Biomechanical evaluation of lumbar spinal stability after graded facetectomies. In an in vitro experiment using fresh human lumbar functional spinal units, the effects of the division of the posterior ligaments (consisting of the supraspinous/interspinous ligaments) and graded facetectomies were investigated. The graded facetectomies consisted of unilateral and bilateral medial facetectomies, and unilateral and bilateral total facetectomies. Six kinds of moments were applied and ranges of motion (ROM) and neutral zones (NZ) were determined three-dimensionally by stereophotogrammetric methods. Range of motion was not affected by the division of the supraspinous/interspinous ligaments for all load modes. In flexion, ROM increased slightly after unilateral medial facetectomy. In right axial rotation, ROM increased after left unilateral total facetectomy. Range of motion was not affected, even by bilateral total facetectomies, in extension and lateral bendings. This study suggested that medial facetectomy does not affect lumbar spinal stability, and conversely, total facetectomy, even created unilaterally, makes the lumbar spine unstable. 