Pelvis and Thigh Osteology

Pelvis and Thigh Osteology

Introduction

• The lecture focuses on the bony architecture of the pelvis and upper thigh, specifically the osteology of this region, including the proximal femur (thigh bone) and the pelvic bones.

• The pelvis is discussed to provide a complete picture of the lower limb, even though a detailed examination of the pelvis will occur later in the course.

• The hip joint discussion is deferred to a later session, but clinical implications, such as hip fractures, will be addressed.

Pelvic Bones (Os Coxa or Innominate Bone)

• The os coxa forms the bony pelvis and acts as a wedge-shaped stone in the superolateral portion of the arch, similar to how the sacrum serves as the keystone.

• The os coxa is composed of three separate bones that fuse in the third decade of life: the ilium, ischium, and pubis.

Ilium

• The ilium is the largest and most superior of the three bones.

• The ala (wings) of the ilium project laterally, forming the iliac crests, which serve as attachment points for abdominal musculature.

• The iliac crests terminate in the anterior superior iliac spine (ASIS) and posterior superior iliac spine (PSIS), which are also muscle attachment sites.

• Below these are the anterior inferior iliac spine and posterior inferior iliac spine, additional sites for muscle attachments.

• The auricular surface of the ilium articulates with the sacrum medially. Its irregular convolutions minimize joint movements.

• The ilium fuses with the other pelvic bones at the acetabulum, the socket for the head of the femur.

• The acetabulum includes:

  • The articular lunate surface: makes direct contact with the head of the femur.

  • The acetabular fossa: opens inferiorly as the acetabular notch, which is separated from the head of the femur by a small gap.

Ischium

• The ischium is located posteriorly and fuses at the acetabulum.

• Key structures include the ischial tuberosity.

• The ischial ramus projects anteriorly, laterally, and inferiorly towards the acetabulum.

Pubis

• The pubic bone is located anteriorly.

• The body of the pubis lies anteriorly with superior and inferior rami projecting towards the acetabulum and the ischial ramus, respectively.

• These rami form the obturator foramen, which facilitates neurovascular communication with the medial compartment of the thigh.

Additional Notes

• The collective protrusions and processes of the os coxae will be identified in small group osteology sessions and discussed in relation to muscle attachments.

Sacroiliac Joint

• The sacroiliac joint, located between the sacrum and ilium, has limited motion and plays a role in shock absorption from ground reaction forces during walking and running and helps resist pelvic fractures.

• It consists of two parts:

  • Anteriorly: a synovial joint between the ilium and sacrum.

  • Posteriorly: a syndesmosis, where bone tuberosities are connected through thick ligaments without a true joint space.

• The sacroiliac joint is reinforced by intrinsic and extrinsic ligaments, including:

  • Deep interosseous ligaments: bind the posterior tuberosities.

  • Posterior sacroiliac ligaments: provide additional reinforcement.

  • Anterior sacroiliac ligaments: reinforce the anterior synovial joint.

  • Iliolumbar ligaments.

  • Sacrospinous ligament: runs from the sacrum to the ischial spine.

  • Sacrotuberous ligament: runs from the sacrum to the ischial tuberosity, blending with the common hamstring origin.

Role of Sacrospinous and Sacrotuberous Ligaments

• The fifth lumbar vertebra and axial load are anterior to the sacroiliac joints, creating a tendency for the sacrum to rotate anteriorly relative to the ilium.

• The sacrospinous and sacrotuberous ligaments resist this rotation by anchoring the posterior aspect of the sacrum to the inferior aspect of the pelvis.

Sciatic Foramen and Obturator Canal

• The greater sciatic notch on the posterior surface of the ilium and ischium is transformed into the greater sciatic foramen by the sacrospinous and sacrotuberous ligaments.

• The lesser sciatic notch, inferior to the ischial spine, becomes the lesser sciatic foramen due to the same ligaments.

• These foramina allow passage of muscles and neurovascular components between the pelvic cavity and the gluteal region.

• The obturator foramen is mostly closed by the obturator membrane, which provides a site for muscle attachment, leaving the obturator canal for passage of the obturator nerve and vessels.

Femur

• The lecture discusses the prominent features of the femur, even though the distal components will be addressed later.

Key Features

• Head of the femur: directed superomedially towards the acetabulum.

• Neck of the femur: elongated compared to the anatomical neck of the humerus, compensating for the depth of the acetabulum.

• Greater and lesser trochanters: oblique bony masses serving as muscular attachment sites.

• Shaft of the femur: angles down towards the knee.

• Gluteal tuberosity: on the posterior surface, serves as an attachment point for the gluteus maximus muscle.

• Linea aspera: a prominent ridge of bone on the posterior surface, serving as an elongation site for numerous muscle attachments.

• Femoral condyles: articulate with the tibia to form the knee joint.

• Epicondyles: located near the condyles, serve as sites for tendon and ligament attachment.

• Adductor tubercle: proximal to the medial epicondyle, allows attachment of a prominent muscle in the medial compartment of the thigh.

Geometric Orientation of the Femoral Neck

• Angle of inclination: the angle between the neck and shaft of the femur in the coronal plane, typically between 120 and 135 degrees.

  • Coxa valga: a higher angle of inclination, common in young individuals.

  • Coxa vara: a smaller angle of inclination, common in the elderly and a confounding factor in hip fractures.

• Torsion angle: the degree to which the neck projects posteriorly relative to the femoral shaft in the transverse plane.

Hip Fractures

• A hip fracture is a general term for any fracture to the femoral neck or proximal shaft of the femur, often outside the joint capsule.

• Radiography confirms the fracture and misalignment of the fractured heads.

• The distal end is typically displaced superiorly due to muscle contraction and laterally rotated due to muscle imbalance.

• Patients present with severe pain in the lateral pelvic region and inability to bear weight on the affected limb, which appears shortened and laterally rotated.

• Distal fractures can be surgically corrected with rods and plates.

• Fractures affecting the articular surface or complicated by osteoarthritis may require a total hip replacement.

Bone Mineral Density and Hip Fractures

• Hip fractures are common in the elderly due to loss of bone mineral density.

• The femur consists of outer cortical bone and inner cancellous bone.

• Cancellous bone's porous nature reduces the overall weight of the bone.

• Bone mineralization follows the compression and tension lines of force in a weight-bearing position.

• Loss of bone mineralization and organic collagen matrix makes bones brittle and subject to fracture.

• Deficient bone mineral density in younger populations is less likely to cause fracture due to a higher concentration of organic matrix, which provides flexural strength.

• Selective loss in bone mineralization without loss of organic matrix makes the bone more flexible than brittle.

• Severe bone demineralization, as seen in rickets (associated with vitamin D deficiency), can cause permanent bowing of weight-bearing structures.