Imaging of the acetabular labrum: a review

Introduction The acetabular labrum is a critical structure within the hip joint. Abnormality of the acetabular labrum is a significant cause of pain, and it plays an integral role in the development of osteoarthritis. This article reviews the normal anatomy and anatomical variants of the acetabular labrum and describes the technique for performing magnetic resonance arthrography. This article also reviews the diagnostic criteria and classification of tears of the acetabular labrum. Conclusion Magnetic resonance arthrography offers the best imaging of the acetabular labrum. The acetabular labrum has little anatomical variation. Irregularity of the labrum and fluid is associated with labral tear. Magnetic resonance arthrography is specific to the diagnosis of the acetabular labrum. Introduction Hip magentic resonance (MR) arthrography has a high accuracy in the detection of labral tears1–3 and is the imaging modality of choice in patients in whom there is a strong suspicion of labral lesions. Abnormalities of the acetabular labrum include partial tears, complete tears and labral detachment, with detachments being more common than tears4,5. There are reports of successful assessment of the labrum without joint distention6; however, in a study by Czerny et al. the sensitivity and accuracy of MR imaging (MRI) for non-distended joints was 30% and 36%, respectively when compared with surgical findings. Sensitivity and accuracy of MRI increased to 90% and 91%, respectively, after joint distention with gadolinium contrast7. The aim is to review the imaging of the acetabular labrum. Discussion MR arthrography technique MR arthrography of the hip is performed as a two-stage procedure. The first step is performed under fluoroscopic guidance. The preferred approach for hip arthrography is a direct anterior approach, targeting the lateral femoral neck or the lateral femoral head. The intra-articular needle position is confirmed by injection of iodinated contrast material. Joint distention is achieved by injection of a dilute solution of gadolinium8. The standard dilution of gadolinium for joint distention is 2.5 mM of gadopentetatedimeglumine9. For a single hip, this dilution may be achieved by mixing 0.1 mL of gadopentetate dimeglumine in 20 mL of normal saline; however, different gadolinium products may contain different gadolinium concentrations. The capacity of the hip joint is 8–20 mL. With the use of intra-articular gadolinium, axial, coronal and sagittal T1-weighted fat-suppressed images are acquired through the hip joint using a surface coil and field of view of 14–16 cm. It is important to use at least three imaging planes to ensure that all portions of the labrum are adequately assessed. An assessment of the entire pelvis is performed with T1 and short tau inversion recovery coronal images that include the symphysis pubis and the sacrum, increasing the field of view to 36–40 cm (Figure 1). Anatomy The acetabular labrum is a fibrocartilaginous structure, which rims the acetabulum anteriorly, superiorly and posteriorly, and which evolves into the fibrous transverse ligament inferiorly10. The labrum may be separated from the transverse ligament by a defect that is filled with articular cartilage. This defect may be confused with a tear on MR and MR arthrographic images11. The labrum serves to deepen the acetabulum; however, its role in maintaining stability is probably less critical than that of the glenoid labrum. On MR arthrography, the normal labrum is triangular in appearance with sharply defined margins. The labrum is thinnest anteroinferiorly and thickest posterosuperiorly and has typically low signal intensity on all imaging sequences. The capsule of the hip joint inserts directly at the base of the labrum anteriorly and posteriorly, superiorly the capsule inserts several millimetres above the labral attachment. Inferiorly, the capsule blends with the transverse ligament12 (Figure 2). * Corresponding author Email: [email protected] 1 Department of Radiology, Mater Misericordiae University Hospital, Eccles Street, Dublin 7, Ireland 2 Department of Orthopaedics, Mater Misericordiae University Hospital, Eccles Street, Dublin 7, Ireland Figure 1: Arthrogram of the right hip showing needle tip placement along the lateral aspect of the femoral neck (arrow). Tr au m a & Or th op ae di cs