SECTION EDITOR Masseter muscle fatigue before and after rehabilitation with implant-supported prostheses R. Jacobs, DDS, PhD, a D. van Steenberghe, MD, PhD, b and I. Naert, DDS, PhD c Faculty of Medicine, Catholic University of Leuven, Leuven, Belgium To establish whether different implant-supported prosthetic reconstructions influence jaw muscle resistance to clenching efforts, sustained submaximal (50%) clenching was performed in two groups of patients who were edentulous in one or both jaws. One group consisted of patients who were rehabilitated by means of an overdenture on two to four implants connected by a bar; the other group consisted of patients who were rehabilitated with an implant-supported fixed prosthesis on four to seven implants in either the maxilla or the mandible. Results indicated that after rehabilitation with implant-supported fixed reconstructions, the myoelectric output level increased over time, which was parallel to a higher bite force level. For the other group, such a time-dependent change in electromyographic amplitude did not occur. On the other hand, power spectrum analysis revealed a significant downward shift of the mean power frequency during sustained clenching after rehabilitation with implant-supported overdentures but not with implant-supported fixed prostheses. The absence of a spectral shift in the latter group probably expressed a fear of biting too hard and fracturing the prosthesis. (J PROSTHET DENT 1995;73:284-9.) It has been assumed that implant-supported pros- theses lead to an improvement of masticatory muscle per- formance. 1One parameter of muscle performance is fatigue resistance during sustained clenching. In a cross-sectional study on different types of rehabilitation, fatigue occurred in patients with natural teeth, complete dentures, and im- plant-supported overdentures, but fatigue was not present in patients with fixed prostheses on implants. The absence of fatigue in patients with fixed prostheses probably reflected a fear of biting too hard and fracturing the pros- thesis because of the absence of periodontal feedback. 2 In this follow-up study, the occurrence of change in fatigue resistance after installation of implants was investigated. Muscle fatigue has been defined in different ways and has led to different hypotheses about the underlying mechanisms. 3-6Time-dependent changes in both myoelec- tric signal amplitude and spectral parameters may be ap- plied to monitor deterioration of muscle performance. 6 For jaw muscles, the surface electromyogram (EMG) analyzed Sponsored by the National Fund for Scientific Research (NFWO Belgium). aResearch Assistant, NFWO, Department of Periodontology (lab- oratory of Oral Physiology). bprofessor, Department of Periodontology, The P-I Br~nemark Chair in Osseointegration. cprofessor, Department of Prosthetic Dentistry. Copyright | 1995 by The Editorial Council of THE JOURNAL OF PROSTHETIC DENTISTRY. 0022-3913/95/$3.00 + 0. 10/1/61130 by means of power spectrum analysis indicates that the power shifts to lower frequencies when the subject is maintaining a constant clenching level. 7 The median fre- quency (mean power frequency, MPF) is one of the most reliable spectral parameters to analyze such shifts. 7 The MPF is related to the bite force level. Lower MPF values are usually observed at higher bite force levels, ss~ Denture wearers develop a reduced bite force compared with that of subjects with natural dentitions. 11 After rehabilitation with osseointegrated implant-supported prostheses, bite force increases. I, 12-14 It could therefore be hypothesized that the increased bite force levels for implant-supported prostheses involve decreased MPF values. This study investigated resistance to fatigue and shifts in the power spectrum of the masseter muscle during a sustained submaximal (50 %) clenching effort before and after rehabilitation with osseointegrated implants. This investigation determined whether masseter muscle fatigue was dependent on the type of the prosthetic superstructure supported by implants. MATERIAL AND METHODS Subjects Seventeen women participated in the experiment (Table I). All subjects denied having any known heart disease or hypertension, which might make constant clenching effort unsafe because of the muscle heart reflex) 5 They gave in- formed consent and ethical committee approval was ob- tained. 284 THE JOURNAL OF PROSTHETIC DENTISTRY VOLUME 73 NUMBER 3