The shape of the proximal femur influences acetabular wear patterns over time

Abstract

Background: Femoroacetabular impingement has been proposed as a cause of early osteoarthritis, but it is not known how this develops over time or whether the shape of the proximal femur influences this risk.

Questions/purposes: (1) Which areas of the acetabulum are worn more frequently by individuals with a cam deformity of the proximal femur? (2) Do observed acetabular wear patterns differ based on the etiology of the cam deformity? (3) Do wear patterns of individuals with a cam deformity differ based on an individual's age?

Methods: We examined 645 corresponding femora and acetabuli from the Hamann-Todd Osteological Collection and determined the offset and alpha angle using photographs; 370 specimens met inclusion criteria and were examined for signs of wear and the locations of wear were recorded. Specimens were separated into eight subgroups based on age either younger than 40 years or older than 60 years, alpha angle greater or less than 55°, and degree of anterior head-neck offset. We compared the prevalence of wear between groups in each location.

Results: Individuals with abnormal geometry of the proximal femur demonstrated different wear patterns from individuals with normal geometry. There were few differences in wear patterns identified based on the etiology of the femoral deformity. Abnormal femoral geometry was associated with more frequent wear primarily at the anterosuperior acetabulum for individuals younger than 40 years of age and globally for individuals older than 60 years of age.

Conclusion: Femoral geometry appears to influence the pattern of acetabular wear occurring over time.

Fig. 1A–C

The morphology of the femoral neck was observed using an axially directed photograph perpendicular to the femoral neck. The alpha angle and degree of posterior positioning of the femoral head with respect to the neck axis were measured for each specimen using MATLAB software. Specimens were then divided into groups: the Bump group (A) included specimens with alpha angle > 55° and the head not offset more than 0.7 mm behind the axis of the neck; the Offset group (B) included specimens with any alpha angle and the femoral head offset more than 1.4 mm behind the axis of the neck; the Cam group included all specimens in the Bump and Offset groups; the Normal group (C) included specimens with alpha angle < 55° and the head not offset by any amount behind the axis of the neck.

Fig. 2

Acetabular specimens were observed for evidence of wear, which included osteophytes, erosions, and pitting. These changes could be found in several areas within the same acetabulum. An acetabulum exhibiting a large amount of wear is shown at left with the contralateral acetabulum from the same specimen showing little evidence of wear.

Fig. 3

The grading system we devised for mapping acetabular wear divides the acetabulum into 15 zones. Five zones each comprise the center rim of the semilunar cartilage (C), articular surface (A), and the outer acetabular rim (R), and these correspond to the numbers on a clock face. We examined the anterior (3:00–5:00), anterosuperior (1:00–3:00), superior (11:00–1:00), posterosuperior (9:00–11:00), and posterior (7:00–9:00) areas of the acetabulum. Wear in these areas was noted and recorded to create a map for each specimen.

Fig. 4

A comparison of wear patterns between specimens of all ages with and without a cam deformity was performed. Specimens demonstrating a cam deformity were found to have more frequent appearance of acetabular wear centrally from 7:00 to 3:00, on the articular surface from 11:00 to 1:00, and at the rim from 11:00 to 3:00.

Fig. 5A–B

A comparison of acetabular wear patterns based on both morphology and age was performed. (A) Specimens younger than 40 years of age in the Cam group showed more frequent wear than those in the Normal group at the acetabular rim from 11:00 to 3:00. There was also a trend toward more frequent wear (p < 0.10) at the articular surface from 11:00 to 5:00. Specimens older than 60 years of age in the Cam group showed more frequent wear centrally from 7:00 to 3:00, on the articular surface from 9:00 to 1:00, and at the rim from 7:00 to 1:00. (B) No large differences between the Bump and Offset groups were found in specimens younger than 40 years of age, but a trend toward more frequent acetabular rim wear from 9:00 to 11:00 in the Offset group was identified. For specimens older than 60 years of age, more frequent wear was identified in the Offset group centrally from 1:00 to 3:00, and a trend toward more frequent wear was identified on the articular surface from 11:00 to 1:00. A trend toward more frequent wear in the Bump group was identified on the articular surface from 1:00 to 3:00.