Lumbar Spine Surgery

Part 1: Lumbar anatomy
Part 2: Lumbar degenerative conditions
Part 3: Lumbar surgery options

Part 1: Basic Anatomy of the Lower Back

Bones
The lumbar spine, or low back, consists of 5 vertebrae numbered L1 through L5. These vertebrae (vertebral bodies) are the largest in the spine and support the head and trunk. For example, the L5 vertebra transfers upper body weight through the sacrum and pelvis into the legs. The sacrum consists of 5 naturally fused vertebrae and provides a stable platform for the spinal column. Although the bones of sacrum are fused, they are numbered S1 through S5. The pelvis is often referred to as the hip.

Each lumbar vertebra shares a basic structure:

• Vertebral body - a large kidney-shaped bone; when viewed from above
  
• Pedicles - horizontal cylinders of bone that project outward off the back of a vertebral body
  
• Lamina - a thin plate of bone that forms an arch behind a vertebral body
  
• Articular processes - upward and downward bony projections from the lamina
  
• Facet joints - smooth cartilage-lined structures at the ends of the articular processes
  
• Transverse processes - long, thin bony sideway projections at the junction of the pedicle and lamina

The Discs (Intervertebral Discs)
Between each vertebral body is a disc. Discs are numbered according to their spinal position. For example, the L4-L5 disc is found between the 4th and 5th lumbar vertebrae. The low back discs are the spine's largest. The discs match the kidney-shape of the vertebrae. There are 2 parts to each disc. The outer layer consists of rings of tough, yet flexible cartilage called the annulus fibrosus. Inside the annulus is a jelly-like substance called the nucleus pulposus. The nucleus contains mostly water to help maintain hydrostatic disc pressure (balance). Discs perform 2 important functions:

• Act as spinal shock absorbers between the vertebral bodies
  
• Function as flexible pivots to help provide motion between the vertebrae

Spinal Cord, Cauda Equina and Nerves
The spinal cord is a firm, yet delicate structure of nerve tissue. It cannot be easily moved without the potential for serious neurological injury. The spinal cord ends at L1 (first lumbar vertebra). Below L1, encased in a thick sack of fluid similar to a water balloon, are nerves that make up the cauda equina. Cauda equina is Latin meaning horse's tail, which is what the human cauda equina resembles. The cauda equina is somewhat flexible and may be moved slightly to gain access to the intervertebral discs during posterior (back) low back spine surgery. Spinal nerves exit the cauda equina through small holes called lateral foramina and control leg function and sensation.