Femoral version as it affects turnout of the feet. a Normal anteversion of the femoral neck, demonstrating relationship to the distal femur (knee) with the foot pointed straight ahead. b Increased anteversion of the femoral neck with the foot initially pointed straight ahead, demonstrating impingement at the hip as the hip externally rotates. c Increased retroversion of the femoral neck with the foot initially pointed straight ahead, demonstrating increased ability to externally rotate at the hip without impingement and thus ability to achieve greater turnout
Femoral head: The “ball” of the hip; the highest portion of the femur.
Femoral retroversion: The femoral head and neck are posteriorly directed with respect to the axis of the femur; this manifests as outward twisting of the femur and relative outward turning of the knees and feet (see Fig. 7.1c).
Femoroacetabular impingement (FAI): Increased coverage of the femoral head by the acetabulum; see pincer and cam impingement.
Hip capsule: A membranous envelope of tissue that surrounds the hip joint, acting as a seal for the joint space.
Hip dysplasia: Inadequate coverage of the femoral head by the acetabulum.
Intra–articular: Inside the joint; in this context, pertaining to the bony structure of the femoral head and acetabulum and their articulation with each other, cartilage, and labrum of the hip joint.
Labrum: Horseshoe-shaped ring of connective tissue that rims the acetabulum; acts like a rubber seal to increase the stability of the hip joint.
Ligament: A band of fibrous connective tissue that connects two bones, when crossing a joint can contribute to the stability of the joint.
Pincer impingement: Hip impingement caused by excess growth of the upper rim of the acetabulum.
Sub-spine impingement: Occurs when the AIIS contacts the femoral neck during extreme hip flexion.
Tendon: a cord of strong fibrous collagen tissue attaching a muscle to a bone.
Version: The relation of the femoral head and neck with respect to the axis of the femur; see: femoral anteversion, retroversion.
It has been said that the feet are the end goal in a dancer; however, the art begins with turn out of the hips. As such, the dancer’s hip is unique in the extreme range of motion required of it and the subsequent stresses placed on the hip and its surrounding structures. These stresses can have a particularly pernicious effect on young dancers, due to their ongoing growth and development and the potential for exposure to ill-conceived methods of training. Appropriate prevention, diagnosis, and treatment of common hip injuries are important to help the young dancer return to dance, and more importantly prevent further injury.
Dance training is often begun at an early age, and time spent in training increases considerably during adolescence, placing increased stress on the developing hip. It is estimated that 35% of adolescent girls participate in some form of dance . Hip injuries have been reported to comprise from 7 to 50% of injuries in dancers [2–4]. This can have long-term consequences, as hip pain is nearly three times more common in retired ballet dancers than in non-dancers . Years of dancing may also contribute to the development of early osteoarthritis in dancers with chronic hip pain . It is clear that hip pain and injuries affect a significant portion of dancers and can persist even after dance activities have ceased.
Risk Factors in Dancers
Not only are dancers required to achieve extremes in range of motion, they must also sustain flexibility and strength within these extremes. The young dancer may lack the neuromuscular control and strength required by such demands. Bennell et al.  compared 77 novice dancers ages 8–11 (average age 9.6 years) to similarly aged controls (non-dancers) and found that initially the dancers had significantly less strength in hip flexors, internal and external rotators, and adductors than the control group. At 1-year follow-up of the same two groups , the dancers had an interval preferential increase of strength in their hip external rotators, abductors, and adductors; all muscle groups that are targeted in their training. However, the selective strengthening of isolated muscle groups may lead to muscle imbalance and susceptibility to injury .
Some dancers may not possess the anatomic makeup required to achieve certain ballet positions, such as “perfect” (180°) turnout. Degree of turnout at the hip is determined by skeletal anatomy, hip capsule laxity, and muscle strength. The skeletal anatomy that dictates hip rotation is version, i.e., the relation of the femoral head and neck with respect to the axis of the femur. Increased retroversion is conducive to greater hip external rotation and ultimately increased turnout (Fig. 7.1). However, this angle is primarily predetermined by genetics and also changes during development . Newborns have an average of 40° of femoral anteversion, which decreases by age 8 to the average adult value of 15° .
While some studies have shown that ballet dancers have increased turnout angles and hip external rotation compared to controls [4, 11, 12], these studies were done on advanced dancers and are unlikely to represent the young dancer population. Bennell et al.  examined the degree of hip external rotation in novice dancers compared to controls and found that the dancers had on average 5° less external rotation. Increased turnout and external rotation seen in more advanced dancers are likely due to a combination of training and selection.
A common technique used by less experienced dancer to achieve turnout is “screwing the knee.” The feet are placed in 180° of turnout with the hips and knees flexed, and the rest of the body is then positioned to accommodate this alignment without moving the feet. This results in compensatory stress seen not only at the hips but also at the knees and lower back, which may predispose the dancer to injury .
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 . In addition to increasing the acetabular volume by 33%  and acting like a suction cup to resist joint distraction and improve hip stability , the labrum is believed to act as a seal to preserve the intra-articular joint fluid , which lubricates the joint surfaces and allows for a smooth gliding motion.
Capsular and Ligamentous Structures
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 . The ligaments are the iliofemoral, ischiofemoral, and pubofemoral, which are extra-capsular, and the intra-capsular ligamentum teres . 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.
The functional muscle groups and associated muscles of the hip
Iliopsoas (Iliacus + Psoas), rectus femoris, sartorius, pectineus
Hamstrings (semitendinosus, semimembranosus, biceps femoris)
Adductor magnus, gluteus medius and minimus (anterior fibers)
Piriformis, obturator internus and externus, superior and inferior gemellus, quadratus femoris
Gluteus maximus, gluteus medius, gluteus minimus, tensor fascia lata
Adductor longus, adductor brevis, adductor magnus, gracilis
Intra-articular Causes of Hip Pain
The extremes of motion and force demanded of the dancer’s hip can lead to pain and injury even in the presence of normal anatomy. Certain structural variations, such as hip dysplasia or femoroacetabular impingement (FAI), may increase susceptibility to injury.
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 . Decreased coverage can lead to instability and increased shearing between the femoral head and the anterolateral rim of the acetabulum . Increased forces caused by this shearing can then lead to labral tears, as well as injury to the adjacent cartilage.
Femoroacetabular Impingement (FAI)
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 . 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 . Monazzam et al.  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.
Hip Labral Tears
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 . 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.  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.  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 . These injuries occur when the bone is subjected to repeated abnormal stresses, and the risk is increased in females with amenorrhea . Thus, young dancers engaging in intensive dance training, who may also have altered eating habits and poor nutrition, are at particular risk.