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Biomedical subjects

M A Spencer

Publications and source records attributed to M A Spencer.

6 recordsLinked to original sources
PubMed1999

Constraints on masticatory system evolution in anthropoid primates.

It is well established that some observed patterns of force production in the primate masticatory system match those predicted by a simplified lever model. This model is also commonly invoked in adaptive explanations of craniodental diversity. However, systematic studies of the predictive power of this model are missing, leaving open the possibility that factors not traditionally included in the model alter the function and evolution of the masticatory system. One such factor was proposed for mammals generally by Greaves ([1978] J. Zool. (Lond.) 184:271-285), who argued that the temporomandibular joint (TMJ) was poorly suited to being pulled apart. In this constrained lever model, the avoidance of joint distraction leads to limitations on masticatory system form and function. The goal of the present study was to quantify masticatory system diversity in anthropoid primates for comparison with these predictions. Results indicate that all sampled taxa exhibit a form that is consistent with selection against regular distraction of the TMJ. Also apparent from observed patterns of scaling is a regular interaction among a limited set of cranial and dental dimensions, in accordance with the constrained model. However, the data indicate that specific positional relationships among the muscles, joints, and teeth differ from those predicted by Greaves (1978). The pattern of deviation suggests that selection has favored a conservative masticatory system configuration that safeguards the TMJ from distraction during the dynamic processing of irregular foods. The resulting buffered model leads to alternative hypotheses regarding the response of the masticatory system to dietary selection pressures. It may, therefore, improve our understanding of the adaptive significance of primate craniofacial form.

Animals↗
PubMed1998

Force production in the primate masticatory system: electromyographic tests of biomechanical hypotheses.

Studies of the influence of dietary selection pressures in living and extinct primate taxa frequently interpret cranial diversity using a simple lever model. When this model is applied to functional or evolutionary questions, it is commonly assumed that the muscles of mastication vary little in activity during biting at points along the tooth row. A pattern of smoothly increasing maximum bite force magnitudes is therefore predicted as the bite point is moved posteriorly along the dental arcade. Diverse adaptive explanations are mapped onto this pattern. In this study, the activity of the superficial masseter and anterior temporalis muscles in humans was quantified during high magnitude bite force production at points along the tooth row. These data indicate that there are substantial changes in muscle activity with bite point, and that the standard lever model is therefore an incomplete description of masticatory force production. Maximum muscle force magnitudes were found to be greatest during first molar biting and to decrease as the bite point moved anteriorly and posteriorly. Additionally, relative balancing and working side muscle activity changed by bite point. This latter pattern is consistent with the predictions of Greaves' constrained lever model, which assumes that masticatory muscle activity is restricted by the need to maintain compressive forces at both temporomandibular joints. However, these results also imply that additional factors influence muscle activity--such as dental morphology, mandibular kinematics, and the need to safeguard against joint distraction during diverse loading conditions--and that the constrained lever model of Greaves is therefore also incomplete. These considerations suggest that masticatory system morphology in primates will respond differently to dietary selection pressures than is commonly hypothesized. Intepretations of cranial morphology in fossil taxa may therefore also differ.

Adult↗
PubMed1995

Progressive hemifacial atrophy. A natural history study.

PURPOSE: To describe two very different natural history courses in 2 patients with hemifacial atrophy. Progressive hemifacial atrophy (Parry-Romberg syndrome, Romberg syndrome, PHA) is characterized by slowly progressive atrophy, frequently involving only one side of the face, primarily affecting the subcutaneous tissue and fat. The onset usually occurs during the first 2 decades of life. The cause and pathophysiology are unknown. Ophthalmic involvement is common, with progressive enophthalmos a frequent finding. Pupillary disturbances, heterochromia, uveitis, pigmentary disturbances of the ocular fundus, and restrictive strabismus have also been reported. Neurologic findings may be present, but the natural history and progression of ocular findings are often not described in the literature. METHODS: We studied the records and present findings of 2 patients with progressive hemifacial atrophy who were observed in our institution over a 10-year period. RESULTS: Both patients showed progression of ophthalmic findings, primarily on the affected side. One patient has had chronic uveitis with secondary cataract and glaucoma, in addition to retinal pigmentary changes. She also had a third-nerve paresis of the contralateral eye and mild seizure activity. The other patient had mild uveitis, some progression of unilateral retinal pigmentary changes, and a significant increase in hyperopia in the affected eye, in addition to hypotony at age 19 without a clear cause, but with secondary retinal and refractive changes. CONCLUSION: Ocular manifestations of progressive hemifacial atrophy are varied, but can progress from mild visual impairment to blindness.

Adult↗
PubMed1993

Biomechanical analysis of masticatory system configuration in Neandertals and Inuits.

Considerable debate has surrounded the adaptive significance of Neandertal craniofacial morphology. Numerous unique morphological features of this form have been interpreted as indicating an adaptation to intense anterior tooth use. Conversely, it has been argued that certain features related to muscle position imply a reduced mechanical advantage for producing bite forces on the incisors and canines. In this study, hypotheses about morphological specializations for anterior tooth use have been derived from a biomechanical model of Greaves (1978). These hypotheses were tested by performing separate pairwise comparisons of Neandertals and early Homo sapiens, and Inuits and Native Americans from Utah. Inuits are known to have produced repeated and high magnitude forces on their anterior dentition and therefore serve as a good model for a hominid adapted to intensive anterior tooth use. Biomechanically relevant dimensions of the masticatory system were measured using a computer-driven video analysis system and compared between the two taxa in each comparison. The results of this study reveal a number of similarities between the morphological specializations exhibited by Neandertals and Inuits that can be related to intensified anterior tooth use. The hypothesis that Neandertals were poorly designed for producing masticatory forces is rejected. Specializations that differ between the two groups are interpreted as being the result of differential functional demands placed on the postcanine dentition in Neandertals and Inuits. It is suggested that many of the unique morphological features of the Neandertal face are a response to intensified use of the anterior dentition and the need to retain a sufficiently large postcanine occlusal area necessary for a relatively high attrition diet.

Adaptation, Physiological↗