Study Notes on Pelvis and Hip Complex

PELVIS AND HIP COMPLEX KIN 365 – APPLIED BIOMECHANICS

General Notes

• The Hip/Pelvic Girdle Complex functions as a closed kinetic chain. This means that forces generated from the ground travel upwards through the lower body, and forces from the upper body travel downwards, all converging and distributing within this complex. Think of it like a central hub or a well-engineered bridge where loads are continuously transmitted and absorbed across interconnected segments.
• The movement of the Hip/Pelvic Girdle is foundational for distal joint movement, initiating motion and providing critical stability and balance for the entire kinetic chain.

GENDER DIFFERENCES

• Characteristics of Female Pelvis:
  • Generally lighter and thinner than the male pelvis.
  • Wider in area compared to the male pelvis.
  • The sacrum is wider and flatter, influencing the overall pelvic inlet and outlet dimensions.

PELVIC GIRDLE MORPHOLOGY

• The pelvic girdle forms the robust bony base that supports the spine and connects the lower limbs, consisting of the following fused bones:
  • Ilium
  • Ischium
  • Pubis
  • In conjunction with the sacrum, these bones create the complete pelvic ring.

BONE FEATURES: THE FEMUR

• The femur, as the longest and strongest bone in the body, is integral to the hip complex. Key morphological features include:
  • Greater Trochanter (located on the femur, a prominent site for muscle attachments).
  • Femur Morphology (anterior view):
  • Head: The spherical component that articulates with the pelvis.
  • Neck: Connects the head to the shaft, a critical region for load bearing and common site for fractures.
  • Lesser Trochanter: Another key site for muscle attachment, particularly for hip flexors.
  • Adductor Tuberosity: A bony prominence linked to muscle insertions.
  • The posterior view of the femur highlights additional features crucial for muscle attachments and joint mechanics.

HIP REGION JOINTS: THE INTERLOCKING CONNECTIONS

• The hip region comprises three major joints, each playing a distinct role in movement and stability:
  • Pubic Symphysis: The cartilaginous junction connecting the two pubic bones anteriorly.
  • Sacroiliac Joint (SI Joint): The articulation between the sacrum and the ilium.
  • Coxofemoral Joint (Hip Joint Proper): The primary ball-and-socket joint connecting the head of the femur to the acetabulum of the pelvis.

PUBIC SYMPHYSIS

• This joint provides a crucial, though limited, connection between the two halves of the pelvis, offering shock absorption and slight mobility, particularly relevant during activities like childbirth.

SACROILIAC (SI) JOINT

• The SI joint is classified as a synovial joint with gliding or plane characteristics, allowing only limited motion. Think of it like two smooth, slightly irregular plates gently sliding against each other, allowing subtle adjustments rather than broad movements. This limited mobility means it's a common site for low back injuries due to the significant forces it transmits.

LIGAMENTS OF THE SACROILIAC JOINT: THE REINFORCING BANDS

• These strong ligaments act as natural seatbelts, heavily reinforcing the SI joint to manage the immense forces transmitted through the pelvis:
  • Iliolumbar Ligament
  • Sacroiliac Ligament
  • Sacrotuberous Ligament
  • Sacrospinous Ligament

COXOFEMORAL JOINT: THE MAIN HUB

• This robust ball-and-socket joint is designed for both significant mobility and stability to support the body's weight.
  • Acetabulum: The deep, cup-like socket of the pelvis that receives the femoral head. It is formed by the fusion of the ilium, ischium, and pubis.
  • The depth of the acetabulum is significantly enhanced by the acetabular labrum, a fibrocartilaginous ring. Imagine the labrum as a rubber gasket or suction cup, effectively deepening the socket to create a more secure fit for the femoral head and increase joint stability.
  • Femur: The head of the femur, covered by articular cartilage, articulates within the acetabulum. The numerous muscles surrounding the femur and spanning both the knee and hip joints are vital for the joint's stability against the body's weight.

STRUCTURAL SUPPORT IN THE COXOFEMORAL JOINT

• The hip joint relies on specific structures for its remarkable stability:
  • Capitis Femoris (Round Ligament): This ligament, though relatively small, primarily functions to hold the head of the femur within the acetabulum, acting like an internal tether, especially during certain movements.
  • Both the femoral head and acetabulum contain large amounts of spongy, trabecular bone, which effectively dissipates forces. Think of this bone structure as a natural shock absorber, distributing impact forces over a larger area.
  • Approximately $70\%$ of the head of the femur articulates with the acetabulum, indicating a high degree of congruence.

HIP LIGAMENTS AND MOVEMENT: THE GUIDES AND RESTRAINTS

• Extracapsular ligaments are dense fibrous bands that further reinforce the joint capsule and resist excessive movements:
  • The iliofemoral ligament (often described as Y-shaped) is one of the strongest ligaments in the body. It resists over-extension, internal rotation, and, to some extent, external rotation, and also constrains abduction.
  • The ischiofemoral ligament primarily functions to resist adduction and internal rotation.
  • It is notable that these ligaments generally do not restrict flexion, allowing for a wide range of forward bending.

FEMORAL TRIANGLE: A CRITICAL ANATOMICAL LANDMARK

• The femoral triangle is a clinically important region in the anterior thigh.
  • Borders:
  • Inguinal Ligament: Connects the Anterior Superior Iliac Spine (ASIS) to the pubic tubercle. It's a significant thickening of the external oblique aponeurosis.
  • Adductor Longus (medial border)
  • Sartorius (lateral border)
  • Contents: The structures passing through this triangle are vital for neurovascular supply to the lower limb:
  • Femoral Nerve
  • Femoral Artery
  • Femoral Vein
  • Additional structures include superficial and deep lymphatic vessels and associated nerves.

ANGLE OF INCLINATION: FEMORAL NECK ALIGNMENT

• This angle describes the relationship between the femoral neck and the femoral shaft, directly influencing hip biomechanics.
  • It is defined as the angle formed by the femoral neck relative to the femoral shaft.
  • The normal range is typically between $90$ to $135$ degrees, with $125$ degrees considered optimal.
  • Coxa Vara: An angle of inclination less than $125$ degrees. This can lead to increased stress on the femoral neck and altered joint mechanics.
  • Coxa Valga: An angle of inclination greater than $125$ degrees. This may reduce the effective lever arm of the hip abductors and increase joint contact pressure.

ANGLE OF ANTEVERSION: FEMORAL NECK ROTATION

• This angle describes the rotation of the femoral neck in the transverse plane, relative to the femoral condyles.
  • Normally, it ranges from $15$ to $25$ degrees anteriorly relative to the femur, influencing how the femoral head sits in the acetabulum and affecting hip rotation capabilities.

COXOFEMORAL MUSCULATURE: THE ENGINE OF MOVEMENT AND STABILITY

• The muscles surrounding the hip joint are organized by their primary actions, working synergistically to produce movement and provide dynamic stability to the complex.

PRIMARY HIP FLEXORS

• These muscles are the main drivers of hip flexion:
  • Iliopsoas: The most powerful hip flexor, composed of:
  • Psoas Major
  • Psoas Minor
  • Iliacus
  • Functions: Primarily hip flexion, with an assistive role in external rotation.
  • Rectus Femoris:
  • Origin (O): Anterior inferior iliac spine (AIIS) and the groove above the acetabulum.
  • Insertion (I): Superior aspect of the patella, connecting via the patellar ligament to the tibial tuberosity (making it a strong knee extensor as well).

ASSISTIVE HIP FLEXORS

• These muscles assist in hip flexion along with their other functions:
  • Tensor Fasciae Latae (TFL),
  • Sartorius,
  • Muscles that cross anterior to the hip joint (e.g., some adductor muscles).

ADDUCTORS AND OTHER MUSCLE GROUPS

• The adductors bring the thigh towards the midline:
  • Adductor Longus:
  • Origin (O): Anterior pubis just below its crest.
  • Action (A): Primary contributor to adduction, also assists in hip flexion.

HIP EXTENSORS

• These muscles are responsible for moving the thigh posteriorly:
  • Gluteus Maximus (the most powerful hip extensor),
  • Biceps Femoris (part of the hamstrings),
  • Semimembranosus (part of the hamstrings),
  • Semitendinosus (part of the hamstrings).
  • These muscles contribute to hip extension and also play roles in external rotation (Gluteus Maximus) or knee flexion (hamstrings).

HIP ABDUCTORS

• These muscles move the thigh away from the midline and are crucial for pelvic stability during gait:
  • Gluteus Minimus,
  • Gluteus Medius (a primary stabilizer during single-leg stance),
  • Tensor Fasciae Latae,
  • Sartorius.
  • Function primarily to produce abduction and contribute significantly to stabilizing the pelvis on the femur.

INTERNAL AND EXTERNAL ROTATORS

• These muscles control the rotation of the femur within the hip joint:
  • Internal Rotators:
  • Tensor Fasciae Latae (TFL),
  • Gluteus Minimus,
  • And other muscles that produce internal rotation when the hip is flexed.
  • External Rotators: A group of deep, small muscles often referred to as the "deep six" external rotators, including:
  • Piriformis,
  • Obturators (Obturator Internus and Externus),
  • Quadratus Femoris,
  • Gemelli (Superior and Inferior).
  • These are primarily involved in external rotation of the hip.

COMMON HIP INJURIES: UNDERSTANDING COMPLICATIONS

• Understanding the anatomy helps in comprehending common injuries:
  • Hip Replacement: Approximately $1$ million operations annually involve total hip joint replacement, where the damaged femoral head and/or acetabulum are substituted with artificial components, often due to arthritis or trauma affecting the ball-and-socket joint.
  • Muscular Low Back Pain: Often arises from tight hamstrings, which can pull on the posterior superior iliac spine (PSIS), leading to an eccentric loading of the lower back muscles and subsequent discomfort or pain over time. This illustrates the interconnectedness of the pelvic girdle and lumbar spine.
  • Contusions (Bruises): Generally result from direct impacts to bony prominences. Severe contusions can lead to pathologies like hip pointers (bruise to the iliac crest) and myositis ossificans (bone formation within muscle tissue) if deep bruises are not treated properly.
  • Snapping Hip & IT Band Friction Syndrome (ITBFS):
  • Snapping Hip often involves tendons, such as those of the gluteal muscles or Tensor Fasciae Latae, snapping over the greater trochanter or iliopsoas tendon snapping over bony prominences during hip movement.
  • ITBFS occurs when the iliotibial (IT) band, a thick band of fascia, rubs excessively against the greater trochanter or the lateral femoral condyle near the knee, generating friction and inflammation, often exacerbated by repetitive motions.