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Orthotic Cranioplasty: Material and Design Considerations

Joseph F. Terpenning, CO


External cranial remolding dates back to 2000 BC. 1 The rapidity to bring orthoses to market since the 1992 "Back to Sleep Campaign". 2 has produced many "carbon copy" designs, often sacrificing orthotic design considerations that are the specialty of orthotic professionals. Designing an orthosis requires a detailed understanding of material science and how material combinations will affect correction.

CRANIOPLASTY MECHANICS

External forces on the infant's skull act to compress the aspect of the skull that is in direct continuous contact with that substance while intrinsic brain growth expands the cranium laterally along the surface of contact. 3 Turk et al. 4 equated this to a water balloon being placed onto a rigid surface ( Figure 1 ). External forces from the surface combined with gravity pulling the balloon downward cause compression of the contact surface producing lateral displacement of the balloon and distortion of the shape. As the balloon is further filled, it expands parallel to the contact surface.

The fetal skull is incapable of withstanding prolonged exposure to force (that is, surface contact and gravity). In 1982, Kriewall 5 examined sections of infant parietal bone under load for stiffness, modulus of elasticity, and density, concluding that the load-bearing capacity of the fetal skull is directly related to the stiffness of the bone and the bone's thickness. These findings support the work of Kelly and Littlefield 6 linking early orthotic intervention with greater success. In addition to the increases in bone stiffness and higher load-bearing capacities, the growth rate of the skull is significantly reduced, reducing the outwardly directed force required for cranial modeling.

When considering two forces, Newton's Law of Reaction states that "For every action, there is an equal and opposite reaction," producing equilibrium. When considering a single force exerted onto a surface, one must consider the structural properties of the surface. Although counterforce by a material is not a true force, the resistance returned by the material is real. For example, a section of steel and a section of foam will possess markedly different load-bearing capacities under force.

Four historical methods for cranial molding stand out in the literature. 7 The least effective was manual manipulation by a parent. Force applied by the molder's hands lacks strength and longevity to produce alteration of the cranium. More effective was the use of tightly woven cords to "bind" the skull to re-direct the cranium. The most effective method incorporated boards positioned along the frontal and occipital aspects of the skull held together by tightly wrapped cords that were adjusted to produce deformation. The latter two examples cite a cranial-surface relationship in which the surface enacted on by the infant's skull was of greater stiffness and rigidity than the skull, and the duration of this interaction was of sufficient length of time to allow growth and gravity to influence the counterforce and the cranial shape.

The material of an orthosis must be of sufficient stiffness and rigidity to withstand the force acted on it by the cranium over a given timeline and deliver a consistent counterforce to overpower the bone's load-bearing capacity. The material of a cranial molding orthosis must be biocompatible and hypoallergenic. The material of choice must resist force from two directions. As Figure 2 illustrates, a thermoplastic should possess strength and rigidity to provide adequate force at the sites of linear contact without deflecting. 8 Any deflection will allow the prominences to advance and retard the normalization of the skull. The orthosis must also resist gravity's pull of the child's head and orthosis onto the surface of contact (that is, bed, carseat, stroller). This could mimic the deformational effect of the cranium.

Heat is absorbed through conduction into the material of the orthosis. A child will perspire in an orthosis and experience slightly higher-than-normal temperatures at sites of direct contact. Perspiration could also adversely interact with polymers that possess the ability to uptake water molecules into their polymer chains, potentially altering the structure of the polymer. Monomers are combined into multiple con- figurations to create large, complex polymers, yielding plastics of differing mechanical properties. 9 The polymer chains of thermoplastics form weak bonds, allowing them to slide past one another under energy. The bonds reform during cooling, allowing for the formability of thermoplastics.

Orthoses can be combined with an internal liner of Aliplast (Voltek, Brebbia, VA) or Pelite (Sekisui America, New York, NY), or with no liner for direct contact with the skull. Traditionally, band-style orthoses combine a rigid polymer with an internal liner, whereas helmet-style orthoses maintain direct polymer–skin contact. The Clarren-style orthosis (Orthomerica, Orlando, FL) is the only helmet-style orthosis cleared for use that combines a rigid polymer shell with a foam liner.

Copolymer Molding Orthoses

Copolymer is a polypropylene–polyethylene blend, usually of a 9:1 ratio, possessing strength and resistance of both materials. Polyethylene foam (Volara) is a biocompatible foam irradiated with electrons during manufacturing. Polyethylene foam liners encompass the underside of the orthosis, contacting the infant's skull when the orthosis is donned ( Figure 3A, B ). Persistent contact by a foam liner against the skull, while creating a soft interface, presents the potential for an initial spike in temperature and perspiration. Although the child's inherent thermal-regulating system will recalibrate the temperature, perspiration could introduce bacteria and microorganisms into the padding.

Surlyn Molding Orthoses

Surlyn 9020 manufactured by Dupont (Towanda, PA) is an ethylene/methacrylic acid copolymer neutralized for toughness with the use of a zinc cation. This polymer is chosen for its optical clarity and smoothness when thermoformed ( Figure 3C ). Surlyn orthoses are categorized as helmet-style devices in which the cranium is covered circumferentially and over the vertex. Furthermore, minimal padding is applied for the prevention of migration and in certain cases to enhance the control of the cranial prominences.

Polyethyleneterephthalate Glycol Molding Orthoses

Polyethyleneterephthalate glycol (PETG), commonly known as Durr Plex, is a copolyester (polycyclohexylene–methylene– terephthalate) manufactured by Eastman Chemical Company (Kingsman, TN). Polyester can be used to create either a thermoplastic (PETG) or a thermoset, depending on the structural configurations. As a thermoplastic, PETG possesses a high rigidity and resistance to deformation. PETG is among the more brittle polymers and could fracture easily under blunt force contact or as a result of excessive thinning during fabrication.

DESIGN CONSIDERATIONS

Table 1 provides mechanical data for the major polymers used. 10–13 One difficulty comparing polymers through mechanical data is the inconsistency in the data. Analysis of data for the major polymers reveals trends regarding a polymer's tolerance when exposed to mechanical, thermal, and moisture influences and their effectiveness as molding orthoses.

The flexural modulus is a measure of a material's stiffness when flexed. The flexural test (ASTM test D790) measures the force required to bend a material under a three-point loading condition. Surlyn's flex modulus of 14,000 psi is an order of magnitude smaller than that of copolymer (180,000 psi). PETG possesses the highest modulus at 310,000 psi. PETG has the highest density (1.27 g/cm 3 ) and rates higher than copolymer on the Rockwell R Scale Hardness Test (ASTM test D785) (Wilson-Shore Instruments, Instron Corp., Canton, MA).

Thermal behavior is a more difficult comparison. The heat deflection point is the temperature at which a standard test bar deflects a specified distance under a load, determining the short-term heat resistance of a material. This identifies materials that could sustain light loads at short exposures to high temperatures and those that lose rigidity over a narrow range. Surlyn 9020 maintained the lowest value of 104°F, whereas PETG and copolymer were more resistant with levels reaching 164°F and 173°F, respectively. Although PETG is more rigid than copolymer, the latter is more thermal-resistant. Although this test factors higher-than-normal temperatures encountered by a cranial molding orthosis, the data suggest how a polymer reacts to prolonged exposure to a lesser temperature (body heat). A more accurate measure is the Vicat Softening Temperature (ASTM test D1525), which reflects the point of softening when a material is used in an elevated temperature application. Only Surlyn and PETG have undergone this test, with resulting values of 135°F and 181°F, suggesting that PETG possesses a higher resistance to persistent heat exposure than Surlyn.

Water absorption analysis (ASTM test D570) is used to determine the amount of moisture absorbed by a material. The data suggest how materials perform in water or humid environments. Copolymer absorbed the least amount of water (0.01) as compared with PETG (0.2%). Surlyn 9020 does not present with data for analysis, although it has been suggested that water absorption is a greater issue for the structure of Surlyn as a result of the neutralizing cation.

IMPLEMENTATION OF THE ORTHOSIS

How a cranial molding orthosis is used is equally important as its material. Much debate exists around terms such as "dynamic" and "passive," and "active" and "static," and what they mean to orthotic cranioplasty. Active versus static traditionally describes the inherent function of the cranial molding orthosis and what action the orthosis itself performs. When viewed as a device containing no motorized parts, circuits, or mechanisms, band- and helmet-style orthoses are categorized as static. Dynamic and passive indicate similar behaviors, because many people equate mechanics to motion when considering these terms as descriptors of a cranial molding orthosis. Littlefield 13 describes "dynamic" as a technique rather than a structural property, the "process" of continuously monitoring an input and output, and altering the input to provide a change or optimization of the output ( Figure 4 ). The orthosis is set in place providing an initial force. This setting is reviewed periodically, noting the achieved correction, and adjusted to either increase or decrease the counterforce delivered and the resulting correction. This relationship can be precisely adjusted by the closure strap of the orthosis and monitored/adjusted. This "dynamic management" could occur in any bivalved orthosis secured by an adjustable strap, requiring that the feedback is assessed and addressed by the practitioner. The Clarren-style orthosis and nonhinged devices could still be categorized as passive designs.

It has been noted by many practitioners that Surlyn orthoses possess a lifespan of 2 to 3 months before the need for replacement. The shell deflects and assumes a shape analogous to the patient's deformity. This could result in multiple devices and a reduction in the success of the treatment. Analogous occurrences are observed in band-style orthoses and others containing an internal liner. Reducing the maximal oblique fronto-occipital dimension (bossings) before fabrication, with the goal of providing a greater force and hold at these prominences, compresses the polyethylene foam and creates a higher rigidity to heighten the magnitude of counterpressure. Although this delivers a higher counterforce to the cranium, the foam undergoes deformation as a result of its intolerance to pressure. Rigid, hinged orthoses using Surlyn or PETG do not require precompression or tight donning. The material is of a higher rigidity with the ability to deliver greater holding forces over a longer period. Materials with greater rigidity provide more efficient correction for a longer timeline, reducing the number of orthoses necessary. These materials possess a risk of skin ulceration if improperly used, and great care must be taken during fabrication and patient monitoring.

CONCLUSION

Orthotic cranioplasty is the resurrection of an ancient methodology. Continued research is needed to truly understand how correction of the cranial alignment is achieved and how an orthosis can be designed to deliver the highest efficiency of correction. Comparative studies analyzing the effectiveness of the known orthotic designs are the best method of truly assessing the effectiveness of the materials used today.

ACKNOWLEDGMENTS

The author thanks the members of the AAOP craniofacial consensus meeting; and Stephen Baker, MD, Craig Dufresne, MD, Jeffrey Posnick, MD, DDS, and Charles Thorne, MD, for their input and insight on this topic.

Correspondence to: Joseph F. Terpenning, CO, Eastern Cranial Affiliates, 1600 Wilson Blvd., Suite 200, Arlington, VA 22209; e-mail: jterpenning@infinitetech.org .


JOSEPH F. TERPENNING, CO, is affiliated with Eastern Cranial Affiliates, Arlington, Virginia.

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