Risk of Bone Fracture in Resurfacing Hip Arthroplasty at Varus and Valgus Implant Placements

It is possible to have a varus or valgus placement of resurfacing hip implants after resurfacing hip arthroplasty based on clinical reports. The likelihood of accidents such as sideways falls during the recovery process after arthroplasty is higher for the patient due to gait adaptation and weaker lower body condition. Hence, a computational study has been conducted to predict the risk of bone fracture with different implant placements during sideways fall accidents. A CT image of a young adult with hip osteoarthritis was imported into biomechanical software to develop the 3D inhomogeneous femoral bone model. A model of the Birmingham Hip Resurfacing implant with the properties of cobalt-chromium alloy was inserted into the femur during the reconstruction of the arthroplasty, which mimics the procedure of clinical practice. The loading and boundary conditions were implemented to simulate the sideways fall accident, and the prediction of bone fracture was based on the formation of failure elements. The loading magnitude was applied based on the patient’s body weight, ranging from the patient’s body weight (1 BW) to five times the patient’s body weight (5 BW). The fracture location was predicted to occur at the neck and trochanteric area of the femur, with the greatest damage occurring to the bone model implanted with varus placement. Our finding concludes that the varus placement of the resurfacing hip implant should be avoided whenever possible in clinical practice to sustain bone survivability.

Bone fracture; Damage formation criterion; Resurfacing hip arthroplasty; Sideways fall; Varus and valgus placement

    Resurfacing hip arthroplasty (RHA) is a hip replacement method applied to young adults with end-stage hip osteoarthritis (OA) disease (Isaac et al., 2006; Quesada et al., 2008; Amanatullah et al., 2010; Wagner et al., 2012). Previous studies have discussed the positive surgical outcomes of young adults with hip OA who have undergone RHA (Vail et al., 2006; Mont et al., 2007; Lavigne et al., 2008; Shimmin et al., 2008). Despite that, complications after RHA still exist and have been reported by clinical institutions. Based on the clinical reports, the greatest complication that happens to patients who undergo RHA is bone fracture (Freeman, 1978; Freeman et al., 1978; Shimmin and Back, 2005). Other factors  might  contribute  to  the  failure,  including  patient,  post-operative,  and  surgical factors, which have been discussed previously (Shimmin and Back, 2005; Sershon et al., 2016).

However, it is believed that the placement of the RHA implant inserted into the femur during the surgical procedure might have a huge impact on the bone condition. Biomechanical factors, such as stress shielding, might also lead to implant loosening and be oriented toward improper placement during the recovery process (Goshulak et al., 2016). Improper placement of the implant might increase the tendency of early bone fracture after arthroplasty.

Since there is a possibility of improper implant placement occurring in RHA, the prediction of bone fracture in emergency cases such as sideways falls might further the understanding of bone failure after RHA. Patients who underwent hip arthroplasty had a higher risk of falling during the recovery process due to gait adaptation and instability (Beaulieu et al., 2010) and might face some difficulties in avoiding environmental hazards (Brunner et al., 2003). The extreme loading exerted on the hip area during a sideways fall can initiate a sudden impact on the area and might lead to the greatest failure, which is bone fracture. Although many CT-FEA studies have discussed the effects of different implant placements after RHA, no study to date has discussed its consequences in the case of an accident. Thus, the current study aims to predict the bone fracture mechanism of intact femurs and femurs that are associated with different RHA implant placements during sideways fall accidents.

    The present study demonstrates the risk of femoral bone fracture in the case of a sideways fall accidents. As hip arthroplasty is needed for late-stage hip OA patients, the placement of the implant has a huge impact on the survivability of the femoral bone. Although all femurs are predicted to fracture at the highest load applied (5 BW), the femur implanted in valgus placement shows the lowest fracture formation when compared to the varus and straight placements. The result shows that the valgus placement of the RHA implant might have a preventive effect against fracture where the possibility of fracture is reduced by 44% and 34% compared to the varus and straight placement conditions. The increase of failure elements as the implant is oriented from valgus to varus suggests that the risk of bone fracture is higher when the implant is positioned in the varus placement zone.

This research was supported by Universiti Teknologi MARA, UiTM under Grant No. 600- IRMI/PERDANA 5/3 BESTARI (103/2018). We thank and acknowledge the Ministry of Education, Malaysia, and our colleagues from the Faculty of Medicine, UiTM, who provided insight and expertise that greatly assisted the research.

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