A Comparative Study of Two Treatment Protocols Using Immediate Loading Implants in the Rehabilitation of the Edentulous Jaws with Full Fixed Prostheses: 1 Year Interim Results

Introduction

The introduction of immediate function in implant dentistry was an important landmark, as it induces important physical and psychological positive effects on the patients benefitting from it Malo, et al. [1], and excellent success rates, technical benefits and the simplification of this procedure has been widely reported in previous systematic reviews [2, 3, 4, 5]. Using this treatment modality, long-term high survival rates have been reported with over 94% at 10 years for full- arch [6, 7] over 97% at 10 years for partial rehabilitations [8, 9], and over 95% with up to 8 to 12 years for single tooth [10, 11].

Several authors have reported promising clinical outcomes when placing immediate fixed reconstructions in edentulous patients supported by four splinted implants; immediately loaded full-arch fixed prostheses supported by two axial and two tilted implants in both arches have proven to be a successful long-term treatment option [12].

The immediate loading of tilted implants with a provisional restoration has been proposed as a simpler, more predictable, less expensive, and less time-consuming method to treat the atrophic jaws, the aim of the present retrospective study was to compare, after 1 year in function, a definitive reinforced PMMA resin prosthesis (G1 group) with an immediate definitive milled CAD-CAM metal-framework prosthesis (G2 group) in patients with completely edentulous jaws: the first-one were immediately loaded and supported by axial and tilted implants, the other-one were loaded within 72 hours and supported by only axial implants.

Material and Methods

All patients in this retrospective study were treated in two clinics between October 2012 and June 2014.The patients who met the inclusion criteria were identified from the medical records. The study was conducted in accordance with the ethical standards of the 1964 Helsinki declaration and its later amendments. The study follows the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines [13]. Informed consent was obtained from all individual participants included in the study and the study was approved by the ethics committee of the Catholic University of The Sacre Heart, Rome. The study included 55 patients (29 females and 26 males), with an age range of 41 to 85 years and mean age of 63.7 years.

The patients have been divided in 2 groups

• Group 1-Test group (G1): immediate restoration with multiunit abutments and screw retained bridges at abutment level
• Group 2-Control group (G2): restoration with a screw retained milled framework at implant level produced within 72 h upto 7 days after implant placement The 0-hypothesis of this study has been, that both protocols of the prosthetic restoration show similar results concerning the success rate of the implants and the clinical parameters, like bone loss and soft tissue related complications.

In G1 a minimum of 4 implants has been placed (blueSKY Bredent medical, Senden, Germany), where the posterior implants have been placed tilted, according to the “All-on-four” protocol [14]. The implants have been restored immediately with the corresponding straight and angulated multi-unit abutments (SKY fast & fixed Abutment, Bredent medical, Senden, Germany).

In G2 a minimum of 4 implants has been placed straight. In the lower jaw the implants have been Replace tapered Groovy RP (Nobel Biocare) and in the upper jaw with Nobel Speedy Groovy (Nobel Biocare). These implants have been restored with milled frameworks at implant level were us abutments are used that are fused with the bar of the prosthetic structure.

Discussion

This retrospective study aimed to investigate and evaluate the clinical and radiographic outcomes of immediate loading implant rehabilitation in patients with at least one edentulous arch. Marginal bone Loss (MBL), implant success (I-CSR) and prosthetic success (SR-prosthesis) were used to compare two different rehabilitation modalities.

One of the disadvantages of the study is certainly its retrospective nature, but fortunately there were no dropouts among patients and all of them presented the necessary information at the programmed controls. In this study 100% of the radiographs were available at the baseline and according to the strict follow-up protocols, all patients had at least 3 months, 6 months, 1 year radiographs, therefore it was possible to calculate the MBL intercourse. This factor is crucial because, as demonstrated by Browaeys, et al. [28], the average bone loss can vary greatly depending on the periods that are taken into consideration: analyzing this average in 700 implants, loaded within 10 days of insertion with full- arch prosthesis, between 0 and 3 months the authors point out a loss of about 1 mm, while between 3 and 6 months it was close to zero. In our study the values between 0-3 and 3-6 months were respectively (0.36 ± 0.49) mm and (0.60 ± 0.65) mm in G1 and (0.56 ± 0.63) mm and (0.99 ± 0.98) mm in G2. These values do not show a clear trend as described by Browaeys, et al. [38] but this value seem to show a continuous remodeling in the first 6 months, which will be attenuated at a 1 year from implant placement.

The difference of MBL around axial and tilted implants at 1 year was not statistically significant. Our result is in line with that of other studies, the observed MBL is modest and comparable with previous studies both for axial and inclined implants. In a recent systematic review, the MBL for axial implants varied between 0.6 and 1.2 mm and between 0.34 and 0.92 mm for inclined implants, with no apparent difference between the mandible and maxillary [29].

Several authors Zechner, et al. [30], Van De Velde, et al. [31], Collaert & De Bruyn [32], Cooper, et al. [33], Dierens, et al. [34], Vandeweghe, et al. [35] found no statistically significant differences in the bone levels of the mesial and distal sites of the implant, both for intraoral RX and OPT, both at baseline and in follow-ups. Therefore, also the implants in which one of the 2 sites mentioned was illegible were considered in the study.

In a comparative study where intraoral Rx and OPT were compared it was demonstrated that the CEJ (cement enamel junction) was identified with a discrepancy of 0.1 mm and the results of some studies Sivasriyanond & Manson-Hing; Brägger, et al. [36, 37] in which the authors attested the accuracy of the image of the endoral Rx (0.1 mm) higher than the OPT (0.2 mm), of only 0.1 mm, we asked ourselves about the clinical relevance of differences of <0.2 mm and it was decided to use the measurements on digital OPT.

Lambert, et al. [38] defined the learning curve for implant placement, showing that implants inserted by inexperienced operators (<50 implants) fail with a double frequency. In our study, surgical procedures were performed by an operator with more than 20 years of experience in the field of implant surgery (> 50 edentulous rehabilitations).

As already reported, the primary stability at the time of implant placement is fundamental for immediate loading and it is reported that insertion torque values below 30 Ncm can lead to implant failure. For this reason, it is not possible to establish a priori a standardized position of the distance between implant-abutment junction and bone peak, since at the time of insertion the clinician can choose to place the implant more coronally/apically depending on what is his experience in order to obtain a correct primary stability.

While in G2 the patients received a permanent prosthesis directly, the patients in G1 received a temporary prosthesis, having in any case the final objective of receiving a definitive rehabilitation after 6-8 months. The incidence of fracture of PMMA-reinforced prostheses in this study was 3.1% (1/32). Although it is not possible to make a direct comparison with other studies in the literature, as the reinforced PMMA has never been used as material for the temporary prosthesis, it is interesting to report that in the studies where provisional prostheses were delivered in acrylic resin alone or resin acrylics reinforced with metal, the authors reported higher fracture incidences, such as Agliardi, et al. (15.6%), Francetti, et al. (11%) and Maló, et al. (27%).

In our practice, a temporary structure is more appreciated both by the clinician and by the patient, at least in early months. In fact, from the point of view of the clinician it is simpler to manufacture, easily modifiable in case of necessity or easily repairable in case of possible fractures and finally allows to improve the aesthetics following the healing and the maturation of the tissues. The patient appreciates more the use of the provisional protocol because it allows him to receive rehabilitation within 3-4 hours, has a lower cost compared to temporary structures with metal bars, has an excellent appearance and has a lower weight than those reinforced.

In this study, 90.91% of implant failures (intended as removed implants or with peri-implantitis) occurred in G2, whereas 9.09% in G1. Some studies report a greater percentage of failure in the maxilla than in the inferior and in our study, although for a few implants (7), considering both the test group and the control group, the percentage is slightly in favor of the maxilla (57%).

To guarantee the duration of osseointegration, the union (splinting) of two or more implants requires the realization of prostheses that must meet two basic requirements: precision of prosthetic coupling (fitting) and liabilities (passivity of the structure). Ideally a prosthesis should guarantee a correct marginal closure on the abutment (precision of the fit) to limit the marginal bacterial infiltration. At the same time, it should guarantee passive insertion without exerting tension on the implants and therefore on the bone tissue.

However, passivity and fit precision are two characteristics that are difficult to obtain simultaneously. The number of clinical and laboratory steps and the different materials used in the manufacture of implant prostheses are responsible for deformations and therefore errors that are difficult to predict and control. The more the prosthetic bar tends to passivity the more the marginal precision decreases and vice versa, the more the prosthetic bar is precise the less it is passive and the clinician feels high friction on the abutments during insertion. This friction translates into a constant force which is applied to the bone-implant interface and which is added to the masticatory loads: this force is called preload. We must therefore adopt a method that is able to guarantee a good balance between passivity and fit precision, limiting the possibility of generating a potentially harmful preload.

In our study the MUA (inclined or 0°) used in G1 are industrial components, therefore standardized and precise, while in the control group the abutments are milled metal structure, therefore produced only after the impression taking. These steps may entail a certain inaccuracy in the milled implant-abutment coupling, which could lead to the formation of a microgap between the two structures. If we consider that in the industrial production of two mechanical components to be coupled a dimensional error margin (± 10 μm) is tolerated, the accuracy will probably be lower if we add the bar milling variable to this data. Supposing that the best oral hygiene of the patient is given, the possibility that the bacterial plaque accumulates depends on the precision in the abutment-implant connection and on the surface roughness of the abutment. This space, although small, is not cleanable and can easily be colonized by pathogenic bacteria and be a starting point for the development of a possible peri-implant mucositis.

A further variable to consider is the depth of engagement of the two methods. Rehabilitation on a milled bar, in order to have an acceptable engagement depth, should be inserted in systems that are absolutely parallel to one another, otherwise the liability previously described would be lost. This situation is clinically difficult to achieve. For this reason almost all (84%) of the G2’s systems present an external connection and the engagement takes place in a superficial manner. On the contrary, the MUAs are inserted into the implant for a depth of several mm, the passivity is guaranteed as an industrial element, resulting perfectly stable and with micromovements below the threshold values for osseointegration (<150μm).

Moreover, thanks to the presence of MUAs, the connection with the prosthesis takes place at a more coronal level than the control group, where the connection is made directly with the implant platform. This involves: a juxta- gingival abutment-prosthesis connection and not at the implant level; the MUAs are inserted into the implant at the time of surgery. This allows a healing of the tissues around what will be the definitive abutments. Furthermore, it is not necessary to remove them during the control visit, or in case the prosthesis should be removed, unlike the G2.

Regarding delivery times, in addition to the advantages already mentioned above, there is an extremely important factor. If the milled metal structure has inconsistencies that do not allow it to be inserted, it is necessary to restart the processes from the impression taking and wait for the delivery times again, which would result in the loading in a phase that may already be critical for osseointegration (as it would have been 7-10 days), a phase that requires the absolute immobility of the prosthetic structure on the implants, under penalty of the failure of the rehabilitation.

Vervaeke, et al. [39] reported the effects of smoking on initial bone remodeling and early implant failure. The study reports 3 times more implant failure in smokers than non- smokers and the maxilla appear to be more susceptible to MBL than the mandible. In our study there is no statistically significant difference between smokers and non-smokers in terms of implant failure, probably because none of the patients was considered a heavy smoker.

No difference even between patients with previous periodontitis treated and healthy patients in terms of implant failure.

Conclusion

With the limitations of this study, the success in the medium term of the results obtained seem to confirm that full-arch fixed rehabilitations supported by implants can be considered an effective solution for the rehabilitation of the edentulous patient.

The use of a protocol that foresees the provisional prosthetic rehabilitation in reinforced PMMA seems to show promising results, based on the variables considered in this study: an implant success rate (implants do not show peri- implantitis and have not been removed) equal to 97.56%, in contrast with the success rate of 85.98% of G2. This difference was statistically significant. The 1-year MBL (0.8 ± 0.6 mm) is lower than the values present in the most recent systematic reviews of the immediate full-arch load (0.9 ± 0.5 mm).

There was only one fracture within 32 rehabilitations of the reinforced PMMA structure, presenting a success rate of 96.9% temporary rehabilitation, a very high percentage, even comparing it with other studies where solutions without reinforcing metal structure were presented.