The psoas major muscle is fusiform and originates through a series of fibrous arches, from the lateral aspect of the bodies of the last thoracic vertebra and the first four lumbar vertebrae and from the interposed intervertebral discs and the base of the transverse processes of the first four lumbar vertebrae. It continues caudally and laterally through the lumbar and iliac region. It exits the pelvis passing under the inguinal ligament, between the anterior inferior iliac spine and the iliopectineal eminence. It then passes anterior to the hip joint and inserts into the lesser trochanter, after a characteristic rotation in its course, so that its ventral surface becomes medial [4, 5].
The iliopectineal bursa, the largest in the human body, is interposed between its robust distal tendon and the hip joint fibrous capsule [1, 5] (Fig. 15.2).
The fan-shaped iliacus muscle originates from the inner lip of the iliac crest, the two anterior iliac spines, and from the notch interposed between them, the upper 2/3 of the iliac fossa, the iliolumbar ligament, and the lateral portion of the sacral wing.
From this extensive origin line, the muscular fibers converge inferiorly and terminate by partially merging with those of the psoas major muscle, while the most lateral muscle fibers insert on the lesser trochanter without joining with the main tendon.
The iliopsoas muscle is innervated by branches of the lumbar plexus and then the femoral nerve (L1–L4) .
15.3 Symptoms and Clinical Examination
Iliopsoas tendinopathy occurs most frequently as a result of overuse and, as we have already noted, more often in those sports that require repetitive hip flexion and external rotation movements .
More rarely, the pain starts following a single eccentric iliopsoas contraction. In this case the pain is most frequently caused by a myotendinous junction lesion of the iliopsoas itself.
This chapter includes: the insertional tendinopathy with inflammatory characteristics affecting the distal bone-tendon junction at the level of the lesser trochanter; the peritendinitis with possible and frequent involvement of the iliopectineal mucous bursa that is anterior to the hip joint capsule and lies between the iliopsoas tendon and the bone margin of the iliopubic branch; the tendinosis, actual fibrillar tissue degeneration of the tendon; the peritendinitis and tendinosis: the partial or total (very rare) lesion of the tendon itself.
The iliopsoas muscle tendon unit may also be affected by true injuries to the muscle or to the myotendinous junction by direct or, more frequently, by indirect trauma. If not properly rehabilitated, reaching full extensibility and strength recovery, it could then cause tendinopathies of the iliopsoas tendon or lead to “internal snapping hip.”
Internal snapping hip is the patient’s perception of a “clicking” sound in the groin also audible by the operator, which can be more or less painful depending on the degree of inflammation of the iliopsoas tendon [1, 6–8].
The snapping occurs when moving the hip joint from a position of flexion, extra-rotation, and hip abduction to a position of extension, intra-rotation, and adduction, and it can be perceived when rising from sitting to standing, especially with the hip at an acute angle (sitting low) .
The snapping is due to the rubbing of the iliopsoas tendon on the iliopectineal eminence or to the movement of the psoas tendon onto the iliac tendon.
The Thomas test can be positive [9, 10] (Fig. 15.3).
Thomas test. The Thomas test is a physical examination test used to highlight short hip flexor and iliopsoas tendinopathy. The patient lies supine on the examination table and holds the uninvolved knee to the chest while allowing the involved extremity to lie flat. Holding the knee to the chest flattens out the lumbar lordosis and stabilizes the pelvis. If the iliopsoas muscle is shortened, the lower extremity on the involved side will be unable to fully extend the hip, which will remain raised from the couch
This condition must be differentiated from the hip snap due to intra-articular pathologies such as injuries of the acetabular labrum or as cartilaginous loose bodies.
The snapping hip can also be due to an “external snapping hip” caused by the tensor fasciae latae muscle tendon passage on the femoral greater trochanter, diagnosed by the Ober test [1, 9, 10].
In iliopsoas tendinopathies, the pain is located in the groin and below the pubic area with possible radiation to the medial thigh.
Most often the pain occurs subtly and does not prevent any sport activity.
Subsequently a progressive increase in the intensity of pain is reported during sporting activity with pain even at rest. This establishes compensatory postural mechanisms which over time determine changes in pain characteristics giving rise to pain in other locations of the pelvis as well (contralateral, suprapubic, etc.).
It can be difficult to understand whether a groin pain comes from iliopsoas or adductor longus tendinopathy or another tendon which inserts onto the pubis.
The contraction tests against manual resistance can be helpful.
A positive iliopsoas contraction test elicits pain with flexion at the externally rotated hip, and the knee fully extended is positive, while adductor and abdominal contraction against resistance is less painful and shows less functional impairment [11, 12].
The Liedloff test is positive with the patient seated and the knee extended as the patient is asked to lift the heel off the ground causing pain and functional impairment on the affected side.
A characteristic snap is classically caused by passing from a flexed hip position, abducted and externally rotated, to an extended one, adduced and internally rotated.
When evaluating a patient with iliopsoas tendinopathy, the back and pelvis mechanics should be checked.
In fact the psoas major muscle is the only one that connects the spine with the lower limb. Besides being a flexor and a femur rotator, it is also a spine flexor and an important postural muscle activated during many movements and daily activities .
It is in fact a muscle with a prevalence of type 1 fibers (slow twitch) .
In the trunk flexion movement (Piédallu test), patients with reduced iliopsoas extensibility tend to have an anterior ileum with an increased excursion of the ipsilateral PSIS (posterior superior iliac spine)  and a tendency of walking with a slightly flexed knee.
Tests for core stability are also important as they can highlight iliopsoas changes in strength, even unilaterally .
These patients can both present with the typical inguinal symptoms and ipsilateral back pain .
15.4 Differential Diagnosis
The differential diagnosis should be made with hip intra-articular orthopedic pathologies such as FAI (femoral acetabular impingement) [1, 9, 16], acetabular labrum tears, femoral head edema, osteochondral detachments, or systemic conditions such as rheumatic or nonrheumatic osteoarthritis .
Differential diagnoses of the iliopsoas tendinopathies include insertional tendinopathies; proximal myotendinous lesions of the adductor muscle, the rectus femoris, and the sartorius; distal myotendinous lesions of the abdominal muscle; and obturator muscolotendinous pathologies [1, 16].