Total hip replacement (THR) is an effective and cost-effective procedure for people with severe hip osteoarthritis (OA), unresponsive to conservative therapy [1, 2]. It has become a high volume procedure throughout Europe, and annual rates continue to rise [2, 3].

It is generally accepted that the indications for THR are pain and disability in spite of the use of non-surgical interventions such as education, drugs, walking aids and physical therapy [4]. However, it is not clear how severe the pain or disability needs to be before surgery should be undertaken, or when in the course of OA it is most appropriate to perform a THR. Economic modelling [5], some outcome studies [6] and patient views [7] suggest that it might be appropriate to perform surgery early on, before the condition gets too severe. However, doctors tend to be cautious about the use of a potentially dangerous and irreversible intervention too early, particularly because of evidence that it is not universally successful [8] and the accumulating risk with time of late prosthesis loosening necessitating further, more complex surgery. The potential implications of operating relatively early or late in the course of the disease are large, as this could have massive effects on the volume of surgery undertaken, as well as on outcomes.

There have been a number of attempts to develop indications for THR through consensus procedures [9], appropriateness criteria [10], and criteria for the prioritisation of patients waiting for a THR [11, 12]. However, the resulting publications have done little more than emphasise the importance of pain and physical disability, and they struggle to take account of other factors that affect surgical decision making, such as co-morbidities, individual values and aspirations, and psycho-social circumstances. Furthermore, they do not address the issues of relative risk of complications or of a poor outcome, or how these might be affected by variations in THR indications.

The 'EUROHIP' consortium has been addressing the current debate on the utilisation and timing of THR, by studying the indications for primary hip replacement [13, 14], and the process of hip replacement in the different centres involved [15]. In addition, the group has developed a cohort of people undergoing primary hip replacement for primary OA. The overall aims of the 'EUROHIP' cohort study are threefold: 1) to describe the amount of variation in disease severity at the time of primary THR in Europe; 2) to look for determinants of any variation; 3) to examine the effects of variations and their determinants on short-term (one year) patient centred outcomes. In this paper we report on the first two of these aims – describing the cohort and the variations in disease severity observed at the baseline assessment, and exploring some of the determinants of variations in pre-operative disease severity.

This large cohort study of patients with hip joint OA coming to primary total hip joint replacement surgery in European orthopaedic centres, shows that disease severity varies greatly at the time of surgery, indicating that there is no consensus on how 'bad' the patient's diseases should be to warrant surgery. The variation was much wider for symptoms than for radiographic changes, and one of the striking findings from the cohort is the absence of any relationship between the two. Other key findings are that the severity of symptoms at the time of surgery is associated with differences in age, sex, weight, general health, employment status and educational status. These variations appear to be present throughout Europe, and not dependent on individual centres.

The demography of our cohort was as expected. The average age of 69, predominance of retired people, greater numbers of women than men, and slightly raised mean BMI are all in accord with other groups of patients coming to hip replacement [21, 22]. Similarly, the wide age ranges operated on is in keeping with other data suggesting a trend to surgery being undertaken on more older people [3]. The majority of our cohort had disease of other joints (predominantly the other hip and the knees) but that they were otherwise relatively fit: very few had an ASA score of 3 or 4. The location of the hip OA is also in accord with previous descriptions, with a predominance of supero-lateral or supero-medial disease [23]. One feature of the cohort that did surprise us was the relatively short history of pain in the joint to be operated on: 70% of those for whom we have this information reported 5 years or less of joint pain, and only 12% said that they had suffered for 12 years or more. This suggests that the majority of cases coming to surgery progress fairly rapidly.

We used standard, validated instruments to assess disease severity. The WOMAC is one of the most commonly used disease specific measures, which assesses pain, stiffness and function [16]. One of its problems is the fact that the disability domain, which dominates the total score, cannot differentiate between disability due to a single joint, or that caused by disease in other joints (common in our cohort) or co-morbidities. It has been suggested that people should have a total WOMAC score of 39 or more to be considered for joint replacement [24], but 155 patients (12.5%) of our cohort had a WOMAC score of 40 or less, and 16 patients (1.3%) had total WOMAC scores of 20 or less, which would generally be considered to be indicative of very mild disease. The variation in WOMAC scores shown in Figure 2 is striking. The K&L score is the oldest and most widely used index of radiographic severity of OA [19]. One of its many problems is lack of discrimination and a 'ceiling effect', as established OA can only be scored grade 3 or 4 [25]. We attempted to get round this problem by modifying it so that we have 4 grades of severity of established disease – 3a, 3b, 4a, and 4b. In contrast to the wide variation in the clinical severity of the disease in our cohort, it is clear that the vast majority had severe radiographic changes. This raises concerns that the radiographic findings may be having an undue influence on surgical decision-making. It is not clear if radiographic severity has an impact on the postoperative outcome, and if so, which radiographic features are most predictive, there being some conflicting findings amongst recent publications on this topic [26–28]. Follow-up of our cohort should contribute to this debate. In this context it is again interesting to note the lack of any significant relationship between the clinical severity on the WOMAC score and the radiographic severity of the disease. It is well known from epidemiological studies that while radiographic OA is a clear risk factor for symptoms, the correlation between x-ray changes and symptoms is weak, and that some people with severe x-ray changes of OA remain asymptomatic 29,30. However, the relationship between x-ray changes and symptoms in people with severe arthritis coming to surgery has not been studied extensively, and doctors find it hard to believe that there is not some relationship when the disease is severe. We believe that our finding of no association between x-rays and symptoms in this patient group reinforces the need to assess patients on the basis of the impact of the condition on their lives, and not on x-ray severity, when considering them for surgery.

Some of our data on the determinants of the variation in the severity of pain and disability at the time of surgery are worrying. Similar sex differences have been found in several other studies of surgical interventions (women always having more severe disease at the time of surgery than men), and remain largely unexplained [24]. The fact that those who were more unfit (higher ASA status) or more obese, had more severe symptoms than average before they were operated on, is also neither a new nor a surprising finding. However, the strong association with both employment status and educational status are new findings. There are several possible explanations for an association between socio-economic status and surgery, including access and willingness to undergo surgery, as well as bias amongst health care professionals, we believe this inequality needs further investigation.

Our initial exploration of the data to find out if the variations in severity observed, and their determinants, are dominated by events in a single region of Europe, or single centres (data not shown) suggests that these findings are fairly consistent across all countries and centres studied. However, further examination of the data centre by centre is being undertaken. The multi-centre, multi-country nature of the cohort is both a strength and a weakness. The strength is that it will allow us to examine differences in different parts of Europe, the weakness is that the huge variations in the health care systems between, for example, northern and eastern Europe, introduce another source of variation that has nothing to do with the patients' need for surgery.

This cohort study has other strengths and limitations. Its strengths include the relatively large number of patients involved, use of validated outcome measures, and relative paucity of missing data. Limitations include the fact that we do not know how representative the cohort is, as it came from self-selected centres rather than a random selection of orthopaedic centres, and the fact that we do not know how many patients were excluded from the study in each centre, or the reasons for exclusion. In addition, we do not know which potential patients might have been triaged out of the system before being able to see a surgeon, or how many were not put on the waiting list for surgery. In other words, we do not know whether the patients in our cohort are truly representative of those in need of a THR in the community. It is known that the willingness of patients to undergo surgery determines the utilisation of THR [31], and it is likely that this varies in different European countries.

There are no clear indications for THR. Consensus statements simply emphasise pain due to hip disease and functional impairment, in spite of adequate non-surgical treatment [4, 9]. However, clinical practitioners are aware of various other reasons for undertaking or delaying a hip replacement, including age and weight, the social role of the patient (as a carer for example), their psychological status, and the presence or absence of any co-morbidities that might affect surgery or its outcome. In addition, surgeons are aware of the importance of the motivation and expectations of their patients when they undertake surgery. There is an increasing trend for those who pay for surgical interventions, such as total hip replacement, to want to use simple scoring systems in order to 'triage' patients as suitable or unsuitable for surgery. But simple scoring systems, such as the total WOMAC, cannot account for subtleties of the sort noted, and these are crucial to appropriate decision making. It has recently been suggested that a different approach, using the concept of the 'capacity to benefit' from a total joint replacement might be used instead of simple scoring systems [7]. This approach provides surgeons and patients with information on the likely risk of adverse events, as well as the likely degree of benefit, from which to make the judgment on suitability of surgery. Our data show clearly that if a score of severity of pain or function were used to assess suitability for surgery, many patients who are currently being operated on might not be allowed an operation. One of the aims of the 'EUROHIP' consortium is to explore this conundrum further, with more exploration of these and other data to help us understand when it is most appropriate to undertake a hip replacement.

This large prospective cohort study of patients coming to primary total hip replacement for primary osteoarthritis of the hip has shown that the severity of the disease varies widely at the time of surgery. The pre-operative scores on the WOMAC instrument, a widely used measure of osteoarthritis severity that assesses pain, stiffness and function, show that many of the patients appeared to have relatively mild disease, whereas others are very severely affected. In contrast, the radiographic findings showed that almost all patients coming to surgery had severe structural changes in the affected hip. There was no correlation between clinical severity and radiographic severity.

We interpret these findings as follows. First, we believe that the data indicate that surgeons require significant structural changes to be present on the radiographs of their patients before they are willing to suggest that hip replacement is advisable. Second, these data show that simple scoring systems of pain and disability, such as the WOMAC, should not be used to define thresholds for surgical intervention. The lack of correlation between radiographic severity and clinical severity on the WOMAC suggests that the decision needs to be based on patient-related variables rather than the x-ray, and those variables should include the degree of pain and disability. However, a large number of other aspects of the patients' lives need to be considered before surgery is undertaken, including their psychological status, their motivation and expectations, roles in society and social circumstances. We believe that the patients enrolled into this cohort study were being operated on for these complex reasons, and that this explains the huge variation seen in pre-operative WOMAC scores.