This was a single-centre prospective observational study. Patients with osteoarthritis treated with THA were enrolled consecutively at a single institution in Austria from November 2014 to August 2015. Due to the inherently altered biomechanics and the presumed effect on osteolysis, we excluded patients who had undergone previous surgery of the affected hip, had received arthroplasty for other joints of the lower limbs, required bilateral THA, suffered from relevant comorbidities, or were either unable or unwilling to participate in the study. Clinical and radiographic examinations were carried out pre-operatively, and three, 12, and 24 months post-operatively.
The study protocol was approved by the local ethics committee. The institutional review board also approved the study (ethics approval registration number: EK 19/14; issue date: 16 June 2014). The study was also registered in the German Clinical Trials Register (clinical trial registration number: DRKS00017076). We conducted the study in accordance with the study protocol, the latest Helsinki Declaration, and good clinical practice guidelines.
Surgical technique, implant design, and post-operative rehabilitation
All patients received spinal or general anaesthesia and were placed in a supine position. Five senior orthopaedic surgeons performed the procedures, using an anterolateral, muscle-preserving approach between the tensor fasciae latae and the gluteus medius muscles in all cases.
All patients underwent uncemented short-stem THA. On the acetabular side, they received a monoblock press-fit acetabular cup (RM Pressfit vitamys; Mathys Ltd Bettlach, Switzerland) made of vitamin E-infused, highly cross-linked polyethylene. On the femoral side, they received a calcar-guided femoral short-stem prosthesis with a titanium plasma spray and calcium phosphate coating (optimys Stem; Mathys Ltd Bettlach, Switzerland) combined with a ceramic femoral head of 28 or 32 mm (ceramys Hip Head; Mathys Ltd Bettlach, Switzerland).
On the first post-operative day, patients began full weight-bearing under the supervision of a physiotherapist. Active and passive mobilisation with restricted joint flexion was encouraged. Following discharge, patients either underwent outpatient physiotherapy or were transferred to inpatient rehabilitation centres.
Dual-energy X-ray absorptiometry
We measured BMD around the prosthesis pre-operatively (baseline) and at three, 12, and 24 months post-operatively using the bone densitometer Lunar iDXA (GE Healthcare Lunar, Madison, Wisconsin, USA). We recorded absolute BMD values in three regions of interest defined according to a modified DeLee and Charnley model (Fig. 1). Additionally, we calculated the BMD change in each zone by dividing the measured BMD by the baseline value and expressed the ratio as a percentage.
We measured BMD with patients in a supine position and the operated leg internally rotated 20°, which allowed us to prevent errors in measurement . A blinded investigator analysed all DEXA measurements. This investigator did neither participate in the surgeries or in post-operative patient care, nor did he have access to study data.
Radiographic and clinical assessment
Pre- and post-operative radiographic examinations included a standing anteroposterior radiograph of the pelvis and an axial radiograph of the operated hip. Follow-up examinations consisted of anteroposterior and axial radiographs of the affected hip only. Standardised templates, as described by DeLee and Charnley , were used to locate peri-prosthetic abnormalities and bone loss.
Acetabular cup inclination angles were measured post-operatively from the anteroposterior radiographs and differentiated into three categories: <40º, between 40º and 50º, and > 50º.
We used the Harris Hip Score (HHS) to evaluate pain, function, and range of motion. The questionnaire included a visual analogue scale (VAS) from zero to ten to assess pain at rest and under load, with zero representing no pain and ten the worst pain. We also asked patients to rate their overall satisfaction on a scale of zero to ten, with zero representing the lowest and ten the highest level of patient satisfaction.
Descriptive statistics included means, medians, standard deviations, 25% and 75% percentiles, and ranges. We used paired t tests to evaluate BMD differences to baseline. The level of significance was set at 0.05 (two-sided) for all tests. All statistical analyses were performed with SAS version 9.4 (SAS Institute Inc, Cary, North Carolina, USA).