Deformative stress associated with an abnormal clivo-axial angle: A finite element analysis | Surg Neurol Int. 2010 Jul | free full text PubMed article (updated with Figures 5a-d 4/22/15)
The overly elastic connective tissue of EDS patients can allow the brain stem to “sag” and become pinched in the uppermost area of the neck.
This is a very technical article about measuring the clivo-axial angle to predict the success of decompression surgery to reduce biomechanical neuraxial stress within the corticospinal tract, dorsal columns and nucleus solitarius.
Chiari malformation, functional cranial settling and subtle forms of basilar invagination result in biomechanical neuraxial stress, manifested by bulbar symptoms, myelopathy and headache or neck pain.
Definition: Bulbar_palsy – Bulbar palsy refers to impairment of function of the cranial nerves IX, X, XI and XII, which occurs due to a lower motor neuron lesion either at nuclear or fascicular level in the medulla oblongata or from lesions of the lower cranial nerves outside the brainstem
A prospective, Internal Review Board (IRB)-approved study examined a cohort of 5 children with Chiari I malformation or basilar invagination
Definition: invaginate – 1. (Pathology) to push one section of (a tubular organ or part) back into itself so that it becomes ensheathed; 2. (Biology) (intr) (of the outer layer of an organism or part) to undergo invagination 3. (Biology) (of an organ or part) folded back upon itself
Standardized outcome metrics were used. Patients underwent suboccipital decompression where indicated, open reduction of the abnormal clivo-axial angle or basilar invagination to correct ventral brainstem deformity, and stabilization/ fusion
FEA predictions of neuraxial preoperative and postoperative stress were correlated with clinical metrics.
Mean follow-up was 32 months (range, 7-64). There were no operative complications.
Paired t tests/ Wilcoxon signed-rank tests comparing preoperative and postoperative status were statistically significant for pain, bulbar symptoms, quality of life, function but not sensorimotor status.
Clinical improvement paralleled reduction in predicted biomechanical neuraxial stress within the corticospinal tract, dorsal columns and nucleus solitarius.
The results are concurrent with others, that normalization of the clivo-axial angle, fusion-stabilization is associated with clinical improvement.
FEA computations are consistent with the notion that reduction of deformative stress results in clinical improvement. This pilot study supports further investigation in the relationship between biomechanical stress and central nervous system (CNS) function.
Below is a more complete description of the various consequences of an aberrant clivo-axial angle and the biomechanical stresses it causes.
Ventral brainstem compression (VBSC) from basilar invagination is a well-known cause of encephalomyelopathy in the setting of Chiari malformation and in platybasia.
Most of these patients have traditionally undergone suboccipital decompression and expansile duraplasty, but up to a half either failed to show improvement or exhibited a delayed regression despite decompression
In the latter instance, posterior decompression often exacerbates cervicomedullary angulation over the odontoid process.
The importance of this angulation of the brainstem, or “medullary kinking,” has been oft cited
Sawin et al. refer to the “fulcrum effect in basilar invagination, by which traction is applied to the caudal brainstem and rostral cervical spinal cord, producing prominent bulbar dysfunction and myelopathy.”
Kubota demonstrated that a clivo-axial angle of less than 130° was associated with delay or failure to recover after foramen magnum decompression
Numerous series of patients report cervicomedullary kyphosis or ventral flattening in the presence of a kyphotic clivo-axial angle or retroflexed odontoid process.
Medullary kinking and basilar invagination introduce abnormal deformative stresses in the brainstem and spinal cord, which result in neurobiological changes that are believed to underlie the pathophysiology of many of the observed neurological changes in this group of patients.
In a novel approach, the authors applied a finite element analysis (FEA) research tool to compute estimates of preoperative and postoperative mechanical stress within the brainstem and spinal cord in 5 children with medullary kinking due to kyphotic clivo-axial angulation in the context of Chiari malformation or basilar invagination. These stresses were compared with clinical metrics.
The FEA estimations of deformative strain, generated postoperatively, were used to test the hypothesis that reduction of abnormal stresses improved neurological deficits. The 5 patients studied herein underwent open reduction (normalization of the clivo-axial angle) and posterior translation to normalize the craniospinal relationship. This reduction was followed by occipitocervical fusion and stabilization
Correlation of computed mechanical stresses with clinical outcome indices suggested a direct relationship between reduction of deformative stress and clinical improvement.
Mechanical compression at the cervicomedullary junction occurs in
- Chiari 1 malformation,
- or as a result of basilar invagination,
- clival hypoplasia,
- anterior indentation of the pons,
- upward displacement of the brainstem or
- anterior displacement of the foramen magnum.
Here are summaries of previous studies:
- Scoville and Sherman first opined that angulation of the brainstem in basilar invagination caused neurologic signs and disability.
- Breig emphasized that craniospinal flexion increases strain in the brainstem, thus rendering it susceptible to injury.
- Van Gilder reported that a clivo-axial angle of less than 150° was associated with neurological changes.
- Menezes reported progression of disability in many patients following suboccipital decompression for Chiari 1, and attributed the observed bulbar findings to the fulcrum effect of the medullo-spinal junction draped over the dens.
- Milhorat observed that retroflexion of the odontoid process in 96/364 patients and basilar invagination in 44/364 patients resulted in kinking of the medulla in 140 of 364 patients.
- Grabb, Mapstone and Oakes noted failure of suboccipital decompression to relieve the symptoms of Chiari syndrome in 48% of the pediatric and 28% of the young adult patients, and attributed this failure to flattening of the ventral brainstem. Patients with a Grabb-Oakes measurement of greater than or equal to 9 mm were considered to be at high risk for VBSC, and underwent transoral odontoidectomy.
- Kubota et al. found that patients with cervicomedullary syndromes who failed to improve after suboccipital decompression had an average clivo-axial angle of 121.7°, while those who improved rapidly had an average angulation of 142.8°.
- Kim et al., Goel, Botelho and others have demonstrated that normalization of the clivo-axial angle from an average of 127° to 147° by intraoperative manipulation (extension of the clivus with respect to the odontoid to normalize the clivo-axial angle) is an effective treatment for basilar invagination, with demonstrable improvement in neurological function, thus obviating the need for transoral odontoidectomy.
The patients in this series were referred for disabling neurological symptoms, which included headaches, bulbar findings and myelopathy. All subjects shared abnormality of the clivo-axial angle. The authors consider the CAA to be a surrogate measure of deformative stress in the brainstem and upper spinal cord
Neuraxial strain is accentuated with flexion of the craniocervical junction.
Approximately 22° of flexion/ extension occurs at the craniocervical junction.
Upon flexion at the craniocervical junction, there is a lengthening of the brainstem and upper spinal cord (16), which may reach pathological strains in the context of an abnormally acute CAA, or retroflexed odontoid.
The addition of “out-of-plane” loading greatly increases the overall Von Mises stress. As opposed to the in-line strain that occurs with stretching of the spinal cord upon flexion, “out-of-plane” loading is any deformative stress that occurs horizontally upon the neuraxis, due to indentation from deformity, stenosis or disc or horizontal strain from the dentate ligaments perpendicular to the axis of the neuraxis
Compression from the sides of a viscoelastic cylinder, such as the medulla/ upper spinal cord, will create increased longitudinal tension within the neuraxis and perpendicular to the plane of compression. Thus a ventral compression force, like the retroflexed odontoid, “nontraditional” basilar invagination, platybasia and “functional cranial settling” with hereditary connective tissue disease, results in increased intra-axial tension.
The overall deformative stress generated at the craniocervical junction may reach levels where nerve function becomes attenuated; indeed, the axon is rendered nonconductive and develops pathological changes at a strain e = 0.2
Clinical improvement after “relief of brainstem angulation” has been attributed to improvement in blood supply and CSF dynamics. The authors make the argument that clinical improvement primarily relates to alleviation of deformative stress and substantiate this hypothesis with the growing body of neurobiological evidence.
The concept that the spinal cord elongates with flexion of the neck and that medullospinal kyphosis results in deleterious axial strain is well established
The importance of deformative stress in myelopathy and encephalopathy is supported in clinical reports, experimental studies and in the biomechanical and mathematical literature.
The degree of injury appears to be related to the peak strain of the tissue and the loading rate. The cord is initially compliant to stretch but becomes progressively stiffer as the fibers bear tensile load. Axonal injury relates directly to magnitude and rate of strain increase, but even mild stretch can induce progressive neurofilament alteration and delayed axotomy.
Stretching acts upon the Na+ channel mechanoreceptors to increase Na+ influx, causing reversal of the cation exchange pumps and depolarization of voltage-gated Ca++ channels, with subsequent pathological influx of Ca++
Stretching of the axolemma may result in several levels of injury: decreased amplitude and increased latency, a conduction block due to myelin damage, or membrane injury with irreversible changes.
However, the predominant substrate for stretch-induced injury appears to be the axon. Electron micrographs show clumping, loss of microtubules and neurofilaments, loss of axon transport and accumulations of axoplasmic material identified as the “retraction ball.”
Clearly, the importance of biomechanical stresses upon the nervous system warrants closer attention.
Conventional radiographic assessment of basilar invagination does not reveal the more subtle forms of ventral brainstem compression and deformation.
Open reduction of craniospinal deformity (normalization of clivo-axial angle), stabilization and surgical fusion are effective in improving pain and neurological function in subjects with cervicomedullary disorders resulting from deformative stress. The growing body of neurobiological literature supports the concept that deformative stress is important in both direct and epigenetic mechanisms of neurological dysfunction.
We have used FEA to compute neuraxial deformative stress in the context of cervicomedullary disorders due to Chiari malformation and basilar invagination. Surgical correction of the deformity resulted in improvement of computed Von Mises stress in selected anatomical structures, which was concordant with relief of pain and neurological deficits. FEA may offer new insight into the effect of pressure and strain on the neuraxis at the cervicomedullary junction. Further investigations are warranted to validate the concept that deformative stress is an important determinant of neurological function.