Letter to the Editor: Bilateral Femoroacetabular Impingement: What is the Fate of the Asymptomatic Hip?

To the Editor, I read the study by Azboy and colleagues [1] with great interest. In their study, the authors addressed the prognosis of the asymptomatic hip in patients with bilateral radiographic signs of femoroacetabular impingement (FAI) and unilateral symptomatic FAI. I believe it is worth examining their study’s patient inclusion criteria. My concern is that patients with developmental dysplasia of the hip (DDH) were included in their study (25 of 74 patients). The definition of DDH was not shown in the paper, but a patient with lateral center-edge angle of 11° was included, and this made me wonder whether at least some of these patients were experiencing symptoms from DDH rather than FAI. They stated that of the 74 patients including DDH, 60 patients became symptomatic, and most of them underwent subsequent surgical intervention on the contralateral hip. If some of these patients may have had DDH in addition to radiographic FAI, it may be difficult or impossible to attribute the hip pain to FAI, rather than to instability from DDH. The pathomechanism that causes hip damage in DDH are completely different from the mechanism that causes hip injury in FAI [5]. In DDH, the unstable femoral head migrates and subluxates in regions where the femoral head is least covered, while in FAI, the femoral head remains wellcentered, but the free arc of hip motion is limited by either a prominent acetabular margin (pincer type), a misshapen proximal femur (cam type), or a combination of the two (mixed type) [5]. This suggests that as long as DDH is present, hip symptoms are likely to be caused by hip instability, even if concomitant cam deformity is present [7]. It is notable that cam deformity was occasionally found in patients with DDH who underwent periacetabular osteotomy (22%); however, none of them had symptoms of FAI [3]. A systematic review also demonstrated that patients with lateral center-edge angle of < 25° should not be considered to have FAI to differentiate other pathologies such as hip instability [8]. In my view, patients with DDH should be considered separately when examining FAI unless impingement is clearly demonstrated by some kind of objective methods like kinematic analysis. Azboy and colleagues [1] do not recommend any particular intervention for the patients who developed hip pain in the asymptomatic hip; however, evidence suggests that we should be skeptical of isolated arthroscopic FAI procedures that do not address the underlying DDH [2, 4, 6]. Since pelvic osteotomy is needed to definitively treat the pathoanatomy of DDH, we should question whether isolated arthroscopic treatments are effective over a long period for patients with DDH.