Cross-links in Minimally Invasive Spinal Instrumentation: Are they necessary?

Presented at SMISS Global Forum 2014
By Arvind Kulkarni MS(ORTH) Dip(ORTH) FCPS Dip(SICOT)

Disclosures: Arvind Kulkarni MS(ORTH) Dip(ORTH) FCPS Dip(SICOT) None

Introduction:
Single or multiple cross-links (Transverse-Connectors) are routinely used to augment posterior spinal-constructs with the rationale of providing rotational stability. Minimally Invasive Spinal Instrumentation (MISI) is a well-established technique and it is possible to instrument single to multiple motion-segments with longitudinal connectors (Screw and rod systems) .To our knowledge, there have been no techniques to incorporate a transverse connector (cross-link) in a MISI construct.

Aims/Objectives:
The purpose of this study was to critically assess if there is a need to innovate techniques for inserting cross-links to augment MISI constructs.

Methods:
The spinal-constructs of patients of varied etiology that underwent open surgery between July-2007 and July-2011 without the usage of cross-links were evaluated. The immediate post-operative erect-radiographs were compared with the erect-radiographs at the last follow-up by two independent observers (spine fellows not involved in the management of the patients) critically for any rotational instability using the Nash-Moe technique of assessment of vertebral rotation as well as for any ‘parallelogram effect (quadrilateral shift)’. The intra-observer and inter-observer reliability was analyzed.

Results:
There were 208 cases included in the study, during the study period; satisfying the criteria. The total number of motion segments fused was 707 ranging from one to 15 involving various etiologies. The average follow-up was 15 months (12 months to 36 months). Barring one patient with a thoraco-lumbar fracture with rotational instability (AO Type C) that had undergone a short-segment fixation, none of the cases demonstrated any rotational instability in the follow-up radiographs. Interestingly, the rotational instability (parallelogram-effect) in that patient got corrected spontaneously once anterior reconstruction was performed. The intra-observer reliability was 100 % and the inter-observer reliability was 92.83%. This variability was in assessing the grade of vertebral rotation only; none of the levels had a change in rotation irrespective of variation in grade assessment in the final post-op radiograph.

Conclusions:
This study concludes that utilization of cross-links in open constructs may be avoidable and the same can be extrapolated to MISI constructs. The derivations from biomechanical studies do not translate into clinical advantages. The very basis of MISI is minimal damage to the native anatomy, which in itself lends stability to the construct. There is no clinical evidence that cross-links would augment the stability of the constructs. Thus any research or innovation in techniques to incorporate cross-links in MISI constructs appears to be futile.