What happens when a neck manipulation meets a spinal canal already narrowed by ossified ligament? This finite element study models that collision—and finds the simulated stress rises as OPLL occupies more of the canal. Few studies have focused on the biomechanism of spinal cord injury according to ossification of the posterior longitudinal ligament, or OPLL, during cervical rotatory manipulation, or CRM.

The study therefore aimed to explore the biomechanical effects of CRM on the spinal cord, dura matter, and nerve roots with OPLL in the cervical vertebral canal. The specific biomechanical effects of CRM on the cervical spinal cord complex with OPLL had not been studied before, according to the authors’ knowledge.

OPLL is one of the major contributors to myelopathy, and its morbidity is approximately three percent in Asian countries. The effects of CRM on the spinal cord, dura matter, and nerve roots were investigated quantitatively in mild, moderate, and severe OPLL, using finite element analysis.

The craniocervical finite element model with OPLL and vertebral canal contents was constructed on the basis of a previous study. The model simulated OPLL compression on the spinal cord under static and dynamic conditions, which the study identifies as its novelty. Three models were developed by adding mild, moderate, and severe OPLL to a healthy finite element model.

OPLL severity was based on the occupying ratio of spinal canal stenosis: the ratio of OPLL thickness to the sagittal diameter of the cervical vertebral canal. The occupying ratios were twenty percent, forty percent, and sixty percent for mild, moderate, and severe OPLL, respectively.

OPLL was constructed at the C5 and C6 segments because the free space of the spinal cord at the C5 slash C6 segment after CRM in flexion was relatively narrow. Figure one lays out the intact finite-element models of the cervical spine and spinal cord complex.

Transverse views at C5 show mild, moderate, and severe ossification of the posterior longitudinal ligament, while the paired sagittal views show these conditions across the C5 and C6 segments. The labeled anatomy includes the disc, nerve roots, spinal cord tissues, meninges, cartilage, and OPLL, establishing the model’s structural detail and the progressively defined canal-occupying lesion scenarios used for later analysis.

Two simulations were performed in ABAQUS twenty twenty to validate the finite element model. In the first validation, a normal force of zero point eight newtons was applied to the ventral surface of the spinal cord, and the force-displacement relationship was close to a published in vitro study.

In the second validation, axial rotation of plus or minus twenty degrees around the x-axis simulated cervical flexion and extension, and the calculated C3 to C7 spinal cord displacements matched in vivo experimental data. A convergence check was also performed on the mesh of the spinal cord, dura matter, and nerve roots to ensure acceptable mesh density.

The study simulated OPLL compression on the spinal cord under static and dynamic conditions. Static compression was the compression caused by OPLL before CRM, while dynamic compression was the compression during CRM. For static compression, the OPLL model moved by an occupying ratio of twenty, forty, or sixty percent toward the anterior surface of the dura matter while the other parts were fixed.

After static compression, the OPLL model was tied to the posterior surface of the vertebral body, and rotation was performed with mild, moderate, or severe OPLL compression on the spinal cord. Figure two separates the model into static and dynamic compression. In panels A and B, the OPLL model starts inside the vertebral bodies and is moved toward the dura to compress the cervical spinal cord, while the other parts remain fixed.

In panels C and D, the model is tied to the vertebral body after static compression, and cervical rotatory manipulation in flexion is applied to represent dynamic compression. This setup matters because it distinguishes persistent compression from compression occurring during movement.

Under static OPLL compression, the spinal cord in the severe OPLL model experienced higher von Mises stress than the spinal cord in the mild and moderate OPLL models. The maximum von Mises stress under static OPLL compression was lower than the maximum von Mises stress under dynamic OPLL compression.

Under dynamic OPLL compression, average spinal-cord von Mises stress during rightward four-degree z-axis rotation was zero point zero two seven seven, zero point zero three zero one, and zero point zero three seven one megapascals in the mild, moderate, and severe models. Compared with the model without OPLL, stress increased by four point one percent in mild OPLL, thirteen point one percent in moderate OPLL, and thirty-nine point five percent in severe OPLL.

In the severe model, maximum spinal-cord stress was at the C5 slash C6 disc level during the first half of rotation, then at the C1 slash C2 segment during the latter half. Figure three shows boxplots of maximum von Mises stress in the cervical spinal cord and dura mater during rightward four-degree z-axis rotation in cervical rotatory manipulation, across healthy, mild, moderate, and severe OPLL models.

The spinal-cord means are zero point zero two six six, zero point zero two seven six eight, zero point zero three zero zero eight, and zero point zero three seven one one megapascals; the dura-mater means are three point four seven six, three point six zero three, four point zero seven two five seven, and four point two seven two four three megapascals.

The distributions and significance markers show how stress measurements vary with OPLL severity. Figure four maps von Mises stress across the spinal cord, dura mater, and nerve roots for mild, moderate, and severe OPLL during cervical rotation.

The spinal-cord panels separate dorsal and ventral surfaces, while the severe model also marks the maximum stress at the C5/6 compression level; the nerve-root panels identify the maximum in the severe model at C6. These distributions matter because they show where dynamic OPLL compression concentrates mechanical loading around neural tissues.

Table two reports von Mises stress in the C1 through C8 nerve roots as OPLL occupancy rises from zero to twenty, forty, and sixty percent. The authors use double asterisks to mark values significantly different from the model without OPLL at p less than .001, including C6 at twenty percent and several roots at forty and sixty percent.

This matters because the authors identify increased nerve-root stress around the compression area during dynamic OPLL compression, with effects spreading across more segments as occupancy increases. Under dynamic OPLL compression, nerve-root von Mises stress increased by twenty-two point one percent at the C6 segment in the mild OPLL model compared with the model without OPLL.

As OPLL occupying ratios increased, more nerve-root segments experienced increased von Mises stress. In the moderate OPLL model, nerve-root stress was higher than without OPLL by nine point six percent at C5, one hundred nine point five percent at C6, fifty-three point six percent at C7, and thirty point three percent at C8.

The maximum nerve-root stress occurred at C2 in the mild and moderate models, but at C6 in the severe model. Only segmental plateau-shaped OPLL was constructed, so various OPLL types should be investigated because a more angular shape may lead to higher spinal-cord stress.

The OPLL models were based on a healthy model, and the CRM simulations were simplified; this may not be completely consistent with the actual situation, although it helps comparability and reproducibility. Only stress distribution was investigated using the von Mises stress failure criterion, while displacement and ischemia of the spinal cord were not taken into account.

The study can quantitatively predict biomechanical characteristics of the spinal cord complex and provide information about the correlation between neurologic symptoms and mechanical stress due to OPLL. The results suggest that certain OPLL occupying ratios in the vertebral canal can significantly increase stress on the spinal cord complex.

Clinically, the results indicate high risk in performing CRM in the flexion position on patients with OPLL, and caution is warranted when spinal canal stenosis exists. Across these models, OPLL increased stress on the spinal cord complex during cervical rotatory manipulation, with the strongest effects in severe OPLL.

The authors therefore identify flexion-position manipulation as high risk when spinal canal stenosis exists.