After all data had been collected and specimens had been sorted into groups, final demographic information for each group was determined, which consisted of mean age, sex, and race. The mean alpha angle and offset were calculated for each subgroup (Table 1). A Student’s t-test was performed to determine differences in these characteristics of the proximal femur. Acetabular wear data were compared between groups for each of the 15 anatomic zones using a parametric test of two proportions (Z-test) to determine whether there were differences between groups in the frequency of wear appearance in each zone. A total of 370 specimens met inclusion criteria for comparison of acetabular wear patterns based on whether or not a cam deformity was present (216 cam, 154 normal). The comparison of acetabular wear patterns between groups based on whether the cam deformity was caused by a bump on the anterolateral femoral neck or by posterior positioning of the femoral head with respect to the neck axis was conducted by comparing the Bump (n = 94) and Offset (n = 122) groups directly. Differences in acetabular wear patterns based on the presence or absence of a cam deformity, with respect to a patient’s age, were evaluated by comparing groups based on age either younger than 40 years or older than 60 years at the time of death.
The proximal femoral geometry of the Cam and Normal groups differed substantially, with the Cam group demonstrating a much larger alpha angle and more posterior positioning of the femoral head with respect to the neck axis. The acetabuli in the Cam group also showed more frequent wear in several areas when compared with the Normal group (Table 1). Specimens demonstrating a cam deformity had a 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. 4).
The Bump and Offset groups demonstrated differences in the alpha angle and the degree of posterior positioning of the femoral head with respect to the neck axis with the latter of these differences being very noticeable. However, direct comparison between the acetabular wear patterns of the Bump and Offset groups demonstrated no differences (Table 2).
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 (Table 3). 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 (Table 4). The locations of wear differences changed based on whether the specimens were younger than 40 years of age or older than 60 years of age (Fig. 5A). We found no differences between the Bump and Offset groups 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 (Fig. 5B).
The prevalence of a cam deformity within the general population is believed to be high despite the fact that hip pain in younger individuals is comparatively rare . Although unproven, it is also believed that this deformity usually develops over time and may be a cause of hip osteoarthritis, even in those individuals who do not exhibit pain early in life [2, 5]. The current conceptualization of cam impingement, as presented by Ganz et al. , states that as the femoral cam deformity comes into contact with the acetabulum, the labrum is pushed up and the labral-chondral junction is exposed to increased shear stresses. Recent studies have implicated labral damage in the progression to early osteoarthritis, because damage to the labrum alters joint stability, impairs the sealing mechanism of the labrum over the femoral head and neck, and decreases cartilage consolidation . By performing an osteological study of specimens with and without the cam deformity, we have attempted to further substantiate this concept of osteoarthritis progression by answering the following questions: (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 whether the cam shape is caused by a bump on the femoral neck or by abnormal posterior positioning of the femoral head with respect to the axis of the neck? (3) With respect to both of these questions, do the wear patterns of individuals with a cam deformity differ based on an individual’s age?
The conclusions of our study must be viewed in light of several limitations. First, we used a novel grading scheme with limited interobserver agreement: the overall agreement between observers for 900 measurements was greater than 75% (Cohen’s kappa = 0.502). We nonetheless believe our attempt at mapping acetabular wear provides important data despite the lack of validation of the scheme and the limited reliability. Second, the use of an osteological collection limits the conclusions that may be drawn from the data. The specimens we studied lacked articular cartilage and labral tissue, and thus wear needed to have been severe enough to cause changes in the underlying bone to be observed. However, all specimens were prepared in the same manner, and we believe wear significant enough to cause osseous changes is likely to be clinically important. It was also not possible to assess whether these individuals had hip pain or whether they became symptomatic early or late in life, and other factors such as activity level could not be assessed. Third, the orientation of the acetabulum, as it relates to pincer-type FAI, was unknown because it was not possible to make these measurements using disarticulated skeletons. However, despite not knowing the orientation of the acetabulum, we believe our data relating to femoral geometry are of value and are in agreement with previous clinical work . Finally, we did not attempt to quantify the severity of wear at each location. By placing the focus of our investigation instead on the pattern of wear that could be observed, we have provided data that do not attempt to comment on whether a particular deformity caused changes that might have been particularly debilitating in life, but instead simply provide information about the relationship of femoral geometry and the location of acetabular wear.
We observed a difference in the acetabular wear patterns of individual specimens based on whether a cam deformity of the proximal femur was present. The presence of a cam deformity was associated with an increased prevalence of acetabular wear at the anterosuperior rim as well as centrally deep within the acetabulum. This finding is in agreement with earlier work by Goodman et al.  published using the same osteological collection in 1997. These authors graded the severity of global hip osteoarthrosis and found more severe femoral and acetabular changes occurred in individuals with postslip morphology. We believe our study provides further evidence of a wear pattern in FAI that does not strictly involve the anterosuperior acetabular rim and one that deserves further study.
The osteological specimens we studied demonstrated very few differences in wear patterns based on whether the cam deformity was primarily caused by bone buildup or abnormal positioning of the femoral head posteriorly with respect to the femoral neck axis. The prototypical deformity of the proximal femur characterized by abnormal posterior offset occurs in cases of subclinical SCFE, which is considered to be a static deformity occurring early in life. Although the natural history of the development of an anterolateral bump at the femoral head-neck junction is not known, this region of the proximal femur is believed to be the last part of the neck to ossify [11, 15] and the deformity is considered to be more progressive. Given our findings, it may be the case that any abnormal abutment of the proximal femur against the acetabular rim with hip motion has the potential to cause global damage over time. Whether this abutment is caused by the development of bump morphology on the anterosuperior neck or by posterior slippage of the capital femoral epiphysis may be less important.
By addressing age as a factor in the appearance of wear within the acetabulum, we hoped to provide insight into the natural history of cam impingement. Individuals younger than 40 years of age demonstrated differences between the Normal group and the Cam group at the acetabular rim between 11:00 and 3:00 and some increased wear on the articular surface from 11:00 to 5:00. This is consistent with the described injury pattern of FAI. Specimens older than age 60 years at the time of death demonstrated differences between the Cam and Normal groups, which we found both centrally and posteriorly. This may be evidence of global osteoarthritis resulting from loss of the seal of the labrum over the femoral head and possibly of a contrecoup wear pattern, which has been proposed to occur in pincer-type FAI [12, 13]. Although it deserves further study, calcification of the labrum after repeated damage caused by a cam deformity may have contributed to this posterior shifting of wear patterns over time. Our results are in agreement with those of Beck et al. , who performed a similar study in patients with cam-type morphology undergoing surgical dislocation of the hip and noted that the most severe acetabular damage occurred anterosuperiorly but that some of these patients also demonstrated osseous metaplasia or ossification of the posteroinferior labrum. It should be noted that the most commonly observed sites of wear for all groups were at the acetabular rim between 11:00 and 3:00, and in fact this is probably why no differences between the Cam and Normal groups were identified in these locations in older individuals. These appear to be the areas that sustain the greatest attritional damage over time regardless of the shape of the proximal femur.
Overall, our observations suggest differences in acetabular wear patterns are discernible based on whether an individual has a cam deformity of the proximal femur and that these differences change based on age. The etiology of the cam deformity appears to only subtly influence the type of wear experienced by the acetabulum over time, because both an anterosuperior osseous bump on the femoral neck and a postslip deformity are associated with global wear, which is different from that found in osteological specimens without a cam deformity. The increased prevalence of posterior and central acetabular wear in specimens with a cam deformity is an important finding, because it may help to further explain why certain individuals develop global osteoarthritis of the hip. An understanding of the underlying biomechanics of this process may help guide treatment of young patients undergoing hip preservation surgery for FAI.
We thank Lyman Jellema and the Cleveland Museum of Natural History for their assistance in the completion of this study. We acknowledge the contributions of Sean Waldron MD, for making femoral measurements. We also recognize our artist, Matt Streit (www.mattstreit.com), for his contributions to the article.
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