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Biomechanical Risk Factors for Proximal Junctional Kyphosis: A Detailed Numerical Analysis of Surgical Instrumentation Variables

Cammarata, Marco MASc*,†; Aubin, Carl-Éric PhD, PEng*,‡; Wang, Xiaoyu PhD*,‡; Mac-Thiong, Jean-Marc MD, PhD†,‡

doi: 10.1097/BRS.0000000000000222

Study Design. Biomechanical analysis of proximal junctional kyphosis (PJK) through computer simulations and sensitivity analysis.

Objective. To gain biomechanical knowledge on the risk of PJK and find surgical solutions to reduce the risks.

Summary of Background Data. PJK is a pathological kyphotic deformity adjacent to the instrumentation. Clinical studies have documented its risk factors, but still little is known on how it is correlated with various individual instrumentation variables.

Methods. Biomechanical spine models of 6 patients with adult scoliosis were developed, validated, and then used to perform 576 simulations, varying the proximal dissection procedure, the implant type at the upper instrumented vertebra, the sagittal rod curvature, and the proximal diameter of the proximal transition rods. Four biomechanical indices—the proximal junctional kyphotic angle, thoracic kyphosis, proximal flexion force, and proximal flexion moment—were assessed.

Results. The bilateral complete facetectomy, the posterior ligaments resection, and the combination of both increased the proximal junctional kyphotic angle (respectively, by 10%, 28% and 53%) and the proximal flexion force (4%, 12%, and 22%) and moment (16%, 44%, and 83%). Compared with pedicle screws at upper instrumented vertebra, proximal transverse process hooks reduced the 3 biomechanical indices by approximately 26%. The use of proximal transition rods with reduced proximal diameter from 5.5 mm to 4 mm decreased the proximal junctional kyphotic angle (by 6%) and the proximal flexion force (4%) and moment (8%). The increase of the sagittal rod curvature from 10° to 20°, 30°, and 40° increased the proximal junctional kyphotic angle (by 6%, 13%, and 19%) and the proximal flexion force (3%, 7%, and 10%) and moment (9%, 18%, and 27%).

Conclusion. Preserving more posterior proximal intervertebral elements, the use of transition rods and transverse process hooks at upper instrumented vertebra, and reducing the global sagittal rod curvature each decreased the 4 biomechanical indices that may be involved in PJK.

Level of Evidence: N/A

The individual effect of 4 independent instrumentation variables on the proximal junctional kyphosis was investigated through numerical simulations. The biomechanics of the proximal junctional spinal segment could be improved by reducing posterior bony resection, ligament dissection, and sagittal rod curvature, and using tapered transition rods and transverse process hooks at upper instrumented vertebra.

*Department of Mechanical Engineering, Polytechnique Montréal, Downtown Station, Montreal (Quebec), Canada

Research Center, Hôpital du Sacré-Cœur de Montréal, Montreal (Quebec), Canada; and

Research Center, Sainte-Justine University Hospital Center, Montreal (Quebec), Canada.

Address correspondence and reprint requests to Carl-Éric Aubin, PhD, PEng, Department of Mechanical Engineering, NSERC/Medtronic Industrial Research Chair in Spine Biomechanics, Polytechnique Montréal, PO Box 6079, Downtown Station, Montreal, Quebec, H3C 3A7, Canada; E-mail:

Acknowledgment date: September 19, 2013. First revision date: December 19, 2013. Acceptance date: January 9, 2014.

The manuscript submitted does not contain information about medical device(s)/drug(s).

Natural Sciences and Engineering Research Council of Canada (Industrial Research Chair with Medtronic of Canada) funds were received to support this work.

Relevant financial activities outside the submitted work: grant, board membership, consultancy, grants/grants pending, development of educational presentations, patents, royalties, stock/stock options, payment for manuscript preparation, support for research chair and support for spine fellows.

© 2014 by Lippincott Williams & Wilkins