Purpose <p>Instrumented posterior lumbar fixation (PLF) with fusion is the surgical standard for treating various degenerative changes or deformities of the spine. Several studies have shown the risk of adjacent segment pathology following fusion. Therefore, revision procedures are often required which involve extension of instrumentation to adjacent levels. The purpose of this study was to evaluate the stability of multisegmental-instrumentation of the lumbar spine when extended with continuous rods or with connectors to the lower mid thoracic spine.</p> Methods <p>Eight human specimens from Th5 to S1 were used and loaded with moments of 7.5 Nm about the three anatomical axes. A total of eight different PLF configurations were examined, with three conditions in a short Th10-S1 instrumentation and five conditions with a long Th6-S1 instrumentation. Continuous rods, and axial and lateral connectors were used to extend the PLF. Total range of motion (tROM) and total neutral zone (tNZ), as well as the intersegmental range of motion (iROM), were evaluated.</p> Results <p>Overall, all configurations of the PLF demonstrated high primary stability. The iROM was less than 2 degrees in all segments studied where lengthening was performed with continuous rods or connectors. We found only a few significant differences, which are unlikely to be clinically relevant due to their very small magnitudes.</p> Conclusion <p>From a biomechanical point of view, the axial and lateral connectors provide comparable primary stability to a continuous rod system. Thus, the use of connectors is likely to be a suitable approach of extending an existing lumbar fixation to the lower and mid thoracic spine.</p> Level of evidence <p>Biomechanical study.</p>