High-resolution ultrasound in the evaluation of the adult hip

Signs of joint effusion are not readily detected on clinical examination and, therefore, they are best evaluated under US. A joint effusion can be seen as a region of anechoic or hypoechoic fluid which distends the anterior capsule (Fig. 6). It can be assessed by measuring the distance between the anterior femoral neck and the joint capsule. A joint effusion has been described to be present if the distance measured is over 7 mm or if the distance is over 1 mm greater than on the contralateral side⁽⁹⁾. Anterior recess distention and echogenicity have been shown to be unreliable indicators of adult hip joint effusion, either in native or postoperative hips, and arthrocentesis may be needed in some instances, such as infection. Distention may have no fluid but may be due to synovitis. Positioning is also important, as internal rotation can distort the capsule, resulting in an artifactually thickened recess greater than 7 mm. In some instances (i.e. prior surgery), fluid may be loculated^(10,11).

Fig. 6.

A joint effusion in a healthy asymptomatic adult is uncommon⁽⁹⁾. However, in conditions such as inflammatory arthropathy or septic arthritis, a joint effusion is often present; studies have shown that there is a relationship between the presence of a joint effusion and clinical signs⁽¹²⁾. An effusion in cases of osteoarthritis (OA) is thought to be uncommon, so the presence of a large effusion should raise the possibility of rapidly progressive OA⁽¹³⁾. There may also be synovitis, which will be seen as hypoechoic or anechoic intra-articular soft tissue which is non-compressible⁽¹⁴⁾. In addition, synovitis will usually display a degree of Doppler signal.

The ability of US to assess fluid collections is especially useful in screening for pseudotumors in the setting of total hip arthroplasty (THA) or other hardware which may produce artifacts on CT or MRI (Fig. 7).

Fig. 7.

Acetabular labral tears are common and found at arthroscopy in up to 55% of cases of intractable hip pain⁽¹⁵⁾. Tears of the labrum may be seen in the anterior position, and the majority of symptomatic tears are seen within the anterior superior quadrant⁽¹⁶⁾. The findings manifest as a cleft within the anterior labrum, with hypoechoic fluid tracking into the region of tearing. In addition, an associated paralabral cyst may be identified on US imaging as a well-defined hypoechoic region which lies adjacent to the acetabular wall, often connecting to the labrum (Fig. 8).

Fig. 8.

The tendons/musculotendinous complexes of interest in the anterior hip are the iliopsoas and rectus femoris. In tendinopathy, the iliopsoas tendon appears thickened and enlarged, with loss of the usual fibrillar structure. This is a common cause of groin pain following total hip arthroplasty⁽¹⁷⁾.

Partial tears of the iliopsoas tendon tend to occur in athletic injury. Spontaneous full-thickness injury of the iliopsoas tendon is uncommon but has been described in the elderly with associated risk factors including advanced age, steroid use, and chronic disease⁽¹⁸⁾ (Fig. 9). Tears usually occur near the tendinous attachment, at or close to the lesser trochanter⁽¹⁹⁾.

Fig. 9.

The iliopsoas bursa is a large synovial bursa which lies between the iliopsoas tendon and the anterior capsule of the hip. This is the largest bursa in the human body, extending proximally under the iliacus muscle to the pelvic brim. The bursa is usually collapsed and not perceptible on US; its function is to reduce friction associated with the movement of the tendon over the anterior hip⁽²⁰⁾. On US, a distended bursa is deep to the femoral vessels and superficial over the iliopsoas tendon; it may appear compartmentalized when the tendon is bifid or contains multiple slips (Fig. 10). The iliopsoas bursa frequently communicates with the hip joint, so joint disease and joint effusion may led to distension of the bursa⁽²¹⁾.

Fig. 10.

Dynamic US may identify mechanical snapping of the iliopsoas tendon, but this is not seen in all cases of clinical snapping hip. A study looking at static and dynamic US in patients with clinical features of ’snapping hip’ showed that in only 9 out of 40 patients was snapping actually demonstrated on the US study⁽²²⁾. The transducer should be placed in a transverse oblique plane over the anterior hip, at the level of the iliopsoas tendon: the patient should then be asked to place their leg into the frog leg position: flexion, abduction, external rotation position, and then subsequently to extend the leg. During the first phase of the movement, the tendon itself rotates laterally and comes to lie anteriorly within the muscle. As the leg is extended, the tendon should then rotate smoothly back to its posteromedial position. In cases of the snapping hip, the tendon will abruptly flip back to its original posterior position rather than moving in a smooth fashion (Video 1).

The rectus femoris is the most commonly injured of the quadriceps muscles, with injury to the proximal tendon and myotendinous junction often seen in kicking sports such as soccer. The facts that the muscle spans over two joints, and has predominantly type II fibers and a long fusiform shape, all contribute to its susceptibility to injury⁽²³⁾. In young people, avulsion of the rectus femoris origin may be seen at the apophysis at the anterior inferior iliac spine. In this instance, a bony fragment may be identified on US, surrounded by anechoic fluid. Intrasubstance tearing of the proximal tendon and proximal myotendinous junction injury are more common in skeletally mature individuals. Either one head or both may be involved in proximal injury. Features of an acute muscle injury include ill-defined hyper- or hypoechoic lesions within the muscle associated with disruption of the muscle fibers⁽²⁴⁾. Subsequent scar formation with the healed or healing muscle will be seen as a hyperechoic focus within the muscle at the site of previous injury, with a varying degree of retraction of the muscle fibers⁽²⁴⁾.

Greater trochanteric pain syndrome (GTPS) consists of pain and tenderness over the greater trochanter, and it is diagnosed based on a combination of medical history, physical examination, and imaging findings. Risk factors include female gender in the sixth decade, femoral neck-shaft angle less than 134°, and leg length discrepancies⁽³³⁾. While GTPS was traditionally ascribed to trochanteric bursitis, it is increasingly recognized that gluteus tendinopathy and ITB pathology may be more significant contributors^(29,34,35). The pathophysiology may be analogous to shoulder impingement, with attrition of the iliotibial band (ITB) and fascia lata against the abductor tendons being similar to the friction of the acromion against the rotator cuff⁽²⁷⁾. Recent studies also suggest an association with acetabular morphology and hip snapping⁽³³⁾.

Hip abductor or gluteal tendinopathy most commonly occurs near the tendon insertions at the trochanteric facets. It includes peritendinitis, tendinosis, partial tear, and complete rupture, which probably represents a pathologic continuum rather than isolated entities⁽³⁵⁾. Soft tissue edema surrounding the gluteus minimus or gluteus medius tendons represents peritendinitis and is the earliest sign of gluteal tendinopathy⁽³⁵⁾; tendon architecture remains intact. Tendinosis refers to a degenerated tendon and is often seen in middle-aged individuals, both in athletes and sedentary subjects; females are more commonly affected compared to males⁽³⁶⁾. On US, this appears as a thickened and heterogeneous tendon with some loss of the normal fibrillar pattern, either with or without superimposed calcification^(29,34)(Fig. 14). Increased vascularity on color or power Doppler and tendon tenderness on sonopalpation are ancillary signs of tendinosis but are uncommon in clinical practice⁽³⁵⁾.

Fig. 14.

In Bunker and colleagues’ original description of hip rotator cuff tears and subsequent studies thereafter, gluteus tendon tears can occur during the following scenarios: (1) at the time of THA or hemiarthroplasty for fractures of the neck of the femur; (2) avulsion following THA performed via an anterolateral or trans-gluteal approach (3) chronic, non-traumatic tear in pre-existing tendinosis; and (4) trauma or spontaneously. Gluteus tendon tears often occur as the by-product of long-standing tendon degeneration or tendinosis rather than an acute traumatic event^(26,37,38). A partial tear is seen as focal discontinuity in the fibrillar pattern of the tendon⁽³⁴⁾. It can also present with discrete anechoic foci or intrasubstance clefts⁽²⁹⁾. A complete tear is a full-thickness interruption of the tendon fibers that may either be fluid-filled or depicted by loss of tendon mass⁽³⁴⁾. A “bald” facet and avulsed bone fragment are compatible with a complete tear^(26,35). Lengthening of the gluteus medius tendon between the musculotendinous junction and the insertion onto the greater trochanter which exceeds 2 cm is also an indirect sign of a tear⁽³⁵⁾. The anterior fibers of the gluteus medius tendon are the most common site of both partial and complete tears⁽²⁹⁾. The gluteus minimus tendon is rarely torn in isolation and is more commonly injured along with the gluteus medius tendon (Fig. 15)⁽²⁹⁾. Atrophy of the corresponding muscle may be seen in chronic partial or complete tears but it is nonspecific^(26,38). Previous investigations have shown age-related progression from tendinosis to tears with associated advancement in muscle atrophy in the absence of injury^(26,38). Cortical irregularity and enthesopathy of the greater trochanter are present in a vast majority of gluteus tendinopathy cases⁽²⁹⁾. Gluteus tendon tear is also an important cause of persistent hip pain after THA (Fig. 16). US is especially valuable as a post-operative imaging tool, allowing evaluation of the tendon without susceptibility artifact from the prosthesis or other hardware (Fig. 17).

Fig. 15.

Fig. 16.

Fig. 17.

Abductor tendinopathy may present with bursal distension or inflammation (i.e. bursitis)⁽²⁹⁾. The association of gluteus tendinopathy with trochanteric bursitis has been likened by a few authors to rotator cuff abnormalities with subacromial bursitis⁽³⁹⁾. Caution must therefore be practiced in diagnosing isolated bursitis without careful scanning and scrutiny of the abductor tendons. On US, a normal bursa is collapsed and just represents a potential space⁽⁴⁰⁾. In such cases, its position may only be inferred from adjacent structures, with the greater trochanteric bursa being superficial to the gluteus medius tendon and deep to the ITB. Murray and colleagues recommend dynamic US to localize this bursa by alternatively internally and externally rotating the hip. This maneuver demonstrates the gluteus medius and minimus tendons sliding back and forth under the relatively static iliotibial band, and may be especially useful during US-guided injections⁽⁴⁰⁾. Gentle transducer pressure must be applied to avoid displacement or compression of the small amount of bursal fluid. The primary advantage of MRI over US is demonstrating bony changes in the trochanter and marrow edema. Bursal location is variable; it may be located adjacent to the superoposterior facet of the greater trochanter or between the lateral facet and the iliotibial band⁽³⁵⁾. Compared to MRI, which shows gluteus tendinosis or trochanteric bursitis even in asymptomatic patients⁽⁴¹⁾, US enables concomitant clinical assessment by eliciting tenderness upon transducer pressure over the area of concern⁽³⁴⁾. Image-guided procedures performed to address lateral hip symptoms are directed towards managing GTPS either by steroid injection to the trochanteric bursa (Video 2) or dry needling the tendons⁽⁴²⁾.

External snapping hip syndrome refers to abnormal “jerky” movement or transient subluxation of the junction between the iliotibial band and the anterior margin of the gluteus maximus muscle over the greater trochanter⁽⁴³⁾. Most cases are idiopathic, though femoral osteochondroma and other space-occupying masses can be implicated^(43–45). Young individuals, especially those who engage in high-level athletic activities. are at increased risk⁽⁴³⁾. On US, findings include a thickened and hypoechoic iliotibial band, and a fluid-filled, inflamed trochanteric bursa⁽⁴³⁾. Dynamic evaluation of snapping is best performed on transverse scans (Video 3). Normal dynamic hip flexion and extension show a smooth gliding movement of the iliotibial band and anterior margin of the gluteus maximus muscle anteriorly during flexion and posteriorly during extension over the greater trochanter⁽⁴³⁾. Painful snapping is characterized by transient entrapment of the ITB and gluteus maximus against the posterolateral aspect of the greater trochanter during early flexion. With further flexion, the ITB and gluteus maximus escape and then suddenly move forward over the anterior edge of the lateral facet of the greater trochanter, producing the audible “snapping”⁴³⁾. The snapping may also be elicited while standing and leaning on the symptomatic side or during hip extension⁽⁴³⁾.

The lateral aspect of the hip is a common site of the Morel-Lavallée lesion, or posttraumatic hematoma/seroma that extends along the trochanteric region and the proximal thigh. Motor vehicle collisions are the most common mechanism; low-velocity crush injury and contact sports such as football and wrestling are other causes⁽⁴⁶⁾. In the lateral hip, the superficial tissues (subcutaneous fat and dermis) are characteristically mobile compared to the relatively firm and immobile ITB, placing these tissues at risk of shearing injury, with accumulation of hemolymphatic fluid between these planes⁽⁴⁶⁾.

This lesion can occur in isolation but is more commonly associated with underlying fractures⁽⁴⁶⁾. Clinical symptoms are pain, swelling, and a soft fluctuant area of contour deformity, which may have skin mobility⁽⁴⁶⁾ (Fig. 18). Injury to the cutaneous nerves may result in decreased cutaneous sensation over the area of internal degloving. On US, they appear as nonspecific, compressible and avascular fluid collections with heterogeneous echogenicity, dependent on the stage of evolution of the blood products⁽⁴⁶⁾(Fig. 19). Acute and subacute (<1 month) lesions have a heterogeneous appearance with irregular margins, whereas chronic lesions (>18 months) are frequently more homogeneous, with smooth margins and flat or fusiform shape⁽⁴⁶⁾. Light pressure with the transducer should be utilized when examining the collection to prevent compressing the bloody effusion away from the field-of-view⁽⁴⁾. Collections with atypical features – such as marked heterogeneity – may appear similar to soft-tissue tumors, and may be further evaluated on MRI⁽⁴⁾. On MRI, Morel-Lavallée lesions are classified into six types according to T1 and T2 appearances and enhancement features. Type I is a serohematic effusion or a seroma. Type II is a subacute hematoma, while type III is more chronic and organized hematoma. Types IV and V are both perifascial lesions. Type IV presents with perifascial dissection with closed fatty laceration, while type V manifests as a perifascial pseudonodular lesion. Type VI is an infected lesion with variable sinus tract formation, internal septations, and thick enhancing capsule. Although MRI is more optimal for classification, US contributes to treatment through image-guided intervention. US-aspiration, either with or without injection of sclerosing agents (doxycycline, erythromycin, alcohol, bleomycin, tetracycline, talc), has been shown to be helpful when compression banding fails⁽⁴⁶⁾.

Fig. 18.

Fig. 19.

US of the posterior hip is performed primarily to assess for hamstring tendon pathology. Other structures to be evaluated are the ischiofemoral space in the setting of impingement, the sciatic nerve and surrounding bursae. The patient is placed prone on the examination bed; the knee may need to be flexed to facilitate internal and external rotation during assessment of ischiofemoral impingement.

The gluteus maximus muscle is the most superficial structure at the posterior aspect of the hip. The deep gluteal space lies deep to this, bound by the posterior cortex of the femoral neck, the joint capsule and posterior acetabular column anteriorly, the ischial tuberosity and common hamstring origin inferiorly, the sacrotuberous and falciform fascia medially, the linea aspera and greater trochanter laterally, and the inferior margin of the greater sciatic notch⁽⁴⁷⁾. The ischial tuberosity and greater trochanter are easily appreciated on US. The space contains the short external rotator muscles (piriformis, superior gemellus, obturator internus, inferior gemellus, and quadratus femoris), fat, and multiple neurovascular structures, most importantly the sciatic nerve.

The common hamstring tendons originate from the ischial tuberosity. The biceps femoris and semitendinosus tendons attach to the posteromedial facet as a conjoint tendon, and the semimembranosus tendon attaches anterolaterally⁽⁴⁸⁾. Distal to the attachment, at the proximal third of the femur, the conjoint tendons migrate superficial to the semimembranosus tendon to end up laterally^(49,50) (Fig 20). The adductor magnus tendon is considered a ‘mini-hamstring muscle’ with an ischiochondylar origin that is indistinguishable from the hamstring tendons both functionally and on US⁽⁵¹⁾.

Fig. 20.

Depending on the patient’s body habitus and depth of the soft tissues, the structures of the posterior hip are visible to various degrees on US. The gluteal crease serves a useful landmark. A transducer with sufficient penetration placed transversely should allow identification of the subcutaneous fat and the gluteus maximus muscle, and deep to this, the ischial tuberosity as a horizontal hyperechoic line. The hamstring tendons are identified in cross-section as ovoid hypoechoic structures superficial to the hyperechoic cortex of ischial tuberosity, and in long axis as a short hyperechoic striated structure merging into the muscle belly. In cross section, the sciatic nerve appears as hyperechoic structure with a honeycomb appearance, lateral to the hamstring tendons. On US, the ischiofemoral space is typically interrogated at the level of the quadratus femoris muscle (Fig. 21). This is performed by moving the transducer slightly cranial and laterally from the ischial tuberosity until the ischiofemoral space with the intervening quadratus femoris muscle and sciatic nerve are visualized.

Fig. 21.