Bone level changes at axial- and non-axial-positioned

Theofilos Koutouzis Jan L. Wennstro¨m

Bone level changes at axial- and nonaxial-positioned implants supporting fixed partial dentures A 5-year retrospective longitudinal study

Authors’ affiliation: Theofilos Koutouzis, Jan L. Wennstro¨m, Department of Periodontology, Institute of Odontology, The Sahlgrenska Academy at Go¨teborg University, Go¨teborg, Sweden Correspondence to: Jan L. Wennstro¨m Department of Periodontology Institute of Odontology The Sahlgrenska Academy at Go¨teborg University Box 450 SE 405 30 Go¨teborg Sweden Tel.: þ 46 31 7733189 Fax: þ 46 31 7733791 e-mail: [email protected]

Key words: bone loss, fixed partial dentures, loading, oral implants Abstract Aim: The aim of this study was to retrospectively analyze the potential influence of implant inclination on marginal bone loss at freestanding, implant-supported, fixed partial dentures (FPDs) over a 5-year period of functional loading. Material and methods: The material comprised 38 periodontally treated, partially dentate patients with a total of 42 free-standing FPDs supported by implants of the Astra Tech s

System . Mesio-distal inclination of the implants in relation to a vertical axis perpendicular to the occlusal plane was measured with a protractor on standardized photographs of the master cast. The two tail quartiles of the distribution of the implants with regard to the implant inclination were defined as axial-positioned (mean 2.41; range 0–41) and non-axialpositioned implants (mean 17.11; range 11–301), respectively. For FPDs supported by two implants, both the mesial–distal and buccal–lingual inter-implant inclination was measured. The primary outcome variable was change in peri-implant bone level from the time of FPD placement to the 5-year follow-up examination. Comparison between axial- and non-axialpositioned implants was performed by the use of a Mann–Whitney U-test. Spearman’s correlation analysis was used to analyze relationships between inter-implant inclination (mesial–distal and buccal–lingual) and 5-year bone level change on the FPD level. Results: The 5-year mean bone level change amounted to 0.4 mm (SD 0.97) for the axial and 0.5 mm (0.95) for non-axial-positioned implants (P40.05). For the FPDs supported by two implants, the mean inter-implant inclination was 9.21 (1–361) in the mesial–distal direction and 6.71 (0–241) in the buccal–lingual direction. Correlation analysis revealed lack of statistically significant correlation between inter-implant inclination (mesial–distal and buccal–lingual) and 5-year bone level change (r ¼ 0.19 and r ¼ 0.32, respectively). Conclusion: The study failed to support the hypothesis that implant inclination has an effect on peri-implant bone loss.

Date: Accepted 10 August 2006 To cite this article: Koutouzis T, Wennstro¨m JL. Bone level changes at axialand non-axial-positioned implants supporting fixed partial dentures. A 5-year retrospective longitudinal study. Clin. Oral Impl. Res. 18, 2007; 585–590 doi: 10.1111/j.1600-0501.2007.01386.x

Based on clinical observations it has been suggested that there is a positive relationship between excessive loading and periimplant bone loss (Lindquist et al. 1988; Quirynen et al. 1992; Rangert et al. 1995). Laboratory tests utilizing finite element analysis demonstrated that, when applying lateral or oblique loads, the highest stress concentration occurs at the marginal part of

c 2007 The Authors. Journal compilation c 2007 Blackwell Munksgaard

the implant (Borchers & Reichart 1983; Clelland et al. 1993, 1995; Papavasiliou et al. 1996; Holmes & Loftus 1997; Kitamura et al. 2004). With regard to the biological effects of such stress concentrations, however, animal experiments revealed conflicting results. In a dog study, Barbier & Schepers (1997) studied the effect of axial and non-axial loading condi-

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Koutouzis & Wennstro¨m . Axial- vs. non-axial-positioned implants

tions induced by either a bilaterally supported fixed partial dentures (FPDs) or a cantilever FPD on two implants. Based on the histological observations that non-axial loading for 7 weeks gave rise to a more dynamic remodeling of the surrounding cortical and trabecular bone tissue, the authors extrapolated that a longer experimental period of loading could have resulted in marginal bone loss. In fact, using a rabbit tibia model, Duyck et al. (2001) reported that dynamic excessive loads perpendicular to the implant axis caused crater-like bone loss around the marginal part of the implant. The hypothesis that excessive dynamic load can trigger bone resorption through the induction of micro-damage in the bone may also be supported by observations made by Isidor (1996, 1997), demonstrating that excessive occlusal load in a lateral direction caused implant failure due to loss of osseointegration in five out of eight implants during an 18-month observation period. However, it was also observed that the bone crest remained at a position close to the margin of the implant without signs of triangular-shaped bone craters. The latter observation corroborates findings made in several other experimental studies (Hu¨rzeler et al. 1998; Miyata et al. 1998; Gotfredsen et al. 2001a, 2001b, 2001c, 2002; Heitz-Mayfield et al. 2004) showing no detrimental effect on the marginal bone following excessive loading. In the most recent of these studies (Heitz-Mayfield et al. 2004), the effect of excessive oblique load on osseointegration of implants placed in beagle dogs were examined. Single crowns in supraocclusion with the opposing maxillary teeth were connected in one side of the mandible, while no crowns were placed on the implants in the contra-lateral side. The authors reported that no differences were found in clinical, radiographic or histological parameters between implants in supra-occlusion and unloaded controls after the 8-month experimental period. Clinical trials designed to evaluate the potential influence of oblique loading direction in relation to the implant axis on periimplant bone stability are comparatively few. Aparicio et al. (2001) reported data derived from examinations of 29 maxillary FPDs in 25 patients supported by 101

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Clin. Oral Impl. Res. 18, 2007 / 585–590

Bra˚nemark implants, 59 inserted in an axial and 42 in a tilted direction. No significant difference in marginal bone level change between tilted and axial positioned implants could be observed over the 5 years of follow-up, a finding that in large is supported by observations made by Krekmanov et al. (2000) and Calandriello & Tomatis (2005). Balshi et al. (1997) evaluated in a 3-year study the performance of angulated abutments used to compensate for a non-ideal implant inclination. The data reported indicated no increase in failure rates with the use of angulated abutments. The aim of the present clinical study, involving periodontally compromised patients, was to retrospectively analyze the potential influence of implant inclination on marginal bone loss at freestanding, implant-supported, FPDs over a 5-year period of functional loading.

Material and methods The patient material evaluated in the present study originated from a longitudinal prospective study of implant treatment in periodontally compromised patients (Wennstro¨m et al. 2004). The patients, who exhibited advanced chronic periodontitis, had received comprehensive periodontal treatment of the remaining dentition before the implant placement, and were after completion of the restorative therapy maintained on an individually designed supportive care program. The original sample included 51 partially dentate patients with a total of 56 freestanding FPDs supported by Astra implants (Astra s Tech Dental Implant System, Mo¨lndal, Sweden). Based on the inclusion criteria of 5 years of follow-up, 45 patients (50 FPDs) were available for the present study. Three FPDs had been lost due to implant fractures, while three FPDs belonged to patients who were lost to the 5-year followup examination (for details see Wennstro¨m et al. 2004). Additional seven patients (eight FPDs) had to be omitted as the original master casts were not available for implant inclination measurements. Thus, the final patient sample to be included in the present analysis comprised a total of 38 patients with 42 FPDs (111 implants). The characteristics of the patient material are given in Table 1.

Table 1. Characteristics sample Number of patients Gender (male/female) Smokers Mean age (years) Mean number of remaining teeth

of

the

patient

38 20/18 10 59.5 (9.7) 18.5 (3.9)

Mean value (SD).

Out of the 42 FPDs included in the analysis, 24 (57%) were placed in the maxilla. Fifteen FPDs were supported by two implants and 27 FPDs were placed on three implants. Twenty-two (52%) out of the 42 FPDs were designed with a cantilever extension. All implants used had a diameter of 3.5 mm while the length varied between s 8 and 19 mm. Standard, Uni-abutments s (Astra Tech Dental Implant System; Astra Tech AB, Mo¨lndal, Sweden) of varying length were used. The prosthetic treatment was performed in accord with the manual provided by the manufacturer. All FPDs were screw retained and delivered about 4 weeks after abutment connection. Baseline for the present study was considered to be the time of FPD installation. The original study protocol was approved by the ethics committee at the Sahlgrenska Academy, Go¨teborg University, and informed consent was obtained from all subjects.

Implant inclination measurements

Standardized photographs were taken of the original master casts with a digital camera (Nikon, Coolpix 4500; Nikon Corp., Tokyo, Japan). The segment of the jaw where implant installation had been performed was photographed with the upper and lower casts in occlusion (Fig. 1). A second photograph of the cast carrying the implants, and with guide pins abutment pick-up in place, was taken with identical projection geometry. By means of a computer program (Adobe Photoshop 6.0; Adobe Systems Inc., San Jose, CA, USA), the second image was then trimmed to only show the cast and the guide pins and superimposed with precision on the image of the two casts in occlusion. Thus, a final image of the two casts in occlusion with the guide pins revealing the inclination of the implants in relation to the occlusal plane was



Fig. 1. Illustrations describing the photographic process for performing implant inclination measurements. (a) Upper and lower casts in occlusion, (b) upper cast with guide pins abutment pick-up in place and (c) the final image produced by superimposing image (b) on image (a).

Comparison between axial- and non-axialpositioned implants was performed by the use of the Mann–Whitney U-test. Spearman correlation analysis was carried out on FPD level with respect to inter-implant inclination (mesial–distal and buccal–lingual) and 5-year bone level changes. In all analyses a P-value of o0.05 was considered as being statistically significant. Fig. 2. Photographic illustrations of the inter-implant inclination measurements in mesio-distal and buccolingual directions.

Results obtained. The computer-generated images were printed and the inclination in the mesial–distal direction of each individual implant, in relation to a vertical axis perpendicular to the occlusal plane, was measured with a protractor. For cases with an FPD supported by two implants, an additional photograph of the cast with the guide pins in place was taken in a transversal direction (Fig. 2). Assessments of the inter-implant inclination in both mesial–distal and buccal–lingual directions were performed. The methodological error of the whole recording procedure as well as the interexaminer reproducibility for inclination assessments was determined. The process from taking photographs to implant inclination measurements was repeated for five randomly selected cases. The mean difference between duplicate recordings was 0.151 (SD 1.34). Inter-examiner reproducibility of the implant inclination measurements was determined through double assessments of one randomly selected implant from each patient. The mean difference between the two readings was 0.071 (SD 0.85). Radiographic examination

Radiographs of all implant sites, obtained by the use of a standardized parallel long-

cone technique and custom-made stents, were taken at the time of insertion of the FPDs and at the 5-year follow-up examination (Department of Oral & Maxillofacial Radiology, Go¨teborg University). In the radiographs, the location of the marginal bone level – in relation to the marginal edge of the fixture – was assessed by the use of a magnifying lens ( 7) to the nearest 0.1 mm at the mesial and distal aspects of the implant. Two radiologists who were unaware of the purpose of the study performed all bone level assessments. The error of the radiographic assessment was determined through double recordings at one randomly selected implant from each patient representing the 5-year follow-up examination. The mean difference between the two readings was 0.04 mm (SD 0.33). Data analysis

The primary outcome variable was the change in peri-implant bone level from the time of FPD placement to the 5-year follow-up examination. For data description, the two end quartiles of the distribution of the implants with regard to the inclination in the mesial–distal direction were chosen to represent axial-positioned implants and nonaxial-positioned implants, respectively.


The frequency distribution of implants with regard to the inclination in relation to the occlusal plane is given in Fig. 3. The implant inclination in the mesial–distal direction showed a skewed distribution with a mean value of 8.41 (median 61, range 0–301). The tail quartiles of the distribution defined as axial-positioned implants showed a mean angulation of 2.41 (range 0–41), while the mean value for nonaxial-positioned implants was 17.11 (range 11–301). Twenty-two of the axial-positioned implants were placed in the maxilla and 14 in the mandible, whereas for the non-axial-positioned implants the corresponding numbers were 18 and 15 (Table 2). The mean length of the axialpositioned implants was 12.8 mm and of the non-axial-positioned implants 12.7 mm (P40.05). The mean bone loss during the 5 years in function (Fig. 4) was for the axial- and non-axial-positioned implants 0.4 mm (SD 0.97) and 0.5 mm (0.95), respectively. Thirty-nine percent of the axial-positioned implants demonstrated no bone loss after 5 years in function, compared with 37% of the axial-positioned implants. The prevalence of implants that had experienced 1 mm of peri-implant bone loss was 30% for the axial-positioned and 33% for

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Table 3. Spearman’s correlation analysis with respect to inter-implant inclination (mesial–distal and buccal–lingual) and 5year bone level changes on FPD level (n ¼ 17)

Distribution of implants based on mesialdistal inclination 16 Axial

14

Coefficient

Non-axial

10

Inter-implant inclination Mesial–distal 0.19 Buccal–lingual 0.32

8

FPD, fixed partial dentures.

12

P-value 0.46 0.22

6 4 2 0 0

4

8

12

16

20

24

28

Mesial-distal implant inclination Fig. 3. Distribution of implants according to mesial–distal inclination. The two tail quartiles were selected to represent axial-positioned implants (n ¼ 36; mean 2.41, range 0–41) and non-axial-positioned implants (n ¼ 34; mean 17.11, range 11–301).

Table 2. Characteristics of the axial and non-axial positioned implants

No. of implants Mean mesial–distal inclination Mean implant length Jaw (maxilla/mandible)

Axial-positioned implants

Non-axial-positioned implants

36 2.41 (1.2) 12.8 (2.7) 22/14

33 17.11 (5.7) 12.7 (2.8) 18/15

positioned implant did not reveal any significant difference compared with the contralateral implant site or the axial-positioned implants; mean bone loss 0.4 mm (1.14). The inter-implant inclination for the FPDs supported by two implants varied between 11 and 361 (mean 7.41, SD 8.8) in mesial–distal direction and between 01 and 241 (mean 6.91, SD 7.3) in buccal– lingual direction. No significant correlations were found between inter-implant inclination (mesial–distal or buccal–lingual) and 5-year bone level changes (FPD level; Table 3).

Discussion

Mean value (SD).

Bone level change - 5 years 100

80

Axial positioned Mean – 0.4 (0.97)

60

Non-axial positioned Mean – 0.5 (0.95)

40

20

0 –4

–3

–2

–1

0

1

2

Bone level change (mm) Fig. 4. Cumulative percentage of axial- and non-axial-positioned implants according to the peri-implant bone level change during 5 years. Mean value (SD).

the non-axial-positioned implants. No statistically significant differences in marginal bone change were found between axial-

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and non-axial-positioned implants. Furthermore, analysis of the bone loss at the site facing the inclination of the non-axial-

The results of the present study failed to demonstrate that implant inclination significantly influences peri-implant bone loss over a 5-year period of functional loading of FPDs in patients who maintain a high standard of oral hygiene. A factor shown to significantly influence the degree of peri-implant bone loss over time is the level of bacterial biofilm control. Lindquist et al. (1988, 1997) demonstrated that patients with poor oral hygiene showed a significantly increased risk for peri-implant bone loss in comparison with subjects with a high standard of plaque control. Further, data in the literature indicate that periodontally compromised patients demonstrate a higher rate of peri-implant bone loss than patients without experience of periodontitis (Hardt et al. 2002; Karoussis et al. 2003). The patients included in the current study, who all were periodontally compromised, maintained a high level of infection control throughout the 5 years of follow-up, as demonstrated by low plaque and mucositis scores (5%) (Wennstro¨m et al. 2004). Hence, the inter-



pretation that the bone loss recorded at 5 years, which as a mean amounted to 0.4 mm, may be attributed to other factors than inflammatory lesions due to bacterial biofilm formation seems reasonable. In the interpretation of the results of the present study one should recall that the classification of axial- and non-axialpositioned implants was based on the assessment of the inclination in only the mesial–distal direction. Inclination in buccal–lingual direction might be of equal importance but was not included because of difficulties experienced in defining the occlusal plane in a transversal direction. With respect to the inter-implant inclination for the FPDs supported by two implants, however, both the mesial–distal and the buccal–lingual inclinations were determined and analyzed in relation to the 5-year bone level change on the FPD level. The reason for selecting only the FPDs supported by two implants in this analysis was that it is more plausible from a biomechanical point of view that inter-implant inclination can have an effect on loading conditions of such FPDs compared with FPDs supported by three or more implants (Rangert 1995). Yet the results revealed lack of statistically significant correlation between inter-implant inclination (mesial–distal and buccal–lingual) and the 5-year bone level change. Neither the analysis performed on the implant level showed any statistically significant difference in bone level change between axial- and non-axial-positioned implants, even though only the tail quartiles of the distribution of the implants in relation to inclination against the occlusal plan were compared. The fact that implant inclination was at maximum 301 might, however, be a factor to consider in the interpretation of the results. Further, all FPDs were carefully designed with respect to occlusal loading in order to minimize the risk for excessive loading, particularly for FPDs including a posterior cantilever. Whether loading directions in relation to the implant axis may influence peri-implant bone stability were also addressed in clinical studies by Krekmanov et al. (2000), Aparicio et al. (2001) and Calandriello & Tomatis (2005), in which implants with a more extreme angulated position than in

the current study were included. The authors reported that implants installed in an axial and a tilted direction showed after 1–5 years of loading no significant differences in bone loss. Taken together the results suggest that, under functional loading conditions, non-axial-positioned implants incorporated in FPDs may not face a greater risk for marginal bone loss than axial-positioned implants. However, one may not extrapolate the findings to single-implant replacements because loading conditions may be different for such implants compared with implants supporting FPDs. Peri-implant bone reactions as a consequence of excessive non-axial loading have been studied in animal experiments (e.g. Isidor 1996, 1997; Barbier & Schepers 1997; Hu¨rzeler et al. 1998; Miyata et al. 1998, 2000; Duyck et al. 2001; Gotfredsen et al. 2001a, 2001b, 2001c, 2002; HeitzMayfield et al. 2004). However, these studies were designed to evaluate excessive loading conditions that may not be comparable with loading conditions during normal function in humans. Notwithstanding this fact, in most of the studies referred to, the loading did not induce marginal bone destruction. Besides loading, several other factors have been suggested to contribute to an increased rate of peri-implant bone loss, e.g. smoking (Weyant & Burt 1993; Lindquist et al. 1997; for a review see Bain 2003), jaw of treatment (Jemt & Lekholm 1993; Naert et al. 2001), implant and abutment length (Naert et al. 2001), and type of prosthetic material used and type of antagonists (Naert et al. 1992, 2001). In a previous publication (Wennstro¨m et al. 2004) on the patient material utilized in the current analysis, it was shown by multivariate analysis that the variables smoking and jaw of treatment had a significant influence on the peri-implant bone level change on the FPD level. However, in view of the fact that the bivariate analyses performed in the current study failed to demonstrate a significant relationship between implant inclination in the mesial– distal direction as well as inter-implant inclination (both mesial–distal and buccal–lingual) and peri-implant bone level change, it was not considered worthwhile to further evaluate these factors in a multivariate model.


As reported in a previous publication on the patient material utilized in the present study (Wennstro¨m et al. 2004), the technical complications experienced over the 5-years of follow-up were three implant fractures, three cases with crown-screw loosening and three cases with minor porcelain fractures. The retrospective analysis of these particular cases revealed no increased incidence of technical complications associated with tilted implants. In fact, only two of the nine FPDs affected by a technical complication involved an implant tilted 4101 (one implant fracture and one screw loosening). In conclusion, the findings of the current 5-year study involving moderately tilted implants ( 301), as well as those reported by others who have clinically investigated the influence of more extreme non-axial loading on peri-implant bone level stability at implants of different design and surface texture (Balshi et al. 1997; Krekmanov et al. 2000; Aparicio et al. 2001; Calandriello & Tomatis 2005), indicate that a tilted position of the implant does not render an increased risk for bone loss during functional loading.

Acknowledgements: The authors want to thank Drs Annika Ekestubbe and Kerstin Gro¨ndahl, Department of Maxillofacial Radiology, The Sahlgrenska Academy at Go¨teborg University, for their invaluable contribution to this study.

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