Finite element investigation of the effect of a bifid arch on loading of the vertebral isthmus

Abstract

The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. Spondylolysis (SL) of the lower lumbar spine is frequently associated with spina bifida occulta (SBO). There has not been any study that has demonstrated biomechanical or genetic predispositions to explain the coexistence of these two pathologies. In axial rotation, the intact vertebral arch allows torsional load to be shared between the facet joints. In SBO, the load cannot be shared across the arch, theoretically increasing the mechanical demand of the vertebral isthmus during combined axial loading and rotation when compared to the normal state. Purpose:To test the hypothesis that fatigue failure limits will be exceeded in the case of a bifid arch, but not in the intact case, when the segment is subjected to complex loading corresponding to normal sporting activities. Study Design:Descriptive Laboratory Study Methods:Finite element models of natural and SBO (L4-S1) including ligaments were loaded axially to 1kN and were combined with axial rotation of 3°. Bilateral stresses, alternating stresses and shear fatigue failure on intact and SBO L5 isthmus were assessed and compared. Results:Under 1kN axial load, the von Mises stresses observed in SBO and in the intact cases were very similar (differences <5MPa) having a maximum at the ventral end of the isthmus that decreases monotonically to the dorsal end. However, under 1kN axial load and rotation, the maximum von Mises stresses observed in the ipsilateral L5 isthmus in the SBO case (31MPa) was much higher than the intact case (24.2MPa) indicating a lack of load sharing across the vertebral arch in SBO.When assessing the equivalent alternating shear stress amplitude, this was found to be 22.6MPa for the SBO case and 13.6 MPa for the intact case. From this it is estimated that shear fatigue failure will occur in less than 70,000 cycles, under repetitive axial load & rotation conditions in the SBO case, while for the intact case, fatigue failure will occur only over 10 million cycles. Conclusion:SBO predisposes spondylolysis by generating increased stresses across the inferior isthmus of the inferior articular process, specifically in combined axial rotation and anteroposterior shear. Clinical Relevance:Athletes with SBO who participate in sports that require repetitive lumbar rotation, hyperextension and/or axial loading are at a higher risk of developing spondylolysis compared to athletes with an intact spine. Key Terms:Spina Bifida Occulta, spondylolysis, finite element analysis, stress fracture, fatigue fracture, lumbar spineWhat is known about the subject: SL of the lower lumbar spine is frequently associated with SBO. A recent investigation has revealed a 3.7 fold Odd ratio for the presence of SL in individuals with SBO. As yet, it is unclear if SBO is a...

Factors associated with failure of bony union after conservative treatment of acute cases of unilateral lumbar spondylolysis

“…A possible reason for the substantial variation in union rate between the reports is that there are multiple confounding factors that have a significant impact on bone union. Previous reports revealed several possible factors affecting bone union after conservative treatments for lumbar spondylolysis, including vertebral level [4], stage [4], contralateral condition [4], bilateral spondylolysis [5], and latent spina bifida [6,7]. Contralateral pseudarthrosis might be an inhibitory factor for bony union because new spondylolysis tends to occur when a bone defect occurs on the opposite side of the vertebral arch [8].…”

Co-existence of spina bifida occulta and lumbosacral transitional vertebra in patients presenting with lower back pain

“…Rarely, neurological manifestations can be observed in those who develop tethering of the spinal cord within the spinal canal. Also, SBO leads to inappropriate distribution of weight and attachment of muscles due to incomplete vertebral arch [ 8 , 9 ].…”

Sacral fatigue fractures in children with sacral spina bifida occulta

“…Several studies in the literature have described a positive association between SBO and spondylolysis 14 – 16 . A recent finite element study showed a bifid arch in SBO and following laminectomy increases load across the isthmus, predisposing toward early fatigue fractures of the isthmus 17 . Similarly, SBO in the sacrum might affect the stress concentration on the sacral alae, predisposing toward stress fractures of the sacrum without repetitive stress.…”