August 1, 2006
by Jacques Bernier, DMD, MSc & Kenneth S. Serota, DDS, MSc
Edentulism persists in a large percentage of the North American population in spite of decades of advances in oral health.1 Unfortunately, when studies designed to assess the reasons for full-mouth extractions are reviewed, in most countries, the biggest predictors or factors leading to edentulism are associated with the cost of dental treatment or related to dental phobias. It can be said that for many patients who became edentulous, the extraction of all their teeth and their replacement by dentures was the least expensive or the least demanding treatment, and was considered by them, at that time, as the most accessible and acceptable mean of restoring function and aesthetics.
In Canada today, nine percent of those 15 years or older are edentulous, while thirty percent of those sixty-five or over are edentulous. In Quebec, approximately fifty percent of the population over sixty years of age is edentulous.2 While this is double the prevalence of edentulism in the United States, it is comparable to the prevalence of edentulism in the Netherlands (Fig. 1). Fortunately, most of these patients are well adapted to their situation and are capable of living a functional and happy life without their natural teeth and with conventional complete dentures. These individuals have learned to coordinate their musculature to keep their dentures in place while they eat and speak. This level of adaptation may be explained by the fact that many of these people lost their teeth as teenagers and necessity and repetition has proven to be the mother of skill. However, such an ongoing long-term continuum of adaptation is very unpredictable.
Even with technological improvement, the mandibular denture remains with limited retention capability and over time it will inevitably debilitate the muco-periosteum and the underlying alveolus. With age, adaptive capacity diminishes and becomes increasingly unpredictable due to time dependent physiological changes i.e. reduced salivary flow, reduced motor skills and increased tissue vulnerability. Over time, many of the edentulous population will require relief from and a better solution than conventional dentures; they deserve to have access to implant dentistry.
From the pioneering works of Dr. Branemark to the investigations led by Zarb and others to the efforts of Henry in Australia, the implant anchored prosthesis has been shown to be a significant improvement over the conventional full denture.3-6 However, much of the evidence supporting this therapy comes from studies that mandate multiple implant placements (Fig. 2). Sadly, insurance co-payment is not an option for many. As such, many edentulous individuals are fiscally constrained to the conventional complete denture.
In 2002, a symposium held at Mc Gill University in Montreal, Quebec brought together many expert clinicians and researchers from around the globe and included a forum with patients who discussed their experience with implant anchored overdentures using only two fixtures.7
The findings showed mandibular 2-implant anchored overdentures (2-IAOD) to be superior to conventional dentures in randomized and non-randomized clinical trials that ranged over a nine year period.
The participants were more satisfied and more comfortable with the 2-IAOD than with conventional dentures, the 2-IAOD was more stable, mastication was easier and speech dramatically improved (Fig. 3). The improvement in diet of those who received the 2-IAOD improved their nutritional state and general health, an important consideration for seniors who are vulnerable to malnutrition. Perhaps of greatest significance was the bio-psychosocial enhancement observed in self esteem and social engagement. They concluded that there is overwhelming evidence that a 2-IAOD or some variant should become the first choice of treatment for the edentulous mandible (Fig. 4) for patients where a fixed solution is not a consideration.
Overdenture treatment is a simple modality, but it deserves consideration and attention; the difference between success and nightmare is razor thin. There are critical factors in all phases of the treatment that can enhance patient satisfaction. This article will address a number of these factors with the use of the 2-IAOD; ideal placement of the implants, determination of the need for a third implant, how to choose the appropriate attachment system based on the specific clinical situation and the use of lingualized occlusion.
POSITIONING OF THE IMPLANTS
Two implants in the anterior mandible generate a rotational axis with a 2-IAOD (Figs. 5A-D). The rotational movement of the overdenture when the patient occludes in the posterior segments is desirable; however, the objective is to avoid this rotational movement when the patient incises. This is the main source of disappointment for many patients and a source of frustration for the dentist. The goal is to have the rotational axis formed by the 2-IAOD to be situated beneath the incisor segment if possible. Ideally, the implants should be placed as anterior as possible while staying in stable soft tissue. Experience has shown that the location in the lateral incisor position enables placement as anterior as possible while keep the implants maximally separated and obviating a substantial posterior rotation.
By placing the implants and the resultant fulcrum more anteriorly, the tendency for the posterior segment of the overdenture to lift during incision is reduced, there is more anterior posterior stability, the durability of the attachment is extended and post insertion visits and complications are reduced thereby reducing maintenance costs (Fig. 6). In addition, placing the implants in the lateral incisor position, approximately 15mm center to center, does not preclude the possibility of placing additional implants if the patients subsequently wishes to be treated with an implant supported prosthesis either fixed or removable (Fig. 7).
There are situations that can not be optimized by placing the implants in the lateral incisor position only. The Angle Class II skeletal relationship is one of the more difficult scenarios to resolve with the 2-IAOD. The Class II skeletal relationship forces the prosthetic dentist to place the labial segment anterior to the residual crest for lip support. As a result, the incisor segment is anterior to the rotation axis created by the two implants. As this distance increases, the risk of a rocking effect of the overdenture increases (Fig. 8). The patient must be informed of the increased risk of lifting of the posterior segments of the denture during incision and that a 2-IAOD with implants in the lateral position can be altered by increasing the number of implants should the result be unsatisfactory. Ideally, this situation mandates a third implant located centrally. The other two implants can then be located in the premolar regions and a low cost overdenture on three ball attachments fabricated (Fig. 9).
The 3-IAOD avoids the cost of doing a laboratory fabricated bar as the shared stress extends the life of the ball attachment and by sharing the stress, it extends the life of the ball attachments which reduces maintenance and cost. Of note, in the case Angle Class III skeletal relationship, the fulcrum is directly beneath the incision segment making the use of 2-IAOD feasible (Fig. 10).
Figure 11 shows the number of patients treated each year since 1991 year using one of three different overdenture modalities. The use of 3-IAOD was instituted approximately six years ago and very rapidly became the preferred choice of a majority of patients. The corresponding decline in the number of 2-IAOD’s shows that given the choice, most patients will choose the 3-IAOD as the perception of the enhanced retention and resistance to injurious rotation about the fulcrum is readily understandable with diagrammatic explanation (Figs. 12A-B).
CHOICE OF ATTACHMENT
The attachment chosen must meet the patient’s expectations for r
etention and stability without having to change or adjust the retention device with undue frequency. Clinical reality determines attachment choice as function of anatomic and prosthetic characteristics as well as stress expectations. The degree of lateral stability provided by the attachment is a primary concern as more resorption means increased vertical height of the prosthesis and thus more pronounced vertical lateral and tilting forces on the attachments (Fig. 13).
There are situations where the greatest degree of stability is provided by the residual ridge; thus the attachments contribute mainly as a retention device. The AstraTech Dental Locator is ideal for this ridge configuration (Fig. 14A). Alternatively, there are situations where the residual ridge cannot contribute to denture stabilization. As such, the attachment is submitted to lateral and tilting forces and the Locator attachment is less suited to this configuration. This usually manifests as rapid loss of retention due to premature wear of the nylon matrix of the Locator Insert (Fig. 14B). As the overdenture increases dimensionally, it becomes prudent to use the biggest ball attachment that fits into the denture. The 3.4mm diameter ball abutment with a rubber O-Ring in the cap attachment seems to work best in these situations. Not only is O-Ring change relatively infrequent, but the feeling of soft and reliable retention it provides the patient last forever without significant variation so the patient basically doesn’t feel the need for adjustment (Figs. 14C-D).
The stability of the opposing maxillary complete denture is critical when doing an IAOD. The inter-arch occlusal scheme must eliminate deflecting contacts in both centric and dynamic parafunction. The use of a lingualized occlusion requires that the maxillary lingual cusp function as the main supporting cusp in harmony with the occlusal surfaces of the lower teeth. From the position of maximum intercuspation, the maxillary lingual cusps glide over the central fossa of the mandibular teeth with an absence of deflection during lateral and protrusive movements. By using a lingualized occlusion, it is relatively simple to provide the patient a free entry into centric occlusion and a good bilaterally balanced occlusion. Ideally, there should be no contact between the anteriors during maximum intercuspation (Figs. 15A-C).
The population requiring complete dentures in North America will increase from 33.6 million (1991 determination) to 37.9 million by 20208. A removable option is not second class treatment in comparison to fixed treatment for a significant majority of the population. As confirmed by the McGill Statement on the Edentulous, the benefits of implant prostheses for totally edentulous patients is not dependent on the number of implants placed. The response in terms of function, self esteem, social functioning and overall quality of life is the same regardless of the number. The implant anchored overdenture can become one of the most ubiquitous treatment modalities in prosthetic dentistry. As a profession we need to seek a baseline for standard of care and research and develop low cost alternatives to the conventional denture. The need for this prosthesis will hopefully be negated with advances in the biologic not biomimetic sciences, but until then…
The views disclosed in this article are not necessarily those of the editor, or Oral Health.
Dr Bernier is a graduate from the University of Laval, Qubec Canada in 1985. He recently completed a Masters Degree of Science with his thesis on “Bio-Psycho-Social Benefits of Implant Therapy on Edentulous patients”. He maintains a private practice in Quebec City dedicated to implantology. He is also actively involved in research and education in this area.
Dr. Serota is the founder of ROOTS, a cybercommunity dedicated to endodontic learning and excellence and in private practice limited to endodontics in Mississauga (www.endosolns.com).
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