Mandibular condylar growth alterations after unilateral partial facial paralysis: an experimental study in the rabbit.
In a previous study in the rabbit, the authors defined the macroscopic growth alterations after unilateral partial facial paralysis. Dry skull measurements revealed a reduced premaxillary, maxillary, mandibular, and anterior corpus length with a simultaneous increase in mandibular ramal height on the paralyzed side. The authors hypothesize that these mandibular growth alterations are, among others, caused by alterations in condylar growth activity and that an altered occlusal relationship may be involved in the adaptive condylar growth response after facial paralysis.A total of 84 New Zealand White rabbits were used for this study. The animals were randomly assigned to either a control group that was not operated on (n = 28), a group that underwent a sham-operation (n = 28), or an experimental group (n = 28). In the sham-operation group, the facial nerve was dissected as in the experimental group but was left intact. In the experimental group, a left-side partial facial paralysis involving the midfacial muscles was induced by an operation at the age of 12 days. After different follow-up time intervals of 3.5, 7, 14, 21, 28, 42, and 56 days, four control, four sham-operation, and four experimental animals (all randomly selected) were killed for histomorphometric measurements of the left control and sham condyles and the left-side and right-side experimental condyles. No significant differences between the control and sham-operation groups were found. The other results revealed that shortly after the paralysis in the experimental group, as compared with the controls, a decrease in condylar growth activity was seen before a catch-up increase in activity, as expressed by the time-sequenced decrease and increase in the height of the functional and hypertrophic chondroblast layer. The response on the right side was analogous, though less intense. It is suggested that the mandibular ramal growth alterations might be the result of a chain of adaptations involving the lateral pterygoid muscle and the condylar growth activity. The unilaterally restricted length increment of the maxillary snout, as a result of the loss of tensile forces caused by paralysis of the midfacial musculature, necessitated an adaptation in the position of the mandible to maintain a normal occlusal relationship. Subsequently, the function of muscles involved or influenced by an altered mandibular position, such as the lateral pterygoid muscle, were changed. These altered muscle activities induced condylar growth adaptations, which in turn explained the alterations in mandibular ramal growth.