Greenville Spinal Orthosis -- Polypropylene

WILLIAM DEWEY FRIDDLE, C.P.O. LAWRENCE P. BROWN, M.D.


Summary

The Greenville Spinal Orthosis is a vacuum-formed polypropylene cast. It is used postoperatively for spinal fusions, particularly in scoliosis. It is also used for bracing scoliotic curves. The clinical advantages and technical considerations for fabrication are discussed in this article.

Application

The Greenville Spinal Orthosis (GSO) is a polypropylene cast which is vacuum-formed over a positive mold of the patient. It is used postoperatively for the immobilization of our fusion patients ( Figure 1 and Figure 2 ), as a night brace ( Figure 3 ), or as a low-profile brace ( Figure 4 ). It can be formed in any manner in which a conventional plaster cast or brace could be formed. The plastic cast is ideal for long-term casting of young patients to prevent progression of curves until the child becomes old enough for spinal fusion.

The obvious advantages of the plastic casts are manifold.

Advantages to the doctor

 

  1. Cast work for the surgeon is minimal. The initial mold requires very little time compared to the full application of a surcingle or other cast.
  2. X-rays taken through the plastic cast are essentially of the same quality as X-rays out of any cast; therefore, the maturation of fusion is easily determined.
  3. The jacket is usually made unremovable, but it can be made removable.
  4. The cast is expandable if necessary, and to date no patient has outgrown the cast despite wearing it for a full 10-month period of postoperative immobilization.
  5. To date there has been no breakage of a cast.
  6. Posterior windows can be cut for wound care if necessary or to palpate the fusion mass to aid in evaluating healing.
  7. When contemplating the GSO as a brace, the correction to be gained can be determined by X-ray through the plaster before going to the~expense of GSO fabrication; and if correction is unsatisfactory, a different type plaster device can be tried.

 

Advantages to the patient

 

  1. The cast is light, weighing approximately 680 g (1 and 1/2 lbs.) for a full surcingle cast, or Risser-type cast, from chin to pelvis.
  2. The patient can swim, bathe, and change the lining shirt readily.
  3. The smooth fit of the cast has prevented sores. It has eliminated further hospitalization for cast changes. The only occasional complication is mild skin maceration, which has been largely eliminated by multiple holes placed in the cast, by the use of cotton shirts, and by the occasional use of hydrocortisone ointment to the involved areas. In general, we and our patients have been extremely satisfied with the polypropylene cast.

 

Fabrication

The surgeon makes a light, unpadded, plaster cast over stockinette only ( Figure 5 ), using corrective forces (i.e., traction, localizers, surcingles) and the positioning desired. Final correction can be assessed by X-ray at this point.This cast is frequently made on the fifth or sixth postoperative day and is immediately bivalved, removed, and given to the orthotist, who uses it to form a plaster positive of the patient. The positive mold is modified as desired and covered with one layer of stockinette. Alliplast 4E is used for padding in the final plastic cast ( Figure 11 ). Alliplast 4E is shaped and tacked to the plaster positive with small nails ( Figure 8 and Figure 9 ). We like to relieve the anterior iliac spines, throat, and anterior chest window, using felt applied to the plaster positive under the stockinette ( Figure 8 ).

The GSO is made in two side-to-side halves ( Figure 1 , Figure 2 , Figure 3 , Figure 4 , Figure 6 , Figure 8 , Figure 10 , and Figure 11 ). The modified padded positive is placed side-up on the vacuum-forming table and positioned to a chosen depth in multiple steel ball bearings ( Figure 8 , Figure 9 , and Figure 10 ). The ball hearings control the depth of polypropylene forming ( Figure 6 , Figure 9 , and Figure 10 ) by preventing the polypropylene from being unevenly vacuumed around the positive. A vacuum of 380 mm Hg (15 in.) is used, and the ball bearings allow air to evacuate through the opening in the center of the rubber diaphragm ( Figure 7 ). This opening is loosely plugged with stockinette to prevent escape of the ball bearings. Stockinette ( Figure 8 , Figure 9 , and Figure 10 ) is used to cover the ball bearings to prevent the upper ball bearings from being incorporated in the polypropylene.

Three-mm (1/8-in.) sheets of polypropylene are heated to 204 degrees C (400 degrees F) for approximately 10 minutes. This is done in a large oven with the polypropylene laid on a sheet of Teflon-coated fiberglass used for bakery conveyor belts to prevent adherence. The heated polypropylene is laid over the positive and becomes the upper seal ( Figure 10 ) fbr the vacuum system when the upper wooden frame is pressed down over it.

After the first half (side) is made, the excess polypropylene is trimmed ( Figure 6 ), allowing for the desired anterior and posterior midline overlaps ( Figure 4 and Figure 11 ). This first trimmed plastic side is again positioned on the positive; and both are placed back in the hall bearings, this time "turned over" with the plastic down in the ball hearings to the point of desired overlap, usually 5 cm (2 in.) ( Figure 10 and Figure 11 ). The second side is then formed. Rough trimming, including windows and punched ventilation holes ( Figure 2 and Figure 4 ), is done by the orthotist. The plastic cast is now ready for trial application on the patient. Duct tape and rapid rivets are used for securing the two halves together. Final trim lines are established by the surgeon, and these are cut and huffed smooth, using a 5AS Tycro burnishing wheel at 900 rpm.

Size adjustments, side to side, can he achieved by allowing expansion or separation of the overlap, or by narrowing the inside flap of the overlap, thereby allowing the sides to come closer together. The final rapid rivets are applied ( Figure 1 and Figure 4 ): and the anterior and posterior external seams are plastic welded with a Kamweld welder, Model 34A W with a KS-1 speed tip, used at medium heat and 3.5 psi. Air is the gas used in the welder. The rapid rivets and plastic seams can be "popped open" with a screwdriver for cast removal. The cast can readily be reapplied.

Stretchable knit sleeveless turtleneck shirts ( Figure 1 and Figure 2 ) are used as liners and are changed as often as desired by the patient.

Corrective Alliplast 4E pads ( Figure 2 ) can be bonded to the original polypropylene cast and used to exert corrective forces. These can later be supplemented by gluing or riveting additional pads to exert further correction.