Population-based prevalence of multiple radiographically-defined hip morphologies: the Johnston County Osteoarthritis Project

Abstract

Objective: To provide the first prevalence estimates of different radiographic hip morphologies relevant to dysplasia and femoroacetabular impingement in a well-characterized USA population-based cohort.

Methods: Cross-sectional data were from the baseline examination (1991-1997) of a large population-based prospective longitudinal cohort study (The Johnston County Osteoarthritis Project). HipMorf software (Oxford, UK) was used to assess hip morphology on anteroposterior (AP) pelvis radiographs. Weighted, sex-stratified prevalence estimates and 95% confidence intervals for four key hip morphologies (AP alpha angle, triangular index sign, lateral center edge angle (LCEA), and protrusio acetabula) were derived and further stratified by age, race and body mass index (BMI).

Results: A total of 5192 hips from 2596 individuals were included (31% African American, 43% male, mean age 63 years, mean BMI 29 kg/m²). Cam morphology was seen in more than 25% of men and 10% of women. Mild dysplasia was present in about 1/3 of men and women, while pincer morphology was identified in 7% of men and 10% of women. Femoral side (cam) morphologies were more common and more frequently bilateral among men, while pincer morphologies were more common in women; mixed morphologies were infrequent. African-Americans were more likely to have protrusio acetabula than whites.

Conclusion: We report the first population-based prevalence estimates of radiographic hip morphologies relevant to femoroacetabular impingement (FAI) and dysplasia in the USA. These morphologies are very common, with ¼ men and 1/10 women having cam morphology, 1/3 of all adults having mild dysplasia, and 1/15 men and 1/10 women having pincer morphology in at least one hip.

Keywords: Cam/pincer morphology; Epidemiology; Femoroacetabular impingement; Osteoarthritis; Radiography.

Figure 1

Flowchart of participants and hips included in the analysis

Figure 2. Assessment of hip morphologies in HipMorf

A) Alpha (α) Angle. Point 1 is the center of the congruent circle placed over the femoral head. Point 2 is placed on the superior cortex of the femoral neck as it passes out of the circle placed over the femoral head. Alpha angle (α) is marked as the angle between the femoral neck axis and the line joining Point 1 and Point 2. B) Triangular (Gosvig) Index A line is drawn along the midline of the femoral neck axis. A distance equating to ½ the radius of the femoral head is measured (mm) along this axis from the femoral head centre, at which point a line is drawn at a perpendicular angle to this axis and which extends to the outer cortex of the femoral head. This line represents the triangular index as a binary outcome; present if R > r + 2mm, absent if R < r +2mm. C) Lateral Center Edge Angle (LCEA) A vertical axis is drawn perpendicular to the inter-teardrop line. The lateral centre edge angle is measured between this vertical axis and a line drawn between the femoral head centre (point 3) and the lateral edge of the sourcil (point 4). This line was drawn to the native sourcil, excluding any osteophytes or overgrowth that may have been present. D) Protrusio Acetabuli The medial margin of the femoral head touches or overlaps the ilioischial line.