Protrusio Acetabuli in Ankylosing Spondylitis Patients with End-Stage Hip Involvement
Protrusio acetabuli (PA) is a significant complication observed in various pathological conditions affecting the hip joint. This condition arises from the medial displacement of the acetabulum, leading to the femoral head protruding into the pelvic cavity. PA can result from a wide range of causes, including inflammatory, traumatic, genetic, metabolic, infectious, and idiopathic factors. Inflammatory arthritis (IA) is one of the most common causes of PA, with conditions such as rheumatoid arthritis (RA), juvenile idiopathic arthritis (JIA), ankylosing spondylitis (AS), and psoriatic arthritis (PsA) frequently associated with this complication. The inflammatory process in these conditions leads to the destruction and weakening of the bone surrounding the hip joint, causing migration along the joint-reaction vector and resulting in PA.
Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the axial skeleton, but it can also involve peripheral joints, including the hips. In AS patients, hip involvement can progress to end-stage disease, characterized by severe pain, limited range of motion, and significant functional impairment. Total hip arthroplasty (THA) is often required to alleviate symptoms and restore function in these patients. However, the presence of PA in AS patients with end-stage hip involvement presents unique challenges for surgeons, particularly in the placement of the acetabular component and reconstruction of the hip center of rotation (COR).
This study aimed to investigate the prevalence of PA in AS patients with end-stage hip involvement and to explore its relationship with clinical characteristics and potential predisposing factors. The study was conducted at Beijing Jishuitan Hospital, where a total of 670 consecutive hips with AS were reviewed from 2005 to 2020. After applying strict inclusion and exclusion criteria, 532 hips were enrolled in the study. The inclusion criteria required a diagnosis of AS based on the 1984 modified New York criteria and end-stage hip involvement necessitating THA. Patients with a history of congenital, developmental, metabolic, or endocrine hip disease, previous hip surgery, deep infection, trauma, tumor, or combined IA and connective tissue disease other than AS were excluded. Additionally, cases with poor-quality radiographic recordings were also excluded.
Patient demographics, clinical parameters, and laboratory data were collected retrospectively. Demographic information included gender, side of hip involvement (left or right), and body mass index (BMI). Clinical parameters encompassed age at THA, age at onset of AS, diagnosis delay, disease activity, functional status, extra-articular manifestations (EAMs) such as uveitis, psoriasis, and inflammatory bowel disease (IBD), and smoking habits (current or past). Disease activity was assessed using the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), while functional status was evaluated using the Bath Ankylosing Spondylitis Functional Index (BASFI). Patient-reported outcomes (PROs) were measured using the Short Form-12 (SF-12), and hip pain and function were assessed using the Harris Hip Score (HHS). Laboratory parameters included serum erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), hemoglobin (HGB), and albumin (ALB).
Pre-operative anteroposterior (AP) radiographs of the pelvis were evaluated for bony ankylosis and PA using the criteria proposed by Sotello-Garza and Charnley. PA was graded as follows: grade I (mild) with an acetabular-ilioischial distance (AID) of 1-5 mm, grade II (moderate) with an AID of 6-15 mm, and grade III (severe) with an AID greater than 15 mm. Additional radiographic parameters measured included the center-edge angle (CEA) of Wiberg, obturator foramen ratio (OFR), canal flare index (CFI), and neck-shaft angle (NSA). These measurements were performed using Mimics software (version 16.0) by a single reviewer blinded to the patients’ clinical information.
The statistical analysis was conducted using SPSS software (version 25.0). Descriptive analyses were performed for categorical variables (percentages and frequencies) and continuous variables (mean and standard deviation or median and quartile). Intergroup differences were compared using independent sample Student’s t-tests or Mann-Whitney U tests for continuous variables and chi-squared tests for dichotomous variables. A multivariate logistic regression model was used to identify significant factors associated with PA, and the odds ratio (OR) with 95% confidence intervals (CIs) and associated P values were determined. The area under the curve (AUC) value was calculated to assess the accuracy of the model.
The study found that 103 out of 532 hips (19.4%) met the criteria for PA, with 87 hips (16.4%) classified as grade I and 16 hips (3.0%) as grade II. Compared to the non-PA group, the PA group had a significantly higher percentage of IBD (9.7% vs. 4.2%, P = 0.024), a higher BASFI (62.0 ± 19.1 vs. 57.1 ± 19.4, P = 0.014), a lower SF-12 physical component summary (PCS) (30.5 ± 7.7 vs. 33.1 ± 8.5, P = 0.003), and a lower HHS (30.0 ± 14.3 vs. 33.9 ± 13.2, P = 0.020). Radiographic analysis revealed a significantly higher CEA in the PA group (41.0 ± 10.6 vs. 36.5 ± 10.3, P = 0.001). Multivariate logistic regression identified IBD (OR = 0.335, 95% CI: 0.147-0.764, P = 0.009), SF-12 PCS (OR = 0.966, 95% CI: 0.940-0.992, P = 0.012), and HHS (OR = 0.982, 95% CI: 0.966-0.998, P = 0.026) as significant parameters associated with PA. The AUC value for the multivariate regression model was 0.765.
The findings of this study highlight the prevalence of PA in AS patients with end-stage hip involvement and its association with disease-specific functional status and clinical severity of hip involvement. The presence of PA poses technical challenges for surgeons performing THA, necessitating special reconstruction techniques to address issues such as bone stock restoration, hip COR reconstruction, and implant survivorship. The study also identified IBD as a significant protective factor against PA, which may be related to active medication therapy, particularly the use of biological agents in this patient population.
Despite the valuable insights provided by this study, several limitations should be acknowledged. The study’s single-centered, retrospective, and non-longitudinal design may limit the generalizability of the findings. The sample consisted exclusively of AS patients with end-stage hip involvement, and the dynamic changes of PA and its impact on functional status could not be adequately evaluated. Additionally, potential selection bias may have influenced the results, and not all related factors contributing to PA development were assessed. Factors such as HLA-B27 status, history of pharmacological interventions, and rehabilitation were not included in the regression model analysis.
In conclusion, PA is a common radiographic finding in AS patients with end-stage hip involvement and is associated with general functional status and clinical severity of hip involvement. The presence of PA presents technical challenges for surgeons, and special reconstruction techniques should be considered to optimize outcomes in these patients. Further research is needed to explore the dynamic changes of PA and its impact on functional status, as well as to identify additional factors contributing to its development in AS patients.
doi.org/10.1097/CM9.0000000000001792
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