Objective. The authors analyze the causes of destabilization of transpedicular spinal instrumentation and prove the measures of its prevention basing on biomechanical modeling results and clinical data. Material and Methods. Experimental study included three series of 10 tests and one series of 12 tests with human spine cadaver specimens. The stability of injured spinal segments after transpedicular instrumentation was studied under mechanical load similar to that experienced by the human spine. Clinical study included the outcome analysis of transpedicular instrumentation in 107 patients with unstable thoracic and lumbar spine injuries. Results. The bone tissue mass around screws inserted in a cranial vertebra for two-segment spinal fusion proved to be the weakest place in a system consisting of a fourscrew transpedicular metal construction and spinal segments. The static mechanical strength of the injured vertebral motion segment stabilized with transpedicular device turned to be lower by 8-42 % (depending on loading conditions) than that of a corresponding intact vertebral motion segment. The identified factors negatively affecting the mechanical stability of transpedicular fixation were the following: osteoporosis, incomplete correction of deformity, motion coordination disorder due to neurological deficit, excessive weight, and postoperative regimen breach. A differentiated approach to reposition and transpedicular instrumentation for significant spinal deformities was offered allowing the restoration of anatomical interrelations and stable fixation in the spine regardless of the time of trauma.

The strength as well as rigidity indices of the spine segments, subjected to osteosynthesis with a transpedicular system, were studied experimentally with respect to vertical compressing and bending kyphosis-producing loads. A technique of modeling bending loads on segments experimentally as well as a technique of modeling vertical loads on segments experimentally was used for the studies. The anatomic preparations of the blocks of segments T12-L2 and Т9-Lз were used with completely preserved discs and ligamentous structures, with the help of which instable injuries and transpedicular osteosynthesis (TPO) were simulated. Two stages of the destabilization process of spinal segments transpedicular metal fixator system were singled out. Local destruction starts when the load is 1300-1400 N or 59,5-68 NM. Complete destabilization occurs for the loads of 2300-2500 N or 68-85 NM.

Study of T9—L3 spinal segment blocks using anatomic preparations has showed that in instable injury of T12 the rigidity of T11—L1 segments under conditions of transpedicular osteosynthesis with four screws spinal system is on average 25% and 14.7% lower than the normal rigidity of the intact T11—L1 segments in relation to bending kyphotic loads and lateral bending loads, respectively. The rigidity of synthesized spinal segments to lateral bending loads is 1.9 times lower than the rigidity to sagittal bending loads. With use of metalwork the rigidity indices of the synthesized spinal segments are on average 1.2 times higher as compared with the rigidity of the intact spine.