R. F. Akeel, BDS, MDS
Longon Hospital Medical College, Turner Street, London El 2 AD

 
Improving the ability of patients to masticate food is one of the goals of performing prosthodontic restorations and is essential in achieving patient satisfaction. Different methods of assessing this function are di­cussed as are the factors considered important in achieving good masticatory efficiency

Methods of Analysis
Fractional Sieving
The majority of the tests developed so far depend on the fractional sieving of the chewed food or material in the analysis. The general principle is to pass the masticated material through a series of sieves of decreasing mesh size. Essentially, the more efficient the mastication, the greater the quantity of material that will pass through the finest sieve. The quantity of material collected is estimated by volume or by weight.
Manly and Braley18 concluded that using a 10 or 20 mesh U.S. standard screen in sieving peanuts is more sensitive than using finer screens, and that mastication is a selective process which tends to grind larger particles more than the fine particles. As yet, we do not know the optimal size of food particles to be swallowed, and consequently, there are no criteria for selection of the size of the mesh screen.
Area of Pieces
Dahlberg17 proposed this method of analysis using gelatin as a test material. As one of the functions of mastication is to enlarge the food particle area exposed to the digestive juices, measuring the area of pieces will be a reliable method for assessing masticatory efficiency. Kayser and Hoeven21 used a similar principle but with carrots as a test food.
Gunne26 modified the previous method and he estimated masticatory efficiency by calculating the area of the chewed material (gelatin hardened by formalin) indirectly. He placed the chewed material in a water-soluble dye which, when diffused into the material, will result in a reduction in the concentration of the dye in the surrounding solution. This concentration is registered with the help of a photometer. A close correlation was found between the gelatin particle area and the reduction of the dye solution concentration.
Extraction of Sugar
Heath25 proposed a method of estimating masticatory effectiveness using chewing gum. This material does not break up into particles during mastication, rather a principle of measuring the percentage loss of sugar on chewing was developed to determine a value for the masticatory effectiveness. Although chewing gum is considered a soft material, it has been shown that it can be an effective method for both dentate individuals and denture wearers.25'33
Another method, developed by Nakasima et al28 is claimed to be simple, reliable, and rapid. It implies chewing on a rubber capsule under standardized conditions. This capsule contains a specific amount of pigment-coated granules which, when crushed, will leak out the pigment. The leaking of pigment inside the capsule is positively correlated to the energy used. The masticatory ability, defined as the joules of work performed, was calculated by measuring the concentration of dye in an aqueous solution of the capsule's contents.
Subjective Evaluation
This method of evaluation has attracted a lot of interest from researchers. Different types of questionnaires have been used in several studies often in combination with an objective masticatory efficiency test. The questions usually used include the perceived ease of chewing different foods and about the dietary intake. The self-evaluation of mastication includes the psychological effect on the subject and the resultant satisfaction or dissatisfaction. These data cannot be obtained with the objective methods.25'2734"37
Comparisons of Different Test Methods
The difficulty in chewing different types of food and materials, beside the difference in methods of analysis, makes it difficult to compare results from different test methods. However, Helkimo et al38 found a close correlation among results obtained from comminution tests. Poyiadjis and Likeman33 also found a significant correlation between masticatory performance results of two different tests but only in denture wearers.
Contrary to previous results, Krysinski et al39 found neither direct comparisons nor exact conversions of results between three different methods. This observation has been substantiated by Gunne,40 who compared a surface area determination method with a comminution method. He found that a value from one method corresponded with a large range of values in another method and vice versa. He concluded that each method measures one aspect of mastication.
When the subjective evaluation of masticatory efficiency was compared with the objective test results, no positive correlation was found.25,56 Many people with poor masticatory efficiency, as judged from objective test methods, will still judge their masticatory function as good. It is concluded that self-assessment of chewing ability is, in general, too optimistic when compared with results of functional tests.41
Effect of Loss of Dentition
Manly and Braley18 tested the masticatory performance of more than one hundred dentitions, some complete, some with the third molar missing and some with two molars missing. The results revealed significant differences between the three groups. The mean masticatory performance score was lower with a reduction in the number of molars. However, they also found that the variation in performance among persons with the same number of teeth was very large. They concluded that counting the number of teeth is of little value in predicting how well any individual can masticate food.
Number of teeth was correlated with masticatory efficiency in subsequent investigations but the large variation in performance still dominated all studies.22,27,42
Effect of Malocclusion
Variation from the normal or ideal intercuspation of teeth can reduce the chewing efficiency.43 Changes in occlusion, as a result of wearing orthodontic appliances, have resulted in lower masticatory efficiency test scores.44,45 Patients with severe mandibular prognathism, or developmental deformities, showed a relatively low value in the masticatory efficiency test.46,47
Omar et al4B found a significant correlation between the masticatory efficiency scores and the occlusal status indices. In their study, the occlusal status indices included aspects of malocclusion.
Effect of Occlusal Contacts
Yurkstas and Manly49 developed a method to quantify the extent of the grinding surface of natural teeth by using semiopaque wax. An impression of the occlusal surfaces of teeth is taken and the amount of light that passes through this wax will determine the food platform area.
They found a great variability in the food plat-form area ot similar teeth according to the type of intercuspation. However, they found a significant correlation between masticatory performance and occlusal contact area. Occlusal contact area was related to masticatory performance in a curvilinear manner and it was the major factor in accounting for variations in masticatory performance. This relationship has been demonstrated again in a later study by Luke and Lucas.43
Later on, Helkimo et al22 proposed the number of occluding pairs of teeth as a relatively reliable measure of chewing efficiency, instead of the occlusal contact area, which is difficult to judge. Helkimo's finding was substantiated by other investigators although different methods of counting were used.42,48
It can be concluded that the maintenance of occlusal contacts is essential for masticatory function.
Effect of Bite Force
Bite force measurement has been used frequently in the literature. The method usually used is to ask the patient to bite with different force levels (usually maximum) on a fork containing strain gauges. Bite force was found to be positively correlated to masticatory efficiency25,38,50 and it can vary according to the state of dentition.51
According to this method, it was found that denture wearers have substantially lower bite force than dentate subjects.25,51
Glantz and Stafford52 pointed out that instruments, like the bite fork, have a major influence on the recorded bite force value. By the use of strain-gauged maxillary complete denture, they showed that the maximal bite force in denture wearers registered in the intercuspal position was much higher than that registered by the bite fork method.
Effect of Prosthetic Replacement
Loss of teeth, in general, can lead to a reduction in masticatory efficiency. The question was whether the restoration of missing teeth by prosthetic replacement can restore the masticatory function back to normal. This issue has been studied by many investigators. Most of the interest was focused on the evaluation of masticatory efficiency objectively in complete denture wearers. However, the function of dental arch restored by fixed or   removable   partial   denture   is   considerably improved.53,54 In another study,55 it was found that chewing performance in subjects with cross-arch fixed partial dentures was almost as good as in subjects with complete healthy dentitions.
Masticatory Efficiency in Denture Wearers
According to many studies, masticatory efficiency is lower in denture wearers compared to dentate subjects.41 Masticatory efficiency in denture wearers is about one- fourth18 to one-sixth56 relative to dentate subjects. Due to the reduced ability of dentures to pulverize food, Kapur and Soman56 thought that the grinding of food is random in complete denture patients and, thus, the reduction takes place homogeneously in all particle sizes. This is contrary to the selective process of chewing in dentate subjects, where bigger particles (greater than 0.84 mm) are subdivided faster than smaller ones.18
Effect of Quality of Dentures and Residual Ridges
An enhanced denture quality would be expected to improve the masticatory efficiency in complete denture wearers. However, this has been refuted by some investigators. Optimizing the old denture, or the construction of a new one, gave no or only a slight improvement in masticatory efficiency after an adaptation period lasting up to six months.50,57 Even when the follow-up period was extended to 18 months in another study,36 the same result was found. Nevertheless, in contrast to previous studies, a direct improvement in masticatory efficiency was found after the patients were provided with new dentures15 and after three months' adaptation.33
Different denture base forms were tested for difference in masticatory performance.58 No significant difference was noted. The investigators also showed that reduction of the extension of upper and lower denture bases in different areas failed to affect the chewing efficiency of patients, except when an overall reduction in extension of the periphery of the lower denture base had been made.
Heath25 has shown that the height of residual mandibular bone is correlated to the masticatory effectiveness in house-bound pensioners. In another investigation,-58 the average masticatory performance ratio in subjects with good ridges was higher than in subjects with poor ridges, though the difference was not statistically significant.
Patients who underwent surgical reconstruction of their deficient residual ridges had improved masticatory performance during the rehabilitation period with new dentures.37 The subjective evaluation by the patients was also improved.
Effect of Cusp Form and Occlusal Contacts
Teeth with occlusal markings were found to be more effective in mastication than flat teeth.59 This was also observed by Kapur and Soman,60 who studied the influence of occlusal pattern on masticatory performance in complete denture wearers. An acrylic resin block was used for the posterior teeth and 15 different occlusal patterns were cut into these blocks. In general, it was found that posterior teeth with occlusal markings were more effective in pulverizing the test food (carrots and peanuts) than those with no markings.
Non-anatomical teeth were found less efficient in mastication than 20 degrees61 or 33 degrees16 denture posterior teeth.
In a series of investigations on the effect of denture factors on masticatory performance, Kapur and Soman62 studied the influence of the shape of the polished surface of complete dentures on chewing efficiency. They found no obvious differences in subjects' ability to chew with different denture forms. They also found that the position of the occlusal table in a complete denture can effect masticatory performance.63 If the platform was over the crest of the lower alveolar ridge at the height of the lower canine and parallel to the flat portion of the ridge, then the most effective chewing situation existed to masticate peanuts and raw carrots. A significant reduction in chewing efficiency of subjects resulted when the food platforms were moved in dentures with teeth positioned buccal to the crest of ridges.
The reduction of the occlusal contact area of denture teeth will reduce the masticatory performance in denture wearers.64 The loss in masticatory efficiency was directly, but not proportionally, related to the potential contact area.
Patients with Osseointegrated Implant Bridges
The treatment of edentulous patients by means of bridges on osseointegrated implant yields long-term success.65 Lindquist and Carlsson66 evaluated the masticatory function of 27 edentulous patients with denture adaptation problems. They were first given optimal conventional complete dentures and then fixed prostheses on osseointegrated oral implants in the lower jaw. Functional evaluation was made with the old denture, after the insertion of the optimal denture, and 2 months and 3 years after insertion of the osseointegrated implant bridge.
No significant improvement of masticatory function was found after conventional denture treatment. However, after insertion of the fixed mandibular implant bridge, a marked improvement was noted in the patients' assessment of their chewing ability and in the results of chewing efficiency tests and bite force measurements, with a strain-gauged bite fork placed between the denture teeth. The test results were further improved after three years.
It was concluded that treatment of dissatisfied complete denture wearers, with osseointegrated implant bridges, will dramatically improve such oral functions as bite force and chewing efficiency as well as their oral well-being. This is in agreement with a previous study by Haraldson,67 where a sub-stantial functional rehabilitation was found in patients treated with this type of therapy.