In order to evaluate biomechanically the efficacy of four types of posterior instrumentation for the stabilization of isthmic spondylolisthesis of the lumbosacral spine, mechanical non-destructive cyclic testing in axial compression, flexion, extension, and rotation was performed on six fresh lumbosacral spines from calves. Each segment contained four motion segments, including the lumbosacral junction. Isthmic spondylolisthesis was created by sectioning the pars interarticularis of the sixth lumbar vertebra and all posterior ligaments between the fifth and sixth lumbar levels. Eight constructs were tested sequentially: (1) the intact spine, (2) the destabilized spine, (3) the spine fixed with Harrington double-distraction rods, (4) the spine treated with transpedicular Cotrel-Dubousset instrumentation with a transverse approximating device, (5) the spine treated with Steffee transpedicular screws and plates, (6) the spine treated with posterior lumbar interbody arthrodesis, (7) the spine treated with Cotrel-Dubousset instrumentation and posterior lumbar interbody arthrodesis, and (8) the spine treated with Steffee instrumentation and posterior lumbar interbody arthrodesis. One motion segment was involved in each construct, except for the spine that was fixed with Harrington instrumentation, which involved three segments. Strain across the supraspinous and anterior longitudinal ligaments was measured with two extensometers that were attached at the spondylolisthetic level and at the intact motion segments adjacent to the fixed level. Harrington instrumentation was the least rigid construct under any type of loading except axial compression (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)