Musculoskeletal ultrasound of the hip

INTRODUCTION — Diagnosis of hip region disorders can prove challenging based on history and physical examination alone. Ultrasound has proven to be a useful clinical tool, especially when patient complaints are dynamic in nature (ie, snapping hip), as static imaging may be unrevealing. Although the relatively deep location of many hip structures creates challenges, appropriate transducer selection and image optimization allows for adequate imaging of many structures.

This topic will describe a systematic approach to complete sonographic evaluation of each hip quadrant. Topics devoted to hip pain and specific hip conditions are found separately:

●General hip pain assessment and diagnosis – (See "Approach to hip and groin pain in the athlete and active adult" and "Approach to the adult with unspecified hip pain" and "Approach to hip pain in childhood" and "Musculoskeletal examination of the hip and groin".)

●Specific hip conditions and injuries – (See "Greater trochanteric pain syndrome (formerly trochanteric bursitis)" and "Overview of common hip fractures in adults" and "Evaluation and management of slipped capital femoral epiphysis (SCFE)" and "Osteitis pubis".)

USES, ADVANTAGES, AND LIMITATIONS OF HIP ULTRASOUND — Below, the advantages and limitations of the ultrasound examination of the hip are reviewed; the general advantages and limitations of musculoskeletal ultrasound are discussed separately. (See "Musculoskeletal ultrasound of the shoulder".)

Ultrasound offers many advantages about the hip. Its superior spatial resolution allows for detailed evaluation of muscle, tendon, and nerve disorders that may not be apparent on other imaging modalities. Mechanical complaints of snapping or clicking can be assessed in real time, and the location of pain can be correlated with precise anatomic structures by palpation with the ultrasound transducer (ie, sonopalpation). Another advantage of ultrasound is the lack of artifact associated with orthopedic hardware, allowing for evaluation of surrounding structures for impingement or irritation. When a specific structure is identified as a potential cause of pain, this can be verified by performing an anesthetic injection under ultrasound guidance.

Hip ultrasound has some limitations. Many hip structures lie deep below soft tissues, which can lead to suboptimal imaging, especially when combined with an unfavorable body habitus. Evaluation of intra-articular structures is an inherent limitation of ultrasound technology, and, although ultrasound may play an important role in evaluation of hip joint disorders, other advanced imaging modalities may be required for complete evaluation.

HIP ANATOMY — For the purposes of sonographic evaluation, the hip is divided into anatomic quadrants: anterior, lateral, medial, and posterior. These quadrants are divided by the long-axis of the femoral neck.

●Anterior – Structures evaluated in the anterior hip quadrant include: the hip joint (picture 1), including femoral head, neck, capsule, anterior synovial recess, and anterior labrum; femoral nerve (figure 1); femoral artery and vein (figure 2); iliopsoas muscle, tendon, and bursa (figure 3 and figure 4); sartorius muscle and tendon; tensor fascia lata muscle and tendon (figure 5); lateral cutaneous nerve of thigh; and rectus femoris muscle and tendon.

●Lateral – Structures evaluated in the lateral hip quadrant include: gluteus maximus and tensor fascia lata insertion onto the fascia lata/iliotibial band (figure 5); gluteus minimus muscle and tendon (figure 6); gluteus medius muscle and tendon; greater trochanteric bursa (subgluteus maximus bursa) (figure 7).

●Medial – Structures evaluated in the medial hip quadrant include: femoral nerve, artery, and vein (figure 1 and figure 2); adductor longus muscle and tendon (figure 8); adductor brevis muscle and tendon; adductor magnus muscle and tendon; gracilis muscle and tendon; distal iliopsoas tendon; pubic bone and symphysis (figure 9); distal rectus abdominis muscle and tendon (figure 10).

●Posterior – Structures evaluated in the posterior hip quadrant include: gluteus maximus muscle and tendon (figure 11 and figure 6 and figure 12); gluteus medius muscle and tendon; deep short external rotators (as indicated) (figure 13); hamstring muscles and tendons (figure 14); ischial tuberosity and ischiogluteal bursa (figure 6); sciatic nerve (figure 13); posterior hip joint (as indicated).

Hip anatomy is discussed in greater detail separately. (See "Musculoskeletal examination of the hip and groin", section on 'Anatomy and biomechanics'.)

ULTRASOUND EXAMINATION OF THE HIP

Guidelines, structures to image, and positioning — The sonographic examination of the hip is organized into quadrants: anterior, medial, lateral, and posterior. It is often appropriate to focus the sonographic examination on the quadrant of the patient's complaints, but other structures may need to be evaluated as clinically indicated, and some limited examinations may involve more than one quadrant (such as assessment of the hip joint). A list of the structures to be evaluated in each quadrant is provided above. This list is based upon the American Medical Society for Sports Medicine (AMSSM) Recommended Sports Ultrasound Curriculum for Sports Medicine Fellowships and the American Institute of Ultrasound in Medicine (AIUM) Guidelines for Performance of the Musculoskeletal (MSK) United States Examination [1-3]. The approach described is consistent with that of the European Society of Musculoskeletal Radiology [4,5]. (See 'Hip anatomy' above.)

A high frequency (eg, 12 to 5 MHz) linear array transducer is preferred for the diagnostic evaluation of the hip region, unless a large body habitus makes this infeasible. However, it is common to require a combination of both high frequency linear and low frequency (eg, 5 to 1 MHz) curvilinear array transducers for complete evaluation in the adult. All structures should be imaged in both short and long axis and any possible pathology confirmed with orthogonal imaging. Doppler evaluation may be needed to evaluate possible synovitis, bursitis, and intratendinous neovascularization, as commonly seen in chronic tendinosis.

Anterior hip region — To examine the anterior hip, the patient lies supine with their lower limb in a neutral position.

Sagittal oblique (parallel to long-axis of femoral neck) — The transducer is aligned in the sagittal oblique plane, distal and medial to the anterior superior iliac spine (ASIS) (picture 2). Alternatively, the transducer can be placed in the transverse plane at the mid-femur and moved superiorly until the femoral head is visualized, and then rotated into a sagittal oblique plane.

With the transducer in this position, the femoral head is visualized as a hyperechoic structure with a characteristic curved appearance. The superior portion of the image shows the acetabulum, acetabular labrum, and articular cartilage (image 1). The acetabular labrum has a characteristic fibrocartilaginous appearance, seen as a homogenous hyperechoic triangular structure. Careful scanning technique is required to eliminate anisotropy when evaluating the acetabular labrum. Superficial to these structures, overlying the femoral head, is the anterior capsule profile.

The anterior synovial recess can be visualized inferior at the junction of the femoral head and neck and is composed of an anterior and posterior layer (image 1). The anterior layer is relatively thicker, as it is reinforced by the zona orbicularis and capsular ligaments. The normal thickness of the anterior synovial recess is between 4 to 6 mm [6,7]. However, these values are based on a pediatric population (normal: 4.9 mm; synovitis: 9.9 mm), as studies in adults have not been undertaken [7]. Thus, the sonographer should not rely on absolute values, but rather should compare the thickness with the contralateral side, and also assess for a hypoechogenic region between both layers (anterior and posterior) of the anterior synovial recess suggestive of an effusion.

Transverse oblique — Once scanning is completed in the sagittal oblique plane, the transducer is rotated 90 degrees into the transverse oblique plane. The transducer is then moved superiorly in line toward and parallel with the inguinal ligament (picture 3). In this position the hyperechoic line of the anterior inferior iliac spine is visualized laterally, and the iliopectineal eminence medially (image 2). The iliopsoas muscle is superficial to the iliac bone. The iliopsoas tendon resides in the deep medial portion of the iliopsoas muscle and deep to this tendon is the iliopsoas bursa. The femoral nerve and femoral artery can be visualized from this position medial to the iliopsoas muscle (image 2). The iliopsoas bursa may be distended and seen as an anechoic fluid collection deep and medial to the iliopsoas tendon. A connection between the iliopsoas bursa and hip joint has been described [8,9]. Thus, when an iliopsoas bursopathy is noted, intra-articular causes should be considered, particularly osteoarthritis [10]. Plain radiographs obtained prior to the ultrasound examination allow the clinician to identify signs of hip osteoarthritis and enable correlation with findings of iliopsoas bursitis seen on ultrasound.

To visualize the femoral vessels completely, the transducer is moved medially. In this position the femoral nerve is the lateral-most structure (medial to iliopsoas muscle), and the femoral artery is medial to the nerve. The femoral vein resides deep and medial to the femoral artery. Color or Doppler imaging can be used to identify the femoral artery and vein (image 3).

Transverse — The ASIS should then be localized with palpation, and the transducer placed over it in a transverse orientation (picture 4). Centering the ASIS over the middle of the screen, the tensor fasciae latae (TFL) tendon is lateral to the ASIS, and the sartorius tendon is medial to the ASIS (image 4). Moving the transducer distally, both tendons can be scanned in short-axis (transverse plane), providing views of the myotendinous junction and the muscle (image 5). By rotating the transducer and moving laterally (TFL) or medially (sartorius), both tendons and muscles can be visualized in long-axis (sagittal plane) (image 6 and image 7).

The lateral cutaneous nerve of thigh (formerly referred to as the lateral femoral cutaneous nerve) travels between the sartorius and tensor fasciae latae muscles, and can be visualized in the transverse plane when scanning the proximal portion of these muscles (image 8). The nerve can be traced proximally to the inguinal ligament. When doing so the transducer is rotated into a transverse oblique plane parallel to the inguinal ligament.

The rectus femoris muscle can be visualized deep to the sartorius muscle while scanning in the transverse plane (image 9). Once the muscle is visualized, the transducer can be moved superiorly to bring the myotendinous junction and the direct and indirect heads of the rectus femoris tendon into view (image 10). With the transducer centered over the direct head of the rectus femoris tendon at its origin on the AIIS (image 11), the transducer can be rotated 90 degrees into the sagittal plane to visualize the direct head of the rectus femoris tendon in long-axis (image 12). Moving the transducer laterally reveals the refraction shadow of the indirect head of the rectus femoris tendon (image 13). From this position the transducer is rotated into a coronal oblique plane (picture 5), with the hyperechoic line of the acetabulum visualized superiorly, to provide the best view of the indirect head of the rectus femoris tendon in long-axis (image 14). The transducer can be moved distally over the rectus femoris muscle in short-axis to visualize the superficial tendon of the rectus femoris (direct head), and central tendon/aponeurosis of the rectus femoris (indirect head). The tendons and rectus femoris muscle can also be visualized in long-axis (sagittal plane).

Dynamic scanning of snapping hip — In cases where internal, extra-articular snapping hip (coxa saltans) is suspected, a dynamic ultrasound scan of the anterior hip in the transverse oblique plane can be performed to visualize the iliopsoas muscle and tendon. The position of the transducer is identical to that described above for imaging these structures. In this position, the patient can be asked to reproduce the movements that create the snapping sensation, during which the abrupt movement of the iliopsoas tendon can be visualized with ultrasound and felt through the transducer (movie 1); the anatomy depicted in the movie can be reviewed in the image below it [11]. If the patient cannot reproduce the snap themselves, they can be brought into neutral starting from a flexed, abducted, and externally rotated hip position.

Lateral hip region

General examination — Ultrasound examination of the lateral hip region is performed with the patient lying on their side and the hip flexed 20 to 30 degrees. The scan begins with the transducer in a transverse plane relative to the lateral mid-femur (picture 6). From this position, the transducer is moved superiorly, until the bony protuberance of the apex of the greater trochanter between the anterior and lateral facets is visualized as a hyperechoic line (image 15).

Anterior to the apex is the anterior facet of the greater trochanter, and in a transverse oblique plane the gluteus minimus tendon is visualized in short axis, with the gluteus minimus bursa deep to the tendon (image 16). The transducer can then be rotated 90 degrees, with the anterior facet appearing as a hyperechoic line (image 17). In this view, the gluteus minimus tendon is visualized in long-axis; superficial to the gluteus minimus tendon is the gluteus medius muscle, which is visualized in oblique long-axis.

After returning the transducer to the transverse plane over the apex of the greater trochanter, the transducer is moved posteriorly to visualize the hyperechoic line of the lateral facet, and the gluteus medius tendon (short-axis) inserting onto the lateral facet (image 18). Deep to the gluteus medius tendon is the potential space for the gluteus medius bursa. The transducer can be rotated 90 degrees to image the gluteus medius tendon in long-axis (image 19).

While imaging the gluteus medius tendon insertion onto the lateral facet in short-axis, the transducer can be moved further posterior to visualize the hyperechoic line of the superior posterior facet, where a thicker (in comparison to lateral facet) portion of the gluteus medius tendon inserts (image 20) [12]. From this position, the transducer can be rotated 90 degrees, to visualize the tendon in long-axis. While imaging the gluteus medius tendon insertion onto the superior posterior facet in short-axis, the gluteus maximus muscle can be visualized superficially. In between the gluteus maximus muscle and gluteus medius tendon is a hypoechoic line representing the anterior extension of the trochanteric bursa (subgluteus maximus bursa). The remainder of this large bursa resides along the posterior facet deep to the gluteus maximus muscle. The tissue interface where the trochanteric bursa resides can be accentuated with passive internal and external rotation of the hip.

While imaging the trochanteric bursa and gluteus medius muscle in a transverse plane, the attachment of the gluteus maximus tendon to the iliotibial band can also be visualized (image 18). The iliotibial band is superficial to the gluteus medius and minimus tendons, and can be visualized by moving anteriorly in a transverse plane form the point of insertion of the gluteus maximus tendon to the iliotibial band. As the transducer is moved anteriorly, the myotendinous junction of the tensor fasciae latae is visualized in short-axis as it inserts onto the iliotibial band. The transducer should then be rotated 90 degrees and translated superiorly to visualize the iliotibial band in its long-axis and evaluate the origin at the ilium (image 21).

Dynamic scanning of snapping hip — In cases where external snapping hip (external coxa saltans) is suspected, the apex of the greater trochanter is visualized in a transverse scan. The patient is asked to reproduce their symptoms (eg, shift weight onto affected limb while standing, or flex and extend the hip while lying on their contralateral side). While the patient reproduces their symptoms, the ultrasonographer looks for abrupt movement of the iliotibial band over the greater trochanter (movie 2).

Medial region

Abducted-externally rotated (frog leg) — The adductor mass can be palpated in the medial region of the proximal thigh. The transducer is placed in the transverse plane in this region (picture 7), and running from superficial to deep the adductor longus, adductor brevis, and adductor magnus muscles are visualized in short-axis (image 22). The anterior obturator nerve can be visualized in the fascial plane between the adductor longus and brevis muscles. The posterior obturator nerve can be visualized in the fascial plane between the adductor brevis and magnus muscles. The transducer is then moved superior to visualize the myotendinous junctions of the three adductors, and their tendinous origin at the pubic tubercle. With one end of the transducer fixated on the pubic tubercle, the transducer can be rotated 90 degrees to visualize the three adductor tendons in long axis (image 23).

If the transducer is then moved superiorly the insertions of the adductor tendons and rectus abdominus tendon to the pubic tubercle is visualized (image 24). The fibrocartilaginous plate superficial to the pubic tubercle is visualized at this point, which is contiguous with the rectus abdominus tendon insertion and adductor longus tendon insertion. Moving the transducer further superior will visualize the rectus abdominus tendon and muscle in long-axis. At this position, the transducer can be rotated 90 degrees to visualize the rectus abdominus muscle in short-axis (image 25). On the medial border of the rectus abdominus muscle is the linea alba, separating the muscle from the contralateral rectus abdominus muscle. By moving the transducer medially to position the linea alba in the middle of the screen, and sliding the transducer inferior, the pubic symphysis and bones are brought into view (image 26).

Flexion-abduction-external rotation (FABER) — The distal iliopsoas tendon is best viewed with the patient in the FABER position [13]. The transducer is placed over the femoral head in a coronal oblique plane, to approximate the course of the distal iliopsoas tendon (picture 8). In this plane the lesser trochanter is visualized as a hyperechoic line; the distal iliopsoas tendon can be visualized inserting onto the lesser trochanter (image 27).

Posterior region (prone with or without pillow under hips) — The transducer is placed in the transverse plane on the mid-posterior thigh (picture 9). In this plane, in short-axis the semimembranosus muscle is visualized medially, with the semitendinosus muscle lateral to it, and the long head of biceps femoris muscle lateral to the semitendinosus muscle (image 28). The sciatic nerve can be visualized deep to the junction of the semitendinosus muscle and long head of biceps femoris muscle, and the short head of biceps femoris muscle deep to the long head of biceps femoris muscle. Scanning inferior to the mid-thigh reveals enlargement of the short head of biceps femoris and semimembranosus muscles, and a reduction in size of the long head of biceps femoris and semitendinosus muscles. Conversely, when scanning superior to the mid-thigh, the former two muscles become smaller, and the latter two are relatively larger.

As the transducer is moved superior from the mid-thigh, the semitendinosus and long head of biceps femoris tendons converge to become the conjoint tendon, with the former medial to the latter (image 29). The semimembranosus tendon, initially deep to the semitendinosus tendon, courses obliquely to originate lateral to the conjoint tendon off the ischial tuberosity. The sciatic nerve, which is deep to the conjoint tendon and lateral to the semimembranosus tendon inferiorly, courses lateral with the semimembranosus tendon as the transducer is moved superiorly (image 29). The ischiogluteal bursa is the potential space between the ischial tuberosity and the gluteus maximus muscle.

The transducer is then moved lateral from the hamstring origin at the ischial tuberosity to visualize the hyperechoic contour of the lesser trochanter of the femur (image 30). The muscle coursing between these structures is the quadratus femoris and the sciatic nerve is readily identifiable lying just superficial to the muscle at this level. This represents the ischiofemoral space which can be prone to impingement. Slight superior translation of the transducer will bring the femoral head into view and the posterior hip joint can be evaluated for presence of effusion. The lateral edge of the transducer is then fixed on the greater trochanter, and the medial edge of the transducer is pivoted superior toward the greater sciatic foramen. This will bring the piriformis muscle into view (image 31). The piriformis muscle can be seen coursing through the sciatic foramen, deep to the gluteus maximus muscle and inserting onto the greater trochanter.

To differentiate the piriformis muscle from the overlying gluteus maximus muscle, the hip can be passively internally/externally rotated, causing the piriformis muscle to lengthen and shorten, with no concomitant movement of the gluteus maximus muscle. At this position, the sciatic nerve runs deep to the piriformis muscle. From this position, the lateral edge of the transducer is fixated on the greater trochanter, and the medial edge is pivoted superiorly to the crest of the ilium. The transducer is then moved superiorly until the medial edge of the transducer is on the crest of the ilium. In this position, the gluteus maximus muscle is visualized in long axis and superficial to the gluteus medius muscle (long-axis). The muscles can be traced distally to the greater trochanter region in long-axis, or short-axis, by rotating the transducer 90 degrees.

ADDITIONAL ULTRASOUND RESOURCES — Instructional videos demonstrating proper performance of the ultrasound examination of the hip and related pathology can be found at the website of the American Medical Society for Sports Medicine: anterior hip US examination, lateral hip US examination, posterior hip US examination, medial hip US examination, sports US hip pathology, US guided interventional procedures of the hip. Registration must be completed to access these videos, but no fee is required.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Musculoskeletal ultrasound".)

SUMMARY AND RECOMMENDATIONS

●Use and limitations of ultrasound hip examination – Ultrasound is a useful clinical tool for the assessment of many injuries and conditions of the hip, especially when complaints are dynamic (eg, snapping hip). Although the relatively deep location of many hip structures creates challenges, appropriate transducer selection and image optimization allows for adequate visualization of many structures. Nevertheless, incomplete evaluation of intra-articular structures is an inherent limitation of ultrasound technology. While ultrasound may play an important role, other advanced imaging techniques may be required for complete evaluation. (See 'Uses, advantages, and limitations of hip ultrasound' above.)

●Anatomy – An understanding of the anatomy of the hip and surrounding structures is essential for interpreting ultrasound images. A brief outline of the structures assessed as part of the fundamental hip examination is provided above; detailed descriptions of hip anatomy and biomechanics are provided separately. (See "Musculoskeletal examination of the hip and groin", section on 'Anatomy and biomechanics'.)

●Transducer selection – A high frequency linear array transducer is preferred for evaluation of the superficial structures of the hip when body habitus allows. However, a low-frequency curvilinear array transducer is often required for adequate imaging of deep structures or in larger patients. (See 'Guidelines, structures to image, and positioning' above.)

●Examination performance – Sonographic evaluation of the hip is organized into a quadrant system as described in the text. The major quadrants and structures examined include the following:

•Anterior – Structures evaluated in the anterior hip quadrant include: the hip joint (picture 1), including femoral head, neck, capsule, anterior synovial recess, and anterior labrum; femoral nerve (figure 1); femoral artery and vein; iliopsoas muscle, tendon, and bursa (figure 3); sartorius muscle and tendon; tensor fascia lata muscle and tendon (figure 5); lateral cutaneous nerve of thigh; and rectus femoris muscle and tendon.

•Lateral – Structures evaluated in the lateral hip quadrant include: gluteus maximus and tensor fascia lata insertion onto fascia lata/iliotibial band (figure 5); gluteus minimus muscle and tendon; gluteus medius muscle and tendon; greater trochanteric bursa (subgluteus maximus bursa) (figure 7).

•Medial – Structures evaluated in the medial hip quadrant include: femoral nerve, artery, and vein; adductor longus muscle and tendon (figure 8); adductor brevis muscle and tendon; adductor magnus muscle and tendon; gracilis muscle and tendon; distal iliopsoas tendon; pubic bone and symphysis; distal rectus abdominis muscle and tendon.

•Posterior – Structures evaluated in the posterior hip quadrant include: gluteus maximus muscle and tendon (figure 11 and figure 6 and figure 12); gluteus medius muscle and tendon; deep short external rotators (as indicated) (figure 13); hamstring muscles and tendons (figure 14); ischial tuberosity and ischiogluteal bursa; sciatic nerve (figure 13); posterior hip joint (as indicated).