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The pathophysiology of the various types of spondylolisthesis includes:

• Congenitally dysplastic joints
• Hyperextension of the lumbar spine
• Degenerative joint disease (arthritis)
• Trauma
• Bone disease
• Genetics

Congenitally Dysplastic Joints

Children who are born with dysplastic (underdeveloped) facet joints, which affect the stability of the vertebrae, may develop slippage of one vertebra over another. The presence of dysplastic (abnormal development) sacral facet joints is accompanied by forward movement of the L5 vertebra leading to stress on the pars which may then fracture and result in slippage of the L5 over the sacrum. The neural arch typically compresses the nerve as it slips.

Hyperextension of the lumbar spine

Hyperextension of the lumbar spine is typically the cause of isthmic spondylolisthesis through several mechanisms, including:

• Traumatic mechanism - Hyperextension of the lumbar spine is commonly found in children and adolescents who play impact sports and for whom repetitive impact causes stress fractures or fatigue of the pars. Up to 40% of athletes with spondylolysis recall having some type of back injury.
• Weak pars interarticularis - Researchers believe that most children with spondylolysis may be born with a weak pars that makes them more vulnerable to stress fractures that ultimately may evolve into spondylolisthesis. Biomechanical studies suggest that the pars is the weakest part of the posterior neural arch which may predispose it to injury with certain repetitive motions.
• Abnormal bone healing - If normal bone healing does not follow stress microfractures, then the pars can become elongated and cause imbalance setting the stage for slippage. Elongation of the pars can also be a congenital condition.

Degenerative Joint Disease

Chronic degenerative disk disease and arthritis (degenerative joint disease) cause instability between the affected vertebrae that leads to slippage most commonly at the level of L4-L5. Nerve roots coming out of L5 are typically compressed due to stenosis and narrowing of the spinal canal. Progression of slippage occurs in approximately 30% of people, but it usually does not exceed Grade I. Degenerative spondylolisthesis is not related to spondylolysis and may be related to the pars only in terms of the overall degenerative process in the joints.

Some researchers theorize that degenerative spondylolisthesis may also be related to increased laxity of ligaments due to hormonal changes.

Trauma

Trauma can lead to spondylolisthesis by a variety of different mechanisms:

• Fracture of the neural arch
• Fracture of the pars interarticularis
• Fracture of the articulating processes of the vertebrae
• Fracture of the facet joints
• Traumatic subluxation or dislocation of the facet joints

Bone Disease

Certain bone diseases can cause abnormal mineralization of the bone which results in remodeling (effort of the bone to repair itself) or attenuation (reduced density of bone). When the vertebrae are affected, instability ensues that can lead to spondylolisthesis. Some examples of bone disease include:

• Syphilis - a sexually transmitted disease (STD) which in the late stage can cause bone damage
• Paget's disease - causes a malfunction in the normal process of bone breaking down (resorption) and rebuilding (regeneration)
• Albers-Schonberg's disease - an inherited disorder characterized by an increase in bone density