Anatomy of the Hip Joint

Hip Joint Structure

• The hip joint, also known as the femoroacetabular joint, is a synovial "ball and socket" joint with 3 degrees of freedom: flexion/extension, abduction/adduction, and internal/external rotation.

Function of the Hip Joint

• The hip joint supports the load of the head, arms, and trunk.
• It transmits force and facilitates locomotion.
• It works in both non-weight bearing (NWB) during open kinematic chain movements and weight bearing during closed kinematic chain movements.

Acetabulum

• The head of the femur articulates with the acetabulum, which is directed laterally, anteriorly, and inferiorly.
• The acetabular labrum increases the congruence of the joint.
• The transverse acetabular ligament is also present.

Anatomical Variations

• The angle of inclination between the neck and shaft of the femur can vary between humans, with normal values ranging from 120-125°.
• Variations include coxa vara (105°) and coxa valga (140°).
• These variations can influence movement.

Hip Joint Stability

• Static stabilizers include the acetabular labrum, transverse acetabular ligament, strong capsule, and ligaments (iliofemoral, pubofemoral, ischiofemoral, and ligamentum teres).
• Dynamic stabilizers include muscles and myofascial tissue.

Acetabular Labrum

• The acetabular labrum is a wedge-shaped fibrocartilage that deepens the socket and increases the concavity of the acetabulum.
• The hip joint is more stable than the shoulder joint due to the depth of the acetabulum.

Ligamentum Teres

• The ligamentum teres directly connects the head of the femur to the acetabulum.
• It provides stability, proprioception, and coordination, and also supplies blood to the hip joint.

Osteokinematics of the Hip

• The hip joint moves in an open kinematic chain, with the femoral head moving on the acetabulum.
• The femur is convex, and roll and glide movements occur in opposite directions.

Flexion and Extension

• Flexion involves the femur spinning posteriorly with posterior pelvic tilt.
• Extension involves the femur spinning anteriorly with anterior pelvic tilt.

Kinetics of Flexion

• Anterior muscles involved in flexion include psoas/iliacus, rectus femoris, sartorius, and TFL.

Kinetics of Extension

• Posterior muscles involved in extension include gluteus maximus, gluteus medius, gluteus minimus, piriformis, semitendinosus, semimembranosus, and biceps femoris.

Limits of Flexion and Extension

• Limits of flexion include posterior chain soft tissue tension and structural hit of the knee with the chest.
• Limits of extension include iliofemoral ligament tension and anterior chain soft tissue tension.

Abduction and Adduction

• Abduction involves the femur rolling superiorly and gliding inferiorly.
• Adduction involves the femur rolling inferiorly and gliding superiorly.

Limits of Abduction and Adduction

• Limits of abduction include soft tissue tension from adductor muscles, pubofemoral and iliofemoral ligaments, and articular limitation.
• Limits of adduction include iliofemoral ligament tension.

Kinetics of Hip Abduction

• Primary motor muscles involved in hip abduction include gluteus medius and TFL.
• Secondary motor muscles include gluteus minimus, piriformis, gluteus maximus, obturator externus, and gemellous superior and inferior.

Kinetics of Hip Adduction

• Primary motor muscles involved in hip adduction include adductor magnus, adductor longus, adductor brevis, and gracilis.
• Secondary motor muscles include quadratus femoris and obturator externus and internus.

Internal and External Rotation

• Internal rotation involves the femur rolling anteriorly and gliding posteriorly.
• External rotation involves the femur rolling posteriorly and gliding anteriorly.

Kinetics of External Rotation

• Primary motor muscles involved in external rotation include piriformis, obturator externus and internus, gemellous superior and inferior, and quadratus femoris.
• Secondary motor muscles include gluteus maximus and posterior fibers of gluteus medius.

Kinetics of Internal Rotation

• Primary motor muscles involved in internal rotation include adductor magnus, adductor longus, and adductor brevis.
• Secondary motor muscles include TFL, gluteus minimus, gluteus medius, and pectineus.

Limits of External and Internal Rotation

• Limits of external rotation include iliofemoral and pubofemoral ligament tension.
• Limits of internal rotation include ischiofemoral ligament tension and tension of the external rotator muscles.