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Spondylolisthesis occurs when one vertebra slips over the vertebra just below it. Though it can occur at the level of cervical vertebrae, it occurs most often at the level of the lumbar vertebrae (the focus of this Guidebook). Its symptoms can range from being asymptomatic (no symptoms) to severe. The slippage may result in lordosis (swayback) for some people and in kyphosis (roundback) where the upper spine falls over the lower spine in others (this is a late manifestation of high-grade slippage). Most cases of spondylolisthesis are not severe and do not involve high levels of slippage. Spondylolisthesis can be caused by various conditions but the symptoms are basically similar and include:

• Lower back pain
• Stiffness in the back
• Localized pain or tenderness in the back just above the pelvis
• Tight hamstrings
• Pain in the thighs and legs (radiculopathy)
• Pain in the buttocks

The standard accepted classification system for spondylolisthesis is that of Newman, Wiltse, and McNab. This system describes the etiology of five types of spondylolisthesis and also highlights the anatomic lesion responsible for the slip. The five types of spondylolisthesis include:

• Dysplastic or congenital spondylolisthesis
• Isthmic spondylolisthesis
• Degenerative spondylolisthesis
• Traumatic spondylolisthesis
• Pathologic spondylolisthesis

Recently, a sixth type of spondylolisthesis has been recognized, namely iatrogenic spondylolisthesis.

Dysplastic or Congenital Spondylolisthesis

This type of spondylolisthesis accounts for up to 20% of treated cases. Slippage is related to a congenital malformation of spinal structures, usually a neural arch defect of L5 or the upper sacrum or dysplastic sacral facet joint. Spina bifida (incomplete closure of the embryonic neural tube) is frequently present (seen in up to 94% of cases of congenital spondylolisthesis) and scoliosis (spinal curvature) at the level of the slip is noted in up to 50% of cases of congenital spondylolisthesis. There is a high rate of nerve root involvement such as compression, associated with dysplastic spondylolisthesis. Dysplastic spondylolisthesis usually causes pain during adolescence but not during childhood.

Isthmic Spondylolisthesis

This type is due to progression of spondylolysis (the presence of a defect or fracture of the pars) that becomes unstable and causes slippage of one vertebra (usually L5) over the one below it (S1). Isthmic spondylolisthesis is the main focus of this Guidebook. There are three subtypes of isthmic spondylolisthesis:

• Subtype IIA - a defect due to a fatigue fracture of the articulating segment and associated with complete bony separation. This is more common in individuals under 50 years of age.
• Subtype IIB - elongation of the articulating segment due to repeated microfractures, but no separation. Bony areas may form around the improperly healed microfractures. The result is an elongated pars which may eventually break and may transform to Type IIA over time.
• Subtype IIC - acute fracture of the articulating segment

Spondylolysis and isthmic spondylolisthesis are discussed in greater detail below.

Degenerative Spondylolisthesis

This type of spondylolisthesis is due to chronic degeneration of the vertebral facets and is considered in most cases as secondary to osteoarthritis. It usually occurs later in life after the age of 50 and is the cause of spinal stenosis (narrowing of the spinal canal). The location of degeneration is most frequently at the level of L4-L5 (6-10 times more frequent than other locations), though it can also be present at the L3-L4 level. More infrequently, it may take place at the L5-S1 level. It is diagnosed in females approximately five times more frequently than males. Degenerative spondylolisthesis is the most common type of spondylolisthesis in adults. In some people, there may be a combination of disk degeneration and/or pars injury that causes the slippage. The degree of listhesis (slippage) usually does not progress beyond Grade I.

Traumatic Spondylolisthesis