The hip is typically described as a “ball and socket” joint, with the femoral head forming the ball and the acetabulum comprising the socket. The acetabulum is formed by three pelvic bones—the ilium, ischium, and pubis—which join at the triradiate cartilage, a Y-shaped growth plate that fuses at the age of 14–16 years. The femoral head rotates within the acetabulum, with the tapered femoral neck serving as the connection between the head and the rest of the femur. The congruence of the spherical head within the cuplike acetabulum allows for the extent of range of motion seen in the hip.

The labrum is a horseshoe-shaped structure formed of dense connective tissue and fibrocartilage that rims the acetabulum [14]. In addition to increasing the acetabular volume by 33% [15] and acting like a suction cup to resist joint distraction and improve hip stability [16], the labrum is believed to act as a seal to preserve the intra-articular joint fluid [17], which lubricates the joint surfaces and allows for a smooth gliding motion.

The hip joint is reinforced by the capsule and four ligaments. The capsule is composed of circular fibers forming a collar around the femoral neck, called the zona orbicularis, as well as longitudinal fibers traveling parallel to the neck and carrying the blood vessels that supply the intra-articular structures [18]. The ligaments are the iliofemoral, ischiofemoral, and pubofemoral, which are extra-capsular, and the intra-capsular ligamentum teres [19]. Together with the hip capsule, these ligaments facilitate hip range of motion while simultaneously providing additional stability to the joint.

The muscles surrounding the hip function along three primary axes of motion: flexion–extension, internal–external rotation, and abduction–adduction. A list of the functional muscle groupings and primary participating muscles is provided in Table 7.1.

Dysplasia refers to inadequate coverage of the femoral head by the acetabulum. This may be due to either a shallow acetabulum, or an increased femoral-head-to-neck angle (coxa valga), effectively creating a femoral head that lacks coverage relative to a normal acetabulum. The degrees of rotation of the acetabulum and femoral head (anteversion or retroversion) relative to each other and to the rest of the lower extremity can also affect the total coverage of the femoral head by the acetabulum [20]. Decreased coverage can lead to instability and increased shearing between the femoral head and the anterolateral rim of the acetabulum [21]. Increased forces caused by this shearing can then lead to labral tears, as well as injury to the adjacent cartilage.

FAI is the opposite scenario to hip dysplasia. Again, the abnormality can originate from either the acetabulum or femoral head, or a combination of both [22]. Acetabular abnormality is dubbed pincer impingement, and refers to excess coverage of the femoral head by the acetabulum. Cam impingement refers to abnormal shape of the femoral head and neck junction, with decreased offset (difference in diameters) between the femoral head and neck. This can lead to abnormal forces and loading between the femoral head/neck junction and the rim of the acetabulum, even when the hip is taken through a normal range of motion [23]. Monazzam et al. [24] performed CT review on 225 patients age 2–19 years and found that (1) the prevalence of cam and pincer morphology is similar in adolescent and adult populations, and (2) this morphology occurs as early as age ten and 12, respectively, in this cohort.

Labral tears may be a source of hip pain either in isolation or in association with other injuries, such as damage to adjacent cartilage or inflammation of the surrounding muscles and tendons. Multiple types of sensory nerves and free nerve endings have been identified within the labrum, including pain receptors [25]. Therefore, even in the absence of other abnormalities, a labral tear can cause hip pain.

More commonly, the labral tear is seen in conjunction with other insults. Harris et al. [26] noted that chondral wear and labral tears in the hip often occur in the same location. Although there are no definitive studies demonstrating that labral tears result in hip arthritis, there are several basic science studies [16, 27] demonstrating that labral tears lead to increased strain within the articular cartilage. In theory, this could lead to degeneration of the joint and resulting arthritis, but further studies are necessary to demonstrate a true causative relationship.

Labral tears may also lead to micro-instability within the hip. Crawford et al. [28] demonstrated that a labral tear decreased the force required to distract the hip by 60%. The micro-instability resulting from a labral tear may cause the other stabilizing structures around the hip, specifically the surrounding musculature, to overcompensate, resulting in injuries.

Though a relative rarity, young dancers are at risk for stress reaction and stress fracture of the femoral neck [29]. These injuries occur when the bone is subjected to repeated abnormal stresses, and the risk is increased in females with amenorrhea [30]. Thus, young dancers engaging in intensive dance training, who may also have altered eating habits and poor nutrition, are at particular risk.