Prevalence and development of hip and knee osteoarthritis according to American College of Rheumatology criteria in the CHECK cohort

Osteoarthritis (OA) is associated with joint pain and functional limitation and is a leading cause of disability among older people. OA is considered the most common form of arthritis from which 15–18% of the population suffers [1]. Approximately 22% of the general population suffers from knee pain, and knee and hip pain are even more common in older people [2, 3]. This generally leads to consultation with a physician: e.g., 33% of the population with knee pain in the UK consults a general practitioner (GP) in primary care [4]. One reason for consultation is that patients with knee pain are looking for a definite diagnosis [5]. However, no clear clinical diagnostic primary care tools are available. Diagnosis of OA is often based on radiological evidence and/or on recommendations formulated by OA experts active in secondary care [6].

The diagnosis of OA in patients suffering from knee or hip pain in primary care would become easier if well-defined criteria were used. The American College of Rheumatology (ACR) has developed different criteria for the classification of OA of the knee and hip in order to promote uniformity in reporting OA in epidemiological and intervention studies. These criteria were developed using combinations of clinical, clinical/laboratory, and clinical, laboratory and radiographic criteria [7, 8]. Although these criteria were developed primarily for epidemiological purposes rather than for clinical use, the ACR criteria are commonly used as a diagnostic tool in secondary care. Because the criteria were developed in secondary care with patients with (mostly) rheumatoid arthritis (RA) in the control group, these criteria might primarily distinguish patients with OA from patients with RA. Furthermore, it has been suggested that the criteria are probably mainly diagnostic for late-stage OA [9]. More uniform and early diagnosis of OA would provide a better window of opportunity for interventions and a clear diagnosis could also help to motivate patients for often-difficult lifestyle changes involved with such a diagnosis.

The present study aimed to evaluate the prevalence of ACR criteria in subjects with knee and hip complaints and whether they will develop evident OA according to the ACR criteria for hip and knee OA. This study also aimed to determine predictive factors for the development of knee/hip OA according to the ACR criteria, during 5-year follow up. These predictive factors may help to diagnose OA at an earlier stage in primary care and thereby promote earlier treatment according to established guidelines.

This study demonstrates that the majority of patients presenting for the first time with hip pain fulfill the combined ACR hip OA criteria, both clinical or combined ACR criteria, and that 40% of patients not fulfilling those ACR criteria will develop evident OA according to the clinical or combined ACR criteria for the hip after 5 years. For this last subgroup we identified the following predictive factors: morning stiffness, painful internal rotation, hip flexion < 115° and an ESR < 20 mm/h. Combinations of these signs and symptoms have an even higher predictive value. In first presenters with knee pain, up to 92% do fulfill the clinical or combined ACR criteria, at baseline. For this reason, the number of participants with knee symptoms not fulfilling the ACR criteria was, in fact, too small to assess predictors of OA development. This study is unique in having such a large group of first presenters. We would like to argue that the CHECK cohort represents people in (Dutch) primary care presenting for the first time with hip and knee complaints and suspected of having early OA.

We were surprised by the large percentage of participants fulfilling ACR criteria at baseline in participants with hip complaints, and that this was even more pronounced in participants with knee complaints. In a previous open population-based knee pain cohort that included persons with chronic knee pain, 47% were not diagnosed with OA at baseline [13]. This proportion is larger than our proportion of participants without OA at baseline. This difference could be due to the younger age (mean age 45 years) and lower BMI in that cohort. In that same study, the majority (86%) of persons developed OA during the 12-year follow up [13]. In our study, a smaller proportion of participants with pain in the hip (40%) and knee (55%) were diagnosed with either hip or knee OA according to the ACR criteria during follow up. However, this result could be related to the shorter follow-up period in our study.

The predictive factors we identified to be associated with the development of hip OA are consistent with the previous literature. Morning stiffness and limited internal rotation are known predictors for total hip replacement in primary care [14, 15]. Age and pain levels, however, were not statistically significant in the final model in the current study whereas other studies found these to be predictive [14, 15]. This could be explained by our relatively young cohort with generally quite low pain levels (Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) pain score 27.2 on a scale of 0–100, numeric rating scale (NRS) 3.7, Table 1) such as can be expected in a cohort with early OA. Limited hip flexion and ESR < 20 mm/h were not identified previously as risk factors for the development of HOA but were statistically significant in our final model. A possible explanation could be that higher ESR was related to inflammatory diseases at baseline that were not evident at the time of inclusion.

In contrast to previous studies we were unable to identify predictors of the knee pain that develops into knee OA [16, 17], even when we performed a separate analysis for the clinical and the combined ACR criteria. Also, in these subgroup analyses, no variables were significantly associated with development of knee OA, except for borderline significant results for morning stiffness. The large percentage of patients with knee pain who fulfill the ACR criteria at baseline is probably the main reason for not finding significant predictive factors based on OR. However the predictive values show that morning stiffness would probably have had good prognostic value if we had had greater statistical power.

As expected, the criteria associated with fulfilling the ACR criteria at follow up, in either the combined or separate analysis for the clinical and the combined ACR criteria, were all sub-items of the ACR criteria. This indicates that pain in combination with one or more of these sub-items of the ACR criteria might be indicative of future OA.

There are currently no clear diagnostic criteria for OA in primary care, e.g. the ACR criteria are widely used in epidemiologic research but not validated in primary care. Most discussions focus on the use of radiographic outcomes [18]. For example, Kellgren and Lawrence (K&L) grade ≥ 2 is accepted as a cutoff for OA in epidemiological studies and (possibly) in secondary care. The cutoff of K&L grade ≥ 1 is useful in epidemiologic studies to predict progression, but its use is not advised in primary care because knee radiography has no additional value in the assessment of individual patients with knee pain [19–22]. However, in the present study we chose to examine not only clinical features but also radiographic features, because of the availability and still frequent use of radiography in primary care. Our study clearly showed that radiographic features do not predict fulfillment of ACR criteria, nor when assessed in subgroups of clinical or combined ACR criteria (data not shown.)

The prevalence, incidence, and predictors of the incidence of OA are clinically important findings, because they implicate that most persons aged 45–65 years of age presenting to a GP with no other hip or knee disease could be diagnosed with clinical OA at that time or are prone to developing clinical OA within the following years. This could help to provide a clear diagnosis, which contributes to early treatment according to the guidelines that are available for both hip and knee, whereas undiagnosed knee and/or hip pain is usually treated according to the best insight of the individual physician [23–25]. For patients diagnosed with OA, first-step treatments (e.g. education, lifestyle advice, and acetaminophen) should be started, due to their beneficial effects in the early stage of the disease process [26].

Our study offers a unique population to study hip and knee pain in first presenters, because the patients included are comparable with patients who would present to a primary care physician and therefore this study helps in addressing the diagnostic challenge of hip and knee pain in primary care. A limitation of our study is that a substantial number of variables were tested in the analysis. Due to the limited number of OA cases identified, we could justify testing only 2–5 variables per analysis per category when building the explorative models. However, clinically relevant variables were used (defined prior to our analyses) that were previously applied in epidemiological/clinical research and no new predictors were introduced. Further, data reduction methods were used by means of restrictions based on p values by pre-analyzing the predictors in their categories. Other predictors of OA could remain unexposed due to this lack of power.

The majority of people presenting with hip pain for the first time fulfill the clinical or combined ACR criteria and 40% of the patients not fulfilling those ACR criteria will develop OA according to the clinical or combined ACR criteria for the hip after 5 years. Predictive factors for the development of HOA are morning stiffness, painful internal rotation, hip flexion < 115° and an ESR < 20 mm/h. In first presenters with knee pain, up to 92% already fulfill the clinical or combined ACR criteria. No predictive characteristics were identified for the development of knee OA in those not fulfilling ACR criteria.

We would suggest that future studies validate whether patients with hip complaints aged > 45 years with the characteristics of morning stiffness, painful internal rotation, hip flexion < 115° and an ESR < 20 mm/h indeed have early OA. It also needs to be validated whether first presenters with knee complaints aged > 45 years indeed have early KOA.