FERRARI, CAGIDIACO, AND BERTELLI ments to best serve patients by providing conservative preparations with maximal retention for resin-bonded restorations. The recommended thickness of a retainer is 0.3 to 0.5. 3,4 By reducing the enamel 0.2 to 0.4 mm, the possibility of traumatic occlusal forces to the retainer is reduced or eliminated. This reduces stress on the resin bond. CONCLUSIONS The extent and thickness of enamel permits adequate reduction for acid-etched retainers. The gingival layer of enamel should serve only to receive a bevel. The data reported in Table I may serve as a guide to determine the depth of reduction of enamel. REFERENCES 1. Livaditis GJ. Cast metal resin-bonded retainers for posterior teeth. J Am Dent Assoc 1980;101:926-9. 2. Livaditis GJ. Resin-bonded restorations: clinical study, lnt ,i Perio Rest Dent 1981;l:70-q. 3. Livaditis GJ. Resin-bonded restorations: Preparatkm o! tht~ abutment teeth. Int J Perio Rest Dent !982;4:3%4 ";~. 4. Simonsen R, Thompson VP, Barrack G. Etched c~st restoration~ clinical and laboratory techniques. Chicago: QUintessence Pub- lishing Co, Inc, 1983. 5. Howe DF, Denehy GE. Anterior fixed partial dentures utilizing the acid-etched technique and a cast metal fiamework. ,J PRos- THET DENT 1977;37:28-31. 6. Ferrari M, Bertelli E, Borracchini A. Considerations an micro- scope electronique a balayage sur le ponte Maryland. Dubrow nite, Yugoslavia: XXIX Congress du GIRSO, April ] 985. Reprint requests to: DR. MARCO FE~RA~I ASSOCIAZIONE ODONTOIATRICA EMILIO CAGIDIACO PIAZZA A'rT,AS 19 LIVORNO .57125 ITALY I I I I II I I I I I Ill Illlll II ..... A system periods with decre vat.rice Petros T. Koidis, D.D.S., Ph.D.,* Simon V. Zmuidzinas, D,D,S.,** and James G. Butch, D.D.S., M.S.*** Ohio State University, College of Dentistry, Columbus, Ohio The relative pause in the electrical activity of mastica- tory muscles elicited by an exteroeeptor stimulation, usually a chin tap, is called the silent period (SP). The SP has a prolonged duration in patients with cranio- mandibular disorders compared with the SP duration (SPD) of normal subjeetsJ -s Because of this difference, SPD measured in milliseconds has been suggested as a possible clinical diagnostic aid. Several investigators within the past 10 years have questioned directly and indirectly the potential influence of different variables on the duration of the evoked SPs. Gillings and Klineberg9 concluded in their study that Semifinalist, John J. Sharry Research Award competiuon. American College of Prosthodontists. *Postdoctoral resident Advanced Prosthodontic Education, Section of Restorative and Prosthetic Dentistry. **Postgraduate student, Section of Orthodontics. ***Professor, Sections of Periodontics and Orthodontics. significant differences in duration were generally observed after different procedures. In addition, it has been reported that statistical trends show increased SPD for increased tap forces and decreased SPD with increased bite forces,t~ Higher levels of tap force were correlated with an increased. SPD,", ,2 whereas the SPD was shown to be inversely related to the magnitude of the bite force,u However, Bailey et al. 4 demonstramd that no significant changes on the SPD could be ascribed to any of the following variables: position, forte and angle of tap, degree of opening of the jaw, and bite force. Other investigators also deny any influence of bite force on the SPD. 1r Although controversy about the influence of stimulus variables on the recorded SPDs still exists, there is a general agreement about the hig!~ variability in recorded durations. As a consequence, SPDobjeetively seems to have a relative value in the [ of diagnostic findings? 6 Never&eh .... hypothesized that some ~ar~,aI~ility in the duration will 110 JULY 1987 VOLUME 58 NUMBER 1