The primary objective of this biomechanical study was to investigate the effect of decompressive surgery, specifically dorsal laminectomy and discectomy, on the stability of the lumbosacral joint in the dog. Different size laminectomies were compared with respect to their effect on lumbosacral stability. A total of eighteen lumbosacral motion units were collected from cadavers and divided into three groups. Group 1 was a control group and received no modification, Group 2 specimens received mini-dorsal laminectomies and discectomies (lamina of L7 caudal to the dorsal spinous process excised, lamina of S1 not affected) while Group 3 specimens received standard dorsal laminectomies and discectomies (75% of L7 lamina and 50% of S1 lamina excised). All specimens were potted in aluminium tubing and mounted in a four-point bending jig and tested in a load cell. Specimens were stressed to 21° in dorsiflexion and ventroflexion. The relevant surgical modification was then performed and the specimens re-tested to 21° in dorsiflexion and ventroflexion. All specimens were then tested to failure in ventroflexion. Force and angular displacement was recorded and used to obtain load-deformation curves for each specimen (5 curves for each specimen). From the load-deformation curves the stiffness (gradient of the graph) was determined at three set angles of deflection. These points were 6°-8°, 12°-16° and 18°-20°. The percentage change in stiffness for each specimen in both dorsiflexion and ventroflexion was obtained. Peak force at failure and angular deformation at failure were obtained when tested to failure in ventroflexion. When examining the overall stiffness of the specimen (dorsiflexion and ventroflexion and all angles of deflection) mini-dorsal laminectomy was shown to result in a 48.3% reduction in stiffness (P < 0.001) while standard dorsal laminectomy and discectomy resulted in a 59.8% reduction in stiffness (P < 0.001). These results were statistically significant. The difference between the two different types of laminectomies could be described as approaching significance (P=0.066). Larger group size would be required to determine whether this is in fact statistically significant Dorsal laminectomy combined with discectomy does have an effect on the stability of the lumbosacral joint. This may contribute to the relatively high recurrence rate following surgical treatment of degenerative lumbosacral stenosis especially in large breed highly active dogs. The study provides further support for decompressive surgery combined with a stabilisation technique when treating degenerative lumbosacral stenosis. It also provides potential support for the use of mini-dorsal laminectomies. / Dissertation (MMedVet)--University of Pretoria, 2009. / Companion Animal Clinical Studies / unrestricted