Rapidly Progressive Arthritis in Femoroacetabular Impingement: Patient Characteristics and Risk Factors for Total Hip Arthroplasty by the Age of Forty.

Background: Femoroacetabular impingement (FAI), particularly cam-type, is now well accepted as a risk factor for the development of hip osteoarthritis (OA). However, many hips with FAI morphology will never develop hip pain or OA, identifying that our current understanding of FAI disease progression remains limited. The purposes of this retrospective case-control study were to (1) report the patient and disease characteristics of patients with rapidly progressive FAI requiring hip arthroplasty by the age of 40 and (2) to identify patient and imaging factors associated with rapidly progressive FAI. Methods: Cases were retrospectively identified from an arthroplasty registry as patients 40 years old or younger with underlying FAI deformity and end stage OA requiring primary total hip arthroplasty. Patients were excluded for known DDH, AVN, SCFE, inflammatory arthritis, and previous ipsilateral surgery. Controls were identified from a hip preservation database as patients with symptomatic FAI undergoing surgical intervention over the same time period, and were matched 2:1 by gender and age. Alpha angles were calculated on frog-leg lateral and anteroposterior (AP) radiographs with both inclusion and exclusion of any osteophytic prominences (representing minimum and maximal possible underlying FAI morphology). Patient characteristics, radiographic parameters, and baseline patient reported outcomes were compared between the two groups using student's t-tests. Results: The rapidly progressive FAI cohort of 31 patients had a mean age of 35.8 years at surgery and was 39% female and 61% male. Alpha angles were significantly larger compared to controls when osteophytes were included (Frog: 74.7±10.8 vs. 57.2±12.7°, p<0.001; AP: 91.7±10.7 vs. 61.2±19.4°, p<0.001), but not when osteophytes were excluded (Frog: 61.2±11.1 vs. 57.2±12.7°, p=0.15; AP: 64.9±17.1 vs. 61.3±19.4°, p=0.38). Except for UCLA activity score, all baseline outcome measures were significantly lower for rapidly progressive FAI cases (p<0.001 for all). Conclusions: When compared to controls with symptomatic FAI, rapidly progressive cases did not demonstrate major differences in cam deformity magnitude. Thus severity of bony deformity may only be one aspect of a multifactorial etiology of hip OA progression in FAI.