Immediate Loading of Unsplinted Implant Retained Mandibular Overdenture: A Randomized Controlled Clinical Study

Complete edentulism creates many challenges, which often affect the quality of life and health of those involved. In most industrialized countries, the number of edentulous people positively correlates with mean age.¹  Being edentulous for a prolonged period results in an atrophic alveolar ridge, which in turn creates a challenge for oral rehabilitation for those patients with removable prostheses. Moreover, patients with prolonged edentulism report difficulty eating many types of food and subsequently have poor nutritional habits.^1,2 

The predictability of dental implants has changed prosthetic rehabilitation of mandibular edentulous patients. Dental implants in the anterior mandible have shown a high survival rate as proven by longitudinal clinical studies.^3–5  Recent evidence suggests that the use of dental implants to retain mandibular overdentures significantly improved not only clinical outcomes, but also quality of life. Further, there was no significant difference in long-term cost compared with conventional dentures.⁵  Thus, implant retained mandibular overdentures were considered the standard of care in the treatment of mandibular edentulism in cases where an implant supported fixed prosthesis is not the treatment of choice.³ 

Since their introduction, dental implants have been investigated for osseointegration healing time, using various loading protocols based on clinical situations and bone quality. Branemark's initial recommendation called for a healing period of 4–6 months before loading mandibular implants.⁶  Gallucci et al⁷  published consensus statements and clinical recommendations for implant loading protocols. Immediate loading was defined as the implant-supported restoration is placed within 48 hours of implant placement and is functionally in occlusal contact with the opposing dentition.⁷ 

Current studies showed high survival rates of threaded and microtextured implants that used with a minimum diameter of 3 mm for the support of overdenture prostheses in early or conventional loading protocols.^8–10  The overall cumulative survival rate after 10–27 years of follow-up in these studies was 92.6%.^8–10 

Schimmel et al¹¹  listed criteria for immediate loading: insertion torque (≥30 Ncm), implant stability quotient (ISQ) value (≥60), 2 or more implants in the mandible, or 4 or more implants in the maxilla.¹¹  Also, splinting of implants and the type of attachment system had no effect on 12-month implant survival rate compared with unsplinted implants.¹¹ 

Conventional treatment with implant-retained overdentures (IODs) suggested that attachments be installed no earlier than 3 months after implant placement.¹²  During the initial phase of healing, patients were sometimes instructed not to wear the removable dentures for several weeks to avoid premature loading of the implants, which is believed to impair the osseointegration process.¹²  Additionally, several appointments for adjustments and/or soft relining of the treatment dentures are often required during this healing period. These restrictions and frequent appointments are, at best, inconvenient for the patient.

Survival and success of dental implants are dependent on many factors including, but not limited to implant surface, quality of bone, implant stability during healing, and loading protocol. Research suggested that implants with enhanced surface microtopography can achieve secondary stability at earlier healing times.^13,14  Several clinical studies on different implant systems showed that the loading time in complete mandibular implant retained overdentures can be safely shortened without compromising the osseointegration process and implant success rate.^15–21 

Since their introduction in 2001, Locator attachments (Zest Anchors Inc, Escondido, Calif) have been used successfully.^{15–17,22,23}  Delayed or early loading of implant-retained mandibular overdentures (IOD) showed equivalent success rates when the IODs opposed maxillary complete dentures.^15–18  Several studies on immediate functional loaded and splinted implant-supported overdentures have reported high success rates.^21,24–32  However, there is a lack of comparative studies in the literature on immediately loading of two unsplinted implant retained mandibular overdentures with Locator type attachments.^{15–17,27,33–35} 

Romanos et al³⁶  suggested that immediate functional loading of dental implants improved bone regeneration and enhances bone remodeling at the bone–implant interface and that new-bone formation can be achieved and bone resorption controlled. Such a mechanism promotes healing and reduces treatment time.^36,37 

Several studies validated the use of the Osstell (Gothenburg, Sweden) instrument in evaluating implant primary stability in determining the loading protocol.^38–42  Resonance frequency analysis (RFA) was emerging as a valuable tool for the implant surgeon to determine the viability of a particular implant by providing a relatively unobtrusive method for assessing the stability of the newly placed implant. On the other hand, RFA measurements may be applied as a predictor of implant success for immediately loaded implants. Both intraoperatively and postoperatively, RFA was considered a noninvasive technique that can be used repeatedly for quantitative stability measurements and RFAs can be assessed for any implant system.^43,44 

The torque of a dental implant is measured in Newton centimeters (Ncm).⁴⁵  Insertion torque is the continuous measurement of torque while the implant is being placed. The torque increases if the size of the osteotomy decreases or if the bone quality increases. Additional factors that affect the torque are the sharpness of the implant-cutting tip, the surface properties of the implant, lubrication of the preparation provided by blood, and the design of the implant.⁴⁵ 

The primary aim of this study was to compare the survival of implants immediately loaded with mandibular overdentures using two Locator attachments vs implants with delayed loading. The secondary aim was to clinically and radiographically compare the peri-implant tissue response around implants immediately loaded with mandibular complete overdentures to implants and overdentures with delayed loading.

This study was a single site, randomized, controlled, and prospective clinical study. The methodology was reviewed by an independent statistician. Twenty edentulous patients were enrolled from the clinics of the University of Kentucky, College of Dentistry. Ten subjects per group is reasonable for a pilot study, providing an 89% probability of estimating the true mean for bone quality and implant stability to within half a standard deviation in each group. These means, along with the observed standard deviations and within-subject correlations that we obtained, will prove essential for designing the larger, confirmatory study that constitutes the next phase of this research. This study received Institutional Review Board approval (IRB#12-0931-F1V) and was conducted in accordance with the Helsinki Declaration of 1975, as revised in 2000. Each subject provided signed informed consent before participation.

Each subject was randomly assigned to one of the treatment groups: the test group (n = 10) received immediately loaded, unsplinted implant retained mandibular complete overdentures (20 implants). The control group (n = 10) received delayed loaded, unsplinted implant retained mandibular complete overdentures (20 implants).

To be included in the study, all subjects must have demonstrated an understanding of the study and willingness to participate as evidenced by voluntary written informed consent. The minimum age was at least 22 years old. Subjects also had to demonstrate that they were willing, able, and likely to comply with all study procedures and restrictions. Moreover, all subjects were competent in written and spoken English and able to follow directions. They had to be in satisfactory general health with no clinically significant and relevant abnormalities of medical history, and subjects must have expressed interest in receiving a 2-implant retained mandibular complete overdenture, as well as be available for the follow-up examinations.

Subjects were excluded from the study if they were pregnant or breastfeeding or could not undergo oral surgery procedures for any reason. Subjects were excluded if they were undergoing chemotherapy or radiation, uncontrolled diabetes, had seriously impaired cardiovascular or pulmonary function, had immune-compromised diseases, kidney, or liver disease, or had a history of severe multiple allergies.

All participants received new maxillary and mandibular complete dentures before implant placement. Two implants were inserted under local anesthesia, using 2% Lidocaine HCL with 1:100,000 epinephrine (Dentsply Sirona, New York, NY) following the routine administration of prophylactic antibiotic medications 1 hour before the surgical procedure. Based on penicillin allergy, 2 g amoxicillin or 600 mg clindamycin was administered. A modified crestal incision was made, and full-thickness flaps were conservatively reflected to the mucogingival junction to reduce the possibility of postsurgical swelling and to allow surgical access to the implant sites. The implant positions were predetermined with a clear duplicate of the mandibular dentures used as positioning guides and adjusted at the canine areas to allow for introduction of pilot drills and guiding pins. Two osteotomies at the sites of the mandibular canines were prepared according to the protocols specified by BioHorizons (Birmingham, Ala). For both test and control groups, tapered internal laser lock 3.8- × 12-mm implants were inserted, and peak insertion torque was measured immediately with a minimum of 35 Ncm. ISQ measurements were recorded by using resonance frequency analysis (RFA) using Osstell mentor (Osstell, Baltimore, Md). Using the manufacturers' guidelines, Smartpegs type 4 (Osstell) were attached to the implant platform and the mean of 2 consecutive ISQ values, recorded from the facial aspect and then from the distal or mesial aspect, was calculated. Locator abutments (Zest Dental Solutions, Carlsbad, Calif) were connected to the implant fixtures with 15 Ncm torque in the test group, and cover screws were placed into implant fixtures in the control group. The flap was sutured with 4-0 Vicryl (Ethicon-coated Vycril synthetic absorbable, Ethicon Inc, Blue Ash, Ohio) sutures to achieve tension-free primary closure. The complete dentures were modified by creating a space in the intaglio surface for the metal housing of the attachment to have no contacts between the metal housing and the denture base. The attachments were picked up intraorally with Quick-Up autopolymerizing resin (Voco, Indian Land, SC). A cone beam computerized tomography (CBCT) scan was made immediately after implant placement for research purposes for both groups (i-CAT Cone Beam 3D Imaging with settings: diameter 16 cm, height 6 cm, mandible/resolution 0.3 mm, voxel 8.9 seconds/mAs = 18.54/KVp = 120, acquisition time 8.9 seconds, Henry Schein Dental, Albany, NY). For the control group, the Locator abutments were placed, and intraoral attachment pick up was performed 3 months following implant placement (Figures 1 and 2).

The recommendation for postoperative care included amoxicillin, 1 500-mg tablet every 8 hours for 10 days, and analgesics, mainly ibuprofen, up to 600 mg every 6–8 hours as needed. If the patient was allergic to penicillin, 300 mg clindamycin twice a day was prescribed instead. Sutures were removed 2 weeks after surgery. Patients were advised to clean the surgical area gently by rinsing with 0.12% chlorhexidine gluconate twice daily for 2 weeks. Follow-up evaluations were done at 2 weeks, 3 months, and 12 months.

The following outcome measures were evaluated.

A “surviving implant” is when the implant remains in the jaw and is stable and when the subject's treatment is functionally successful even though all the individual success criteria, as defined by Smith and Zarb, are not fulfilled⁴⁶: (1) does not cause allergic, toxic, or gross infectious reactions either locally or systematically; (2) offers anchorage to a functional prosthesis; (3) does not show any signs of fracture or bending; (4) does not show any mobility when individually tested by tapping or rocking with a hand instrument; and (5) does not show any signs of continuous radiolucency on an intraoral radiograph using a paralleling technique strictly perpendicular to the implant-bone interface.

All implant stability values were recorded at the time of implant placement and at 3 and 12 months. ISQ has a nonlinear correlation to micro mobility, whereas micro mobility decreases more than 50% from 60 to 70 ISQ.

The insertion torque for every implant was recorded at the time of surgery. Values recorded were 35 Ncm and higher.

CBCT was made at implant installation as baseline and at 12 months postoperatively to evaluate bone changes around the implant (Figures 3 and 4)

The zone of keratinized mucosa (KM) was measured using a UNC 15 Periodontal probe (Hu-Friedy Manufacturing Co, Chicago, Ill) at the time of implant placement and 3 and 12 months.

The gingival index (GI) has been modified and adapted (mGI) for application around oral implants (Tables 1 and 2). However, soft tissue texture and color around implants depend on the normal appearance of the recipient tissues before implant placement and may be influenced by the material characteristics of the implant surface. Furthermore, difficulties in recording mucosal inflammation have been reported, such as nonkeratinized peri-implant mucosa normally appearing redder than keratinized tissue.^47,48 

Mombelli et al⁴⁹  modified the original plaque index introduced by Silness and Loe to assess biofilm formation in the marginal area around ITI implants (mPI) (Institut Straumann, Waldenburg, Switzerland). Aspe et al⁵⁰  assessed oral hygiene levels according to a 3-point scale and reported a significant relationship between oral hygiene and peri-implant bone resorption over an observation period of 6 years.

The results were analyzed and reviewed by an independent statistician. The statistician suggested for this pilot study that 10 subjects per group was reasonable. The results of this pilot study can be used for power calculation for a phase 2 randomized controlled clinical study on immediate loading implant retained overdenture with longer follow-up. Although there was a small sample size and a short period of follow-up for this pilot study, the results presented are clinically relevant.

Independent-samples t-tests were conducted to compare KM measurements between the treatment and control group at each of the three time intervals (baseline and 3 and 12 months). At baseline, there was a significant difference in the KM measurements for the treatment group (M = 5.45 mm, SD = 0.826) and control group (M = 4.80 mm, SD = ±0.951; P = .027). At 3 months, there was not a significant difference in the measurements for the treatment group (M = 5.00 mm, SD = ±1.085) and control group (M = 4.20 mm, SD = ±1.508; P = .071). At 12 months, there was a significant difference in the KM measurements for the treatment group (M = 5.06 mm, SD = ±0.873) and control group (M = 3.70 mm, SD = ±1.809; P = .007; Table 3).

For both the control and treatment groups, no significant differences were found between baseline, 3 months, and 12 months as determined by 1-way analysis of variance (ANOVA) [control group: F(2,58) 3.233, P = .047; and treatment group F(2,56) 1.317, P = .277]. Because there were no significant differences overall, no post hoc comparisons were calculated (Table 4; Figure 5).

A 2-way repeated-measure ANOVA was used to test the interaction between group and time; the interaction was not statistically significant [F(2,35) 2.326, P = .113], illustrating that changes in the KM measurements over time for the treatment group were not significantly different than the changes over time for the control group.

Independent-samples t-tests were conducted to compare bone level measurements between treatment and control groups. Reference points used for crestal bone changes were the inferior point of the mandible to the platform of each implant. At baseline, there was a significant difference in the bone level measurements from the reference point for the treatment group (M = 26.71 mm, SD = ±3.54) and control group (M = 24.17 mm, SD = ±4.37; P < .0001). At 12 months, there was a significant difference in the bone level measurements from the reference point for the treatment group (M = 25.86 mm, SD = 3.46) and control group (M = 22.84 mm, SD = ±4.82; P < .0001). There was also a significant difference in change in bone level measurements from baseline to 12 months for the treatment group (M = −0.65 mm, SD = ±1.69) and control group (M = −1.33 mm, SD = ±1.473; P = .009; Table 5; Figure 6).

Independent-samples t-tests were conducted to compare Osstell measurements between the treatment and control group at each of the 3 time points. At baseline, there was a significant difference in the RFA measurements for the treatment group (M = 79.88, ISQ, SD = 3.44) and control group (M = 74.45, ISQ, SD = 6.66; P < .020). At 3 months, there was a significant difference in the RFA measurements for the treatment group (M = 80.08, ISQ, SD = 3.08) and control group (M = 74.73, ISQ, SD = 6.77; P = .004). At 12 months, there was a significant difference in the Osstell measurements for the treatment group (M = 79.56, ISQ, SD = 3.35) and control group (M = 76.33, ISQ, SD = 5.60; P = .040; Table 6; Figure 7).

For the control group, there was no significant difference between baseline, 3 months, and 12 months, with an increase in ISQ after loading, as determined by 1-way ANOVA [F(2,58) 0.517, P = .60]; thus, no post hoc comparisons were calculated. For the treatment group, there was no significant difference between baseline, 3 months, and 12 months as determined by 1-way ANOVA [F(2,56) 0.12; P = .85). No post hoc comparisons were calculated (Table 7).

A 2-way repeated-measures ANOVA was used to test the changes intragroup over time. The interaction was not significant [F(2,35) 0.847; P = .438], demonstrating that the changes in the Osstell measurements for the treatment group and control group were not significantly different over time.

In summary, ISQ, measured using Osstell RFA, showed that, at baseline, there was a significant difference in the ISQ measurements for the treatment group (M = 79.88, SD = ±3.437) and control group [M = 74.45, SD = ±6.657; t(38) = 3.238; P = .020].

At 3 months, there was a significant difference in the ISQ measurements for the treatment group (M = 80.08, SD = ±3.083) and control group [M = 74.73, SD = 6.770; t(36) =3.080; P = .004]. At 12 months, there was a significant difference in the measurements for the treatment group (M = 79.56, SD = ±3.351) and control group [M = 76.33, SD = ±5.601; t(36) = 2.127; P = .040]. At baseline, there was a significant difference in the gingiva measurements for the treatment group (M = 5.45, SD = 0.826) and control group [M = 4.80, SD = 0.951; t(38) = 2.308; P = .027].

After 12 months, implant survival rate was 100% for both immediate load and delayed load groups.

At 12 months, the modified gingival index was higher in the control group than the test group, with no significant difference between groups. The modified plaque index was higher in the test group than the control group, with no significant difference between groups.

Throughout the course of the study, 2 patients had fractured mandibular dentures that needed repair. Two additional patients developed gingival recession due to lack of keratinized mucosa and bone loss along the facial aspects of the implants, which necessitated further surgical intervention using soft tissue grafting (free gingival graft) around implants.

Several subjects required frequent follow-up visits for damaged locator attachments or for locator abutments replacement with shorter or longer heights following complete healing (Table 8).

The purpose of this investigation was to clinically and radiographically compare the implant success rate and peri-implant tissue response between the immediate loading protocol and delay loading protocol for unsplinted implant retained mandibular overdenture, using the following metrics: marginal bone levels, modified plaque index, gingival index, and ISQ, and success rate.

Bone level changes were measured at 4 sites (mesial, buccal, distal, and lingual) around the implants. There was a significant difference in change of the bone level measurements from baseline to 12 months for the treatment groups, indicating that immediate loading implant retained overdenture maintained more crestal bone compared with delayed load implant retained overdenture. This difference of crestal bone loss around the neck of implants could be due to the immediate bone response over time to healing and reorganization along with functional stresses.^36,51–53 

Although many host factors cannot be controlled by the operator in general, the choice of the implant design and surface in conjunction with the surgical techniques are operator dependent and can affect implant stability. Primary stability is an important prerequisite in achieving osseointegration and considered a useful predictor for successful osseointegration.⁵⁴ 

Endosseous implant stability is the capacity of the implant to withstand loading in axial, lateral, and rotational directions. Achievement of initial primary stability depends on quality of bone, surgical technique, implant design, and prosthetic design.⁵⁵  The strengths of this study were based on these fundamental factors that used to maintain stability of the studied implants.

Bone quality was classified into 4 categories, ranging from dense to spongy bone. Implants placed in dense bone (types 1 and 2) show better initial stability than those placed in poorer quality bone (types 3 and 4).⁵⁶  Therefore, this may be considered a main strength factor to have an equivalent success rate between immediate and delayed loading groups because all implants placed in this study were in type 1 symphysis bone.

Dental implants with laser-ablated coronal microgrooves reduce peri-implant crestal bone loss (CBL). However, laser microgrooves appear to inhibit apical migration of crevicular epithelium and to promote true attachment of peri-implant gingiva. The formation of an interface between connective tissue and the implant collar that is more like that of a natural tooth will improve the long-term performance of dental implants.⁵⁷  In this study, we used Laser-Lok BioHorizons implants that are characterized by only surface treatment shown to attract a true physical connective tissue attachment onto the surface of dental implants.⁵⁸  The implants used in this study were tapered to provide excellent primary stability, maximum bone maintenance, and soft tissue attachment for optimal esthetics. The tapered implant achieved these benefits from its anatomically tapered dental implant body, aggressive buttress threads, and advanced Laser-Lok surface technology. These implants have a deep 1.5-mm internal hex connection with a lead-in tapered bevel to create a rigid connection and a stable biologic seal. Moreover, implants with textured surfaces achieve better anchorage in bone tissue compared with implants with machined surfaces.⁵⁹ 

Generally, in all bone quality, reducing the diameter of implant osteotomy by 5% will increase in torque value by 15%–20%, and this will improve bone density to optimize primary implant stability.⁶⁰  Rough surfaces provide more favorable conditions for stabilizing the fibrin clot that forms on the surface of a freshly inserted implant for better osseointegration. Rough implants tend to yield higher bone-to-implant contact (BIC) values than machined ones.⁶¹ 

A reduction in abutment diameter (ie, platform switch [PS]) resulted in the translation of less stress to the crestal bone in the microthread implants.⁶² 

The microthreaded design was found to be more effective in reducing shear stress under off-axis loading, which dominates in the oral cavity. However, higher peak compressive stress and strain around the microthreaded implant were found to be localized in a smaller bone volume. The biomechanical rationale of the microthreaded design may reduce the risks of marginal bone loss caused by overloading.^63,64 

Although the design of this study was to test using unsplinted implants to retain mandibular complete overdenture with resilient attachments for locator abutments, which were considered not rigid for the immediate loading protocol, we used several surgical and restorative techniques along with the implant design to maximize primary stability of implants that were controlled by the operator, such as undersizing the osteotomy, tapered implant design, widened crestal portion, threaded implant design, buttress thread design, platform switch design, and rough surface. Patient compliance was another important factor in this study. All patients were instructed to stay on soft food for 6 weeks after surgery, as this is the most critical time for osseointegration to be established. Quality of life and patient satisfaction were not measured in this study. This is another limitation of this pilot study. The mean marginal bone loss values observed for both groups at 12 months remain below the normal range of values reported in the literature (1.5–2 mm in the first year).^65–68  The number of disconnecting and reconnecting Locator abutments to measure ISQ for this research purpose along with raising the flap for implant placement may influence the rate of crestal bone loss around the neck of the implants, and this is one of the limitations of the study. A recent study by Rodriguez et al⁶⁹  found that implants with a PS design are associated with less CBL during the healing process and as their abutments are disconnected than are non–platform-switched (NPS) implants with a comparable number of disconnections and reconnections.⁶⁹ 

During the period of the study, 1 patient in the test group did not complete the follow-up appointments at 3 and 12 months. There was no significant difference in implant success between the 2 groups in the current study. These finding agrees with previous studies^15,52  where there was a 100% success rate when unsplinted implants with Locator attachments were placed in the mandibles to retain an overdenture using the immediate loading protocol.

A systematic review conducted by Zygogiannis et al⁵³  found that studies demonstrated similar outcomes between conventional and immediate loading protocols in cases of mandibular implant overdentures including ball attachments, O-rings, bar attachments, and unsplinted implants.

Forty implants were inserted in this study, with >35-Ncm torque in both immediate and conventional loading groups. All had >64 ISQ values, which is required for successful immediate loading procedures and indicates a high degree of primary stability. Although preliminary, these findings provide a platform for future clinical research. Larger cohorts and long-term follow-up are recommended.

The limitations of this study were the short-term clinical trial and follow-up period of only 1 year. Bias was due to the nature of the study with the inability to blind study examiners. The free-handed implant placement technique was used, along with raising flaps for implant installations. Moreover, there was a small sample size for this pilot study. Future research investigations can increase the sample size with long-term follow-up data. A future phase 2 research study sample size can be selected based on the results of this pilot study. It can be estimated that a sample size of 41 patients per group (anticipated 10% dropouts) will be necessary to provide 90% power with a type I error of .05, for a difference of 0.4 mm (SD = 0.53 mm) in peri-implant vertical bone loss between groups, which would be statistically significant.

The computer-guided surgery technique is a more accurate and reliable tool to be considered for future studies. This technique can be done with or without raising a flap for implant installation. The flapless technique may preserve more crestal bone around the neck of the implants. Future research should consider the differences between these techniques for long-term results.

Immediate loading the unsplinted implant in different bone qualities (ie, in maxillary edentulous patients) is another project for future research.

Patient satisfaction and quality of life are important and were not measured in this pilot study. Oral health, as related to quality of life (OHRQoL), can be used in the future to establish new prosthetic rehabilitation strategies. The Oral Health Impact Profile (OHIP) questionnaire is a valid and reliable instrument for assessment of OHRQoL. Future research considering patient satisfaction can be assessed using the Oral Health Impact Profile in Edentulous Adults questionnaire.

Moreover, this study was conducted at a single-site university setting. Multicenter studies may be needed to verify the validity of this pilot study results.

An independent statistician reviewed the conclusions and suggested a longer healing time with a larger sample size. Immediately loaded implants retaining mandibular overdentures using Locator attachments showed higher stability and less bone and soft tissue loss after 12-month follow-up and could be considered an alternative treatment modality. Implementing the immediate loading protocol for the mandibular implant retained overdentures using Locator attachments would shorten the treatment time, which could lead to better patient satisfaction. More research is needed to verify these pilot study results.

Abbreviations

B

buccal

BIC

bone to implant contact

BIV

bone–implant volume

CBCT

cone beam computerized tomography

D

distal

GI

gingival index

IOD

implant-retained overdenture

ISQ

implant stability quotients

ITI

International Team of Implantologists

KM

keratinized mucosa

L

lingual

M

mesial

NPS

non–platform switch

PI

plaque index

PS

platform switch

RFA

resonance frequency analysis

This study was partially funded by BioHorizons. The authors thank Dominique Zephyr for performing statistical analysis and reviewing the methodology, results, and conclusions.

The authors declare no conflicts of interest.