MR Imaging of the hip: Avoiding pitfalls, identifying normal variants. (2024)

The hip is a complex ball-andsocket joint comprising theacetabulum, proximal femur, and articular cartilage. In addition, thecapsulolabral tissues and surrounding muscles and tendons stabilize thehip, dictate its range of motion, and enhance its function. Familiaritywith the spectrum of magnetic resonance imaging (MRI) appearances of thehip is necessary to recognize normal variants and distinguish them fromtrue pathologic conditions.

This article provides a brief overview of considerations intailoring protocols of the bony pelvis and hip to optimize detection ofboth intra- and extra-articular hip pathology, followed by a review ofcommon labral, osteocartilaginous, and soft tissue variants that can bemistaken for true pathology.

Imaging protocol

Dedicated MRI protocols for imaging the pelvis and hip vary amonginstitutions, but there are key elements common to most. In patientswith non-specific hip pain, even unilateral symptoms, largefield-of-view (FOV) images of the pelvis (30 cm or more) are commonlyperformed, spanning the region from the iliac crests to the level of thelesser trochanters. This prevents other pelvis pathology that can mimichip pain from being overlooked. Coronal T1-weighted images without fatsuppression are best for revealing detailed anatomy and evaluating bonemarrow and musculature. Large FOV fluid-sensitive sequences, eitherT2-weighted with fat suppression or short tau inversion recovery (STIR),are obtained in the coronal and axial planes and are used to evaluatefor fractures, fluid collections, and injuries involving tendon ormuscle. Larger slice thicknesses (5-7 mm) with small interslice gaps canbe permitted for these large FOV images. For pelvic imaging, dependingon patient body size, either a torso phased array coil or a larger bodycoil can be used. Depending on the referring clinician, dedicatedimaging of the symptomatic hip may be performed only based on theclinical examination and clinical question.

Dedicated imaging of the symptomatic hip should be performed usinga surface coil, such as a cardiac phased array coil, with the smallestpossible FOV (15-20 cm) extending from the top of the anterior inferioriliac spine to the bottom of the lesser trochanter. Small-FOV imageswith fluid-sensitive sequences (proton density [PD] or T2-weighted withfat suppression) provide the best evaluation of the acetabular labrumand articular cartilage on non-arthrographic examinations. Small slicethickness (2-3.5 mm) with minimal or no interslice gap is important forthese acquisitions. Plane selection in hip imaging is challenging, asthe hip joint is a highly curved structure with closely opposed corticalsurfaces and relatively thin articular cartilage, making it particularlysusceptible to partial volume averaging. (1) Traditional (axial,sagittal, and coronal) planes are used, as are offaxis planes (obliqueaxial and oblique sagittal). Routine radial imaging to assess labral andfemoral head morphology can be performed to visualize multiple segmentsof the labrum and articular cartilage in cross section. (2) For MRarthrography, multiplanar small-FOV images with T1- and T2-weighting,both with fat suppression, help to delineate fluid- or contrast-filledclefts in the labrum or cartilage and paralabral cysts. Indirect MRarthrography, which entails imaging after administering intravenousgadolinium followed by exercising the extremity of interest, has beenreported accurate in labral tear detection and is utilized in somecenters. (3,4) MR arthrography can be helpful to the surgeon, asanesthetic is administered as part of the injectate; this can provideadditional information on pain relief and has been found to be 90%accurate in those with intra-articular pathology in at least one study.(5)

Conventional MRI and MR arthrography are both excellent detectorsof extra-articular pathology of the hip, whereas the latter has beenshown superior in detecting intra-articular pathology, particularlylabral tears. Meta-analysis data suggest pooled sensitivity andspecificity of 66% and 79%, respectively, for conventional MRI indetecting labral tears, compared to 87%-97% and 64% for MR arthrography.(6,7) MR arthrography is also superior for evaluating the articularcartilage, with sensitivities ranges of 71-92% compared to 58-83% forconventional MRI. (8-10) Examinations performed at 3.0T provide higherresolution and greater signal-to-noise compared to those performed at1.5T, with some data suggesting conventional MRI at 3.0T has similar toslightly higher detection for labral tears than MR arthrographyperformed at 1.5T. (11) Protocols using 3D acquisitions may potentiallyreduce scan time without sacrificing diagnostic accuracy. One study ofMR arthrography of the hip at 3.0T demonstrated that sensitivity andspecificity for labral pathology did not differ significantly betweenconventional multiplanar 2D acquisitions and a single isotropic 3Dsequence reconstructed into multiple planes. (12)

Labral pitfalls

Normal labrum

The labrum is a fibrocartilaginous structure attached to theperipheral acetabulum. It facilitates appropriate acetabular formationduring skeletal maturation, stabilizes the femoral head by deepening theacetabulum, and seals synovial fluid within the hip joint. The normallabrum is most frequently triangular in morphology but demonstrates anincreasingly rounded and irregular shape in asymptomatic hips withincreasing age. (13) The labrum reportedly is absent in 3% of patients.(14) There is also a wide range of normal labral signal characteristics,including intermediate signal intensity on T1-, proton density-, andT2-weighted images and, less commonly, high signal intensity onT2-weighted images.

Diagnostic criteria for labral tears includes the presence oflabral distortion, high signal intensity on T2-weighted imaging, orgadolinium contrast extending into the labral substance oracetabular-labral junction. An adjacent paralabral cyst is a usefulsecondary sign of a labral tear and should increase diagnosticconfidence or raise suspicion for a labral tear. (15) The most commonlocation of labral tears is anterosuperior. (16-18)

Sublabral sulcus

Normal labral variants that manifest as high T2-weighted signal inor adjacent to the labrum can be mistaken for a labral tear. One commonvariant, the sublabral sulcus, is seen in up to 25% of patients. (19) MRimaging characteristics include a linear shape with smooth edges,location at the chondrolabral junction (undermining the labrum ratherthan extending into it), not full-thickness, and absence of signalchanges elsewhere in the labrum (Figure 1). The sublabral sulcus canoccur anywhere in the labrum, but it most commonly appearsposteroinferiorly (48%) or anterosuperiorly (44%), followed byposterosuperiorly (4%) and anteroinferiorly (4%). (19) Several studieshave questioned whether sublabral sulci actually represent partial orpartially healed labral tears, including one published by Magerkurth etal, who found no sublabral sulci with either MR arthrography orarthroscopy in a small series of patients younger than 17 years old.(20) In distinguishing between a labral tear and a sulcus, a labral tearshould be suspected if the abnormality is located within theanterosuperior aspect of the labrum, accompanied by appropriate clinicalhistory, and intra-articular anesthetic brings pain relief.

Perilabral sulcus

A perilabral sulcus or recess is a normal potential spaceinterposed between the labrum and overlying joint capsule. These aremost conspicuous in the superior joint space, where the joint capsuleinserts several millimeters superior to the labrum (Figure 2), but theycan also be seen along the anterior and posterior joint capsule.Perilabral sulci are easy to detect if the joint is distended with ajoint effusion or contrast, as is the case in MR arthrography. However,if the joint is not distended, a small volume of trapped fluid in theperilabral recess may mimic a paralabral cyst. (21) If the perilabralsulcus is large, it may mimic capsular stripping.

Osteocartilaginous pitfalls

Os acetabuli

An os acetabuli is a small accessory ossification center locatedadjacent to the acetabular rim present in 2-3% of the population. (22)On radiography, they appear as a well-corticated ossicle adjacent to theacetabulum, most common anterosuperiorly. On MRI, these ossicles followmarrow signal intensity and are separated from the acetabulum by a thincartilage rim (Figure 3). At least one sequence without fat suppressionis necessary to discern the typical appearance of marrow within theossicle and to ensure the ossicle is not confused with a labral tear.When located in the acetabular fossa (os acetabuli centrale), they maymimic an intra-articular body. (23) Os acetabuli can also be acquired assequelae of incompletely healed fractures or ossification of theacetabular labrum. These structures are typically asymptomatic; if largeenough, however, they may contribute to symptoms of femoroacetabularimpingement.

Supraacetabular fossa

The supraacetabular fossa (SAF) is a defect in the subchondral boneat the 12 o'clock position of the acetabular roof, with anincidence in the population of approximately 10%. On average, the SAF islocated 8 mm lateral to the acetabular notch in the coronal plane.Though the overlying articular cartilage is intact, its location at theweightbearing portion of the acetabulum poses a risk for this variant tobe incorrectly described as an osteochondral or chondral defect of theacetabular cartilage. It can be differentiated from a pathologic lesionof the acetabulum by its smooth margins, absence of subjacent bonemarrow edema, and absence of adjacent cartilage irregularity or signalheterogeneity (Figure 4). The average size of a SAF is 5.2 x 4.5 mm inwidth and 3 mm in depth. (24) SAF can be either partially or completelyfilled with cartilage. A SAF filled with cartilage may bearthroscopically occult but may fill with synovial fluid or contrast onMR arthrography. (24,25)

Stellate crease

Another variant of the acetabular roof is the stellate crease,which is typically an arthroscopic finding only. While the SAF is afocal defect of bone, the stellate crease is a focal defect of articularcartilage. It is located more medially within the acetabular roof thanthe SAF, immediately adjacent to the acetabular notch (Figure 5). It isa thin, star-shaped focal bare area devoid of articular cartilage andcan be mistaken for a pathologic defect at MRI and arthroscopy. Itscharacteristic shape, location, clean margins, and lack of associatedbone marrow abnormality help distinguish the stellate crease from a truecartilage abnormality. (21,24,26,27) In addition, acquired cartilagedefects should not be limited to just this location.

Synovial herniation pit

A synovial herniation pit, also known as "Pitt'spit," is commonly encountered on radiography as awell-circ*mscribed round or oval lucency with a surrounding narrow zoneof sclerosis located in the subchondral or subcortical anterosuperiorfemoral neck. On MRI, it demonstrates central T2-weighted hyperintensitywith a corresponding low signal intensity rim (Figure 6). These lesionsare occasionally associated with marrow edema. Synovial herniation pitsare typically less than 1 cm but have been reported to enlarge overtime. (28) Pathologically, these lesions are composed offibrocartilaginous tissue and were originally hypothesized to representsequelae of extra-articular structures such as the iliopsoas tendon oriliofemoral ligament exerting pressure on the underlying synovium,resulting in herniation of synovium into the underlying bone through acortical defect. (29) More recently, synovial herniation pits arepostulated to represent sequelae of repeated contact between the femoralhead-neck junction and acetabulum, supported by at least one studyshowing a higher incidence in patients with femoroacetabular impingementcompared to the normal population (33% versus 5%, respectively). (30)Given the risk of progression to primary osteoarthritis in patients withfemoroacetabular impingement, identification of a synovial herniationpit warrants close attention to femoroacetabular morphology. (31)

Soft tissue pitfalls

Iliopsoas bursa

The iliopsoas bursa is located lateral to the femoral artery andvein and deep to the myotendinous portion of the iliopsoas muscle. It isthe largest bursa in the body, measuring up to 3 x 6 cm. (32) It ispresent in most individuals and directly communicates with the hip jointin approximately 15% of cases. (32) When distended with fluid or debris,it assumes a characteristic appearance of one or two ovoid or teardropshaped collections on either side of the iliopsoas tendon (Figure 7).The bursa travels along with the iliopsoas tendon toward the tendonattachment on the lesser trochanter. The bursa can also extend cephaladinto the iliac fossa, where it can become quite large. Though iliopsoasbursitis can be a cause of hip pain, it is important to considercommunication between the hip joint and iliopsoas bursa, as a distendedor debris-filled bursa may reflect intraarticular pathology rather thanbursitis. (33,34) It is also important not to mistake the bursa for aparalabral cyst, cystic mass, or pelvic lymphadenopathy. It is theauthors' experience that paralabral cysts invariably lie lateral tothe iliopsoas tendon, whereas bursitis usually starts medial to thetendon or envelops it from both sides.

Obturator externus bursa

The obturator externus bursa is a potential space located betweenthe obturator externus tendon and the ischiofemoral capsular ligamentsat the posterior aspect of the hip joint capsule. It communicates withthe hip joint in all cases, and is considered by some to represent anarticular recess rather than a true bursa. (35,36) On MRI, the obturatorexternus bursa can be a site where intra-articular bodies and jointfluid collect. MR arthrography can distend the capsule and fill thebursa with contrast (Figure 8); it should not be mistaken for a rent orpartial tear of the joint capsule.

Plicae and pectinofoveal fold

Several normal synovial reflections in the hip can be mistaken forintra-articular pathology. The ligamentous plica is located within theacetabular fossa at the acetabular attachment of the ligamentum teres.The neck plica parallels the femoral neck, most commonly along theanterior joint capsule. The labral plica is found at the inferomedialmargin of the acetabular labrum. The ligamentous and labral plicae areparticularly problematic, since they can mimic tears of the ligamentumteres and acetabular labrum, respectively. Plicae are thin, linear, andsmooth, in contrast to the irregular and undulating appearance of a tornligamentum teres or the triangular or trapezoidal shape of a tornsection of labrum. Plicae are considered to be embryonic remnants andare seldom symptomatic unless they become entrapped by or betweenadjacent structures. The labral plica, due to its proximity to thelabrum and transverse ligament, is considered to have the highestpotential to produce symptoms. (21,27) In contrast, the pectinofovealfold has no symptomatic potential. The pectinofoveal fold is a linearthickening of the medial hip joint capsule which contains branches ofthe retinacular arteries and the medial circumflex femoral artery. Itextends inferiorly from the more superomedial joint capsule to itsinferior attachment, either on the inferomedial femoral neck or on thejoint capsule itself (Figure 9). It can be smooth or irregular, and ispresent in more than 95% of patients at MR arthrography. (37)

Peritrochanteric edema

Lateral hip pain is a common indication for hip imaging. Along withclinical history and physical examination, MRI can contribute to thediagnosis of "greater trochanteric pain syndrome," which is acause of lateral hip pain posited to be due to a variety of causes,including peritrochanteric bursitis, external snapping hip, and tears ortendinopathy of the gluteal tendons. Though frank tendon tears andimaging findings of bursitis have been shown to correlate with symptomsof lateral hip pain, the finding of peritrochanteric edema alone,identified as increased T2-weighted signal but not a true fluidcollection paralleling the greater trochanter on axial or coronal MRimages (Figure 10), does not correlate with hip pain. (38,39) Itsfrequency increases with patient age and is frequently bilateral.Although gluteal tendinopathy occurs more commonly in women, there is nogender predilection associated with peritrochanteric edema. (39)Peritrochanteric edema is extremely common in patients undergoing hipimaging, and may represent the mildest end of a spectrum of lateral hippathology at its asymptomatic stage. When it occurs in isolation,particularly bilaterally and in an older patient, it can be relegated tothe findings section of a report and should not be used to suggestfindings of greater trochanteric pain syndrome. (40)

Summary

Several labral, osteocartilaginous, and soft tissue normal variantsexist around the hip joint that may mimic, and must be distinguishedfrom, true pathologic conditions. Awareness of these conditions canprevent unnecessary evaluation or intervention. Additionally, althoughthese entities are typically asymptomatic, some normal variants mayherald an underlying process such as abnormal biomechanics, which maypredispose the hip to disease. MR arthrography can increase specificityand diagnostic confidence in uncertain cases.

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Brian Y. Chan, MD; Hailey Allen, MD; Kirkland W. Davis, MD, FACR;and Donna G. Blankenbaker, MD

Dr. Chan and Dr. Allen are Radiologists at the University of UtahSchool of Medicine, Salt Lake City, UT; and Dr. Davis and Dr.Blankenbaker are Radiologists at the University of Wisconsin School ofMedicine and Public Health, Madison, WI. Dr. Blankenbaker and Dr. Davisare Consultants for and receive royalties from Elsevier.