Immediate vs conventional loading of variable-thread tapered implants supporting three- to four-unit fixed partial dentures in the posterior maxilla: 1-year interim results of a split-mouth randomised controlled trial

Immediate vs conventional loading of variable-thread tapered implants supporting three- to four-unit fixed partial dentures in the posterior maxilla: 1-year interim results of a split-mouth randomised controlled trial

MENA Clinical Dentistry

23. January 2019

Habib L Abi-Aad, Fadi I Daher, Hani I Dimassi, Giampiero Cordioli, Zeina AK Majzoub

Purpose: To compare the outcome of immediately loaded and one-stage conventionally loaded variable-thread tapered implants in the posterior maxilla.

Materials and methods: This study was designed as a split-mouth randomised controlled trial. Twenty-six patients missing teeth bilaterally in the posterior maxilla received three to four implants in each of the posterior sextants. Bone quality was recorded based on Misch criteria (D1-D4) and insertion torque values were measured using a manual wrench. The implants on one side were immediately loaded with a temporary resin fixed partial denture on definitive multi-unit abutments. The implants in the contralateral side received definitive multi-unit abutments according to the one-stage unloaded protocol. Three to 3.5 months following implant placement, the implants were restored with metal-ceramic fixed prostheses. Outcome measures were implant and prosthesis failure rates, complications, and peri-implant bone level changes at 1 year following delivery of the definitive prostheses.

Results: Two patients dropped out prior to the delivery of definitive prostheses. Four implants supporting a four-unit immediately loaded prosthesis failed in one patient, 3 months following delivery of the definitive prostheses. None of the conventionally loaded implants or prostheses failed. There were no significant differences in the proportions of implant and prosthesis failures (difference = 4.2%; 95% CI -4.2 to 12.6%; P = 0.999). In the immediately loaded group, four early prosthetic complications occurred during the provisionalization phase (three small resin chippings and one prosthetic screw loosening). No other complications were reported. The difference in the rate of complications between the two interventions was not statistically significant (difference = 16.7%; 95% CI -1.2% to 35.6%; P = 0.125). The 1-year peri-implant marginal bone level changes were evaluated in 23 patients (77 immediately loaded and 76 conventionally loaded implants). On average, patients lost 0.42 mm at the immediately loaded and 0.46 mm at the conventionally loaded implants, the difference being statistically not significant (difference = 0.044 mm; 95% CI -0.27 to 0.18 mm; P = 0.701).

Conclusions: Immediate loading of 3- to 4-unit fixed partial prostheses supported by variable-thread implants in the posterior maxilla can yield good and similar 1-year results to one-stage conventionally loaded implants. (Quintessence Int 2019; 50:–19; published in Eur J Oral Implantol 2018;11(3):337–350)

Introduction

Immediate loading of dental implants has been demonstrated to be a predictable procedure with high success rates in multiple clinical applications, including full edentulism in the maxilla1-5 and mandible2,3, single tooth replacement in the anterior6 and posterior7-10 areas, and both maxillary11 and mandibular12 overdentures. In a Cochrane systematic review evaluating the different times for loading dental implants13, the results from 15 randomised controlled trials (RCTs) comparing immediate with conventional loading showed no evidence of differences in implant failures in the first year.

Most investigators advocate adequate implant primary stability with insertion torque values (ITVs) of at least 35 Ncm and implant stability quotient (ISQ) values of at least 60 when considering immediate loading14. The ITV threshold of 35 Ncm seems to be one of the prerequisites for successful immediate loading procedures13. While numerous studies have investigated immediate loading in partially edentulous patients with at least two adjacent teeth missing15-32, relatively limited information is currently available relative to immediate loading specific to the posterior maxilla33-43, where optimal implant stability is more difficult to achieve because of poor bone quality44. The 2008 ITI Consensus Meeting45,46 suggested that successful outcome of immediate loading in the posterior maxilla can be achieved in selected patients and under certain circumstances, as it appears to be technique sensitive. A more recent systematic review and meta-analysis47 investigating implant loading protocols for partially edentulous patients with extended edentulous sites (i.e. at least two adjacent teeth missing) concluded that immediate loading presents similar implant survival rates to conventional and early loading protocols. However, the authors suggested that further research is required before immediate loading in partially edentulous patients can be integrated in everyday practice. Evidence-based conclusions relative to threshold values for implant primary stability, bone quality and quantity needed, or impact of occlusal loading forces need to be further investigated.

Variable-thread tapered implants have been reported to consistently achieve high primary stability in poor or poor-to-medium bone quality48-50 and to yield favourable outcomes under various loading protocols48,49,51-62. Specific data related to the application of this implant design under immediate loading conditions in the posterior region of the maxilla is still lacking.

The aim of this split-mouth RCT was to compare 1-year post-loading clinical outcome and peri-implant marginal bone levels (MBLs) of variable-thread tapered implants placed in healed sites and loaded immediately with unilateral 3- to 4-unit temporary prostheses vs one-stage conventionally loaded implants placed contralaterally in patients with bilateral partial maxillary posterior edentulism. The hypothesis was that there are no differences between immediately loaded and traditionally loaded one-stage implants in the posterior maxilla. This report presents the preliminary data up to 1 year following definitive loading. This article is reported according to the CONSORT (Consolidated Standards of Reporting Trials) statement for improving the quality of reports of randomised controlled trials (http://www.consort-statement.org/) and CONSORT 2010 statement: extension checklist for reporting within person randomised trials (https://doi.org/10.1136/bmj.c869).

Materials and methods

Trial design

This study was designed as a split-mouth randomised controlled trial with 1:1 allocation ratio of the two maxillary posterior sextants in each patient to be treated according to the immediate or conventional loading protocol. However, the number of immediately and conventionally loaded implants was not equal within each patient and among patients since the number of missing teeth and the baseline anatomical characteristics of the posterior maxilla were not exactly identical.

Participants

Systemically healthy adult patients who were candidates for maxillary bilateral posterior implant-supported fixed partial dentures (FPDs) were recruited for the study from the patient population attending the postgraduate Periodontics department at the Lebanese University, Faculty of Dental Medicine, Hadath, Lebanon. Patients were selected based on the following inclusion criteria:

  • Three to five consecutive teeth missing contralaterally in the posterior maxilla posteriorly to the canines;
  • Teeth at the implant sites must have been extracted or lost at least 9 months before the date of implant surgery;
  • Occlusal scheme allowing contacts on existing teeth in lateral excursions;
  • Adequate sub-sinus bone height of at least 8 mm at the implant sites and bucco-palatal bone width allowing implant placement without bone augmentation procedures;
  • Presence of an acceptable occlusal plane opposing the planned implant sites;
  • Low smile line with no display of the gingival margins at the planned fixed prostheses during a full smile. This type of lip line would conceal potential aesthetic complications associated with facial soft tissue recession exposing the metallic body of the one-time definitive abutment used in the study.

Patients with the following local or systemic conditions were not included in the trial:

  • Systemic conditions contraindicating implant ­surgery;
  • Pregnancy or lactation;
  • Ongoing or history of pathologies or medications affecting bone metabolism;
  • Head and neck irradiation;
  • Oral inflammatory and autoimmune diseases;
  • Presence of osseous lesions in the planned ­implant sites;
  • Persistent infection in the areas intended for implant placement;
  • Previous bone augmentation surgery;
  • Severe parafunctional habits (bruxism or clenching);
  • Lack of interocclusal prosthetic space;
  • Lack of motivation.

The smoking status (number of cigarettes per day) was recorded, but not considered as contraindication for implant therapy.

All patients were given detailed information about the study procedures and the evidence-based outcome of implant therapy in the posterior maxilla. Their informed written consent to participate in the study and to use their data for research purposes was obtained at least 7 days prior to surgery. The study protocol and informed consent were in full accordance with the ethical principles outlined in the Declaration of Helsinki on clinical research as revisited in 2013 and were approved by the Ethical Committee of the Lebanese University (CUEMB85/4/2017).

Interventions

Prior to the initiation of the study, all patients underwent a full dental clinical examination, panoramic radiographs and cone-beam computed tomog­raphies where needed. Full-mouth scaling and hygiene instructions were performed approximately 1 month preoperatively. In the presence of untreated periodontitis, patients received full-mouth scaling and root planing and subsequent re-evaluation at 3 months to ensure the establishment of periodontal health. In addition, a complete comprehensive treatment plan including restorative and endodontic treatments was performed as needed.

Surgeon’s calibration relative to bone quality rating was performed once, 1 week before the first surgical implant procedures63. Implant surgery was carried out in the postgraduate clinic of the Lebanese University under aseptic conditions with a prophylactic antibiotic therapy of 2 g of Amoxicillin (or equivalent) 1 h preoperatively.

Following crestal incisions and elevation of a full-thickness mucoperiosteal flap, implant recipient sites were prepared using the bone-quality adjusted drilling sequence, according to the manufacturer’s recommendations. Subjective bone quality was assessed based on the surgeon’s tactile sensation during high-speed drilling at 800 rpm with the 2 mm pilot twist drill and recorded as D1 to D4 according to Mish criteria64, with D1 represented by thick cortical and dense cancellous bone, D2 corresponding to a combination of thick cortical bone and coarse cancellous bone, D3 combining thin cortical and dense cancellous bone, and D4 with a very thin to non-existing cortical layer and fine trabecular bone. Variable-thread tapered implants (NobelActive, Nobel Biocare AB, Göteborg, Sweden) of 10, 11.5, 13, and 15 mm lengths were used in the study. Implant diameter (3.5, 4.3, and 5 mm) was selected according to the bucco-palatal width of the implant sites. When sub-sinus bone height was approximately 8 mm, implant length was chosen so that a modest portion of the implant protruded into the sinus cavity, thereby allowing engagement with the sinus floor cortex. Implant shoulders were positioned at crest or subcrestally according to the anatomical configuration of the implant sites and thickness of the crestal gingival tissues. Insertion torque values were measured at placement using a customised, calibrated manual torque wrench that can measure torque values from 10 Ncm up to 70 Ncm with 5 Ncm incremental markings (RT, Medical Research & Technologies, Albignasego, Padova, Italy). Maximum insertion torque was recorded during the last quarter turn before the desired insertion depth was reached and measurements were rounded to the closest 5 Ncm. Implant insertion and assessment of implant stability were performed by one surgeon highly experienced in implant surgery and immediate loading techniques.

Multi-unit abutments (MUA) (Multi-unit Abutment System, Nobel Biocare) of adequate vertical height and angulation were connected to all implants and the surgical sites sutured. For the implants assigned to the immediate loading protocol, final impressions with previously prepared customised trays were made (ImpregumTM PentaTM Soft Polyether Impression Material, 3M ESPE, St. Paul, Minnesota, USA) and a screw-retained temporary non-reinforced resin FPD without intermediate pontics or cantilevers was delivered within 48 to 72 h postoperatively. The temporary FPDs had flat cusps and a narrow occlusal platform compared with natural dentition (Figs 1a to c). None of the implant-supported temporary FPDs were connected to natural teeth. The occlusion was carefully checked using 40 µm articulating paper and adjusted to allow very light occlusal contacts in maximum intercuspation. Lateral working and non-working contacts were eliminated. All prosthetic procedures were performed by the same experienced prosthodontist.

Postoperatively, the antibiotic course of amoxicillin was continued with 1 g twice daily for 6 days and analgesics were prescribed as needed and according to patient’ preferences. Patients were instructed not to brush the treated areas and to rinse three to four times daily for 1 min with chlorhexidine digluconate 0.12% for 4 weeks. Patients were also asked to follow a soft diet for 1 month. Suture removal was carried out 2 weeks postsurgery. The prostheses were manually checked for stability and patients given further instructions relative to oral hygiene and diet. The use of removable prosthesis was not allowed during the entire healing period.

Three to 3.5 months following implant placement, final impressions at abutment levels were taken (Impregum Penta) and within 2 to 4 weeks definitive restorations consisting of screw-retained metal-ceramic FPDs without pontic elements or distal/ mesial cantilevers were delivered on all implants. All MUA abutments were maintained unchanged throughout the study even when an apical shift of the gingival complex occurred buccally.

Following delivery of the definitive FPDs, all patients were enrolled on a 6-month maintenance programme, with recall visits for prophylaxis and reinforcement of oral hygiene instructions.

Outcome measures

The outcome measures were:

  • Prosthesis failure defined as an impossibility to place the planned prosthesis because of implant failure, loss of the prosthesis subsequent to implant failure, or prosthesis replacement for any reason.
  • Implant failure defined as any implant mobility, infection and/or progressive peri-implant bone loss necessitating implant removal and/or mechanical complications rendering the implant unusable. Stability of individual implants was assessed after removing the FPDs using the handles of two metallic instruments at 1 year following definitive loading, or when the patients reported problems.
  • All biological (peri-implant mucositis, peri-implantitis, etc) and prosthetic complications (MUA fracture, framework fracture, prosthetic screw loosening or fracture, abutment screw loosening or fracture, chipping or fracture of veneering material) were recorded as they were reported or at the recall intervals by the outcome assessor together with the prosthodontist.
  • Peri-implant marginal bone level (MBL) changes: intraoral periapical radiographs (Kodak 2100 Intraoral X-Ray System, 60KV, 7 mA, 0.25 s) with digital sensors (Kodak RVG 6100) and using the paralleling technique were taken for both implant groups at the appointment of provisional prostheses delivery, 2 to 3 days after implant placement (baseline), and 1 year following delivery of the definitive prostheses. Radiographs were repeated if bone levels were difficult to identify. The radiographs were exported as uncompressed TIFF files at maximum sizes into a personal computer and the measurements were made in a darkened room on a 15-inch LCD monitor with the resolution set at 1593 ×1024, using the PhotoShop 7.0 measuring tool. The vertical radiographic distance between the implant-abutment connection and the most coronal point of bone-to-implant contact mesially and distally to the implant was measured to the nearest 0.1 mm. Measurements accounted for the radiographic distortion by applying ratios calculated as the measured radiographic dimension between the first two coronal threads of the implants divided by its known distance (pitch). In the presence of a radiographic double marginal bony contour, assessments were made to the most apical level. The values of MBLs were recorded as 0 mm when the most coronal bone-to-implant contact was levelled with the implant shoulder or was located coronally to it. The values were considered positive when the first bone-to-implant contact was apical to the implant-abutment connection. Mesial and distal MBLs were averaged for each implant and at patient level and group level.

A single outcome assessor who did not participate in the surgical or restorative procedures served as the blinded rater for the clinical outcome and radiographic measurements. He was calibrated by measuring MBLs at the mesial and distal aspects of 15 implants on five random radiographs at five different instances with an interval of 1 day between assessments. High intra-observer agreement of 0.931 (0.888-0.963; 95% CI) and 0.985 (0.975-0.992; 95% CI) was calculated for single and average measurements respectively using intraclass correlation coefficient test.

Statistical analysis

Power analysis was performed in the planning phase of the study to calculate the sample size according to differences in marginal bone level changes. A difference of 0.25 mm in MBL with a standard deviation of 0.5 mm was assumed34,39 with significance level set at α = 0.05 (type I error) and power set at 0.8 (ß = 0.2, type II error assuming a medium effect size of 0.55. For differences in means, a difference of 0.25 mm in MBL with a standard deviation of 0.5 mm was assumed34,39 (same type I error and power). The required sample size was 29 patients contributing with one test and one control implant. Taking into consideration the difficulties in recruiting patients with the stringent inclusion criteria of the present study, the authors decided to include patients that could be recruited during a period of 1.5 years.

A first computer-generated randomisation list created by one of the authors was initially programmed for a total of 20 patients prior to the initiation of the clinical procedures based on the expected number of enrolled patients. Subsequently, a second list was generated for the last block of six enrolled patients. The randomisation lists were kept on the password-protected portable computer of the author in charge of randomisation, and allocation was only revealed to the surgeon and prosthodontist following completion of implant insertion.

Differences in the proportion of patients with implant failures, prosthesis failures, and complications (dichotomous outcomes) were compared using the exact McNemar’s test. The paired t test was used to compare 1-year MBL mean changes (continuous outcome) at patient level. Pearson’s chi-square analysis was applied to detect differences between immediately and conventionally loaded implant groups relative to their baseline characteristics. Statistical analysis was performed using the Statistical Package for the Social Sciences (IBM SPSS for Windows, Version 24.0; IBM Corp., Armonk, NY, USA).

Results

A total of 71 patients were screened for eligibility. Thirty-eight patients were not enrolled for not fulfilling the required inclusion criteria: 17 had their premolar and/or molar teeth extracted within the 4 months preceding the screening visit and were reluctant to wait for implant therapy; nine had extrusion of one or more teeth in the opposing posterior mandible resulting in a poor occlusal plane that was difficult to manage prosthetically; eight had adequate interocclusal prosthetic space only in the premolar area; and four had persistent radiolucencies at the apical aspects of the potential implant sites. Seven patients were not willing to participate for logistic reasons (five for frequency and hours of appointments and two for distance from their residency to the university campus). Twenty-six patients fulfilling all the inclusion requirements were enrolled at the end of the recruitment phase (Fig 2). All these patients were treated according to the allocated type of interventions. Deviations from the experimental design occurred in three participants where the final prostheses were delivered at 5 months in two patients and 6 months in one patient. The patients were recruited and treated between March 2013 and February 2015 at the Lebanese University with the last surgery performed in February 2015 and the last definitive prosthesis delivered in June 2015.

Fig 2 Flow diagram.

 

Tables 1 and 2 summarise main baseline patient, implant and prosthetic interventions characteristics in the 26 patients. The patient population included 14 women and 12 men ranging between 34 and 67 years with a mean age of 49.5 (9.7) years at implant placement. The mean age was 45.8 (7.5) years for females and 54.2 (10.3) years for males. Eighty-seven implants were immediately loaded and 86 were inserted according to the one-stage conventional loading protocol (Fig 2). No significant baseline imbalances between the two groups were demonstrated except for bone quality (P (Pearson’s chi-square test) = 0.039) and implant length (P = 0.022). A higher percentage of conventionally loaded implants were placed in D4 bone (69.8% for the conventionally loaded vs 52.9% for the immediately loaded implants). Nearly half of the implants in the immediate loading group were longer than 11.5 mm vs less than 30% in the conventional loading group.

Table 1 Frequency distribution of patients and corresponding implants according to gender, smoking status, and number of implants per patient.

 

Patient characteristics Number of patients (%) Number of test implants (%) Number of control implants (%)
Gender
Female 14 (53.8%) 47 (54.0%) 47 (54.7%)
Male 12 (46.2%) 40 (46.0%) 39 (45.3%)
Smoking status
Non-smoking 12 (46.2%) 39 (44.8%) 39 (45.3%)
1 to 9 cigarettes per day 0 (0%) 0 (0%) 0 (0%)
10 to 20 cigarettes per day 5 (19.2%) 17 (19.5%) 17 (19.8%)
> 20 cigarettes per day 9 (34.6%) 31 (35.6%) 30 (34.9%)
Number of implants per patient
6 14 (53.8%) 42 (48.3%) 42 (48.8%)
7 7 (26.9%) 25 (28.7%) 24 (27.9%)
8 5 (19.2 %) 20 (23.0%) 20 (23.3%)
TOTAL 26 87 86

 

Table 2 Implant characteristics.

Implant characteristics Number of immediately loaded implants (%) Number of conventionally loaded implants (%) Total number of implants (%)
Number of implants per FPD
3 51 (58.6%) 54 (62.8%) 105 (60.7%)
4 36 (41.4%) 32 (37.2%) 68 (39.3%)
Bone quality
2 5 (5.7%) 6 (7.0%) 11 (6.4%)
3 36 (41.4%) 20 (23.3%) 56 (32.4%)
4 46 (52.9%) 60 (69.8%) 106 (61.3%)
Opposing dentition
FPD on implants 9 (10.3%) 9 (10.5%) 18 (10.4%)
FPD on teeth 17 (19.5%) 16 (18.6%) 33 (19.1%)
Removable denture 30 (34.5%) 29 (33.7%) 59 (34.1%)
Natural dentition 31 (35.6%) 32 (37.2%) 63 (36.4%)
Implant length
10 mm 14 (16.1%) 22 (25.6%) 36 (20.8%)
11.5 mm 29 (33.3%) 40 (46.5%) 69 (39.9%)
13 mm 40 (46.0%) 21 (24.4%) 61 (35.3%)
15 mm 4 (4.6%) 3 (3.5%) 7 (4.0%)
Implant diameter
3.5 mm 23 (26.4%) 17 (19.8%) 40 (23.1%)
4.3 mm 53 (60.9%) 50 (58.1%) 103 (59.5%)
5 mm 11 (12.6%) 19 (22.1%) 30 (17.3%)
Implant position
First premolar 21 (24.1%) 20 (23.3%) 41 (23.7%)
Second premolar 24 (27.6%) 21 (24.4%) 45 (26.0%)
First molar 26 (29.9%) 24 (27.9%) 50 (28.9%)
Second molar 14 (16.1%) 17 (19.8%) 31 (17.9%)
Third molar 2 (2.3%) 4 (4.7%) 6 (3.5%)
Insertion torque
< 10 Ncm 0 2 (2.3%) 2 (1.2%)
[10-20] 18 (20.7%) 22 (25.6%) 40 (23.1%)
[25-35] 17 (19.5%) 24 (27.9%) 41 (23.7%)
[40-55] 13 (14.9%) 8 (9.3%) 21 (12.1%)
[60-70] 35 (40.2%) 24 (27.9%) 59 (34.1%)
> 70 Ncm 4 (4.6%) 6 (7.0%) 10 (5.8%)
Allocated side
Right side 41 (47.1%) 45 (52.3%) 86 (49.7%)
Left side 46 (52.9%) 41 (47.7%) 87 (50.3%)

FPD = fixed partial denture

The type of antagonist dentition was similar between the two implant groups (Table 2). More than half of the study implants were opposed by natural teeth or tooth-supported FPDs (55.2% and 55.8% of immediately and conventionally loaded implants respectively). Approximately 10% of the implants in both groups had implant-supported FPDs for antagonists. Removable partial or complete dentures opposed approximately 34% of both immediately and conventionally loaded implants.

Two enrolled patients dropped out during the definitive prosthetic phase for unidentified reasons (did not return phone calls and messages reminding them of their appointments). Four immediately loaded implants splinted in one 4-unit FPD failed in a 34-year old female smoker (> 20 cigarettes per day), 3 months following delivery of the definitive prostheses. The characteristics of the four failed implants and the three non-failed contralateral conventionally loaded implants are summarised in Table 3. The failed implants were inserted with ITVs of 35 Ncm (one implant) and 65 Ncm (three implants). They were aligned in a straight configuration at placement without any buccal or palatal offset, and were opposed by natural teeth. These implants did not demonstrate any radiologic or clinical signs of peri-implant inflammatory changes during the temporary phase and up to the diagnosis of implant failure. The three contralateral conventionally loaded implants in the same patient achieved 65 Ncm at insertion and had opposing natural dentition. The patient refused replacement of the failed implants.

Table 3 Characteristics of the four failed immediately loaded and the three non-failed conventionally loaded implants.

Bone quality (D1-D4) Baseline insertion torque (Ncm) Implant length (mm) Implant ­diameter (mm) Implant location Opposing dentition 1-year peri-implant bone change (mm)
Test implant 1 Test implant 2 Test implant 3 Test implant 4 D3 D3 D3 D3 65 65 65 35 13 13 11.5 11.5 3.5 4.3 4.3 4.3 First premolar Second premolar First molar Second molar Natural teeth Natural teeth Natural teeth Natural teeth ___
Control implant 1 Control Implant 2 Control Implant 3 D4 D4 D4 65 65 65 11.5 11.5 11.5 4.3 5 5 First premolar Second premolar First molar Natural teeth Natural teeth Natural teeth 0.50 1.75 0.65

None of the conventionally loaded implants in the patient sample was lost. There were no statistically significant differences in the proportions of patients with implant or prosthesis failures (difference = 4.2%; 95% CI -4.2 to 12.6%; P = 0.999). The patient with failed implants on one side was considered a patient failure and her implant-related data were not included in the subsequent bone level analysis (Fig 2) since the paired t test requires data for both immediately and conventionally loaded sides.

None of the implants developed biologic complications during the study period. In the immediately loaded group, three temporary FPDs had small resin chippings at their occluso-palatal aspects during the provisionalization phase and were polished without any further intervention. One prosthetic screw in a 3-unit temporary FPD became loose 2 months following delivery of the provisionals and was tightened. No other prosthetic complications occurred in the two implant groups up to 1 year following delivery of the final prostheses. The difference in the rate of complications between the two interventions was not statistically significant (difference = 16.7%; 95% CI -1.2% to 35.6%; P = 0.125).

MBLs could not be measured at the distal aspect of the most posterior implant in two patients. The mesial bone level at these two implants was used as bone level measurement for the implant. One year following delivery of the definitive prostheses (Figs 3a to d), the immediately loaded implant group lost 0.42 mm (0.45) of peri-implant bone vs 0.46 mm (0.30) for the conventionally loaded implant group, the difference being statistically non significant (difference = 0.044 mm; 95% CI -0.27 to 0.18 mm; P = 0.701).

When the number of implants with baseline ITVs ≤ 20 Ncm incorporated in FPDs was considered (Table 4), 11 out of the 26 temporary FPDs (42.3%) and 10 out of the 23 definitive FPDs (43.5%) in the immediately loaded group had at least one implant with baseline ITV ≤ 20 Ncm. Two 3-unit temporary FPDs had all their supporting implants inserted with baseline ITVs ≤ 20 Ncm.

Table 4 Distribution of fixed partial dentures according to the number of incorporated implants with baseline ITVs ≤ 20 Ncm

Test temporary FPDs (26 patients) Test definitive FPDs (23 patients) Control definitive FPDs (23 patients)
3-unit FPDs
0 implant per FPD with ITV ≤ 20 Ncm 11 (64.7%) 10 (62.5%) 6 (40%)
1 implant per FPD with ITV ≤ 20 Ncm 4 (23.5%) 4 (25%) 6 (40%)
2 implants per FPD with ITVs ≤ 20 Ncm 0 (0.0%) 0 (0%) 2 (13.3%)
3 implants per FPD with ITVs ≤ 20 Ncm 2 (11.8%) 2 (12.5%) 1 (6.7%)
TOTAL 17 16 15
4-unit FPDs
0 implant per FPD with ITV ≤ 20 Ncm 4 (44.4%) 3 (42.9%) 2 (25%)
1 implant per FPD with ITV ≤ 20 Ncm 2 (22.2%) 1 (14.3%) 4 (50%)
2 implants per FPD with ITVs ≤ 20 Ncm 3 (33.3%) 3 (42.9%) 0 (0%)
3 implants per FPD with ITVs ≤ 20 Ncm 0 (0.0%) 0 (0%) 1 (12.5%)
4 implants per FPD with ITVs ≤ 20 Ncm 0 (0.0%) 0 (0%) 1 (12.5%)
TOTAL 9 7 8

FPD = fixed partial denture

Discussion

This split-mouth RCT was designed to evaluate whether treating partial edentulism in posterior maxilla exhibiting 8 mm or more of residual sub-sinus bone height with immediately loaded multiple implants is associated with similar outcomes than implants placed according to the one-stage conventional protocol. Only two RCTs designed to test the same hypothesis in the posterior maxilla are available – one with a split-mouth design34 and the other of parallel group design39. Other RCTs with similar objectives and parallel group design included single implants in addition to multiple posterior implants or incorporated mandibular and/or anterior sites20,65. A number of parallel group RCTs compared immediately vs early-loaded implants mainly in the posterior jaws19,21,22,26,31. One split-mouth RCT evaluated immediate vs early loading in first and second premolars and first molar sites of only eight patients43. Finally, one split-mouth RCT compared two types of implants inserted bilaterally and treated with immediate loading in the posterior jaws30. The implant failure rate observed in the present interim analysis at 1 year following definitive prosthesis delivery (4.2%) is within the range of failure rates of immediately loaded implants reported in the RCTs limited to the posterior maxilla (2.7%34 and 5.5%39). In addition, the lack of significant differences in failure rate and peri-implant bone loss between the two loading protocols in the posterior maxilla confirms the findings of the same two above-mentioned RCTs.

In the present study, no threshold ITV was applied to allocate the implants to the immediately loaded group and none of the implants that achieved ITVs ≤ 20 Ncm in the allocated immediate loading sites were excluded from the study or left unloaded. In addition, sites with poor bone quality (D4) and second/third molar sites were included in the experimental design. Nearly 43% of the immediately loaded temporary FPDs had at least one implant with baseline ITV ≤ 20 Ncm. Two immediately loaded 3-unit FPDs had all their supporting implants inserted with ITVs ≤ 20 Ncm. Implants are likely to integrate irrespective of the torque values applied if not loaded immediately66. When immediate loading is considered, large variability exists relative to the recommended threshold ITVs and relative number of implants with low/high ITVS incorporated within immediately loaded FPDs. ITVs between 25 and 45 Ncm have been suggested for immediate functional loading with values ≥ 30 Ncm for single crowns and ≥ 20 Ncm for splinted implants27,67-71. While immediate loading in single tooth indications is considered an unsafe approach if the implant is placed with an ITV < 35 Ncm72,73, the importance of achieving torque values higher than 20 Ncm for immediate loading in splinted prostheses has been questioned74. Degidi et al74 assessed 1-year osseo­integration and crestal bone remodelling of immediately loaded implants inserted with low ITVs. Each of the 13 full-arch prostheses included in the study (nine supported by six implants and four by seven implants) had at least three test implants inserted with ITVs ≤ 20 Ncm and two control implants with ITVs ranging between 25 and 50 Ncm. The authors concluded that the test implants inserted with mean ITVs of 12.6 Ncm, but splinted with other implants presenting higher ITVs (mean of 35.6 Ncm) can achieve and maintain osseointegration and have similar success rates and crestal bone remodelling to control implants. Several differences exist between the present study and that of Degidi et al74. The latter used a rigid framework cross-arch splinting in both upper and lower jaws while the present study used acrylic provisional restorations without rigid metal reinforcement in 3- to 4-unit maxillary posterior FPDs. In addition, the lateral forces in full-arch prostheses cannot be eliminated while their effects can be substantially reduced in posterior FPDs when the occlusal scheme allows contacts on existing teeth. Finally, the prosthetic load conditions are significantly different between full-arch restorations supported by multiple implants positioned on a curved line versus posterior FPDs where implants are placed in a straighter configuration and potentially subject to greater bending75. Stabilisation of one implant inserted with low ITVS (15 Ncm) by splinting it to another five stable implants within the context of a full-mouth immediately loaded temporary rehabilitation has also been reported by Calandriello et al76. Considering the relatively low number of immediately loaded prostheses in the present study, definite conclusions cannot be drawn relative to the maximum number of implants with low ITVs and those with higher ITVs to be incorporated within 3- to 4-unit FPDs when using immediate loading protocols. Furthermore, guidelines relative to the use of rigid splinting in such clinical conditions cannot be extrapolated. Further larger scale RCTs specifically designed to test these hypotheses are required with standardised ITV measurement techniques.

The four failed immediately loaded implants incorporated within a 4-unit FPD in one patient were inserted with ITVs ranging between 35 and 65 Ncm in D3 bone quality. These values fall within or exceed the recommended ITV figures for immediately loaded single and multi-unit implant-supported FPDs. Although implant failure under immediate loading conditions tends to implicate implants with low ITVs77,78, implants with high torque values were reported to fail in medium bone density26. It is also interesting to note that most studies considered implants suitable for immediate loading only if ITVs were relatively high and excluded implants that did not achieve adequate primary stability47. Subsequently, immediately loaded implants that failed in these studies were likely to have been stable at insertion. The predictability of immediate loading in partial edentulism should be interpreted with caution since most available data does not include clear information on patient selection criteria and intention to treat analysis47. It is difficult to attribute the failures in the present study to single factors such as smoking or linear arrangement of the implants75 since implant success relies on a complex multi-factorial equation involving anatomical, surgical, and environmental parameters. It is the author’s subjective opinion that implant failure occurred for implant overload potentially associated with non-adherence to soft diet or other unidentified biomechanical factors.

Large variability exists concerning how to provide occlusal contacts in immediately loaded FPDs79. Whether functional or non-functional occlusal schemes are applied, the results of a recent systematic review77 suggest that the type of occlusal loading does not seem to affect the survival of immediately loaded implants or their marginal bone loss. This information however does not apply specifically to immediate loading of non cross-arch multi-unit FPDs in the posterior maxilla. In the absence of such data and from a clinical point of view, it was appealing in the present study to apply precautions in designing the occlusal schemes, i.e. very light centric occlusal contacts using 40 µm articulating paper, complete discharge in lateral excursions, narrow and flat occlusal tooth platform, absence of cantilever extensions, control of functional and parafunctional forces by recommending a soft diet for the first month postoperatively and excluding heavy bruxers.

The main limitations of the present study include the relatively small sample size and the high level of experience of the surgeon and prosthodontist. Both factors may limit generalisation of the present results to general practitioners and to all patients with similar characteristics. Finally, D4 bone, implant lengths of 10 and 11.5 mm, and second/third molar locations were less frequently encountered in the immediately loaded than in the conventionally loaded implant groups. Although random allocation of immediate and conventional loading protocols was rigorously observed in all patients, such baseline imbalances can occur in studies with limited sample sizes.

Conclusions

The present study’s outcomes suggests that the 1-year clinical and radiographic outcomes are comparable between immediately loaded and one-stage conventionally loaded variable-thread tapered implants in healed sites in the partially edentulous posterior maxilla. Implants inserted with low baseline ITVs ≤ 20 Ncm can be incorporated into immediately loaded FPDs when they are splinted to other implants presenting similar or higher ITVs in the same prosthesis. However, confirmation of this approach as a safe treatment modality should derive from larger scale RCTs.

Acknowledgements

The authors gratefully acknowledge the helpful comments provided by Nobel Biocare Scientific Affairs after reviewing the first version of this paper.

Conflict-of-interest statement

This study was independently designed by the investigators but was supported by Nobel Biocare Services AG, Kloten, Switzerland (grant number 2010-954) in the form of free surgical kits, implants and prosthetic components. No other financial contribution was received by any of the authors or their institutions. The authors declare that there is no affiliation or any other conflict of interest to the sponsor and that Nobel Biocare did not in any way interfere with the conduct of the trial or the publication of its results.

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Authors

Habib L Abi-Aad DCD, CES, Lecturer, Department of Prosthodontics, Lebanese University, Faculty of Dental Medicine, Hadath, Lebanon

Fadi I Daher DCD, CES, Lecturer, Department of Periodontics, Lebanese University, Faculty of Dental Medicine, Hadath, Lebanon

Hani I Dimassi MPH, PhD Biostatistics, Associate Professor, Lebanese American University, School of Pharmacy, Department of Pharmaceutical Sciences, Byblos, Lebanon

Giampiero Cordioli MD, DDS, Private Practice, Padova, Italy

Zeina AK Majzoub DCD, DMD, MScD, Professor, Department of Periodontics, Lebanese University, Faculty of Dental Medicine, Hadath, Lebanon

Correspondence: Zeina Majzoub, Via Paruta, 33, 35126 Padova (PD), Italy, Email: dr.zeinamajzoub@yahoo.it