Hip Biomechanics

The biomechanics of the hip joint are fundamental to understanding hip arthroscopy and hip preservation. The femoral head and acetabulum provide the main bony constraint, while the labrum, capsule, cartilage, muscles, and surrounding pelvis work together to keep the joint stable and distribute load. Hip preservation procedures aim to restore this balance when it is disrupted.

Hip stabilizers: The hip is a highly constrained ball-and-socket joint with motion in three planes: flexion-extension, abduction-adduction, and internal-external rotation. Its deep socket gives it greater inherent stability than joints like the shoulder, but bony morphology is just one of the components of hip stabilization. Static stabilizers of the hip include the acetabular labrum, joint capsule, capsular ligaments, ligamentum teres, and the fluid seal. Dynamic stabilizers of the joint include surrounding muscles and fascia such as the hip abductors, adductors, external rotators, hamstrings, rectus femoris, gluteus maximus, and the tensor fascia lata.

The femoral head is not fixed in one position during movement. Small translations occur as the hip rotates, and the location and direction of contact change with hip position and load. Normal morphology and constraints allow these movements to occur without the femoral neck contacting the acetabular rim prematurely. Abnormal morphology can either cause excessive contact, as in femoroacetabular impingement, or inadequate containment, as in acetabular dysplasia and instability.

Motion is shared between the hip, pelvis, and spine: Hip motion does not occur in isolation. Flexion and extension of the lower limb are coupled to movement of the pelvis and lumbar spine. Changes in the slope of the sacrum and angle of the pelvis can alter acetabular version which affects hip mechanics. For example, in activities such as sitting or deep squatting, posterior pelvic tilt helps the acetabulum rotate away from the femoral neck and increases functional clearance for hip flexion. Conversely, anterior pelvic tilt reduces this clearance and can make anterior femoroacetabular contact occur earlier.

Good spinopelvic mobility can compensate for a relatively small loss of hip clearance, while a stiff lumbar spine or pelvis may require more motion to occur directly through the hip. Patients with lumbar spine issues or prior fusion may therefore have more limited hip flexion.

Reactive Joint Forces and joint loading: There are two primary forces acting across the hip, body weight and the abductor muscles. These two forces act across the hip joint at different distances with the lever arm for body weight being about 2.5 times longer than the lever arm for the hip abductors. The lever arm for body weight is from the center of the hip joint to the center of the pelvis, while the lever arm for the abductors is from the hip center to the greater trochanter. Actions of daily life can cause repetitive large loads to be placed on the hip, which can exacerbate even minor bony defects. Studies from hip arthroplasty show that an action as simple as walking up stairs can put 2-3 times the force of body weight through the hip joint.

Articular cartilage and contact mechanics: The articular cartilage of the femoral head and acetabulum provides a low-friction surface that distributes compressive load over a broad area. Cartilage is highly hydrated. Under load, pressurization of fluid within the cartilage contributes substantially to load support and reduces stress on the solid cartilage matrix. For this system to work well, the femoral head must remain well contained and the load must be distributed over an adequate contact area.

Changes in coverage alter contact mechanics. A shallow dysplastic acetabulum provides a smaller weight-bearing surface, concentrating load near the acetabular rim. This increases stress on the lateral cartilage and labrum and helps explain why labral hypertrophy and labral tears are common in dysplasia.

The opposite problem is excessive or abnormally oriented coverage. In acetabular overcoverage, the femoral neck contacts the acetabular rim earlier in the arc of motion, producing repetitive compression of the labrum. A defect in the femoral head-neck junction can worsen these mechanics. The aspherical head-neck junction enters the acetabulum during flexion and rotation, generating shear at the chondrolabral junction and potentially separating acetabular cartilage from the subchondral bone. These repetitive impingements are the bony basis for femoral acetabular impingement, which is discussed in depth in later chapters.

Biomechanics of the labrum: The labrum is an important mechanical structure. It deepens the acetabulum, increases the effective surface area of the socket, contributes to stability, and forms a seal against the femoral head. The labral seal helps maintain fluid pressurization within the joint. This supports smooth joint movement by the fluid acting as a lubricant and preventing articular cartilage from experiencing prolonged direct contact. The labrum also resists distraction and translation of the femoral head. Cadaveric studies show increased femoral head displacement after labral resection, particularly when the capsule has also been violated.

Biomechanics of the capsule: The capsule is one of the major static stabilizers of the hip. Its fibres are arranged so that many of the capsular ligaments tighten as the hip moves into extension, creating a strong restraint against anterior translation and excessive rotation. The iliofemoral ligament is the dominant anterior restraint limiting extension and external rotation. The pubofemoral and ischiofemoral ligaments provide additional support. The zona orbicularis consists of circular capsular fibres that wrap around the femoral neck. It functions like a locking ring and contributes to resistance against distraction.

Accessing the intraarticular hip joint in arthroscopy requires a capsulotomy or capsular release. This decreases capsular stability and is particularly important in hips that may rely on soft tissues for extra stability. Extra caution should be given to protecting the capsule in patients with borderline acetabular coverage, as the capsule and other soft tissues can help compensate for the decreased bony stability.