The lumbar spine, or lower back, supports the upper body and facilitates various movements. The L4 and L5 vertebrae are important components in this region. They bear considerable body weight and are important for activities like bending, twisting, and lifting.
Understanding L4 and L5
The L4 and L5 vertebrae are positioned in the lower lumbar spine; L4 is the fourth lumbar vertebra, and L5 is the fifth and lowest, situated directly above the sacrum. Their primary function is providing structural support for the upper body, including the torso, head, and arms. These vertebrae also allow flexibility in the lower back, enabling a wide range of motion. Between each vertebra lies an intervertebral disc, acting as a shock absorber. These discs cushion the spine during movement and protect the spinal cord and exiting nerve roots.
The Nerves Affected by L4 and L5
The L4 and L5 nerve roots emerge from the spinal canal at their respective vertebral levels. These nerve roots are part of the sciatic nerve, the body’s longest and widest nerve, originating from the lower lumbar (L4-L5) and upper sacral spinal nerves (S1-S3). The sciatic nerve descends through the posterior aspect of the thigh before dividing into its two main branches: the tibial nerve and the common fibular (peroneal) nerve.
The L4 nerve root innervates the quadriceps muscles, responsible for knee extension. It also provides sensory input to the inner side of the leg and foot. The L5 nerve root innervates muscles for ankle dorsiflexion (lifting the front of the foot), including the tibialis anterior, extensor hallucis longus (big toe extension), and extensor digitorum longus (toe extension). It also contributes to hip abductor muscles (e.g., gluteus medius and minimus), which move the leg away from the body’s midline, and parts of the hamstring muscles for hip extension and knee flexion. Sensory innervation from the L5 nerve root covers the top of the foot and the area between the first and second toes.
Common Symptoms of L4 and L5 Issues
When the L4 or L5 nerve roots experience compression or irritation, specific symptoms can arise. Pain is a frequent complaint, often described as burning, sharp, or radiating, extending from the lower back through the buttocks and down into the legs. This radiating pain, commonly known as sciatica, can vary depending on whether the L4 or L5 root is affected.
Sensory disturbances are also common, including numbness, tingling, or a “pins and needles” sensation in the lower extremities. If the L4 nerve root is affected, numbness may occur on the inner side of the leg and foot, while L5 nerve root issues can lead to loss of sensation in the big toe and the back of the foot. Motor deficits can manifest as weakness in specific leg muscles, such as difficulty extending the knee if the L4 root is compromised. Compression of the L5 nerve root can lead to weakness in ankle dorsiflexion, making it difficult to lift the front of the foot, a condition known as foot drop. This can cause the foot to drag or slap the ground during walking.
Conditions Impacting L4 and L5
Several spinal conditions commonly affect the L4 and L5 vertebrae and their associated nerve roots. A herniated disc occurs when the soft center of an intervertebral disc bulges or ruptures, compressing nearby nerve roots. Over 90% of herniated discs occur at the L4-L5 or L5-S1 disc space, potentially impinging on L4 or L5 nerve roots.
Spinal stenosis is the narrowing of the spinal canal or nerve root passages, which can pressure L4 and L5 nerve roots. This narrowing often results from bone spurs or degenerative changes.
Spondylolisthesis occurs when one vertebra slips forward over another, commonly L4 over L5. This slippage can compress nerves as the spinal canal or nerve root openings narrow. Degenerative disc disease, a natural part of aging, involves spinal disc wear and tear. Discs lose water and elasticity, leading to thinning and cracking. This L4-L5 degeneration can cause pain and symptoms from reduced shock absorption and nerve irritation.