Follow-up examination of patients with mini-implants for the stabilization of existing removable partial dentures

DOI: 10.3238/dzz-int.2020.0038-0049

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Keywords: double crown mini-implant removable partial denture satisfaction stability strategic survival

 

Introduction: The aim of this study was to evaluate the clinical performance of mini-implants (MI), which were used for the stabilization of double crown retained removable partial dentures (RPDs), after a middle-term period of service in a dental practice. Additionally, implant stability and patient satisfaction with the dentures were evaluated.

Material and Methods: Patients who had received 10 to 13 mm long MI with diameters of 1.8, 2.1, and 2.4 mm and ball attachments for supplementary support of their existing double crown retained RPDs at least 3 years ago were included in this study. After patient chart and medical history analysis as well as the completion of an 8-item questionnaire on satisfaction with the RPD (Likert scale 1 to 5) by the participants, an experienced dentist independently examined the periodontal/peri-implant conditions; this involved measurement of implant stability by using the Periotest and the Osstell device. In addition to descriptive statistics, survival analyses based on the Kaplan-Meier and Cox regression analyses were used to estimate possible risk factors for implant loss.

Results: Out of 70 reachable patients, 66 study jaws in 57 patients were examined. The duration between the time of implant placement and the follow-up examination ranged between 3 and 9 years for the examined 77 MI in 25 upper jaws and 113 MI in 41 lower jaws. The MI in 20 jaws with good bone quality (insertion torque ≥ 35 Ncm) were loaded immediately using matrices (housing with O-rings), while the other RPDs were initially soft-relined for 3–4 months. The 5-year-survival rates of the MI in the maxilla and mandible were 97.4 % (3 failures) and 86.9 % (13 failures, one fracture), while the tooth survival rates were 88 % and 88.9 %, respectively. The Cox regression analyses revealed no statistically significant effect of possible risk factors on implant failure (tooth status, smoking habits, diabetes mellitus, loading modus). In 18 of the study participants, a total of 40 MI were placed subsequent to implant or tooth loss. The aftercare of the RPDs comprised of 8 O-ring replacements and 26 denture base relinings. The complications included denture base (n = 17), secondary crown veneering (n = 11) and artificial denture teeth (n = 2) fractures. The mean Periotest values were 5.5 and 6.7 (P = 0.078), while the mean Osstell values were 38 and 33 (P < 0.0001), in the maxilla and mandible, respectively. The majority of participants were very satisfied with their RPD (80 % in the maxilla, 70 % in the mandible) and nobody was dissatisfied.

Discussion: The lower MI survival rate in the mandible compared with the maxilla comes as a surprise and is contrary to previous studies performed on edentulous jaws. The complications were manageable, despite implant losses and denture fractures. The stability values of MI were lower than those of standard-diameter implants.

Conclusion: Strategic MI under double crown retained RPDs are a recommendable therapeutic option in the dental practice. Prospective randomized clinical studies are required to investigate this therapeutic alternative.

 

University Medicine Greifswald, Polyclinic for Dental Prosthetics, Geriatric Dentistry and Medical Materials Science: Prof. Dr. Torsten Mundt; Dr. Christian Schwahn

Practice The ProDentists, Schwerin: Dr. Jörn Kobrow

Translation from German: Christian Miron

Citation: Mundt T, Kobrow J, Schwahn C: Follow-up examination of patients with mini-implants for the stabilization of existing removable partial dentures. Dtsch Zahnärztl Z Int 2020; 2: 38–49

Peer-reviewed article: submitted: 23.08.2019, revised Version accepted: 07.01.2020

DOI.org/10.3238/dzz-int.2020.0038–0049

Introduction

Dental implants for the stabilization of removable partial dentures (RPD) have become an accepted therapy alternative [2–4, 13, 15, 16, 30, 31]. In addition to providing distal support for free-end dentures [4] and increasing the number of primary abutments prior to new prosthetic restoration [2, 3, 13, 15], implant placement under an existing RPD is an interesting alternative [30]. Abutment extractions and/or their unfavorable distribution can lead to problems with denture retention. In a prospective study, in a total of 11 patients with unfavorable distribution and a low number of abutment teeth in one jaw, the subsequent incorpo­ration of retaining elements on implants led to an improvement in the oral health-related quality of life [30] and chewing efficiency [31]. After 6.5 years, all implants and RPDs were still functional although some abutment teeth had to be extracted (89 % tooth survival rate) [16].

Despite the fact that this is less expensive than making a new superstructure, the associated costs are still relatively high. Moreover, the use of implants with standard diameters (> 3.5 mm) is lim­ited due to bone atrophy after tooth extraction and the resulting narrowing of the alveolar process. Implants with reduced diameters (3–3.5 mm) are not always indicated for single attachment. Finally, augmentative procedures to improve the bone volume are not only associated with risks for patients with systemic diseases, but they are also frequently rejected, particularly by older patients because of the longer treatment duration as well as the greater effort required [29].

Mini-implants (MI) with an even smaller diameter (< 3 mm) are usually one-piece, and therefore, a no-load osseointegration is hardly possible. They are mainly used to stabilize complete dentures by means of ball attachments. For this, 6 MI in the upper jaw and 4 MI in the lower jaw are recommended [14]. The most recent systematic reviews have reported high survival rates (> 95 %) after an average period of 3 years and low bone resorption rates (< 1.2 mm) in the edentulous mandible [12, 14, 23]. Contrary to this, after immediate loading in the edentulous maxilla, the MI rate of failure was unacceptably high at 32 % [14]. If the bone quality is poor, or more specifically, the insertion torque < 35 Ncm, the dentures should first be hollowed out in the area of the ball attachments and lined with soft material. This apparently leads to fewer failures [9, 20].

In addition to the insertion torque as a measure of primary strength, implant stability can also be determined longitudinally with Perio- test measurements or resonance frequency analyses [22]. For immediate loading and for follow-up checks, reference values as for two-piece standard diameter implants are desirable. However, previous Periotest measurements on MI have shown different mean values of < -3 [7] and > 5 [25]. For resonance frequency analysis of one-piece MI with ball-shaped heads, only data from an animal experiment (rabbit lower leg bone) with a specially designed attachment have been published so far. In direct comparison with two-piece standard implants, the differences between the values were not significant [5].

Meanwhile, there are now 2 studies with an observation period of 12 and 6 months, respectively, on the successful application of MI for better support of RPDs in the presence of remaining anterior teeth (Kennedy Class I) [6, 28]. To date, there have only been case reports on the use of MI as strategic abutments to improve load distribution and retention under existing RPDs in the conditions of few or unfavorably distributed residual teeth [19, 27]. The results from a prospective, randomized 3-year study on the same topic, where the design has been published so far, are still pending [18].

Therefore, a retrospective exami­nation was initiated on patients from a dental practice who had received MI for the stabilization of double crown-retained RPDs for a longer time. Following implant placement for a minimum period of 3 years, clinical performance, implant stability and patient satisfaction with the dentures were evaluated.

Material and treatment methods

Study participants

The study initiated by the Greifswald University Hospital was financially supported by the company 3M Deutschland GmbH (Germany) and received the vote (BB 025/13) from the responsible ethics committee. Patients were invited to a dental practice in North Rhine-Westphalia, where they had received mini-implants (Mini Dental Implant, MDI, 3M ESPE, Seefeld, Germany) as supplementary abutments under existing RPDs at least 3 years ago (Figure 1). Meanwhile, MDIs are distributed by another company (Condent, Hannover, Germany). Patients who could not be expected to take part in the study due to general medical conditions and who did not give written consent to participate in the study were excluded. The neutral losses to follow-up (deceased, seriously ill and those who had moved out of the catchment area of the practice) were subtracted from the gross sample size so that the difference, being the net sample size, could be used to determine the response. Drop-out was a (multiple) failure to attend the examination dates or a refusal to participate in the study. The study participants were examined by a trained and experienced dentist who was not involved in the treatment of the patients.

Therapy

In cases where the denture retention was insufficient such as after abutment tooth extraction, or primarily due to an insufficient number or distribution of remaining teeth, patients received subsequent MI for additional stabilization of their RPDs. The number and position of the implants was determined based on the distribution of the remaining teeth and the existing vertical bone height, which is limited distally by the maxillary sinus and the inferior alveolar nerve. Insertion was largely performed transgingivally or, as in a few cases, subsequent to the mobili­zation of a small mucoperiosteal flap and preparation of the implant site with a 1.1 mm thin pilot drill at different depths (one to two thirds of the implant length); the drilling depth was of course dependent on bone quality. In practice, only MI with lengths of 10 and 13 mm and diameters of 1.8, 2.1 and 2.4 mm were used. In the patient example in Figures 2 and 3, the use of standard implants would only have been possible by employing procedures to widen the bone bed such as splitting or augmentation, or with a reduction of the narrow part of the alveolar ridge. Immediate loading was made on MI having a sufficient insertion torque (approx. 35 Ncm). For this purpose, the dentures were hollowed out above the ball attachments and the matrices (metal housings with O-Rings) were incorpo­rated using self-curing acrylic resin either direct intraorally or indirectly using an impression and a model. If the insertion torque was insufficient, the dentures were first soft relined and the housings were directly or indirectly incorporated after approximately 3 months.

Investigation parameters

The medical findings prior to implantation were based on the documentation in the patient‘s chart and the postoperative panoramic X-ray. All treatments, technical and biological complications on teeth, implants, the superstructure as well as any post-implantations between the primary implant placement and the follow-up examination were also recorded.

The study jaws were classified according to the residual dentition which was present at the time of primary implant placement [18]: one quadrant is edentulous (class 0), in one or both quadrants there are either only incisors (1), or the canine is missing and only one posterior tooth (2), the canine is missing and two posterior teeth (3), only the canine and no posterior tooth (4) or the canine and one posterior tooth (5).

During the follow-up exami­nation, a medical anamnesis was first performed; diseases, medication and smoking habits were recorded. The patients were divided into smokers, former smokers (quitting smoking 5 years before the fol­low-up examination), and never smokers. With the help of a validated questionnaire, the satisfaction with the prosthetic restoration in the study jaw was determined based on the grading system used in German schools; 8 questions regarding general satisfaction, retention, position stability, resilience, speaking, eating, appearance and ease of cleaning of the denture were asked. The answers were marked according to a Likert scale of very good (1), good (2), neither good nor bad (3), bad (4) to very bad (5) [1].

In addition to the dental and prosthetic status, the following clinical parameters were assessed on teeth and implants:

  1. Modified plaque index according to Mombelli [17] ranging from grade 0 (no plaque) to grade 3 (massive plaque)
  2. Probing depth: 4 measuring points (mesial, vestibular distal, oral) were carefully probed (< 0.2 N) with the periodontal probe PCP-12 (Hu-Friedy)
  3. Bleeding on probing: yes/no
  4. Periotest value (Periotest device, Medizintechnik Gulden, Germany): The measurements were made at right angles to implants (center of ball-shaped head). The lower the Periotest values were, the more fixed the implants were.
  5. Resonance frequency analysis (Osstell, Gothenburg, Sweden): A smartpeg prototype developed by the former manufacturer of MDI was placed on the spherical head and fixed below the spherical equator with a lateral screw (Figure 4). The hand-operated probe stimulated the Smartpeg. The resonance was recorded by the Osstell measuring device. The implant stability quotient (ISQ) indicated the resonance frequency (kHz) on a clinically applicable scale of 1–100 ISQ. The higher the ISQ was, the more fixed the implant was. The Smartpeg attachment is being tested for the first time in a clinical study. Reference values are therefore not yet available.

Statistical analysis

 

In some study participants, both jaws were treated, but at different time points. Thus, the upper and lower jaws were evaluated separately. In addition to descriptive statistics, the survival probabilities of implants and teeth were calculated using Kaplan-Meier analyses and subgroups were compared using log-rank tests. Pos­sible predetermined factors for implant failure (age, gender, type of incomplete dentition, smoking, diabetes mellitus, loading mode) were evaluated with Cox regression anal­yses. The software used was Stata/MP software, release 14.2 (Stata Corpo­ration, College Station, TX, USA). The significance level for the statistical tests was set at 0.05.

 

Results

Patient characteristics

From the original 98 patients (35 men, 63 women) with strategic MI, 28 were no longer reachable; 9 were deceased, 11 were seriously ill and 8 moved to another and/or unknown location. Of the remaining 70 patients, 13 refused to partici­- pate in the study (18.6 % drop-out). In the end, 57 study participants (35 women, 22 men) with 25 upper jaws and 41 lower jaws were included. The general characteristics are shown in Table 1.

Characteristic

Men (n = 22)

Total (n = 57)

Total (n = 57)

n

(%)

n

(%)

n

(%)

Smoking habits

Never smoker

Former smoker

Smoker

 9

10

 3

(40.9)

(45.4)

(13.6)

21

 6

 8

(60.0)

(17.1)

(22.8)

30

16

11

(52.6)

(28.1)

(19.3)

Cardiovascular diseases

12

(54.5)

18

(51.4)

30

(52.6)

Diabetes mellitus

 3

(13.6)

 2

 (5.7)

 5

 (8.8)

Anticoagulant medication

 7

(31.8)

 7

(20.0)

14

(24.6)

Rheumatoid arthritis

 0

  (0)

 5

(14.3)

 5

 (8.8)

Cancer

 1

  (4.5)

 3

 (8.6)

 4

 (7.0)

Number of medications per day

 0

 1

 2

 3

>3

 9

 5

 3

 2

 3

(40.9)

(22.7)

(13.6)

 (9.1)

(13.6)

 8

 8

 6

 3

10

(22.8)

(22.8)

(17.1)

 (8.6)

(28.6)

17

13

 9

 5

13

(29.8)

(22.8)

(15.8)

 (8.8)

(22.8)

Table 1 Characteristics of study participants

All study jaws were treated with double crown-retained RPDs and 9 of the participants received strategic MI in both jaws. In the antagonist jaws, double crown-retained RPDs (n = 12), clasp-retained RPDs (n = 4), complete dentures (n = 14, exclusively upper jaw), precision attachment-retained RPDs (n = 2) or fixed restorations on teeth (n = 15) or implants (n = 1) were found. In 42 study jaws, no tooth (class 0, n = 18), exclusively anterior teeth (class 1, n = 16), at most one posterior tooth (class 2, n = 7) or 2 posterior teeth (class 3, n = 1) were present in at least one quadrant before implantation. In 24 study jaws, the dentures were supported on both sides at least on canines (classes 4 and 5).

At the time of implant insertion in the upper and lower jaws, the average age of the participants was 64 ± 9.7 years and 66.4 ± 9.1 years, respectively, without any relevant gender differences. The average time between initial implant insertion and examination was 5.5 ± 1.8 years in the maxilla and 5.3 ± 1.9 years in the mandible with a minimum duration of 3.1 and a maximum duration of 9.7 years for both jaws. In the upper and lower jaws, 77 MI and 113 MI were inserted, respectively. Most frequently, 2 implants were placed in both jaws (Table 2).

Number

Upper Jaw

Unterkiefer

Total

Implants

Number

(%)

Number

(%)

Number

(%)

1

1

 (4)

 2

 (5)

 3

 (5)

2

9

(36)

19

(46)

28

(42)

3

6

(24)

 9

(22)

15

(23)

4

7

(28)

 9

(22)

16

(24)

5

0

 (0)

 2

 (5)

 2

 (3)

6

2

 (8)

 0

 (0)

 2

 (3)

Total

25

41

66

Table 2 Number of implants per jaw at the time of first implant placement

MI with lengths of 10 mm (n = 5) and 13 mm (n = 185) were placed in the tooth areas between 15 and 25 as well as 36 and 46 (a total of 10 molar implants) in the upper and lower jaws, respectively. Most frequently, implants were placed in the first premolar and central incisor areas. In the maxilla, 61 MI with a diameter of 2.4 mm, 10 of 2.1 mm and 6 of 1.8 mm were used. In the mandible, 88 MI with a diameter of 1.8 mm, 20 MI of 2.1 mm and the remaining 5 MI of 2.4 mm were used. In 9 upper jaws (36 %) and 11 lower jaws (26.8 %), the MI were immediately loaded with the housings.

Implant and tooth survival/post-operative care

According to the Kaplan-Meier analysis, the 5-year survival rate of MI was 97.4 % in the maxilla (3 losses due to missing/lost osseointegration) and 86.9 % in the mandible (13 losses due to missing/lost osseointegration, one fracture). The log-rank test, without regard to the person level, showed a statistically significant difference between the jaws (P = 0.0481). As can be seen in Figure 5, the vast majority of losses were recorded in the first year (n = 12). The statistical evaluation did not take into account 14 and 26 replaced implants subsequent to tooth and/or implant loss in the maxilla and mandible, respectively.

A Cox regression analysis on possible factors influencing implant failure was only meaningful for the mandible due to the number of events and patients; it did not reveal any significant effects of age, gender, gap dentition classification, smoking, diabetes mellitus, loading mode on implant failure (Table 3). Also, diabetics did not lose implants.

Hazard Ratio (95%-Confidence Interval)

 

Lower Jaw (14 Results; adjusted for 41 clusters from patients)

Risk factor

Reference- Category

Not adjusted

Adjusted for age

Adjusted for age and gender

Age (≥ 70 years)

< 70 years

0.73 (0.21–2.47)

---

0.76 (0.23–2.52)

Female Gender

Male

1.28 (0.39–4.23)

1.19 (0.37–3.77)

---

Dentition classification

Continuous

0.80 (0.58–1.12)

0.81 (0.58–1.11)

0.81 (0.58–1.13)

Smoking

Never/Ex-Smoker

2.17 (0.64–7.30)

2.49 (0.60–10.4)

2.46 (0.61–10.0)

Diabetes mellitus

No

(0)

(0)

(0)

Delayedloading

Immediate-loading

4.49 (0.57–35.5)

4.46 (0.56–35.7)

4.51 (0.53–38.3)

Table 3 Cox regression analyses of possible factors for implant failure

During the entire period of study, 19 out of 106 upper teeth and 18 out of 170 lower teeth were lost. The 5-year survival rate of teeth was 88.0 % in the maxilla and 88.9 % in the mandible based on Kaplan-Meier estimates.

None of the 66 dentures had to be renewed until the follow-up examination. Prosthetic aftercare measures included replacement of O-rings a total of 8 times, 26 denture relinings in connection with tooth extractions, MI losses or replacement of MIs, 9 times replacement of denture teeth, as well as, 17 and 11 repairs following the fracture of the denture base and double crown veneering, respectively.

Clinical examination

In the maxilla, 57 % of the MI were plaque-free (plaque index degree 0), while the other MI showed a thin plaque film (degree 1). In the mandible, 39 % of MI were plaque-free (grade 0), 51 % had a thin plaque film (grade 1), 9 % showed visible plaque (grade 2) and 1 % had massive plaque deposits (grade 3). From the remaining teeth, 20 % in the maxilla and 25 % in the mandible were plaque-free. However, 19 % of the teeth in both jaws displayed visible plaque (grade 2), but no massive plaque deposits.

The maximum probing depth around implants and teeth was on average 2.5 mm and 3.2 mm, respec­tively. Slightly higher values were recorded in the maxilla (Table 4 and Table 5).

Jaw

Number

Mean

Standard deviation

Min

1st Quartile

Median

3rd Quartile

Max

Upper jaw

 76

2.7

1.0

1.0

2.0

3.0

3.0

 8,0

Lower jaw

105

2.3

1.2

1.0

2.0

2.0

3.0

10,0

Total

181

2.5

1.1

1.0

2.0

2.0

3.0

10,0

Table 4 Maximum probing depths around implants

 

Jaw

Number

Mean

Standard deviation

Min

1st Quartile

Median

3rd Quartile

Max

Upper jaw

 93

3.5

1.1

1.0

3.0

3.0

4.0

 8.0

Lower jaw

151

3.0

1.1

1.0

2.0

3.0

4.0

10.0

Total

244

3.2

1.1

1.0

2.5

3.0

4.0

10.0

Table 5 Maximum probing depths around teeth

After careful probing, 58 % of implants and 34 % of teeth in the maxilla and 40.5 % of implants and 37 % of teeth in the mandible showed sulcus bleeding.

On average, the Periotest measurements yielded slightly lower values of 5.3 ± 5.6 in the upper jaw compared to 6.7 ± 6.4 in the lower jaw (Figure 6). However, the difference was not statistically significant after a Box-Cox transformation of the values for a symmetrical distribution (P = 0.078). The box-cox plots revealed a large upward dispersion with values smaller than 0 being rare. The mean ISQ values (Osstell) in the upper jaw (38 ± 9.4) were higher than those in the lower jaw (33 ± 10.9) (P = 0.001) (Figure 7).

When the Periotest and Osstell values are correlated, the Pearson correlation is -0.87 and the Spearman correlation is -0.82; this indicates a high correlation (Figure 8). Further anal­yses show an interaction between jaw and diameter (P = 0.0092) after the Box-Cox transformation of the Perio­test values. The highest values were found in the mandible with 1.8 mm thick implants (P = 0.0006). In the maxilla, the differences in Periotest values between implant diameters were not significant (P = 0.5828). Here, however, only 6 MI with a diameter of 1.8 mm were included. There was also an interaction between jaw and diameter (P = 0.0095) for the Osstell values. The 1.8 mm MI in the mandi­- ble showed statistically significant lower values than the thicker MI (P < 0.0001). In the maxilla, the differences were again random (P = 0.5886). Repeated problems occurred when using the Smartpeg attachment. When the peri-implant mucosa reached very close to the sphere, fixation of the attachment with the lateral screw was not always easy to control.

Satisfaction with the prosthetic treatment

The evaluation of one mandibular denture is missing. The overwhelm­ing majority of the participants answered the individual questions on satisfaction with the prosthetic treatment of the study jaw with very good or good. Only a few were not quite so satisfied and no study participant was dissatisfied (Table 6). These ratings are reflected in the cumulative scores. From the sample of study participants, almost half with maxillary dentures and about one third with mandibular dentures answered all questions with „very good“ (cumulative score = 8, Table 7).

Item

Upper jaw: Number of answers n (%)

Lower jaw: Number of answers n (%)

Very good

Good

Neither good nor bad

Very good

Good

Neither good nor bad

General Satisfaction

20

(80)

 5

(20)

0

(0)

28

(70)

11

(27)

1

(3)

Retention

20

(80)

 5

(20)

0

(0)

31

(77)

 9

(22)

0

(0)

Stability

17

(68)

 7

(28)

1

(4)

30

(75)

10

(25)

0

(0)

Support

20

(80)

 3

(12)

2

(8)

29

(73)

11

(27)

0

(0)

Speaking

21

(84)

 4

(16)

0

(0)

35

(87)

 5

(13)

0

(0)

Eating

18

(72)

 6

(24)

1

(4)

29

(73)

11

(27)

0

(0)

Appearance

14

(56)

11

(44)

0

(0)

25

(62)

15

(38)

0

(0)

Cleanability

15

(60)

 9

(36)

1

(4)

20

(50)

20

(50)

0

(0)

Table 6 Answers to questions relating to study participant satisfaction with dentures by jaw

 

Sum score

Upper jaw

Lower jaw

n

(%)

n

(%)

 8

11

 (44)

14

  (35)

 9

 2

  (8)

 9

 (22,5)

10

 5

 (20)

 3

  (7,5)

11

 0

  (0)

 1

  (2,5)

12

 1

  (4)

 5

 (12,5)

13

 0

  (0)

 0

  (0)

14

 2

  (8)

 3

  (7,5)

15

 2

  (8)

 2

  (5)

16

 1

  (4)

 3

  (7,5)

17

 1

  (4)

 0

  (0)

Total

25

(100)

40

(100)

Table 7 Sum scores relating to study participant satisfaction with dentures by jaw

Discussion

The use of MI as supplementary abutments under existing RPDs is a successful medium-term therapy option. The lower survival rate of MI in the mandible compared to the maxilla was surprising. Apart from repairs following the fracture of denture bases, the aftercare of the dentures required relatively low effort because no RPD had to be renewed during the period of observation. The presence of plaque (80 % of the teeth in the upper jaw and 75 % in the lower jaw) together with probing depths around teeth (more than half ≥ 3 mm) are indicative of a periodon­tally involved dentition with partly active inflammation (bleeding on probing in about one third of the teeth and about half of MI). In order to measure implant stability, the Osstell device with corresponding Smartpegs can be used in addition to the Periotest device. However, the values for MI are higher with the Periotest and lower with the Osstell compared to standard diameter implants; moreover, the values are also influenced by MI diameter, at least in the mandible. The questionnaire revealed that the vast majority of patients were very satisfied or satisfied with the prosthetic treatments.

Like any retrospective study, the present evaluation also has limitations that must be taken into account when interpreting the results. For instance, the initial periodontal situation was not known. Also, there were no regular X-ray controls. The distribution of the remaining teeth in the study jaws varied considerably and the number of additionally inserted MI was also variable, partly due to the limited vertical bone in dorsal jaw regions [8, 23, 24]. The study population was broadly diversified and it included patients with underlying diseases, smokers, or subjects displaying bruxism. Lastly, the retrospective patient‘s chart analysis showed that the attitude with respect to coming for dental check-ups varied considerably among the participants.

The latter aspects can also be considered a strength of the study because the results reflect the perfor­mance of MI, and their prosthetic treatment, under normal practice conditions without prior selection. The data were collected by a dentist with more than 20 years of professional experience, who had no experience with MI before his training prior to the beginning of the study. Further strengths of the study are the minimal (3 years) and mean (5.5 years) observation period, as studies of at least 5 years duration on MI are still rare [9, 23, 26, 27]. Despite the fundamental retrospective design, all implants were clinically examined and the current subjective satisfaction with the prosthetic restoration was determined using a validated measuring instrument [1].

The MI 5-year survival rate of 86.9 % in the mandible is lower than in previous studies on MI-supported overdentures for edentulism, where 2 to 5-year survival rates were 93–100 % [9, 12, 23, 24]. Possible reasons for this are: First, periodontal inflammation of the remaining teeth has been shown to negatively affect osseointegration and lead to implant loss or peri-implantitis [10]. Secondly, in the MI studies on edentulous mandibles, all patients had complete dentures in the maxilla; this is in contrast to the present study, which included 14 complete dentures, 20 RPDs and 6 fixed restorations in the maxilla. This could contribute to an overload of the MI during the healing phase. The high failure rate in the first 6 months after insertion in the lower jaw supports this assumption.

In contrast to prospective studies with MI rates of failure of up to over 30 % after 2–3 years in the edentulous maxilla [8, 14, 24], the survival probability in the present study was 97.4 % after 5 years with a total of 3 losses. In the prospective studies mentioned above, all MI were immediately loaded with the housings, regardless of bone quality or insertion torque. In the present study, 64 % of the maxillary RPDs were milled out above the ball attachments and relined with a soft material. The MIs with the housings were loaded only after 3 to 4 months; this mirrors another retrospective study where the MI survival rate in the edentulous maxilla was 94.3 % [20]. The Cox regression analysis to determine potential risks for implant failure in the mandible had too small of a sample size in the subgroups. The confidence intervals of the hazard ratios indicate the possible negative influence of smoking and initial soft relining or poor bone quality on implant survival.

The 5-year tooth loss rates of 12 % in the maxilla and 11 % in the mandible confirm the results of a similar study where the 6.5-year rate of loss of abutment teeth was 11 % with standard diameter implants and ball attachments as supplementary anchors for 6 telescopic dentures in the maxilla and 5 in the mandible [16]. However, none of the delayed loaded implants were lost in this study. Similar results are shown in 2 recent systematic reviews of combined tooth and implant-supported RPDs. The 1 to 10-year survival rates of implants were 92–100 % and those of teeth bearing clasps, ball anchors or double crowns as retaining ele­ments were 79–100 % [2]. The calculated 95 % confidence intervals were 97–100 % for implants and 85–98 % for teeth where exclu­sively double crowns on teeth and implants were used [15].

Among the prosthetic aftercare measures, 26 relinings from a total of 66 dentures with an average observation period of 5.5 years is comparable with the study mentioned above, in which conventional implants were delayed loaded following strategic placement under double crown dentures [16]. In this study, 6 of 11 dentures were relined. However, in contrast to the present study with only 8 O-silicone ring changes for retention improvement, all matrix inserts of the standard implants were adjusted multiple times, or in some cases, even replaced several times in the course of the 6.5 years. This can be explained by the different retention and wear mechanism of the 2 types of matrices. Conversely, the number of denture base, veneering and denture tooth repairs were comparable between the 2 studies and affected approximately half of the dentures. It can be assumed that the subsequent incorporation of the matrices into an existing denture can lead to denture base and framework weakening.

In the present study, subsequent to 37 tooth extractions and 17 MI losses, a total of 40 implants were placed in a number of 18 study participants; in many cases, the implants were placed at the same or another site with the aim of keeping strategically important positions for denture retention. On the one hand, this was again a surgical procedure. On the other hand, the patients were fa­miliar with this minimally invasive surgery with low postoperative morbidity [12, 14, 26] and for which the costs also remained manageable.

The clinical data indicate a patient population with prior periodontal disease of the remaining teeth and numerous active inflammations (bleeding on probing on more than one third of the teeth). Less than a quarter of the teeth were plaque-free and more than half showed maximum probing depths ≥ 3 mm. The fact that about half of the MI showed bleeding on probing should be interpreted with caution; this is because an injury to the mucosa can still be caused even by careful probing in healthy peri-implant mucosa [11].

The stability measurements of the MI yielded higher Periotest values (interquartile range 2–7) and lower ISQ values (30–43) through resonance frequency analysis than osseointegrated standard diameter implants (Periotest: < 1, ISQ: > 60) [22]. According to the manufacturer, these values would indicate insufficient osseointegration. The Periotest values are within the range given by Stepanovic et al. [25] as a mean value = 6 ± 6 for osseointegrated 1.8 mm thick MI in the edentulous mandible. In another study using 1.8 mm MI, however, a Periotest mean value of -3.7 was found [7]. In this latter study, it may be that the plunger of the Periotest device was not directed towards the center of the ball, but rather towards the square base, thus leading to reverse oscillations with a smaller amplitude.

The smaller Osstell values are comparable with the measurements of orthodontic MI (2 x 9 mm), which use a special axially screwed Smartpeg [21]. However, the values are about 30–40 % below the values obtained with identical MI and a similar Smartpeg prototype after insertion into the lower leg bones of rabbits [5]. The connection of this smartpeg to the implant appears to be more stable. Its attachment fits the insertion square of the MI perfectly and thus bridges the thin neck that carries the ball. This could explain the relatively high Osstell values of about 60, which were in the range of standard implants. The high scattering of values with a wide interquartile range of the Osstell measurements in the present study is due, among other things, to the occasional uncertain fixation of the Smartpegs by the lateral screw in deep inserted MI. Further studies are needed to validate the Osstell measurements with an optimized Smartpeg for MI with ball attachments.

The lower stability values of MI compared to conventional implants are probably due to the dimensional differences. This assumption is supported by the trend towards a higher stability of maxillary MI compared to mandibular ones, as the 2.4 mm MI were mainly used in the maxilla. In addition, the 1.8 mm MI showed higher Periotest and lower ISQ val­ues in the mandible than the 2.1 and 2.4 mm MI. In the upper jaw, only a total of 6 MI with a diameter of 1.8 mm were used.

Patient satisfaction with the prosthetic treatment was chosen as a subjective parameter. The predominantly very good to good values according to the grading system used in German schools in this study are consistent with those of longitudinal studies, where patient satisfaction according to similar criteria (general satisfaction, comfort, stability, hygiene, esthetics, chewing ability) increased noticeably after supporting free-end dentures with posterior implants [4]. In another study on jaws with few residual teeth, after strategic placement of standard diameter implants, not only the subjective chewing ability but also the objectively measured chewing efficiency was improved based on a test diet [31].

Conclusion

In light of the limitations of a retrospective investigation, the use of MI for subsequent stabilization of double crown-retained RPDs is a viable medium-term therapy option in a general dental practice setting. Apart from a few fracture repairs, the aftercare effort was low and no denture had to be renewed. In the event of tooth or implant loss, subsequent MI were frequently used. The stability values based on the Perio­test and resonance frequency analysis were lower for MI than for standard diameter implants. The vast majority of patients were very satisfied with the prosthetic treatments. Prospective randomized studies with MI used in this indication are required.

Conflicts of interest:

The first author uses the implant system in dentistry and receives fees for lectures and further training on mini-implants, including from the implant manufacturer. There are no conflicts of interest for the co-authors.

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Prof. Dr. Torsten Mundt

University Medicine Greifswald

Centre for Dental, Oral and Maxillofacial Medicine

Polyclinic for Dental Prosthetics, Geriatric Dentistry and Medical Materials Science

Walther-Rathenau-Str. 42a, D-17475 Greifswald

mundt@uni-greifswald.de

(Photo: T. Mundt)


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(State: 11.03.2020)

Latest Issue 2/2020

In Focus

  • Which “band-aid” is appropriate for the dentin wound of permanent teeth?
  • Reconstruction and masticatory rehabilitation of a bilateral maxillary defect
  • Follow-up examination of patients with mini-implants