The influence of spinal fusion length on proximal junction biomechanics: a parametric computational study
Proximal junctional kyphosis and failure are frequent complications in adult spinal deformity surgery with long fusion constructs. The aim of this study was to assess the biomechanics of the proximal segment for fusions of various lengths.
A previously established musculoskeletal model of thoracolumbar spine was used to simulate full-range flexion task for fusions (modeled by introduction of rigid constraints) with lower instrumented vertebrae at L5 or S1 and upper instrumented vertebrae (UIV) at any level above, up to T2. Inverse dynamics simulations with force-dependent kinematics were performed for gradually increasing spinal flexion in order to predict global and segmental range of flexion, maximum passive moment, segmental compression and shear forces, which were compared to the uninstrumented case.
For long fusions, with the UIV at T11 or higher, the model predicted an increase in segmental flexion (by 33–860%, or 1.6°–4.7°) and passive moment (by 39–1370%, or 13–31 Nm) at the proximal junction—generally increasing with fusion length. While the maximum shear force was 57–239% (135–283 N) higher for the proximal junction at the upper thorax (UIV at T6 or above), the compression forces were reduced by up to 44% (375 N).
The length of the instrumentation has an important effect on the proximal segment biomechanics. Despite the limitations of the current model, musculoskeletal modeling appears to be a promising and versatile method to support planning of spinal instrumentation surgeries in the future.
KeywordsSpinal fusion Proximal junctional failure Biomechanics Musculoskeletal model Thoracolumbar spine
This study has been supported by a research grant from Mäxi Foundation, Switzerland.
Compliance with ethical standards
Conflict of interest
S. J. Ferguson received research project support from DePuy Johnson & Johnson. D. Ignasiak, T. Peteler, T. F. Fekete and Daniel Haschtmann declare that they have no conflict of interest.
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