Theories of Craniofacial Growth: From Brodie to Petrovic
Brodie’s Genetic Theory
– What we sometimes assume to be genetic may be acquired and superimposed on a genetic foundation common to parents and progeny.
– There is evidence to support polygenic inheritance greatly limiting our ability to explain facial dimension from the study of parents.
– The old argument about heredity versus environment has changed from the question of which is more important to how, and in what way, does the environment alter the original form laid down by heredity.
– Even if the size of facial bones were inherited in a Mendelian fashion, that inherited pattern is altered by environmental influences, some epigenetic and some general, to such an extent that in the patient the underlying genetic features cannot be easily detected.
Van Limborgh’s Hypothesis
– Intrinsic Genetic factors – Genetic factors inherent to the skull tissues-
– Local epigenetic factors – Genetically determined influences originating from adjacent structures (brain, eyes, etc.)
– General Epigenetic factors – Genetically determined influences originating from distant structures (sex hormones)
– Local environmental factors – Local non-genetic influences originating from the external environment (local external pressure, muscle forces)
– General environment factors – General non-genetic influences originating from the external environment (food, oxygen supply).
Sicher’s Hypothesis (Sutural Dominance Theory)
– Sicher deduced from the many studies using vital dyes that the sutures were causing most of the growth. He said, in fact, “…The primary event in sutural growth is the proliferation of the connective tissue between the two bones.”
– If the connective tissue proliferates, it creates space for the appositional growth at the borders of the two bones.
– Replacement of the connective tissue was necessary for the functional maintenance of the bones.
– He said sutures of the nasomaxillary and vault produced forces that separated the bones, just as the synchondrosis expanded the cranial base and the epiphyseal plates lengthened long bones.
Disadvantages
– Primarily because translatory growth of the bones continues normally either in the absence of sutures or through extirpation in experimental animals
– In untreated cleft palates, though the suture is not present, growth still takes place
Moss’s Hypothesis (Functional Matrix Theory)
– Moss feels that bone and cartilage lack growth determination and grow in response to the intrinsic growth of associated tissues, noting that the genetic coding of craniofacial skeletal growth is outside the bony skeleton.
– He terms the associated tissues “Functional matrices.” Each component of a functional matrix performs a necessary service—such as respiration, mastication, speech—while the skeletal tissues support and protect the associated functional matrices.
– Moss divides the skull into a series of discrete functional components, each comprised of a functional matrix and an associated skeletal unit, designing the functional matrices as either periosteal or capsular.
– He stresses the dominance of non-osseous structures of the craniofacial complex over the bony parts. Moss claims that the growth of the skeletal components, whether endochondral or intramembranous in origin, is largely dependent on the growth of functional matrices.
Moss cont’d
- Each functional cranial component has skeletal and functional matrices.
- The skeletal unit’s biomechanical role is to protect and/or support its specific functional matrix. Each skeletal unit is further divided into microskeletal and macroskeletal.
- The functional matrix carries out the function. Ex. Muscle, gland, teeth, etc. Functional matrices are further divided into periosteal matrices and capsular matrices.
- The mandible is a macroskeletal unit. The coronoid process, condyle, and angle of the mandible are all microskeletal units.
- Periosteal matrices include muscles and teeth; they act on the microskeletal units directly, and they bring about transformation or active growth. Their net effect is to alter the form (size and shape) of their respective units.
- Capsular matrices act on the macroskeletal unit, i.e., the whole mandible, and they bring about translation or passive growth. They do so by changing the volume of the capsule within which the functional cranial components, e.g., the oral cavity, have inner oral epithelium and skin outside. The mandible is translated passively.
Scott’s Hypothesis (Cartilaginous Theory)
– Scott, noting the prenatal importance of cartilaginous portions of the head, nasal capsule, mandible, and cranial base, felt that this development is under intrinsic genetic control.
– He especially emphasized how the cartilage of the nasal septum during its growth paced the growth of the maxilla.
– Sutural growth, Scott felt, came in response to the growth of other structures, including cartilaginous elements, the brain, the eyes, and so forth.
Petrovic’s Hypothesis (Servosystem Theory)
- Using the language of cybernetics, Petrovic reasoned that it is the interaction of a series of causal changes and feedback mechanisms that determines the growth of the various craniofacial regions.
- According to the servosystem theory of facial growth, control of primary cartilages takes a cybernetic form of command, whereas, in contrast, the secondary cartilages comprise not only the direct effect of cell multiplication but also indirect effects.
- In his experiments, Petrovic detected no genetically predetermined final length for the mandible.
- The direction and magnitude of condylar growth variation are perceived as quantitative responses to the lengthening of the maxilla.