The hip joint comprises the ball of the femoral head and the socket of the acetabular cup. This articulation serves the functioning musculoskeletal system as an important shock attenuator and force generator through most weight-bearing movements. When considering the forces that traverse the joint with the appreciable mobility of the hip, it is easy to understand why this joint is a frequent source for arthritic pain.

Draovitch et al introduced the concept of viewing the hip joint in terms of anatomic layers, in order to guide examination and treatment of painful conditions. This perspective is useful in its simplicity as well as leading the clinician toward appropriate rehabilitation methods. The layer concept presents the hip joint in four layers: (1) osteochondral, (2) inert, (3) contractile, and (4) neuromechanical. The functional anatomy of the hip joint will be presented in this format.

The hip is a ball-and-socket type joint with six degrees of movement. Often, the dialogue regarding hip motion is presented from the perspective of the ball of the femoral head moving within the acetabular cup. However, because most human motion is performed in weight-bearing positions, the clinician should consider the motion of the hip joint as the free pelvis moving over a fixed femoral head. This can be designated as acetabulofemoral movement, rather than femoroacetabular motion. Neuromuscular stability and control of the pelvis is imperative for proper functioning of the hip joint.

The osteochondral layer is comprised of the femur, acetabulum, and the pelvis and associated cartilaginous layers. The pelvis is the entire conjoined unit of the two innominate bones. The acetabular socket is the convergence of the three bones of each innominate: the ilium, ischium, and pubic bones each with its own degree of variability. These variances can alter the congruency of the joint. It is valuable for the clinician to understand the variability, deformities, and limits of the bony structure of the hip joint in order to individualize a rehabilitation program. These irregularities of the hip joint can involve one or both hips asymmetrically and vary from established norms, making examination and treatment challenging even for the most astute clinician.

The position and depth of the acetabulum are variable. The acetabulum may be oriented anteriorly or posteriorly in the transverse plane, which is termed acetabular anteversion or retroversion, respectively. Additionally, it can face superiorly or inferiorly in the sagittal plane. The depth of the socket can vary, leading to a shallow or deep acetabulum.

The femoral head is spherically shaped and protrudes from the femoral neck. The sphericity of the femoral head can differ and affect the congruency of the femoroacetabular joint. Abnormal sphericity may cause instability of the hip joint with limited motion. When the sphericity, in relation to the head–neck junction, exceeds normative values in the head–neck offset measurement, a CAM deformity exists.

The angle at which the neck courses between the femoral head and shaft is known as the angle of inclination. A steeper neck, with an increased angle of inclination, is known as coxa valga. Coxa vara is the term used to describe a decreased angle of inclination.

The neck can possess variable-angle torsion, labeled femoral torsion. This “twist” of the femoral shaft, in relation to the femoral neck, is measured by comparing an imaginary line drawn transversely through the femoral neck and shaft, and a line drawn through the epicondylar axis. Excessive femoral torsion—anything greater or less than 15° to 20°—can be biased anteriorly or posteriorly. Femoral torsion can be measured radiographically or by use of the Craig test in the clinic. Abnormal femoral torsion has been extensively related to several other orthopaedic injuries and problems.

Bony incongruences will affect the kinematics of the joint. Deformities, whether congenital or acquired, may lead to tears of the acetabular labral complex. Labral tears are frequently present in arthritic joints. The cartilage of the hip joint is subject to wear and degradation. The cartilage of the acetabulum covers the lunate surface in an upside-down “U-shaped” manner, and is meant to attenuate the forces travelling from the lower extremity up through the femur and femoral head, and into the pelvis and torso. Highest compression forces during the gait cycle have been shown to occur during midstance, when the entire lunate surface is in contact with the femoral head.

The hip joint functions as a traditional ball-and-socket joint. The axis of rotation is centered through the femoral head. In the classical description of hip kinematics, motion is described in relation to the femur’s movement within the acetabulum, or femoral-on-pelvic movement. However, as previously stated, most locomotion requires the pelvis to move over a fixed femur, or as pelvic-on-femoral motion.