A new Morphological Classification of the Anterior Inferior Iliac Spine: Relevance in Subspine Hip Impingement

Abstract

MORALES-AVALOSSUMMARY: Femoroacetabular impingement syndrome (FAI) is a clinical entity that has been recognized in recent years as a frequent cause of pain and the early development of hip arthrosis. Subspine hip impingement is characterized by the prominent or abnormal morphology of the anteroinferior iliac spine (AIIS), which contributes to the development of a clinical picture that is similar to FAI. The aims of this study were to propose a new morphological classification of the AIIS, to determine the prevalence of the different AIIS morphologies based on this classification and to correlate the presence of said morphologies with different gender and age groups. The sample consisted of 458 hemipelvises from individuals of known age and sex (264 men and 194 women). Each specimen was analyzed to determine the prevalence of each of the different morphologies of the AIIS based on the classification proposed as Type 1: the presence of a concave surface between the AIIS and the acetabular rim; Type 2A: the presence of a flat surface between the AIIS and the acetabular rim; Type 2B the presence of a convex surface between the AIIS and the acetabular rim; and Type 3: the AIIS protrudes inferiorly toward the anterior acetabulum. A prevalence of 69.87% was determined for Type 1 AIIS (320/458). In regard to abnormal morphology, prevalences of 17.90% (82/458), 3.71% (17/458) and 8.52% (39/458) were determined for type 2A, Type 2B and Type 3, respectively. The prevalence of abnormal AIIS morphology was 30.30% (80/264) in male specimens and 29.90% (58/194) in female specimens. This study demonstrates the prevalence of the different morphologies of the AIIS, providing information that will be useful in determining the role of the AIIS in the emergence of subspine hip impingement.

Femoroacetabular Impingement: What the Surgeon Wants to Know

“…140 Because most patients have articular pathology, it is difficult to recognize if the symptoms are related to EAI or purely to FAI. According to AIIS morphology, it can be divided into different types 141,142 : type I, usually of normal morphology, with a notch or contact surface between the AIIS and the acetabular rim; type II when the AIIS extends to the level of the rim, with a flat surface (type IIa) or convex surface (type IIb); and type III when the AIIS extends distally to the acetabular rim. Hetsroni and colleagues correlated these three types with different restrictions of ROM.…”

Anterior Inferior Iliac Spine (AIIS) and Subspine Hip Impingement

“…the concave nature of the ilium wall provides recessed space for soft tissue recoil when under compression during flexion, adduction and internal rotation of the hip 4,10,11 . Amar et al reviewed 50 CT scans of 50 patients (100 hips) without hip pathology to investigate the size, position and location of the normal AIIS.…”

“…Anatomical studies have demonstrated considerable variability in the morphology and location of the AIIS in the absence of previous trauma or avulsion 4,10 . Traction hypertrophy may result from the repeated strain placed across the AIIS from the attachment of the direct head of rectus femoris during running, cutting and kicking sports (especially during the adolescent development years); a bony response to the traction stimulus producing thickening and prominence of the AIIS which may lead to restriction of hip motion and femoro-acetabular impingement (Figure 2) 3,5 . Other congenital and developmental abnormalities of the AIIS have been described including the "Pelvic digit or pelvic rib"; this is considered to arise from a congenital abnormality during embryonic bone development and has been reported in a number of regions around the pelvis 33 including the sacrum 35 , the ilium (Iliac rib) 36 , coccyx 37 , and ischium.…”

A comparison between ultrasound-guided AIIS injection and radiography in the diagnosis of subspine impingement in patients with FAI

“…Among them, the latter two types are usually related to SSI symptoms. Type II was further divided into Type IIA and Type IIB according to whether the AIIS protruded forward [ 11 ]. However, an increasing number of studies have suggested that symptomatic subspine impingement does not completely depend on AIIS morphology.…”