Because there is no shock absorbency within the bone surrounding dental implants, potentially damaging occlusal forces rise very quickly on an implant prosthesis’ occlusal surface. And, because the proper locations of any occlusal force excess is not quantifiably and reliably described to the operator by the articulating paper markings 5,6,7,8,9,10,11, there is no predictable occlusal force control when insertion occlusal adjustments are performed without of measurement of the occlusion. Therefore, the regions of occlusal force excess are often not removed during the insertion occlusal adjustment procedures. Hence, rapid occlusal surface dental material breakage is often clinically observed.4
Using Computerized Occlusion on the Complete Arch Implant Supported Prosthesis
Mitigating occlusal force excess while improving the overall occlusal balance on complete arch implant supported prostheses can be readily achieved with measurement of the overall occlusal force distribution utilizing the T-Scan III Computerized Occlusal Analysis System (T-Scan III for Windows® Tekscan, Inc. S. Boston, MA, USA)(Figs 1, 2, 3). Computerized Occlusal Analysis technology records, and displays occlusal contact relative occlusal force and tooth contact timing data which occur during functional mandibular movements .These functional movements are recorded intraorally with an ultra-thin, electronically charged, Mylar-encased sensor, that is connected to a computer via a USB interface (Figs. 1,2). The desktop software then displays the tooth contact sequences in .003 second increments with their changing occlusal forces, described by both the percentage of the maximum occlusal force obtained within the recording, and by a color-coded scale (Figs. 2, 3).
The T-Scan III system can be used to detect implant prosthesis occlusal force excess, and guide its’ correction (Figs.4-9). Figures 4 and 5 show two 20 year-old completely implant supported opposing hybrid restorations, sitting upon 6 abutments each. These hybrids have recently been resurfaced with new denture teeth and pink acrylic, and their occlusion is to be finished with T-Scan III guided occlusal adjusting. Figures 6 (maxillary occlusal view matched to T-Scan III 2-Dimensional window) and 7, detail the articulating paper markings resultant from preliminarily occlusal adjustments made to the hybrid prostheses at insertion. At this point in the insertion process, there are more paper markings through the left side than are present on the right side and the largest paper makings are present in the left Canine and Central Incisor regions.
What cannot be seen within the paper mark distribution, is the moving occlusal force summation (known as the COF Trajectory) that begins near the left upper canine area (tooth #11; corresponding ISO #23) (Fig. 8a), crosses slightly anteriorly (Fig. 8b), and then moves posteriorly to finish in the left anterior quadrant with a overall force imbalance of 60.8% left - 39.2% right (Fig. 8c). This poorly directed occlusal force summation, if not corrected, will during occlusal function, repeatedly torque the prosthesis thru its’ lifespan, by overly depressing the left anterior region while simultaneously lifting the right posterior region.
Note that at the end of the pre-operative closure sequence (Fig. 8c) there are very large forces present on teeth #s 3 and 4 (corresponding ISO teeth #s15 and 16), despite the fact that there are very small paper marks on these same teeth observed in figure 4. This is a clear example of how both large articulating paper marks like those present on teeth #s 11 and 9 (corresponding ISO teeth #s23 and 21)(Fig. 6) and very small marks like those on teeth #s 3 and 4 (corresponding ISO teeth #s15 and 16), can both demonstrate high occlusal forces.
Figures 9a-c shows the T-Scan III corrected endpoint obtained after 7 recordings and computer-guided adjustment sequences. Post-operatively, the COF Trajectory starts just left of the center of the prostheses (Fig. 9a), travels to the right across the midline (Fig. 9b), and moves to the posterior slightly where its’ movement ends near the midline. The corrected right to left force imbalance is now only .2% (50.8% right – 49.2% left) (Fig. 9c).
After the completion of computer-guided occlusal adjustments, when the prosthesis is placed under functional occlusal loading, instead of being repeatedly torqued to the left anterior, the prostheses will now be seated upwards into the center of the palate and downwards into the floor of the mouth. Thru their lifespan, this occlusal endpoint will be far more preservational of all the components of these prostheses, thereby improving the longevity of the implants, the metal substructures, and the acrylic denture teeth.
Using Computerized Occlusion on the Distal Extension Implant Prosthesis
Natural teeth move vertically and horizontally significantly more than implants because of the resiliency of the periodontal ligament.12,13 In mixed implant–natural tooth occlusal schemes, this discrepancy in movement results in a force transmission difference whereby an implant prosthesis, which moves significantly less, can stop the natural teeth nearby from completely depressing into their periodontal ligaments. Because the implant prosthesis is the least mobile element when compared to its’ neighboring natural teeth, the implant prostheses will absorb more of the occlusal forces than their neighboring natural teeth, making it more prone to breakage and/or deosseointegration.
Therefore, in mixed implant–natural tooth occlusal schemes, the optimal clinical scenario is where the implant prosthesis occludes after the natural teeth by enough elapsed time for the teeth to depress partway into their Periodontal Ligament fibers. This is when the teeth will begin to meet resistance by the surrounding alveolar housing. Ideally, the natural teeth will physiologically move in response to the applied occlusal load before the implant prosthesis commences occluding.14 The clinical benefits of applying the Time Delay principle were reported by Stevens15. He demonstrated how the delaying of the occlusal contacts on a longstanding distal extension implant prosthesis that had previously lost significant bone support around the supporting implants, could regenerate the lost bone. 15
Establishing a Time Delay is a precise occlusal adjustment that requires time measurement to successfully accomplish.14 The operator should first establish implant prosthesis - natural tooth contact time simultaneity in patient self-closure, by improving the COF Trajectory position and length such that the trajectory is a short, nearly straight line, that is centered along the T-Scan III’s 2-Dimensional Window arch-half midline. The Occlusion Time for the entire arch should be < .2 seconds from 1st to last contact, during a properly balanced patient self-closure into complete intercuspation (Figs. 10). 16
Then to delay the implant prosthesis during subsequent patient self-closures, the closure occlusal contacts on the implant prosthesis need to be gently “shaved”, so that only a small amount of occlusal materials is removed during subsequent adjustment sequences.14 This shaving is accomplished by lightly brushing the occlusal surface where the paper marks indicate occlusal contact exists, using a medium course round-diamond bur. Gentle shaving insures only a very small amount of closure contact occlusal surface material is abraded, so as not to lose the contact in total, but to slightly delay it from making initial contact.
The following Time Delay example required four recording and adjustment sequences after measurable simultaneity was established to hold back the loading of 3 different implant prosthesis. There are opposing 3 and 4-unit distal extension implant prostheses in the posterior right quadrants, and another 2-unit distal extension prosthesis located in the lower left posterior quadrant that opposes upper previously crowned natural teeth. The remaining anterior teeth are non-restored (Figs 11a-c). Note how the articulating paper marks (Fig. 11c) on the installed maxillary prosthesis, although widespread, do not measure or describe the contact timing sequence for the operator.
The post-treatment computer-guided occlusal result is depicted in Figures 12a-d, which depicts 4 sequential Movie frames that illustrate the established delay. At 2.869 seconds, there are light-force (blue) early contacts present on all of the natural teeth that are anterior to the implant prostheses, while no contacts are present on the implant prostheses. The COF Trajectory starts anteriorly near the right anterior teeth because they are the 1st teeth to occlude (Fig. 12a).
At 2.947 seconds (Fig. 12b), the right 4-unit implant prosthesis begins to make light force contact (blue) while the anterior natural teeth are contacting more forcefully than at previously (light green, yellow, light blue).
At 3.044 seconds (Fig. 12c), all the anterior teeth are making moderately forceful contact just as the middle left posterior teeth begin to rise in force. There are light blue columns present on teeth #s 11 and 12 (corresponding ISO teeth #s 23 and 24) while at the same time, the right posterior implant prosthesis is demonstrating mostly low force (blue) contacts. At this point in the patient self-closure, the left posterior 2-unit implant prosthesis has not yet begun to make contact. Then finally at 3.417 seconds (Fig. 12d), both the right and left posterior implant prostheses rise to low-moderate force levels (light blue) and maintain these reduced force levels into static intercuspation. The desired endpoint has been achieved as the anterior natural teeth reach near maximal occlusal forces prior to the initial force rise on the 3 different implant prostheses.
The definitive Time Delay is [3.417 seconds (Static Intercuspation) - 3.081 seconds (both implant prostheses in low-force contact)] = .336 seconds.
Occlusal adjustment procedures performed upon implant supported prostheses can be guided by computerized occlusal analysis relative occlusal force and contact time-sequencing data, to accomplish precise, ideal, measurable occlusal endpoints. Because the T-Scan III System records the elapsed time in .003 second increments, and changing relative occlusal contact force evolution that occurs during any recorded occlusal event, it is possible to manipulate the sequence of the timing of closure occlusal contacts, while simultaneously controlling any regions of damaging relative occlusal force excesses. Its’ use with implant prostheses can improve prosthesis occlusal surface material longevity, and limit the possibility of the supportive implants undergoing deosseointegration.
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This article was adapted from; Kerstein RB. Appendix in Tarantola G, Clinical Cases in Restorative and Reconstructive Dentistry. September 2010, Wiley-Blackwell, Hoboken, NJ USA.
This material is reproduced with permission of John Wiley & Sons, Inc.