Rod derotation vs. direct incremental segmental translation : A biomechnical analysis

2009

BACKGROUND CONTEXT

Scoliosis is corrected by different maneuvers applied to the spine via a mechanical constructs, with rods usually bent to desired sagittal profile. Basic techniques involve vertebral translation, rod derotation, direct vertebra derotation, compression and distraction, and in situ rod contouring. In order to maintain the correction rods are fully seated and locked into the slot of each implant, making it difficult to fine-tune the implant-rod relative location and control the force distribution amongst the implants. Direct incremental segmental translation (DIST) was proposed to provide a better control on the vertebra location with respect to the rod. The most distinguishing point of this concept is the ability to translate each implant toward and fixed on the rod from any distance and at any angle.

PURPOSE

Compare the forces at the bone-screw interface during scoliosis correction using rod derotation vs. DIST.

STUDY DESIGN/SETTING

We analyzed the biomechanics of two instrumentation paradigms: rod derotation technique (RDT) vs. a direct incremental segmental translation approach (DIST).

PATIENT SAMPLE

Reduction techniques documented using pre- and post-op radiographs as well as intra-operative video of surgical maneuvers of 10 cases were used to develop a model for computer simulation of correction techniques for scoliosis.

OUTCOME MEASURES

Computer simulation of scoliosis correction.

METHODS

A common curve pattern (thoracolumbar curve) for adolescent idiopathic scoliosis was chosen. Simulations with both the RDT and DIST techniques were performed using the same patient biomechanical model built using biplanar X-rays, with same instrumentation levels and rod shape. The correction maneuvers and resulting effects were analyzed and compared.

RESULTS

The vertebra position relative to the rod for the DIST is determined by 5 independent variables (position, orientation) vs. 2 for the RDT; thus increasing the possible correction of the connected vertebra. The DIST allows the spine deformity to be reduced by either gradually pulling the spine towards the rod through helical connections or translating it by pivoting the posts. Load at the vertebra-implant connection was on average 18% lower for the DIST, and better distributed (lower STD).

CONCLUSIONS

The direct incremental segmental translation approach allows more control with better load sharing amongst implants.

Significance

This analysis provides insight into the different biomechanical effects of the 2 instrumentation paradigms.