This review was undertaken primarily to provide a summary of previously reported estimates of the radiographic prevalence of primary hip OA in the general adult population that could be referenced in future research on this topic. This goal was accomplished by summarizing estimates from previous review articles and conducting a literature search to uncover additional studies published in the past decade.
This dual strategy (primary and secondary sources) allowed us to provide a comprehensive review of the topic with estimates spanning several decades. However, we acknowledge including such estimates without verifying the primary studies of review articles may have introduced bias in those estimates if they were incorrectly reproduced in the previous review articles referenced. Although our systematic review focused on summarizing estimates of prevalence according to various factors of interest (eg, gender, age), we were unable to establish any causal relationships between those factors and radiographic primary hip OA. We were also unable to examine the effects of possible confounders (eg, obesity, physical activity) not discussed in previous estimates of prevalence and cannot determine potential interactions between various suspected risk factors. Additional observational and experimental studies attempting to isolate the effects of specific risk factors on the prevalence of radiographic primary hip OA would be required to confirm the associations noted in this review before instituting measures aimed at primary prevention.
We also focused exclusively on radiographic primary hip OA to minimize variations resulting from self-reported symptoms or physician diagnosis without imaging. However, even with these eligibility criteria in place, a lack of consensus was noted when defining hip OA in the studies reviewed. For instance, the mean prevalence was higher in studies using K&L than those using minimum JSW, the two most commonly reported methods of radiographic diagnosis. Differences were also noted based on the exact definition of hip OA within each method of diagnosis with an inverse association between prevalence and severity. This makes it essential for investigators to report not only the method of radiographic diagnosis when discussing estimates of the prevalence of hip OA, but also the precise definition used (ie, K&L grade, minimum threshold for JSW).
The overall estimates of radiographic primary hip OA uncovered in this review varied 30-fold from the lowest (0.9%) to highest (27.0%), even when examining ostensibly similar general adult populations. The mean of the estimates (8.0%) was substantially higher than the median (5.3%), indicating the data were positively skewed as a result of a few relatively high estimates. This variability underscores the inherent weakness of relying on any single estimate of prevalence when making health services decisions about hip OA and should encourage stakeholders to instead examine a range of studies that together may provide a more accurate representation of the epidemiologic characteristics of this condition.
The majority of estimates of radiographic primary hip OA uncovered in our review had been published in the past decade, perhaps as a reflection of the growing interest in this topic or as a result of the relative scarcity of large, population-based epidemiologic studies in earlier decades. Although an increase in hip OA over time has been theorized given that an aging population, at least in developed countries, is placing increasing demands on its weightbearing joints as a result of obesity and physical activity, we observed no such trends. This may have been the result of the paucity of data in the 1940s and 1950s with estimates from only two studies in each decade providing means twice as high as those reported in subsequent decades. However, by focusing on recent decades for which a greater number of estimates is available, the mean prevalence did in fact rise consistently from 4.0% in the 1970s to 8.6% in the 2000s. Ascribing meaning to such trends is quite challenging as a result of underlying study heterogeneity both over time and within each decade. Although an association between obesity and hip OA has been suggested [1
], other studies refuted these findings [30
]. This may suggest the association, if any, is weak or may simply take more time to become apparent in the literature given the relatively recent rise in obesity and time lag required for publishing the results of prospective studies.
Although it has been proposed hip that OA is less frequent in Asians [10
] and the mean prevalence was in fact lower in the few studies from Asia compared with those from North America or Europe, no firm conclusions can be drawn given the small number of estimates available. If a lower prevalence of hip OA does in fact exist in Asia, it may be partially attributable to lifestyle factors. It has been postulated, for instance, the frequent kneeling or squatting common to life in Asia may protect against hip OA [34
]. Other possibilities suggested for lower hip OA in Asia include a lower rate of acetabular dysplasia in Asians as well as differences in physical activity, obesity, and genetic factors [43
]. Limiting the language of publication to English for studies considered in this review may have resulted in fewer studies reporting on nonwhite populations.
Given the growing multicultural nature of North America and Europe, it is increasingly difficult to use the country of origin for a study as a proxy for the race or ethnicity of its participants. To separate the effects of genetic and environmental factors requires studying hip OA in adults of different ethnicities living in a similar area. In the United States, it was reported those of Asian origin had lower rates of hip arthroplasty than whites [10
]. Although this may imply potential differences in the prevalence of hip OA related to ethnicity, other factors may also be involved in the decision to proceed to arthroplasty. For example, it was reported that blacks with hip OA in the United States were less likely to view arthroplasty as an effective intervention [18
]. Differences in surgical rates may therefore be associated not only with prevalence, but also with socioeconomic factors and expectations about the healthcare system [18
The mean prevalence of radiographic primary hip OA among the studies found was higher in men than women. However, this finding is not consistent with the literature. In fact, several studies have reported women are at greater risk of developing hip OA [1
]. It has been suggested that men have a higher prevalence of hip OA before age 50 years, after which women have a higher prevalence [11
]. This was somewhat supported in our study, in which the mean prevalence of radiographic primary hip OA was higher for men in two of three age groups before 50 years (ie, 40–44 years, 45–49 years) but in only three of eight age groups after age 50 years (ie, 55–59 years, 60–64 years, 70–74 years). The higher incidence of hip OA in women after age 50 years may be related to hormonal changes from menopause [1
]. Bolstering this hypothesis are findings from several studies reporting a protective effect for estrogen replacement therapy and hip OA [10
The mean prevalence of radiographic primary hip OA from studies that included participants with a minimum age of younger than 55 years was slightly higher than those requiring a higher minimum age. Although this finding is at odds with advanced age being an independent risk factor for hip OA [1
], a more likely explanation is that the minimum age required for study participation is not indicative of the actual age of enrolled participants. Studies have previously suggested the prevalence of symptomatic hip OA increases substantially after age 50 years in both genders, possibly as a result of changes in chondrocytes, ligaments, musculature, and joint viscoelasticity [1
] independent of other postmenopausal hormonal changes seen in women. Our study supports these findings with a gradual rise in mean prevalence with advancing age, becoming more pronounced after age 60 years.
One study reported the validity, reliability, and applicability of seven commonly used methods of radiographic diagnosis for hip OA [46
]: (1) K&L; (2) Croft; (3) minimum JSW; (4) JSW according to Resnick and Niwayama [51
]; (5) American College of Rheumatology criteria; (6) radiographic hip OA with pain; and (7) radiographic index grade according to Lane et al. [35
]. The minimum JSW method of diagnosis had the highest level of intra- and interrater reliability, the highest association between radiographic findings and joint pain, and good applicability compared with the other methods; neither the Croft nor the American College of Rheumatology criteria method had high reliability and validity [46
]. It was also reported a minimum JSW of ≤ 1.5 mm had a stronger association with hip pain than did the presence of osteophytes (56% versus 34%). Given the mean JSW in normal hips is approximately 4 mm [26
], using criteria of minimum JSW of ≤ 3.0 mm may introduce more error than using lower thresholds that more easily distinguish hip OA.
The K&L method has previously been criticized for being inconsistent in its interpretation [26
] and placing undue emphasis on the presence of osteophytes, which correlate poorly with hip pain [46
]. Although the limitations of K&L for hip OA have long been apparent [26
], familiarity with this method prolongs its use in epidemiologic studies. Considering that K&L is the most commonly used method of diagnosis for hip OA [46
], a greater understanding of its validity and relation to clinical presentation is warranted. Despite these shortcomings, one group of authors has suggested the K&L is appropriate to define hip OA for epidemiologic studies [47
The type of radiograph used to establish a diagnosis of hip OA should also be considered when reporting prevalence of radiographic hip OA, because many of the epidemiologic studies reviewed were conducted in the general adult population without hip pain and therefore relied on incidental findings of hip OA after colon, pelvic, or abdominal radiographs. It has been suggested that these views may not provide sufficient details to evaluate minor structural changes in the hips [28
]. Similarly, it has been claimed the nonweightbearing radiographs used in some of these studies may make joint space narrowing less evident than the preferred weightbearing radiographs [37
]; this could particularly impact the minimum JSW method of diagnosis.
Based on the difficulties we encountered when attempting to summarize data from previous studies as a result of their heterogeneous methods, it became apparent that a gold standard should be developed to identify radiographic primary hip OA and that its use should be promoted in future studies to facilitate comparisons of results over time and across different study populations.
Further complicating the issue of the method of diagnosis for hip OA is the uncertain association between radiographic and clinical findings. Structural changes in the hip consistent with OA that may be apparent on radiographs are expected with advanced age and may not necessarily be accompanied by symptoms of the disease [3
]. One study examined the association between specific radiographic findings other than those used in the method of diagnosis and the prevalence of hip OA [30
]. When defining hip dysplasia according to acetabular depth ratio, femoral head extrusion index, or center-edge angle, the authors found an association between the presence of hip dysplasia and hip OA [30
]. Other studies have reported much higher rates of radiographic hip OA than symptomatic hip OA [22
]. This suggests symptomatic hip OA confirmed by radiographic findings of hip OA according to an established method of diagnosis may be more relevant clinically than either method alone [40
Making the association between findings of radiographic hip OA and the need for eventual surgical management through arthroplasty presents an even greater challenge. A prospective cohort of 320,192 male Swedish construction workers was conducted from 1971 to 1992 and reported a total of 1495 THAs during that time [32
]. Although the association between obesity and prevalence of radiographic primary hip OA is uncertain, body mass index was strongly associated with the incidence of THA in that study [32
]. This suggests obesity impacts the severity and clinical outcome of hip OA rather than the occurrence of this disease [32
]. Unfortunately, there was no attempt to examine radiographic findings of hip OA predictive of the need for THA in that study.
The mean prevalence of radiographic primary hip OA from studies of higher methodological quality was slightly superior to that from studies of lower methodological quality. However, these results may have been unduly influenced by the presence of outliers because the study with the highest methodological quality also had the highest reported prevalence [7
]. This suggests the methodological quality of the study plays a minor role in the observed prevalence rates with only a slight trend toward higher quality studies reporting generally higher prevalence rates.
This review uncovered numerous previously published reports of the estimated prevalence of radiographic primary hip OA in the general adult population over the past seven decades. The study methods were heterogeneous, reporting on diverse populations with differing eligibility criteria, various age and gender distributions, assorted methods of diagnosis, and divergent criteria used to satisfy those methods. Predictably, the reported estimates varied considerably among these studies. Nevertheless, it appears radiographic primary hip OA is routinely present in approximately 5% to 10% of the general adult population. Higher estimates of prevalence were generally reported in more recent studies, in studies from North America and Europe, in men, although this difference disappeared when also taking age into account, in studies using the K&L method of diagnosis, in studies with lower thresholds used to define OA, and in studies of lower methodological quality. Stakeholders should understand the various factors that may influence the reported prevalence of radiographic hip OA before planning for the demand in related health services. Future studies reporting on the prevalence of radiographic primary hip OA should endeavor to clearly define their methods and use previously validated methods of diagnosis. Establishing a gold standard for the measurement and reporting of the prevalence of radiographic primary hip OA is needed to facilitate comparisons of results across multiple studies, both past and future.