RÉSUMÉ
<p><b>OBJECTIVE</b>To observe the influences of pedicle screws in various insertion depth on the adjacent segment disc degeneration following lumbar spinal fusion. To explore the relationship between the internal fixation rigidity and incidence of adjacent segment disease.</p><p><b>METHODS</b>Sixteen hybrid male Bohr goats of 10 months old, weighting between 25 and 30 kg, were randomly devided into a control group (N group), and 3 experimental groups, each group had 4 goats. The L4 vertebra of each goat in the experimental groups was fractured, L3-L5 segments were internal fixed with pedicle screws followed by intervertebral joint fusion by a posterior approach. Three experimental groups were devided according to the length of pedicle screws applied, vertebras of goats in L group were internal fixed by the screws at the length of 25 mm, for M group and S group, 20 mm and 15 mm, accordingly. The goats in the control group were treated without any operation. Biomechanical changes and MRI index of upper unfused segment (L2) were measured 24 weeks after operation, and histological changes were observed as well.</p><p><b>RESULTS</b>The pressure and straining of L2 vertebral body and intervertebral disc of L group increased more than N group (P < 0.05), and degenerated cell counting in nucleus pulposus increased as well (P < 0.05). However, MRI index remain unchanged (P > 0.05).</p><p><b>CONCLUSIONS</b>Rigid internal fixation increases the pressure and straining of vertebral body and intervertebral disc of upper adjacent segment, accelerating the degeneration process following lumbar spinal fusion in goats.</p>