Intimate relationship between instability and degenerative signs at L4/5 segment examined by flexion–extension radiography

Relationship between lumbar disc degeneration and segmental instability has remained controversial. Using instability factors that found close relations with symptoms in flexion–extension radiographic study, their relationship with degenerative findings was examined. More than (≥) 3 mm slip in neutral position (SN), ≥3 mm translation (ST), and ≥10° angulation (SA) at L4/5 segment were defined as instability factors and were applied on 447 patients who had low back and/or leg pain and satisfied inclusion criteria for accurate measurements. Radiologic findings for degeneration were disc height including three groups with different disc heights divided by mean ± 1 standard deviation, length of the anterior spur formation, presence of vacuum phenomenon, and endplate sclerosis. As results, group with SN factor was the oldest in age and the lowest in disc height; in contrast, group with SA was the youngest in age and the highest in disc height. The group with ST showed a mid-standing position in both age and disc height. These findings indicate that instability factors are intimately related to age and disc height. The three different disc height groups showed more anterior slip according to the progression of the disc height diminution. Presence of the apparent spur formation and/or vacuum phenomenon had an intimate relationship with the ST factor. Disc height was the most important in the examined parameters and showed an intimate relationship with age and instability factors. Although the etiology is still unknown, clinical common knowledge, that a diminution of disc height with progressive degeneration had a close relation with anterior vertebral slippage, was firstly confirmed. This study allows comprehensive understanding of segmental instability and is useful for considering surgical indications.