Introduction

Magnetic resonance (MR) imaging of the hip provides a platform to assess the osseous structures and surrounding soft tissues and is thus an invaluable tool in the assessment of both the hip joint and its associated structures. It is therefore important to develop a thorough grounding in the normal anatomical appearance of the pelvis and hip on MR imaging in order to thereafter appreciate the more common disorders that will regularly present in clinical practice.

Because a range of diagnostic modalities is available to the investigating clinician, it is also important that they have an understanding of the applications of each technique and its clinical appropriateness. Often, a patient’s principal presenting complaint will be that of low back pain, with associated symptomatology in the hip, groin or thigh.

Depending on the size of the field of view, the hips may be included on MR imaging of the lumbar spine; however, in order to explore the hip’s anatomy and pathoanatomy in detail, a focused MR imaging study, initially of both hips followed by a dedicated protocol of the hip in question should be performed, to optimize the detection of any pathology and to help direct the most appropriate clinical management of the patient.

History and examination

As with any patient examination, the eliciting of a detailed and accurate history is paramount and lays the foundation for the performance of an appropriate examination and establishment of a working diagnosis and differential. The most crucial element of the history in this area of the body is the precise localization of the area of pain: many patients who complain of ‘hip pain’ do not in fact have pain in their hip and, when questioned, will point to their buttock, gluteal fold or iliac crest; similarly, patients who do have hip pathology will often complain of symptoms in their groin, or the side or front of their thigh – not infrequently described as ‘sciatica’! The use of pain diagrams can help in this regard, but, at the very least, the patient should be asked to point to where they feel pain or discomfort.^1,2

The nature and precipitation of the pain can also give important clues as to the underlying cause of the patient’s complaint. Progressive, insidious pain and stiffness in the groin (often in association with a flexion contracture) and/or the buttock is indicative of arthritis; this will typically be made worse by walking and relieved by sitting. By contrast, a sacroiliac syndrome, which presents with buttock pain that may refer to the groin, will often be acute with an obvious precipitating cause and be relieved by walking and aggravated by sitting.

Sporting injuries often have very localized pain produced by highly specific activities: soccer players may complain of groin pain but only when passing the ball in certain directions; this can help differentiate injury to the adductor longus from rectus femoris or psoas. Increasing pain on repetitive activity is also typical of soft tissue injuries such as bursitis, which commonly affects the greater trochanteric area: often the patient will complain of being unable to sleep on the affected side.

With experience, the clinician will normally be able to recognize the atypical presentation of more unusual conditions such as osseous, intrapelvic and spinal lesions. These can, of course, mimic everyday injuries and the clinician should always be alert for an apparently benign condition that fails to respond to normally successful management protocols or is relentlessly progressive. Pain or clicking in the hip of a child is always a cause for concern and usually an indication for diagnostic imaging.³

Associated symptoms such as clicking or popping can also be associated with specific conditions⁴ and a familial history of pelvic conditions is often very revealing: many conditions such as degenerative coxarthrosis, inflammatory arthropathies, connective tissue disorders and dysplasia have familial tendencies.^5–9

Examination should commence with observation of the patient both stationary and walking. Obesity (a body mass index greater than 30) is a significant predisposing factor towards coxarthrosis,¹⁰ which will often be accompanied by an obvious flexion contraction causing an inability to stand with a straightened leg on the affected side; other forms of antalgia can also help identify the source of a patient’s pain both in posture and gait.

Although there are myriad orthopaedic tests for the hips and sacroiliac joints, few have any proven validity.^11–14 Range of motion, however, often is enough to demonstrate hip pathology, particularly painful reduction of internal rotation. The Patrick/FABER test (external rotation of the hip with the leg in the ‘figure-4’ position: Flexion, Abduction and External Rotation) has been shown to have good reliability as has digital palpation in the identification of greater trochanteric bursitis – the same is also true of the many myofascial trigger points that frequently coexist with pelvic girdle dysfunction and with the posterior margin of the sacroiliac joint.^12,15,16

Identification of sacroiliac syndrome is more problematic; however, multiple positive provocation tests have been shown to have a measure of diagnostic reliability; these are detailed in Table 5.01^17–19 alongwith tests that the authors have also found useful when similarly used in combination. It is, however, important not to perform too many tests; if three or four have already proved positive, there is little to be gained from continuing and the patient may well have an adverse reaction – the tests are called provocative for a reason and can eventually aggravate the patient’s symptoms. It should also be kept in mind that the pelvis is a closed loop kinematic chain and that comorbidity with lumbosacral facet joint dysfunction is high.²⁰

Table 5.01 Suggested provocation tests for the sacroiliac joint
Provocative tests with evidence of diagnostic reliability

Differential diagnosis

Once the differentiation between intrapelvic conditions, hip pain and sacroiliac syndrome has been made, the diagnosis should be further refined. The main differential with sacroiliac syndrome is with myofascial syndromes in the muscles of the gluteal region and lumbosacral junction and with lumbar spine injury, which can often refer or radiate to the buttock. As these conditions are often comorbid, the clinical challenge is to ascertain the extent of the contribution from each.

Figure 5.01 • The thigh thrust sacroiliac joint provocation test.

(Reproduced from Manual Therapy¹⁷ with permission.)

Figure 5.02 • The compression sacroiliac joint provocation test.

(Reproduced from Manual Therapy¹⁷ with permission.)

When it comes to the hip joint, the main aim of the differential diagnosis is to distinguish between intra-articular pathology, extra-articular pathology, and those conditions that can mimic hip pain. These are summarized in Table 5.02.^3,21,22

Table 5.02 Causes of pain around the hip joint

The clinician should also be alert for paediatric cases presenting as knee pain without discernible cause. Two common hip conditions often present in this manner: slipped capital femoral epiphysis and Legg–Calvé–Perthes disease, and both have prognosis directly related to early detection.²³

Technique and protocols

During any MR imaging evaluation, patient co-operation is critical. For evaluation of the hip, the patient should lie supine with mild internal rotation of the feet. Symmetry of rotation of both feet is important when both joints are examined simultaneously, which they often are to compare the appearance of the trochanters and adjacent muscles.^24–26 Two types of images are available: a screening examination of both hips, or a higher-detail study of a single joint (Figure 5.03).

Figure 5.03 • MR imaging of the hip using two different protocols in paediatric patients. In (A), MR imaging of the left hip has been performed using a gradient-weighted sequence in the coronal plane. The specificity of the MR imaging to include just one hip joint allows for the close inspection of this region, particularly with respect to the labrum; however, in (B), an MR image of both hips in the coronal plane using a fluid-sensitive sequence allows for the comparative evaluation of both hips albeit with a reduction in the anatomical detail, but which remains a very useful technique to study the hips. Both techniques have advantages and disadvantages.

The study begins with a plan scan (or ‘scout’ view) through the level of the femoral head and will usually include axial and coronal views (Figure 5.04). T1-weighted and fluid-sensitive images will be obtained in multiple planes. Depending on the pathology suspected, sagittal views may also be acquired (Figure 5.05). Fat-suppression techniques are very commonly used in the hip and pelvis to show early changes in marrow signal.^24,25,27,28

Figure 5.04 • Three MR imaging scout views demonstrating planes of imaging for (A) axial pelvis (T1-weighted), (B) axial hip (T1-weighted in a paediatric patient), and (C) coronal pelvis gradient echo.

Figure 5.05 • MR imaging using T1-weighted, spin echo images of the hip in the sagittal plane. These sequences are useful in the evaluation of the subchondral bone, in conditions such as avascular necrosis, or in cases of acetabular labral tears. Figure (A) demonstrates the scout view for the resulting sagittal sections (B–D). Note that the anterior aspect of the patient is to the reading left in these figures, which progress from medial to lateral.

Intravenous contrast is not routinely utilized but may be helpful to differentiate cystic from solid masses or early ischaemic changes and in the evaluation of labral pathologies that have proven inconclusive on normal MR protocols. In these situations, findings may be accentuated even more if paired with fat-suppression techniques.^25,27,29,30

Evaluation of the sacrum, sacroiliac joint and superior bony pelvis anatomy requires different positioning for the patient and should be ordered separately. The patient is often asked to lie supine with the knees slightly bent and hip joint mildly flexed; this position precludes good visualization of the iliofemoral joint. The planes of imaging for these areas is also different; in cases where sacroiliac arthropathy is suspected, images obtained in an oblique coronal plane parallel to the joint may be helpful (Figure 5.06). Intravenous administration of contrast can help identify early sacroiliitis. If pathology is suspected in the posterior soft tissues, imaging in the prone position may be also considered, to limit compression of the soft tissues.^{24,25,27,31,32}

Figure 5.06 • MR imaging depicting scout view of the sacrum. Particularly note the angulation of the plane of imaging which allows this plane to capture not only the sacrum but also a portion of the hip region which may reveal previously occult lesions.

Normal anatomy and common variants

MR imaging displays osseous and soft tissue structures with great clarity; the osseous anatomy is best evaluated on the T1-weighted sequences. The acetabular fossa is a deep pocket that covers approximately 40% of the surface of the femoral head. The thick fibrocartilaginous labrum and transverse acetabular ligament completely encircle the acetabulum. Both structures will be demonstrated as low intensity areas on most of the MR sequences.³³

The femoral head is generally spherical and covered by articular cartilage with the exception of the insertion point of the ligamentum teres: the fovea centralis. Covering the proximal femur is the articular capsule extending from the supra-acetabular region to the femoral neck.^34,35 It is a normal finding to see a small amount of fluid inside the joint space (Figure 5.07). Adjacent to the capsule is one of the largest synovial bursae of the body: the iliopsoas bursa. It is seen as a high intensity area on fluid-sensitive images, sandwiched between the muscle and the capsule. In a small percentage of the population, it may even communicate with the joint.³⁶