Posterior Interbody Fusion versus Posterolateral Fusion for Treatment of Segmental Lumbar Spinal Instability

Instability of the spine has been defined as an abnormal movement in the motion segment as a response to applied loads. Segmental instability occurs when an applied force produces displacement of part of a motion segment exceeding that found in a normal spine. Frymoyer et al defined segmental instability as ”a condition where there is loss of spinal stiffness, such that normally tolerated external loads will result in pain, deformity, or place neurological structures at risk.”Frymoyer has described a classification system for degenerative segmental instability. It is based on radiographic findings, as well as a history of previous spine surgery. It is classified into primary and secondary instabilities. Primary instabilities include axial rotational, translational, retrolithetic instability, degenerative scoliosis and internal disc disruption. Secondary instability may follow discectomy, laminectomy or spinal fusion.Instability exists when sudden aberrant motions such as a visible slip or catch are observed during active movements of the lumbar spine or when a change in the relative position of adjacent vertebrae is detected with palpation. An anterior slip of 5 mm or more in the thoracic or lumbar spine or a difference in the angular motion of two adjacent motion segments more than 11 degrees from T1 to L5 and motion greater than 15 degrees at L5-1 compared with L4-5.