The dynamic recording of occlusal forces related to mandibular movement and masseter muscle activity in implant stabilised overdenture wearers
The initial phase of this study was to develop a force transducer to investigate occlusal forces during the process of mastication. The strain gauge transducer was designed to attach to an Astra implant. A commercial pressure transducer was also used under one distal saddle of the denture. The calibration method was designed to record unilateral masticatory forces regardless of the site of loading. Five edentulous subjects with implant stabilised mandibular overdentures and maxillary complete dentures were selected for the study. Mandibular movements and unilateral Masseter muscle EMG were recorded. Experiments were designed to establish within subject differences related to cycle type, food type, EMG-force relationship, chewing side and cycle phase. Maximum biting force on a bite gauge and in the intercuspal position were also recorded. Details of force production during the closing and occlusal phases improved the description of the movement cycle and it's relationship to food breakdown. A classification of chewing cycles is proposed based on the force exerted in the closing and occlusal phases. This provides objective criteria for separating crushing, reduction and mush cycles. Peak forces varied between subjects, but are characteristic for the type of food within subjects. The different strategies used by subjects appeared to be partly related to the force capacity of the individual. The duration of force in the closing phase was related to the stage in the chewing sequence and indicated the degree of bolus resistance. A longer force duration in the closing phase was usually accompanied by a shorter occlusal duration and consequently an early occurrence of force peak relative to occlusion. The progressive change of jaw gap at the beginning of force was consistent with the expected reduction of particles through the masticatory sequence. The jaw gape at maximum velocity correlated with maximum jaw gape but both did not decrease progressively 2 Abstract through chewing sequences. Conversely, the jaw gape at which force exceeded 5N showed patterns of progressive decrease especially with frangible foods. Integrated force and Emg showed high correlations during mastication, better than peak values. These correlations were, however, weaker than those found in static unilateral biting. The slope difference found between the two conditions contraindicate the use of Emg activity in static unilateral biting as an index for measuring masticatory forces. This study has, thus, validated a method for investigating masticatory forces and shown their value for analysis of dynamic aspects of the loads that occurs during mastication.