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Pain in the hip is the experience of pain in the muscles or joints in the hip region, a condition commonly arising from any of a number of factors. Sometimes it is closely associated with lower back pain.
Common etiology include:
Pain in the groin, called anterior hip pain, is most often the result of osteoarthritis, osteonecrosis, occult fracture, acute synovitis, and septic arthritis; pain on the sides of the hip, called lateral hip pain, is usually caused by bursitis; pain in the buttock, called posterior or gluteal hip pain, which is the least common type of hip pain, is most often caused by sacroiliac joint dysfunction as well as sciatica (whether from a hemorrhaged spinal disk or a tense piriformis muscle). Herpes zoster (shingles) may also cause posterior hip pain.
Plain radiography allows us to categorize the hip as normal or dysplastic or with impingement signs (pincer, cam, or a combination of both). Besides these, pathologic processes like osteoarthritis, inflammatory diseases, infection, or tumors can also be identified (Figure 1).
Figure 1.
Despite the widespread of ultrasound, pelvis X-ray is still frequently used to diagnose and/or monitor hip dysplasia or for assessing other congenital conditions or bone tumors.
The most useful lines and angles that can be drawn in the pediatric pelvis assessing DDH are as follows:
In Group A, which has a better prognosis, there are no loss of height in the lateral third of the femoral head and little density changes; in Group B, there is lucency and lateral height loss of less than 50%; and in Group C, the most severe form, there is more than 50% loss of lateral height. Group B/C is considered when the loss of lateral pillar height is at 50%. People who are over the age of 8 years at the time of onset and have a hip in the lateral pillar B group or B/C border group have a better outcome with surgical treatment than they do with conservative treatment. Group B hips in children who are less than 8 years at the time of onset have a very favorable outcome unrelated to the treatment, whereas Group C hips in children of all ages usually have poor outcome unrelated to the treatment.
Measurements of hip dysplasia in adults are quite different from those in children.
There are other pathological conditions that can affect the hip joint and radiographs help to make the appropriate diagnosis. Acute bacterial septic arthritis can be diagnosed by radiographs when a fast regional osteoporosis and destructive monoarticular process develops (Figure 1(f)). In case of tuberculous or brucella arthritis it is manifested as a slow progressive process, and diagnosis may be delayed.
Synovial chondromatosis can be confidently diagnosed by X-ray when calcified cartilaginous chondromas are seen. However, other synovial proliferative processes, such as pigmented villonodular synovitis, require MRI for accurate diagnosis, although noncalcified synovitis can be suspected in radiographs by indirect signs, such as soft tissue swelling and/or erosions in the femoral head, femoral neck, or acetabulum (Figure 7).
Figure 7:
Radiological signs of transient osteoporosis of the hip include localized osteoporosis of the femoral head and neck (Figure 8). Nevertheless, final diagnosis has to be made with MRI to differentiate it from avascular necrosis and from insufficiency or stress fractures of the femoral head or neck. In case of AVN, radiographs can only demonstrate delayed or advanced signs. Staging according to Ficat classification ranges between normal appearance (stage I), slight increased density in the femoral head (stage II), subchondral collapse of the femoral head with or without “crescent” sign (stage III), and advanced collapse with secondary osteoarthritis (stage IV). In the case of stress or insufficiency fractures X-ray sensitivity has been proven to be much lower than MRI, which is currently the gold standard.
Figure 8:
MRI findings correlate with prognosis in LCPD. These include extent and distribution of epiphyseal necrosis, subchondral ossified nucleus fracture, involvement of the lateral pillar, and disturbance of physeal growth, including presence of transphyseal neovascularity or bridging.
Recent studies have been focused on the role of diffusion weighted MRI because it does not need contrast medium administration. ADC ratio of the femoral metaphysis was positively correlated with the Herring classification. ADC ratio superior to 1.63 indicates bad prognosis with 89% sensitivity and 58% specificity.
In adults, MRI is currently playing a definite role in the assessment of osteoarthritis. Although traditionally belonging to the arena of radiographs, the role of MRI has been stressed after the term femoral acetabular impingement was coined in 2003. Growing interest has been focused in accurate diagnosis of the acetabular and femoral morphological abnormalities that may lead to early osteoarthritis.
MR imaging is considered paramount to these objectives, mainly when surgery is considered, due to the ability of MRI to portray the whole section of the femoral neck surface, as well as to image the labrum and articular cartilage.
Diagnosis of impingement can only be achieved if, besides imaging findings, there are also clinical symptoms and positive impingement maneuvers.
Most of the angles and measurements described in the plain radiograph section can be accurately reproduced on MRI. In addition, the superiority of MRI resolution with intra-articular contrast allows detection of labral and chondral abnormalities that may influence the choice of medical, percutaneous, or surgical management (Figure 9).
Figure 9:
MR arthrography has proven superior in accuracy when compared to native MR imaging. It is considered the best technique to assess the labrum. Knowledge of the normal variable morphology of the labrum helps to differentiate tears from normal variants. A triangular shape is most commonly seen in 66% of asymptomatic volunteers, but round, flattened, and absent labra can also be found in asymptomatic populations. MR arthrography has demonstrated sensitivity over 90% and specificity close to 100% in detecting labral tears. Loose bodies are demonstrated as filling defects surrounded by the hyperintense gadolinium.
Association between labral tears and chondral damage has been demonstrated. This underscores the interaction between cartilage and labrum damage in the progression of osteoarthritis. Chondral damage to the posteroinferior part of the acetabulum as a contrecoup lesion occurs in approximately one-third of pincer cases secondary to persistent abutment on the anterior part of the joint leading to a slight posteroinferior subluxation. This is considered a bad prognosis sign.
MR arthrography can also demonstrate ligamentum teres rupture or capsular laxity, which are debated causes of microinstability of the hip. Elongation of the capsule or injury to the iliofemoral ligament or labrum may be secondary to microtrauma in athletes. MR can demonstrate abnormalities in these cases, such as increased joint volume or a ligamentum teres tear (Figure 9).
Intra-articular osseous causes of pain include several conditions: avascular necrosis (AVN), transient osteoporosis of the hip (TOH), tumors, and stress or insufficiency fractures. All these entities may present with a pattern of bone marrow edema characterized by decreased signal intensity on T1 weighted images and increased signal intensity on fluid sensitive sequences, such as fat saturated T2-weighted or STIR images. When there is no evidence of a focal lesion associated with the edema pattern, TOH is suspected. When a band of low intensity is seen inside the edematous area, the shape and length of this band become important. It is generally convex to the articular surface in the case of subchondral stress or insufficiency fractures, whereas it is concave, circumscribing all of the necrotic segment, in cases of AVN. When doubts do persist, gadolinium-enhanced MRI tends to show that the proximal portion beyond the band is enhanced in fractures but is not in AVN.
MRI has been shown to have 100% sensitivity and specificity in prospective studies of occult hip fractures. These fractures were diagnosed by bone marrow edema and a low signal fracture line, mainly on T1 or T2 weighted images (Figure 10).
Figure 10:
In synovial proliferative disorders, MRI demonstrates synovial hypertrophy. In the case of PVNS, characteristic foci of low signal intensity related to hemosiderin deposition are better seen on gradient echo T2* images (Figure 7). In the case of synovial osteochondromatosis, the synovial hypertrophy is accompanied by intermediate signal cartilaginous loose bodies and/or low signal calcified loose bodies.
Quite often, CT is widely available unlike MRI, especially in the acute setting. CT is performed in this setting when doubt about the existence of a fracture persists following plain radiograph. Modern multidetector computed tomography (MDCT) shows results comparable with MRI for detecting occult fractures.
Due to the submillimeter resolution of MDCT arthrography, many authors consider this technique complementary to MR arthrography. It may even have superior sensitivity in detecting cartilage pathology, but lesser detecting labral tears.
CT can also be used to obtain accurate measurement of the femoral version and torsion. The femoral version is measured by an angle formed between a line through the femoral head-neck axis and another horizontal line drawn between both ischial tuberosities. Normal values range between 5 and 25°. Retroversion is considered abnormal.
Femoral torsion is the angle between a line along the femoral head and neck axis and a second line that is touching the posterior border of both femoral condyles. The normal value at birth is approximately 32° and decreases gradually with age. In adults, the normal value ranges from 10° to 20°.
Ultrasound allows categorizing pediatric hips, according to Graf’s criteria, in four main types: normal, immature, and dysplastic (subluxed and dislocated). This classification is based on measurements of the acetabular inclination angle (alpha), cartilage roof angle (beta), and infant age. The femoral head coverage can also be determined by dividing the length of the femoral head covered by the acetabular fossa and the diameter of the femoral head. Its lower normal limits are 47% for boys and 44% for girls (Figure 11).
Figure 11:
In a recent study, including newborns with high clinical suspicion for DDH (Ortolani/Barlow test, asymmetry in abduction of 20° or greater, breech presentation, leg-length discrepancy, and first-degree relative treated for DDH), hip sonography led to a change in clinical diagnosis in 52% of hips and to a change in management plan in 32% of hips. It obviated further follow-up in 23%, strengthening its role as an important technique reassuring the clinical diagnosis.
During childhood, ultrasound is a quick method to assess hip pain and quite often may be used to avoid use of irradiating techniques, such as radiography or CT. Ultrasound allows evaluation of joint effusion, synovial thickening and neovascularity, the bone/cartilage contour, and the femoral head-neck alignment. Although sonography is extremely sensitive in detecting increased synovial fluid, it is nonspecific and cannot be used with accuracy to determine the type of fluid. Transient synovitis of the hip, despite being the most frequent cause of pain in children between 3 and 10 years, remains a diagnosis of exclusion. It usually shows anechoic fluid, but echogenic fluid can also be found. The effusion is considered pathologic when it is measured at >2 mm in thickness. The differential diagnosis is wide, including osteomyelitis, septic arthritis, primary or metastatic lesions, LCPD, and SCFE. Discrimination from septic arthritis is challenging, often requiring joint aspiration. In septic arthritis, US is able to demonstrate a hip joint effusion, synovial thickening, and cartilage damage, although the appearances are nonspecific.
A step between the head and the physis can be detected in children with SCFE, while abnormalities in the femoral head contour may suggest the presence of LCPD. In both cases, radiographs are mandatory to confirm diagnosis and severity (Figure 12).
Figure 12:
In adults, the most common application for US is to detect tendon or muscle injuries, effusion or synovitis within the hip joint or its adjacent bursae. Joint effusions may be due to many intra-articular processes and this may need another imaging technique to achieve a specific diagnosis.
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