The effect of posterior polyester tethers on the biomechanics of proximal junctional kyphosis: a finite element analysis.
OBJECTIVE Proximal junctional kyphosis (PJK) remains problematic following multilevel instrumented spine surgery. Previous biomechanical studies indicate that providing less rigid fixation at the cranial aspect of a long posterior instrumented construct, via transition rods or hooks at the upper instrumented vertebra (UIV), may provide a gradual transition to normal motion and prevent PJK. The purpose of this study was to evaluate the ability of posterior anchored polyethylene tethers to distribute proximal motion segment stiffness in long instrumented spine constructs. METHODS A finite element model of a T7-L5 spine segment was created to evaluate range of motion (ROM), intradiscal pressure, pedicle screw loads, and forces in the posterior ligament complex within and adjacent to the proximal terminus of an instrumented spine construct. Six models were tested: 1) intact spine; 2) bilateral, segmental pedicle screws (PS) at all levels from T-11 through L-5; 3) bilateral pedicle screws from T-12 to L-5 and transverse process hooks (TPH) at T-11 (the UIV); 4) pedicle screws from T-11 to L5 and 1-level tethers from T-10 to T-11 (TE-UIV+1); 5) pedicle screws from T-11 to L-5 and 2-level tethers from T-9 to T-11 (TE-UIV+2); and 6) pedicle screws and 3-level tethers from T-8 to T-11 (TE-UIV+3). RESULTS Proximal-segment range of motion (ROM) for the PS construct increased from 16% at UIV-1 to 91% at UIV. Proximal-segment ROM for the TPH construct increased from 27% at UIV-1 to 92% at UIV. Posterior tether constructs distributed ROM at the UIV and cranial adjacent segments most effectively; ROM for TE-UIV+1 was 14% of the intact model at UIV-1, 76% at UIV, and 98% at UIV+1. ROM for TE-UIV+2 was 10% at UIV-1, 51% at UIV, 69% at UIV+1, and 97% at UIV+2. ROM for TE-UIV+3 was 7% at UIV-1, 33% at UIV, 45% at UIV+1, and 64% at UIV+2. Proximal segment intradiscal pressures, pedicle screw loads, and ligament forces in the posterior ligament complex were progressively reduced with increasing number of posterior tethers used. CONCLUSIONS Finite element analysis of long instrumented spine constructs demonstrated that posterior tethers created a more gradual transition in ROM and adjacent-segment stress from the instrumented to the noninstrumented spine compared with all PS and TPH constructs. Posterior tethers may limit the biomechanical risk factor for PJK; however, further clinical research is needed to evaluate clinical efficacy.
Department of Orthopaedic Surgery, New York University/Hospital for Joint Diseases.
Shay
S
Bess
NuVasive, Inc., San Diego, California.
Jeffrey E
JE
Harris
NuVasive, Inc., San Diego, California.
Alexander W L
AW
Turner
Department of Orthopaedic Surgery, Hospital for Special Surgery, New York, New York.
Virginie
V
LaFage
Department of Neurosurgery, University of Virginia Medical Center, Charlottesville, Virginia; and.
Justin S
JS
Smith
Department of Neurosurgery, University of Virginia Medical Center, Charlottesville, Virginia; and.
Christopher I
CI
Shaffrey
Department of Orthopaedic Surgery, Hospital for Special Surgery, New York, New York.
Frank J
FJ
Schwab
Atlanta Brain and Spine Care, Atlanta, Georgia.
Regis W
RW
Haid
D001696:Biomechanical Phenomena; D020342:Finite Element Analysis; D006801:Humans; D007738:Kyphosis / Q000503:physiopathology* / Q000601:surgery; D017846:Longitudinal Ligaments; D008159:Lumbar Vertebrae / Q000503:physiopathology; D008954:Models, Biological; D065289:Pedicle Screws; D011312:Pressure; D016059:Range of Motion, Articular; D013312:Stress, Physiological; D013904:Thoracic Vertebrae / Q000503:physiopathology
ISL = interspinous ligament; PJF = proximal junctional failure; PJK = proximal junctional kyphosis; ROM = range of motion; SSL = supraspinous ligament; TPH = transverse process hooks; UIV = upper instrumented vertebra; biomechanics; proximal junctional kyphosis; spine deformity