Could isthmic approach to far lateral disc hernias cause instability?

The aim of this study is to investigate the relationship between the degree of instability and the amount of isthmus excised during transpars (isthmic) approach which is accomplished with partial resection of the pars interarticularis used in distal lateral disc herniation. Thirty-six ovine lumbar spine segments were used in the study. 25% and 50% of the right side isthmus of each spinal segment were excised. Flexion, lateral bending and axial rotation tests were performed in both groups. There was no statistically significant difference found between the groups for yield moment, stiffness and ultimate load in flexion tests (p = 0.262, p = 0.749 and p = 0.200, respectively). Statistically significant difference was found between the groups for yield moment, stiffness and maximum load in lateral bending tests (p = 0.016, p = 0.010 and p = 0.016, respectively). There was no statistical difference found between the groups for yield torque and stiffness in axial rotation tests (p = 0.855 and p = 0.314). These results show that a significant loss of resistance especially during the lateral bending loading was occurred with increasing resection portion of isthmus. With the load applied during the lateral bending of the pars interarticularis, the vertebra resected by 50% percent fractured significantly easier in comparison to the vertebra resected by 25% percent. Pars interarticularis is an important structure with an important role in stability. It is presumed that the more defect is created during the drill-up of the pars interarticularis, the more instability will be occurred. •Partial resection is needed in the transpars approach to pars interarticularis.•Either 25% or 50% of the right side of isthmus of ovine spinal segment were excised.•Yield moment, stiffness and maximum load differed between 2 groups in lateral bending.•There is a relationship between instability and the amount of isthmus resected.•Pars interarticularis has an important role in vertebral stability.