the effect of posterior thoracic spine anatomical structures on motion segment flexion stiffness study design this in vitro human cadaveric study tested the loss of thoracic motion segment flexion stiffness after sequential posterior upper instrumented vertebra anchor placement techniques and posterior column destabilizationobjective this study was designed to determine the possible destabilizing effects of upper thoracic instrumentation anchor site preparationsummary of background data proximal junctional kyphosis after instrumentation and arthrodesis for scoliosis and related spine deformities has recently been reported to range from 10 to 46 the effect of posterior skeletal dissection associated with upper instrumented vertebra anchor placement on adjacent motion segment flexion stiffness has not been previously studied methods twentythree intact thoracic motion segments were obtained from 6 human cadavers biomechanical testing was performed with each motion segment flexed to approximately 32 degrees at a rate of 01 hz with corresponding torques recorded data were collected after a series of 6 posterior procedures differences with p value 001 were considered significant and those with p value 005 marginally significantresults supratransverse process hook supralaminar hook pedicle screw placement or pedicle screw removal done bilaterally produced similar small range 209603 nonsignificant reductions in motion segment flexion stiffness but when totaled these 4 procedures resulted in a significant 1631 loss of flexion stiffness the fifth procedure of supraspinous and interspinous process ligament transection added a marginally significant 659 incremental loss of flexion stiffness supralaminar hook site preparation combined with supraspinous and interspinous process ligament transection resulted in a marginally significant 1262 incremental loss of flexion stiffness transection of the remaining posterior structures facet joints and all other posterior soft tissue structures produced a significant additional flexion stiffness loss of 4472 the anterior column alone provided only 3239 of the total motion segment flexion stiffness transection of all posterior stabilizing structures similar to a smithpetersonchevronponte resection decreased motion segment flexion stiffness significantly 6761conclusion posterior thoracic skeletal structures involved in upper instrumented vertebra exposure andanchor placement were found to contribute to adjacent segment flexion stiffness although stiffness loss was small after individual procedures the effects were additive for routinely used combinations