University of Vermont Probes Scoliosis Mysteries

UNIVERSITY OF VERMONT TEAM PROBES SCOLIOSIS MYSTERIES WITH ALGOR FEA

Adolescent Idiopathic Scoliosis (AIS) is a spinal deformity which develops during the growth spurt. It consists of both lateral curvature of the spine and axial rotation of the vertebrae. In the United States, school screening programs identify possible problems in approximately 4% of children. However, only about one in 1000 of these children ever requires treatment. The treatment of scoliosis may consist of wearing a brace for several years and/or having surgery to fuse the spine.

Three Questions

For many years, medical researchers have been working to answer three major questions about scoliosis. What causes the disease? What mechanical factors are responsible for its progression into a serious deformity? And, can current surgical techniques be improved by the application of mechanical principles?

A team of researchers from the University of Vermont is using Algor Finite Element Analysis software to search for the answers to these questions. The team is a unique combination of medical and engineering professionals. The principal investigator is Dr. Ian Stokes, biomechanics researcher in the Department of Orthopedics and Rehabilitation. He is joined by Dr. Jeffrey Laible, professor in the Department of Civil Engineering and Mr. Mack Gardner-Morse, orthopedic engineer.

Series of FEA Models

The team has developed a series of three finite element models to aid them in their research. According to Mr. Gardner-Morse, "The first model relates to the etiology (origin or causes) of scoliosis. A clinical X-ray study showed that many patients have longer ribs on the convex side of the scoliosis. This suggested a growth abnormality as a possible etiology of the disease. The model was used to demonstrate the effects that asymmetric rib growth might have on spinal curvature."

"The second model was designed to study various hypotheses about the mechanisms of the progression of scoliosis and its resulting spinal shape. The third model," he continues, "is being developed to simulate the surgical treatment of the disease. First, to see if the model can predict the amount of correction that might be possible with surgery. Second, to see if the outcome can be improved by variations in surgical techniques."

Models From X-Rays

"The geometry for these models is obtained from stereo X-rays of scoliosis patients," says Mr. Gardner-Morse. "A computer program reads the stereo X-ray geometry and outputs the nodal geometry in an Algor SAP format. The model utilizes beam elements to represent the ribs, sternum and the costal cartilage which connects them. Stiffness matrices, taken from published experimental data, describe the vertebrae and discs which make up the spine. Truss elements are used to represent the intercostal ligaments between the ribs."

University of Vermont researchers Dr. Ian Stokes (left) and Mr. Mack Gardner-Morse.

Heat Simulates Growth

One of the key challenges in developing the models was the question of how to simulate growth. Since the disease manifests itself most strongly during the adolescent growth spurt, this simulation was critical to the success of the tests. In the words of Mr. Gardner-Morse, "Growth is modeled by analogy with thermal expansion. The load inputs consist of nodal temperatures which are asymmetric in the rib cage. Also, we incorporate a mechanism by which forces acting on bony elements modulate their growth."

Analyses

The analyses performed on the models were very complex and consisted of a series of recursive (small step) static analyses. According to Mr. Gardner-Morse, "The results from each step were stored as load cases in binary files. Fixed end actions, used to simulate load-modulated growth, were calculated by a program which read the files from the previous step and output the beam data in Algor format. The nodal geometry was advanced by the Algor ADVANCE program. A third program updated the stiffness matrices based on the new geometry and calculated the forces necessary to grow (elongate) the spine. The new geometry, beam data and stiffness matrices were then combined with the truss data to form a new input file."

The research team implemented this iterative process with a batch file which could be run without operator intervention. Similar static analyses were used on the second model with loadings of either temperature, global forces or prescribed displacements using boundary elements. The third model uses recursive static analyses to model the large rotations which take place during scoliosis surgery.

FEA Model Simulates Surgery
This photo shows a finite element model of the rib cage and spine of a scoliosis patient. The model was created from an actual stereo X-ray.
In this photo, the FEA model is used to predict the level of improvement that might be expected in the patient's spine after surgery has been performed.
Here, we see a model created from the same scoliosis patient's stereo X-ray after surgery. In this case, the simulation closely matches the actual results.

Results

"The analyses of the first two models show that elastic deformations alone do not produce the deformity seen in scoliosis," says Mr. Gardner-Morse. "While a rib length asymmetry might initiate a scoliosis, it does not cause it to progress to the large magnitudes seen clinically. The third model is still under final development.

"We like using Algor FEA software because of its ability to incorporate experimental stiffness matrices into the models using general stiffness elements," he continues. "We also appreciate the recursive processing and fixed end actions which allowed us to simulate the mechanical modulation of growth. Algor software offers an easy interface for our own software which can write files in Algor SAP input format and read the binary output files. This permits our own post-processing, which allows comparisons with real world, stereo X-ray data. Finally, we like the graphical interface for viewing the results."

Mr. Gardner-Morse adds that the team is looking forward to getting the latest update to Algor's AccuPak High-End and Nonlinear Stress, Vibration and Mode Shape Analysis Package which includes the new nonlinear beam element.