Introduction
The purpose of this study was to explore the contributions of occlusion, maximum bite force, and chewing cycle kinematics to masticatory performance.
Methods
A prospective cross-sectional study was performed on 30 subjects with Class I occlusion. Masticatory performance was measured with the test food Cuttersil (Heraeus Kulzer, South Bend, Ind) and the fractional-sieve technique. Blu-Mousse (Parkell Biomaterials, Farmingdale, NY) bite registrations were used to measure occlusal contact areas. The American Board of Orthodontics occlusal discrepancies were measured on the subjects’ dental models. Maximum bite forces were recorded with a custom transducer, and 3-dimensional chewing cycle kinematics were tracked with an opto-electric computer system and Optotrak software (Northern Digital, Waterloo, Ontario, Canada).
Results
Masticatory performance was most closely correlated with occlusal contact area, indicating larger contact areas in subjects with better performance. Occlusal contact area and occlusal discrepancies were also related to bite force and chewing cycle kinematics. Maximum bite force was positively related with masticatory performance.
Conclusions
Although masticatory performance is related, both directly and indirectly, to a number of morphologic and functional factors, it is most closely related to occlusal factors.
The goal of orthodontic treatment is to improve the patient’s life by enhancing jaw function and dentofacial esthetics. In a recent survey, orthodontists and general practitioners rated good function, rather than morphology or esthetics, as the most important feature of an acceptable occlusion. Impaired masticatory function can adversely affect quality of life. Although orthodontists claim that improving masticatory function is a major goal, no standard assessment is routinely performed before or after treatment to determine whether improvements occur.
Masticatory performance is the best overall measure of masticatory function. It quantifies a patient’s ability to break down food based on the size of food particles after a specified number of chewing cycles. Cuttersil (Heraeus Kulzer, South Bend, Ind), a condensation silicone impression material, is the test food of choice and is considered to be the standard for measuring masticatory performance. Masticatory performance has been shown to be influenced by 3 main factors; teeth, masticatory muscle strength, and jaw movements. Although these factors have been studied individually, the relationships among them and their relative contributions to masticatory performance have yet to be determined.
It has been well established that masticatory performance is related to occlusion and occlusal contact areas. Subjects with malocclusion have lower masticatory performance than those with normal occlusion. Occlusal areas of contact and near contact (ACNC) have been shown to be positively correlated with masticatory performance.
Maximum bite force also affects masticatory performance. Hatch et al showed that bite force was directly related to masticatory performance, although its impact was not as strong as the number of functional teeth. Julien et al found that maximum bite force, along with body size and occlusal contact area, explained 72% of the variation in masticatory performance among children and adults.
Whereas chewing cycle kinematics might be expected to influence masticatory performance, this association has not been well studied. Wilding and Lewin showed that kinematic variables were significant determinants of chewing performance. It has also been shown that certain characteristics of muscle activity and jaw movement might be associated with improved masticatory performance.
The purpose of this study was to explore the relative contributions of occlusion, occlusal contact areas, maximum bite forces, and chewing cycle kinematics to masticatory performance. To limit variation and spurious associations, the study was designed to focus on healthy, young adults with Class I occlusion.
Material and methods
A prospective, cross-sectional study was designed to evaluate the relationship between occlusion, maximum bite force, chewing cycle kinematics, and masticatory performance. Thirty subjects (15 men, 15 women) were chosen from the students and staff at Baylor College of Dentistry in Dallas. The inclusion criteria included age between 22 and 32 years and Class I molar relationships. Class I occlusion was chosen because malocclusion has repeatedly been shown to be an important determinant of masticatory performance. Subjects were excluded based on the following criteria: (1) missing teeth (excluding third molars), (2) symptoms of temporomandibular dysfunction including pain and crepitus, (3) active orthodontic or periodontic treatment, (4) full-coverage dental restorations or tooth replacements, and (5) more than 2 surface restorations on the right first premolars or right first molars. Each subject received an oral examination to assess occlusion, temporomandibular joint function, and state of dentition. Informed consent was obtained according to the guidelines for human research of the Institutional Review Board at Baylor College of Dentistry.
The artificial test food used in this study was Cuttersil, a condensation silicone impression material. An acrylic plastic template was used to standardize the size (20 mm in diameter and 5 mm thick) of the Cuttersil tablets. After hardening for at least 1 hour, the Cuttersil tablets were cut into quarters and packaged for each subject. Each subject was given 2 quarter tablets per trial and instructed to chew naturally, on the right side only, for 30 chewing cycles. After that, the subjects were instructed to stop chewing, expectorate the sample, and rinse with water until all particles were removed from the mouth. The procedure was repeated 7 times, until approximately 10 g of Cuttersil had been chewed and expectorated into a filter.
The chewed sample was dried in an oven for 1 hour at 80°C and then separated using a series of 7 sieves with mesh sizes of 5.6, 4.0, 2.8, 2.0, 0.85, 0.425, and 0.25 mm, stacked on a mechanical shaker and vibrated for 2 minutes. Once the sample was separated, the contents of each sieve were weighed to the nearest 0.01 g.
Cumulative weight percentages (defined by the amount of the sample that could pass through each successive sieve) were calculated for each subject. From these percentages, the median particle size (MPS) was estimated by using the Rosin-Rammler equation.
where Q w is the weight percentage of particles with a diameter smaller than x (maximum sieve aperture) and b is the broadness of the particle distribution. The MPS ( x 50 ) is the aperture of a theoretical sieve through which 50% of the particles can pass.
To measure occlusal contact areas, 2 impressions of the right buccal segments were taken with Blu-Mousse, a vinyl polysiloxane impression material (Parkell Biomaterials, Farmingdale, NY). The impression material was expressed directly onto the mandibular occlusal table from the second molar to the first premolar. The subjects were then instructed to bite down firmly into maximum intercuspation and to hold that position for 30 seconds. To measure ACNC, the impressions were trimmed and optically scanned (mandibular occlusal surface facing down) on a Twain Pro flatbed scanner (Epson, Long Beach, Calif). A block of known length was included in every scan so that the area measurements could be calibrated to the actual size.
The software program UTHSCSA Image Tool (University of Texas Health Science Center, San Antonio) was used to manually trace the occlusal table area of the first and second premolars and molars. This software program automatically calculated the surface area of the occlusal table and the frequency distributions of pixels corresponding to each of the 256 gray scales (GS).
A step wedge of known thickness made of the Blu-Mousse impression material was scanned and used to calibrate the relationship between the 256 GS and the thickness of the occlusal registration. The thickness along the step wedge was measured with digital calipers (accurate to .001 μm). Based on the curve-fitting function in SPSS software (version 15, SPSS, Chicago, Ill), the relationship between the GS and the step-wedge thickness followed a curvilinear pattern (R = 0.9855, P <0.001), expressed by the following formula:
thickness = 0 . 0147 + ( . 0005 ∗ GS ) + ( . 00000021 ∗ GS 2 )
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