Courtesy: Dr. Nico Kamosi
Overdentures (OD), Implant Retained Overdenture (IROD),
Biomechanics, Implant Supported Overdenture (ISOD), Implants-Supported Fixed Prosthesis (ISFP), Success rate (SR), Survival rate (Sr)
Over-denture is a simple and favorable prosthetic treatment modality for elderly patients as less number of implants involving limited traumatic surgical invasion and less clinical chair side and financial resources are required. There are instances where the provision of the conventional implant-supported fixed prosthesis (ISFP) is not clinically feasible due to the extensive atrophied residual alveolar bone where only a few implants could be placed in the front region of jaws rendering provision of implant retained overdentures (ISOD) a viable and predictable treatment modality. This alternative also facilitates replacement of lost hard and soft tissue especially in patients with congenital/acquired defects in order to improve aesthetic, phonetics as well as to avoid food entrapment.
Most of the old patients often ask for better stability of their prosthesis, particularly in the lower jaw. In patients with compromised manual dexterity, overdentures would be less demanding for the practice of self-performed plaque control.
Disadvantages of IR/ SOD
Soft tissue complication due to the adverse effect of mucosal coverage, initiating or accentuating severe gagging reflex, patient intolerance to dentures due to psychological/physiological reasons in some cases together with requirements for higher maintenance involved are some of the inherent back draws which may render the provision of IR&S OD design less appropriate in some individuals. In some cases, inadequate restorative space over the abutments renders the OD bulky what is not acceptable for some patients. To avoid this problem, the technician may reduce the acrylic body of denture over the abutments which render the construction less resistance to fatigue fracture. In case of the very well preserved alveolar ridge, the patient may not benefit from specific advantages of overdentures and an ISFP may be a more appropriate option.
Clinical aspects of IR/S OD & ISFP
The inherent problems such as divergent and buccally directed implant alignment, long teeth, open interdental spaces and incongruent implant distribution/ tooth position render the provision of maxillary ISFP rather challenging. To address the aesthetical requirement for such morphological discrepancies, gingival-mask (epithesis), as well as buccal flanges for ISFP (Mericske-Stern et al. 1998b), has been satisfactorily fabricated.
The optimum implant placement in overdenture cases is more consistent with the available bone quality and quantity. The denture base also facilitates phonation by restoring the phonetic zone. Some literature reports a higher survival rate (stability) for the OD than for the actual implants. They also reveal that the survival rate in the mandible is more favorable compared to the maxilla, where failures are mainly related to poor bone quality, short implants, and long lever arms. The different anchorage systems of overdentures (balls, bars, and magnets, single attachments) do not seem to influence the implant failure rates (Jemt et al. 1996).
Bio-mechanical aspects in IR/S OD & ISFP
Naert et al. (1997) reported a cumulative success rate of 97% in their 9-year longitudinal study for the IROD. There was no difference observed in SR for the mandibular IROD, whether the two implants were connected by a bar or left individually. It should be born in mind that these data relate to the IROD where OD is mucosally supported in the distal areas.
The increasing demand for fabrication of maxillary IR/S OD suggests a strong need for this modality of therapy when treating specific selected cases of edentulous maxilla. However, the provision of IR/S OD should not be considered a remedy to salvage the failing fixed prosthesis. To improve the success rate of this modality, meticulous surgical protocol, well-designed rigid supra-structure supported by adequate number and size of the implants (minimum 4 well-distributed) is crucial to meet the functional demand of OD in each individual case.
Biomechanical parameters of the implant supported and implant retained prostheses are influenced by the design of the sub-structure, supra structure, as well as impression technique, applied. It should be emphasized that biomechanical principles applied for IROD is very different from that of for ISOD, where 4 implants are connected with a rigid bar providing a semi cross-arch stabilization. As it has been shown that a rigid bar (Dolder bar) on telescopic attachment provides support and stability comparable to the framework in fixed prostheses (Mericske-Stern et al. 1998b).
Prosthetic and maintenance aspects of IR/S OD & ISFD
As there are more components (abutments, bars, clips, anchors, female retainers) involved in the construction of the overdentures, more technical and clinical complications are reported requiring more service and maintenance (Taylor 1998). Denture base and underlying soft tissue deterioration are also more specific to the overdentures. Therefore, the clinician should explain the importance of the routine maintenance service and possible non-routine repairs required after provision of overdentures in the context of the cost-benefit discussion.
Fractures and loosening of the abutments (mainly in maxilla due to non-parallel implants), retaining screws, bar clips (mainly in the mandible), female parts of ball anchors and fracture of the bar and its extension, acrylic tooth, resin, prosthesis, and framework are complications related to IR/S OD:s.
Soft tissue hyperplasia, stomatitis, soreness as well as food impaction may require adjustments of the prostheses or relining of the overdenture, occlusal correction, tooth re-arrangement and redesigning of overdenture (Jemt et al. 1996). Hyperplasia has been mainly associated with the maxillary OD while marginal soft tissue recession has been observed more around the mandibular OD.
Fatigue fractures, impaired phonation, and aesthetic problems are the observed complications in maxillary ISFP which require redesigning of prostheses, while lip biting is more prevalent in conjunction with the lower ISFP. In contrast to overdentures, the need for all kind of services may decrease over time for ISFP (Jemt 1991).
Improvement of masticatory comfort and efficiency, phonetics and aesthetics favor the treatment modality of IR/S-OD. Subjective criteria such as hygiene procedures, oral comfort, and ease of handling of the prosthesis are determinant factors for patient preferences.
Comparison of the patients’ experience before and after oral rehabilitation with IR/S OD reveals a higher overall satisfaction which decreases slightly over a period of time. This so-called “combination syndrome” due to loose maxillary conventional full denture with anterior resorption and unstable posterior contacts, is described following the provision of inter-foraminal IROD (Lechner & Mammen 1996).
The literature suggests that mandibular IROD is a predictable and successful treatment modality which improves quality of life especially in patients who have already experienced wearing of their conventional dentures satisfactory and successful. Where short implants are placed due to the severely atrophied jaw, it is recommended to insert more than two implants and connect them by a rigid bar to retain and support the prosthesis with complete denture design.
The success rate of dental implants retaining and supporting overdentures may vary but have been shown to be similar to that of fixed prosthesis. However, only after the assessment of cost/benefits, risk benefits, patients’ preferences, advantages and disadvantages of fixed contra removable option for each specific case, the application of each modality may be indicated and justified to achieve an optimal clinical outcome in the best of patients’ interests.
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