Participating centres and patients
The 'EUROHIP' consortium includes 20 orthopaedic centres in 12 different European countries. The overall design was endorsed by representatives from each centre in 2002. It was agreed that we would recruit a large multi-centre cohort of patients undergoing primary THR willing and able to complete self-administered questionnaires recording demographic variables and pre-operative levels of pain, stiffness, mobility and quality of life (using standard, validated questionnaires) as well as their expectations of the operation. One year after surgery they would be sent a similar questionnaire in the post. In addition, we agreed to obtain pre-operative radiographs and to record data about the operative procedure, including the type of prosthesis used. Inclusion criteria included a diagnosis of OA of the hip, primary hip replacement, and signed informed consent; exclusion criteria included causes of hip disease other than OA, severe mental illness or dementia, and patients unwilling or unable to take part in the study.
Data collection
Each centre obtained local ethical approval, if required. The study protocol and data collection forms were designed in Bristol, UK and Ulm, Germany by the study PIs (PAD and KD) and the study co-ordinator (SW). The patient questionnaire was piloted for acceptability in Bristol and modified accordingly before being sent to Ulm for translation and distribution. Questionnaires were sent to each centre for translation and returned for checking before printing and distribution with a set of instructions. All data forms included the birth year and initials of the patient, as well as a centre ID and an individual patient number, to ensure unique identification while maintaining patient anonymity. Full identification records of all patients were kept separately in each centre. Completed questionnaires were photocopied locally and then returned to Bristol, where the database was constructed.
Prior to surgery, patients completed a questionnaire about age, sex, home circumstances, employment, education, current medications, duration of pain in the hip to be replaced and problems in other joints. In addition, they completed the WOMAC [16] and EQ5D [17] questionnaires (the EQ5D data has not been included in any of the analyses presented below).
The Western Ontario and McMaster Universities (WOMAC) OA Index
The WOMAC index (version 3.1) was used to assess the severity of symptoms. This consists of 24 items in 3 subscales: pain (5 items), stiffness (2), and physical function (17) [16]. Component items are measured on a 5-point Likert scale with higher scores indicating greater symptom severity (0 = none, 1 = slight, 2 = moderate, 3 = severe, and 4 = extreme). Missing data were treated as follows: if ≥ 2 pain, both stiffness, or ≥ 4 physical function items were not completed, the items were regarded as invalid, and the subscale score not calculated; where 1 pain, 1 stiffness, or 1–3 physical function items were missing, the average value for the subscale was used in place of the missing item. A total score was calculated for each subscale, and a normalised score (0 indicating no symptoms and 100 indicating extreme symptoms) then calculated for each subscale, by summing up the total score of each subscale, multiplying it by 100, and dividing by the possible maximum score for the scale. A total score out of 96 was created by combining the 3 subscales. This was then converted into a normalised score out of 100, as described in the WOMAC user's handbook.
The surgical teams were asked to complete a form recording the patient's height and weight (from which BMI was calculated), side of surgery, duration of arthritis, date wait-listed and date of surgery. The form also asked for prosthesis type, ASA status – a standard measure of fitness for surgery, scored in this study from 1 (normal, healthy) to 4 (life-threatening systemic disease) [18].
The Kellgren and Lawrence (K&L) Radiographic scores
The K&L score was used to assess structural disease severity. A pre-operative anterior posterior (AP) radiograph of the pelvis was obtained from all patients within 6 months of surgery. In order to standardise readings all films were examined by one of three observers within the co-ordinating centre (Bristol) who undertook training sessions together. These three observers all read 20 randomly selected films on two occasions, in a random order, to test their inter- and intra-rater reliability using Kappa scores. They collected data on the hip to be operated on, including the standard K&L grade (0–4) [19] and the intra-articular pattern of disease distribution (supero-lateral, supero-medial, medial or concentric) and whether the hip disease appeared hypertrophic (excessive osteophytes and new bone formation) or atrophic (extensive bone loss).
As most of the radiographs showed advanced OA, we divided K&L grades 3 and 4 further by adding data from the individual scores of joint space narrowing and bone attrition (assessed using the OARSI atlas – [20]). A K&L grade 3 radiograph with joint space narrowing (grade 1 on the OARSI atlas) was graded 3a, those with more severe joint space narrowing (OARSI atlas grade 2) 3b. Similarly, a K&L grade 4 radiograph (which has complete loss of joint space, graded 3 on the OARSI atlas) was divided into 4a if there was no bone attrition seen, either in the femur or acetabulum, and 4b if there was any bone attrition noted in any part of the joint.
Database management and statistical analysis
Each centre was given 18 months to collect data. A 'Microsoft Access' database was set up in Bristol where all patient data was entered and checked by trained staff. In 2006, when the study had almost been completed and the database was about to be closed, the analysis plan was agreed and some remaining missing data obtained from participants. The database was then closed and a professional database manager carried out routine data cleaning.
Stata 9.2 was used for all statistical analyses (Stata Corp., College Station, TX). The main outcomes in the analysis were the three WOMAC subscale scores (pain, stiffness, function) and the combined total WOMAC score. Exposure variables considered in analyses were: Age (< 50, 50–69, 70+), Gender, Obesity (not obese [BMI < 30], obese [BMI 30–39], morbidly obese [BMI > 40]), Employment status (employed, retired, retired early, other), Education after leaving school (none, diploma or equivalent, degree, postgraduate degree), ASA status (1, 2, 3, 4), and Kellgren & Lawrence grade (0, 1, 2, 3, 4) of the hip operated on.
Univariate linear regression analysis was performed to explore the association between the outcome with each exposure, and a multivariate regression analysis then carried out to control for confounders. The distribution of WOMAC scores was assessed to examine the assumption of normality. Wald tests were used to explore linear trends, by fitting models with the variable as a score. To assess for non-linear trends, likelihood ratio tests were used, comparing a model with a categorical variable to that with the variable as a score. Effect modification was considered using likelihood ratio tests to examine for interaction between age, sex and obesity.