Magnetic resonance arthrography of the hip: technique and spectrum of findings in younger patients

Magnetic resonance(MR) imaging is the reference imaging technique in the evaluation of hip abnormalities. However, in some pathological conditions—such as lesions of the labrum, cartilaginous lesions, femoroacetabular impingement, intra-articular foreign bodies, or in the pre-operative work-up of developmental dysplasia of the hip—intra-articular injection of a contrast medium is required to obtain a precise diagnosis. This article reviews the technical aspects, contraindications, normal appearance and potential pitfalls of MR arthrography, and illustrates the radiological appearance of commonly encountered conditions.


Introduction
The development of magnetic resonance (MR) imaging has allowed it to become the standard of reference in the work-up of bone and soft tissue abnormalities of the hip. However, some pathological conditions-such as lesions of the labrum, cartilaginous lesions, femoroacetabular impingement, intra-articular foreign bodies or in the preoperative work-up of developmental dysplasia of the hiprequire intra-articular injection of a contrast medium, i.e. to perform MR arthrography, in order to obtain a precise diagnosis. This article's objectives are to refresh the technical aspects, contraindications, normal appearance and potential pitfalls of MR arthrography; and in addition, to illustrate the radiological findings of commonly encountered conditions.

Intra-articular access
In our institution, MR arthrography involves two steps: first, intra-articular injection under fluoroscopic guidance, then MR imaging. The different steps for intra-articular injection are as follows: & intra-articular injection under fluoroscopic guidance, 12 ml of iodinated contrast medium (Iodixanol Visipaque 270 mg I/ml, GE Healthcare) mixed with 0.06 ml gadolinium chelates (Gadopentetate Dimeglumine Magnevist 0.5 mmol/ml, Bayer) [1] or 10-15 ml from a prefilled syringe of gadolinium chelates (Gadoteric acid Artirem 0.0025 mmol/ml Guerbet) The cutaneous access location is found lateral to the femoral vessels and under the inguinal ligament. It is chosen at the preference of the operator, generally at the superior or middle part of the femoral anatomical neck (Fig. 1). Injection at the superior part of the neck may cause less pain, but would risk more extra-articular leakage of contrast medium compared with injection of the middle part [2]. MR imaging should ideally be performed less than 45 min after arthrography because of the decline of the signal-to-noise ratio (SNR) over time, although a 1-h delay may be tolerated [3,4].
Miller proposed a technique consisting of a fluoroscopically guided landmarking of the cutaneous access point, then intra-articular access in the MR suite [5]. Intraarticular access may also be guided by MR fluoroscopy [6].

MR apparatus
MR operating systems nowadays have magnetic field strengths from 0.2-3 Tesla (T). Because it has a higher SNR, a greater magnetic field (3 T) allows for higher spatial resolution in the same time period or faster acquisition with identical spatial resolution [7,8]. Evaluation of the labrum and cartilage would be better with a 3-T magnetic field [9]. Phased-array coils are needed for parallel imaging techniques [simultaneous acquisition of spatial harmonics (SMASH) or sensitivity encoding (SENSE)]. These reduce imaging time and the specific absorption rate [7,10] with, according Ryan et al. [11], no discernible difference in image quality or SNR between images acquired with and without SENSE.

MR sequences
Our routine protocol (Table 1) for MR arthrography of the hip at 1.5 T includes fat-saturated T1-weighted 3-mm slices in axial oblique, coronal and sagittal planes; fatsaturated T2-weighted 4-mm slices in the coronal plane; and fat-saturated three-dimensional (3D) gradient echo T1weighted 1-mm slices allowing for multiplanar and radial reconstructions, which could help in the detection of acetabular labrum lesions [12] and improve the analysis in its weight-bearing region [13]. The field of view used is at a narrow 14-18 cm, focusing on the symptomatic hip. Some authors opt for fat-saturated proton density (PD)-weighted oblique axial slices parallel to the femoral neck to analyse the head-neck junction [14].

Contraindications
As with all MR studies, absolute contraindications related to magnetic field exposure need to be respected. Principally, these are implantable material (cardiac stimulators, neurostimulators, cochlear implants etc.), certain types of cardiac valves, ferromagnetic material and metallic foreign bodies. Osteosynthesis material is not an absolute contraindication to MR studies, but when situated in close proximity to the region of interest, magnetic susceptibility artefacts may worsen image quality to the extent of rendering image interpretation impossible. For that reason, we elect not to perform MR arthrography of the hip for patients with osteosynthesis material at the level of the cotyle or at the upper extremity of the homolateral femur.
Precautions related to intra-articular injection of contrast media Intra-articular access is an invasive procedure, and systematic precautions have to be followed: & Informing the patient about the different steps of the procedure, of potential allergic haemorrhagic or infectious complications, and obtaining consent & Verifying absence of anticoagulation or anti-platelet aggregation medication & Verifying the absence of allergies to the products used When strict aseptic precautions are respected, infectious complications are avoided. Arthrography may be responsi- ble for a temporary flare-up of joint pain, reaching its peak 4 h post-injection, independent of the quantity of local anaesthetic previously injected [15]. Minor adverse reactions are rarely reported, such as headache, vertigo, vasovagal episode or urticaria [16], but Saupe and co-workers' series of 1,085 MR arthrographies revealed no major adverse reactions [15].

Normal anatomy and variants
Normal anatomy of the hip The hip joint is an enarthrosis joining the femoral head with the iliac bone (Fig. 2). The articular surfaces are formed by the femoral head, a sphere of 45 mm in diameter; and the acetabulum, a semi-circular cavity in the shape of a horse-shoe closed off inferiorly by the transverse ligament. These are covered by an articular cartilage 1-3 mm thick [17,18] except at the centre of the acetabulum and at the level of the fovea, where the ligament of the femoral head (ligamentum teres) is inserted. The acetabular labrum is a triangular fibrocartilage inserted at the periphery of the acetabulum (Fig. 2). It increases the depth of the acetabulum [19], and is thicker in its postero-superior compared with its antero-inferior aspect [20,21]. The articular capsule forms a fibrous sleeve around the whole joint between the base of the labrum and the femoral neck. Normally, there is a small perilabral recess between the labrum and the capsule, shown with contrast medium [22] (Fig. 3). On axial slices, the articular capsule rests in front of the iliofemoral ligament and the ischiofemoral ligament, at the back. The normal hip joint capacity is 8-20 ml [23,24]. Located anteriorly, the ilio-psoas bursa communicates with the hip joint in 10-15% of cases [25] (Fig. 4).

Normal variants and pitfalls
Anatomical variants of the labrum have been described and they should not be interpreted falsely as abnormal: & The junctional zone between the acetabular labrum and the transverse ligament is generally indistinguishable. An irregularity of this zone is possible and may give a pseudotear aspect to the labrum [26]. & Most often the labrum is triangular in shape, but it may be rounded or flat in asymptomatic patients [27,28]. It appears that with age, even in the absence of symptoms, the proportion of triangular labra diminishes [29,30], and intralabral T1 intermediate and T2 hypersignal anomalies appear [27,30]. Complete absence of the labrum is possible [26,28,30]: the antero-superior labrum may not be visualised in 10% of cases; this proportion increases with age [28]. & The existence of a sublabral recess is possible (Fig. 5).
In MR imaging, it appears as a well-defined linear interposition of contrast medium partially separating the underside of the labrum without any perilabral anomalies [31]. However, many locations have been described: Petersilge et al.n [24] believe it is always in an anterosuperior position; on the other hand, Studler et al. [31] describe it in an antero-inferior location. Dinauer et al. [32] also reported the possibility of a postero-inferior sublabral recess. & There is a possibility of a sulcus between the labrum and the articular cartilage at the level of the acetabular fossa [20,33].

Abnormality of the acetabular labrum
Abnormality of the acetabular labrum may be traumatic, degenerative or related to hip dysplasia [34][35][36]. The clinical presentation typically consists of clicking, locking and pain on flexion and internal rotation of the hip. Conventional MR imaging of the hip is not effective, except in the study by Mintz et al. [37], in the evaluation of labral tears, while MR arthrography has shown good sensitivity and specificity in the detection of acetabular labral tears [22,24,26,33,38,39]. Acetabular labral lesions are considered to be precursors of coxarthrosis.
Early diagnosis and precise determination of the location and extent of labral tears may allow for a timely surgical intervention [22]. In 1996, Czerny et al. [33] proposed the following MR imaging classification of acetabular labrum lesions in increasing stages of severity:    [41]. The author proposed to describe the morphology of labral tears rather than using a specific classification scheme: & An irregular labrum, i.e. irregular margins without a tear (Fig. 6a) & A flap-type labral tear, may be partial or complete (Fig. 6b) & A longitudinal peripheral labral tear, located at the base of the labrum, may be partial (Fig. 6c) or complete (Fig. 6d) & A thickened and distorted labrum, in keeping with the instability of the labral lesion A linear or curvilinear increased signal at the insertion of the labrum is compatible with a partial tear, but needs to be interpreted with caution, in light of other clinical information, as it can be found in asymptomatic patients (Fig. 7): in a series of 52 examinations in asymptomatic patients, Cotten  Axial fat-saturated T1-weighted arthro-MR image. This curvilinear hyperintense line visible on the articular side of the anterior labrum (white arrow) consistent with a partial tear can be asymptomatic et al. [28] described four cases of labral tear (7.7%). The presence of a paralabral cyst is an indirect sign of complete labral tear (Fig. 8) [42]. Paralabral cysts may lead to acetabular erosions [43] and appear to be more frequent in cases of developmental dysplasia of the hip [44].
In young patients presenting with pain, a diagnosis of developmental dysplasia of the hip on plain film is often associated on MR imaging with acetabular labrum abnormality showing hypertrophy, irregularity, intermediate or hypersignal (Fig. 9), an inverted horizontal or trapped appearance, and sometimes a tear. With time, direct contact between the abnormal labrum and the femoral chondral surface leads to the formation of bony protrusions and the equivalent of femoroacetabular impingement. Progressively, it evolves to coxarthrosis.
In 30% of cases, labral lesions are associated with chondropathy [45]. Most labral tears involve the anterior and superior labrum [41]. Using the clock-face method to localise and describe the extent of a labral tear is more precise, besides more than 40% of labral tears extend beyond a quadrant [41]: on a sagittal slice, 12 o'clock is above, 3 o'clock in front, 6 o'clock below and 9 o'clock behind (Fig. 6c).

Femoroacetabular impingement
Femoroacetabular impingement (FAI) is a cause of acetabular labrum abnormality and early coxarthrosis; and is potentially treatable surgically [46][47][48][49]. Clinically, as in labral tears, patients with FAI may complain of pain on hip flexion and internal rotation. A limited range of motion on internal rotation of the hip should also point to the diagnosis. Early on, radiographs may be normal or may show subtle anomalies: a prominent anterior portion of the head-neck junction, anterior synovial herniation pits, the presence of an os acetabuli, ossification of the acetabular border, coxa profunda, acetabular protrusion or acetabular retroversion [48,50].
According to the mechanism thought to be responsible for each, FAI is classified into two types: the cam type, where the antero-superior part of the anatomical neck of the femur is hypertrophied and convex in shape impinging between the labrum and acetabulum; and the pincer type, where there is excessive coverage of the femoral head by the acetabulum (Fig. 10). The cam type is more frequent compared with the pincer type, but many cases of FAI are caused by both morphological anomalies and are not pure forms of either type [51].

Cam-type FAI
In this type of FAI, the so-called "pistol grip deformity" of the anterior portion of the femoral head-neck junction causes a compression and abnormal stress on the anterior labrocartilaginous complex on flexion and internal rotation. It is most frequently found in young athletic males [46,47].
Cam-type FAI may be congenital or due to developmental dysplasia of the hip, slipped capital femoral epiphysis, trauma or avascular necrosis [14] but contrary to slipped capital femoral epiphysis and growth abnormalities of the femur congenital dysplasia is a rare cause of cam impingement because the head-neck junction in these patients is usually normal.  & A flattening or a convexity of the antero-superior part of the normally concave femoral head-neck junction (Fig. 11) [49]. The so-called "pistol grip deformity" is the nickname for a pronounced deformity of the femur and should not be used for slighter deformities with abnormal head-neck offset. & An alpha angle exceeding 55 degrees (Fig. 11) [52].
The alpha angle is measured on an axial-oblique image between the axis of the femoral neck and a line connecting the femoral head centre with the point of change in the radius of the curvature on the anterior part of the femoral head. Its normal value is 42 degrees. The pistol grip deformity increases the alpha angle. The alpha angle may be underestimated if the contour anomaly is anterosuperior or superior, some recommend measuring the alpha angle on radial reconstructions around the femoral neck axis [53]. & In general, an earlier, more severe and extended involvement of the antero-superior cartilage [54] com- pared with the pincer-type FAI [48]; and may be associated with subchondral oedema shown by high signal intensity on fat-saturated T2-weighted and fatsaturated PD-weighted sequences. & There is often extensive antero-superior labral involvement [48,54]. & Fibrocystic changes and subchondral microcysts in the antero-superior part of the femoral neck, not to be mistaken for physiological anterior synovial invaginations [55].
The therapeutic approach aims at early restoration of the normal morphology of the hip joint before the setting in of chondropathy and coxarthrosis. Some even recommend treating patients presenting with impaired flexion-internal rotation of the hip in association with MR imaging signs of FAI, without pain symptoms [56]. Surgical interventions for cam-type FAI include osteoplasty of the anterior portion of the femoral neck-head junction (Fig. 12), relieving the cause of impingement, arthroscopic debridement of labral tears and possible removal of foreign bodies [57]. On postoperative follow-up, an alpha angle of 43 degrees measured on standard plain films allows for FAI-free motion of the hip, but only MR arthroscopy with radial reconstructions may formally exclude a femoral head-neck junction curvature anomaly [58].

Pincer-type FAI
Found preferentially in middle-aged women [46,48], this type of FAI is characterised by a morphological anomaly of the acetabulum causing excessive coverage of the femoral head: coxa profunda, acetabular protrusion or acetabular retroversion [59,60]. Acetabular anteversion, normally at an angle of 22.5 ± 3 degrees [61], is measured on an axial slice through the centre of the femoral head, however this angle may vary with pelvis flexion [62]. Rarely, it may be caused by trauma or labral ossification. Radiologically, retroversion manifests as the crossing sign and by the visibility of the ischial spine in the pelvis [63,64]. It causes direct contact between the anterior acetabular border and the femoral head-neck junction, thereby explaining preferential involvement of the labrum and a relatively preserved cartilage [46,48,51]. Later in the course, lesions to the postero-inferior cartilage by a contrecoup mechanism may appear [47]. Surgical management includes anterior acetabular osteoplasty for retroversion and circumferential osteoplasty in the case of a protrusion, in combination with labral lesion debridement [57]. Acetabular retroversion may be treated by periacetabular inverted osteotomy [60].

Abnormality of the cartilage
As the articular cartilage of the hip is thin, it is difficult to study with standard MR imaging. Sensitivity and specificity of MR arthrography in the detection of chondral lesions using T1-weighted spin-echo sequence are estimated to be about 62-79% and 77-94% [65,66] but multi-detector computed tomographic (MDCT) arthrography is superior to MR arthrography in terms of sensitivity, specificity and interobserver reproducibility [67]. Even if 3D double-echo steady-state sequence optimised for cartilage imaging improves lesion conspicuity, it doesn't seem to improve diagnostic performance [65]. As the articular cartilage is so thin and because of the direct apposition of the femur and acetabulum, it is sometimes difficult to differentiate femoral from acetabular cartilage. Inferior limb traction may improve articular cartilage analysis [68,69], but it remains controversial [70]. Cartilage changes can be categorised into: normal, signal heterogeneity, fissuring, thinning to <50% of the normal thickness, thinning to >50% of the normal thickness and full thickness cartilage loss (Fig. 13) [71]. Early chondral anomalies are most often found in the antero-superior part of the joint, first on the acetabular side [66,72]. Among 100 patients with mechanical symptoms, Neumann et al. [71] has estimated the prevalence of cartilage lesions to be 76%, predominantly cartilage thinning to <50%. He also confirmed in vivo the association first reported in cadaveric studies [44] between labral tears and cartilage loss. Cartilage lesions are also correlated with bone marrow oedema [71]: analogous to the knee, bone marrow oedema may be indicative of more rapid progression [73] but has to be confirmed in longitudinal studies. Chondral lesions evolve to complete erosion of the articular cartilage, subchondral cyst formation and subchondral osteosclerosis. Before hip arthroscopy the aim of MR arthrogram is to assess the presence and severity of cartilage loss. It also could be useful in evaluating new chondroprotective or chondroregenerative drugs.

Intra-articular foreign bodies
The prevalence of intra-articular foreign bodies of the hip is low. They may be responsible for pain, locking, clicking and limited articular range of motion, and may or may not be associated with an osseocartilaginous defect. MR and MDCT arthrography allow for the differentiation of intraarticular foreign bodies from osteophytes, synovial folds or hypertrophic synovitis: classically, intra-articular foreign bodies are surrounded by contrast medium, but only partially in the case of osteophytes and synovial folds (Fig. 14) [74]. They are accessible to arthroscopic treatment.

Abnormality of the ligamentum teres
Abnormality of the ligamentum teres (ligament of the head of the femur) is rare. It may present as complete or partial Fig. 15 a, b Complete tear of the ligamentum teres in an 29-year-old man. a Coronal reconstruction from a fatsaturated 3D gradient echo acquisition: the acetabular insertion of the ligamentum teres is interrupted. The ligamentum teres has an abnormal intermediate signal. b Sagittal fat-saturated T1-weighted arthro-MR image through the acetabular bottom shows a laminated ligamentum teres completely vertically divided into two parts (white arrows) Fig. 14 Axial oblique fat-saturated T1-weighted arthro-MR image shows a hypointense structure wholly surrounded by contrast medium consistent with an intra-articular foreign body (white arrow tear (Fig. 15). Degenerated ligamentum teres can be seen in young patients often with a history of avascular osteonecrosis or slipped capital femoral epiphysis in association with coxarthrosis [14,75].

Conclusion
MR arthrography is a high performance imaging tool in the work-up of abnormalities of the acetabular labrum and articular cartilage, intra-articular foreign bodies and in femoroacetabular impingement. Although still an invasive procedure, because there is intra-articular injection of contrast medium, this imaging technique has shown excellent tolerance. Its indications have widened in the recent years since the development of conservative surgical interventions of the hip such as arthroscopy and surgery for femoroacetabular impingement.