Experimental study of muscle reattachment following surgical detachment.

The process of muscle reattachment was studied in rhesus monkeys using electromyographic and histologic techniques. The attachments of the temporal muscle were exposed on both sides and the muscle was detached from its origin on the right side in ten rhesus monkeys. EMG activity was recorded by fine wires placed intramuscularly within the anterior, middle, and posterior parts of the muscles before and after detachment. The control EMG activity recorded in the attached muscle showed a wide range of values, and there were no clear trends. Following detachment the range of values was also wide, and the differences in activity between the detached muscle and the contralateral attached muscle were not outside the range of difference normally observed between right and left sides. EMG activity was greater before detachment in 31.2% of the recordings and was greater after detachment in 29.4% of the recordings. Biopsies of bone and muscle were taken from normal attachment sites on the left sides and also from the right sides following detachment at one, two, three, four, six, and eight weeks and examined by light microscopy. New bone spicules were seen developing from the surface of the bone after two weeks that were oriented in the direction of the muscle fibers. The reattachment process is one in which new bone is formed on the surface of the bone and develops toward the end of the muscle to envelop the reorganizing tendon. Bone formation occurring on the surface of bones at the ends of muscles is not dependent on tension or viscoelastic properties of the muscle.

Adaptation in the function of pharyngeal constrictor muscles.

The palatopharyngeus and pharyngeal constrictor muscles were studied by electromyography (EMG) and by direct observation with a flexible fiberoptic scope in the anesthetized as well as in the alert rhesus monkey. The muscles were monitored to determine the change in their discharge with nasal obstruction, head posture, head extension, and swallowing. The results indicated that certain regions of the middle and inferior pharyngeal constrictors never discharged during deglutition. Extending the head could induce a tonic discharge in fibers of the middle pharyngeal constrictor for the duration of head extension. Placement of water in the hypopharynx not only induced a sustained laryngospasm but also a tonic discharge in the select fibers of the superior and middle pharyngeal constrictors. Changing from a supine to an upright posture, or obstructing the nasal cavity, could induce a rhythmic discharge. These studies indicate that there are functional components of fibers within each of the anatomically recognized pharyngeal constrictors.

Experimentally induced neuromuscular changes during and after nasal airway obstruction.

Neuromuscular changes were studied by electromyography in rhesus monkeys which adapted to nasal obstruction for 2 years and then in the succeeding year recovered to nasal respiration. Obstructing the nasal passage with silicone plugs induced specific behavioral responses which remained for the duration of nasal obstruction and were lost within 8 days after removal of the plugs. Animals demonstrated individual variations, but more than 80% consistently maintained a lower mandibular posture for the entire 2-year period. Rhythmic mandibular, tongue, and upper lip movements were evident in fewer than 60% of the animals. Certain craniofacial and tongue muscles (the genioglossus, dorsal tongue fibers, digastric, geniohyoid, dilator naris, and vertically oriented fibers of the superior orbicularis oris, that is, lip-elevator fibers) were recruited rhythmically and remained rhythmically active throughout the entire 2-year period of nasal obstruction. This rhythmic activity ceased within 1 week after removal of the nose plugs. A tonic or consistent discharge was also induced in the genioglossus, dorsal tongue fibers, the geniohyoid, superior orbicularis oris, and lip-elevator fibers over the entire 2 years of nasal obstruction. Not all muscles lost their tonic discharge after removal of the nasal plugs. The genioglossus, geniohyoid, inferior orbicularis oris, and lip-elevator fibers discharged tonically during the recovery period. These data suggest that nasal obstruction can induce neuromuscular changes which extend beyond the period of obstruction and remain after the original stimulus for neuromuscular change has been removed.

Morphologic response to changes in neuromuscular patterns experimentally induced by altered modes of respiration.

The present experiment was designed to test whether specific recordable changes in the neuromuscular system could be associated with specific alterations in soft- and hard-tissue morphology in the craniofacial region. The effect of experimentally induced neuromuscular changes on the craniofacial skeleton and dentition of eight rhesus monkeys was studied. The neuromuscular changes were triggered by complete nasal airway obstruction and the need for an oral airway. Alterations were also triggered 2 years later by removal of the obstruction and the return to nasal breathing. Changes in neuromuscular recruitment patterns resulted in changed function and posture of the mandible, tongue, and upper lip. There was considerable variation among the animals. Statistically significant morphologic effects of the induced changes were documented in several of the measured variables after the 2-year experimental period. The anterior face height increased more in the experimental animals than in the control animals; the occlusal and mandibular plane angles measured to the sella-nasion line increased; and anterior crossbites and malposition of teeth occurred. During the postexperimental period some of these changes were reversed. Alterations in soft-tissue morphology were also observed during both experimental periods. There was considerable variation in morphologic response among the animals. It was concluded that the marked individual variations in skeletal morphology and dentition resulting from the procedures were due to the variation in nature and degree of neuromuscular and soft-tissue adaptations in response to the altered function. The recorded neuromuscular recruitment patterns could not be directly related to specific changes in morphology.

Sequential neuromuscular changes in rhesus monkeys during the initial adaptation to oral respiration.

Experimental induction of oral respiration in primates altered the neuromuscular use of specific craniofacial muscles. Obstruction of the nasal passage in the rhesus monkey induced changes in the electromyographic discharge (EMG) of both mandibular and facial muscles during the first 6 months of adaptation. Eighteen craniofacial muscles were studied with regard to their type of neuromuscular pattern. The EMG discharge was analyzed in terms of whether it had a rhythmic discharge or a continuous recruitment of motor units. The results of the investigation revealed that certain muscles in the control monkey using nasal respiration could be rhythmically or continuously active, but no significant trend was apparent with either pattern over time. In contrast, a significant number of muscles became rhythmically active within the first month of adaptation to oral respiration in the experimental animals. The rhythmic pattern was evident in key muscles that actively depressed the mandible, protruded the tongue, altered the shape of the tongue, and raised the upper lip. Continuous activity was induced in the first month within the suprahyoid region and tongue but later, by the fifth month, in specific lip and elevator muscles. These results suggested that the neuromuscular system adapted immediately to nasal obstruction but would vary as to (1) which muscles would be important in the initial adaptation, (2) the mode of adaptation, and (3) the time when a particular pattern first began to be used.

Electromyographic analysis of the functional components of the lateral pterygoid muscle in the rhesus monkey (Macaca mulatta).

Electromyographic recordings (EMG) obtained from intramuscular electrodes inserted in the lateral pterygoid muscle (lat. pt.) of 29 juvenile and adolescent monkeys were assessed in relation to the recorded activity of three jaw elevating muscles, temporalis, masseter and medial pterygoid, and a jaw muscle depressor, the geniohyoid. The lat. pt. was recruited in one of three patterns: (1) only during mandibular depression, (2) during both depression and elevation, or (3) only during elevation of the jaw. The discharge of fibres in the lat. pt. during closing movements occurred bilaterally in 9 of 21 recordings. Simultaneous recordings from the anterior temporalis, medial pterygoid and the ipsilateral lat. pt. indicated completely different discharge patterns of the three muscles. In 9 recording sessions, two pairs of electrodes were placed in the same lat. pt. The discharge patterns from the two pairs were different in 5 of the recordings. In one region, the fibres were active only during depression, whereas in the other region fibres discharged only during elevation of the mandible. The data support the concept of two groups of fibres with separate functions within the lat. pt. and suggest that adaptation to oral respiration utilizes the two groups in distinctive patterns.

Primate experiments on oral respiration.

Oral respiration associated with obstruction of the nasal airway is a common finding among patients seeking orthodontic treatment. The primate experiments reported here are part of a series designed to test some of the current hypotheses regarding the relationship between mouth breathing and dental malocclusions, that is, between deviations in orofacial muscle recruitment and jaw morphogenesis. Mouth-breathing was developed in the animals of this experiment by obstruction of the nasal passages with silicon nose plugs. The experiments showed that the monkeys adapted to nasal obstruction in different ways. In general, the experimental animals maintained an open mouth. Some increased the oral airway rhythmically, while others maintained the mandible in a lower position with or without protruding the tongue. All experimental animals gradually acquired a facial appearance and dental occlusion different from those of the control animals. From these and the previously reported primate experiments in this laboratory, it can be deduced that orthodontic appliances in general affect the morphology of the orofacial structure in two ways: by direct force and by sensory stimulation. (1) The appliance exerts a direct physical force which alters the strain distribution in the bone and elicits bone remodeling and tooth movement. (2) The presence of the appliance initiates the sensory input which triggers a neuromuscular response. This change in neuromuscular activity, in turn, affects both muscle development and bone remodeling. The fixed orthodontic appliance may work mainly on the first principle. Certain removable appliances may have a significant effect based on the second principle.

Influence of immediate dentures on oral motor skill and speech.

An investigation was designed to test whether the precision of finely coordinated orofacial movements is influenced by the transition from natural to artificial dentition. The oral motor skill of 16 patients was tested before extraction and within 2 weeks of wearing immediate dentures. Speech recordings were employed to detect changes in voice quality and speech sound production. Results showed that the loss of natural teeth and the wearing of immediate dentures did not alter sensorimotor pathways significant to the precision of oral motor skills. It was also determined that physical characteristics of dentures are significant factors in the production of acceptable speech.

The bilateral response of the temporal muscle in the rhesus monkey (Macaca mulatta) to short-term detachment of the muscle and increased loading of the mandible.

The electromyographic background activity of the temporal muscle was observed after short-term detachment of the muscle and with increased loads on the mandible. Detachment of the muscle increased its mean spike discharge within the first 24 days but without significant changes in the spectral components. Loading the mandible decreased both the spike discharge and spectral components.

Experiments on the influence of oriented stress on bone formation replacing bone grafts.

A cleft maxilla is healed when it has the attributes of a normal maxilla. Experimental and clinical observations suggest that bone perimeters can be extended by biophysical stimuli induced through functional forces when the tissue is under tension. An experiment was designed to test whether the establishment of bone across the cleft is similarly influenced. Bone most consistently replaced split-rib grafts when tension was applied to the segments of surgically produced maxillary clefts. Additional observations on this model indicate that a suture can be developed in the bone bridge across the cleft margins. While the parameters are not established, the hypothesis cannot be rejected that tension facilitates the extension of bone inducing stimuli and influences the differentiation of tissue in intra-alveolar transplants.

The re-do pharyngeal flap.

Twelve patients with persistent hypernasal speech after a previous pharyngeal flap operation had a second pharyngeal flap operation performed by the high-attached technique. Six of them achieved complete or near complete correction of hypernasality in connected speech. Four showed speech improvement of a lesser degree. Two patients had no significant improvement.