What Are the Reference Values and Associated Factors for Center-edge Angle and Alpha Angle? A Population-based Study

Abstract

Background: The available evidence regarding normal ranges for the center-edge angle and the alpha angle derives from a few small studies, and associated factors such as sex and anthropometric factors have not been well evaluated. Knowing more about normal values for these parameters is critical, because this can inform decisions about when to perform elective hip preservation surgery. Population-level studies would provide considerable clarity on these issues, but to our knowledge, no such studies are available.

Questions/purposes: The purposes of this study were (1) to use MRI in patients of a population-based study to establish normal values for the alpha and center-edge angles in the normal adult hip; and (2) to determine whether age, sex, or anthropometric variables were associated with differences in these values.

Methods: We used MRI images (1.5 T) of 3226 participants of the Study of Health in Pomerania (SHIP). SHIP is a population-based study that started with 4308 participants in 1997. Participants were recruited randomly from official inhabitant data files as a stratified cluster sample of the population from a defined region in northeastern Germany. To ensure a representative epidemiologic cohort, stratification variables were sex, age, and city of residence. Between 2008 and 2012, 1094 of these participants underwent whole-body MRI with pelvic sequences in the second followup. In parallel, the next cohort, SHIP-Trend, started with the same protocol in which 2132 participants underwent MRI examination. Reference values were calculated by quantile regressions for the 2.5 and 97.5 percentiles. Associations with the demographic features sex, age, weight, height, body mass index (BMI), and waist circumference were analyzed by bivariate linear regression models.

Results: The mean center-edge angle was 31° (± 7°) with a corresponding calculated normal range of 17° to 45°. The mean alpha angle was 55° (± 8°) with a corresponding calculated normal range between 39° and 71°. Men (30° ± 7°) had a lower center-edge angle than women (32° ± 8°) (p < 0.001, β = 1.4°; 95% confidence interval [CI], 0.9°-1.9°) and a higher alpha angle (57° ± 8° versus 52° ± 7°, p < 0.001, β = 5.7°; 95% CI, 5.2°-6.3°). Moreover, a higher center-edge angle for the left side was found (33° ± 8° versus 30° ± 8°, p < 0.001, β =3.2°; 95% CI, 3.0°-3.4°). In addition to sex, we found that age, height, waist circumference, and BMI affected both center-edge angle and alpha angle. Weight was associated with the alpha angle as well. Because of these associations, age- and sex-adjusted reference values with belonging formulas were calculated.

Conclusions: The range of normal center-edge and alpha angles is quite wide. Therefore, only markedly abnormal angles may be associated with pathology. Moreover, center-edge angle and alpha angle are associated with age, sex, and anthropometric factors, which have to be taken into account for better interpretation.

Clinical relevance: The association of abnormal radiographic values with true clinical hip pathology is tenuous at best. Assuming that a patient with an abnormal radiograph requires treatment is unwise. The clinical picture has to be substantial for therapeutic decisions.

Fig. 1

The flow diagram shows the cohort from inclusion to the final study population.

Fig. 2

Wiberg’s center-edge angle is composed of a and b; a = line perpendicular to the connection line between each femoral head center (C₁ und C₂); E = lateral acetabular margin; b = line through C₁ and E.

Fig. 3

The alpha angle (α) is composed of A and B; A = axis through the departure of the radius (r) of the femoral head; B = femoral neck axis; C = center of the femoral head.

Fig. 4 A-D

Sex- and age-dependent reference values for center-edge angle and alpha angle based on fractional polynomials are shown. Reference values of center-edge angle for women (A) and men (B) increase with age. The formulas for women are 10.04 + 0.181 × age for the lower limit and 31.68 + 0.241 × age for the upper limit. For men, the lower limit is calculated by 11.91 + 0.116 × age and the upper limit by 32.15 + 0.223 × age. The adjusted reference values for alpha angle increase for women (C, formula lower limit: 37.19 + 0.080 × age, upper limit 60.31 + 0.110 × age), whereas they stay nearly steady for men (D, formula lower limit 43.99 + 0.008 × age, upper limit of 74.58 + 0.036 × age). All reference values are given with corresponding 95% CIs.

Fig. 5

The association of center-edge angle and alpha angle with increasing body weight is shown. The alpha angle decreases, whereas the center-edge angle stays steady.

Fig. 6

The association of center-edge angle and alpha angle with increasing body height is shown. The alpha angle increases, whereas the center-edge angle decreases.

Fig. 7 A-B

Means and SDs of center-edge angle for women (A) and men (B) of previous studies and the present study are shown for comparison.

Fig. 8 A-B

Means and SDs of alpha angle for women (A) and men (B) of previous studies and the present study are shown for comparison. Toogood et al. [49] just presented mixed data for women and men (*).