JBJS | Skin Surface Pressure Beneath an Above-the-Knee Cast: Plaster Casts Compared with Fiberglass Casts*

The Journal of Bone & Joint Surgery, Volume 79, Issue 4

Articles | April 01, 1997

Skin Surface Pressure Beneath an Above-the-Knee Cast: Plaster Casts Compared with Fiberglass Casts*

JON R. DAVIDS, M.D.†, GREENVILLE; STEVE L. FRICK, M.D.‡, CHARLOTTE, NORTH CAROLINA; ED SKEWES, C.P.O.†; DAWN W. BLACKHURST, M.S.§, GREENVILLE, SOUTH CAROLINA

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Investigation performed at Shriners Hospital for Children, Greenville; Carolinas Medical Center, Charlotte; and the Division of Medical Education and Research, Greenville Hospital System, Greenville

The Journal of Bone & Joint Surgery. 1997; 79:565-9

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Abstract

Complications related to immobilization in a cast after an injury or an operation may be related to the materials used for the cast or to the techniques of application, or to both. To evaluate the widely held clinical opinion that the use of a fiberglass cast is dangerous and inappropriate when subsequent swelling of the extremity is anticipated, we studied the skin surface pressures that were generated beneath above-the-knee casts made with different materials and applied with different techniques.A prosthetic model of the lower extremity was designed with an expandable calf compartment to simulate swelling after an injury or an operation. With use of this model, we measured the skin surface pressure beneath a plaster-of-Paris cast, a fiberglass cast that had been applied with a standard technique, and a fiberglass cast that had been applied with a stretch-relax technique. The highest mean skin surface pressure after application of the cast (p < 0.001) and after simulated swelling of the limb (p = 0.04) was generated by the fiberglass cast that had been applied with a standard technique. The lowest mean skin surface pressure after application of the cast (p = 0.006), simulated swelling of the limb (p < 0.001), and all subsequent steps of the experimental protocol (p < 0.001) was generated by the fiberglass cast that had been applied with the stretch-relax technique.The mean skin surface pressure generated by the plaster cast and by the fiberglass cast applied with the standard technique did not return to the value before application of the cast until anterior and posterior longitudinal cuts had been made in the cast and the cast had been spread at those cuts. When the fiberglass cast had been applied with the stretch-relax technique, the mean pressure returned to the baseline value after only an anterior longitudinal cut and spreading at that cut.The principal pitfall of the use of a fiberglass cast is related to the technique of application. When the fiberglass cast had been applied with the standard technique, it generated a mean skin surface pressure that was higher than that associated with the plaster cast and it accommodated simulated swelling poorly. When the fiberglass cast had been properly applied, with the stretch-relax technique, it generated a mean skin surface pressure that was significantly lower (p = 0.006) than that associated with the plaster cast and it better accommodated simulated swelling without the need to sacrifice the structural integrity of the cast.

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Introduction

Introduction | Materials and Methods | Results | Discussion | References

With the introduction of synthetic cast materials (such as fiberglass mesh impregnated with polyurethane resin) in the 1980's, orthopaedic surgeons were presented with the first viable alternative to plaster of Paris (cotton-weave knit impregnated with anhydrous calcium sulfate) for immobilization of an extremity in a cast after an injury or an operation. Although plaster of Paris is easy to apply and mold and is relatively inexpensive, it is also messy, is slow to reach maximum strength after application, has a poor weight-to-strength ratio, and hardens by means of an exothermic reaction that can burn the skin^{2,5,12,14,23,27}. Synthetic cast materials have been described as being cleaner to apply, more difficult to mold, quicker to set, stronger, and lighter than traditional plaster^{1,4,6,18,19,22,23}. Additional advantages that have been ascribed to synthetic cast materials include relative radiolucency and resistance to water^18,22,25,26.

Biomechanical analysis of the intrinsic properties of cast materials has consistently demonstrated that plaster has a considerably greater elastic modulus and a substantially lower ultimate yield strength than synthetic materials^1,4,18,23. The recognition that plaster is stiffer and synthetic materials are stronger has led to the clinical recommendations that the former be used in the acute period after an injury or an operative procedure, when immobilization is essential, and that the latter be utilized later, during the course of rehabilitation, when a lighter and stronger cast facilitates mobilization of the extremity^1,4,17,20,23. Additional concerns that synthetic materials generate higher pressures on and within the extremity and accommodate less swelling have led some investigators to discourage the use of casts made of such materials in the acute period after an injury or an operation^{11,13,17,20,23}.

A review of the literature revealed many recommendations concerning the materials and techniques for the application of casts but few objective, clinically relevant studies that substantiated these opinions^{2,7,9,11,16,23-25}. At our institution, a variety of materials and techniques are utilized for postoperative immobilization, reflecting the individual preferences of the attending surgeons. A retrospective review was conducted at our institution to assess the early and late complications associated with an above-the-knee cast applied in the operating room after an operation on a lower extremity of children who had cerebral palsy. This group was selected for the analysis because immobilization is extremely challenging, as the result of associated spasticity, contractures, altered sensation, altered mental status, and a limited ability to communicate. The review failed to reveal a clinically important difference in the complications associated with plaster casts and those associated with casts made of fiberglass, which had been applied with the stretch-relax technique (to be described). The findings of that clinical review prompted us to design an experimental study, with use of a prosthetic lower-extremity model with an expandable calf compartment that simulated swelling after an injury or an operation, to assess the skin surface pressures generated beneath casts made of different materials and applied with different techniques.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study. Johnson and Johnson, Charlotte, North Carolina, donated the supplies used in the study.

†Motion Analysis Laboratory, Shriners Hospital for Children, 950 West Faris Road, Greenville, South Carolina 29605.

‡Carolinas Medical Center, 1000 Blythe Boulevard, Charlotte, North Carolina 28203.

§Research Department, Greenville Hospital System, 701 Grove Road, Greenville, South Carolina 29605.

†Motion Analysis Laboratory, Shriners Hospital for Children, 950 West Faris Road, Greenville, South Carolina 29605.

‡Carolinas Medical Center, 1000 Blythe Boulevard, Charlotte, North Carolina 28203.

§Research Department, Greenville Hospital System, 701 Grove Road, Greenville, South Carolina 29605.

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Figs. 1-A and 1-B: Components of the prosthetic model of the lower extremity. Fig. 1-A: The model consisted of inflatable bladders, an aluminum and wooden endoskeleton, and a polyethylene mold of a lower extremity.

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Fig. 1-B Tubing was attached to the bladders, which were positioned on the endoskeleton and embedded within prosthetic foam. The shell was removed, and three layers of prosthetic nylon stocking were placed on the model.

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Fig. 2 The mean (and standard deviation) skin surface pressures that were determined at each step of the experimental protocol. (One millimeter of mercury is equivalent to 0.13 kilopascal.)

Materials and Methods

Introduction | Materials and Methods | Results | Discussion | References

The prosthetic model was constructed by first fabricating a polyethylene mold of the lower extremity, with the knee flexed 10 degrees, of a ten-year-old boy who had cerebral palsy (Fig. 1-A). The endoskeleton consisted of an aluminum conduit (diameter, 0.5 inch [approximately thirteen millimeters]) to which a wooden and polyethylene foot was attached in a plantigrade position. Three inflatable bladders (100 to 500 milliliters each) were attached to the endoskeleton in positions consistent with the major anatomical compartments of the thigh, leg, and foot. Flexible tubing (diameter, three millimeters) was attached to each bladder and positioned parallel to the aluminum conduit, exiting at the proximal end of the model. The tubing was connected to a two-way stopcock and a disposable transducer monitoring kit (Abbott Critical Care Systems, Chicago, Illinois), which allowed for the infusion of fluid and the measurement of the pressure within the bladders. For the current study, only the bladder representing the posterior tibial compartment was inflated to simulate the swelling that may be seen after an injury or an operation. After correct positioning of the endoskeleton within the polyethylene shell, two-part prosthetic foam (Liquid Silicone Polymer A-2370; Southern Prosthetic Supply, Alpharetta, Georgia) was mixed and poured into the shell. Once curing was complete, the shell was removed and three layers of prosthetic nylon stocking (Otto Bock Orthopedic Industry, Minneapolis, Minnesota) were placed on the model (Fig. 1-B).

Water was infused into the bladder representing the posterior tibial compartment to achieve a baseline fluid pressure of ten millimeters of mercury (1.33 kilopascals). A single electronic pressure monitor with three sensor pads (Cast Alert Pressure Monitoring System; Johnson and Johnson, New Brunswick, New Jersey) was placed in a standard position over the anterior, lateral, and posterior aspects of the shank segment of the lower-extremity model. The sensor pads are modified position monitors; the output of the pad depends on the amount of compression of the pad, which is directly proportional to the force applied to the surface of the pad⁹. The position signal is calibrated in terms of the number of millimeters of mercury needed to cause the compression. Extensive laboratory and clinical testing has shown that the results obtained with this device are highly accurate and reproducible (within less than one millimeter of mercury [0.13 kilopascal] for both parameters) throughout a wide range of temperatures^9,17,21.

The experimental protocol consisted of eight steps, with the skin surface pressure determined after each: (1) two rolls of four-inch (10.2-centimeter) cast padding (Webril; Kendall Health Care Products, Mansfield, Massachusetts) were applied over the lower-extremity model and the pressure sensors, with approximately 50 per cent overlapping of the wraps; (2) the cast (plaster, fiberglass with a standard technique, or fiberglass with a stretch-relax technique) was applied; (3) the bladder representing the posterior tibial compartment was inflated with water until the fluid pressure was sixty millimeters of mercury (8.00 kilopascals), to simulate swelling of the limb; (4) an anterior longitudinal cut was made in the midline for the full length of the cast; (5) the cast was spread through the anterior cut at two levels (shank and thigh); (6) a posterior longitudinal cut was made in the midline for the full length of the cast; (7) the cast was spread through the posterior cut; and (8) the cast was removed completely.

The water into which the casting material was dipped was obtained directly from the tap and was maintained at room temperature. The plaster cast was applied from the toes to the proximal part of the thigh of the model with use of three rolls of four-inch (10.2-centimeter) plaster casting tape (Specialist; Johnson and Johnson); during application, the roll of tape was in contact with the model and tucks were made to contour the tape to the model. The fiberglass casts were also applied from the toes to the proximal part of the thigh, with use of three rolls of four-inch (10.2-centimeter) fiberglass mesh (Delta-lite Fiberglass Casting; Johnson and Johnson). Two techniques were used to apply the fiberglass casts. The standard technique involved application of the mesh in a fashion similar to that for the plaster cast except that the tape was stretched, not tucked, to contour it to the model. With the stretch-relax technique, the roll of fiberglass casting tape was pulled away from the model, which stretched and unwound the tape. The tension on the unwound portion then was relieved, and the tape was laid on and around the model. Contouring was achieved by slightly re-stretching the tape as it was applied.

Ten trials were performed on each type of cast; testing was done four hours after application of the plaster cast and immediately after application and setting of the fiberglass casts. The mean skin surface pressures (and standard deviations) were determined after each step of the protocol. The data were analyzed with use of repeated-measures analysis of variance. The levels of significance for over-all tests were based on the Greenhouse-Geisser adjusted tests. To adjust for multiple pairwise comparisons, a p value of 0.01 was used to assess significance.

Results

Introduction | Materials and Methods | Results | Discussion | References

The mean skin surface pressures after application of the cast padding were comparable among the three groups (p = 0.09) (Fig. 2).

Plaster cast compared with fiberglass cast applied with standard technique: The mean skin surface pressure was significantly higher (p < 0.001) after application of the fiberglass cast with the standard technique than after application of the plaster cast. The mean pressure after simulated swelling also was higher in association with the fiberglass cast, but the difference, while clinically important, was not significant (p = 0.04) with the numbers available. There were only slight differences between the pressures generated by the two types of cast after each of the remaining steps of the protocol (anterior cutting of the cast, spreading of the cast at that cut, posterior cutting of the cast, spreading of the cast at that cut, and removal of the cast). The mean skin surface pressure generated by either cast did not return to the level before application of the cast until the anterior and posterior longitudinal cuts had been made and the cast had been spread at the cuts.

Plaster cast compared with fiberglass cast applied with stretch-relax technique: The mean skin surface pressure after application of the cast (p = 0.006) and after simulated swelling (p < 0.001) was significantly lower when the fiberglass cast had been applied with the stretch-relax technique than when the plaster cast had been applied. The pressures generated by the fiberglass cast were consistently and significantly lower (p < 0.001) than those generated by the plaster cast after anterior cutting, after spreading at that cut, after posterior cutting, and after spreading at that cut. As already noted, the mean skin surface pressure generated by the plaster cast did not return to the baseline value until after the anterior and posterior longitudinal cuts had been made and the cast had been spread at each cut; in contrast, the mean pressure generated by the fiberglass cast applied with the stretch-relax technique returned to the baseline value after only anterior cutting and spreading at that cut.

Discussion

Introduction | Materials and Methods | Results | Discussion | References

Although the principle of immobilization after an injury dates back to the ancient Egyptians, the first recorded medical use of a cast (made of a mixture of lime and egg whites) was by Arabic physicians in the ninth century A.D.^12,15. Treatment with this type of cast remained the standard of care until 1798, when calcium sulfate (plaster of Paris) was introduced as a cast material^15,24. In 1852, the currently used plaster bandage was developed, with the plaster added to the cotton-mesh gauze by hand immediately before application¹⁵. By 1927, the addition of binders (to keep the plaster embedded within the gauze) allowed for the commercial manufacture of premade, ready-to-apply plaster bandages¹². The literature after the introduction of these more recent advances in techniques for application or in cast materials generally reflected an initial skepticism and theoretical concern about potential complications^7,15,16,23. These same concerns were expressed in the 1980's and the 1990's after the introduction of synthetic materials for casts^{1,3,9,13,17,18,25}. The incorrect application of material properties to structural properties (and by inference, clinical performance), in combination with anecdotal or experimental biases, has led to the common clinical perception that the use of synthetic casts immediately after an injury or an operative procedure is dangerous and inappropriate^1,13,17,23.

The principal pitfall of synthetic cast materials is related to the technique of application. Fiberglass mesh has a relatively low modulus of elasticity and a wide range of elastic deformity. The polyurethane resin can be tacky, making it difficult to unwind the roll of mesh. When the mesh is applied with the standard technique used to apply a plaster cast (that is, keeping the roll in contact with the limb), the mesh may be placed around the limb under substantial tension, which can lead to a constricting cast. Furthermore, stretching the mesh rather than tucking or folding it to contour it to the limb, while producing an aesthetically appealing cast, may also contribute to the tightness of the cast. The preferred technique for application (the stretch-relax technique), with which we have had extensive clinical experience, involves pulling the roll of fiberglass mesh away from the limb, which stretches and unwinds the mesh. After a segment is unwound, the tension is relaxed, and the mesh is laid on and around the limb. Only slight re-stretching is needed for contouring. To reduce pressure generated beneath the cast further, an anterior longitudinal cut is made immediately and the cast is spread; this is possible because of the rapid setting and superior strength of the synthetic material. Often, a fiberglass cast will spread spontaneously after the anterior longitudinal cut has been made, reflecting the intrinsic properties of the fiberglass mesh.

The validity of these observations and the efficacy of the stretch-relax technique, as well as the danger associated with the inappropriate application of synthetic casts, were confirmed by the findings of the current study. We chose to measure the skin surface pressure beneath the cast because this parameter presumably is a marker of the pathophysiological mechanism by which a tight cast causes substantial pain and which, if not corrected, can lead to the most serious of all cast-related complications, a compartment syndrome^10,11,19. It should be noted that the measurement of the skin surface pressure is not an absolute parameter on the basis of which to make the clinical diagnosis of an impending compartment syndrome, nor does relieving the cast constriction guarantee prevention of a compartment syndrome. In the current study, the highest skin surface pressures after application of the cast and after simulated swelling were generated by the fiberglass cast that had been applied with the standard technique. The lowest pressures at each step of the experimental protocol were generated by the fiberglass cast that had been applied with the stretch-relax technique. The mean skin surface pressure generated by the plaster casts and by the fiberglass casts applied with the standard technique did not return to the value before application of the cast until the anterior and posterior longitudinal cuts had been made and the cast had been spread at those cuts. In the clinical setting, casts that have been cut and spread in such a fashion generally are unstable and must be reinforced or replaced once the swelling has resolved. When the fiberglass cast had been applied with the stretch-relax technique, the mean skin surface pressure returned to the baseline value after only anterior cutting and spreading at that cut, which does not compromise the structural integrity of the cast. The relative performances of the plaster and the fiberglass casts were consistent with what is known about the intrinsic properties of each material^1,4,18.

The introduction of new cast materials and techniques for their application should be followed by critical analysis of performance and assessment of outcome. In the current era of short hospital stays and outpatient operations, safe postoperative immobilization is critical because the patient often is recovering at home and cannot be easily checked by the physician. The findings of the current study do not support the widely held clinical opinion that synthetic casts are inherently more dangerous than plaster casts. In fact, when properly applied, a fiberglass cast is less stiff (allowing it to better accommodate swelling) and stronger than a plaster cast. These properties should make a fiberglass cast the preferred choice for immobilization in the acute period after an injury or an operation.

NOTE: The authors thank Edward Sugarman, M.D., and Nick D'Antonio, P.E., for lending the Cast Alert Pressure Monitoring System used in the study.

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Introduction | Materials and Methods | Results | Discussion | References

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