The hip joint is classified as an enarthrosis, or ball and socket joint. The slightly incongruous articulation occurs between the acetabulum, which is less than a hemisphere, and the femoral head, which is normally about two-thirds of a sphere. It is a complex joint and allows rotational movement in three anatomic planes: sagittal, coronal, and transverse. The capsule, which is reinforced by the iliofemoral, pubofemoral, and ischiofemoral ligaments, contains and stabilizes the joint, protecting it from the extremes of motion. The capsule tightens in extension, providing maximum passive stability, and loosens in flexion. In flexion, the 27 separate musculotendinous units that cross the hip joint work both individually and in groups to provide positional dynamic stability. These muscles are unique in their large volume and length, spanning the hip and knee joint both anteriorly and posteriorly, and allowing the hip joint to generate large forces through an extremely broad range of motion between 240 to 300 degrees (Table 32-3).

Understanding the normal and abnormal growth and development of the bony hip joint in relation to its muscular, ligamentous, and capsular support is paramount in comprehending the injuries and pathologic conditions that can affect it. Both the acetabulum and proximal femur develop from multiple primary ossification centers, and their final shape is influenced by a dynamic interaction between the developing bone, joint position, and its response to internally and externally transmitted forces (15,16). The physeal growth sections of the iliac, pubic, and ischial portions of the acetabulum join centrally through a common epiphysis, the triradiate cartilage. This common epiphysis is responsible for relatively spherical expansion of the acetabulum during growth, and simultaneously accommodates uninterrupted congruency with the femoral head as it enlarges. In addition to growth of the head, complex structural dynamics occur during development of the proximal femur, which includes elongation and anteversion of the femoral neck, differentiation of the extra-capsular greater and lesser trochanteric apophyses, and transition of the neck-shaft angle (17). Hip joint structural maturation occurs from before birth to approximately age 16 to 18, when the majority of the physeal growth plates have closed. Critical time frames include; formation of acetabular morphology by age 8 to 9, structural quiescence from 9 to 12, rapid growth of the capital femoral epiphysis from age 13 to 15, and closure of the triradiate cartilage and femoral epiphysis by age 16 (16,18). These time frames are important, as they correspond to the types of hip injuries that can occur in the growing athlete.

The normal function of the hip joint is requisite for successful sports performance, and is based on the anatomic bony architecture that provides the foundation for movement and inherent mechanical stability. The prime function of the hip joint is to act as a fulcrum to provide a mechanical advantage to the muscles moving the leg, stabilizing the pelvis, and holding the body upright. The biomechanical study of hip kinematics and kinetics describes this relationship and defines how the joint interacts with the surrounding soft tissues and environment to generate motion and accommodate the static and dynamic forces that are generated and absorbed. The explanation of joint reactive forces and gait analysis is beyond the scope of this chapter, but should be reviewed by the physician evaluating and treating hip injuries and disorders. Awareness of the normal or “ideal” biomechanical configuration can be helpful in assessing radiographs, providing clues to the mechanism of injury, making a difficult diagnosis, and planning surgical correction.

Deviations in the bony anatomy of the hip can result in biomechanical alterations that amplify force transmission, affect the strength and moment-generating capacity of the muscles, and can increase susceptibility to injury and early degeneration (19). Developmental dysphasia of the hip (DDH) is the most frequently encountered example, and describes a variety of structural problems and malformations that can also result in increased susceptibility to instability, subluxation, and/or dislocation of the joint (20). In the athletic population, the less severely affected joints are typically diagnosed after an injury, and should be evaluated for variations in acetabular position/joint center location, femoral neck anteversion angle, head-neck angle, neck length, and resultant leg length discrepancy.

Patients with suspected hip fractures should always go through the trauma initial assessment (Advance Trauma Life Support). Special attention should be placed on the ipsilateral upper extremity to rule out possible associated injuries. At the same time, careful evaluation of the lower extremity should be performed to identify other injuries. A neurovascular assessment needs to be done as soon as possible. When a life-threatening condition is identified, it should be managed first. The patient should be transported as soon as possible to a facility that can perform definitive evaluation and treatment.

During the physical exam, pain and deformity around the hip joint are the hallmarks for a fracture, which usually present as shortening with deformation in flexion and rotation, depending on the fracture pattern. The athlete is disabled with severe groin pain and an inability to bear weight on the affected limb. When a hip fracture is suspected, AP and lateral radiographs should be performed and will usually reveal the fracture pattern. However, occult or stress fractures of
the femoral neck may require additional imaging to establish the diagnosis. MRI is the most commonly used diagnostic test after radiographs, and helps to identify the location and verticality of the fracture (23,28). When the hip pain has been present for several days, a bone scan may be useful to identify stress fractures and pathologic fractures associated with tumors and/or infections.