Inclination and version were effectively the same in normal hips (51° and 24°), cam-type hips (53° and 23°), and pincer-type hips (51° and 25°) (Table 1). We detected no difference in acetabular inclination (p = 0.324) or version (p = 0.455) between cam and pincer hips There was a small gender difference, however, as female hips were more anteverted than male hips (26° versus 22°; p = 0.008).
FAI is a major predisposing cause of coxarthrosis, yet quantification of the acetabular component of this condition has remained elusive. We therefore set out to establish whether there were distinct and measurable features of the acetabulum in cam and pincer impingement that would aid the surgeon in the assessment of this condition and guide treatment strategy. We specifically set out to confirm whether the acetabulum in cam impingement could be distinguished reliably from the acetabulum in pincer impingement, and if so, whether it was the version or the depth that best characterized the difference between these conditions.
We acknowledge limitations of this study. First, it is a small study, involving only 60 hips, with 20 cam and 20 pincer cases retrieved sequentially from our database and 20 normal hips from a CT colonoscopy database. However, even with the small numbers, we found differences between the two groups. Second, as our study is based entirely on the 3-D anatomy from CT scans, it cannot contribute to any appreciation of the role of the articular cartilage or acetabular labrum. Although it is likely that both play a major role in joint health and disease, we are not yet able to model them reliably, therefore, we cannot comment on their contribution. Third, by excluding hips that already had developed coxarthrosis to ensure accurate modeling of the bone surfaces, severe examples of cam or pincer disease presenting late have been eliminated. After articular cartilage and bone are lost, the earlier shape of the joint also is lost. We therefore lose the ability to determine forensically exactly what joint shape existed before destruction of tissue occurred. By using joints that have not yet lost height, and shape, we are at least able to measure joint surfaces reliably, Longitudinal studies must be performed to validate this further. Fourth, our observations are based on a plane generated from 52 rim points taken all around an acetabulum already oriented with respect to the APP. The plane generated is artificial; the rim has a predictable waveform (Fig. 4), not a plane, but this ARP is highly reproducible if sufficient points are taken around the rim. Fifth, by focusing our study on the acetabulum, we run the risk of missing some mixed-type pictures. However, the dominant clinical problem we wished to address is that of cam impingement in which surgeons see an additional rim lesion causing a mixed picture. By selecting hips with cams based only on their alpha angle, we have ensured they were really cams, whereas none of the pincer group had abnormally high alpha angles. The acetabular characteristics in this second group therefore are those of pure pincer hips for comparison. Normal hips from a colonoscopy database provide an additional comparison group. Sixth, our data were acquired from subjects who were supine and imaged in extension. The structural relationship between the lumbosacral spine and the pelvis, and motion between these two body segments are thought to play a role in the genesis of hip disorders . We are unable to contribute to that area of investigation, as our study focuses solely on morphologic features of the acetabulum and the direct angular relationship between the acetabulum and pelvis.
Decreased acetabular version or retroversion is described as a factor leading to FAI [3, 14, 16, 21, 22, 32, 42-44]. However, we found no difference in the average or range of orientation among cam, pincer, and normal acetabula in our study population to support the concept of retroversion as a cause of FAI, whereas depth of socket did distinguish these two pathologic entities. The radiographic description of retroversion differs from our 3-D method, which may explain this apparently contradictory conclusion. Using plain AP radiographs, retroversion is determined by the prominent iliac eminence overlying a shallow ilioischial recess, effectively relying on two measurements, which can vary substantially with pelvic tilt and with the center of the xray beam. When version is described as the angle of the average greater than 52 points all around the rim and corrected for relation to the APP, we have not found what we consider real retroversion.
These findings in a small group of cam hips need to be confirmed in larger studies, but if substantiated, they have implications for bone-conserving surgical treatment of FAI. Acetabular rim resection, with or without labral repair and reattachment, may be indicated for pincer impingement. However, the acetabular rim profile of every cam hip we have studied is lower than a normal hip right across the impingement zone between 11 and 4 o'clock (Fig. 5) and markedly lower than a pincer hip. Resection of any acetabular surface in this weightbearing zone in such a cam hip therefore runs the risk of rendering the acetabulum insufficient and increasing the load on the already reduced surface area, increasing the risk of osteoarthritis . We therefore cannot support any acetabular rim resection in a patient whose acetabulum matches the characteristics we have described. Rim resection is clearly indicated in pincer FAI, but we have not yet seen such overcoverage in a cam hip. We deduce from this that cam hips may only develop in shallow sockets. This is in contrast to the most cited references regarding the incidence of mixed-type impingement [1, 3].
This method has potential for use in planning rim resection in pincer impingement and for use with surgical navigation or robotic assistance. It also could be used to quantify the extent to which a surgical plan was completed. The detailed description of morphologic features of the rim also naturally lends itself to improvements in device design . Furthermore, the method may help to quantify the 3-D morphologic features of acetabular dysplasia.
The distinction in acetabular geometry in these two types of impingement adds to the growing body of work examining the structural causes of coxarthrosis. We provide a novel, robust, and reliable method of assessing and quantifying the abnormal acetabulum. Based on these findings, we are apprehensive that acetabular rim resections in patients with cam hips may lead to progressive edge loading of an already shallow socket, and we await evidence of the clinical results of this practice.
We thank Robin Richards for developing the software and Louise Brown for statistical advice.
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