Can Implant-Retained Fixed Prostheses Trigger Bone Response in the Posterior Region of Edentulous Mandibles? A 32-Month Cone-Beam Computerized Tomography Study Analyzing Bone Height and Density

Totally edentulous individuals rehabilitated with conventional complete dentures (CD) often report adaptation-related difficulties, mainly due to the lack of retention and stability of the prostheses.¹  Retention and stability issues with CD are aggravated by bone resorption, as the lack of functional mechanical stimuli after losing natural teeth results in bone remodeling and reabsorption of the residual ridge.^2–4  Continuous use of CD further aggravates this situation, since the remaining bone does not receive enough mechanical stimulus to decrease, stabilize, or even reverse bone resorption. Some studies have shown that prolonged use of CD (15–25 years) results in excessive mandibular bone resorption.^5,6  Over time, this often results in severe bone atrophy, especially in the mandibular region.^7,8 

Rehabilitation with implants is one of the primary treatment options for rehabilitation of totally edentulous patients.⁹  The McGill Consensus has adopted overdentures retained by 2 mandibular implants¹⁰  as the minimum protocol for rehabilitation of totally edentulous individuals, and treatment with an implant-supported full-arch fixed prostheses retained by 4 or 5 implants is also recommended.¹¹  Both types of rehabilitation are capable of exerting more adequate and efficient mechanical stimulus on the bone tissue than CD. The presence of implants can potentially revert the residual ridge reabsorption, thus contributing to a greater preservation of the residual bone ridge.^12,13  Wright et al¹¹  used panoramic X rays to compare posterior mandibular bone resorption around implant-retained mandibular prostheses with resorption around overdentures. They found that posterior bone resorption rates around overdentures stabilized while annual rates of bone gain of 1.6% were recorded around implant-retained mandibular prostheses. Reddy et al.¹⁴  observed that implant-retained mandibular prostheses promoted a residual bone growth of approximately 17% during the first year of function, followed by stability in the following 3 years. The authors found that the use of implants to avoid posterior bone loss seemed to be more beneficial in individuals with severe mandibular atrophy due to the higher bone height increase when compared with individuals with moderate atrophy.

Bone density changes in the posterior region have also been studied. Ichikawa et al¹⁵  analyzed radiographs up to 2 years after implant installation and found an improvement in bone density and bone formation. The use of implants guarantees remarkable preservation of the residual alveolar ridge, ranging from ridge preservation to bone apposition.^12,16–21  However, the extent of this preservation is still unknown.

Although panoramic radiographs have been used to study these parameters,^11,14,22  many studies have demonstrated the difficulty of standardizing these radiographs to measure qualitative and quantitative changes in bone tissue after rehabilitation with implant-supported full-arch fixed prostheses. Georgescu et al¹⁸  carried out studies comparing concomitant cone-beam computerized tomography (CBCT) with panoramic radiographs as effective methods for quantitative and qualitative evaluation of the mandible region and found that the CBCT measurements are more accurate than those provided by radiographs and recommended that the density assessment should be analyzed only by CBCT. These authors also concluded that the measurements taken by the panoramic radiographs are overestimated by 6 to 7 mm when compared with the results obtained by CBCT. However, studies^22,23  using concomitant CBCT scans for this purpose are still rare in the literature. Only 1 short-term clinical study¹⁸  has analyzed changes in bone density through concomitant CBCT.

Thus, the objective of this study was to evaluate the long-term behavior of bone tissue in the posterior region of edentulous mandibles rehabilitated with implant-retained fixed prostheses using CBCT.

This longitudinal prospective clinical study was carried out at the ILAPEO Dental Faculty after approval by the Ethics Committee of Pontifical Catholic University of Paraná (PUCPR) (protocol number 5975). The selection process for the volunteers was based on the recruitment of edentulous patients rehabilitated from a previous study¹³ ; the individuals had to be in good general health and available for the scheduled prosthetic maintenance appointments. Exclusion criteria were having diabetes or immunologic conditions, smoking or user of bisphosphonated drugs, or radiotherapy treatment. The sample size calculation was based on the study of Wright et al,¹¹  considering the mean bone height difference between the evaluation periods (mean difference: 2.9; variance: 1.5–4.5, power: 80%, confidence interval = 95%). According to these parameters, the inclusion of 12 individuals was necessary. The sample size was increased by 15% to compensate for potential losses, so that 14 individuals were required. Patients attending ILAPEO who met the selection criteria were invited to join the study, and those who agreed to participate signed an informed consent form. Thus, 14 individuals were rehabilitated by a specialized dentist with mandibular implant-retained fixed prostheses with immediate occlusal loading and torques of at least 45 Ncm. As described in the original follow-up study by Alcântara et al,¹³  all prostheses were installed within 72 hours after implant placement, using a conventional rigid bar framework splinting or a semirigid cantilever extension system with titanium bars placed in the distal abutment cylinders. The prosthesis followed a cantilever extension from 15 to 20 mm. The prostheses were fabricated with acrylic resin teeth. Five external hexagon implants (Neodent) positioned between the mental foramina were installed by an experienced surgeon. In the maxilla, all patients received CD. The prostheses were installed in bilaterally balanced occlusion.

To measure bone dimensions and bone density, CBCT scans were performed immediately after implant installation and at 8, 22, and 32 months postoperatively (T0, T8, T22, and T32, respectively). All images were obtained in a standardized way using the same instrument (Galileos CBCT scanner) by the same trained and calibrated operator, following the manufacturer's recommendations. The acquisition parameters for the CBCT scans were 14 seconds of acquisition, field of view of 15 × 15 × 15 cm³, 42 mA, high contrast, 85 kV, and 0.3-mm voxels. The detector was an image intensifier type, with 12-bit dynamics (4096 gray scale). The acquisition was performed in a standardized way: the patient's head was positioned with the occlusal plane parallel to the ground and the median sagittal plane perpendicular to the ground, while keeping the cephalostat configurations constant. The patients were always wearing their prostheses in the occlusion position during acquisition (Figure 1).

Measurements on the CBCT image were performed using the software Galaxis version 1.7 (Sirona), and the postprocessing parameters used were 0.3-mm cutoff thickness, spaced 0.3-mm apart. First, bone height in the posterior region of the distal implants was measured linearly. After a standard adjustment in the “oblique” cut window, a reference frame was drawn for the linear measurements based on the longitudinal implant axis and the lowest central point of the internal implant screw: the vertical half-line parallels the long axis of the implant and the horizontal half-line perpendicular to the implant axis. From the center of the implant, the 5-, 10-, and 15-mm reference lines were drawn to obtain the linear bone measurements. From each reference point, a line parallel to the vertical half-line was traced to the bone crest, resulting in the linear measurements L1, L2, and L3 (Figure 2).

Bone density was measured using the tool “visualize gray value,” resulting in gray-scale measurements, from 0 to 4096 gray tones (maximum hyperdensity). The circle of the tool (region of interest = 1.5 mm) was positioned with its upper edge at the highest point of the line (of L1, L2, and L3 measurements), resulting in density values (D1, D2, and D3, respectively) (Figure 3). All steps to obtain linear measurements (L1, L2, and L3) and density (D1, D2, and D3) of the distal implant region on the tomographic images of the 14 patients at time T0 (initial), T22 (22 months), and T32 (32 months) were performed by a trained and calibrated examiner (F.N.G.K.F.). Of the 14 rehabilitated patients, 3 could not be analyzed at the 32-month follow-up (T32) due to health issues.

SPSS 22 software (IBM SPSS Statistics 22) was used to process data. Data were assessed for normality using the asymmetry and kurtosis coefficients, followed by the Kolmogorov-Smirnov test. The repeated-measures analysis of variance (ANOVA) test was used to compare bone height and density registered at each linear distance from the distal implants (5, 10, and 15 mm) at each follow-up period (T0, T8, T22, and T32). The post hoc Bonferroni test was used to compare each outcome variable at each position from the implant in each follow-up period. The level of significance was set at α = .05 for all tests. The statistical analysis was reviewed by an independent statistician (W.J.S.).

The sample consisted of 3 male and 11 female patients. The patients' age ranged from 53 to 80 years, with a mean age of 66.5 years.

Table 1 shows the results of repeated-measures ANOVA, and Table 2 lists the means (±standard deviations) for bone height and density in the posterior region. Table 3 shows that there was a statistically significant mean difference (Δ) of 0.44 mm in bone height between T0 and T8 (P ≤ .005), Δ = 0.55 mm between T0 and T22 (P ≤ .005), and Δ = 0.66 mm between T0 and T32 (P = .05). At T32, the mean bone height increased by 8.85% relative to T0. Most of this height increase (5.77%) occurred after 8 months. Bone height differences were also observed in the posterior regions: (1) 0.81 mm between the 5-mm and 10-mm regions (P ≤ .005), (2) 1.97 mm between the 5-mm and 15-mm regions (P ≤ .005), and (3) 1.15 mm between the 10-mm and 15-mm regions (P ≤ .005). A bone growth difference of 28.42% was measured between the 5-mm and 15-mm positions at 32 months of follow-up. There was a significant effect of time and distance on bone density (Table 1), and statistical differences were found only between T0 and T22 (P = 0.03), during which a 5.76% mean bone density increase was observed.

Several studies have attempted to understand the behavior of bone tissue as a function of mechanical stimuli provided by different types of prosthetic rehabilitation.^11–14  The present study observed a significant increase in bone height in the posterior region during 32 months of follow-up, while the observed increases in bone density were not significant, except at T22. In general, CBCT scan images provide more reliable results than panoramic radiographs, which are prone to bone height overestimation.

The mean gain in bone height observed after 8 months was 5.77%, and this increased to 8.85% after 32 months. Although the clinical impact is small on average, bone changes were clinically observed in half our sample population, and in some cases, prothesis readjustments below the cantilever region were required to reestablish comfort during chewing and facilitate access for hygiene maintenance. Nakai et al⁴  and Reddy et al¹⁴  also observed bone gain in the posterior region of mandibles rehabilitated with implant-retained prostheses using tomographies and panoramic radiographs, respectively. Nakai et al⁴  found bone gains of 3.3% to 8.6% after a 66-month follow-up. A similar result was found by Reddy et al,¹⁴  who observed a marked increase in bone in the first 12 months. Taylor¹⁹  also observed about 2.5- to 3.0-mm bone gain after 2 years and 8 months, although the results were reported without standardization of the panoramic radiographs.

In this study, significant differences in posterior bone height were also observed at different distances to the most distal implant in the posterior mandible region. The highest bone gain (+14.83%) was observed between the 10- and 15-mm regions. After 32 months, the total mean gain across all analyzed regions (5–15 mm) was 28.42%. In the study by Reddy et al,¹⁴  there was a greater bone growth at the 15-mm position after the first year. These authors found bone height gains at all positions (5, 10, 15, and 20 mm) as in the present study, with a peak in bone growth in the first year succeeded by stability in the following 3 years.

With regard to bone density in the posterior region, a significant qualitative improvement of 5.76% was observed after 22 months. Most studies available in the literature do not assess changes in bone density in the posterior region. However, one study found that there was no statistically significant difference in bone density of totally edentulous individuals with mandibular atrophy after a follow-up period of 8 months.¹³  Our study found no difference in bone density after the 8-month follow-up in our nonatrophic sample population. The measurement methodology for qualitative bone changes remains a controversial topic. Georgescu et al¹⁸  compared CBCT with panoramic radiographs for quantitative and qualitative evaluation of mandibular bone tissue. Fifty-one patients were analyzed for quantitative analysis of the alveolar crest and 81 patients for qualitative analysis. The authors concluded that the CBCT measurements were more accurate than those provided by the panoramic radiographs and that the density assessment should be analyzed using only CBCT, because panoramic radiographs overestimate results by 6 to 7 mm when compared with CBCT. The advantages of CBCT for measuring bone tissue dimensions and density are well documented in the literature.^22,23  Compared with spiral CT, CBCT offers images with fewer artifacts, greater comfort, and patient acceptance, with lower cost and a significantly reduced radiation dose, almost 20% of the total dose of spiral CT.⁶ 

Thus, the present study demonstrated that, in our sample population, bone behavior in the posterior region of mandibles rehabilitated with implant-retained prostheses responded favorably to the occlusal biomechanical stimulus generated by this type of rehabilitation with a gain in bone and bone density. This favorable biomechanical behavior can be observed clearly only in long-term studies and can be attributed to increased stability and retention provided by the implant-retained fixed prosthesis, which results in a more efficient load distribution. This avoids the compression of the mucosa, thus preserving the residual bone tissue. Unlike in totally edentulous CD wearers, the masticatory load ends up being transmitted directly to the bone via the implants, resulting in a functional response of the bone tissue according to Wolff's law.^20,21 

The limitations of this study include that the follow-up time was restricted to 32 months and no interlaboratory comparisons of CBCT measurements were performed. In addition, the study sample was recruited as a convenience sample by inviting all patients who visited the Dental Clinic of the ILAPEO and met the inclusion criteria to participate in the study; the validity of our findings for other populations should be tested by replicate studies and studies with large sample sizes in different populations.

The bone tissue in our sample population responded positively to occlusal loading after rehabilitation with implant-retained prostheses in the mandible, with a progressive linear increase of the posterior residual ridge height during 32 months of follow-up. A transient increase in bone density measured with CBCT occurred at 22 months.

Abbreviations

ANOVA:

analysis of variance

CBCT:

cone-beam computerized tomography

CD:

complete dentures

CT:

computerized tomography

D1:

density value: 5 mm

D2:

density value: 10 mm

D3:

density value: 15 mm

IRFP:

implant-retained fixed prosthesis

L1:

linear measurements: 5 mm

L2:

linear measurements: 10 mm

L3:

linear measurements: 15 mm

T0:

time immediately after implant installation

T8:

8 months after implant installation

T22:

22 months implant installation

T32:

32 months implant installation

The authors would like to thank Prof. Wander José da Silva (Department of Prosthodontics and Periodontology, Piracicaba Dental School, State University of Campinas, Piracicaba, SP, Brazil) for reviewing the statistical analysis of the study.

The authors declare no potential conflicts of interest with regard to the authorship and/or publication of this article.