inferiorly on the lateral femoral cortex (Table 1). The guide wire was set at the correct angle of varus and valgus to avoid neck notching during reaming. The resurfacing heads were placed at 117? and 137?, respectively. In the anatomically and valgus notched groups, a standardised 4-mm deep notch was created by a ribbon saw in the superior neck, just distal to the base of the resurfacing head (Table 1). The implants were firmly positioned, but not cemented.

Orientations and proper seating of the resurfacing heads were verified using radiography and computer tomography. The femurs were evaluated according to the International Standard (ISO 7206-8) for testing a stemmed hip prosthesis under combined bending and torsion.9 This standard specifies a testing orientation of 9?�1? flexion and 10?�1? adduction, at which maximal loading is experienced by the femur during the normal human gait cycle.

A vertical load was applied to the superior surface of the resurfacing head and gradually increased at a displacement of 0.1 mm/s until the femoral neck fractured, using a MTS 858 servo-hydraulic testing system (Fig.1). The load applied, displacement, and load to failure (in Newtons) were measured. The stiffness was calculated as the rate of change of load applied with respect to displacement (N/mm). The total energy was calculated as the integral of the load applied with respect to the total displacement (N.mm).

Differences between groups was determined using analysis of variance. Comparison between groups was made using a least significant difference post-hoc test, with a p value of

RESULTS

The load to failure and the stiffness of the construct was significantly reduced by 29 to 57% (p

In the intact, anatomic, and valgus groups, the fracture line was away from the bone-prosthesis junction and near the lateral trochanteric region of the neck, running at an oblique angle to the load path. In the varus, anatomically notched, and valgus notched groups, the fracture line started at the superior edge of the bone-prosthesis junction or at the notch and ran almost parallel to the load path (Fig. 2). The orientation of the fracture line was correlated with the energy to failure and the load to failure, with the former groups having a higher fracture energy.