Single crown restorations supported by 6‐mm implants in the resorbed posterior mandible: A five‐year prospective case series

Abstract Purpose To assess clinical performance of single restorations supported by 6‐mm long implants in the posterior mandible after 5 years in function. Materials and Methods Twenty‐one consecutive patients with the absence of premolars or molars in the posterior mandible and an estimated bone volume of at least 6 mm in width and an estimated height of 8 mm between the top of the ridge and alveolar nerve were included. Each patient received one or more 6‐mm implants. Custom‐made titanium abutments with cemented zirconia‐based porcelain crowns were placed after a 3‐month osseointegration period. Data of clinical examinations and radiographs were assessed at placement of the restoration and 12 and 60 months thereafter. The patients answered a questionnaire to score the satisfaction before treatment and after 12 and 60 months with the restoration in function. Results Implant survival was 100%. Five‐years' mean marginal bone loss was 0.14 mm (SD: 0.4). Indices scores for plaque, calculus, gingiva, and bleeding were low as well as mean pocket‐probing depth. Patients' satisfaction was high. Conclusion Five‐year follow‐up data of this limited case series study revealed that 6‐mm dental implants inserted in the resorbed posterior mandible provide a solid basis for single tooth restoration.


| INTRODUCTION
In the posterior region of the mandible, the bone height above the mandibular nerve often frustrates the use of standard length implants (≥10 mm). Either surgical reconstruction of the planned implant site by vertical bone grafting techniques in combination with implants of standard length has to be applied or shorter implants should be used.
Felice and colleagues 1 and Esposito and colleagues 2 stated being in favor of the use of shorter implants as such an approach reduces surgical interventions, treatment time and morbidity. Moreover, as it is a less complicated approach, the treatment outcome is presumed to be even more reliable.
In the systematic review of De N. Dias and colleagues, 3 it was reported that survival rates of implants of ≤8 mm in length are comparable to those of longer implants in combination with vertical reconstructive surgery. Even more importantly, the use of short implants is presumed to be a significant asset in cases where there is a lack of bone for placement of longer implants in the posterior mandible as vertical bone augmentation procedures in that area should be avoided. 2 Another recent review even states that in case of limited mandibular bone height short implants are favored because of a number of advantages for the patients and the clinician. 4 Prospective studies with a medium-term and long-term follow-up on short implants of ≤8 mm in the posterior mandible are scarce, however.
Clinical studies with 5-year results on performance of short implants in the resorbed posterior mandible are limited to those of Rossi and colleagues, 5,6 Pieri and colleagues 7 and Naeni and colleagues. 8 All these studies but one reported the results of a mixture of treatments with short implants in mandible and maxilla. Overall survival rates varied from 86.7% to 95.0%. Due to a difference in bone density, it is not yet shown whether the performance of short implants differs between maxilla and mandible. Only the retrospective study of Pieri and colleagues 7 solely reported on implant treatment in the posterior mandible. The implant-survival rate in that 5-years study was 97.8%. Rossi and colleagues 9 are the only authors reporting about 10-year results on short implants in the posterior maxillary and mandibular region. The 10-year overall survival rate of implants placed in either the maxilla or mandible was 91.7%. A prospective medium-term study, solely focusing on short implants in the resorbed posterior mandible, is missing. Therefore, the present case series study was performed to evaluate the clinical performance after 5-years in function of 6 mm implants restored with non splinted crowns in the posterior region in the mandible. The primary objective of the study was marginal bone level changes by radiological assessments at 5-year followup. Secondary objectives were patients' satisfaction, implant and restoration survival and condition of peri-implant mucosa.

| MATERIALS AND METHODS
The treatment and evaluation procedures applied in this study have been described in detail in the 1-year study of Guljé and colleagues. 10 A summary of the procedures utilized is presented below.

| Outcome measures
Throughout the 5-year follow-up period the following outcome measures were assessed at the evaluation time points (preoperatively, 2 weeks (T1), 12 months (T12), and 60 months (T60) after restoration placement): • Implant survival: implant still present, not mobile and removal not dictated by progressive bone loss, infection or fracture.
• Restoration survival: restoration still present, not renewed and renewal not dictated by extensive fracture or inferior aesthetics.
• Radiographic evaluation: radiographs were taken with an individualized X-ray holder to make the radiographs individually comparable. Crestal bone changes were measured, both distally and mesially, from a reference point to the crestal bone margin. The reference point was the junction between the machined bevel and the micro threads at the neck of the implant. Bone loss was F I G U R E 1 Part of rotational panoramic radiograph with 6-mm implant in position 46, 2 weeks after implant placement presented as the worst value for mesial and/or distal changes between 2 weeks, 12 months, and 60 months after restoration placement.
• Clinical evaluations: plaque accumulation was measured with the modified Plaque Index 11 and bleeding tendency with the modified Sulcus Index, 11 assessment of peri-implant inflammation according the Gingival Index, 12 presence of dental calculus and pocket probing depth to the nearest millimeter using a manual periodontal probe.
• crown-implant ratio: clinical crown-implant ratios were calculated on digitized casts as described by Meijer et al. 13 and Guljé et al. 14 • Patient satisfaction: patients validated the treatment result with an overall mark (on a 10-point rating scale) and were asked to answer a questionnaire composed of questions or statements on a 5-point rating scale ranging from (score 1) "very satisfied" and "in agreement" to (score 5) "very disappointed" and "not in agreement".

| Statistical method
The same observer (F.L.G.) did analysis of the radiographs and data   (Table 2) (Figures 2 and 3). Scores of the indices for plaque, calculus, gingiva, and bleeding were low and did not change over time (Table 3). Also, mean probing depth was favorable (2.6 ± 0.7 mm) and did not change during follow-up. Mean crown-implant ratio was 2.23 with a SD of 0.40. No technical complications (eg, porcelain chipping, screw loosening) and no biological complications (eg, peri-implantitis) were encountered during the 5-year follow-up. Patient's satisfaction was very high after treatment and remained at that high level during follow-up (Table 4).

| DISCUSSION
In the present study placement of 6-mm implants in the posterior region of a resorbed mandible appears to be a solid solution to support single restorations. The implant survival rate after 5-year was 100%, marginal bone loss was minimal, peri-implant health favorable and patients' satisfaction high. F I G U R E 2 Intraoral radiograph of 6-mm implant with restoration, 1 year after restoration placement (same patient as depicted in Figure 1) The high implant survival in the present study is the best comparable with results of the retrospective study of Pieri and colleagues 7 that solely reported on implant treatment in the posterior mandible.
The implant-survival rate in that 5-years study was 97.8%. However, in the latter study the short implants were splinted to neighboring implant-supported restorations. The high density of mandibular bone, and therefore a high bone-to-implant contact area, could be a reason for the high survival rate in both studies. Also, restoration survival was 100%, which favorable outcome is, probably, due to the materials used and the firm connection between implant and abutment leading to reduction of major complications. Lemos [5][6][7][8] In the present study and in the study of Pieri and colleagues 7 bone level implants were used, whereas in the other 6-mm studies tissue level implants were applied. In earlier days, a microgap at bone level was seen as a risk for bone loss, but this was refuted in the systematic review of Vouros and colleagues. 17   F I G U R E 3 Intraoral radiograph of 6-mm implant with restoration, 5 years after restoration placement (same patient as depicted in Figure 1) augmentation group as a control ethically questionable to our opinion.
Nevertheless, the medium-term results of the present study support the use of short implants since it offers excellent results with a simple and safe treatment procedure as well as that no complications were observed.
Another limitation of this study is that a limited sample size was used. Although medium results are excellent, more studies with possibly larger patient populations are needed to strengthen the conclusions.

| CONCLUSION
Within the limitations of this study, the 6-mm OsseoSpeed 4.0 S implants with a single restoration placed in the posterior resorbed mandible provide a stable solution with healthy peri-implant soft tissues and a high patient satisfaction after a 5-years follow-up period.