INTRODUCTION

Mastication represents the first stage of the digestive process, during which foods are physically broken down into smaller particles to increase their surface area, thereby facilitating enzymatic processing during later stages of digestion.1,2 Mastication can be measured by several means, including masticatory ability, efficiency, and performance. Masticatory ability is a subjective measure, a perception of how well subjects think they break down foods.3 Efficiency pertains to the number of masticatory cycles (ie, number of chews) required to reduce foods to a certain size,4 and masticatory performance, the most common and powerful measure used, pertains to the particle size distribution of food chewed after a standardized number of cycles.4,5

Several factors influence masticatory performance, including body size, bite force,6 number of functional tooth units,7 occlusal contact area,8 and malocclusions.9 Although not as potent a factor as the mutilated dentition, malocclusions can negatively affect subjects' ability to process and break down foods.10 Unfortunately, most of the studies evaluating the relationship between malocclusions and mastication are not conclusive in terms of identifying the subtle influences of the different types of malocclusions on masticatory performance.

Given this background, a systematic review was warranted, focusing on the relationship between malocclusions and masticatory performance. Furthermore, a qualitative analysis of the methodological soundness of the studies in the review was performed.

MATERIALS AND METHODS

Selection Criteria

The inclusion and exclusion criteria are given in detail in Table 1.

Table 1 Initial Inclusion and Exclusion Criteria for the Retrieved Studies

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Data Collection and Analysis

Data were collected on the following items: author, year of publication, study design, study groups, methods/measurements, and outcome measurements. The articles were separated into two main topics: (1) the influence of malocclusion treatment (orthognathic surgery) (Table 2) and (2) the influence of malocclusion type and severity (Table 3).

Table 2 Masticatory Performance Studies in Orthognathic Surgical Patients^a

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Table 3 Masticatory Performance Studies Regarding Different Malocclusion Types^a

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In addition, to document the methodological soundness of each article, a quality evaluation was performed with respect to preestablished characteristics,11,12 evaluating the following eight variables: (1) study design (randomized clinical trials, prospective trials, or controlled clinical trials [CCTs]—3 points; clinical trials [CTs]—1 point); (2) adequate sample size—1 point; (3) adequate selection description—1 point; (4) valid measurement methods—1 point; (5) use of method error analysis—1 point; (6) blinding in measurement—1 point; (7) adequate statistics provided—1 point; and (8) confounders included in analysis—1 point. Each study was categorized as low (0–5 points), medium (6–8 points), or high (9 or 10 points). The data extraction and quality scoring from each article were assessed independently by two researchers who selected the articles by reading the title and abstracts. All of the articles that appeared to meet the inclusion criteria were selected. One hundred percent agreement was obtained in this phase between the two researchers. The reference lists of the selected articles were also searched manually for additional relevant publications that might have been missed in the database searches.13

RESULTS

The search strategy yielded 78 articles. After selection according to the inclusion/exclusion criteria, 12 articles qualified for the final analysis (Tables 2 and 3). The research quality and methodological soundness were high in one study, medium in 10 studies, and low in one study (Table 4). The most serious shortcomings were the CT and CCT designs with small sample sizes and inadequate description of selection criteria. Lack of method error analysis and the absence of blinding in measurements were other examples of shortcomings. However, the choice of statistical methods was explained in all articles. Considering the use of confounding variables, only two studies did not report any factor. In all other reports, confounding variables, such as surface electromyography, occlusal contacts, bite force, signs and symptoms of temporomandibular joint dysfunction, and anthropometric measurements, were declared. All measurement methods used in the studies were valid.

Table 4 Quality Evaluation of the Retrieved Studies^a

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DISCUSSION

This systematic review aimed to select all clinical trials verifying the relationship between malocclusion and masticatory performance. No previous review study could be found. Twelve studies were retrieved. From a methodological point of view, it was notable that all of the studies used examination methods without blinding design. In all studies, the methods used to detect and analyze the relationship between malocclusion and masticatory performance were valid and well known. However, great variations in test food and number of cycles were observed, which could make comparisons among all studies difficult.1,14 On the other hand, studies15–17 published by the same group were found, allowing more comprehensive conclusions based on their results.

The influence of malocclusion treatment (orthognathic surgery) on masticatory performance showed that mastication was still hampered in comparison to results obtained from controls, even after surgical correction. In addition, surgical correction did not improve masticatory performance significantly.15,16,18 However, it was suggested that after surgery, some time is needed in order for the muscles to adapt to the new bone position. The muscle fibers are stretched and may also decrease bite force when compared to the situation before surgery.15,19 This fact was confirmed when a significant increase in masticatory performance was noted 5 years after surgery.17

According to these results, it is important to consider that longitudinal studies with short postsurgical time evaluation should be observed cautiously, since the musculature may need a long time to readapt the new incorporated modifications. It seems that at least 5 years are needed to measure a real improvement in masticatory performance.17

In addition, simultaneous evaluation of number and area of occlusal contacts, bite force, muscle thickness, amount of lateral jaw movement, pain, and muscle activity are encouraged in order to control all co-variables after orthognathic surgery, once masticatory performance may be influenced by all of these factors.6,20,21

The influence of malocclusion type and severity on masticatory performance was also investigated. In general, malocclusions caused decreased masticatory performance.8,10,22–27 Undoubtedly poor masticatory performance is associated with fewer intermaxillary tooth contacts. In addition, a diminished muscle activity was noted.22 A reduced platform to grind the food affects masticatory performance.6 Subjects with a reduced occlusal contacts area cannot pulverize their food to the same extent as subjects with more occlusal units, in a fixed number of chewing strokes. Fontijn-Tekamp et al.28 reported that the number of occlusal units was the most important factor that affected the median particle size of masticatory performance and the swallowing threshold. Similar results were previously reported.7 Occlusal contacts promote mandibular stability at maximal intercuspation29 and have an influence on chewing function8 and masticatory muscle activity.30

Masticatory performance is also influenced by bite force. It is believed that bite force increases with teeth in occlusal contact.31,32 In all selected studies, only Tate et al.23 evaluated bite force and masticatory performance in orthodontic patients. The differences with regard to bite force in preorthognathic surgery patients and Class I molar relation was not statistically significant. However, in this study, sample size and selection description were considered inadequate for drawing any further conclusions. Therefore, the correction of malocclusion through orthodontic treatment becomes an important resource with which to improve occlusal contacts and, consequently, masticatory performance.

CONCLUSIONS

• Malocclusions cause decreased masticatory performance, especially as it relates to a reduced occlusal contacts area.

• The influence of malocclusion treatment (orthognathic surgery) on masticatory performance is only measurable 5 years after treatment.