Coxitis in Axial Spondyloarthritis: The Unmeasured, Yet Functionally Most Important, Radiographic Progression
Axial spondyloarthritis (axSpA) is a chronic inflammatory condition primarily affecting the spine and sacroiliac joints. While spinal involvement often dominates clinical attention, extra-spinal joint involvement, particularly in the hips, can impose significant physical and functional burdens on patients. This article delves into the underappreciated yet critical role of hip involvement in axSpA, exploring its epidemiology, pathophysiology, clinical implications, and therapeutic considerations.
Hip involvement in axSpA, particularly in ankylosing spondylitis (AS), is a major concern due to its profound impact on mobility and quality of life. The hip joints, along with the shoulders, are considered “root joints” in AS, playing a pivotal role in daily activities. Hip involvement is the most frequent extra-spinal manifestation in AS and juvenile spondyloarthritis (SpA), severely affecting young patients and posing a high risk of major disability. Notably, hip damage can occur independently of spinal damage, and in older patients, hip involvement has been shown to contribute more significantly to functional decline than spinal involvement.
Epidemiological data reveal that hip involvement varies widely across the spectrum of SpA, with prevalence rates ranging from 27% in psoriatic arthritis to 53% in juvenile SpA. Asian patients exhibit a higher prevalence of hip involvement. Despite these findings, there is a lack of prospectively collected data on the severity of hip damage in AS and the broader SpA spectrum. Total hip replacement (THR) is a transformative procedure for young patients with severe hip involvement, significantly improving quality of life. However, THR carries risks such as joint infection and the potential need for surgical revision. The number of THR procedures in AS patients has increased from 1999 to 2013, particularly among older patients. This trend may reflect advancements in surgical techniques and reduced post-surgical complications rather than an increase in end-stage hip damage.
The pathophysiology of joint damage in AS involves inappropriate osteogenesis, leading to the formation of syndesmophytes and enthesophytes in the spine and extra-spinal joints, respectively. In contrast, hip inflammation in axSpA presents as a destructive erosive process, characterized by bone cysts, femoral head collapse, and irreversible femoro-acetabular joint damage. This process resembles the synovitis seen in other chronic inflammatory arthropathies, such as rheumatoid arthritis, with prominent cell influx, effusion, and joint erosion. Theoretically, these similarities suggest common pathophysiological mechanisms, potentially enabling different therapeutic approaches for spinal versus extra-spinal involvement in SpA.
The hip joint’s relatively low blood supply and limited anastomosis to the femoral head make it particularly vulnerable to rapid avascular necrosis following compromised blood supply. Groin pain is a critical clinical indicator for suspected avascular necrosis, necessitating immediate intervention. Early hip damage in SpA is a poor prognostic sign, independent of spinal radiographic progression. Relying on symptoms like groin pain may be too late to prevent further joint damage. THR offers painless walking and accelerates return to work, providing significant individual and societal benefits. However, avoiding or postponing THR remains a clinical objective.
In clinical practice, hip involvement in SpA is assessed through symptoms, physical examination, and radiography. The BASRI score for the hip, while sometimes used, lacks correlation with clinical findings and exhibits low sensitivity to change. Non-steroidal anti-inflammatory drugs (NSAIDs) provide symptomatic relief in early axSpA stages, but conventional synthetic disease-modifying antirheumatic drugs (csDMARDs) offer no benefit in halting disease progression. Power Doppler ultrasonography and magnetic resonance imaging (MRI) have shown reductions in inflammatory and structural hip changes following treatment with tumor necrosis factor inhibitors (TNFi). Retrospective observational studies suggest that TNFi may reduce radiographic progression in the spine, raising the possibility of similar effects on hip damage in SpA.
Early referral and diagnosis are critical in SpA to prevent hip joint destruction, particularly in younger patients. Despite the importance of early recognition, there are no published recommendations on when and which imaging procedures should be used to detect early hip damage in SpA. Prospective trials, even short-term, could provide valuable data on interventions to halt hip damage. Given the ethical concerns of long-term placebo arms, randomized protocols with early escape options to the interventional arm could yield relevant data, potentially aided by hip imaging.
Persistent groin pain in SpA patients warrants immediate rheumatologist evaluation and imaging studies to detect hip compromise. If imaging data align with clinical signs of hip damage, prompt initiation of biologic DMARDs (bDMARDs), particularly TNFi, should be considered. While there are no explicit guidelines recommending early bDMARD use in AS patients with groin pain, case series and literature reviews suggest that TNFi may prevent clinical and radiological progression of hip damage in AS.
In conclusion, hip involvement in axSpA represents a significant yet underappreciated aspect of the disease, with profound implications for patient mobility and quality of life. Early recognition and intervention are crucial to prevent irreversible joint damage and the need for THR. Prospective studies and updated treatment guidelines are needed to optimize the management of hip involvement in SpA, ensuring better outcomes for patients.
doi.org/10.1097/CM9.0000000000001743
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