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The Biomechanics of Nocturnal Brace Design

Hooper, C. Ralph Jr. CPO

JPO Journal of Prosthetics and Orthotics: October 2003 - Volume 15 - Issue 4 - p S36-S39
SECTION II: ORTHOSIS BIOMECHANICS
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C. RALPH HOOPER, JR., CPO, is affiliated with Carolina Orthotics and Prosthetics, Inc., Charleston, SC.

Correspondence to: C. Ralph Hooper, Jr., CPO, 285 Meeting Street, Charleston, SC 29401; e-mail hrandj@aol.com.

Time-modified brace wear for the nonoperative treatment of idiopathic scoliosis (IS) is not a recent development in treatment regimens. In fact, some authors believe that time-modified brace wear has been around as long as full-time (23 hours per day) brace wear. 1–8 This belief was substantiated by the patient’s own modification of the prescribed treatment regimen. DiRaimondo and Green, 8 in a review of a 7-year brace wear program, concluded that only 20 percent of the patients were fully compliant with the recommended protocol. It also was noted that high-school-aged patients were wearing their braces only 12 hours per day, regardless of the treatment recommendation. 8 Kehl and Morrissy 9 concluded that a patient on a full-time brace wear program is more likely to discontinue brace use altogether when told that the brace will not work unless it is worn 23 hours per day. It was also concluded that the patient who refuses to wear the brace to school senses futility in wearing the brace at all. Part-time brace wear programs allow the patient to comply with the physician’s recommendations for brace wear and should increase compliance.

Part-time brace wear was prescribed as a treatment method in 1979 by Dr. Frederick E. Reed, Jr., 10 who began prescribing 8 hours of brace wear using the Charleston bending brace. Part-time brace wear was also prescribed by Drs. Charles T. Price and Max F. Riddick 11 in 1979 using a modified Wilmington brace, by Dr. N. E. Green 2 in 1986 using the Boston and Milwaukee braces, Dr. Nanni J. Allington and Bowen 12 in 1983 using the Wilmington brace, and D. E. Katz et al. 5 in 1997 using the Charleston bending brace.

Studies continue to compare the results of part-time and nighttime brace wear to the natural history and to full-time brace wear, prompting part-time brace and nighttime brace wear to become more and more a part of the mainstream treatment regimen. This author is not aware of a study that indicates part-time brace wear does not alter the natural history of adolescent idiopathic scoliosis. Winter 13 questioned the theory in 1994 by publishing The Pendulum Has Swung Too Far: Bracing for Adolescent Idiopathic Scoliosis in the 1990s.

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BIOMECHANICS OF NOCTURNAL BRACE DESIGN

Full-time braces such as the Boston, Wilmington, or Milwaukee were, and still are, expected to correct a scoliosis curve as great as 50 percent while the patient is wearing the brace. 14–16 If we consider these braces are designed to be worn in an upright position as well as a recumbent position, we can appreciate the brace that achieves 50 percent in-brace correction. While we evaluate the full-time brace worn in the upright position, we must consider the forces necessary when we create a vector to displace the spinal column toward the center line, and an opposing vector to maintain the patient in an upright position with the head over the pelvis. The limiting factors in the placement of the vectors are the axilla proximally and the iliac crest distally. When applying a three-point vector for curve correction, the magnitude of the vectors increases as the distance between the vectors decreases, thus causing a significant amount of pressure to be applied to correct the curve 50 percent. Although it has been demonstrated that the amount of in-brace correction is directly proportionate to the final outcome, 14 it must also be considered that in most cases the force necessary to correct a curve more than 50 percent may be unbearable to the patient. 17–25

Nighttime brace design eliminates two vector placement factors present during full-time brace wear:

  1. Limitation of the iliac crest as the most distal placement of a vector.
  2. Maintaining the patient in an upright position with head over pelvis.

With the elimination of two limiting vector placement factors, a nocturnal brace is able to increase the distance between the vectors, thus decreasing the force applied to the vector. With the elimination of the need to position the patient in an upright position with the head over the pelvis, a nocturnal brace can create a fulcrum from one or more vectors around which the spine may be rotated. The placement of these vectors and forces applied enable the nocturnal brace to achieve much better in-brace correction of the scoliosis curve. Studies have shown that a properly designed and fit nocturnal brace should achieve 90 percent to 100 percent in-brace correction of the primary curve, and in some cases more than 100 percent. 1,3–7 The orthotist’s ability to manipulate the vectors to achieve the desired correction is the most important element of nocturnal in-brace correction. 26

Figures 1 and 2 illustrate a simple three-point vector application to correct a single right lumbar curve. The vector at the pelvis is first applied and remains static; the vector at the apex of the curve is applied and is dynamic until it shifts the spine into the desired position, then becomes static, forming a fulcrum around which the last vector rotates the spine with a dynamic pressure.

Figure 1

Figure 1

Figure 2

Figure 2

Figures 3 and 4 illustrate a more complicated double three-point vector application to correct a double curve with thoracic and lumbar components. With the elimination of the patient having to be in an upright position with the head over the pelvis, and the advantage of applying a vector to position the pelvis and lower lumbar vertebra, correction of both curves is achieved.

Figure 3

Figure 3

Figure 4

Figure 4

As we consider in-brace curve correction and review the literature that indicates better in-brace results produce better final results, we must also consider the basic principals of compression and distraction as described by Hueter and Volkmann. 27 If a brace reduces a curve only 50 percent, the resultant curve has a concave and convex aspect, and thus we have compression at the concave aspect and distraction at the convex aspect. It must then be concluded that the compression and distraction forces decrease proportionately as the curve is reduced in magnitude, and if the curve is corrected beyond 100 percent (overcorrected), the compression and distraction forces are negated.

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COMPARING THE RESULTS

Early part-time and nighttime brace wear studies used the existing protocol of previous full-time brace wear studies and included single and double curves with Cobb angle magnitudes as great as 49°. However, more recent studies indicate that certain groups of patients respond better to nighttime brace wear than do others. 1,2,5 Katz et al. 5 noted that single curves with a Cobb angle of less than 35° responded comparably to treatment with the Boston brace worn full-time and the Charleston brace worn only at nighttime. Trivedi and Thomson 28 also noted that patient selection was important in the use of nocturnal wear with the Charleston brace.

Part-time and nighttime brace wear studies have overwhelmingly indicated that both alter the natural history of IS. Bowen et al. 26 reported that nighttime bracing of flexible curves demonstrated a success rate statistically comparable to the Wilmington full-time brace. Kehl and Morrissy 9 demonstrated success with part-time brace wear and indicated that curves of less than 35 percent and absent of vertebral wedging would respond as well with part-time bracing as full-time bracing. Allington and Bowen 12 reported no statistical difference in the results of curves ≥40° when treated with part-time bracing or full-time bracing. Federico and Renshaw 1 reported observing results of nighttime treatment with the Charleston bending brace to be at least as good as reported results for full-time brace treatment for curves ≤40°. 3 Trivedi and Thomson 28 demonstrated favorable results in altering the natural history of adolescent IS (AIS) using nighttime treatment with the Charleston bending brace, and suggested that single curves responded best to nighttime-only treatment.

There have been many investigators of part-time and nighttime brace wear for the patient with AIS, and all investigators agree that both alter the natural history of AIS. The confusion and disagreement that have been reported are focused on patient selection, curve location, curve magnitude, and curve type (single or double). Nocturnal bracing has been a part of every treatment regimen since bracing for AIS began. During the process of weaning from brace wear, nocturnal wear is the last to be discontinued. Price et al. 6 have demonstrated that growth hormones are more active during the sleep hours, so the benefit of nocturnal bracing is heightened. d’Amato et al. 4 described nighttime bracing as a stronger dose of correction during a shorter period because of the far greater amount of in-brace correction nocturnal braces are able to achieve.

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DISCUSSION AND CONCLUSION

Since 1979, investigators have accepted the fact that a large number of patients will not comply with a full-time brace wear treatment regimen. Time-modified treatment regimens using braces designed to be worn in an upright and recumbent position have proven to be successful in altering the natural history of AIS. Nighttime brace treatment regimens have proven to be successful in altering the natural history of AIS. There is still controversy regarding the comparison of part-time and nighttime treatment regimens to full-time brace wear treatment regimens. The data reviewed suggest that single curves with Cobb angle magnitudes of less than 35° respond statistically as well with nighttime brace treatment. The data also suggest that nighttime braces can achieve much better in-brace curve correction, and the nonupright position offers favorable biomechanical positioning.

Double curves present some difficulty for nighttime brace design. The ability of the orthotist is instrumental for in-brace correction of double curves when using a nighttime brace. The data suggest that double curves with magnitudes of less than 30° can be treated effectively with a well-designed nighttime brace; however, the design of such a brace is difficult for the orthotist who is not well trained in scoliosis bracing.

Continued research of nighttime brace treatment for the treatment of AIS is necessary; however, the data as of this date are promising. If nighttime brace treatment of AIS is accepted as advantageous for small curves and it is determined that larger curves need additional time in a brace, it would appear to this author that the addition of another brace for daytime wear is appropriate. Obviously, the braces offer a vastly different biomechanical approach, and the brace that achieves the most favorable in-brace positioning cannot be worn while the patient is upright.

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REFERENCES

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© 2003 American Academy of Orthotists & Prosthetists