The human pelvis is a basin-shaped ring of bones connecting the vertebral column to the femurs. It consists of two symmetrical halves, and a hemi pelvis, also known as an innominate bone, is one of these halves. Each hemi pelvis serves as the attachment point for a lower limb, connecting to the sacrum to support weight and facilitate movement.
Anatomy of the Hemi Pelvis
Each hemi pelvis appears as a single bone in adults but originates from three separate bones that fuse during adolescence: the ilium, ischium, and pubis. These bones are initially joined by the triradiate cartilage, which ossifies between ages 15 and 17. This fusion creates a solid structure capable of withstanding significant force.
The ilium is the largest, superior component, forming the wing-like structure of the hip. The ischium is the posteroinferior part you sit on. The pubis is the most anterior portion, and the two pubic bones meet at the front at the pubic symphysis.
Where these three bones converge, they form a deep, cup-shaped socket called the acetabulum. This socket serves as the articulation point for the head of the femur (thigh bone). This fit creates the ball-and-socket hip joint, which allows for a wide range of motion while providing stability.
Function Within the Skeletal System
The hemi pelvis has several roles in support, movement, and protection. Its primary function is to bear upper body weight and transfer it to the lower limbs. The pelvic girdle’s rigid nature ensures this transfer is stable, providing a strong foundation.
The structure is also an anchor point for powerful muscles of the trunk, abdomen, back, and lower limbs. Attachments to the ilium, ischium, and pubis generate the force for locomotion, such as hip extension and flexion.
The complete pelvic ring, formed by both hemi pelves and the sacrum, creates a protective basin. This cavity shields organs in the lower abdomen, including the bladder, intestines, and reproductive organs.
Conditions Affecting the Hemi Pelvis
The hemi pelvis can be affected by acute trauma or chronic diseases. High-energy events like car collisions or falls commonly cause pelvic fractures. Because these fractures are near major blood vessels and organs, they can cause extensive internal bleeding and require urgent intervention.
Primary bone cancers can also develop in the hemi pelvis. Chondrosarcoma, a cancer of cartilage cells, is the most frequent type. Other malignancies include Ewing sarcoma and osteosarcoma. Vague symptoms like persistent pain can delay diagnosis.
Congenital issues can affect the formation and alignment of the hemi pelvis. Metabolic bone diseases like osteoporosis also weaken the bone structure, making it more susceptible to fractures from low-impact events, particularly in older individuals.
Surgical Removal (Hemipelvectomy)
When a malignant tumor affects the hemi pelvis, a surgical procedure called a hemipelvectomy may be necessary. This operation involves removing all or part of the hemi pelvis to treat sarcomas when other therapies are not viable. It is a complex procedure performed with the goal of complete tumor removal.
An external hemipelvectomy, or hindquarter amputation, removes the hemi pelvis along with the entire lower limb on the same side. This extensive surgery is indicated when a tumor involves the sciatic nerve or major blood vessels, making limb preservation impossible.
In contrast, an internal hemipelvectomy is a limb-sparing procedure where the affected pelvic bone is resected, but the leg is preserved. This is the preferred approach if the tumor can be removed without sacrificing neurovascular structures. Surgeons then reconstruct the pelvis using bone grafts or prostheses to restore stability.