The overall lifetime risk for symptomatic hip OA was 25.3%, suggesting that one in four Johnston County residents who live to age 85 are at risk of developing symptomatic hip OA. Although it was not a statistically significant difference, the lifetime risk was higher for women (28.6%) than men (18.5%), which is consistent with previous prevalence and incidence studies of symptomatic hip OA16
We found similar lifetime risks for blacks and whites, and the race-specific prevalence of symptomatic hip OA in the JoCo Project cohort also was the same for blacks and whites8
. The race-specific prevalence of radiographic hip OA has been compared in at least four other studies. Two African studies found a lower prevalence among blacks17, 18
, whereas two US studies—a national, population-based National Health and Nutrition Examination Survey I (NHANES I) survey19
and a survey of senior citizen centers in Brooklyn, New York18
—indicated a comparable prevalence among blacks and whites. Hip replacements are a well recognized and effective procedure for reducing pain and improving physical function among people with debilitating hip OA. Some studies have found evidence of greater unmet need for hip replacements among blacks compared with whites20
. Our analysis did not account for differences in symptom severity, an indication for hip replacement. However, the similar race-specific risk estimates suggest an equal need for hip replacements for blacks and whites.
Lifetime risk also was similar across education levels. Education was used as an indicator of socioeconomic status because self-reported income data were missing for a high proportion (20%) of the baseline study sample, which is consistent with many epidemiologic studies21
. At least one previous study has found an association between education and prevalent hip OA22
, but education is not a recognized risk factor for incident disease23
Although lifetime risk was higher for participants with a self-reported hip injury (50.0% [95% CI = 14.4–85.6]) than those without (22.1% [95% CI = 18.3–25.8]), the difference was not statistically significant. Hip injury and onset of hip OA have been linked in previous studies19, 24
. The lack of association in this study may have resulted from the small number of people who reported a hip injury in the radiographically affected hip at baseline.
The association between BMI and total hip replacement is strong 25-27
, but the evidence for association using other definitions of hip OA is equivocal. A meta-analysis of studies examining the association between BMI and hip OA indicated moderate evidence of a relationship (summary odds ratio=2) between BMI and hip OA when all studies were considered (i.e., studies including clinical and radiographic definitions) but no relationship when limited to studies examining radiographic disease only28
. Four longitudinal studies have reported that obesity at age 18 predicts a moderate to strongly increased risk for symptomatic hip OA and hip joint replacements in later life25, 26, 29, 30
. Obesity at age 18 and at the time of hip replacement was independently associated with an increased risk for total hip replacement among women in the Nurses Cohort Study25
. However, another study reported that obesity in early life was associated with an increased risk for hip replacement, but weight gain in the fourth and fifth decades of life did not predict later risk for hip replacement31
. We found similar risks in all BMI analyses which is consistent with evidence from studies examining radiographic hip OA; to date, too few JoCo study participants have undergone hip replacement procedures to reliably estimate an association between BMI and hip replacement. The majority of participants reported being under- or normal weight at age 18. Our analysis of BMI across three time points found that no differences across varying life course BMI trajectories; however, because a small proportion of respondents reported being overweight or obese at age 18, there was only sufficient power to estimate disease risk among respondents who reported being under or normal weight at age 18. In our study, BMI at age 18 was self-reported and is likely subject to recall bias32
. Although there was a substantial difference in BMI at age 18 and baseline, the prevalence of overweight or obesity among all, including younger, adults has increased substantially in recent decades 33, 34
; it is plausible that BMI in this cohort was substantially lower among participants at age 18.
At least seven different definitions of hip OA have been used across epidemiologic studies to classify hip OA, including K-L grades, minimal joint space width (JSW) and Croft’s grade35
. K-L grades are the most common measure35
. Two potential limitations of K-L classification are the emphasis on osteophytes35
and potentially problematic intra- and inter- rater reliability when assessing radiological features relative to a published atlas35,36
. Relative to other measures, K-L grades show lower incidence and similar or lower prevalence of radiographic hip OA36, 37
; a strong association between K-L grades and hip pain among women and people aged ≥65 years (comparable or better than JSW)38
; moderate to high inter-rater and intra-rater agreement; similar or higher predictive validity for total hip replacements compared with JSW and Croft’s grade37,38
; and moderate to strong predictive validity for progression of hip OA, especially among people with hip pain at baseline37, 39
We provide a model-predicted prevalence of OA by age 85 for those who achieve this age or older. This can be reasonably interpreted as the lifetime risk of OA for people who live to at least 85 years. This differs from a definition that estimates the risk of disease for the remaining lifetimes of people who live to varying ages 5,6,7
. However, because age 85 is a reasonable expected lifespan for individuals in the US, this estimate represents an informative, helpful, and relevant quantity which would be meaningful to most individuals, as they see themselves potentially living to that age. Our results are mortality-adjusted in the sense that we assume that for the portion of the sample that has died, they would have had OA in the same proportion as those who lived and are estimated by the model to have OA by age 85.
Our lifetime risk estimates were likely underestimated for five reasons. First, the sensitivity analysis found an estimate of 29.4%. This slightly higher lifetime risk may indicate an association between disease status and nonparticipation at first follow up; physical limitations caused by the onset of symptomatic hip OA between baseline and follow up was one reason for nonparticipation at first follow up. Second, the JoCo OA Project sample comprised men aged ≥45 years and women aged ≥50 years (pelvic radiographs were not obtained for women of reproductive age). The onset of hip OA is very uncommon among people aged 45 years or younger40, 41
. Nevertheless, there may have been cases of symptomatic hip OA in the younger Johnston County population that were not captured in this study.
Third, interviewers determined participants’ history of hip pain through oral questioning at the household interview. Birrell et al. reported that schematics are slightly more sensitive than verbal query in detecting hip pain12
. Therefore, a small proportion of participants in our study were potentially misclassified as not having hip symptoms. As well, we defined symptoms of hip OA as pain in the hip or groin. However, symptoms of hip OA can manifest in other parts of the broad hip region, including the low back. Other sites in the hip region were not included in the analysis because only hip and groin pain were measured at both baseline and at follow up.
Fourth, OA symptoms may be intermittent 42
.. We derived the lifetime risk estimate using symptom status at both baseline and at follow up to increase the likelihood of capturing experience of hip pain, thus reducing misclassification of symptomatic hip OA. Last, a maximum of 11 years of follow-up data were available. We believe that lifetime risk will be higher with increased observation time, as previous studies of lifetime risk have reported higher probabilities with increasing observation time6, 43, 44
While estimating prevalence and incidence among people aged ≥85 years can be problematic because of decreased survival (i.e., small sample sizes at older ages), the lifetime risk statistic is a cumulative measure and uses pooled information from across age groups. Therefore, disease risk at age 85 can be estimated with increased precision. GEE repeated-measures modeling was used to reduce selection bias and to increase statistical power, as data for all cohort members were analyzed, regardless of follow-up status. The proportion of the sample participating in the first follow up (among those who were eligible) was 71%, and 90% of this group completed the x-ray evaluation ()4, 10
We have estimated the lifetime risk of symptomatic hip OA to be one in four and previously reported the lifetime risk of symptomatic knee OA to be nearly one in two. The higher occurrence of symptomatic knee OA compared with hip OA is consistent with higher frequency of knee OA observed using other measures of disease burden (e.g., prevalence and incidence). Various statistical methods have been used to derive lifetime risk estimates for other chronic conditions and there is considerable variability in the characteristics of the samples (e.g., age, race/ethnicity) and sampling frames (e.g., clinic- versus population based). We believe that this substantial heterogeneity precludes comparisons of lifetime risk estimates across conditions.
We recommend caution in generalizing our results to the US population. In 1990, the distribution of age and sex in the baseline Johnston County population was comparable to the US population4, 45, 46
, but the Johnston County population had a higher proportion of black (18% versus 12%), rural (76% versus 25%), less educated (35% versus 25% had not completed high school), and lower-income residents (median income of $25,169 versus $30,056). The differences in race and education may be unimportant because although Johnston County had a higher proportion of blacks and people with less education, we found that lifetime risks were similar by race and educational attainment. The proportion of overweight or obese participants aged ≥45 years in the United States and Johnston County was similar (66% in baseline JoCo OA study sample [1990–1997] versus 63.0% in the United States in 1988–1994 NHANES 47
The lifetime risk statistic is considered an accessible statistic for describing risk to lay audiences. It is familiar to the general public because it has been used to convey the person-level risk of other chronic conditions, such as breast cancer48
. The JoCo OA Project is the only longitudinal, population-based study of OA in the United States that includes blacks and whites of both sexes who are middle aged and older. The uniqueness of this sample has enabled us to generate estimates from a sociodemographically diverse sample. The high lifetime risk for symptomatic hip OA observed in our study further illustrates the substantial public health burden of arthritis across a range of diverse groups.