CORR Insights®: Revisiting the Anteroinferior Iliac Spine: Is the Subspine Pathologic? A Clinical and Radiographic Evaluation

Where Are We Now?

Although much has been written about femoroacetabular impingement (FAI), we are still learning about other impingement syndromes that can occur around the hip joint between the femur and the acetabulum [15]. Subspine impingement is thought to lead to hip pain and restricted ROM in affected individuals [12]. This occurs when a prominent anteroinferior iliac spine (AIIS) or area around the AIIS impinges on the femoral neck or anterior facet of the greater trochanter [14]. Hetsroni and colleagues [5] described an anatomic classification of the AIIS based on three-dimensional CT scans. Using computer simulation and collision models, they concluded that prominent AIIS morphology leads to impingement along the anterior femoral neck and decreased ROM [5]. Recently, isolated AIIS decompression combined with other FAI treatments has shown improved patient reported outcomes in short-term followup [13, 14]. However, a recent study found no differences in AIIS morphology between symptomatic and a cohort of more than 500 asymptomatic hips [3].

The current study by Karns and colleagues [8] has furthered our understanding of the AIIS in hip impingement by extrapolating the prior classification system from CT to radiographs. The authors show that the false profile radiograph is best at characterizing the morphology of the AIIS when compared to CT. This further validates the false profile radiograph as an important radiographic view in evaluating the young adult hip. It evaluates coverage as measured by the anterior center edge angle and the anterior articular cartilage, which is the first area to show degenerative change in the dysplastic hip [6, 16]. Prior studies reporting on motion restriction by Type II and Type III AIIS morphology suggested clinically important motion restriction from this subspine impingement [5]. However, Karns and colleagues [8] found no meaningful correlation between AIIS morphology and hip ROM, and the alpha angle had a stronger association with hip flexion. This is similar to recent studies that showed that femoral morphology is a more-important predictor of limited range of motion than is AIIS morphology [10]. Specifically, cam deformity was associated with decreased hip flexion, and decreased femoral anteversion had the stronger association with decreased internal rotation in flexion than the presence of a cam lesion [10].

In the original description of motion loss in subspine impingement, bony collision models were created using patient CT data [5]. However, more-recent studies [7, 11] with in-vivo motion capture techniques including dual-plane fluoroscopy and ultrasound have suggest these collision models may be inaccurate. Kapron and colleagues [7] demonstrated that bony contact never actually occurs as is inferred by CT-based collision models. In addition, ultrasound has been used to measure hip motion to soft tissue and bony impingement [11]. The current study suggests that the original description of subspine impingement might have been flawed due to the use of these computer-based CT models, as their current clinical data did not show a correlation between ROM and AIIS subtype.

Where Do We Need To Go?

While we are starting to understand the pathophysiology and treatment of nonarthritic hip pain, we still have a long way to go. When evaluating a new diagnosis or anatomic morphology, the investigation should start by determining the incidence in an asymptomatic population and progresses to an investigation on the incidence in patients with hip pain. If the diagnosis is thought to cause arthritis, then natural history studies are needed to determine whether the diagnosis leads to hip joint degeneration in an untreated population. After this, clinical outcomes studies are needed to understand the effect of treating the morphology on patient pain and function as well as the progression of degeneration of the hip joint.

In the setting of subspine impingement, most studies (including the current study) have examined patients presenting with hip pain and those undergoing surgery for FAI [5]. Therefore, there is a selection bias of patients having surgery indicated and then secondarily investigating AIIS morphology. In a recent study, Balazs and colleagues [3] found no difference in AIIS morphology between symptomatic and asymptomatic hips. Given the improved motion-analysis techniques that have been developed over the last decade, more accurate in-vivo measures of hip motion related to AIIS morphology and restriction of ROM or impingement should be investigated.

The natural history data on the effect of hip dysplasia and cam-type FAI strongly suggests an increased risk of joint degeneration with these structural abnormalities [1, 2]. To my knowledge, there is no natural history data on any of the forms of extra-articular hip impingement, including subspine impingement. Further studies should investigate the natural history of a prominent AIIS to understand whether this contributes to hip osteoarthritis from impingement. We need well controlled studies on treatment outcomes to understand the treatment effect of AIIS/subspine decompression. Since hip anatomy is complex, and generally involves more than one anatomic variant that contributes to a patient’s hip pain, controlling for all of these pathologies will be needed to fully understand the effect of a prominent AIIS and its treatment on the disability before and the outcome after hip surgery.

How Do We Get There?

Filling in some of the gaps on our knowledge of subspine impingement and AIIS morphology could start with prior data sets. From motion and impingement perspective, newer and more-accurate imaging modalities compared to simulated CT motion including dual-plane fluoroscopy and ultrasound could be used to investigate the true motion until impingement and how AIIS morphology affects this [7, 11]. Larkin and colleagues [11] have demonstrated how to measure hip motion until bony or soft tissue impingement. This would be a more cost-effective method than dual-plane fluoroscopy and could be used to judge motion to bony and soft tissue impingement between different AIIS subtypes.

Given that subspine impingement is an extra-articular form of hip impingement, it may be a pain generator even if it has no effect on degenerative change in the hip. Prior datasets on the natural history of the hip could be used or emulated to investigate the effect of AIIS subtype on osteoarthritis progression. The best study on this to date that included false profile radiographs was the CHECK cohort [1, 2]. False profile radiographs could be used to categorize AIIS subtype and include this in an analysis looking at progression of osteoarthritis while controlling for cam-type FAI and dysplasia. Patient improvement in pain and function after subspine/AIIS osteoplasty would be best determined in a large cohort of patients undergoing FAI correction. Multicenter cohorts like the ANCHOR [4] and MASH [9] cohorts would likely provide the power needed to control for other acetabular and femoral morphologies to help us understand the effect of AIIS morphology on clinical outcomes while controlling for confounding factors.