The hip joint, and more specifically the coxal joint, is formed by the articulation of the head of the femur and the acetabulum of the pelvis. This joint serves as a bridge between the lower extremity and the pelvic girdle, similar to the glenohumeral joint of the shoulder. Both are ball-and-socket joints that offer mobility in all planes, but comparatively, the weight-bearing function of the coxal joint requires much greater stabilization.
Anatomy
The acetabulum is a deep, cup-like depression located on the inferolateral aspect of the pelvis. This osseous socket is formed where three bones (the ilium, ischium, and pubis) fuse and provides a stable seat for the rounded femoral head. The depression of the acetabulum is further deepened by the acetabular labrum—a fibrocartilaginous collar that snugs around the femoral head and increases joint stability.
The coxal joint is a ball-and-socket synovial joint that has a thick, fibrous joint capsule, as well as a complex network of ligaments surrounding the joint. Within the joint capsule, a relatively small ligament (not pictured) connects the acetabular fossa to the fovea of the femur and encloses the obturator artery. More relevant to movement are the iliofemoral, pubofemoral, and ischiofemoral ligamentous networks located outside of the joint capsule.
The iliofemoral ligament joins the anterior inferior iliac spine and the intertrochanteric line of the femur. It is bifurcated and looks like an inverted Y with a primary function of restricting hip extension and lateral rotation. The iliofemoral ligament is considered to be the strongest of the three major ligament systems of the coxal joint.
The pubofemoral ligament joins the superior pubic rami and the intertrochanteric line of the femur. It is shaped like a triangle and restricts both hip abduction and extension and reinforces the joint capsule anteriorly and inferiorly.
The ischiofemoral ligament joins the body of the ischium and the greater trochanter of the femur. It posteriorly reinforces the joint capsule and has a spiral orientation allowing it to restrict hip flexion, adduction, and medial rotation.
Mechanics
Because the coxal joint is a ball-and-socket synovial joint, movement in all three planes is possible, including hip flexion, extension, abduction, adduction, medial rotation, and lateral rotation. This is also true of the glenohumeral joint, but because the hip functions as a weight-bearing structure and is anatomically significantly more stable, there is less range of motion possible at the hip compared to the shoulder.
Another parallel between the hip and shoulder is the presence of several deep muscles that help steer and dynamically stabilize the joint. The rotator cuff muscles, including the supraspinatus, infraspinatus, teres minor, and subscapularis, surround the head of the humerus and control dynamic joint mechanics in the shoulder. Deep in the hip, the piriformis, superior gemellus, inferior gemellus, obturator internus, obturator externus, and quadratus femoris muscles serve a similar function. These are postural muscles and therefore functionally better suited for endurance activity and aerobic metabolism.
Moving superficially, a multitude of large, prime mover muscles cross and affect the hip, including the psoas, iliacus, rectus femoris, tensor fasciae latae, adductor longus and brevis, sartorius, and pectineus anteriorly; adductor magnus and gracilis medially; and the hamstrings and gluteals posteriorly, with the gluteus maximus extending laterally to the greater trochanter where it joins the tensor fasciae latae at the iliotibial band. All these muscles serve both postural and force-generating functions and, therefore, must be capable of both strength and endurance activity and anaerobic and aerobic energy production.
Pathology and Purpose of Soft-Tissue Intervention
The coxal joint is vulnerable to excessive compression, causing osteoarthritic changes due to high weight-bearing forces in this area. This is also a common site of stubborn postural deviations and compensatory patterns associated with low-back pain and other mechanical issues further down the kinetic chain (foot, ankle, or knee dysfunction). Poor mobility, joint stiffness, and muscle hypertonicity are more likely at the hip compared to the shoulder. Keep in mind that genetic variation, history of trauma, or surgical intervention can create instability at the coxal joint, though it is less common than at the glenohumeral joint.
Proper assessment of both joint mobility and muscle length of stabilizers and prime movers at the hip is useful as part of postural assessment and movement analysis. Ensuring optimal range in all motions available and identifying specific movement limitations is a critical part of treatment and maintenance of optimal health for the lower extremities, pelvis, and spine.
Christy Cael is a licensed massage therapist and certified strength and conditioning specialist. Her private practice focuses on injury treatment, biomechanical analysis, craniosacral therapy, and massage for clients with neurological issues. She is the author of Functional Anatomy: Musculoskeletal Anatomy, Kinesiology, and Palpation for Manual Therapists (New York: Jones & Bartlett Learning, 2010; jblearning.com). Contact her at christy_cael@hotmail.com.