CORR Insights®: What Is the Prevalence of Cam Deformity After Prophylactic Pinning of the Contralateral Asymptomatic Hip in Unilateral Slipped Capital Femoral Epiphysis? A 10-year Minimum Followup Study

Where Are We Now? Bilateral slipped capital femoral epiphysis (SCFE) can occur either simultaneously or sequentially. The frequency of subsequent contralateral slip in patients with primary unilateral slip varies from 8% to 36% from the first admission to skeletal maturity [7, 13, 14]. There is no consensus among orthopaedic surgeons regarding how to manage the contralateral hip; should it be prophylactically pinned or should it be observed with close followup? In the current study, Lerch and colleagues [9] examined the prevalence of cam deformity and femoroacetabular impingement (FAI) after prophylactic pinning of the contralateral hip in patients with unilateral SCFE at a mean followup of 12 years. The authors found that 10 of 33 patients (30%) had a cam deformity, defined by an AP alpha angle > 60° and/or lateral alpha angle > 55°, and four patients with symptomatic FAI (11%) underwent surgical hip dislocation with femoral head-neck offset restoration. The authors concluded that prophylactic pinning prevents contralateral slip, but does not guarantee a lifetime free of hip pain and impingement in all patients. The authors’ conclusions support the findings of Fraitzl and colleagues [2], who described increased alpha angles (mean AP alpha angle = 62°) in contralateral hips with prophylactic pinning and a positive impingement test in 31% of hips at a mean followup of 14 years. In contralateral hips without prophylactic fixation, the frequency of cam morphology was 40% at a mean followup of 36 years [12]. Thus, regardless of whether we perform prophylactic surgery, it seems that approximately one-third of contralateral hips develop cam-type deformities. One problem when comparing the frequency of cam deformities in different studies is the variance in the upper normal limit of alpha angles. For example, the alpha angle is higher in men than in women [4, 8]. In the general population, the upper threshold values of the alpha angle on AP radiographs varied from 57° to 94° [3, 4, 8], whereas the variation was less on lateral radiographs (56° to 70°) [3, 8]. Since there is no consensus about the upper normal limit of the alpha angle, a reasonable compromise would be to use two threshold values [1], an AP alpha angle of 60° as upper limit of normal range and 83° as the limit for severe cam deformity. The long-term natural history of contralateral hips (without prophylactic pinning) in SCFE has been analyzed in three studies with quite similar results. Wensaas and colleagues [12] found that the risk of osteoarthritis (OA) was 9% after 36 years and Jerre and colleagues [7] reported that the OA risk at 32 years was 19%, using a more-permissive definition of OA (3 mm vs. 2 mm joint-space width). In 205 contralateral hips with no slip during adolescence, the OA risk at 33 years followup was 13% [5]. The longest followup after prophylactic fixation is 14 years [2], which is not long enough for proper evaluation of OA. Where Do We Need To Go? The main argument in favor of prophylactic fixation is that it prevents a later slip. The argument against routine prophylactic fixation is that it is unnecessary in most patients—and thus needlessly exposes them to the potential for surgical complications—since the contralateral hip in most patients with SCFE does not slip even without pinning. Ideally, contralateral pinning should be performed in those patients with risk factors for subsequent slip, but not in those patients who are not at increased risk. Although several risk factors have been studied, including young age, obesity, endocrine disorders, and increased posterior sloping angle [10], there is currently no exact and universally accepted method of predicting contralateral slip; this important topic calls for further research. Cam-type deformities after SCFE can cause chondral injuries and labral detachment [11], and a study with longer followup showed that cam deformities after SCFE are associated with OA in hips with previous slip and in contralateral normal hips [12]. Zilkens and colleagues [15] found that 35 of 38 patients (92%), at a mean of 22 years after they presented with SCFE, had developed mild OA (Tönnis Grade 1), but there was no linear correlation between grade of cam deformity and OA. Further research is needed to clarify which hips with cam deformity will develop OA. The indications and methods for surgical correction of cam deformities and FAI also need further research. Is it possible that the symptoms of FAI improve in some patients, such that there should be stricter indications for surgical correction? If so, perhaps observation is preferable to surgery in those patients. Since patients with SCFE are children, even 10 years of followup is not sufficient; followup at 25 years or more is necessary really to know the effects of our treatments. If the long-term risk of OA in patients treated with prophylactic pinning appears to be lower than that of the natural history (which is in the order of 10% to 15%), this would be a strong argument for prophylactic contralateral pinning. How Do We Get There? We can analyze the risk factors for subsequent contralateral slip using national quality registers like the ones recently established in Norway and Sweden [6], potentially making it possible for even small countries to participate in this research. Regarding the risk of contralateral slip, we do not need long-term followup since slips do not progress after skeletal maturity. By contrast, questions about OA—and the longer-term benefits and harms associated with prophylactic surgery—will be harder to address. To determine how surgeons should manage the normal hip contralateral to a hip with SCFE, we need large, randomized studies who present initially with unilateral SCFE. Patients with established risk factors for subsequent slip should be excluded because they will undergo bilateral surgery at their first admission. The rest should be randomized to either prophylactic contralateral fixation or observation (natural history). The two groups should be compared in short-term, mid-term, and long-term studies. In short-term followup, researchers should examine postoperative complications, frequency of subsequent slip and degree of slipping, further operations after the index surgery, hip pain, and function. Slip angles and alpha angles should be measured yearly after the primary admission to skeletal maturity. This would clarify the remodeling process and the development of cam-type morphology and clinical FAI that takes place in the two groups. The mid-term studies should be a continuation of the same randomized studies and have a followup of 10 years to 15 years. Alpha angles and cam deformities and their clinical consequences should be included in addition to indications for and type of additional surgery that have been used in the two groups. Hip pain and function need to be analyzed by proper scoring systems like the Hip disability and Osteoarthritis Outcome Score (HOOS). Without much additional effort, the same radiographic and clinical parameters should be used in an analysis of the hips that were initially treated for SCFE. This would provide valuable information about the outcome of surgical treatment of SCFE. These studies should preferably be extended to long-term studies, but this will naturally take a long time since > 25 years followup is desirable. In the meantime, studies like the current paper and Fraitzl and colleagues [2] should be extended to 25 to 30 years followup to clarify the long-term consequences and management of cam-like deformities and clinical FAI. The combined long-term clinical and radiographic outcomes may help us determine whether routine prophylactic contralateral fixation should be recommended.