To describe differences in radiographic features of hip osteoarthritis (OA) between African American and white men and women.
To describe differences in radiographic features of hip osteoarthritis (OA) between African American and white men and women.
We conducted a cross-sectional analysis of radiographic hip OA using baseline data from the Johnston County Osteoarthritis Project, using Kellgren/Lawrence (K/L) grade, and the presence, location, and severity of 4 individual radiographic features (joint space narrowing [JSN], subchondral cysts, sclerosis, and osteophytes). Sex-specific logistic regression was used to evaluate associations between race and individual radiographic features, adjusting for age, body mass index, education, and prior hip injury. Robust variance estimators via generalized estimating equations were used to account for correlation between hips from the same individual.
The sample (n = 2,739) comprised 57% women and 31% African American participants. Among women, African Americans and whites had a similar prevalence of hip OA, defined as K/L grade ≥2 (23% versus 22%), but African American women were significantly more likely to have superior or medial JSN, moderate or severe axial JSN, medial or lateral osteophytes, and subchondral cysts. Among men, 21% of African Americans and 17% of whites had hip OA; African American men were more likely to have superior or medial JSN and lateral osteophytes, but were less likely to have axial JSN.
Individual radiographic features and patterns of hip OA differed by race among women and men, suggesting the possibility of anatomic and/or developmental variation in the hip joint. African Americans have an increased frequency of features that have been predictive of hip replacement in other populations, a finding worthy of further study.
Osteoarthritis (OA) is a common, chronic medical condition with ever-increasing effects on the aging population. Over 21% of the adult US population was affected by self-reported doctor-diagnosed arthritis in 2003–2005, primarily OA, with 8.8% experiencing arthritis-related activity limitation (1). These numbers are expected to increase to 25% and 9.3%, respectively, by 2030 (2). OA of the weight-bearing joints is a particularly important type of arthritis due to effects on mobility, and led to 96.8% of the 455,000 knee and 82.5% of the 233,000 hip replacement operations in the US in 2004 (3).
There are known differences in radiographic hip OA by sex (4–7), and differences by race have been identified as well. Although previous estimates of hip OA prevalence among native African populations have been low compared with whites (8, 9), the frequency of hip OA is higher among Americans of African descent compared with native African populations (10–12). Although total hip replacement (THR) rates are lower among African Americans compared with whites (10, 13), it cannot be assumed that this reflects a lower prevalence of hip OA among African Americans, because African Americans are also less likely to undergo knee replacement (14), despite having equal or higher rates of knee OA compared with other groups (15). Most studies of hip OA are based on Kellgren/Lawrence (K/L) grade, a global assessment combining features such as joint space narrowing (JSN), subchondral cysts and sclerosis, and osteophytes. However, this global assessment does not distinguish between medial and lateral involvement, and places emphasis on osteophytes over other radiographic features, especially in the less severe categories. When evaluating the individual features of radiographic OA at the knee, our group identified differences between African American and white individuals beyond those seen for K/L grade (15), but similar studies have not been done for the hip.
Among elderly white women, individual radiographic features of hip OA, including osteophytes and JSN, were predictive of hip OA progression in the Study of Osteoporotic Fractures (16). Both the location of osteophytes in the hip joint and the pattern of JSN had impact on the odds of any radiographic progression and the odds of progression to hip replacement surgery (16). Therefore, these individual features, along with others, may be more useful than the global K/L grade in understanding the disease course of radiographic OA at the hip, and any differences by race may provide similarly important prognostic information. The purpose of this study was to evaluate differences by race among men and women in regard to individual radiographic features of hip OA, after accounting for differences in demographic and clinical factors.
The study sample was drawn from the Johnston County Osteoarthritis Project, an ongoing, population-based study of OA in a rural North Carolina county. The recruitment of participants and overall project design are detailed elsewhere (17). Briefly, the Johnston County Osteoarthritis Project is a prospective longitudinal cohort study of knee and hip OA in civilian, noninstitutionalized African Americans and whites, age ≥45 years, who are residents of 1 of 6 townships in Johnston County for ≥1 year, and who are capable of completing the study protocol, consisting of home interviews and clinic visits. The current analysis is cross-sectional, utilizing data from the baseline evaluation (May 1991 to December 1997). The study was approved by the Institutional Review Boards of the University of North Carolina Schools of Medicine and Public Health, and the Centers for Disease Control and Prevention; all participants provided written informed consent prior to enrollment.
Categorization of racial group, educational attainment, and prior hip injury (assessed separately for right and left hip) were by self-report. Body mass index (BMI; kg/m2) was calculated from weight (kg) and height (cm) measured during clinic visits. All men and women age ≥50 years had a supine anteroposterior pelvic film, taken with the feet in 15 degrees of external rotation. Because women <50 years of age did not have pelvic radiography to avoid pelvic radiation, they were not included in the current analysis. Hip radiographs were read without knowledge of participant clinical status by a single radiologist (JBR) using the K/L radiographic atlas for overall hip radiographic grades (18). As previously described, inter- and intrarater reliability for this reader are high (κ = 0.859 and 0.886, respectively) (19). Radiographs scored as K/L grade 0 (normal) showed no radiographic features of OA; K/L grade 1 (questionable) included a small osteophyte of doubtful significance. Radiographs showing an osteophyte but no JSN were assigned K/L grade 2 (mild); moderate JSN was graded K/L 3 (moderate); K/L grade 4 (severe) was assigned if severe JSN was present, along with subchondral bone sclerosis (18). Hip radiographic OA was defined as a K/L grade of ≥2 in at least 1 hip; unilateral or bilateral involvement was also evaluated.
Four individual features of hip OA were assessed, including JSN (superior, axial, or medial, as defined by Lanyon et al &lsqbr;6&rsqbr;), subchondral cysts (either femoral or acetabular), sclerosis (either femoral or acetabular), and osteophytes (medial and lateral, acetabular and/or femoral). Osteophytes and JSN were graded for severity based on the Burnett et al Radiographic Atlas of OA (20): a normal joint was designated grade 0, mild involvement as grade 1, moderate involvement as grade 2, and severe involvement as grade 3. Due to small numbers in the severe group, the moderate and severe categories were combined for analysis. Sclerosis and cysts were graded as present or absent.
Statistical analyses were conducted using SAS, version 9 (SAS Institute, Cary, NC). Means of continuous variables were compared using Student's t-test, whereas proportions were compared using chi-square tests or Fisher's exact test, as appropriate. All analyses were stratified by sex, based on known significant differences between men and women. For each sex, binomial and multinomial multiple logistic regression models were used to evaluate associations between race and the presence, location, or severity of individual radiographic features. Binomial logistic models were used for binary characteristics such as the presence or absence of sclerosis.
For ordinal outcomes such as the severity of JSN, proportional odds models were initially used to assess evidence of consistently increasing odds of severity with selected covariates (the proportional odds assumption). A proportional odds model assumes that the relationship between the independent variable of interest and levels of outcome are similar across successively more severe levels of the outcome, generating a single odds ratio (OR) describing the comparison between the best category and the other categories combined, and between the best category plus the middle category, compared with the worst category. This is in contrast to a multinomial logistic model, which generates an OR for each comparison between the referent and every other category. In the case that the proportional odds assumption is satisfied, a single OR can represent the odds across all levels of a variable. However, if the proportional odds assumption is not met, then the relationship between the independent variable and the outcome varies by the level of the outcome. An example of this would be if the odds for a mild level of severity were lower among African Americans, but for the moderate/severe level were higher in African Americans. In this case, separate ORs must be used for each outcome level. Multinomial logistic models were used to assess the association of predictors with each category of nominal outcomes or ordinal outcomes for which the proportional odds assumption was not met. Models were adjusted for covariates of age, BMI (kg/m2), education (<12 years versus ≥12 years), and prior hip injury. Robust variance estimators via generalized estimating equations were used to account for correlation between hips from the same individual.
Data from 3,187 individuals were assessed; after exclusion of women <50 years of age, individuals with evidence of inflammatory disease (on hip or knee radiographs) or bilateral hip joint replacement, and those missing K/L grades (including 24 with unilateral joint replacements), 2,739 participants (1,184 men and 1,555 women) remained for analyses (Figure 1). Of the analyzed individuals, 57% were women and 31% were African American.
Characteristics of the sample, stratified by sex, are shown in Table 1. The mean age was higher among women than men, due to the exclusion of women <50 years of age, with no significant differences in age between racial groups by sex. African American women had a significantly higher mean BMI compared with white women; white men had a higher BMI compared with African American men. More white men and white women (66% and 60%, respectively) had a high school education or beyond, compared with African American men and women (46% and 45%, respectively). There was no difference by race for history of prior hip injury among men or women.
|Women (n = 1,555)||Men (n = 1,184)|
|White (n = 1,023)||African American (n = 532)||P*||White (n = 867)||African American (n = 317)||P*|
|Age, mean ± SD years†||64 ± 9||64 ± 9||0.99||61 ± 10||60 ± 11||0.17|
|Body mass index, mean ± SD kg/m2†||28 ± 6||31 ± 7||< 0.001‡||28 ± 5||27 ± 5||0.01‡|
|Completed at least high school, %§||60||45||< 0.001‡||66||46||< 0.001‡|
|Prior hip injury, %§||3||4||0.18||3||4||0.57|
African American and white women had a similar prevalence of radiographic OA, defined by a K/L grade ≥2 in at least 1 hip (23% versus 22%). No differences were seen between white and African American women by specific K/L grades (P = 0.14) (Figure 2). Unilateral disease was more common than bilateral disease in both groups of women, with no difference by race (P = 0.64). In unadjusted analyses, significant differences were identified in several of the radiographic features. Mild axial JSN was seen more frequently among white women compared with African American women (26% versus 20%), and although African American women had more frequent moderate/severe axial JSN, this finding was quite rare in both groups (1–2% affected) (Table 2). African American women had more frequent superior JSN, and slightly higher frequencies of medial JSN, although the numbers for medial JSN were small and not significantly different by race. African American women had a slightly higher frequency of subchondral cysts, but there was no racial difference for sclerosis (Table 2). African American women had an increased frequency of medial osteophytes, especially on the femoral side of the joint, compared with white women (Table 3). Lateral osteophytes were also more frequent among African American women, and more often on the acetabular side, compared with white women. The combination of both acetabular and femoral osteophytes was also increased in African American compared with white women, both laterally and medially. When assessed across the entire joint without separating lateral from medial, African American women were more likely to have osteophytes in any site (47.1% had acetabular only, 5.1% had femoral only, and 16.0% had both) compared with white women (42.5%, 3.6%, and 13.2%, respectively; P < 0.001).
|Feature||Women (n = 3,110 hips)†||Men (n = 2,368 hips)†|
|White||African American||P‡||White||African American||P‡|
|Axial||< 0.001§||< 0.001§|
|None||1,489 (73)||828 (78)||1,366 (79)||539 (85)|
|Mild||539 (26)||214 (20)||359 (21)||86 (14)|
|Moderate/severe||18 (1)||21 (2)||9 (1)||8 (1)|
|None||1,915 (94)||960 (90)||1,594 (92)||533 (84)|
|Mild||123 (6)||95 (9)||126 (7)||93 (15)|
|Moderate/severe||8 (<1)||8 (1)||14 (1)||8 (1)|
|None||2,008 (98)||1,027 (97)||1,728 (99)||625 (99)|
|Mild||31 (2)||29 (3)||4 (<1)||5 (1)|
|Moderate/severe||6 (<1)||4 (<1)||2 (<1)||4 (1)|
|Absent||1,971 (96)||1,006 (95)||1,626 (94)||583 (92)|
|Present||75 (4)||58 (5)||108 (6)||51 (8)|
|Absent||1,834 (90)||953 (90)||1,395 (80)||484 (76)|
|Present||212 (10)||111 (10)||339 (20)||150 (24)|
|Feature||Women (n = 3,109 hips)†||Men (n = 2,368 hips)†|
|White||African American||P‡||White||African American||P‡|
|None||1,909 (93)||961 (90)||1,509 (87)||542 (85)|
|Mild||107 (5)||93 (9)||194 (11)||76 (12)|
|Moderate/severe||27 (1)||8 (1)||31 (2)||16 (3)|
|None||1,905 (94)||961 (91)||1,509 (87)||542 (85)|
|Acetabular only||54 (3)||35 (3)||81 (5)||26 (4)|
|Femoral only||49 (2)||49 (5)||93 (5)||48 (8)|
|Both||27 (1)||16 (2)||50 (3)||18 (3)|
|Severity§||< 0.001¶||< 0.001¶|
|None||836 (41)||359 (34)||849 (49)||251 (40)|
|Mild||1,030 (50)||575 (54)||747 (43)||317 (50)|
|Moderate/severe||179 (9)||130 (12)||138 (8)||66 (10)|
|None||834 (41)||357 (34)||847 (49)||249 (39)|
|Acetabular only||884 (43)||500 (47)||591 (34)||247 (39)|
|Femoral only||73 (4)||47 (4)||101 (6)||32 (5)|
|Both||248 (12)||157 (15)||193 (11)||103 (16)|
In adjusted analyses, no significant differences were seen between African American and white women for K/L grade or laterality (data not shown). After adjustment, estimates did not significantly change from unadjusted models (Table 4). For axial JSN, where the proportional odds assumption did not hold, African American women had more than twice the odds of moderate or severe axial JSN compared with white women, with mild disease being 30% less likely among African American women. The proportional OR for superior JSN indicated that African American women were 70% more likely to have superior JSN, and to have it be more severe, compared with white women. Results were similar for medial JSN, but did not reach statistical significance, likely due to the infrequency of this pattern of JSN. The odds of having subchondral cysts were 50% higher for African American women, but sclerosis did not differ by race (Table 4). African American women had 40% increased odds for more frequent and severe medial osteophytes compared with white women, with twice the odds of medial femoral osteophytes alone, but there were no significant differences by race for medial acetabular osteophytes alone or for the combination of both medial femoral and medial acetabular osteophytes in the adjusted analyses. African American women had more frequent and severe lateral osteophytes compared with white women, and had 30% increased odds for the combination of lateral femoral and acetabular osteophytes, with no differences for isolated femoral osteophytes or acetabular osteophytes in the lateral compartment. For the overall pattern of osteophytes, without separation into lateral and medial compartments, African American women were 30% more likely to have only acetabular osteophytes, 80% more likely to have only femoral osteophytes, and 50% more likely than white women to have both acetabular and femoral osteophytes (data not shown).
|Feature||Women (n = 3,019 hips)†||Men (n = 2,296 hips)†|
|Mild||0.7 (0.6–0.9)‡||0.6 (0.4–0.9)‡|
|Moderate/severe||2.3 (1.1–5.0)‡||2.0 (0.7–5.9)|
|Superior, mild or moderate/severe||1.7 (1.2–2.3)‡||2.0 (1.5–2.8)‡|
|Medial, mild or moderate/severe||1.8 (0.9–3.4)||3.6 (1.0–13.6)‡|
|Subchondral cysts||1.5 (1.0–2.3)‡||1.3 (0.8–2.1)|
|Sclerosis||1.0 (0.7–1.3)||1.3 (0.9–1.7)|
|Medial severity, mild or moderate/severe§||1.4 (1.0–2.0)‡||1.2 (0.8–1.6)|
|Medial, by location|
|Acetabular||1.1 (0.7–2.0)||0.9 (0.5–1.5)|
|Femoral||2.0 (1.2–3.3)‡||1.5 (0.9–2.3)|
|Both||1.0 (0.4–2.5)||1.1 (0.5–2.1)|
|Lateral severity, mild or moderate/severe§||1.2 (1.0–1.5)‡||1.4 (1.1–1.8)‡|
|Lateral, by location|
|Acetabular||1.2 (0.9–1.4)||1.4 (1.1–1.9)‡|
|Femoral||1.5 (0.9–2.4)||1.0 (0.6–1.6)|
|Both||1.3 (1.0–1.8)‡||1.8 (1.3–2.7)‡|
Among the men, 17% of whites and 21% of African Americans had a K/L grade ≥2 in at least 1 hip, with most of the difference appearing in the milder categories (P = 0.02) (Figure 2). In unadjusted analyses, unilateral disease was more common than bilateral in both groups, with no difference by race (P = 0.08). African American men had a lower frequency of mild axial JSN compared with white men (Table 2). Similar to the findings in women, African American men had more frequent superior JSN than white men. There were no differences by race in medial JSN for the men, because the numbers of affected men were very small. There was no difference by race for the frequency of subchondral cysts, but African American men had a slightly higher frequency of sclerosis than did white men (Table 2). Although the men had similar frequencies of medial osteophytes, African American men had significantly more frequent and more often severe lateral osteophytes, especially on the acetabular side of the joint (Table 3). Again similar to the findings for women, the combination of both acetabular and femoral osteophytes was more frequent among African American compared with white men, but only on the lateral side of the joint. When osteophytes were assessed across both medial and lateral compartments, African American men more frequently had only acetabular osteophytes (36.6%) and both acetabular and femoral osteophytes (19.1%) compared with white men (33.4% and 13.0%, respectively; P < 0.001).
In adjusted analyses, despite a trend toward a difference by race for K/L grade ≥2 (OR 1.3, 95% confidence interval [95% CI] 1.0–1.8), there were no differences by specific K/L grades (and therefore by global OA severity) at the hip among men (data not shown). African American men were, however, more likely to have bilateral radiographic hip OA compared with white men (OR 1.4, 95% CI 1.0–1.8). As in the women, unadjusted and adjusted models did not significantly differ, and adjusted ORs are presented (Table 4).
Compared with white men, African American men were significantly less likely to have mild axial JSN, and no differences by race were seen for moderate/severe axial JSN. African American men had twice the odds of more frequent and severe superior JSN compared with white men. There was a borderline significant racial difference in medial JSN among the men, but very few men in either group had this pattern of JSN, and the model fit was questionable. No significant racial differences were seen among men for sclerosis or subchondral cysts. Medial osteophytes also did not differ significantly by race among the men; no differences were seen for medial acetabular or medial femoral osteophytes alone or for the presence of osteophytes on the acetabular or femoral side of the joint in the medial compartment. However, and similarly to the women, African American men had ∼40% higher odds of having, and having more severe, lateral osteophytes, with 80% increased odds for the combination of both femoral and acetabular osteophytes on the lateral side and 40% increased odds for isolated lateral acetabular osteophytes, with no difference for isolated femoral osteophytes. Finally, when osteophytes were considered regardless of medial or lateral compartment, African American men were 40% more likely to have only acetabular osteophytes and 80% more likely to have both acetabular and femoral osteophytes, compared with white men (data not shown).
This study revealed several differences between African American and white participants in specific radiographic features of hip OA, providing additional information beyond the global assessment provided by K/L grading. We again saw a comparable frequency of radiographic hip OA among African Americans and whites, in line with more recent estimates (11, 12). The hips of African American men and women demonstrated an increased frequency and severity of superior JSN compared with the hips of white men and women. African American men and women had less mild axial JSN than white men and women, and African American women had more moderate and severe axial JSN than white women. These findings suggest that there may be basic anatomic or biomechanical differences by race leading to variations in the pattern of radiographic OA. Cysts and sclerosis showed less variation by race than did other characteristics, although African American women were more likely to have subchondral cysts than white women. Osteophytes were overall more frequent in African American compared with white participants, especially in the lateral compartment, and for the combination of both acetabular and femoral osteophytes, suggesting that there was more bone formation among African American participants.
Although it is not immediately evident why there are differences in specific radiographic features by race, there are several possibilities. Our estimates did not change after adjustment for standard confounders, including age, BMI, education, and prior injury to the joint, indicating that differences in these factors do not explain the observed racial differences. The association between BMI and hip OA is generally not as strong as the association between BMI and knee OA (21–23). BMI was not associated with radiographic features of hip OA in a large cross-sectional study of white individuals in Denmark (4). Occupational history was not evaluated in the current study, but has previously been shown to have variable association with hip OA in general, and none with specific radiographic features (4, 24). However, there may be additional unidentified or unmeasured confounders that would account for the difference seen by race. There may be complex biomechanic differences by race in weight bearing and loading of joints that are dependent not only on the hip, but also on the ipsilateral and contralateral knee and foot. Local or systemic differences in bone mineral density, not measured as part of the current study, could contribute to increased osteophyte formation in African American participants. An association between elevated bone mineral density and the presence, severity, and bilaterality of hip OA has been demonstrated, both at the OA-affected joint and systemically (25, 26), suggesting that elevated bone mineral density may be involved in the pathogenesis of hip OA. As African Americans have higher overall bone density compared with whites, this may have a significant role in the observed racial differences.
Radiographic features of OA may be predictive of prognosis over time. Lanyon et al found that the risk of severe hip OA was higher in siblings of index cases with atrophic hip OA and no osteophytosis, as compared with index cases with any degree of osteophyte formation (6). Lane et al showed that after 8 years of followup, elderly women with both acetabular and femoral osteophytes had 5 times the odds of progressing to THR compared with women with no osteophytes, and that women with femoral osteophytes alone had nearly 3 times the odds of progressing to THR compared with women with no osteophytes (16). Additionally, there was an increased risk of progression of JSN and progression to THR at 8 years among women with baseline superolateral JSN (16). Although the generalizability of these results is unknown, in the current study, African American women had increased odds of having the combination of acetabular and femoral osteophytes, more femoral osteophytes (especially in the medial compartment), and more frequent superior JSN (comparable with superolateral in the Lane et al study), raising the possibility that African American women may be at higher risk for progression compared with white women. This is particularly interesting in the context of rates of THR, which are much lower among African Americans (13), previously thought to be related to lower prevalence of hip OA in African Americans. If the prevalence of hip OA is comparable among African American and white participants, and the radiographic features of hip OA are suggestive of worse prognosis among African Americans, then there is a potentially large unmet need for THR among African Americans with hip OA. The current cross-sectional study did not evaluate progression, but this could be assessed in future studies.
Limitations to the analysis include small numbers in the higher K/L grade categories, especially for K/L grades 3 and 4, and in the moderate and severe categories of JSN. Exclusion for inflammatory arthritis was based on hip and knee radiographs, because we did not have serologic data or other diagnostic criteria to assess; it is therefore possible that a small number of individuals with inflammatory arthritis was included, or that a few with OA were excluded. We did not assess hip symptoms, because we were primarily interested in structural differences for the current analysis, but symptomatic features could be assessed in future analyses. Data regarding bone mineral density were not available at the baseline examination. There was a slight difference in sample size between our unadjusted and adjusted models due to missing data for covariates; however, the estimates did not significantly change with adjustment, either with or without adjustment for injury, compared with the unadjusted ORs. Strengths of our study include the large sample size, as well as the single experienced bone and joint radiologist reading all of the films. Additionally, due to the design of the Johnston County Osteoarthritis Project, we are able to recruit adequate numbers of African American subjects to assess differences by race. Anatomic measures are currently being evaluated on a subset of films to look for racial differences in femoral and pelvic anatomy that may contribute to the differences seen in the current study.
In conclusion, African Americans are not spared from hip radiographic OA, and have more frequent and severe superior JSN and osteophytes than whites. These findings may have implications for progression and prognosis of radiographic hip OA, and are suggestive of a potentially large unmet need for hip replacement surgery in African Americans, warranting further investigations into rates of progression and THR in this population.
All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version to be submitted for publication. Dr. Nelson had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study conception and design. Nelson, Braga, Renner, Hochberg, Jordan.
Acquisition of data. Renner, Woodard, Helmick, Jordan.
Analysis and interpretation of data. Nelson, Braga, Renner, Atashili, Helmick, Jordan.
We would like to thank the staff and participants of the Johnston County Osteoarthritis Project, without whom this study would not have been possible.