What Proportion of Patients Undergoing Bernese Periacetabular Osteotomy Experience Nonunion, and What Factors are Associated with Nonunion?
Authors: Selberg Courtney M. , Davila-Parrilla Ariel D. , Williams Kathryn A. , Kim Young-Jo , Millis Michael B. , Novais Eduardo N. 
Published: May 2020

Abstract

Background
The Bernese periacetabular osteotomy (PAO) is one of the most-used surgical techniques to treat symptomatic acetabular dysplasia. Although good functional and radiographic short-term and long-term outcomes have been reported, several complications after PAO have been described. One complication that may compromise clinical results is nonunion of an osteotomy. However, the exact prevalence and risk factors associated with nonunion are poorly elucidated.

Questions/purposes
(1) What proportion of patients have complete bony healing versus nonunion during the first year after PAO? (2) What is the clinical and functional impact of nonunion at a minimum of 1 year after PAO, as assessed by the modified Harris hip score (mHHS) and the Hip Disability and Osteoarthritis Outcome Score (HOOS)? (3) What patient-specific or surgery-specific factors are associated with nonunion at 6 months and at a minimum of 1 year postoperatively?

Methods
Between January 2012 and December 2015, we retrospectively identified 314 patients who underwent PAO at our institution. During this period, 28 patients with a diagnosis different from symptomatic acetabular dysplasia (reverse PAO for acetabular over-coverage: n = 25; PAO for skeletal chondrodysplasia: n = 3) underwent PAO but were ineligible to participate. Hence, 286 patients underwent PAO to treat symptomatic acetabular dysplasia during the study period and were considered eligible. Inclusion criteria were patients with a complete set of postoperative radiographs (AP, Dunn lateral, and false-profile) at 12 months or more postoperatively. Eighteen percent (51 of 286) of the patients underwent staged, bilateral PAOs, but we only included the first PAO. Finally, 14% (41 of 286) of the patients were excluded because they had an incomplete set of postoperative radiographs at 12 months or more. The study comprised 245 patients. Eighty-five percent (209 of 245) of the patients were female and the mean age at surgery was 24 years ± 9 years. The healing status (complete healing vs. nonunion) was recorded for ischial, superior pubic, supraacetabular, and posterior column osteotomies at each subsequent visit. Nonunion was defined as noncontiguous osseous union with a persistent radiolucent line across any osteotomy site and was recorded at 3 months, approximately 6 months, and approximately 12 months postoperatively. Calculation of Cohen’s kappa statistic coefficients showed the classification had perfect interobserver agreement (0.53; 95% confidence interval, 0.12-0.93), but there was moderate intraobserver agreement between those who healed and those with nonunion. The HOOS and mHHS were collected preoperatively and at a minimum of 1 year after PAO. The HOOS contains five separate subscales for pain, symptoms, activity of daily living, sport and recreational function, and hip-related quality of life. The HOOS responses are normalized on a scale of 0 (worst) to 100 (best). The mHHS includes pain and function scales and is overall interpreted on a scale from 0 (worst) to 100 (best). Eighty-six percent (211 of 245) of the patients with a complete set of images at their 12-month visit completed the mHHS and 89% (217 of 245) completed the HOOS. We collected information from the patients’ medical records about their symptomatic status and additional treatment for nonunion. A logistic regression analysis was used to investigate factors associated with nonunion at 6 and 12 months postoperatively.

Results
Only 45% (96 of 215) of the patients had complete radiographic healing of all osteotomy sites at the 6-month visit and 55% (119 of 215) had not healed completely. However, 92% (225 of 245) demonstrated complete radiographic healing of all osteotomy sites at approximately 1 year postoperatively. The proportion of nonunion at a minimum of 12 months after PAO was 8% (20 of 245 patients). There was no difference in the mHHS after 1 year or more of follow-up between patients with nonunion and patients with complete healing after PAO (nonunion mean mHHS: 73; 95% CI, 62-85 versus healed: 82; 95% CI, 80-85; p = 0.13) and HOOS pain (nonunion mean HOOS pain: 80; 95% CI, 71-90 versus healed: 86; 95% CI, 83-88; p = 0.16). Similarly, no difference was identified for HOOS symptoms (nonunion mean: 72; 95% CI, 63-80 versus healed: 78; 95% CI, 75-81; p = 0.11), HOOS activities of daily living (nonunion mean: 86; 95% CI, 78-94 versus healed: 91; 95% CI, 89-93; p = 0.09), HOOS sports and recreation (nonunion mean: 70; 95% CI, 57-83 versus healed: 78; 95% CI, 75-82; p = 0.18); and HOOS quality of life (nonunion mean: 60; 95% CI, 46-75 versus healed: 69; 95% CI, 65-72; p = 0.28). After controlling for potentially confounding variables such as gender, age, chisel type, and preoperative anterior center-edge angle, we found that higher BMI (per 1 k/m2; odds ratio 1.14; 95% CI, 1.06-1.22; p < 0.01), older age (per 1 year; OR 1.05; 95% CI, 1.01-1.08; p < 0.01) and more-severe acetabular dysplasia as assessed by a decreased preoperative lateral center-edge angle (per 1°; OR 1.06; 95% CI, 1.02-1.11; p < 0.01) were independently associated with nonunion of one or more osteotomy sites at 6 months postoperatively. Only age was an independent predictor of nonunion at 12 months postoperatively (per 1 year; OR 1.06; 95% CI, 1.01-1.11; p = 0.02).

Conclusions
Our study helps us to understand radiographic healing during the first year after PAO to treat symptomatic acetabular dysplasia. Fewer than half of the patients had complete healing of their osteotomies at 6 months postoperatively. More than 90% of patients can expect to have completely healed osteotomy sites at 12 months postoperatively. Surgeons should avoid unnecessary interventions if nonunion is observed radiographically at 6 months postoperatively. Although there was no difference in the HOOS and mHHS between patients with nonunion and those with complete healing, further research with a larger cohort is needed to clarify the impact of nonunion on clinical and functional outcomes after PAO. Surgeons should consider using strategies to enhance osteotomy healing in those who undergo PAO, such as optimizing vitamin D levels and using local bone grafts in older patients, those with a high BMI, and patients with severe acetabular dysplasia.

Level of Evidence
Level III, therapeutic study.