A Mechanism of Action of Spinal Orthoses

WILTON H. BUNCH, M.D., PH.D.,*AVINASH PATWARDHAN, PH.D.,*AND VICTORIA M. DVONCH, M.D.*Maywood, Illinois


Introduction

Orthoses for scoliosis have been used since at least the time of Andry, but empiricism has marked their use. With experience, clinicians have developed expectations as to general results, but a theoretical explanation has been lacking. To complicate the scene further, some authors stress the need for correction of the curve if long-term results are to be satisfactory1,3. Others are content to have modest curve reductions but also report good results2,4

This paper develops a method of thinking about spinal curves and the way an orthosis can treat them. This shows that stability of smaller curves can be achieved with little correction, but larger curves require reduction of curve magnitude for satisfactory results.

Methods

The assumption behind the analysis is the application of Euler's Law of Flexible Columns to the spine. Thus we will be concerned with the critical load which a column of some degree of curvature can support without further buckling. This is shown graphically in Figure 1 where the vertical axis is the percentage of the normal load, and the horizontal axis is the degree of curvature. (The development of the model is beyond the scope of this paper, but is given elsewhere5.) The degrees of curvature are approximations based on calculations and assumptions. There is a great deal of variation from one person to another. Therefore, they should be considered as illustrative of an approach, not a "hard" piece of datum. One can see that the stablilty of the curve drops sharply after approximately 30 deg., whereas a curve of about 15 deg. is nearly as stable as a straight spine.

This plot gives some suggestion as to why a curve might progress. For a given child with a 25-30 deg. curve, the weight of the trunk and upper levels may be less than the critical load and the curve is stable. With growth in height and weight, however, the critical load may be exceeded and the curve progresses.

The action of any spinal orthosis to prevent this progression may be considered as two separate but interactive events. These are transverse loading and curve correction which will be considered separately.

Transverse Loading

The effect of a modest transverse force on the critical load is shown in Figure 2 . For a curve of 25-30 deg., the effect of a transverse force without curve correction is to raise the percentage of normal critical load from 50 to about 70 per cent. This is shown in Figure 2 as the vertical bar labeled A. This may be enough to prevent progression and the final result is about the same degree of curvature as initially, but very satisfactory.

In contrast, a curve with a critical load of 0.2 (approximately 60 deg.) shows very little effect from a transverse load. The change is from 0.2 to 0.3 and the spine is still unstable. This is shown as the vertical bar labeled B in Figure 2 . Therefore, orthoses which do not produce much correction should be used only on curves in the lower end of the acceptable spectrum.

Curve Correction

A second approach to increasing the critical load is to correct the curve. The result of reducing a curve of 30 deg. to 20 deg. is to increase the critical load from 0.5 to 0.8 of normal. This is shown in Figure 3 by the arrow and bar labeled A. The curve is reduced in magnitude, which produces an increase in the load which can be carried.

The effect is also pronounced for curves of greater magnitude. Consider a curve of 45 deg. with a critical load of 0.2 of normal. If this can be reduced to 30 deg., the critical load is increased to 0.5 and a much more stable spine is achieved. This is shown in Figure 3 as the arrow and vertical bar labeled B.

Although the comparisons are somewhat simplistic, they illustrate that for any given curvature, a greater effect in the load-carrying capacity can be achieved by reducing the curve magnitude than by transverse loading only. This is particularly true for the larger curves, as can be seen by comparing the height of the bar labeled B in Figures 2 and 3 . We now have a theoretical explanation for the observation that satisfactory results in curves greater than 40 deg. required a reduction of curve magnitude in the brace to about 50 per cent of the initial curve 3.

Combined Effect

Once a curve is partially corrected, the forces which produced that correction can be reset to provide the continued support. The combined effect is greater than either element alone. Figure 4 shows the result of the same 45 deg. curve considered previously. With the total-orthosis effect, the curve was reduced to 30 deg. This included the stabilization of the pelvis, transverse load and, if Milwaukee brace, the neck ring. With this achieved, the transverse load was reset to its original force. This produced a further increase in the critical load to 0.8. This is shown diagrammatically with the two effects separated. Thus, if curve correction and well-applied forces are combined, curves of larger magnitude can be controlled.

Conclusions

This paper does not present a reason why one orthosis is better than another. It does not recommend one at the expense of all others. It does not provide a cookbook to orthotic management. It does explain why some patients and some physicians achieve better results than others. It does explain what is required if curves of 40-50 deg. are to be treated successfully. It does provide a way to think about a patient, an orthosis, an X-ray, and how they all interact.

*Loyola University Medical Center, Stritch School of Medicine, 2160 South First Avenue, Maywood, IL 60153.

References:

  1. Bunch, W. H., and V. Dvonch: Bracing for Idiopathic Scoliosis-Current Concepts, Management of Spinal Deformity, R. Dickson and D. Bradford, editors. Kent, England, Butterworth and Co., Ltd. (In press)
  2. Bunnell, William, P., G. Dean MacEwen and Shanmuga Jayakumar: The Use of Plastic Jackets in the Nonoperative Treatment of Idiopathic Scoliosis. J Bone Joint Surg 62-A: 31-38,1980.
  3. Carr, William A., John H. Moe, Robert B. Winter and John Lonstein: Treatment of Idiopathic Scoliosis in the Milwaukee Brace: Long-Term Results. J Bone Joint Surg 62-A: 599-612,1980.
  4. McCollough, Newton C., Mark Shultz, Nestor Javeck and Loren Latta: Miami TLSO in the Management of Scoliosis: Preliminary Results in 100 Cases. J Pediat Orthop 1:141-152, 1981.
  5. Patwardhan, A., W. H. Bunch and G. Knight: Spinal Stability, A Buckling Analysis. Transactions of the Society of Mechanical Engineers. (In press).
  6. Patwardhan, A., W.H. Bunch, R. Vanderby and G. Knight: A Mathematical Analogue of Spine Orthosis Systems. J Biomech (In press).