Pi Dental Center Scientific Journal Articles and Ongoing Research
Poster Presentation: Presented at 2001 Academy of Osseointegration Meeting (16th Annual Session)
Toronto, Canada, March 22-24.
In Vivo Axial Forces and Bending Moments on Abutments During Jaw Opening
J. A. Porter*, J.B. Brunski*, T. Balshi°, G. Wolfinger°, and R. Winkelman°
*Dept. of Biomedical Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, °Pi Dental Center, Ft. Washington, PA
Objectives: Some have hypothesized that a rigid prosthesis supported by abutments in the molar and anterior regions will restrain jaw flexure during jaw opening and thereby load the implants, perhaps inappropriately. We tested this hypothesis by measuring abutment loading during jaw opening and closing in two human patients.
Methods: Two patients (1 male, 1 female) participated in this study under informed consent. Each patient had four Brånemark fixtures supporting a mandibular, full-arch, metal-backed prosthesis. Each patient had two fixtures in the anterior region and two fixtures in the distal molar region. During one office visit, we removed each patient’s metal-backed prosthesis and replaced it with a nearly identical metal prosthesis that mated with 5.5 mm abutments, two of which were strain-gaged to act as transducers for axial force plus bucco-lingual and mesio-distal bending moments.1 Prostheses were connected to the abutments using recommended gold screws and torques. Wires from each transducer were routed to a battery-powered data acquisition system (iO tech, Cleveland OH) and laptop computer. Transducers were zeroed before jaw opening with the mandible in a closed position. Data was taken during a sequence of 5 to 10 opening-closing events. In a separate test, impressions (Regisil) were taken of the patient’s mouth in closed vs. open positions with the prosthesis removed. Impressions were electronically imaged and analyzed to enable measurements of changes in the relative position of abutments (in the plane of the image) in open vs. closed jaw positions.
Results: Typical axial forces were in the range of 5 to 50 N independent of abutment position (anterior vs. distal). Axial forces tended to be tensile on one distal implant when the values were compressive on the contralateral distal implant, but this was not a consistent finding in light of data from only two patients. Moment magnitudes ranged from 1 to 20 N·cm, with larger values arising when transducers replaced two distal abutments. Impressions from closed vs. open jaw positions revealed a decrease in spacing of distal implants (0.188mm) and anterior implants (0.131 mm).
Discussion and Conclusions: The cross-arch decrease in implant spacing during jaw opening is consistent with Hobkirk and Schwab2, who reported decreases in distal implant spacing of 0.025 to 0.3 mm during jaw opening. Other literature reports similar results with natural teeth.3,4 We are not aware of literature about forces and moments during jaw opening as measured herein. The most comparable data come from ref. 2, which measured lateral forces of 3 to 14 N on distal implants separated by a ring-type load cell; the data are consistent with our findings if we assume that the ref. 2’s lateral forces acted about 0.5 cm above the bone level, e.g., 14 N x 0.5 cm = 7 N·cm. Overall, all of these values of forces and moments are much less than those during typical in vivo loading of oral implants. While our data are limited to only two patients, our data confirm jaw flexion during opening but does not indicate that excessively large forces and moments accompany such opening on distal and anterior fixtures supporting metal prostheses.
References: 1. Rangert B et al.: Vertical load distribution on a three-unit prosthesis supported by a natural tooth and a single Brånemark implant Clin Oral Impl Res 1995;6:40-46. 2. Hobkirk JA and Schwab J: Mandibular deformation in subjects with osseointegrated implants. Int J Oral Maxillofac Implants 1991;6:319-328. 3. Korioth TWP and Hannam AG: Deformation of the human mandible during simulated tooth clenching. J Dent Res 1994;73:56-66. 4. Niederdellmann H: Fundamentals of healing of fractures of the facial skull, Ch. 5, pp. 125-145, in Oral and Maxillofacial Traumatology (E. Kruger and W. Schilli, Eds.), Quintessence, Chicago, 1982.
Support: Nobel Biocare Scholarship Fund (J.A.P., J.B.B.) and Pi Dental Center