Pediatric patients with complex hip pain can be challenging to care for in many ways, as the exact source of pain is often difficult to discern. Extra-articular hip pain in young active children is common, with many possible etiologies. Teasing out pain generators clinically is often one of the most difficult jobs of the clinician caring for young patients with hip pain. Understanding the interplay between the biomechanics of the hip in young children and injury patterns, especially during periods of growth and development, is critical for making an accurate diagnosis and treatment plan of care. Pediatric athletes, in this light, place high physical demands on the hip joint routinely, including increased mechanical load to the physis, as in the ice hockey athlete and gymnast, and supraphysiologic ranges of motion, as in the dancer. Furthermore, rapid periods of growth and development result in peripelvic strength and flexibility imbalances. A comprehensive physical examination of the hip and peripelvic structures is a critical requirement in the care of patients with hip pain and extra-articular pathology. Overreliability on diagnostic studies is problematic because the prevalence of an asymptomatic bony deformity may be as high as 37% and up to 55% in athletes versus 23% in the general population.¹

Nonoperative care of the patient includes a thorough history, comprehensive knowledge of physical examination technique, and proper diagnostic testing. Dynamic hip ultrasonography and selective diagnostic and therapeutic ultrasound (US)-guided injections now play an important role in the assessment and treatment of young patients with hip pain and injury. The aim of this chapter is to review the evaluation and treatment of pediatric patients with extra-articular hip pain, including the role of diagnostic and dynamic ultrasonography and selective injections.

Injury to the hip flexor tendons in young active children is a common cause of extra-articular hip pain. In athletes that rely heavily on the hip joint for speed and skill, injuries to the hip flexor tendons can be even more prevalent. For example, in soccer, groin injuries account for 8% to 18% of all injuries, with an incidence of 0.4 to 1.3 groin injuries per 1000 hours of exposure.²,³,⁴,⁵,⁶,⁷ The iliopsoas musculotendinous unit is located directly anterior to the hip joint and is composed of the iliacus, psoas major, and psoas minor.⁸,⁹ The most common cause of groin injury stems from the adductor, followed by the iliopsoas.⁷,¹⁰,¹¹,¹² The iliopsoas has recently been implicated in 25% of acute groin injuries.² Other studies have, similarly, indicated that iliopsoas disorders were found in 25% to 30% of athletes with acute groin injury and 12% to 36% of athletes with chronic groin pain.²,⁸,¹³,¹⁴ Common injury mechanisms for injury to the iliopsoas involve kicking, sprinting, and change of direction in sports such as soccer, basketball, and handball.²,¹³ In artistic performing athletes, repetitive and extreme range of hip flexion is often inherent to the discipline. Injury and pain related to a snapping iliopsoas, also known as the “dancer’s tendon” was found in 58% of dancers.¹⁵ If not diagnosed and treated appropriately, iliopsoas injuries can become chronic and greatly impair sport performance.⁷,¹⁰,¹⁴

The anatomy of the iliopsoas complex includes the confluence of three main structures, namely, the iliacus, psoas major, and psoas minor.⁸ The main origin of the psoas major includes the vertebral bodies, intervertebral discs, and transverse processes of T12-L5.⁸ The psoas minor originates from the vertebral bodies of T12 to L1 and is present in roughly 60 to 65% of individuals.¹⁶ Recent cadaveric studies have identified five individual muscle bundles comprising the iliopsoas muscle complex,¹⁷ and
US studies have documented four of the five bundles reported in cadaveric studies.¹⁸ Cadaveric studies have reported a bifid tendon in up to 4 out of 24 cases,¹⁷ and magnetic resonance (MR) studies of children revealed bifid tendon anatomy in 26% of cases imaged and bilateral bifid tendon anatomy in 14% of cases.¹⁹

The anatomy of the rectus femoris involves a confluence of two main tendon insertions, designated as the direct head and indirect head. A third head is described arising from the inferior edge of the indirect tendon, merging superficially with the gluteus minimus muscle and deeply with the lateral iliofemoral ligament.²⁰

The comprehensive examination of the iliopsoas/rectus femoris complex is performed in both a static and a dynamic manner. Notably, significant variations exist concerning physical examination tests performed by hip specialists and interpretation of the findings. Careful palpation of the anterior soft tissue structures is an important component of the hip examination. Hammoud et al.²¹ bring to light the layered complex structures of the hip and the importance of recognizing compensatory injury patterns associated with hip joint pathology. In this light, pain with palpation of the iliopsoas tendon complex may be indicative of iliopsoas tendinitis, iliopsoas bursitis/impingement, and or injury and inflammation of the underlying joint structures. Taken together with dynamic provocative physical examination tests and US static and dynamic imaging, clinicians can improve the accuracy of diagnosis in patients with complex hip pain.

The hip tests most commonly performed consistently include flexion external and internal range of motion (ROM); log roll; flexion, adduction, internal rotation (FADIR); straight leg raise against resistance (Stinchfield test); flexion, abduction, external rotation (FABER), femoral anteversion analysis, and standing gait analysis.²² Of these maneuvers, the Stinchfield test is a test of iliopsoas strength, as well as a sign of inter-articular pathology, as the psoas places pressure on the underlying structures, including the acetabular labrum.²² Other helpful tests for iliopsoas pathology include hip extension internal rotation (iliopsoas stretch test), FABER against resistance, and seated resisted straight leg raise (Ludloff test).

Injury to the rectus femoris may present as pain anteriorly, in the case of injury to the direct head and conjoined tendon, and laterally, in the case of injury to the indirect head. Palpation of the direct head along with selective provocation tests for pain, including seated hip flexion against resistance, and Ludloff test, can help support or negate injury to the rectus tendon. Clinically, injury to the rectus femoris complex may present with insidious onset of anterior pain with chronic injury, inflammation, and tear versus acute traumatic rupture of the direct head, with swelling, deformity, and pain anteriorly.

Clinical management of iliopsoas/rectus femoris pathology consists of rest, activity modification, nonsteroidal anti-inflammatories, physical therapy, and corticosteroid injections.⁸ In cases where there is notable tendinopathy, emphasis on eccentric strengthening is important. When nonoperative treatment is unsuccessful, iliopsoas tendon release, along with correction of existing underlying Intra-articular (IA) abnormality can be performed.⁸,²³,²⁴,²⁵,²⁶,²⁷

Iliopsoas impingement commonly occurs in active young females, and is provoked by activities involving hip flexor tendon use, including walking distances, stair climbing and prolonged sitting ordriving.⁸ Iliopsoas snapping is more common in females²⁴ and has been reported to occur in up to approximately 60% of elite-level dancers¹⁵ and 5 to 10% in the general population.²⁸ Physical examination reveals a pain with hip excursion, positive impingement test, positive scour test, iliopsoas provocative tests, including Stinchfield and Ludloff testing. Commonly, there is tenderness to palpation directly over the anterior hip and iliopsoas complex.

Dynamic US evaluation can be very helpful in making the diagnosis. Dynamic hip excursion may reveal iliopsoas tendon snapping over the iliopectineal eminence when the patient flexes, abducts, externally rotates, and extends the hip.²⁸ Deslandes et al. describes movement of the iliopsoas tendon abruptly into the iliopsoas muscle complex during active hip motion.²⁹ Iliopsoas impingement refers to anterior labral injury seen during arthroscopy, as a result of impingement and trauma to the labrum by the iliopsoas tendon.³⁰ Iliopsoas impingement may be demonstrated using dynamic ultrasonography of the hip joint during hip flexion and may show compression of the acetabular labrum by the muscle tendon unit of the iliopsoas and/or rectus femoris. Domb et al. identified 25 patients with direct anterior labral tears at the 3 o’clock position (right hip) in the absence of bony abnormalities.³¹ Similarly, Blankenbaker et al. reported MR findings of iliopsoas impingement in patients diagnosed during hip arthroscopy and found that the labral injury typically occurs at the 3 o’clock position.³⁰ Labral tears occurring at the 3 o’clock position should therefore suggest iliopsoas impingement, especially in the setting of female athletes with normal bony anatomy.

The iliopsoas bursa is a potential space that lies just deep to the iliopsoas muscle tendon complex and superficial to the joint. The iliopsoas bursa is often compressed with repetitive hip motion, resulting in thickening and irritation to the underlying structure. The iliopsoas bursa has been reported to communicate with the underlying joint in 15% of people.⁸ US imaging of the hip is very useful for imaging of the iliopsoas bursa in both the transverse and sagittal planes. Iliopsoas bursitis appears as thickening of the bursa with or without the presence of a hypoechoic center, signifying the presence of fluid.

The normal US anatomy has been documented with four bundles of the iliopsoas muscle (inferior, medial, lateral,
ilioinfratrochanteric muscle) seen in all study subjects, with the psoas major tendon depicted as a thick hyperechoic ovoid structure, when imaged in the transverse plane, resting against the anterior aspect of the superior pubic ramus, acetabular rim, or the upper coxofemoral joint capsule.¹⁸

Iliopsoas peritendinous injection with US guidance for diagnostic and therapeutic reasons is useful and safe when performed by an experienced sonographer. Iliopsoas peritendinous injections have been shown to provide both short-term (6 weeks)³² and sustained (2 to 10 months) pain relief.³³

US of the rectus femoris complex begins with the anterior hip and direct tendon in the neutral position. The axial plane depicts the direct head nicely, beginning at the anterior superior iliac spine (ASIS) and progressing to the anterior inferior iliac spine (AIIS). Imaging the tendon in the longitudinal plane is important and can be done by rotating the transducer 90° in the sagittal plane. This view allows for US examination of the insertion of the tendon as well as morphologic characteristics of the AIIS (see subspine impingement section).²⁰ US of the indirect head can be achieved by scanning at the level of the conjoined tendon in the sagittal plane and applying a curvilinear ascending motion following the path of the indirect head from distal to the origin of the supraacetabular ridge at the lateral hip.²⁰ A lateral approach to imaging the indirect head has been described scanning the lateral hip in the axial plane, just lateral to the AIIS, and localizing the indirect head deep in the gluteus minimus and medius muscles and overlying the supraacetabular ridge and lateral iliofemoral ligament.²⁰,³⁴ Bianchi et al. utilized cadaveric anatomic dissection of the rectus femoris to confirm the presence of the central aponeurosis as a sagittally oriented fibrous band located within the proximal two-thirds of the muscle belly. This same study has been described in vitro US showing the central aponeurosis as a curvilinear hyperechoic structure.³⁵