Biomechanical cadaver study of proximal fixation in a minimally invasive bipolar construct

Abstract

Study design Biomechanical human cadaver study. Objective To determine the three-dimensional intervertebral ranges of motion (ROMs) of intact and hook-instrumented tho- racic spine specimens subjected to physiological loads, using an in vitro experimental protocol with EOS biplane radiography. Summary of background data Pedicle screws are commonly used in thoracic instrumentation constructs, and their biome- chanical properties have been widely studied. Promising clinical results have been reported using a T1–T5 thoracic hook–claw construct for proximal rod anchoring. Instrumentation stability is a crucial factor in minimizing mechanical complications rates but had not been assessed for this construct in a biomechanical study. Methods Six fresh-frozen human cadaver C6–T7 thoracic spines were studied. The first thoracic vertebrae were instrumented using two claws of supra-laminar and pedicle hooks, each fixed on two adjacent vertebrae, on either side of a single free vertebra. Quasi-static pure-moment loads up to 5 Nm were applied to each specimen before and after instrumentation, in flexion–extension, right and left bending, and axial rotation. Five steel beads impacted in each vertebra allowed 3D tracking of vertebral movements on EOS biplanar radiographs acquired after each loading step. The relative ranges of motion (ROMs) of each pair of vertebras were computed. Results Mean ROMs with the intact specimens were 17° in flexion–extension, 27.9° in lateral bending, and 29.5° in axial rotation. Corresponding values with the instrumented specimens were 0.9°, 2.6°, and 7.3°, respectively. Instrumentation sig- nificantly (P < 0.05) decreased flexion–extension (by 92–98%), lateral bending (by 87–96%), and axial rotation (by 68–84%). Conclusion This study establishes the biomechanical stability of a double claw–hook construct in the upper thoracic spine, which may well explain the low mechanical complication rate in previous clinical studies. Level of evidence Not applicable, experimental cadaver study.