Total hip arthroplasty (THA) is one of the most successful surgical procedures performed, being named as the operation of the century [1], and is a cost effective procedure [2]. Despite the overall success of THA approximately 7–9 % of patients will be dissatisfied with their hip, 1 year after the surgery [3, 4]. The greatest predictor of patient satisfaction after surgery is improvement in their functional scores, both their hip specific Oxford Hip Score (OHS) and the Short Form (SF-) 12 physical component summary (PCS) score [3]. The main determinant of change in the OHS is the pre-operative score, with a worse score being associated with a greater improvement [5]. Other factors such as extremes of age, increasing body mass index (BMI), increasing comorbidity have also been associated with a diminished improvement in the OHS [5].

There is conflicting evidence as to whether leg length is a predictor of functional outcome and patient satisfaction after THA [6–8]. White and Dougall [6] conducted a prospective study to 200 patients, which concluded there was no correlation with the Harris hip score, SF-36 score or patient satisfaction with leg length post THA. More recently, Whitehouse et al. [8] affirmed these findings using the OHS, SF-12 score and satisfaction. In contrast to these studies other authors have reported that perceived leg length discrepancy significantly influences the OHS [9, 10] and patient satisfaction [7, 11] after THA.

There is, however, limited literature reporting the effect of implant position and reconstruction of centre of rotation and (femoral and acetabular) offset on the outcome of THA. Despite the theoretical biomechanical benefits of medializing the acetabular component and increasing femoral offset to compensate for this resulting in a more favourable moment arm [12, 13], there is limited literature to support any clinical effect. Studies by McGrory et al. [14] and Asayama et al. [15] demonstrated improved abductor muscle strength and a lower rate of Trendelenburg positive patients with increasing femoral offset, respectively. Judge et al. [5] demonstrated that female patients with an increased offset stem (exeter sizes 44 or more) had a significantly better outcome according to the OHS at 5 years when adjusting for confounding factors such as age, BMI, and pre-operative functional status, but whether this improved outcome was related to an absolute increase in the offset remains unknown.

The primary aim of this study was, therefore, to assess the independent effect of radiographic measures of implant position, relative to pre-operative anatomical assessment, on the functional outcome of THA according to change in the OHS 1 year post-surgery. The secondary aims were to assess the effect of radiographic measures of implant position on non-hip specific functional outcome (SF-12 and EuroQoL) scores and patient satisfaction 1 year post-surgery.

Ethical approval was obtained from the regional ethics committee (Research Ethics Committee, South East Scotland Research Ethics Service, Scotland, 11/AL/0079) for collection, analysis, and publication of the anonymised data.

During a 1 year period (2013) patients undergoing a THA at the study centre had functional outcome data recorded prospectively. Inclusion criteria for this study were: primary osteoarthritis, no deformity (precluding radiographic assessment), pre- and post-operative radiographs, and a cemented prosthesis. Patients undergoing revision during the first post-operative year were excluded. Patients undergoing consecutive bilateral THAs during the study period only had outcome and radiographic measures assessed for their first THA.

The patient demographics, comorbidities, BMI, and patient reported outcome measures were recorded at the pre-operative assessment clinic. Categories of comorbidity included were: heart disease, hypertension, lung disease, vascular disease, neurological problems, diabetes, stomach ulcer, kidney disease, liver disease, depression, back pain, and pain in other joints, which were all recorded as dichotomous variables.

The OHS [16], SF-12 score [17], and the EuroQoL were recorded pre-operatively and at 1 year post-operatively. The OHS consists of twelve questions assessed on a Likert scale with values from 0 to 4, a summative score is then calculated where 48 is the best possible score (least symptomatic) and 0 is the worst possible score (most symptomatic). The SF-12 is a generic assessment tool to measure a patient’s well-being, which is assessed using a PCS and a mental component summary (MCS) [17]. Both the SF-12 PCS and MCS range from 0 % (worst level of functioning) to 100 % (best level of functioning). EuroQoL (EQ) general health questionnaire evaluates five domains (-5D), which include: mobility, self-care, usual activities, pain/discomfort, and anxiety/depression [18]. In this study the EQ-5D-3L version of the EuroQoL questionnaire was used. This questionnaire assesses the five dimensions with the responses recorded at three levels of severity (no problems; some problems or extreme problems). An individual patient’s health state can be reported based on the five digit code for each domain, of which there are 243 possible health states. This index is on a scale of −1 to 1, where 1 represents perfect health and 0 represents death. Negative values represent a state perceived as worse than death.

Patient satisfaction was assessed by asking the question “How satisfied are you with your operated hip?” 1 year after surgery. The response was recorded using a five point Likert scale: very satisfied, satisfied, neutral, dissatisfied and very dissatisfied. Patients who recorded very satisfied or satisfied were classified as satisfied.

Radiographic assessment was performed by a blinded observer using a standardised protocol anterior–posterior radiograph of pelvis and hips pre-operatively and 1 year post-operatively. Radiographic measurement of offset and length for both the femoral and acetabular components were measured according to the methods described by Nunn et al. [19] and Jogger et al. [20] (Fig. 1). Loughead et al. [21] have demonstrated excellent inter- and intra-observer reliability/correlation of these radiographic measurements. Femoral offset was defined as the perpendicular distance from the anatomical axis of the femur to the centre of rotation of the femoral head. Femoral length was defined as the distance from the tip of the greater trochanter to a line perpendicular to the femoral head along the anatomical axis of the femur (Fig. 1). Acetabular offset was defined as the distance from the medial border of the teardrop to the centre of rotation of the acetabulum parallel to Hilgenreiner’s line. Cup height was defined as the distance from the centre of rotation of the acetabulum to a line drawn parallel with each ischial tuberosity, as this has been recently demonstrated to be more reliable than the inter-teardrop line [22]. All measurements were made using digital radiographs [Kodac© picture archiving and communication system (PACS) on a liquid crystal display] and the graphic measuring tools available. The measuring calibration tool was used to ensure equal measures were obtained. These measurements were repeated by a second observer for 30 cases to assess inter-observer variation and again for a further 30 patients to assess for intra-observer variation.

During the study period twelve consultant orthopaedic surgeons performed or supervised all included THAs. All patients underwent a THA using a cemented Exeter V40 (Stryker^®) femoral stem (n = 327) or an Olympia (Biomet, Warsaw, Indiana) femoral stem (n = 32) using a Contemporary acetabular component (Stryker^®). The surgical approach and technique were dependent upon surgeon preference with a higher prevalence of posterior approach (75.2 %,n = 270/359). All surgeons aimed to maintain offset and restore equal leg length, with a stable hip at the end of surgery. All patients received three peri-operative doses of prophylactic antibiotics (cefuroxime). A standardised rehabilitation protocol as per local clinical care pathway was used for all patients, with active mobilisation and full weight bearing on the first day post-operatively. Patients were then reviewed at 6 weeks, 6 months, and 12 months post-operatively as per local protocol.

Statistical analysis was performed using Statistical Package for Social Sciences version 17.0 (SPSS Inc., Chicago, IL, USA). Parametric and non-parametric tests were used as appropriate to assess continuous variables for significant differences between groups. A Student’s t test, unpaired and paired, was used to compare linear variables between groups. Pearson’s correlation was used to assess the relationship between linear variables. Multivariable linear regression analyses were used to identify independent predictors of outcome (change in the OHS). A single measure intraclass correlation coefficient (ICC) was used for the quantification of inter- and intra-observer reliability of the radiographic measurements. The values greater than 0.75 indicate satisfactory reliability [23]. A p value of <0.05 was defined as significant.

A power calculation was performed using data collected for the first 250 patients selected at random (pilot group). Using the OHS (primary outcome measure) and change in the radiographic measures, both of which demonstrated normal distribution, a significant (p = 0.001) correlation coefficient of 0.17 was demonstrated between the change in the femoral offset and the OHS. Using a bivariate normal model the required sample size using a two tailed analysis with an alpha of 0.05 and a power of 0.90 using the 0.17 correlation coefficient required 359 patients to be recruited.

The inter-observer variation for each of the radiographic measurements demonstrated satisfactory reliability (femoral offset ICC 0.91, femoral length ICC 0.86, acetabular offset ICC 0.93, and acetabular height ICC 0.87). The intra-observer variation demonstrated excellent reliability (femoral offset ICC 0.96, femoral length ICC 0.90, acetabular offset ICC 0.94, and acetabular height ICC 0.94).There were significant changes observed in femoral offset and length, and acetabular offset, but not for acetabular height (Table 3). There was a significant correlation demonstrated between change in femoral offset and change in the OHS at 1 year, with increasing femoral offset being associated with a greater improvement in the OHS (Fig. 2). A similar significant correlation with femoral offset was also observed with change in the SF-12 PCS, however no other radiographic measure achieved significant correlation with the other outcome measures assessed (Table 4). There was no significant difference between satisfied (n = 327) and dissatisfied (n = 26) patients for change in femoral offset (p = 0.81 t test) and length (p = 0.80 t test) or acetabular offset (p = 0.28 t test) and height (p = 0.47 t test).

Table 3

Pre- and post-operative radiographic measurements for the study cohort (n = 359)

Radiographic measures (mm)	Pre-operative (mean, SD)	Post-operative (mean, SD)	Difference	95 % CI	pvalue*
Femoral
Offset	45.9 (9.3)	50.5 (7.7)	4.6	3.7 to 5.5	<0.001
Length	6.5 (7.9)	0.4 (6.5)	6.1	5.4 to 6.8	<0.001
Acetabular
Offset	36.4 (8.8)	31.1 (5.7)	5.3	4.4 to 6.2	<0.001
Height	80.7 (9.2)	81.2 (9.9)	0.5	−0.14 to 1.1	0.13

* Paired t test

Table 4

Correlation of change of radiographic measurements with change in each of the outcome score at 1 year compared to pre-operative measures

Outcome measure	Radiographic measures
	Femoral		Acetabular
	Offset	Length	Offset	Height
OHS
Correlation	0.160	0.052	0.049	0.007
p value*	0.002	0.33	0.36	0.90
SF-12 PCS
Correlation	0.173	0.000	0.027	0.002
p value	0.001	0.99	0.61	0.97
SF-12 MCS
Correlation	0.016	0.065	0.038	0.084
p value	0.78	0.22	0.48	0.11
EQ-5D
Correlation	0.060	0.010	0.027	0.008
p value	0.26	0.86	0.61	0.88

* Pearson’s correlation

Bold p values represent significant correlations

Regression analysis was used to assess the independent effect of the four radiographic measures on change in the OHS when adjusting for confounding variables (Table 1). Femoral offset was the only radiographic measure to be achieved statistical significance (r = 0.198, 95 % CI 0.063–0.333, p = 0.004). Interestingly, on combing femoral and acetabular offset, increasing offset was associated with a greater improvement in the OHS (r = 0.10, 95 % CI 0.01–0.19, p = 0.04), but was not as significant as femoral offset independently. Hence, it would seem that overall offset is not as important as femoral offset.

A major limitation of this study was measuring offset using a digital radiograph of the pelvis. Measuring femoral offset using plane radiographic studies is limited by the precision of the technique and is dependent upon the patient position, magnification, and femoral rotation. It would have been of benefit to use Einbildröntgenanalyse (EBRA), which has been shown to have a measurement precision of around 0.8–1 mm [24, 25]. This methodology was not available due to local issues with compatibility with PACS. The most accurate method to measure offset is with a computer tomography (CT) scan [26]. To have obtained a CT for each patient pre- and post-operatively would not be clinically or ethically indicated [27]. However, results from three dimensional CT demonstrate similar average measures to plain radiographs [28, 29], and demonstrates good reliability [30]. Another limitation was the inclusion of several (12) different surgeons using differing surgical approaches (posterior or Hardinge) which may have influenced outcome and implant position. Furthermore, the study was underpowered (0.46) to demonstrate a significant difference in femoral offset between patients who were satisfied compared to those who were not (3.4 mm in this study), using the data from our cohort with a 12 to 1 (satisfied:dissatisfied) ratio, a cohort of 774 patients would have been required.