Year : 2008 | Volume
: 19 | Issue : 1 | Page : 6--11
Intrusion in implant-tooth-supported fixed prosthesis: An in vitro photoelastic stress analysis
Murali Srinivasan1, TV Padmanabhan2,
1 Department of Dentistry and Implantology, Jebel Ali Hospital, P.O. Box 49207, Dubai, United Arab Emirates
2 Department of Prosthodontics, Sri Ramachandra Dental College and Research Institute, Chennai, India
Department of Dentistry and Implantology, Jebel Ali Hospital, P.O. Box 49207, Dubai
United Arab Emirates
Background and Objective: Intrusion of natural teeth is a very common and interesting problem associated with implant-assisted fixed partial prostheses. Various theories have been put forth to explain this phenomenon, most of which revolve around the philosophy of exertion of excessive forces onto the natural tooth in a combination fixed partial denture. This photoelastic study examines the current theories revolving around intrusion and evaluates whether natural tooth intrusion is a definite possibility in an implant-tooth-connected fixed partial prosthesis.
Materials and Methods: A two-dimensional photoelastic method was employed for testing and analysis. Two sets of photoelastic models were fabricated, one depicting a totally tooth-supported situation and the other an implant-tooth-supported situation. A rigid type and non-rigid type of connection were also incorporated into the fixed partial denture used in the both the situations in the study. Loads were applied on the anterior and posterior abutments and the pontic regions in both sets of models and the fringe patterns were photographically recorded for analysis.
Results and Conclusion: The forces were proportionately consistent with the increase in applied loads in both the situations. The use of a non-rigid connection did not show any major significance but in fact may be erroneous. The forces were considerably higher in the implant-tooth-connected situation. The results indicated that the differences in the forces exerted were not light and continuous and may not cause tooth intrusion. Natural tooth intrusion may be caused by reasons other than excessive forces and needs further investigation.
|How to cite this article:|
Srinivasan M, Padmanabhan T V. Intrusion in implant-tooth-supported fixed prosthesis: An in vitro photoelastic stress analysis.Indian J Dent Res 2008;19:6-11
|How to cite this URL:|
Srinivasan M, Padmanabhan T V. Intrusion in implant-tooth-supported fixed prosthesis: An in vitro photoelastic stress analysis. Indian J Dent Res [serial online] 2008 [cited 2020 Apr 8 ];19:6-11
Available from: http://www.ijdr.in/text.asp?2008/19/1/6/38924
The advent of osseointegrated fixtures has greatly overwhelmed the current day treatment options offered to the partially edentulous patient. The advancements in implant dentistry now afford a more predictable outcome in oral implant therapies which can be primarily attributed to the sophisticated diagnostic and planning processes, achievement of excellent healing quotas, advanced surgical protocols, better implant surfaces, availability of more esthetically and mechanically improved abutment superstructures and not to mention the implant designs themselves which have seen a range of modifications over the past two decades; with the result - an implant loss rarely occurs before 10 years of service. 
Implant therapy once considered a specialized therapy has now changed to a standard conventional procedure accessible to a wide spectrum of patients. This can be attributed to the falling prices in the implant components due to the escalated number of practitioners offering treatment, increase in the number of implant manufacturers and the relative media focus popularizing this form of therapy.  Despite the current advancements and affordability, implant therapy is still relatively expensive and the treatment cost is an important consideration in the planning of an implant-retained prosthesis. This escalated cost can significantly be reduced if the use of natural abutments is included in the implant-assisted prosthesis.
Although it has been argued that it is indeed beneficial to connect natural teeth and implants together in a fixed partial denture, ,,, fears of possible negative effects resulting from splinting natural teeth with osseointegrated implants have always played a superior role in not adopting this option as a mode of treatment and the vast majority of implant dentists worldwide avoid splinting the two in a fixed prosthesis.  It has been suggested that the disparity between the physiologically mobile tooth and the relatively immobile implant make the prosthesis behave like a cantilever generating maximum resultant load almost twice as much applied on the implant , leading to implant failure or fracture of the implant abutment or its retaining screw or cement loosening or possibly an uncommon perplexing controversy titled 'Intrusion Phenomenon'. ,, Intrusion phenomenon is described as 'the migration of the tooth to a more comfortable/less stressful position when combined within a fixed partial denture (FPD) with an osseointegrated implant.'
Various theories have been proposed to explain natural tooth intrusion occurring in an implant-tooth-connected fixed prosthesis such as disuse atrophy, differential energy dissipation, mandibular flexion/torsion theory, flexion of the FPD framework, impaired rebound memory, micro-jamming/debris accumulation and ratchet effect, but these theories are highly speculative and none of them have been clinically substantiated.  One of the first theories, disuse atrophy, was based on the idea that a lack of normal stimulation of the periodontal ligament produces atrophy of the periodontal ligament and intrusion of the tooth. The remaining theories relate to excessive forces being placed on the natural tooth, resulting in movement of the tooth to a less stressful position. These forces are placed on the tooth by differential energy dissipation, mandibular flexion and torsion, flexion of the FPD framework, impaired rebound memory, debris impaction or micro-jamming or ratchet effect related to the use of precision attachments. 
Our understanding of natural tooth intrusion as stated by Proffit, is one of the most difficult orthodontic tooth movements to be achieved and, for many years was considered almost impossible. The forces to bring about this orthodontic tooth movement are described as " precise application of light continuous forces and careful management of anchorage teeth".  Hence the possibility of tooth intrusion cannot be achieved unless the forces are light and continuous which definitely is not the situation with the forces acting on an implant- tooth-assisted FPD and warrants investigation.
Therefore purpose of this study was primarily to determine whether natural tooth intrusion can possibly occur in an implant-tooth-connected FPD and whether to consider intrusion phenomenon as a rationale for determining implant-tooth connection.
Materials and Methods
A two-dimensional photoelastic stress analysis was to be employed for this study. Partially edentulous photoelastic models simulating an edentulous situation of missing mandibular left first molar tooth were constructed for two-dimensional photoelastic testing and analysis.
The first sets of models consisted of natural abutments of mandibular left second premolar and mandibular left second molar. For simplicity, a single root configuration was chosen for the molar and the furcation space was eliminated.  In the second sets of models, the second molar was replaced by a cylindrical press fit implant of 4 mm diameter and 13 mm length.
The periodontal ligament (P.L.) of 0.3 mm thickness was simulated on the teeth to be used in the photoelastic models, using polyvinyl siloxane impression material (Speedex, Coltene) of light body consistency [Figure 1],[Figure 2].  Tray adhesive (Kettenbach, Germany) was applied evenly over the root portions of the teeth to help facilitate adherence of the siloxane material to the photoelastic teeth simulants. The thickness of the P.L. was maintained by measuring the Labio-lingual and Mesio-distal diameters prior and post fabrication in three different confirmed locations of the root to ascertain uniformity.
The photoelastic simulants used for the teeth - tooth-colored epoxy resin (Araldite, CIBA), periodontal ligament - polyvinyl siloxane impression material of light body consistency (Speedex, Coltene) and bone-transparent epoxy resin (Araldite, CIBA). A cylindrical press fit type implant was included in the second sets of models at the position of the second molar (posterior abutment). The condition representing complete integration was obtained by pouring the material slowly along the walls of the container until it covered the entire root portions and the resin was allowed to cure for 24 h.
The teeth were prepared in accordance with the fundamentals of tooth preparation for a fixed partial restorative therapy. Conventional restorative techniques were used to fabricate fixed prosthetic restorations. Restorative procedures were accomplished by using transfer type copings with polyvinyl siloxane impression material, addition type (Reprosil, Dentsply) and custom trays. All restorations were fabricated on stone casts. Restorative dimensions were consistent with the base parameters of occlusal plane and form. The dimensions of the restorations were kept constant with the use of a silicone putty index. The occlusal surfaces were made flat as recommended for similar types of two-dimensional photoelastic studies. 
All restorations were fabricated with an Ni-Cr alloy. Two types of restorations were fabricated for each of the test conditions. The restorations differed among them only in the type of connection. The first type was fabricated with a rigid type connector, while the other with a custom semi- precision type movable connector. The semi-precision type connector was placed between the anterior abutment and the pontic. The semi-precision movable connector had a key and a keyway. The key was placed in the distal aspect of the anterior retainer while the keyway was placed in the mesial aspect of the pontic.
Interchangeability of the restorations was verified on the models. The restoration was cemented with Zinc Phosphate cement (Harvard) on the photoelastic models and loading was carried out. Loads were applied in a straining frame, vertical point loads were applied at fixed identified locations on the occlusal surface of the tooth/implant restorations. The loading point locations were identified as follows:
Over centre of Anterior abutment,Over centre of Pontic,Over centre of Posterior abutment (Molar/Implant).
These points were marked with a straight fissure bur for reproducibility and to facilitate point load placement. Loads placed were 0, 50 and 100 lbs over each of the loading point locations.
These loads were selected as they are realistic functional load levels and also provided a satisfactory optical response within the model. A record of residual stresses was noted and monitored prior to loading. The resultant stresses in all areas of the supporting structure were monitored and recorded photographically in the field of a transmission type polariscope. Each loading and observation sequence was repeated at least two times to ensure reproducibility of the results.
The fringe pattern findings and data were collected for the loading subjected to each situation of restorative connection and loading positions.
Vertical point loads were applied on fixed identified locations on the occlusal surfaces of the restoration. Observations for each loading for the different situations in the study were photographically recorded. The fringe orders were read from each individual photograph taken for each subjected load [Figure 3]a[Figure 4],[Figure 5]b. The fringe order readings were calculated corresponding to each abutment for every load subjected and compared individually for each connection type in both totally tooth-supported and tooth-implant-supported situations and were tabulated [Table 1].
During premolar loading
The fringe orders under the anterior and posterior abutment regions were found to increase with increase in loads in both the situations, i.e. totally tooth-supported and implant-tooth-supported situations with an FPD using a rigid connection.
The fringe orders were found to be slightly more under the anterior abutment when a non-rigid connection was used. The fringe orders were slightly reduced under the posterior abutment in both sets of models when an FPD with a non-rigid design was used.
The fringe orders under the implant in an implant-tooth-connected situation were higher than the molar abutment.
During pontic loading
The fringe orders under the anterior and posterior abutment regions increased proportionately to the increase in loads in both sets of models irrespective of the connection type employed in the FPD design.
The magnitude of the fringe orders was however slightly exaggerated in case of the tooth-implant situation in comparison to the totally tooth-supported situation.
During molar/implant loading
The increase in the fringe orders was proportionately consistent with the increase in applied loads for both sets of models and in both types of connections.
However, the magnitude of the fringe orders was exaggerated in implant-tooth-connected situation under both abutments.
The fringe orders under the premolar abutment were slightly less during molar loading when compared to the premolar loading in the totally tooth-supported situation.
The fringe order readings under the given situations for the specific type of connection clearly demonstrate that the fringe orders were considerably less for the totally tooth-supported situation when compared to the tooth-implant connected situation. This may be attributed to the well-advocated physical functions of the periodontal ligament complex, resistance to the impact of occlusal forces - 'Shock Absorber Effect' as documented by Caranza and Newman. The other significant inference derived from the conducted study demonstrated that the non-rigid type of connector placed between the anterior abutment and the pontic transferred more stresses to the anterior abutment and a uniform stress distribution was not present. On yet further analysis, the results showed that the stress transfer was higher under the implant clearly demonstrating the absence of the P.L. complex, in an osseointegrated implant. Based on theoretical assumptions, an integrated implant connected to a natural tooth in a fixed prosthesis would take up more of the load. Misch et al ., documented this in their finite element analysis of tooth-to-implant fixed partial denture design.  Integration of natural teeth into a restoration remains a point of discussion because of the differential mobility between teeth and implants. The Branemark system has strongly recommended against linking natural teeth with osseointegrated fixtures due to movement of the natural abutment within the limits of its periodontal ligament.  Van Steenberghe,  Balshi,  Gunne et al.,  Cavicchia and Bravi  conclude that direct connection between implant and natural teeth causes no periodontal or mechanical problems. Furthermore, linking osseointegrated fixtures to natural teeth may be useful in helping patients to perceive better propioception. This was documented by Cavicchia and Bravi.  The use of non-rigid design has been advocated in fixed partial dentures, which link natural teeth with implants. The disparities in the mobility of the natural teeth and implant logically warrant some sort of stress-breaking or stress-equalizing when both these are combined together in a fixed partial denture. But controversies exist. Schlumberger though has advocated the use of non-rigid connection in the fixed partial denture design, Misch et al .  conclude that a non-rigid connection may be erroneous because of a biomechanical disadvantage. A redirection of stress was found in the bone simulant of the photoelastic model surrounding the tooth root supporting the non-rigid prosthesis which is probably explained by the fact that 'A rigid restorative condition resolves more stress internally before it can reach the supporting tissues, however, a non-rigid connection between a pontic and an abutment would alter this load distribution. With a non-rigid design the stress may be directed through the tooth abutment to the supporting bone rather than being concentrated in the connector or tooth root'. 
Natural tooth intrusion is claimed to occur as a possible complication in a situation where a natural tooth is splinted with an osseointegrated implant within the same fixed prosthesis and supported by findings of many authors, ,, with the plausible explanation being the disparity in the mobility between the physiologically mobile tooth versus the relatively immobile osseointegrated fixture resulting in maximum resultant loads being transmitted to the tooth by the fixed prosthesis which utilizes a natural tooth and implant as abutments. The current accepted theories revolve around these excessive forces being transferred to the natural tooth, causing it to move to a less stressful position, resulting in intrusion of the tooth. Though this is a possible explanation, practically it is possible to achieve tooth intrusion only if the forces are light, continuous and precisely applied, otherwise the hazards of the tooth becoming non-vital or periodontal weakening leading to tooth loss may occur as a resultant of excessive and uncontrolled load application.  Hypercementosis is yet another possible complication of excessive forces being applied to a tooth under occlusion. The possibility of the tooth intruding as a result of excessive force application is hence not clinically viable and the tooth may undergo regressive changes long before intrusion might even occur. If intrusion does occur it may not be because of combination with an implant but may be because of other intrinsic causes as the cause is still unknown and requires investigation. Load transfer due to non-rigid design was not found to be all that significant and was in fact found to be erroneous. It may actually cause intrusion; however, intrusion cannot occur under dynamic uncontrolled forces as exerted on an FPD.
Conclusion and Summary
After careful analysis of the results of this two-dimensional photoelastic study, the following conclusions can be drawn:
The forces transmitted and distributed by an FPD in totally tooth-supported and tooth-implant situations are not light, continuous and controlled forces but can be described as proportionately increasing with increasing loads, thus these forces cannot bring about intrusion without damaging the tooth.The stress-breaking effect of a non-rigid type of connector used in the FPD utilizing implant and natural abutment is not significantly beneficial to prevent intrusion and may be erroneous for the FPD design.On final analysis, natural tooth intrusion in a fixed partial denture utilizing a natural tooth abutment in combination with an osseointegrated implant may not be directly related as indicated in this photoelastic study.
Clinically it is not possible for such a phenomenon to occur because the forces are not controlled. If intrusion does occur it may be because of other unknown intrinsic causes and requires further investigation. There is established and proven clinical evidence that implant-tooth connection in an FPD has a positive, longstanding clinical success and may be actually very beneficial. ,,,, Hence intrusion phenomenon need not be a criterion to discredit in connecting an implant with a natural tooth in a combination fixed partial prosthesis.
The authors recommend the need for further studies before totally disproving this phenomenon or to provide a correct hypothesis as to why it may occur. A long-term clinical follow-up of cases exhibiting this adverse phenomenon may only throw light on this phenomenon.
The authors would like to sincerely thank Prof. V. Bhaskar, Department of Aeronautical Engineering, MIT Campus, Anna University, Chennai, for his invaluable expertise in the field of photoelastics.
|1||Branemark PI, Svensson B, van Steenberghe D. Ten-year survival rates of fixed prostheses on four or six implant ad modum Branemark in full edentulism. Clin Oral Implants Res 1995;6:227-31.|
|2||Weigl P. Implant prosthodontics: What next? Quintessence Int 2003;34:653-69.|
|3||Cavicchia F, Bravi F. Free standing vs. tooth-connected implant supported fixed partial restorations: A comparative retrospective clinical study of the prosthetic results. Int J Oral Maxillofac Implants 1994;9:711-8.|
|4||Van Steenberghe D. Retrospective multi-centre evaluation of the survival rate osseointegrated fixtures supporting fixed partial prostheses in the treatment of partial edentulism. J Prosthet Dent 1989;61:217-23.|
|5||Balshi TJ. Advantages and disadvantages of linking implants to the natural dentition. Oral Maxillofac Surg Clin North Am 1991;3:945-63.|
|6||Gunne J, Astrand P, Lindh T, Borg K, Olsson M. Tooth-implant and implant supported fixed partial dentures: A 10-year report. Int J Prosthodont 1999;12:216-21.|
|7||McGlumphy EA, Campagni WV, Peterson LJ. A comparison of the stress transfer characteristics of a dental implant with a rigid or a resilient internal element. J Prosthet Dent 1989;62:586-93.|
|8||Lundgren D, Laurell L, Falk H, Bergendal T. Occlusal force pattern during mastication in dentitions with mandibular fixed partial dentures supported on osseointegrated implants. J Prosthet Dent 1987;58:197-203.|
|9||Misch CM, Ismail YH. Finite element stress analysis of tooth-to-implant fixed partial denture designs. J Prosthodont 1993;2:83-92.|
|10||Abrams L. The phenomenon of natural root intrusion in combined root-form implant cases. Dent Implantol Update 1996;7:33-6.|
|11||Sheets CG, Earthmann JC. Natural tooth intrusion and reversal in implant-assisted prosthesis: Evidence of and a hypothesis for the occurrence. J Prosthet Dent 1993;70:513-20.|
|12||Sheets CG, Earthman JC. Tooth intrusion in implant-assisted prostheses. J Prosthet Dent 1997;77:39-45.|
|13||Pesun IJ. Intrusion of teeth in the combination implant-to-natural-tooth fixed partial denture: A review of the theories. J Prosthodont 1997;6:268-77.|
|14||Proffit WR, Fields HW. Contemporary orthodontics. 3 rd ed: Mosby: 2000.|
|15||Kuroe T, Itoh H, Caputo AA, Nakahara H. Potential for load - induced cervical stress concentration as a function of periodontal support. J Esthet Dent 1999;11:215-22.|
|16||Breeding LC, Dixon DL, Sadler JP, McKay ML. Mechanical considerations for the implant tooth - supported fixed partial denture. J Prosthet Dent 1995;74:487-92.|
|17||Nishimura RD, Ochiai KT, Caputo AA, Jeong CM. Photoelastic stress analysis of load transfer to implants and natural teeth comparing rigid and semi - rigid connectors. J Prosthet Dent 1999;81:696-703.|