Radiographic Evaluation of the Bone Remodeling on Tilted Osseointegrated Implants: Follow-up at 1 Year

INTRODUCTION

In dentistry, where new technologies are constantly applied leading to a continuous improvement in therapies,^1–9 osseointegrated implants, that represent one of the most evolving branch, are today widely used for partial or total rehabilitation of edentulous spaces.¹⁰

To obtain an optimal anchorage of the implant within the bone plate, the surgeon should take into account the anatomy of the maxillary and mandibular regions: in the maxillary arch, the presence of the maxillary sinuses can sometimes make implant placement difficult in the posterior sectors; in the mandibular arch, the presence of the alveolar neurovascular bundle can make distal implant placement impossible if the level of bone atrophy is high.

To overcome these problems, some procedures of regenerative dentistry have been described: maxillary sinus lift represents a solution that allow implant positioning in the maxillary arch; even in the mandibular arch, it is possible to carry out regenerative procedures, through the use of bone grafts. These procedures are widely used and described in scientific literature, but it is unavoidable that they lead to increase morbidity for the patients.

As an alternative, it is possible to place inclined implants, to obtain sufficient primary stability.

Oblique implants have been adopted to increase retention in residual bone and to avoid anatomical site at risk. Malò et al. and Daverio et al. in their papers pointed out the use of non-bicortical implants.^11,12

In case of excessive pressure, bone resorption has been evidenced over time: fibroblasts appear, and fibrous connective tissue is formed to replace bony tissue. This kind of process is probably the reason that explains cases of peri-implantitis due to masticatory overloading.¹³

Other possible complications include the possibility of irritation of the alveolar nerve that lead to problems of sensitivity and the possibility of penetration of the implant in the maxillary sinus.

Oblique bicortical or non-bicortical screw fractures are rare and regarded as fatigue fractures occurring time after insertion in the presence of sclerotic bone.¹⁴ Oblique implants do not have other noticeable complications.

PURPOSE OF RESEARCH

The aim of this study was to evaluate the degree of vertical peri-implant resorption that occurred around implants inserted both in the mandible and in the maxilla, with an inclination of more than 10°. The negative results were compared with those obtained by calculating the vertical bone resorption around straight implants, inclined less than 10°. In the same cohort of patients, we also evaluated implant survival at a distance of at least 1 year from the insertion of the fixtures.

MATERIALS AND METHODS

For the realization of the study, a sample consisting of 47 patients for a total of 115 implants was selected.

COLLECTION OF RADIOGRAPHIC DATA

Radiographic examination was performed in two postoperative timings:

• T0 corresponding to an initial radiographic check, before prosthetic loading of the implant;
• T1 corresponding to a later postoperative control in a period ranging from 12 months to 24 months after prosthetic loading.

Digital panoramic radiographs have been acquired with a specific image analysis software (Sidexis by Sirona®); to standardize the analyses for each group of patients, vertical measurements of peri-implant bone were all performed using as reference the occlusal plane meaning tracing a line that touched the cusps of the dental elements in the same arch where the inclined implants were located. The analyzed images were all performed with the same orthopantomograph, and using a dedicated software ensured that the patient was positioned with the Frankfurt plane parallel to the floor using the individualized positioner to minimize image distortions and avoid dimensional alterations of the generated images.

The insertion implant angle was then calculated by measuring the angle between the greater axis of the implant and the perpendicular to the occlusal plane.

To minimize the acquisition of distorted measurements, each measurement was detected along an axis parallel to the greater axis of the implant, tangentially to the mesial and distal margins of the implant at T0, before the prosthetic loading.

The measurements thus obtained were then compared with those obtained at T1, with a minimum time of 12 months after implant loading.

In this study, all the implants with inclination equal to or greater than 10° were took into consideration; implants with inclination between 0 and 10 were used as control sample.

The following groups have been identified:

• Implants not inclined: inclination between 0° and 10°;
• Implants with inclination between 10° and 20°;
• Implants with inclination between 20° and 30°;
• Implants with inclination greater than or equal to 30°.

The data thus obtained were classified in Table 1.

STATISTICAL ANALYSIS

The data collected were analyzed using the software Prism 6.0 (GraphPad Software, Inc., La Jolla, CA, USA). The obtained evaluations have been divided into four different groups of study:

1. Group I: Inclination between 0° and 10° (Tables 2 to 4)
2. Group II: Inclination included between 10° and 20° (Tables 5 to 7)
3. Group III: Inclination included between 20° and 30° (Tables 8 to 10)
4. Group IV: Inclination above 30° (Tables 11 to 13)

RESULTS

As shown in the previous tables on the different groups of patients, the “Column statistic of paired t test” allowed us to compare mesial bone level at T0 with the mesial bone level at T1 and, in the same way, the distal bone level in both times.

In all four groups as the normal test was passed, we could perform the “Paired t test of paired t test” that allowed us to obtain a p value and evaluate if there was a statistically significant difference in the degree of bone resorption on implants with increased inclination, compared with those with lower inclination.

In group I, we can conclude that there is a statistically significant difference in at least one of the two sides (distal one) between T0 and T1. In groups II and III, there is a statistically significant difference in the degree of bone resorption on the mesial side with respect to distal side.

In group IV, taking into account the lack of the sample, we can state that there is a statistically significant difference on both sides.

Implant survival at a distance of 1 year was equal to 100% of cases. No neurological complications occurred at the site of the implants; in only eight patients, we noticed the presence of a copious bleeding, which caused the formation of a hematoma remained evident for 10 days (probably because five of these eight patients regularly took antiplatelet therapy).

DISCUSSION

Modern implantology allows to rehabilitate even extreme situations that a few decades ago were impossible to treat with predictability, improving patient’s quality of life.

Necessary condition to obtain success in implantology is sufficient bone structure to guarantee primary and secondary stability of the implants.

Furthermore, with the use of regenerative techniques, patients had the possibility to rehabilitate their dentition using fixed implant-supported prostheses.

In recent years, a technique called “All on four” introduced the possibility to rehabilitate completely edentulous jaws thanks to the insertion of four implants in which the two are distally positioned with an inclination of more than 30°.¹¹

This new technique stimulated numerous studies on the positioning and clinical success of inclined implants.

The question whether tilted implants are more at risk of failure than axially placed implants is receiving increasing attention in the last years, and a periodic review of the different concepts is necessary to refine techniques and eliminate unnecessary procedures.

We tried to examine whether increased inclination could create bone resorption higher than what was highlighted around implants positioned with inclinations less than 30°.

Chrcanovic et al. suggested that the differences in angulations of dental implants might not affect implant survival neither reduce marginal bone levels.¹⁵

Del Fabbro et al. found no significant difference in failure rate between tilted and upright implants, both maxillary and mandibular implants. No implant-supported prosthesis failure was reported. Limited bone loss around the fixtures was reported with no difference between upright and tilted implants. In three studies examined, based on the questionnaires, a full satisfaction for function, phonetics, and esthetics was reported. For these reasons, they stated that the use of tilted implants to support immediately loaded fixed prostheses for the rehabilitation of edentulous arches can be considered a predictable technique, with an excellent prognosis in the short-medium term even if they recommended randomized long-term trials to better state the efficacy of this surgical approach.¹⁶

Menini et al. in the same year stated that the outcomes of upright and tilted implants supporting full-arch fixed dentures for the immediate rehabilitation of edentulous maxillae, after at least 1 year of function in an electronic search of 1,069 articles, showed no significant difference in failure (p value = 0.52).

A nonsignificant difference between tilted and upright implants was found with regard to bone loss, and tilted implants demonstrated a favorable short-term prognosis in full-arch immediate loading rehabilitations of the maxillae. Also, these authors observed that randomized long-term trials are needed to better explain long-term success of tilted vs upright-positioned implants.¹⁷

Similarly, Francetti et al. found that no implant failures were recorded to date, leading to a cumulative implant survival and prosthesis success rate of 100%. Plaque level and bleeding scores showed a progressive decrease over time during the study, with simultaneous increase of satisfaction for both esthetic and function in the majority of patients. No significant difference in marginal bone loss was found between tilted and axial implants at 1-year follow-up. The authors concluded that tilted implants result a viable treatment modality for the mandible.¹⁸

Bellini et al. on the other way, with an experimental study, stated that tilted configurations of osseointegrated implants showed a lower absolute value of compressive stress compared with non-tilted implants, indicating a possible biomechanical advantage in reducing stresses at the bone–implant interface.¹⁹

Our research, similarly and from a clinical perspective, has shown that there is a statistically significant difference in the degree of bone resorption for implants with inclination of more than 30° as assessed from the analysis of the previous tables, even if it is still necessary to take into account the lack of the sample analyzed in the most inclined implants.

CONCLUSION

Fixed rehabilitation technique using osseointegrated implants is a well-established practice.

As already described, some anatomical conditions make necessary to insert inclined implants in areas where bone or peripheral nerve structures do not allow the insertion of an upright implant with a suitable size to guarantee both primary and secondary stabilities. During the planning phase, surgeons must take into account the possibility that this increase in inclination may lead to a more rapid resorption of bone mesially or distally to the implant.

As previously described, the extent of resorption is correlated to the degree of inclination during implant insertion.

CLINICAL SIGNIFICANCES

Previous articles many times found that no significant influence in overall implant survival exists, but an important factor to take into account is the direction of the prosthetic load transmitted to the fixture as this variable could influence the degree of resorption of both mesial and distal bone structure according to both laboratory and clinical data.

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