Department of Radiology, Nuffield Orthopaedic Centre, Oxford, UK
Summary• Age is an important determinant in the aetiology of hip disorders.• MRI is the key imaging technique in a variety of conditions involving the bone,
including occult fracture, stress fracture, avascular necrosis and transientosteoporosis.
• Synovial diseases are well characterized on MRI, including pigmented villonodularsynovitis, synovial osteochondromatosis and inflammatory arthropathies.
• MR arthrography allows assessment of intra-articular pathology, including tears ofthe acetabular labrum.
• CT provides detailed information on bone morphology and may providea definitive diagnosis of osteoid osteoma.
• Ultrasound may be used to evaluate bursitis, joint effusions and snapping hips, aswell as guiding injections.
Cite this article as: Daghir A, Teh J. Imaging the hip. Imaging 2014;23:20120023.
Abstract. In this article, we review the clinical presentationand imaging appearances of a wide spectrum of disorders of thehip. The role of different imaging modalities is highlighted foreach condition.
In this article, we review the spectrum of pathologicalconditions that involve the hip, their clinical presentationand their radiological features. The range of pathologiesinvolving the hip depends greatly on the age of the pa-tient (Table 1). Radiography remains a key radiologicalinvestigation. However, ultrasound, CT and MRI havebecome increasingly routine in the diagnosis of hip dis-orders. MRI demonstrates bone pathology with increasedspatial resolution compared with bone scintigraphy, andit has become the preferred modality for investigation ofoccult fractures, bone marrow oedema syndromes(BMESs) and avascular necrosis (AVN). MRI is also helpfulin the demonstration of synovial proliferative disorderssuch as pigmented villonodular synovitis (PVNS). MRarthrography (MRA) allows exquisite delineation of thelabrum and cartilage, and it has an important role in theassessment of femoroacetabular impingement, which isnow implicated as a major cause of hip osteoarthritis (OA).CT allows detailed assessment of bone morphology, whichis helpful in conditions like femoroacetabular impinge-ment, and it may allow definitive diagnosis of osteoidosteoma. Ultrasound is useful to evaluate soft-tissue ab-normalities, including joint effusions and bursitis, and itpermits dynamic imaging in snapping hips and may be
used to guide needle aspiration and injection. The injectionof a local anaesthetic into the hip joint under ultrasound orfluoroscopic guidance may allow confirmation of the hipas the source of symptoms.1,2
Anatomy
The hip is a ball and socket joint capable of transmittinglarge forces. It allows a wide range of movement, whilemaintaining strong stability such that dislocation occursmuch less frequently than in the shoulder. The cup-shapedacetabulum is formed at the junction of the iliac, pubic andischial bones. The fibrocartilagenous labrum forms a ringat the margin of the acetabulum, thereby increasing itsdepth3,4 (Figure 1). The femoral head has a hemisphericalarticular surface with a central fovea to which the liga-mentum teres attaches. The capsule of the hip joint attachesat the intertrochanteric line covering the anterior femoralneck and most of the posterior femoral neck. The ilio-femoral, ischiofemoral and pubofemoral ligaments re-inforce the fibrous capsule. The transverse ligament andligamentum teres are intracapsular. The latter is a weakligament that transmits the foveal artery, which in adultscontributes little blood supply to the femoral head. Thefemoral head receives most of its blood supply from themedial and lateral femoral circumflex arteries, which forma ring around the base of the femoral neck. These are atrisk when there is an intracapsular femoral neck fracture.The lesser trochanter is the site of attachment of the ilio-psoas tendon. Several muscles insert onto the greater tro-chanter, including gluteus medius, gluteus minimus andpiriformis.
Address correspondence to: Dr Ahmed Daghir. E-mail: [email protected]
A stress fracture occurs following repeated loading ofa bone, which cannot accommodate itself to the forcesapplied to it.5 Two kinds of stress fractures are described:fatigue fractures occur in normal bones undergoing ex-cessive loading, and insufficiency fractures arise inpathologically weak bone undergoing normal loading.5,6
The femoral neck is a common site of fatigue fracturetypically occurring in military recruits and athletes. An-terior hip and groin pain is exacerbated by activity andimproves with rest. The insidious onset of symptomsmay lead to diagnostic delay and may result in the frac-ture becoming displaced.7
MRI is an excellent technique for identification andcharacterization of radiographically occult fracturesdue to acute or chronic trauma.8–12 In our practice, weperform both T1 and short tau inversion–recovery (STIR)coronal and axial sequences.10 The T1 images are helpfulfor demonstrating the low-signal fracture line, whichusually appears perpendicular to the cortex owing to thecausative compressive forces. The STIR images reveal
high-signal bone marrow oedema and also allow assess-ment of soft-tissue injury (Figure 2a). Radiographs mayinitially appear normal and later show periosteal thicken-ing and a sclerotic fracture line (Figure 2b). Bone scintig-raphy provides another means of diagnosis although thesensitivity and specificity is lower than with MRI.8,13
Occult hip fractures
Radiographs are sufficient for the diagnosis of the vastmajority of hip fractures. However, when radiographsare negative or equivocal and there remains a high clin-ical suspicion for an occult hip fracture, a number ofimaging options are available. In our institution, MRI isthe investigation of choice employing coronal and axialT1 and STIR sequences10,12 (Figure 3). CT is an alternativeimaging technique providing isotropic multiplanarreformats although MRI is reported to be more sensitiveand also allows delineation of soft-tissue injury.14 Scin-tigraphy usually allows detection of fractures 24 h fol-lowing injury; however, in the elderly, the sensitivity isfurther improved after a few days.13
Avascular necrosis
AVN, also called osteonecrosis, is common in thefemoral head and has a number of causes, the commonestbeing chronic steroid use, chronic excessive alcohol useand trauma. With interruption of the blood supply, my-eloid cell death follows in 6–12 h. After 48 h, osteocytedeath occurs, and lipocytes die within 2–6 days.15 This isfollowed by an inflammatory response increasing vas-cularity, leading to the formation of granulation tissueand fibrosis. Collapse of the subchondral bone predis-poses to OA. The condition is bilateral in up to 40% ofcases, so it is important to image both hips together.
Early detection of avascular necrosis
Early detection of AVN is important because therapysuch as core decompression may be implemented sooner.Radiographs are of limited use early on, as the typicalfindings on radiographs of subchondral lucency andcollapse occur late in the disease process (Figure 4a). Theinvestigation of choice is MRI, which is more sensitive
Table 1. The typical age of presentation of various hipdisorders
Age Condition
Above the fifthdecade
Occult fractureOsteoarthritisTrochanteric bursitis and gluteusmedius enthesopathy
Third to fifthdecade
Avascular necrosisTransient bone marrow oedemaSynovial proliferative disorders
Figure 1. Normal anatomy on MR arthrogram. Coronal T1 fat-saturated image shows intra-articular gadolinium contrast ashigh signal. The superior labrum (arrow) and transverseligament (arrowhead) are clearly demonstrated. The ligamen-tum teres can be seen attaching to the fovea (open arrow).Hyaline cartilage appears as intermediate signal.
Figure 2. Stress fracture of the femoral neck. (a) Coronal shorttau inversion–recovery image demonstrating high signal in thefemoral neck indicating bone marrow oedema. A low-signalfracture line is shown perpendicular to the bone cortex(arrow). (b) A radiograph taken on the same day shows verysubtle linear sclerosis indicating the fracture line (arrow).
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and specific than scintigraphy.16,17 The protocol shouldinclude T1 and STIR/T2 fat-saturated sequences in at leasttwo planes. Intravenous contrast, although not usuallynecessary, demonstrates regions of reduced enhancementin early AVN. The “double line” sign on T2 weightedsequences is virtually pathognomonic for AVN and isseen in up to 80% of cases. This describes a high-signalline (representing hypervascular tissue) on the necroticside immediately apposed to a low-signal line (repre-senting fibrosis and sclerosis) on the healthy side18
(Figure 4b,c). A joint effusion and bone marrow oedemamay also be present.18–20
Staging and prognosis of avascular necrosis
Staging the severity of AVN may be performed withimaging. Several classifications exist, including theSteinberg classification, which incorporates radiographic,MRI and scintigraphy findings (Table 2).21
Assessing prognosis in AVN using radiographs is oflimited use, since the prognosis is poor once there is ra-diographic evidence of subchondral collapse. The per-centage of weight-bearing femoral cortex involved withAVN on MRI is reported to be the most reliable
parameter in determining outcome.22 The success of coredecompression may be predicted by quantifying thepercentage of involvement of the femoral head. AVNinvolving ,25% of the femoral head appears to benefitgreatly from core decompression.22,23 If .50% is in-volved, the prognosis is very poor despite core de-compression. A good correlation exists between theextent of weight-bearing articular surface affected andfemoral head collapse. In one study, there was a 74% rateof femoral head collapse by 32 months if the region ofAVN involved more than two-thirds of the weight-bearing surface area.24 Conversely, when there are smalllesions confined to the medial anterosuperior portion ofthe femoral head, collapse tends not to occur.
Bone marrow oedema syndromes
Transient osteoporosis of the hip (TOH), also referredto as BMES, typically presents with acute hip pain in theabsence of previous trauma or signs of infection. TOHwas first described in females in the third trimester ofpregnancy, but it is most commonly seen in middle-agedmales.25 The pain is exacerbated by weight-bearing, andthere may be accompanying antalgic gait and musclewasting. The condition is self-limiting, taking an averageof 6 months for symptoms to completely resolve withprotected weight-bearing and symptomatic support.26,27
In some patients, resolution in one joint may be followedby involvement of another, which is referred to as re-gional migratory osteoporosis (RMO).28 In these cases,the commonest pattern is primary involvement of the hipfollowed by secondary involvement of the knee or an-kle.29 There may be temporal overlap such that more thanone joint is involved at a particular time. The patho-physiology of TOH and RMO remains obscure althoughthe role of ischaemia and trauma has been investigated.30
Radiography may demonstrate osteopenia of the femoralhead and neck, although this is a relatively late finding.Scintigraphyexhibits increaseduptakeasa result of increasedbone turnover and inflammatory change, but this is notspecific. MRI is the imaging modality of choice. It demon-strates bone marrow oedema as a diffuse intermediate/lowsignal on T1 weighted sequences and high signal on T2weighted fat-suppressed or STIR sequences several weeksbefore radiographic changes are detectable26,31 (Figure 5).
Bone marrow oedema in the femoral head and neckhas a wide differential diagnosis, including AVN, TOH,
Figure 3. Coronal short tau inversion–recovery image dem-onstrates an undisplaced intracapsular fracture of thefemoral neck that was not detected on radiographs. High-signal bone marrow oedema surrounds a low-signal fractureline (arrow).
Figure 4. Avascular necrosis (AVN) of the femoral head. (a) Radiograph demonstrates subchondral collapse (arrow), a late featureof AVN. (b, c) In a different patient: (b) coronal T1 image demonstrating a subchondral region of low signal (arrow); (c) short tauinversion–recovery sagittal oblique image demonstrating the classical “double line” sign (arrow) of AVN. The high-signal linerepresents hypervascular tissue on the necrotic side adjacent to the low-signal fibrotic/sclerotic line on the healthy side.
subchondral insufficiency fracture (SIF) of the femoralhead, stress fracture of the femoral neck, arthropathy,metastasis, osteoid osteoma and infection28,32,33 (Table 3).There may be difficulty in distinguishing between theMRI appearances of AVN, TOH and SIF. It is importantto differentiate between these conditions, as there areconsiderable implications for prognosis and treatment.The subchondral region of the femoral head is a criticalarea to evaluate. The absence of focal subchondralchanges is predictive of a transient bone marrow oedemalesion.32 AVN typically exhibits a smooth band of sub-chondral low intensity on T1 weighted sequences anda double line sign on T2 weighted sequences, whichrepresent repair tissue around a zone of necrotic bone.SIF of the femoral head is another distinct entity to con-sider. In contrast to AVN, this condition typically occursin elderly females who are osteoporotic and/or over-weight. A linear low-signal band is described in thesubchondral region corresponding to the fracture line.34
Articular collapse may occur in both AVN and SIF.
Osteoid osteoma
Osteoid osteomas are benign neoplasms that usuallyinvolve the long bones, particularly the proximal femurand tibial shaft. The typical presentation is of localizedbone pain that is worse at night and relieved by anti-inflammatory drugs. The tumour consists of a small ni-dus of osteoid tissue (usually ,1 cm) that demonstrates
variable mineralization. This appears on radiographs asa small ovoid lucent defect. The nidus is surrounded byan osteoblastic response resulting in the appearance ofa variable degree of surrounding sclerosis. On MRI, bonemarrow oedema is present which surrounds the nidus(which is sometimes difficult to detect) as a small in-termediate signal focus on both T1 and T2 weightedimages.35 Intra-articular lesions may be accompanied bysynovial thickening and joint effusions with little or nosclerosis.36 Bone scintigraphy is invariably positive butnot specific. The gold standard imaging technique is CT,which accurately localizes the nidus, thus confirming thediagnosis (Figure 6). CT also has an important role inguiding radiofrequency or laser ablation of the tumour.37
Conditions affecting the soft tissues of the hip
Lesions of the acetabular labrum
Labral tears may arise as a result of developmentaldysplasia, femoroacetabular impingement, trauma or re-petitive athletic activity. Patients with labral tears oftenpresent with a catching type pain, sometimes associatedwith clicking, snapping, locking or giving way of thejoint. Flexion and internal rotation of the hip may re-produce pain.
MRA, requiring instillation of gadolinium contrast intothe joint, is the preferred imaging technique for evaluatingthe labrum. It significantly increases the visualization of theacetabular labrum compared with conventional MRI.38–40
The normal labrum morphology is varied. Lecouvet et al41
demonstrated that the commonest shape of the labrum istriangular, present in 66% of asymptomatic volunteers,whilst round labra were detected in 11% and flattenedlabra in 9% of volunteers. An absent labrum was reportedin 14% of volunteers. Signal alterations within the labrumdo not correlate well with degeneration, as intermediateor high intralabral signal intensity on T1 and protondensity-weighted images has been reported in 58% ofasymptomatic labra using conventional MRI.42 The edge
Table 2. Steinberg radiological–clinical classificationfindings for the staging of avascular necrosis (AVN) of thefemoral head
Stages Features
0 Abnormal MRI but normal radiographs andscintigraphy. AVN should be suspected if it hasalready been diagnosed in the contralateralhip
I Abnormal MRI and scintigraphy, but normalradiographs. Patient has mild groin pain. StageI represents the early resorptive stage. Late inthis stage, plain radiographs may showminimal osteoporosis with poor definition ofthe bony trabeculae. Osteoporosis onlyappears when at least one-third of the mineralcontent of the bone has been lost
II This stage represents the reparative stage beforeflattening of the femoral head occurs. On plainradiographs, demineralization is now evident.It may be generalized or patchy and mayappear in the form of small cysts within thefemoral head. Patchy sclerosis may also occur,representing apposition of new bone on deadtrabeculae
III A linear subcortical lucency, indicatinga subchondral fracture, is present, known asthe crescent sign. This may extend into thearticular cartilage at the superolateral aspectof the femoral head. The femoral head initiallypreserves its round appearance, but, later, itdemonstrates collapse
IV There is segmental flattening of the femoralhead but preservation of the joint space
V There is femoral head collapse and degenerativechange
Figure 5. Transient osteoporosis of the left hip in a middle-aged male. Coronal short tau inversion–recovery image dem-onstrates high signal (arrow) indicating bone marrow oedemain the femoral head and neck. The subchondral region isinvolved, which is not always the case in this condition.
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of the labrum may normally overlap the margin of thearticular cartilage, giving an appearance of cartilage un-dercutting labrum. A sulcus may be present at the ante-rosuperior acetabular–labral junction and is consideredby some to be a normal variant.4
Tears are diagnosed on MRA when intrasubstancecontrast material is demonstrated. T1 fat-saturatedsequences are therefore of particular importance whenevaluating the labrum (Figure 7). Contrast that separatesthe labrum and acetabulum is typical of labral de-tachment. Peri-labral cysts are associated with underlyinglabral tears.43,44 These cysts are usually extra-articularand may erode into the adjacent bone.
Femoroacetabular impingement
Femoroacetabular impingement is a recently describedcause of hip pain resulting from morphological abnor-malities of the hip. Two types are described, cam andpincer, although most patients have a combination ofboth types.45 Cam- and pincer-type deformities are notthought to be painful by themselves. Rather, they pre-dispose to damage to the acetabular labrum and cartilage,which is painful. Identifying these morphological abnor-malities has important implications as surgical correctionmay prevent the onset of OA.46,47 Arthroscopic manage-ment involving recontouring of the cam and/or pincerdeformity has been reported to have favourable earlyoutcomes in most patients although the long-term benefitis not known.48 Accurate assessment of the extent of
cartilage disease is important because, in cases of ad-vanced damage, joint-sparing arthroscopic treatment isunlikely to be helpful.
Cam impingement
Cam-type deformity, typically occurring in athleticmales, describes loss of the normal sphericity of the fem-oral head owing to the presence of an osseous bump at thehead/neck junction, which is usually found antero-laterally49 (Figure 8). It is so-called because of the re-semblance to a camshaft in motor engines. Although a camdeformity is often idiopathic, similar morphology mayarise secondarily as a result of conditions includingtrauma, chronic slipped upper femoral epiphysis, previousosteotomy and Perthes’ disease. Repeated contact betweenthe osseous bump and the labrum causes labral tearingand detachment. This process leads to cartilage damageand OA. A triad of findings on MRA has been describedconsisting of a femoral head/neck osseous bump, antero-superior cartilage abnormality and anterosuperior labralabnormality50 (Figure 9). The degree of loss of sphericitymay be quantified using the a angle (Figure 10). This anglecan be measured on an axial oblique MR image or ona cross-table lateral radiograph of the hip. An a angle.50°may be considered abnormal.51
Table 3. Differential diagnosis of femoral head/neckoedema
Avascular necrosisTransient osteoporosis/bone marrow oedema syndromeSubchondral insufficiency fractureStress fracture of the femoral neckInflammatory arthropathyInfectionOsteoid osteomaMetastasis
Figure 6. Osteoid osteoma. Axial CT demonstrates the lucentnidus (arrow) in a typical location in the femoral neck. Notethe surrounding osteoblastic response resulting in sclerosis.
Figure 7. Labral tear. MR arthrogram axial T1 fat-saturatedimage demonstrates linear high signal (intra-articular con-trast) penetrating the acetabular labrum (arrow).
Figure 8. Cam deformity. Radiograph in a 46-year-old maleshows bilateral cam deformities (osseous bumps) of theanterolateral femoral head/neck junctions (arrows).
Pincer-type impingement, more common in middle-aged females, describes focal or diffuse enlargement ofthe acetabulum resulting in overcoverage of the femoralhead.51 Cranial acetabular retroversion, coxa profundaand protrusio acetabuli are types of morphology leadingto pincer impingement. On anteroposterior (AP) radio-graphs of the pelvis, cranial acetabular retroversion ispresent when the cranial part of the anterior acetabularwall is identified lateral to the posterior acetabular wall.Coxa profunda describes the overlap of the acetabularfossa with the ilioischial line, whereas protrusio acetabulidescribes the overlap of the femoral head with the
ilioischial line, which is more severe (Figure 11a). Thedegree of pincer deformity may be measured using thecentre–edge angle on an AP radiograph (Figure 11b). Avalue .40° has been used to define a pincer abnormal-ity.52 A value ,25° in adults indicates abnormal under-coverage often due to developmental hip dysplasia.53 Thecentre–edge angle can also be measured on coronal MRIimages.54 With progressive disease, the labrum may be-come ossified and detach to form an os acetabulum.There is a high prevalence of synovial herniation pits atthe anterosuperior femoral neck in patients with bothtypes of femoroacetabular impingement, although theiraetiology and clinical relevance are yet to be established.55,56
The radiographic findings are of a small rounded lucentlesion with a thin sclerotic margin. The main diagnosticpitfall is to mistake a herniation pit for an osteoid osteoma.57
Ischiofemoral impingement
Ischiofemoral impingement is a newly recognized con-dition, which remains the subject of debate. The conditionis found predominantly in females of middle age.58
Patients typically present with posterior hip pain thatmay radiate towards the lower extremity.59 The space be-tween the ischial tuberosity and lesser trochanter is typi-cally much narrower in patients with this condition than in
Figure 9. Cam impingement. MR arthrogram coronal T1 fat-saturated image shows a cam deformity (arrowhead). Thereis an associated labral tear (arrow) and thinning of thearticular cartilage.
Figure 10. The a angle in cam impingement: MR arthrogramaxial oblique T1 fat-saturated image. The a angle helps toidentify a cam deformity by measuring the loss of sphericityof the femoral head. First, a best-fit circle is drawn outliningthe femoral head. A line is then drawn along the femoralneck axis. A second line is drawn from the centre of the circleto the point at which the femoral neck contour protrudesfrom the circle owing to the cam deformity (arrow). Theangle between these lines is the a angle.
Figure 11. (a) Pincer deformity due to idiopathic protrusioacetabuli in an 82-year-old female. Radiograph shows over-lap of the femoral head (black arrow) with the ilioischial line(white arrowheads). (b) In the same patient: the centre–edgeangle in pincer deformity. A line is drawn connecting bothfemoral head centres. The a angle (*) is then measuredbetween a perpendicular line through the femoral headcentre and a line from the femoral head centre to the lateraledge of the acetabulum.
Figure 12. Ischiofemoral impingement. Axial short tauinversion–recovery image demonstrates bursa-like formation(arrow) in the ischiofemoral space, which is narrow (arrow-heads). There are less severe changes on the contralateral side.
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controls (normally measuring approximately 2 cm).59
Narrowing of the space may be congenital or related toprevious trauma, surgery, joint degeneration or osteo-chondroma. A combination of narrowing of this space andabnormalities of the quadratus femoris muscle (which liesin this space) has been described.60 MRI may demonstrateoedema, focal fatty infiltration and partial tears in thequadratus femoris; additionally, there may be involvementof the adjacent hamstring and iliopsoas tendons and bursa-like formation58 (Figure 12). However, the imaging ab-normalities may sometimes be incidental; for example,there may be bilateral MRI findings in patients presentingwith unilateral pain. Also, positioning of the hip in internalor external rotation during the scan may alter the mea-surement of the ischiofemoral space.
Inflammatory arthropathy
Inflammatory arthropathies such as rheumatoid ar-thritis or ankylosing spondylitis commonly involve the
hip joint. On plain radiographs, joint space loss is pre-dominantly in the axial region, unlike the superior jointspace loss that is typical of OA. Longstanding in-flammatory arthropathy leads to widespread cartilagedamage, resulting in circumferential loss of hip jointspace. Erosions are not a common finding. With ultra-sound, an effusion and synovial hypertrophy are detec-ted early in the course of disease (Figure 13). There arenon-specific findings on MRI, including effusion, syno-vial thickening and peri-articular bone marrow oedema.
Septic arthritis
Septic arthritis of the hip, although rare, is important toexclude owing to the risk of long-term joint damage if leftuntreated. Infection may result from haematogenousspread, direct inoculation or by spreading along the iliop-soas muscle from the spine.61 Using imaging alone, septic
Figure 13. Sagittal ultrasound image shows a moderate hipjoint effusion. Note anechoic fluid (arrow) and convexity ofthe overlying capsule and iliopsoas tendon.
Figure 14. Established synovial osteochondromatosis. Aradiograph of the hip demonstrates multiple small, ossifiedbodies in the hip joint.
Figure 15. Proliferative synovial osteochondromatosis. Anaxial short tau inversion–recovery image demonstrates highsignal synovial hypertrophy (arrows) and faint low signalbodies indicating mineralization (arrowhead).
Figure 16. Pigmented villonodular synovitis of the hip jointin a 29-year-old male. Note the presence of multiple erosionsof the femoral head, neck and acetabulum (arrows), whichare well circumscribed with sclerotic margins.
arthritis may be very difficult to distinguish from a non-infective inflammatory arthropathy. However, there arefindings that are more specific for infection, includingsoft-tissue collections, sinus tract formation and osteo-myelitis. Ultrasound may guide aspiration of an effusionfor laboratory testing.
Osteoarthritis
OA is certainly the commonest cause of hip pain andstiffness in the elderly. The classical findings on radio-graphs of superior joint space loss, osteophyte formation,femoral neck buttressing, subchondral sclerosis and cystformation are well described. In addition to superior jointspace loss, medial joint space loss is more common infemales than males. In early OA, radiographs may appearrelatively normal and, in these situations, MRI may beuseful to determine if there is significant hip pathology.On MRI, the key features of hip OA include joint effu-sions, subchondral bone marrow oedema, labral abnor-malities and cystic subchondral lesions.62,63 There may beassociated features of femoroacetabular impingement(see section Femoroacetabular impingement).
Synovial osteochondromatosis
Primary synovial osteochondromatosis (SOC) is a be-nign monoarticular condition of uncertain aetiology. Itpresents with pain, swelling and movement restriction
often with a long insidious onset. The condition affectsmore males than females. The hip is the third mostcommonly involved joint after the knee and elbow. Inaddition to joints, bursae and tendon sheaths may rarelybe affected. SOC is characterized by synovial metaplasiacontaining multiple nodules of hyaline cartilage. Thesenodules detach and form loose bodies within the joint.64
There is variable calcification and ossification of theloose bodies. Initially, there is a stage of active synovialproliferation eventually leading to inactive synovialdisease and multiple loose bodies.65 SOC commonlygives rise to premature OA. Malignant transformation isexceedingly rare.66
Secondary SOC may occur as a result of trauma, OA,osteonecrosis and neuropathic arthropathy. Differentiat-ing primary from secondary SOC may be difficult clini-cally and radiologically. However, the intra-articularbodies in secondary SOC tend to be larger, fewer andnon-uniform in size compared with primary disease.64
Figure 17. Pigmented villonodular synovitis of the hip joint(the same patient as Figure 16). Coronal T1 weighted (a) andshort tau inversion–recovery (b) images show low-signalsynovial proliferation (arrows).
Figure 18. Amyloid arthropathy secondary to long-termhaemodialysis. Coronal T1 weighted (a) and short tauinversion–recovery (b) images demonstrate low-signal syno-vial thickening (arrows) and erosions (arrowhead).
Figure 19. Iliopsoas bursitis in a patient with rheumatoidarthritis. Coronal short tau inversion–recovery image showsfluid distension of the iliopsoas bursa (arrow).
Figure 20. Gluteus medius bursitis. Coronal short tauinversion–recovery image shows distension of the bursa deepto the gluteus medius tendon (arrow).
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The classical radiographic appearance of multiplesmall calcified bodies around a joint occurs late on in thedisease (Figure 14). In the early stages, there may bea normal appearance or soft-tissue swelling without cal-cification. Joint space widening, erosions and features ofOA may also be present. An apple core appearance of thefemoral neck may be seen with chronic erosions.67
MRI is a useful modality for evaluating SOC. The keyfinding is of synovial hypertrophy, which exhibits a highsignal on T2 weighted/STIR sequences and an in-termediate signal on T1 weighted sequences. Intra-articular septations may be detected. The appearance ofmultiple intra-articular bodies depends on the degree ofmineralization, and these may be purely cartilaginousdemonstrating intermediate signal on T2 weightedsequences, calcified exhibiting intermediate/low signalon all sequences or ossified when fatty marrow signal ispresent68 (Figure 15).
Pigmented villonodular synovitis
PVNS is a benign proliferative synovial condition,which is characterized by recurrent bloody effusions andjoint erosions. The cause is uncertain with some evidencepointing to an inflammatory reaction of the synovium ora benign neoplastic process.69,70 Most patients are aged20–45 years, with an equal incidence between the sexes.71
The condition is typically monoarticular, most commonlyinvolving the knee followed by the hip.71,72 Histologically,PVNS consists of villous or frond-like synovial pro-liferations exhibiting a reddish colour due to haemosiderindeposition. Fibrosis, chronic inflammation and hyaliniza-tion are found in established disease.69,71,72 Immunophe-notypic differences help distinguish PVNS from othercauses of haemosiderotic synovitis (i.e. synovitis due torecurrent haemarthrosis), for example haemophilia.73
Radiographs in the early stages may be normal. Dense(haemosiderin-laden) soft-tissue joint swelling may laterbe detected with recurrent haemarthroses. The jointspace is initially preserved until the later stages whenthere is cartilage damage. Erosions with sclerotic mar-gins may arise as a result of the tight capsule of the
hip67,74 (Figure 16). These may be better appreciated on CTthan on radiographs.
MRI reveals diffuse or nodular thickening of thesynovium with characteristic low to intermediate signalon T1 and T2 weighted sequences owing to the presence ofhaemosiderin.75 In addition, gradient echo sequencesreveal abundant magnetic susceptibility effect, whichreturns very low signal. Synovial proliferation may ex-tend into the iliopsoas bursa. Bone erosions exhibit vari-able signal intensity depending on the presence of fluid,synovium or haemosiderin (Figure 17). The differentialdiagnosis for large low/intermediate signal erosions ofthe hip includes amyloid arthropathy due to long-termhaemodialysis (Figure 18).
Bursitis
Bursae are synovial-lined structures found betweentendons and muscles over bony prominences. Bursal in-flammation, or bursitis, may arise as a result of frictionfrom repetitive activity, trauma, infection or the in-volvement by systemic inflammatory conditions such asrheumatoid arthritis. Gait disturbances and previous hiparthroplasty may contribute to bursitis around the hip.
The commonly encountered types of bursitis aroundthe hip involve the trochanteric, iliopsoas and ischioglu-teal bursae. Around the greater trochanter, bursae arepresent deep to each of the three gluteal muscles.76 Theiliopsoas bursa is the largest bursa in the body andcommunicates with the hip joint in approximately 15% ofindividuals. Patients with bursitis typically present withpoint tenderness. Iliopsoas bursitis may also give rise topain in the anterior knee and thigh owing to irritation ofthe femoral nerve (Figure 19).
Radiographs are usually unhelpful in demonstratingbursitis, although, occasionally, calcific deposits may bepresent.77 Nevertheless, radiographs are usually obtainedto exclude other causes of hip pain such as OA.
Ultrasound plays an important role in the diagnosis ofbursitis as it identifies fluid in the bursa and allows thesonographer to relate findings to symptoms.78 Trochan-teric bursitis appears as a compressible rim-like sac of lowechogenicity over the greater trochanter. Gluteus medius
Figure 21. Internal snapping hip syndrome. (a) A dynamicultrasound image showing a transverse section of theiliopsoas muscle (narrow arrowheads) and iliopsoas tendon(arrow). Before the snap occurs, the tendon is separatedfrom the superior pubic ramus (wide arrowhead) by part ofthe muscle. (b) When the patient performs a specific hipmovement, the tendon abruptly strikes the superior pubicramus accompanied by an audible snap.
Figure 22. External snapping hip syndrome. (a) Dynamicultrasound image shows the gluteus maximus muscle (arrow-heads) in transverse section lying over the greater trochanter(arrow). (b) When the patient performs a specific hipmovement, the gluteus maximus muscle abruptly jerks awaybringing the iliotibial band into contact with the greatertrochanter accompanied by an audible snap.
bursitis occurs deep to the tendon of gluteus medius(Figure 20). Interrogation using Doppler may reveal in-creased flow in the wall of the bursal sac. If there is noevidence of bursitis, the gluteus medius tendon should becarefully evaluated, as enthesitis is a common cause oftrochanteric point tenderness. Ultrasound may also beused to guide therapeutic injections.79
On T2 weighted or STIR images, bursitis demonstratesfluid signal intensity.80,81 The adjacent tendon may alsodemonstrate enthesopathic changes. An advantage ofMRI is in being able to demonstrate tendon and bonemarrow changes, which cannot be detected on ultra-sound. MRI abnormalities in the trochanteric regions are,however, common incidental findings in patients withouttrochanteric symptoms.
Snapping hip syndrome
Snapping hip syndrome describes an audible snap orclick, which is reproduced with specific movements of thehip accompanied by discomfort. The syndrome is commonamongst athletes and dancers.82,83 Snapping hip syndromemay be due to external, internal or intra-articular causes.External snapping hip is usually caused by slipping of theiliotibial band or gluteus maximus muscle over the greatertrochanter.78,82 Internal snapping hip is caused by im-pingement of the iliopsoas tendon over the iliopectinealeminence.84 Intra-articular snapping hip is usually causedby a labral tear or intra-articular loose bodies.82
Ultrasound is the preferred imaging modality for sus-pected extra-articular snapping, as it provides real-timeanatomical visualization of the involved structures, as thepatient reproduces the painful movement. Dynamic so-nography of internal snapping shows abnormal trappingof part of the iliac muscle between the iliopsoas tendonand superior pubic ramus when moving into the frog-legposition; on moving back to the neutral position, thetendon abruptly jerks back, thereby striking the superiorpubic ramus with an audible snap85 (Figure 21). Iliop-soas tendinosis and bursitis may also be found in thiscondition. External snapping hip results from abnormaljerky movements of the iliotibial band or gluteus max-imus over the greater trochanter86 (Figure 22).
MRA is often required for the evaluation of intra-articular causes of snapping hip.
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