University of Groningen

Implant-supported removable partial dentures in the mandibleLouwerse, Charlotte

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CHAPTER 7

GENERAL DISCUSSION,CONCLUSIONS

AND FUTURE PERSPECTIVES

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Introduction

The PhD research described in this thesis was performed to provide evidence for the use of dental implants to support a removable partial denture (RPD) in the mandible in case of a Kennedy class I situation. Based on the studies performed in this PhD study, it can be concluded that providing implants to enhance retention, support and stability of an RPD is a favourable treatment option, considering all parameters that were investigated.

The results from the retrospective study, with a mean observation period of more than 8 years (range 3-16 years) suggest a successful treatment option. Implant survival is 91.7%. Even after a long period of time, patients are very satisfied although some minor technical and biological complications should be taken into account. The outcome can be considered indicative for the long-term prognosis of an implant-supported RPD (ISRPD). Nevertheless, the drawbacks of this and any retrospective study should be emphasised, like the multi-centre design with different clinicians, the lack of a uniform treatment protocol and lack of randomisation, to name but a few. Therefore a prospective randomized clinical trial had to be performed to get insight in different aspects of an ISRPD and to be able to make more valid and sound recommendations for ISRPD treatment in a mandibular Kennedy class I situation. The dilemma of where to ideally position the implants is these cases, the premolar (PM) or the molar (M) region, for which theoretical models and lines of reasoning that lacked clinical verification are available, were another reason to perform the work.

Patient satisfaction

Oral Health Related Quality of Life (OHRQoL) as measured with the dutch of the Oral Health Impact Profile (OHIP-NL49) questionnaire improved significantly when implant-support was provided to a newly made RPD. This is in agreement with the findings of others.1-4 A minimal important difference (MID) of 6 points (95% CL, 2-9) has been suggested for OHIP use in dentistry.5 The MID was defined as ‘the smallest difference in score in the domain of interest which patients perceive as beneficial and would mandate, in the absence of troublesome side effects and excessive cost, a change in patient management. John et al (2009) suggested in the same study that this MID can be used to approach clinical relevance of changes in perceived oral health.5 The observed reduction in OHIP sum-scores and values in the present study amply exceed this threshold: The mean sum scores were reduced with 12.7 and 19.1 points respectively when the RPD was supported by implants in either the molar (M) or the premolar (PM) region.

The OHRQoL is reported to be highly correlated with the technical quality of the

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prosthesis6, suggesting that when a denture of poor quality is replaced by a technically optimized one, acceptance and satisfaction will improve. In the present study of patients with a poorly functioning bilateral free-ending mandibular RPD, solely providing a new RPD also proved effective when considering certain domains of the questionnaire, like functional limitation and psychological discomfort. It may implicate that the new RPD provided without support actually sufficed in increasing the quality of life, but social desirability bias may have played a role as well. Although the researcher performing the investigation was not involved with the surgical or restorative treatment, it is well known that patients tend to over-report good and under-report undesirable outcomes, especially when questionnaires are used. They tend to please their physician or the researcher or seek confirmation for their own decision to undergo the treatment in the first place. This should be considered when interpretating the data.

Adding implant-support significantly improved the overall OHRQoL. In addition, at the start of treatment patients were asked to express their expectations with respect to the anticipated ISRPD on a Visual Analogue Scale (VAS) ranging from 0 (very discontent, major concerns) to 100 (very content, no concerns at all) as a means to evaluate whether or not patients’ expectations were met at the end of the study. This is a rather unconventional approach to investigate patient satisfaction and is hardly ever used in the literature, yet seems rational. The data suggests that patients’ expectations of contentment with an ISRPD were met since no significant difference was seen between expected and actually achieved contentment levels. This is seen as an important indicator of the quality of treatment. It enhances the reputation of the health care provider and implant dentistry in general. It transforms new patients into loyal customers and brings new referrals by ‘word of mouth’. Although patients were very satisfied, food getting underneath the denture was and remains a recurrent complaint despite implant-support, as also was observed by others.2

It is interesting to consider whether or not public awareness of a treatment option being very successful raises expectations, which as a result will be more and more difficult to meet. Put differently: when a treatment option is seen as a last resort that, despite the physicians’ tendency to downplay expectations proves successful without many drawbacks, future patients will be biased towards higher, or even unrealistic expectations.

When a patients’ prosthesis is more comfortable and functional, one would expect that he is less prone to remove it during the day and/or night. In other words, one would expect that the mean wearing time will increase with ameliorated quality of the prosthesis. This is indeed confirmed by the data as measured by temperature sensitive chips. This feature proved a valuable, objective outcome that is presumed to relate strongly to satisfaction. Thermosensitive chips are originally used to improve compliance in young patients wearing removable orthodontic appliances, but can also

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be used successfully in prosthodontic research.The favourable satisfaction rate was also reflected in a high rate of prostheses

still being in functional use after one year. Conventional mandibular free-ending RPD’s are often not worn by patients. In this PhD study only one patient, who encountered severe health problems during the course of the study, could not really get used to his new (IS)RPD and only wore it occasionally.

Clinical performance of implants and Implant-Supported Removable Partial Dentures

No implant loss was observed and clinical data are representative for healthy and stable implant conditions, after one year of observation. The mean probing pocket depth after 1 year of function was 1.69 mm (SD 0.43 mm) for the M implant and 1.76 mm (SD 0.45 mm) for the PM implant which is within the range of the physiological limit of 3 mm7 and matching with the design of the used tissue-level implant. Patients exhibited a mean marginal bone loss of 1.10 mm (SD 0.53 mm) for the M implant and 1.06 mm (SD 0.59 mm) for the PM implant, which is also in line with the studies that followed a group of patients over a longer period of time.8-10

Functional performance

In this study masticatory performance was expressed as the Mixing Ability Index (MAI) which evaluates the ability to mix and knead a food bolus by mixing a paraffin wax tablet with a blue and red layer (see also the method section).11 It has been shown that chewing on two-coloured paraffin wax is a reliable alternative for the often used comminution tests.11-13 In previous studies the parameter ‘masticatory performance’ was operationalized in various other ways with contradictory findings.14-18 This makes the interpretation of those findings and comparison with the present ones troublesome. Nevertheless, the data clearly demonstrate an improvement in masticatory performance when implant-support was provided. The results underscore the findings by others who reported an increase in bite force and mastication after implant-support to an RPD was provided, be it not to the level of that of a fixed implant-borne restoration.4,19-21

Three months functioning with a new denture is tentatively considered long enough for a patient to adapt and this period of evaluation is also used by others.22,23 Nevertheless, it would be interesting to evaluate the masticatory performance over a longer period of time.

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Implant position: molar or premolar support

Although not evident from the OHIP-NL 49 questionnaires or the wearing-time outcome parameter, the outcome of VAS scores on general satisfaction pointed towards a patient preference for support in the molar region. Over half of the patients expressed a strong preference for M implant-support, approximately one third of the patients preferred PM support and the others gave no preference. In light of value-based health care this is considered a meaningful result. Perhaps the other patient-based measures are not sensitive or specific enough to pick this up or differentiate.

The preference for M implant-support may stem from minor differences in stress or discomfort between the two implant positions. In mathematical model studies and in a study with a pressure-sensitive foil it was shown that more distal positions (the first or second mandibular molar) induce lower amounts of stress on the implants themselves and on the residual alveolar ridge, compared to situations where implants were modeled more anteriorly (the second bicuspid position).24-26 From various tested positions, M support is associated with the least amount of displacement of the tissues under load.27-

30 A more distal position is also favoured by Grossman et al. (2009), who recommended the second molar position.31 Hence, the less rotation and the more relieve of mucosal pressure, the better an RPD is tolerated, and our patients seem to agree with this.

With respect to the biomechanics and design of the denture, when the support is brought further to the distal, an anterior clasp or support can usually not be avoided, with aesthetic repercussions. During the course of the present study, a labial retainer to an anterior natural abutment tooth was always present, both when the M and when the PM implant-support was operationalized. Patients were informed that that would be an inherent consequence when they would finally choose for M implant-support and that they could probably do without this anterior clasp in case they would opt for PM implant-support. Evidently patients’ comfort with M implant-support outweighed their dislike of a potentially aesthetically disturbing anterior retainer since the vast majority favoured M implant-support.

Most clinical parameters (both concerning the implants and the teeth) revealed no difference when the implants at the molar or the premolar sites were loaded but one has to bear in mind that the observation period in the present study is relatively short to evaluate clinical performance. Bleeding on probing around the implants was an exception, with a less favourable score being noted around M implants. During the observation period, this has not led to deeper probing depths or increased marginal bone loss, but it is uncertain what the long-term effect may be. From the retrospective study it tentatively concluded that implants in the PM region clinically perform better than implants in the M region when supporting an RPD. One of the reasons might be a compromised cleaning possibility for more distally placed implants.

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As mentioned the present data clearly demonstrate an improvement in masticatory performance when implant-support was provided, but no noticeable differences with respect to the 2 implant positions tested were seen.

Combining all these observations and all in all, it can be concluded from the patient’s point of view that the ISRPD, in a Kennedy class I situation in the mandible, is best supported by implants placed in the molar region in case of the situation of sufficient bone volume. However, healthier peri-implant conditions can be expected around implants placed in the PM region compared to implants placed in the M region. This, together with the fact that, in case of further tooth loss, an implant-retained denture is better served with implants in the position of the premolars, the PM position may be preferred from a clinical perspective. In addition, it is well advised to plan the position of the implants in such a way that a fixed partial denture, be it a single or multiple unit restoration, would still be possible if the patient desires to convert from a removable to a fixed implant-borne restorative solution or an implant-supported full arch denture.

Cost-effectiveness

Achieving high value for our patients is a major goal of health care delivery in general and subsequently also for dental care. Here ‘value’ is defined as the health outcomes achieved per monetary unit spent.32 Porter postulates this goal as a ‘priority achievement for patients’. It unites the interests of all parties involved: patients, health care providers, suppliers and health care insurance companies. They will all benefit, while the economic sustainability of the health care system increases.

When health care is drifting from volume to value, it is important for health care organizations to measure and analyse costs.33 Therefore, an accurate cost accounting system is necessary. Since a uniform strategy is lacking in dentistry we chose to use the opportunity cost, although it is difficult to be encompassing and in measuring all the expenditures necessary for personnel, supplies, treatment room and other costs. For this study the patients’ contexts were not taken into account although it could play an important role in cost-effectivity. The other different approach for cost accounting we used was the Dutch tariff structure. The advantage of this strategy is the possibility to compare two treatment modalities for all parties. The disadvantage however is that is leaves out a lot of aspects of costs like chair time and the number of visits and besides it is impossible to extrapolate the conclusions to other countries.

Besides analysing the cost, quality assessment is the other prerequisite to determine the value of a treatment. In our study we chose several outcome parameters. The Incremental Cost-effectiveness Ratio (ICER) for OHIP-NL49 and MAI using the

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opportunity costs was €80 and €786 respectively. This means that depending on the choice of outcome measure and monetary threshold, supporting an RPD with implants is cost-effective when payers are willing to pay more than €80 per OHIP point gained. Per MAI point gained an additional €786 has to be invested. When using the tariff structure, corresponding ICERs were €94 and €921. These data cannot yet be compared to other studies and therefore are hard to value. It would be recommendable to use these quality parameters for other treatment options as well to be able to compare data and to make a substantiated choice between alternative treatment modalities. The effect of supporting an RPD with implants when expressed in QALYs was negligible, hence an ICER was not determined. In patients with a Kennedy class I in the mandible and a full arch denture in the maxilla, the general health perception was not influenced by any of the treatment phases of this study. Probably this outcome measure is not sensitive enough and it raises doubt regarding the manner in which continuously effort is invested in associating dental issues in all fields of study to overall general health matters. QALYs seem not to be an accurate parameter in dentistry with relatively healthy patients.

The data presented in this PhD study could aid in analyzing costs and standardizing quality assessment, leading to value-based dental care. This will allow patients, health care providers and insurers in decision making from their different perspectives regarding the cost-benefit aspects of implant-supported RPD’s.

Radiographical treatment planning

The implant placement itself in the posterior manable can usually be planned safely on a panoramic radiograph.34 With (ultra) short implants becoming available, the possibilities to provide implant-support to a removable partial denture or provide fixed partial dentures in the resorbed posterior regions are increasing as well. Based on the presented Cohen’s kappa values, it seems that there is almost no coherence between the two diagnostic procedures for the planning of implants in the premolar (PM) region of the mandible. However, this should be interpreted with care. The cross tabulations show that in 90 % of the cases the panoramic radiograph was sufficient to determine the adequacy or lack of bone volume. This phenomenon has been referred to as ‘the paradox of Kappa’ by Feinstein and Chicchetti.35,36 Kappa values can be distorted by an uneven distribution of numbers in a 2 x 2 cross table, as is the case in the present dataset. The overall Odds Ratio (OR) for the anticipated premolar implant position is approximately 76, which indicates a much higher chance for agreement than for disagreement between the two diagnostic procedures.

The overall value of kappa for the molar (M) region shows a rather high sensitivity

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and specificity, implying that the bone volume in that region can be estimated rather accurately on a 2D image. This is confirmed by the overall OR for the posterior position (≈ 43).

Despite substantial agreement between the two diagnostic procedures as expressed by the high percentages of true-positive and true-negative values, numerous cases remain in which patients run a substantial risk of complications or are deprived of a chance to be treated with implants when only a panoramic radiograph had been used for diagnostic aid. The ALARA principle forces clinicians to justify the use of additional 3D radiography on a patient-to-patient basis. After having informed the patient, justification will depend on the willingness of both patient and clinician to accept the false-positive or false-negative classifications in the order of magnitude as presented here, 10-13 % dependent on the region of interest.

Within the limitations of the present study, it is concluded that implant placement in the resorbed posterior mandible can be well assessed on a panoramic radiograph in the vast majority of the cases. The degree of consistency between the two diagnostic procedures, panoramic radiograph and CBCT is observer dependent. Special care should be taken if a knife edge ridge is present in the lateral parts of the mandible, if the bone height is critical or if the course of the mandibular canal is unclear on the panoramic radiograph. In these cases the ordering of a CBCT will be of benefit to the patient and can be justified.

Conclusions

The cumulated findings of the studies presented in this PhD study lead to the following general conclusion. Placing implants, either in the premolar or molar region of the mandible, to support a bilaterally free-ending removable partial denture in patients who expressed complaints regarding their conventional RPD yet would like to continue wearing one, yields favourable results.

The specific conclusions are:- from a 3-16 year retrospective study it appeared that treatment with an implant-supported removable partial denture is a viable option with a high implant survival rate and resulting in satisfied patients, although technical and biological complications should be anticipated;- mandibular implant-support favourably influences oral health related patient-based outcome measures. Although it was not significantly shown for all operationalized patient based outcome assessments, the majority of patients expressed their preference for implant-support in the molar region;

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- implant-supported removable partial dentures and their supporting implants and abutment teeth function without clinically relevant complications after 1 year and significantly improve masticatory function. No marked difference was seen between molar and premolar implant-support.- depending on the choice of outcome measure and monetary threshold, supporting a removable partial denture with implants is cost-effective when payers are willing to pay more than €80 per Oral Health Impact Profile (OHIP) point gained. Per Mixing Ability Index (MAI) point gained an additional €786 has to be invested;- the possibility of implant placement in the resorbed posterior mandible can be well assessed on a panoramic radiograph in the vast majority of the cases compared to assessment on a cone beam computed tomography scan.

Future perspectives

In this PhD-study different aspects of the ISRPD were evaluated during 1 year. The treatment has been found successful, with minimal complications and a high patient satisfaction rate. One year is a relatively short period of time and therefore it is important to follow the patients in this study group during the next years, leading to 5- and 10- year results.

The study group is very homogeneous, which makes the evidence relatively strong compared to heterogeneous groups as found in the literature. On the other hand, it makes extrapolating the results difficult for situations that are not similar. For example, the results reported only apply to the bilaterally partially edentulous mandible. It would be interesting to investigate whether this treatment modality is as suitable for the maxilla in case of a shortened dental arch as well. Another question can be whether the same conclusions can be drawn in case of an opposing natural dentition instead of a complete removable denture.

The investigated group consisted of patients with complaints regarding their RPD. They did not want their remaining natural dentition to be removed, but perceived that they could not function with their conventional RPD either. Another treatment option for this group of patients is a fixed implant-supported partial denture on both sides of the mandible. It would be interesting to conduct a randomized clinical trial in order to compare the implant-supported fixed denture with an ISRPD on aspects like OHRQoL, functioning, technical and biological complications and cost-effectiveness.

From finite element analyses (FEA) it is known that different retention systems result in a different stress distribution on the abutment teeth, implants and the surrounding bone.37,38 This phenomenon should be investigated in vivo as well. In addition, from another FEA it is concluded that it is a good choice to use an implant

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size with maximum bone-to-implant contact.39 This has never been investigated in vivo and in light of the present prospective study with relatively short implants (6-8 mm in length) with good results it is interesting to set up a research in which longer implants are compared with (ultra) short implants. The same can be done with small-diameter implants.

Finally, it would help to know that, whether or not the inserted implants would indeed be suitable support to a complete denture if these patients with a Kennedy class I situation lose their remaining teeth in the future. Therefore it is important to study the long-term prognosis of the implants as well as the abutment teeth, together with treatment options if complications occur.

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