CORR Insights®: Acetabular Retroversion and Decreased Posterior Coverage Are Associated With Sports-related Posterior Hip Dislocation in Adolescents

Where Are We Now?

In their analysis, Novais and colleagues [8] compared the postreduction CT scans of adolescents with sports-related posterior hip dislocation to CT scans of age- and gender-matched controls that had been obtained for suspected appendicitis. Only 27 patients (over the course of 16 years at a tertiary referral center) met the study criteria, demonstrating the infrequency of this injury. Despite its rarity, posterior hip dislocation can be a devastating injury for the athlete, particularly since a substantial minority of individuals will develop avascular necrosis of the femoral head.

Prior studies have shown an association between femoral acetabular impingement (FAI) morphology and traumatic dislocation [2, 5, 6, 9]. It is believed that cam-type femoral morphology combined with increased anterior acetabular coverage from a pincer lesion creates a fulcrum-like effect on the hip joint allowing the femoral head to dislocate posteriorly with less force [9]. Although researchers have previously used radiographs or CT scans to define morphology of the hip joint in patients with posterior dislocation, most of these have been in an adult rather than adolescent population [3].

Novais and colleagues found an association between acetabular retroversion and decreased posterior coverage with sports-related posterior hip dislocation in adolescents by measuring the acetabular anteversion both superiorly and centrally, as well as by evaluating posterior and anterior acetabular sector angles [8]. Using this comprehensive technique, they demonstrated that decreased posterior wall coverage was associated with dislocation. However, the authors did not find a difference between matched cohorts for anterior acetabular sector angle, which potentially argues against the previously described fulcrum theories and favors deficiency of coverage as the more-important acetabular factor for instability. If this is true, any surgical treatment to prevent further instability would favor reorientation osteotomy rather than pincer osteoplasty.

Where Do We Need to Go?

Despite the findings from the current study, gaps in our knowledge and several questions remain. Although Novais and colleagues evaluated morphology [8], the mechanics of the hip joint must be considered from both the femoral and acetabular perspective. A combined assessment of femoral head-neck concavity and femoral version plus the acetabular analysis of data from the current study is needed given the possibility of a fulcrum effect.

And, if hip morphology predisposes athletes to low-energy dislocation, what do we do with this knowledge? The mounting evidence suggests we should at least carefully evaluate hip morphology in those athletes who experience a dislocation [9]. But we have no guidance regarding the next step. Should surgical correction be considered? If so, is osteoplasty of cam and pincer morphology enough or would periacetabular osteotomy be warranted? Is there a role for counseling “at risk” athletes with morphology who have not suffered a dislocation?

How Do We Get There?

A biomechanical or computer-generated three-dimensional (3-D) motion analysis model could be used to fully understand the variables that contribute to osseous stability in the hip. The fulcrum and posterior undercoverage theories could be compared in these cases. Wassilew and colleagues [10] did create a 3-D motion analysis model to investigate contrecoup cartilage injury and subluxation in cases of cam and pincer impingement, but not specifically to compare decreased posterior coverage to anterior overcoverage in cases of dislocation. A specific model for posterior instability could be used to demonstrate whether PAO, rim osteoplasty, or cam resection have a role in reducing the risk of dislocation by correcting abnormal morphology. If correction is shown to be helpful in a model, then clinical application of this would be supported and further reported. This would likely be done on a small scale given the rarity of this injury, such as in existing case reports for repeat dislocators [1, 4]. Again, given the infrequency of this injury, it is unlikely that we will screen for this morphology in all contact athletes. It is also unlikely that we would suggest surgical measures to correct morphology in an asymptomatic individual. But if a contact athlete were to obtain radiographs for femoroacetabular impingement and retroversion was observed, the evidence may eventually push us to counsel these individuals about their risk of injury in contact sport (such as in the patient with congenital cervical spine stenosis who suffers an episode of transient quadriplegia).