Prevalence, Risk Factors, and Clinical Impact of Bony Cysts in the Dysplastic Hip Undergoing Periacetabular Osteotomy: An Exploratory Study

Background: Formation of bone cysts in the acetabulum or femoral head is common in symptomatic acetabular dysplasia, but the prevalence and significance of cysts in hips undergoing PAO is poorly understood. The purposes of this study were (1) to determine the prevalence of cysts in hips undergoing PAO, (2) to identify predictors of the presence of cysts, (3) to determine if the presence or location of cysts was associated with intra-articular damage and (4) to determine if the presence or location of cysts impacted early clinical outcomes after PAO. Methods: We performed a retrospective review of 270 consecutive hips (249 patients) who were diagnosed with symptomatic acetabular dysplasia and underwent PAO without previous ipsilateral hip surgery. Preoperative low-dose CTs were utilized to identify the presence of cysts, and patient-reported outcome measures (PROs) were recorded at baseline and 1-year minimum follow-up (223 hips, 83%). The prevalence of cysts was reported, and associated factors were determined by univariate analysis (Student's T-test/Mann-Whitney U test for continuous measures, Chi-squared test/ Fischer's exact test for dichotomous measures). PROs and clinical "composite failure", defined as reoperation or failure to reach both the MCID and PASS, were similarly compared. Results: CT analysis of 270 dysplastic hips undergoing PAO demonstrated 17.0% (n=46) with acetabular (13.7%) or femoral cysts (4.4%). Hips with cysts were older (31.0±9.2 years) than those without cysts (24.1±7.9 years, p<0.001), reported higher Tonnis OA grade (p<0.001), showed higher rates of femoral chondromalacia (p=0.008), and had greater acetabular inclination (16.9° ±6.1) compared to hips without cysts (14.8° ±5.9, p=0.046). At 1-year minimum follow-up (223 hips, 83%), neither the presence nor location of cysts significantly impacted PROs. Cystic hips showed an increased but statistically insignificant difference in reoperations, conversion to THA, or composite failure outcomes. Conclusion: Seventeen percent of dysplastic hips undergoing PAO had acetabular (13.7%) or femoral (4.4%) cysts on CT. Cysts in dysplastic hips were associated with increased Tonnis grade and acetabular inclination but did not significantly impact early PROs. Greater composite failure rates failed to show statistical significance, suggesting that patients with cystic dysplasia should continue counseling for hip-preserving treatments. Level of Evidence: III.