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Orthotic Modeling of the Developing Skull

Charles W. Kuffel, CPO, FAAOP

The use of orthotic modeling to achieve symmetry of the developing asymmetric or disproportionate skull has been highly criticized during the past few years. The resistance of some members of the medical community, including many insurance carriers, as to the efficacy of cranial remolding appears to be largely the result of misinformation and lack of education regarding this intervention.

The practice of altering bone structure during periods of rapid bone growth is not a new concept and has been practiced for many years throughout the world by numerous cultures. ^1,2 Alterations and deformational changes along cranial structures were described as early as 1791 by Söömmering. He hypothesized that skull growth occurs along suture lines and within fontanels. ³ In 1851, Rudolph Virchow established the principle that growth of the cranium was restricted in a plane perpendicular to the fused suture and was enhanced in the corresponding parallel plane. ^3–5 Described in 1868, Wolfe's Law holds that every change in the form or function of a bone is followed by adaptive changes in its internal architecture and its external shape. Along with published research, far less developed cultures have inherently understood that externally applied forces during rapid bone growth elicit permanent changes in structure. ^1,2 A thorough understanding of how the developing cranium is modeled during orthotic management is necessary for the development of orthotic protocols regarding this matter.

DEVELOPMENTAL ANATOMY

The human cranial anatomy is far different during the developing neonatal and early postnatal periods than during adolescent and early adult periods.

The neonatal cranium can be broken down into two portions: the neurocranium that encloses the brain and the viscerocranium that represents the face and mandible. The neurocranium can be further divided into the cranial vault and the cranial base. Included in the neonatal neurocranium are the paired parietal, temporal, frontal, sphenoid, and ethmoid bones, and the unpaired occipital bone. The most recent research states that the neurocranium achieves 63% of its mature size by birth, 88% by 1 year, and 95% by 10 years. ⁴ The rapid volumetric increase in the neurocranium during the first year of life is primarily the result of rapid brain growth.

The externally applied pressures of the birthing process and the internally applied pressures of rapid brain expansion during the first year of life are made possible by the ability of the cranial vault flat bones to overlap and expand. The cranial flat bones are interconnected through sutures, fibrous tissues uniting the flat bones of the cranium. These sutures act as the major sites of bone growth and expansion along the leading margins of the cranial bones during craniofacial development. ^3,4,6–8 The sutures of the cranial vault consist of the single metopic and sagittal sutures, and the paired coronal, lambdoid, and squamosal sutures. The metopic, or frontal, suture separates the frontal bone into two halves. ^6,8 The metopic suture is the first to ossify, which typically occurs during the second year of life. ³ The sagittal suture separates the parietal bone into two halves and extends from the anterior fontanel to the posterior fontanel. The sagittal suture remains patent throughout infancy. The paired coronal sutures separate the frontal bones from the parietal bones and typically remain patent throughout infancy. The lambdoid sutures separate the posterior edge of the parietal bones from the occipital bone. The lambdoid sutures remain unossified throughout infancy. Finally, the squamosal sutures create a fibrous band at the superior border of the squamous part of the temporal bone. They articulate anteriorly with the greater wing of the sphenoid bone, superiorly to the parietal bone and posteriorly to the occipital bone. The varied orientation of the cranial sutures allows for the rapid expansion of the cranium in many directions in response to rapid brain growth.

The cranial fontanels are fibrous membrane-covered gaps created from the meeting of more than two cranial vault flat bones. ^4,6,9 The six fontanels of the developing cranium are the single posterior fontanel, the paired sphenoidal fontanels, the paired mastoidal fontanels, and finally the single anterior fontanel. The posterior fontanel is created at the juxtaposition of the two parietal bones and single occipital bone. The posterior fontanel is the first to close and is typically not discernible by 2 months of age. ^4,6,9 The paired sphenoidal fontanels are formed at the intersection of the frontal, occipital, temporal, and greater wing of the sphenoid bones. The sphenoidal fontanels close at or around 3 months of age. ^4,6 The two mastoidal fontanels are created at the juncture of the occipital, parietal, and temporal bones. The mastoidal fontanels close at around 12 months of age. ^4,6 The anterior fontanel is created where the two frontal bones and parietal bones are juxtaposed. The anterior fontanel is the most prominent and is the last to close at the median age of 13.8 months. ^4,6,9

During neonatal growth of the cranium, the cranial vault sutures act as intramembranous bone growth sites. The rapid volumetric increase of the brain during the early postnatal period causes the displacement of the flat bones of the neurocranium, which responds with osseous formation along the sutural edge. ⁷ For the sutures to remain patent, the sutures must remain unossified. The timed expansion and closure of the eight sutures and six fontanels of the developing cranium allow for the symmetric formation of the cranium into adolescence and adulthood.

OSSIFICATION PROCESS

The ossification process of the developing cranium is possible through bone formation along cranial suture lines occurring in two distinct ways: intramembranous ossification and endochondral ossification. Intramembranous ossification is de- fined as direct bone formation, whereas endochondral ossifi- cation begins with a cartilage before bone development. ^3–9 The neurocranium is primarily formed through intramembranous ossification, whereas the viscerocranium and cranial base are formed primarily through endochondral ossification. ³ The membranous neurocranium is derived from mesenchyme, which develops and expands at five primary ossifi- cation centers, the paired frontal, paired parietal, and single occipital plate. ^4,8 The expansion of the five primary ossification centers is initiated by an increase in brain volume and proceeds radially through growth of opposing flat bones. ^4,7,8 Ohman and Richtsmeier ³ have described how cancellous bone initially forms. As the trabeculae thicken and the bone becomes less porous, it becomes compact bone. As compact bone continues to develop and the flat bones of the developing cranium begin to oppose each other, a suture is created. The sutures act as intramembranous growth sites stimulated by an increase in intracranial pressure caused by rapid volumetric change. As the brain increases in size, the cartilaginous bone growth sites widen and respond by adding cancellous bone at the edges of the cranial bones. The sutures lay down bone to maintain the same approximate gap between the opposing flat bones. ⁸ During cranial flat bone development, sutures must remain in an unossified or patent state to allow for the proper formation of the neurocranium.

Research has indicated that the ossification of sutures and the closure of fontanels occur as intracranial pressure no longer produces expansion of the five primary ossification centers. As brain growth slows, the cartilaginous bone growth sites at the sutural fronts continue to lay down cancellous bone, which eventually overlap opposing bones, thereby creating continuous flat plates. ^3,4,7–9 The importance of the cessation of suture patency is evident, as orthotic modeling becomes noneffective after this period. If new bone formation is too rapid, premature fusion of one or more of the sutures results in cranial asymmetry, also known as craniosynostosis. If the suture ossifies prematurely, the increase in brain volume and internally applied pressures will permanently deform the cranium in an asymmetric manner. If the cranial sutures fail to ossify in a timely fashion, abnormally large fontanels and sutural abnormalities will exist. ⁸

ASYMMETRY VS. DISPROPORTION

The asymmetry of cranial scoliosis is best viewed from a transverse viewpoint. The transverse cranial vault presentation of a child with plagiocephalic asymmetry typically appears as a parallelogram shape. The occipital bone is flattened on one side with possible ipsilateral frontal protrusion or bossing. The ear on the side of the posterior flattening could or could not be shifted anteriorly as a result of asymmetry of the cranial base. ^2,10–15 The dolichocephalic and brachycephalic head shapes are characterized by disproportion of the anterior–posterior and medial–lateral measurements. Although variation in cranial breadth and width are normal, the cranial index will determine the severity of a brachycephalic or dolichocephalic head shape. The cranial index is the number that results in a ratio of cranial width versus cranial length multiplied by 100 (maximum width X 100/maximum length). If the resulting number is more than 81, then the cranium is brachycephalic. If the resulting number is less than 75.9, the cranium is termed dolichocephalic.

Many infants will present with cranial disproportion and some degree of asymmetry of one or more of the cranial quadrants when viewed transversely. When placing these infants into asymmetric or disproportionate classifications, it is best to use the cranial index to first determine if the patient has a disproportionate cranium. If the cranial index is less than 75.9 or more than 81, the child has asymmetry secondary to the disproportion. If the infant's cranial index falls between 76 and 81, then the observable disproportion is secondary to the asymmetric plagiocephaly.

PRIORITY OF FOCUS

The priority of focus refers to the portion of the asymmetric or disproportionate cranium that should be addressed initially. The timed closures of the eight sutures and six fontanels of the neurocranium should prioritize orthotic modeling.

The priority of focus for the disproportionate brachycephalic or dolichocephalic cranium is somewhat straightforward. If the infant presents with brachycephaly, the priority of focus is anterior–posterior. Conversely, when an infant presents with dolichocephaly, the priority of focus is medial– lateral. When orthotically managing positional plagiocephaly that affects one quadrant, the priority is to direct growth in the quadrant that possesses flattening.

Orthotically managing the infant with positional plagiocephaly that affects frontal and occipital quadrants is somewhat more difficult to prioritize. Although frontal asymmetry presents as the most evident deformity, the posterior fontanel is the first to close and therefore the first to model. Many neurosurgeons believe that the frontal asymmetry should be addressed initially, because this is the most evident. If the infant presents with frontal asymmetry that affects orbital placement, ear symmetry, and jaw placement, the asymmetry has the potential of causing medical complications during the maturation process. Hair growth will mask posterior asymmetry, but frontal asymmetry has the potential of causing medical complications and is most noticeable throughout life.

Some healthcare providers believe that if an infant presents with two-quadrant plagiocephaly, both areas should be addressed simultaneously because two areas of flattening are present. Fitting and rotation problems often exist when the orthosis contacts the cranium frontally and contralaterally posterior. If it is possible to achieve a satisfactory fit of the orthosis with two voids and two points of contact, that could be the best way to prioritize management.

When addressing two-quadrant positional plagiocephaly from a purely growth and developmental perspective, it is evident that the posterior flattening should be addressed initially, because this is the first area to ossify. Once the posterior flattening has achieved symmetry, the voids are shifted to the contralateral frontal region. By addressing quadrants at different times, problems associated with rotation and excessive movement within the orthosis are kept to a minimum. The decision to orthotically manage two-quadrant plagiocephaly and which quadrant to address initially is best determined through an informed group decision. The referring physician, therapist, orthotist, and parent must agree on the area of asymmetry that causes the most concern and has the greatest potential for creating developmental complications.

CONCLUSION

The manner of correction and priority of focus of the cranium are largely dependent on the developing cranial anatomy. If any of the cranial sutures or fontanels have begun to close or ossify, the region of ossification will respond slowly or not at all to orthotic modeling attempts. Failure to provide the desired correction of asymmetry or disproportion might not be related to orthotic management techniques, but to timing of treatment.

Correspondence to: Charles W. Kuffel, CPO, FAAOP, Pongratz Orthotics and Prosthetics, 2530 E. Thomas Road, Phoenix, AZ 85016; e-mail: Charlie@pongratzop.com .

CHARLES W. KUFFEL, CPO, FAAOP, is affiliated with Pongratz Orthotics and Prosthetics, Phoenix, AZ.

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