Hip & Pelvic Girdle MSK Positioning

Hip & Pelvic Girdle MSK Positioning

Musculoskeletal Anatomy of the Hip & Pelvic Girdle

Accurate positioning and protocol setup require a comprehensive understanding of the complex functional anatomy of the pelvic girdle and coxofemoral articulation.

The Acetabular Labrum & Osseous Articulation

The hip joint (coxofemoral joint) is an enarthrodial (ball-and-socket) synovial articulation formed between the cup-shaped acetabulum of the pelvis and the spherical head of the femur. The osseous acetabulum is formed by the embryological confluence of three pelvic bones—the ilium, ischium, and pubis—which fuse during adolescence at the Y-shaped triradiate cartilage:

• Lunate Surface: The articular portion of the acetabulum is an inverted horseshoe-shaped perimeter lined with smooth hyaline articular cartilage.
• Acetabular Fossa: The central, non-articular deep depression of the acetabulum. Devoid of hyaline cartilage, it contains a fibroelastic adipose cushion (the pulvinar) and houses the origin of the ligamentum teres (ligamentum capitis femoris), which conveys the foveal artery (a small branch of the obturator artery) to the femoral head.
• Acetabular Labrum: A triangular, fibrocartilaginous ring anchored firmly around the osseous rim of the acetabulum. The labrum deepens the acetabular socket by approximately 20% to 30%, increases joint volume coverage, and acts as an airtight suction seal that retains intra-articular synovial fluid. This hydrodynamic seal maintains negative intra-articular pressure and distributes axial compressive loads evenly across the cartilage surface.
• Transverse Acetabular Ligament: At the inferior margin of the acetabulum, the osseous rim is interrupted by the acetabular notch. The labrum is deficient here and is bridged across the notch by the dense fibrous transverse acetabular ligament, creating a fibro-osseous tunnel through which nutrient vessels enter the joint.
• Labral Pathology Zones: Labral tears occur predominantly along the anterosuperior labrum (between 10 o'clock and 2 o'clock), where repetitive mechanical shear stresses and impingement forces reach their maximum.

The Femoral Head, Neck & Trochanteric Anatomy

• Femoral Head: A smooth, nearly perfect sphere (comprising roughly two-thirds of a globe) directed superiorly, medially, and slightly anteriorly. It is covered entirely in hyaline cartilage with the exception of the small central pit, the fovea capitis femoris, where the ligamentum teres inserts.
• Femoral Neck: A flattened pyramidal strut of bone connecting the femoral head to the shaft, transmitting body weight down the skeletal chain. In adults, the normal angle formed between the longitudinal axis of the femoral neck and the femoral shaft (the neck-shaft angle or angle of inclination) measures approximately 125 to 130 degrees. Angles less than 120 degrees constitute coxa vara, while angles exceeding 135 degrees constitute coxa valga.
• Femoral Anteversion: The longitudinal axis of the femoral neck does not lie in a true coronal plane; rather, it is angled anteriorly relative to the trans-epicondylar axis of the distal femur by approximately 15 degrees (normal range 10°–20°). This forward torsion has profound implications for patient positioning in MRI.
• Greater Trochanter: A prominent quadrilateral osseous projection on the lateral proximal femur. It features distinct facets that serve as insertion points for the powerful hip abductors:
  • Lateral & Superoposterior Facets: Insertion of the gluteus medius tendon.
  • Anterior Facet: Insertion of the gluteus minimus tendon.
  • Tip & Superior Margin: Insertions for the deep lateral rotators (piriformis, obturator internus, and gemelli muscles).
• Lesser Trochanter: A conical posteromedial osseous projection providing insertion for the confluent iliopsoas tendon (psoas major and iliacus), the primary dynamic flexor of the hip joint.
• Intertrochanteric Line & Crest: The anterior junction between neck and shaft is marked by the subtle intertrochanteric line, while the prominent posterior ridge is the intertrochanteric crest, featuring the quadrate tubercle for the quadratus femoris muscle.

The Capsular Ligamentous Complex

The hip joint is enclosed by an exceptionally strong fibrous capsule reinforced by three primary intrinsic ligaments that spiral around the articulation:

Hip Capsular Ligaments:
1. Iliofemoral Ligament ("Y-Ligament of Bigelow") ➔ Anterior ➔ Limits Hyperextension
2. Ischiofemoral Ligament                        ➔ Posterior ➔ Limits Internal Rotation
3. Pubofemoral Ligament                         ➔ Inferior  ➔ Limits Hyperabduction & Extension

1. Iliofemoral Ligament ("Y-Ligament of Bigelow"): Located anteriorly, originating from the anterior inferior iliac spine (AIIS) and adjacent acetabular rim, bifurcating into two thick bands that insert along the entire intertrochanteric line. It is the strongest ligament in the human body . It becomes taut during hip extension, preventing the trunk from falling backward without requiring active muscular contraction during standing.
2. Ischiofemoral Ligament: Reinforces the posterior capsule, originating from the ischium below and behind the acetabulum and spiraling superolaterally over the femoral neck to insert onto the greater trochanter. It limits internal rotation and adduction.
3. Pubofemoral Ligament: Positioned anteroinferiorly, originating from the superior pubic ramus and blending with the inferior capsule and iliofemoral ligament. It prevents excessive abduction and extension.
4. Zona Orbicularis: A ring of dense, circular deep capsular fibers constricting the waist of the femoral neck like a collar, functioning as a biomechanical locking ring that prevents subluxation.

Abductor Musculature & the Deep Gluteal Space

• Gluteus Medius & Minimus: The gluteus medius originates from the outer iliac wing and inserts onto the lateral and superoposterior facets of the greater trochanter. It is the primary dynamic abductor of the hip and stabilizes the pelvis in the coronal plane during single-leg stance. Muscular tear or avulsion produces a Trendelenburg gait (pelvic drop toward the unsupported contralateral side). The gluteus minimus lies deep to the medius, inserting on the anterior trochanteric facet to assist in abduction and internal rotation.
• Greater Trochanteric Pain Syndrome (GTPS): A common clinical syndrome characterized by lateral hip pain, previously termed "trochanteric bursitis." MRI demonstrates that GTPS is primarily caused by gluteus medius and minimus tendinopathy or tears (analogous to rotator cuff tears in the shoulder), often accompanied by secondary distension of the subgluteus maximus and subgluteus medius bursae.
• Piriformis Muscle & Sciatic Nerve: The piriformis muscle originates from the anterior surface of the sacrum (S2–S4), passes through the greater sciatic foramen, and inserts onto the superior apex of the greater trochanter. The sciatic nerve (L4–S3), the largest nerve trunk in the body, typically exits the pelvis directly beneath the inferior border of the piriformis (the infrapiriform foramen). In piriformis syndrome (deep gluteal syndrome), muscle hypertrophy, fibrosis, or anatomic entrapment compresses the sciatic nerve, causing buttock pain and radiating sciatica.