Biomechanical evaluation of segmental instability in degenerative lumbar spondylolisthesis

Abstract

Here we investigated the biomechanical properties of spinal segments in patients with degenerative lumbar spondylolisthesis (DLS) using a novel intraoperative measurement system. The measurement system comprised spinous process holders, a motion generator, a load cell, an optical displacement transducer, and a computer. Cyclic displacement of the holders produced flexion-extension of the segment with all ligamentous structures intact. Stiffness, absorption energy (AE), and neutral zone (NZ) were determined from the load-deformation data. Forty-one patients with DLS (M/F = 15/26, mean age 68.6 years; Group D) were studied. Adjacent segments with normal discs in six patients (M/F = 3/3, mean age 35 years) were included as a control group (Group N). Flexion stiffness was significantly lower in Group D than in Group N. The NZ, however, was significantly greater in Group D than in Group N. Thus, compared to normal segments, spinal segments with DLS had a lower flexion stiffness and a higher NZ. NZs in Group D were, however, widely distributed compared to those in Group N that showed NZ <2 mm/N in all cases, suggesting that the segment with DLS is not always unstable and that the segments with NZ >2 mm/N can be considered as unstable.

Fig. 1

The new intraoperative measurement system, comprising spinous process holders, a motion generator, an optical displacement transducer, and a personal computer

Fig. 2

Comparison of biomechanical parameters between the segments with spondylolisthesis and those with normal discs. Olisthesis segments with spondylolisthesis, normal segments with normal discs

Fig. 3

A relationship between stiffness and neutral zone. Open box normal (Group N), closed box olisthesis (Group D). Dotted line shows NZ = 2 mm/N

Fig. 4

Biomechanical parameters and representative cases of normal segments (33-year-old woman) and of L4 degenerative spondylolisthesis (73-year-old female). Mid-sagittal view of T2-weighted MRIs (arrows represent the measured segment) and load-deformation data are illustrated. AE absorption energy, NZ neutral zone