Effectiveness of Maxillary Overdentures Supported by Implants: Maintenance and Prosthetic Complications

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Peter Kiener, Dr Med Dent, Assistant Professor, Department of Prosthodontics, School of Dental Medicine, University of Berne, Switzerland, Matthias Oetterli, Dr Med Dent, Assistant Professor, Department of Prosthodontics, School of Dental Medicine, University of Berne, Switzerland, Ernö Mericske, Dr Med Dent, Assistant Professor, Department of Prosthodontics, School of Dental Medicine, University of Berne; and Private Practice, Berne, Switzerland, Regina Mericske-Stern, PhD, Dr Med Dent, Associate Professor and Acting Chair, Department of Prosthodontics, School of Dental Medicine, University of Berne, Switzerland
Reprint requests: Dr Regina Mericske-Stern, Department of Prosthodontics, School of Dental Medicine, University of Berne, Freiburgstrasse 7, CH-3010 Berne, Switzerland. Fax: + (41-31) 632 49 33. e-mail: regina.mericske@zmk.unibe.ch
Purpose: The aim of this study was to evaluate the prosthetic complications with implant-supported overdentures in the maxilla. Materials and Methods: Forty-one patients (mean age 61 years) were consecutively admitted for treatment from 1991 to 1998. A total of 173 ITI implants were placed. Four to six implants either connected with a bar (34 overdentures) or with single anchors (seven overdentures) supported the denture, and only a few parties had fewer than four implants. The overdentures had a horseshoe design and were reinforced by a cast-metal framework. The mean observation time was 3.2 years. Oral hygiene and periimplant parameters were regularly assessed, and records were kept of prosthetic maintenance service. All prosthetic complications encountered were classified related to (1) implant components and anchorage devices, (2) mechanical and structural failures of dentures, or (3) denture-related adjustments. Results: Three implants did not osseointegrate, and five implants were lost after loading. Thus, the overall survival rate of the implants was 95.5%. Altogether, 85 prosthetic complications were encountered. The most frequent finding was retightening of the bar screw and adjustments of the bar retainers. Repair of dentures was not frequent and was mostly related to broken teeth. No fracture of dentures was observed. Renewal of dentures occurred twice, once after loss of all implants in one patient. Thirty-nine overdentures had been continuously worn; thus, the overall denture stability was 95%. Mucosal irritation and need for occlusal adjustment were the most frequent findings in the first year. Over time, a decrease of complications was observed. Conclusion: Planned maxillary overdentures supported by implants are a successful treatment modality on a short-term basis. Int J Prosthodont 2001;14:133–140.

References

A variety of studies with reliable results on fixed prostheses using Brånemark implants (Nobel Biocare) in the maxilla and mandible1–3 have been published; however, some authors reported on only a limited number of maxillary prostheses.4 Clinical and scientific evidence to support the use of overdentures instead of fixed prostheses in the maxilla as a predictable treatment modality was missing.

The implant prosthodontic treatment of the edentulous maxilla is challenging and determined by inherent problems, such as reduced bone quality and quantity and divergent implant axes.5 Adverse morphologic effects that are specific to the maxilla, eg, long teeth or incongruent implant and tooth position, may be more easily eliminated by the use of maxillary overdentures.6 Until now, maxillary overdentures were selected first to substitute for failing fixed prostheses. Overdenture studies give evidence for this treatment strategy.4,7,8 Thus, low success rates are reported for maxillary overdentures; however, it would be more correct to account for them as failures of fixed prostheses because the original treatment plan could not be followed.9 Reliable long-term results with planned maxillary overdentures are not yet published, and available documentation still shows a slight tendency to an increased failure rate7,10,11 compared with other prosthetic indications. In most studies, Brånemark implants were used, and results with the ITI system (Straumann) are limited to a small number of maxillary implants.12,13 Studies on fixed prostheses have mostly used protocols and technical procedures for fixed prostheses as described by Brånemark et al.14 There is still a lack of comparable guidelines for overdenture fabrication, and evidence-based concepts of design are lacking. However, some authors11,15–18 have described individual methods and techniques.

Various parameters to monitor implants and criteria to evaluate short- and long-term success or failure of implants were defined by various authors. They have included the suggestion to use lifetable analysis.19–22 Evaluation standards to describe the prosthetic treatment outcome are still missing or individually interpreted. Therefore, parameters for comparable assessment of prosthetic problems and maintenance are needed and should be defined.23,24 The aim of the present study was to report on prosthetic complications and maintenance of maxillary overdentures supported by ITI implants.

Materials and Methods

Patients

Forty-one patients, 17 men and 24 women, with an edentulous maxillary jaw were consecutively admitted for treatment with implant-supported overdentures and included in the study. The mean age at the time of overdenture connection was 61.2 years, ranging from 40 to 89 years. They had been treated from 1991 to 1998. At the time of the data collection, the overdentures had been in situ for at least 1 and up to 8 years, with a mean of 3.2 years. Most patients wished to have better retention of their maxillary complete dentures or often asked for fixed prostheses.

Inclusion criteria were: (1) adequate width and height of maxillary bone for placement of four implants; (2) no severe systemic problems, patient was able to undergo surgery; (3) no heavy smoking (< 15 cigarettes per day), no abuse of drugs; (4) good oral hygiene and periodontal health; and (5) periodontal treatment of the mandibular teeth had to precede implant surgery.

Vertical bone augmentation by graft procedures or sinus floor elevation had not been performed; however, simultaneous lateral bone augmentation using nonresorbable membranes became necessary for several implants during the surgical placement.

Careful treatment planning was based on the individual anatomic-morphologic situation, patients' demands, and esthetic and cosmetic aspects.17,18 In the opposing mandible, 16 patients had natural teeth, and four patients had a combination of natural teeth with fixed prostheses supported by implants. One patient had a fixed prosthesis supported by implants only, nine patients had a removable partial denture, and 12 patients also had an implant-supported overdenture in the edentulous mandible.

The patients were classified into three time groups according to the observation period:

  • Group 1: < 2-year observation period, including nine patients with 35 implants
  • Group 2: 5-year observation period, including 25 patients with 68 implants
  • Group 3: > 5-year observation period, including seven patients with 67 implants

Treatment Protocol

The standard protocol was to place four to six well-spaced, screw-shaped ITI implants, most often in the zone between the left and right first premolars. Seven patients had fewer than four implants placed. Two patients with good bone quality and quantity that would have allowed for placement of multiple implants received only two implants, provided with ball anchors, for economic reasons. Because of unfavorable anatomic-morphologic conditions, five patients had only three implants placed, all connected with a bar according to the standard protocol. Table 1 gives an overview of the number of implants per overdenture. Table 2 shows the distribution of implant lengths and diameters. Prior to surgery, optimum location of the implants was assessed using radiographs and a provisional tooth setup. If well-fitting and properly designed complete dentures did already exist, these were duplicated and used for treatment planning. Pantographic radiographs with metallic landmarks, and in a few cases computed tomographic (CT) scans, were prescribed. For fabrication of the dentures, a one-stage impression technique with individual cast trays that covered the entire palate was applied. When the overdentures were completed and delivered to the patient, radiographs were taken and periimplant parameters assessed. Following this, all patients were included in a strict maintenance care program with at least two annual checkups performed by the dental hygienist and, if necessary, by the dentist. Oral hygiene and periimplant parameters were regularly assessed. Simultaneously, prosthetic maintenance service and prosthetic complications encountered were registered, and detailed annual records were kept for all patients.

Table 1 No. of Implants and Overdentures
No. of supporting implants per overdenture No. of overdentures Total No. of implants*
2 2 4
3 5 15
4 26 104
5 8 47
Total 41 170

*Does not include early implant failures.

 

Table 2 Dimensions of Implants Placed (mm)

Diameter (mm) 6-mm length 8-mm length 10-mm length 12-mm length Total No. of implants*
4.1 2 23 37 32 94
3.3 -- 20 32 21 73
4.8 2 4 -- -- 6
Total 4 47 69 53 173

*Includes early implant failures.

Thirty-three patients received an overdenture connected by a U-shaped Dolder bar. Prefabricated bar segments (Cendres et Métaux) were soldered to gold abutments that precisely fit on the octagonal abutments. Corresponding to all bar segments, bar retainers were mounted into the overdentures by the technician during the laboratory procedures. Twenty-four bars were provided bilaterally with short distal extensions, and one bar segment was provided with a mesial extension, which resulted in a total of 49 extensions. Figure 1 shows the different types of anchors. Extensions that did not exceed 5 to 7 mm in length were added to bars if the distal implants were located in a rather anterior position (eg, canine) because of the anatomic situation. Eight overdentures with a total of 29 implants were connected to single abutments, either ball anchors (19 implants) or telescope copings (ten implants). Ball anchors were used to reduce the costs, and for two patients telescopes were prescribed because little space was available for placement of a bar. U-shaped bars and telescopes provide a rigid retention mechanism, and the overdenture is mostly implant supported. If only two ball anchors are used for denture fixation, this results in a resilient retention, allowing for a rational movement of the denture with equal implant and mucosal support. With multiple implants (≥ four), the resilient retention mechanism of ball anchors becomes highly limited and resembles a more rigid retention.

Fig 1a U-shaped bar with distal extensions.

Fig 1b (above right) Bar with long segments, no extensions.

Fig 1c (right) Single ball anchors.

Regardless of the anchorage device, the overdenture itself had a horseshoe design and was reinforced by a cast framework (Fig 2). Semianatomic teeth were mounted with a lingualized and bilaterally balanced occlusion concept. Table 3 gives an overview of the overdentures that were placed.

Fig 2a Denture with horseshoe design and cast-metal framework.

Fig 2b Inner surface of the denture with bar retainers.

 

Table 3 Implants and Overdentures Related to Observation Period

Period (y)*
No. of implants
No. of overdentures
Single anchor
Bar anchorage
1
35
9
4
31
2 to ≤ 5
107
25
21
86
> 5
28
7
4
24
Total
170
41
29
141

*Mean 3.2 y.

Prosthetic Parameters

Maintenance was recorded in annual intervals according to the parameters described below.

  1. Complications related to implant components and anchorage devices: loosening of abutments; loosening of bar screws; broken, loose, or lost female retainers followed by placement of new retainers; tightening of female retainers; fracture of bar or bar extension
  2. Mechanical and structural failures of the overdenture: fracture of denture base resin; fracture of teeth; fracture of cast framework; need for changes of prosthetic design followed by fabrication of new prosthesis
  3. Prosthesis-related adjustments: sore spots; relining of overdenture; occlusal adjustments; changes of tooth arrangement for esthetic reasons; excessive wear of teeth; hyperplasia of soft tissue underneath the bar

All patients were recalled in late 1999 for a clinical examination of the implants and assessment of the overdentures. One patient could not attend this clinical examination. Radiographs were taken, and periodontal parameters were recorded for comparative analysis. Results obtained from this clinical and radiographic assessment of the implants will be published separately.

Statistical Analysis

Descriptive statistics were used to analyze the frequency and type of prosthetic complications. The stability, ie, the overall survival rate of the prostheses, was calculated as the percentage of renewal of prostheses related to the total number of overdentures originally inserted. The estimated time lag between the insertion of the overdenture and the first complication and between the first and second complications, respectively, was calculated by the method of Kaplan and Meier.25 These curves also depict the degree of probability that no complications will occur.

Results

A total of 173 ITI implants were placed in 41 patients. Three implants, 1.7% of all placed implants, did not osseointegrate. Because these three implants were not regarded to be necessary for denture connection, new ones did not replace these. Five implants were lost during the loaded phase; thus, the late failure rate was 3% related to the number of loaded implants. Within the second year of loading, four implants were lost in the same patient who had lost two implants during the healing phase. The implant length in this patient was twice 6 mm (early failure), twice 8 mm, and twice 10 mm (late failure). They showed slight mobility. After 6 years, another implant (8 mm) was found with a marginal infection that could not be successfully treated with antimicrobial therapy. All but one lost implant had a standard diameter, and no implant with a length of 12 mm was lost. While a bar had connected all four implants lost in one patient, the biologic failure was a single implant with a ball anchor. At the time of the clinical examination, 95.5% of all implants originally placed were still in situ.

Altogether, the number of prosthetic findings, including maintenance, repairs, and adjustments, was 85; this results in a mean of 2.1 per overdenture during the mean observation period of 3.2 years. Overdentures with extension bars exhibited a mean of 2.4 complications, 1.7 if no extensions were added, and 1.2 in the presence of single anchors. About 50% of all complications were detected in eight patients, ie, 20% of all patients. Problems with the anchorage system were the most common finding, in particular retightening of the bar screw or repair of the female retainers. Repair of the denture was not frequent and was mostly related to broken teeth. Renewal of the denture occurred twice. Once, the fabrication of a complete denture became necessary after the loss of all four implants in one patient. One denture was remade because of poor retention of the telescopes and a poor esthetic result. Thirty-nine overdentures had been continuously worn, and the overall denture stability was 95%. At the end of this study, two patients--one from group 2, one from group 3--decided to go for a fixed prosthesis because a sufficient number of implants with an adequate distribution over the arch favored this treatment option as well.

Early after delivery of the dentures to the patients, removal of sore spots and occlusal adjustments (remounting of the denture into the articulator) became necessary. While for the first year the mean was one complication per overdenture, this value decreased to 0.7 for the second year and ranged between 0.2 and 0.5 for the following years. The type and frequency of all prosthetic findings are listed in Table 4. Altogether, eight patients had been free of any complication during the entire observation period. One patient belonged to group 1, six patients to group 2, and one patient to group 3. If treatment of sore spots within the first days and weeks after delivery of the denture is not considered a complication, 16 patients were free from any problems. Figure 3 shows the estimated time lag between the delivery of the denture to the patients and the occurrence of prosthetic complications. Further, the probability that the overdenture would remain free of any problems is depicted by the curve. The figures also show that the probability of encountering some problems was rather high.

Table 4 Overview of Prosthetic Complications
Overall
First year
(41 overdentures)
Second year
(34 overdentures)
Anchorage system

Loosening of abutment

3
1
2

Retightening of occlusal screw

15
6
5

Broken, loose, lost retainers

9
3
2

Retightening of retainer

2
1
0

Broken bars or extensions

2 bars/8 extensions
5
2
Repair of denture

Fracture of resin denture base

1
0
0

Fracture of teeth

6
1
3

Fracture of cast framework

0
0
0

Redesign or new denture

2*
1
0
Adjustment of denture

Sore spots

16
12
0

Reclining of denture

2
1
1

Occlusal adjustment

11
9
2

Rearrange of teeth

1
1
0

Excessive wear of teeth

3
2
0

Hyperplasia under bar

4
1
0
Total
85
44
17

*One denture was changed to a complete denture after loss of all implants, and one was changed for esthetic reasons.

 

Fig 3a Estimated time between delivery of the overdenture and first prosthetic complication.

Fig 3b Estimated time between first and second complications.

Discussion

Most studies on maxillary overdentures aim at describing the treatment outcome related to the implant survival and in part to the assessment of prosthetic maintenance and failure. The choice of the implant system itself, design characteristics of the overdenture, and retention devices may result in specific prosthetic complications.

In contrast to the mandible, loss of maxillary implants appears to be frequent, particularly in combination with overdentures. Here, failure is often associated with short implants, ie, those ≤ 10 mm.26 If a distinction between emergency situations and planned overdenture cases is made, the survival rate for implants supporting an overdenture is significantly enhanced.9,11,27,28 The present article exclusively dealt with planned cases. In fact, the overall survival of the loaded implants was favorable, although over 50% of the implants had a length of 10 mm or less. One patient in the present study lost all implants, which corresponds to reports saying that a few patients lost a lot or all of the implants lost.10,27,28 As a consequence of loss of multiple implants, remake of dentures may become necessary. Altogether, in the present study the overdenture stability was 95%, significantly higher than reported by others.16,29 In accordance with the findings of a review article,26 complications with the implants in the present study were not related to the type of attachment system used for denture fixation.

In contrast to other results,9,11 the type of attachment for denture fixation did have an influence on prosthetic complications in the present study. Bars with extensions appeared to be more susceptible to prosthetic complications because some fractures of distal bar extensions were observed. This mostly did not impair wearing of the dentures and was often not repaired. With respect to the prosthetic results, a distinction should be made between normal maintenance and complications of implant-related components or prostheses that were not related to the implants. Complications may vary widely, from a simple removal of a sore spot to the complete remake of a denture.5 If need for maintenance because of normal wear of implant and prosthetic parts becomes excessive, such service procedures have to be attributed to complications and failures of prostheses. Thus, this differentiation between maintenance and complications is in part a quantitative one and still needs criteria for assessment. It has also been suggested to calculate the number of appointments as a measure of maintenance and repairs.7,30 These studies reported that six to nine appointments were necessary in the first year because of technical complications. In part, they had the character of pilot studies, including training cases, which may explain an increased need for maintenance. The present results exhibit a mean of one prosthetic complication in the first year, with a tendency to decrease. Such a decrease after the first year was observed by various authors.4,31

In accordance with the findings of other studies,16,27,28,32 problems with the retentive devices and bar clips were rather frequent. However, such adjustment and repair could easily be resolved,11 and the activation and retightening was mostly carried out during the recall appointments of the patients. Fracture of dentures and denture resin as reported by various authors,7,9,30 particularly in connection with bars,11 was not observed in the present study. It appears that the cast-metal framework did enhance the mechanical properties and stability of the denture. Thus, this specific problem of fracture can be dependent on the design and method of fabrication of the overdentures.

It was claimed that in general, fewer problems are encountered with fixed prostheses than with overdentures.31 However, many studies have reported on fatigue fractures related to implants, abutments and screws, and loosening of screws1–4,31 in connection with fixed prostheses. Denture fractures have also been observed.1–3 In the present study, speech problems were not observed, and the buccal denture flange seemed to be advantageous for speech production. Only one patient received a new denture because of a poor esthetic result. This is in accordance with other overdenture studies, in which, however, esthetic problems appeared to be more frequent.7,9,30,32 Impaired phonetics and speech problems were encountered in up to 35% of patients provided with fixed prostheses, and esthetic problems were also found.4,30,33

On the basis of the present results, comparisons of prosthetic complications related to the different retention devices are not reliable because the majority of the patients received rigid bars. If the prosthetic concept with multiple implants and connecting rigid bars is applied, denture support is mostly provided through the implants, similar to fixed prostheses. The cast-metal framework may additionally contribute to the stiffness and rigidity of the prostheses. Further, the dentures had a horseshoe design, which means a limited contact surface between denture base and underlying soft tissue. In fact, comparative in vivo force measurements on maxillary implants with either overdentures or fixed prostheses revealed highly similar force patterns during the registration of static and functional forces such as maximum biting, clenching, or chewing food.34

With maxillary overdentures, hyperplasia, mucosal irritations, and stomatitis seem to be specific problems.7,16,23,28,30 One may assume that the overdenture most often had a complete denture design in these studies. While in the first year sore spots were removed in several patients of the present study, stomatitis was rarely found. This may be the result of the open-horseshoe design of the overdentures, which uncovers a large portion of the palate. However, in three patients, hyperplasia underneath the bar that required treatment was repeatedly found.

Conclusions

  • The present study shows that maxillary overdentures supported by implants may be a successful treatment modality on a short-term basis.
  • It was demonstrated that in planned overdenture cases, implant survival was significantly enhanced, with > 95% successful implants.
  • Multiple implants connected by rigid bars for denture retention may be favorable with respect to implant survival and denture stability.
  • Although some maintenance service was regularly needed and minor prosthetic complications were encountered, stability of the overdentures was high. A decrease of complications over time was observed. Extension bars seem to be more susceptible to complications.
  • If comparisons of prosthetic treatment outcomes are to be made between different types of prostheses supported by implants, adequate evaluation criteria are needed.