prosthetic | TECHNIQUES
Is complete denture stability during chewing possible? PART II of II By Max Bosshart, Master Dental Technician, Prosthetist Keith E. Doonan, BDS, MClinDent (Pros), FRACDS Matthias Bickel, Dr. med. dent., PhD, Prosthodontist
“The method developed by Professor Gerber greatly improves success... these basic rules are valid whether applied to com plete dentures, overdenture cases on eithe either r implants or natural tooth roots and to removable partial denture cases with free end saddles...”
“To obtain unilateral chewing stability: forget about balanced occlusion, it doesn’t work, but balanced occlusion is essential during parafunctional mandibular movements to prevent unbalanced loads on the supporting alveolar ridges.” This blunt statement shall guide us through some critical steps when making complete dentures, overdentures and free-end saddle removable partial dentures. The present article shows how one can achieve optimal chewing comfort for edentulous patients. patient s. It takes us back more more than half a century when some renowned prosthodontists made significant observations that led to a new concept, namely a modified lingualised occlusion. Furthermore, it reveals some of the reasons why dentures may fracture fracture when this concept is not used.
3. Balanced occlusion The arr arrang angeme ement nt of the tee teeth th acc accord ording ing to the Gerber System is still recognised as being a balanced anc ed occ occlus lusion ion.. We sha shall ll als also o rec recogn ognize ize the importance of molar teeth in parafunctional jaw movement (Figures 10 and 11) as denture wearers
do grind their teeth. Apart from unilateral chewing stability as described above, continuous posterior contacts are important during non-functional tooth conta con tacts cts.. Th Thee ev even en dis distri tribu butio tion n of the for forces ces ov over er the complete maxillary and mandibular alveolar ridges will protect the natural tissues from overload or at least diminish the forces to a minimum. During all functional and parafunctional movements, movements, the elimination of all premature contacts is indispensable. Conclusion: The Gerber System offers a multilatera lat erally lly bal balanc anced ed occ occlus lusion ion in a lin lingua gualis lised ed fashion to reach unprecedented denture stability.
4. Centric relation and the gothic arch tracing device When reorganizing a patient’s occlusion, dentists who attach importance to the structure of the temporomandibular joint consider centric relation as the position of choice. A number of methods exist for recording centric relation which include light chin point guidance, bilateral manipulation and the gothic arch tracing. Furthermor Furth ermore, e, there are impor important tant diff differenc erences es between recording centric relation in dentate and
prosthetic | TECHNIQUES
Figures 10 and 11. Beside the static orientation of the posterior teeth, balanced occlusion is an absolute must to protect the soft and hard natural tissues from local overload, occurring especially during parafunctional jaw movements.
Figure 12a. Gothic arch tracing of an edentulous case to determine centric relation.
Figure 12b. The central bearing pin distributes forces evenly over the denture bearing area.
edentulous individuals. The periodontium allows for only limited functional movements of the teeth in the range of 56-108 microns17 buccolingually and around 28 microns 18 axially. In comparison, wax rims suffer from two sources of movement, firstly the gross movements, which can be in the range of thousands of microns as a result of their lack of attachment to the underlying mucosa. The clinician may attempt to minimise this by holding the lower wax rim in place. The second type of movement occurs even when the wax rims can be held accurately in place and is due to the visco-elastic nature of the mucosa, which allows for movement in the range of several hundred microns. Finding centric with mobile wax rims on an unstable supporting tissue such as the edentulous oral mucosa can be very unpredictable. Registration of a physiological centric relation is however of major importance in the edentulous situation and the stability of dentures is directly related to it. It is symptomatic that maxillary dentures drop down during function irrespective of a perfect impression, when centric is not correct. Intra oral gothic arch tracing devices (Figure 12a) allow centric to be recorded without concurrent manual manipulation by the clinician. Furthermore, the occlusal forces during a gothic arch tracing are directed through a central pin allowing for the even distribution
and stability of the wax rims (Figure 12b). A common problem of guiding the patient manually into centric position is that if wax rims are even slightly incorrectly adjusted, they will contact primarily on a high spot upon closing, causing the lower denture to displace resulting in an incorrect centric record. This may go unnoticed even when the lower lip is subsequently retracted to confirm the lower wax rim is correctly in place. When using intra oral gothic arch tracing devices, the wax rims are not in contact with each other eliminating this problem of premature contacts of the wax rims. During the gothic arch tracing, the patient is instructed to bite together with firm constant pressure whilst making forward, backward and lateral movements. Whilst biting firmly, the patient cannot forcibly retrude the mandible past the physiological centric position. It has also been shown that centric relation is affected by the patient position. 19 A supine position will result in a slightly more retrusive centric relation record as compared to a centric relation record taken in the upright position. 19 The authors consider that effective mastication is an important aim in constructing complete dentures and that patients usually sit upright whilst eating. Therefore it is proposed that a physiological centric relation is best recorded with the patient in the upright position.
prosthetic | TECHNIQUES
Figure 13. Frontal view of a left condyle. The roof shape form is clearly visible and challenges articulator design. A ball and straight lines are simply not good enough to mimic physiologic movements.
Figure 14. Sagittal view of a TMJ. Due to the shape of the posterior wall, it is obvious that the retrusive movement must also be directed caudal (downward).
This is contrary to other techniques where a supine position is advocated. Watanabe 19 reported that centric relation recorded with gothic arch recording plates was 0.46mm (SD 0.23) more retruded when the patient was supine as opposed to sitting upright. This difference is accommodated by the Condylator’s ability to retrude 1.5mm past the point of centric relation to create balanced contacts during forced retrusive movements posterior to centric relation. Conclusion: The Gothic Arch Tracing Device is the most reliable instrument to establish centric in the edentulous jaw.
Figure 15. This figure is a combination of two pictures. The upper picture shows an artificial fossa as originally designed by Gerber in 1948 and the lower a natural condyle. Note the medial and external angulations of both are very similar.
Retrusive movement A simple test shows if a retrusive movement exists. With the head in an upright position and by keeping the teeth slightly closed, incline the head backwards, a slight sliding of the mandibular teeth can be observed. With the condyle centred in its physiologic place (centric relation), any further retrusive movement will be oriented caudally (downwards). There is no other way for the condyle than down because of the posterior wall (Figure 14).
The Fischer angle 5. The articulator is an important instrument in prosthodontics Many articulators use a ball to represent the condyle and a straight line plane to represent the glenoid fossae. Many articulators also attempt to reproduce movements and parameters such as the immediate side shift (ISS), Fischer angle, condylar angle, protrusive and lateral excursive movements. Immediate side shift (ISS) is a 3dimensional movement and can be more accurately reproduced in an articulator that reproduces the 3-dimensional shape of the condyle in the coronal plane as shown in Figure 13. Lateral movements, simulated without an ISS produce too steeper buccal facets on the mandibular molars, leading to hyper-balances. 13 The angle between the sagittal protrusive condylar path during protrusion and the physiological mediotrusive path during lateral excursions is called the Fischer-Angle and is more accurately represented on articulators with an anatomically shaped condylar analogue (Figure 15). Condylar angles, latero-excursive and protusive pathways can be measured and traced on the patient but only the end points can be transferred to an articulator with bite registrations. The movements between these end points on the articulator are only an estimate of the physiological movements that take place in the patient. Taken together, this sets the task, namely mimicking natural articulation that an articulator needs to achieve.
During lateral movements, the roof shaped glenoid fossae (Figure 13) is providing guidance for the condyle. The angular difference (in the sagittal plane) between a pure protrusive movement and the mediotrusive (inward) movement during lateral excursion of the non-working side condyle produces the so-called Fischer angle. The Fischer angle and ISS are influenced by the shape of the condyle in the coronal plane (Figure 15). Few articulators reproduce this morphology of the condyle. This angle becomes important during the simulation of the ISS. If these angles of the medial and the lateral walls were missing, the occlusion would become flat as seen in many dentures. Chewing efficiency becomes insufficient. Conclusion: Articulator design is not merely a mechanical issue, but helps to construct dentures that allow physiological movement with appropriate chewing efficiency.
Final remarks In order not to detract from the principles of the Gerber System we have, on purpose, not gone into too many details. It is important to understand the forces during mastication and the way they interact (Who knows why, knows how!). Of course many other factors are to be observed, from medical history to the very important aesthetical aspects, impression taking and many more.
prosthetic | TECHNIQUES The method developed by Professor Gerber15 greatly improves the success when all the relevant aspects are respected. Those aspects contain all the steps of a treatment, beginning with the first appointment, continuing with the pre-prosthetic treatment, first and second impressions, bite registration, set-up of the teeth providing aesthetic and static/dynamic requirements, try-in, properly executed laboratory procedures and remounting with perfect selective grinding-in. All these basic rules are valid whether applied to complete dentures, overdenture cases on either implants or natural tooth roots and to removable partial denture cases with free end saddles.16
Acknowledgements We thank Dr A. Johnson, Mr M. Boxhoorn for editing and Mrs A. Bruelhart for her assistance with the translations.
References 11. Gobert, B.: Variations cliniques implantaires avec l’Enregistrement Intra-Oral Gerber. Revue Implantologie, Mai 2006; 39-46. A. Girot, Megève France. 12. Gerber, A.; Steinhardt G.: Dental Occlusion and Temporomandibular Joint, 1989. Quintessence Publishing Co. Chicago. 13. Gerber, A.: Condylator Modell 4. Der Zahntechniker r. 6, 1959; 2 -19; Schw. Zanhtenchnikervereinigung, Zürich. 14. Gibbs, C., Lundeen, C., Mahan, P., Fujimoto, J.: Chewing movements in relation to border movements at the first molar. J Prosth. Dent. 1981: 46(12); 308-322. Mosby (Elsrevier, Amsterdam). 15. Hampson, E.L., Askew, P.A., Tanner, A.N., White, G.E.: A technique for constructing full dentures using the Gerber articulator and Condyloform teeth (I and II). Quintessence International 4 and 5, 1973 (45 – 54, and 45 – 51); Chicago/Berlin.
16. Geering, A., Kundert, M., Kelsey, Ch.: Complete Denture and Overdenture Prosthetics. Thieme Medical Publishers, Inc., New York. 1993. 17. Rudd, K.D., O’Leary, T.J., Stumpf, A.J.: Horizontal tooth mobility in carefully screened subjects.Periodontics,2:65-68,March 1964. 18. Parfitt, G.J.: Measurement of the physiological mobility of individual teeth in an axial direction. J Dent Res,39:608-618,May 1960. 19. Watanabe, Y.: Use of personal computers for gothic arch tracing: Analysis and evaluation of horizontal mandibular positions with edentulous prosthesis. J Prosthet Dent ,82:562-72,1999.
About the authors Max Bosshart is an internationally renowned Master Dental Technician and licensed Prosthesist who works in private practice in Einsiedeln, Switzerland and is an advisor and instructor at Condylator, Switzerland and Merz Dental, Germany. Keith Doonan received his BDS in 1993 from Royal London Hos pital Dental School where he graduated as the Dental Society President. He later received his Masters in Prosthodontics from Kings College London. He is a fellow of the Royal Australasian college of Dental Surgeons and a part-time visiting Senior Lecturer at the University of Queensland Dental School. Dr Matthias Bickel is former Professor of Prosthodontics at the University of Queensland and works in private practice as a Specialist Prosthodontics registered in Australia and Switzerland. His special interest is in using the Gerber System to rehabilitate patients in need of fixed and removable dentures including implant overdentures.
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