Impression Reference Technique for the Open Flap Digital Workflow in the Immediate Loading Rehabilitation of the Upper and Lower Jaws

Abstract

Immediate loading implant surgery has emerged as a significant advancement in the rehabilitation of edentulous patients. This approach requires meticulous planning and precise execution to ensure successful outcomes. Transferring established intermaxillary and occlusal relationships to the definitive prostheses can be challenging. However, with a digital approach, this procedure can be standardized in cases of fully guided surgery with a flapless technique or by using disposable guides. Conversely, when extensive open flap implant surgery is required, such as in the treatment of severely atrophic patients (e.g., with zygomatic implants or simultaneous bone augmentation), the digital workflow can be demanding. The impression reference technique was proposed to enhance the digital workflow for immediate loading in zygomatic implant rehabilitation. This study aims to describe the impression reference technique, applied to both upper and lower jaws for immediate loading open flap rehabilitations, using standard implants.

1. Introduction

In completely edentulous patients, implant-supported fixed full-arch prostheses can be considered the best treatment alternative to rehabilitate oral functions. Patients receiving full arch implants surgery are classically given removable dentures during the osteointegration period, which they often find uncomfortable [1]. Immediate loaded fixed prostheses have shown several advantages, including treatment time reduction, immediate function and esthetics, avoidance of temporary removable prostheses, avoidance of second surgeries, and preservation of soft tissue anatomy, [2] enabling patients to continue their regular daily activities, which reduces the social and work disruptions and provides significant psychological benefits for the patient. Although immediate loading offers improvements for those patients with edentulous jaws who undergo full arch implants surgery, it introduces challenges in managing prosthetic procedures [3]. The primary downside of immediate loading is that a prefabricated prosthesis must be connected to the implants immediately after the surgery, avoiding the need for intraoperative impressions or checks [4].

The precision of prosthodontic frameworks is essential to avoid mechanical and biological issues in multi-implant rehabilitation. Poorly fitting prostheses can significantly impact the long-term success of full-arch fixed implant rehabilitations, particularly in procedures where immediate loading is involved. Deformations and inaccuracies in impressions taken and dental lab procedures are major contributors to these misfits in implant prosthodontics [5].

Traditional methods for creating prostheses for edentulous patients using implants include taking impressions, crafting plaster models, and manual fabrication. However, digital technology is now employed to address the challenges and inefficiencies of these conventional approaches, such as lengthy diagnostic processes and extended treatment times [6]. Digital systems use scanned images, computer-aided design (CAD), and computer-aided manufacturing (CAM) to enhance the accuracy by reducing the operator variability and eliminating material-related issues like dimensional changes in impressions and cast materials. This technology also improves patient compliance, especially for those with severe gag reflexes or difficulty opening their mouths and enhances communication between the dental team and patients [7,8].

However advanced digital prosthetic workflows require exact and detailed virtual planning models that include detailed and accurate information on the implant positions, soft tissue anatomy, and occlusion. The direct generation of virtual prosthetic models for restoring full arches and, specifically, edentulous arches, has been described as challenging and often requires a combination of multiple intraoral or extraoral scans. These scans span the entire arch and are needed to feed the model with accurate information on the occlusion and intermaxillary relations, and their registration is often impeded by the limited availability of anatomic landmarks [9,10].

In flapless surgery, the use of a surgical guide that fits perfectly with the soft tissue allows for the digital matching of virtual prostheses and subsequent CAD-CAM manufacturing [11]. Conversely, when bone remodeling is necessary, stackable guides can be utilized to assist surgery involving a crestal flap while maintaining the advantages of a digital workflow. However, these types of guides require fixed support on the buccal plate, which precludes the use of an extensive flap.

To address this issue, the impression reference technique, already employed for zygomatic implants, can be used to allow for full skeletonization. This method may be preferred by surgeons, particularly for atrophic patients who do not need zygomatic implants or to regenerate the bone level [12,13,14], as it provides a direct view of the residual bone and the anatomy of the surgical field. Thus, the benefits of a digital workflow for immediate loading are preserved.

This study describes the impression reference technique applied to the atrophic maxilla and mandible, combining the advantages of fully open flap surgery with a digital workflow for immediate loading.

2. Materials and Methods

The aim of the diagnostic phase was to create a prototype of the definitive prostheses. The pre-visualization of the final outcome was useful to assure the proper clinical parameters to rehabilitate function and aesthetic of the patients. Two different atrophic patients required implant supported rehabilitation, one of them on the lower jaw (patient A) and one on the upper jaw (patient B); they were treated with a fully open flap and the impression reference technique.

2.1. Digital Workflow

A prosthetic prototype was used to take standard radiography and digital impression with the purpose of creating a virtual patient model that involved all the functional and aesthetic information.

The first task involved functionally relining the prostheses using polysulfide elastomeric impression material (Permlastic Regular Body; Kerr Dental, Brea, CA, USA). The relined prostheses were then digitized with an intraoral scanner (TRIOS; 3Shape, Copenhagen, Denmark) outside of the mouth. This technique allowed for the precise clinical capture of the soft tissue positions and essential prosthetic details [3]. Patients wore the relined prostheses while the antagonist arch configurations were scanned, and the occlusal bites were recorded. The initial phase ended with the functional testing of the relined prostheses, evaluating clinical parameters such as the vertical dimension, midline alignment, occlusion, occlusal plane, smile line, tooth size and position, horizontal and vertical overlap, and lateral black corridors to decide whether new prostheses were needed.

Once the clinical requirements were met by the diagnostic prototypes, the existing prostheses were digitized, and 3D printed copies were created for use as radiographic templates. These templates were produced using CAD/CAM technology (Model 2.0 resin; NextDent 3D Systems, Soesterberg, The Netherlands), and approximately eight 2 mm radiopaque quartz spheres were added to the exterior surface. A double-scan method was utilized for radiographic examination: initially, patients underwent CBCT scans while wearing the diagnostic templates in occlusion, followed by scanning the templates alone using the same CBCT unit (NewTom, Cefla, Imola, Italy). Specialized implant planning software (BlueSkyBio, New York, NY, USA) was then used to align the bone volumes with the radiographic templates, simulating prostheses and soft tissues, to develop virtual patient models as shown in patient A for the lower jaw surgery (Figure 1).

For implant placement, anatomical landmarks were employed to determine the optimal emergence of the implants and then checked using a prosthetic open guide supported by the impression reference. Implant positions and approximate lengths were determined and validated on the virtual prostheses. These procedures were performed for both the upper and lower jaws.

2.2. Impression Reference Technique Device

The impression reference (IR) is a prototyping tool featuring central bushings, designed by the authors using DentalCAD 2.4 software (exocad, Darmstadt, Germany) and produced with a digital light processing (DLP) 3D printer (Model 5100 printer/software, 1920 × 1080 resolution; NextDent 3D Systems). The known points, angles, and bushings enable the software to seamlessly align the stereolithographic (STL) file with the CBCT scan in the following steps [3] (Figure 2).

An impression reference positioning stent was manufactured to be used in the first step. It was constructed as a copy of the prosthesis containing parts connected each other via a snap-on connection (OT, rhein, etc.) (Figure 3).

A prosthetic template reproducing the final prosthesis contour (open guide) was manufactured with the possibility of being connected on the IR after the remotion of the IR positioning stent (Figure 4).

2.3. Surgical Procedure

Under local anesthesia (4% articaine with adrenaline [1:100,000], and 2% mepivacaine with adrenaline [1:100,000]), the impression reference (IR) was positioned with the specific stent and then fixed along the midline of the mandibular crest or the midline of the palate depending on the jaw to rehabilitate on each patient. Bone screws measuring 8 or 11 mm in length (Global D, Brignais, France) were used for the IR fixation (Figure 5). Prior to further steps, a complete intraoral scan of the edentulous upper or lower arch and the fixed IR was conducted. A full-thickness flap was then raised using two crestal and two vertical mesial incisions for the mandibular bone and two crestal and two vertical distal incisions for the maxillary bone, while preserving the retromolar trigone area and the maxillary tuberosity. Once the bone was exposed, rotary instruments were utilized to prepare the implant sites following the manufacturer’s protocol (Global D, Brignais, France). Implant entry points were based on anatomical landmarks previously identified through 3D planning for optimal implant emergence using the customized open guide attached to the IR via a clip (Figure 6). Six implants were manually placed to ensure primary stability. Optimal conical abutments were then fixed on the implants, and healing caps were screwed on to facilitate suturing (Vicryl 4-0; Ethicon, Somerville, NJ, USA).

After the healing caps were removed, scan abutments were positioned for a second intraoral scan. The digital impression captured the buccal soft tissues and crestal aspects of the scan abutments, including the retromolar trigone or maxillary tuberosity, with the IR affixed (Figure 7). Once data acquisition was complete, and the quality of the impression was assessed, the IR was removed, and the healing caps were repositioned, a post operative ortopantomography was taken to assure the correct positioning of the implants (Figure 8).

2.4. Laboratory Analysis

Dedicated digital dental modeling software (exo-cad) was utilized to align the preoperative soft tissue scans with those taken while the IRs were in place. This alignment enabled fitting the soft tissues to the bases of the diagnostic prototypes, facilitating subsequent prosthetic projection alignment. The postoperative IRs and scan abutments were also aligned by matching the pre- and postoperative STL files, resulting in complete virtual models of the implant positions, virtual prostheses, and associated soft tissues (Figure 9).

Accuracy check templates (Model 2.0 resin; NextDent 3D Systems) were printed using a Model 5100 printer and software at a layer height of 50 microns to verify the fit of the framework. The metal frameworks were designed and manufactured in cobalt–chrome with 3D software (SUM3D; CIMsystem, Milan, Italy). Aesthetic coatings were applied using pre-colored multilayer polymethylmethacrylate (PMMA) (breCAM.multi-COM, Bredent, Senden, Germany) (Figure 10). Upon completing the milling and printing processes, pre-milled titanium abutments (Gemini LINK IN System; New Ancorvis, Bologna, Italy) were cemented into the metal frameworks using Panavia SA universal cement (Kuraray Noritake, Tokyo, Japan). STL models containing implant analogues were also produced to verify the accuracy of the framework.

Finally, the aesthetic coatings were set by relining them with acrylic resin. All prostheses were verified using printed check guides and then refined. Adequate space was maintained around the implants to ensure good oral hygiene.

2.5. Prosthesis Generation

Both fixed, screw-retained, and metallic PMMA-based prostheses were delivered within 72 h. They were inserted following the removal of the healing abutments, and their accuracy and integrity were verified. Product precision was assessed through direct probing and radiographs taken at the implant connections., with each screw tightened individually while applying unilateral pressure alternately. Once stability was confirmed, and the potential bias from postoperative swelling was checked, a proper occlusal balance in both static and dynamic states was ensured. The goal was to minimize the lateral loading by distributing it across the entire framework. Bilateral contact in occlusion was required, as was canine guidance or group function during lateral movements. A minimal cantilever of less than 5 mm was permitted, and adequate space was provided to maintain hygiene. A dynamometric ratchet was used to tighten the screws at 15 Ncm, and the screw holes were sealed with Teflon strips and composite material (Mosaic; Ultradent, Cologne, Germany).

After the prosthesis delivery (Figure 11), a panoramic radiograph was taken (Figure 12), and the sutures were removed 15 days postoperatively. Occlusion checks and screw tightening were performed every 15 days for the first 2 months. Patients were instructed to follow a soft diet for the first 3 months.

3. Results

The integration of CAD/CAM technology streamlined the entire workflow for delivering screw-retained immediate loaded prostheses with a precise passive fit across both patients. Notably, this advancement allowed for the replication of diagnostic prototypes in the final prosthetics, ensuring that the aesthetic and functional characteristics identified during the diagnostic phase were faithfully preserved in the final outcome.

4. Discussion

The transition from traditional to digital workflows in implant dentistry represents a significant advancement in achieving precision and efficiency in prosthetic rehabilitation. This study introduces the impression reference technique, specifically designed to address the challenges of immediate loading in open flap implant surgeries, particularly in severely atrophic maxillae and mandibles. By integrating digital technologies with traditional surgical approaches, this method aims to enhance the accuracy and predictability of prosthetic outcomes.

Immediate loading of implants has been shown to offer substantial benefits, including a reduced treatment time, immediate function, and improved esthetics, which contribute to patient satisfaction and psychological well-being [15,16]. However, the success of immediate loading is highly contingent upon the precision of prosthetic frameworks, with any inaccuracies potentially leading to significant mechanical and biological complications [17,18]. Digital workflows have emerged as a solution to these challenges, offering improvements in accuracy, efficiency, and patient comfort.

Traditional methods of prosthesis fabrication, involving manual impressions and plaster models, have been associated with several limitations, including inaccuracies due to material dimensional changes and operator variability [19]. The advent of digital technologies, such as intraoral scanners, CAD, and CAM, has revolutionized this process by minimizing these issues [20,21]. Digital impressions facilitate a more precise capture of the patient’s oral anatomy, reducing errors associated with conventional impression materials and improving the overall accuracy of the prosthetic fit [22,23].

Despite these advantages, fully digital workflows can encounter difficulties when extensive open flap surgeries are required, such as in cases involving severe bone atrophy or simultaneous bone augmentation [24,25]. The impression reference technique, as described in this study, offers a valuable solution by integrating digital planning with the benefits of an open flap approach. This technique allows for detailed visualization of the surgical field and accurate transfer of prosthetic plans, addressing the limitations of both digital and conventional methods [26,27].

A key aspect of the impression reference technique is its ability to combine digital and traditional elements effectively. The use of a prototyping tool with predefined reference points and bushings enables the precise alignment of digital models with the surgical field, improving the accuracy of implant placement by allowing the use of stackable guides that facilitate the proper insertion of the implants and prosthetic fitting [28,29]. This hybrid approach preserves the advantages of digital workflows, such as the enhanced precision and reduced need for postoperative adjustments, while accommodating the practical demands of open flap surgeries [30,31].

Several studies have highlighted the efficacy of digital workflows in various implant procedures. For instance, Mangano et al. demonstrated the benefits of digital impressions in improving the accuracy of full-arch restorations, which aligns with the findings of this study [32,33]. Similarly, research by Smith and Brown and Joda and Brägger has shown that digital workflows can streamline the prosthetic fabrication process and enhance the overall treatment efficiency [34,35]. The integration of these technologies into the impression reference technique further supports its effectiveness in immediate loading scenarios.

Furthermore, the application of digital technologies in implant dentistry is not limited to impression techniques alone. Advances in 3D imaging and digital planning software have significantly improved the precision of implant placement and prosthetic design [35,36]. For example, the use of stackable guides and shape memory abutment systems has facilitated predictable and accurate immediate loading, as noted in the recent literature [37,38]. These innovations complement the impression reference technique by ensuring that digital and physical elements align seamlessly, contributing to the overall success of the rehabilitation process.

The study also underscores the importance of maintaining accuracy throughout the workflow, from the initial planning to the final prosthetic delivery. The use of CAD/CAM technologies for fabricating prostheses and verifying their fit through printed check guides exemplifies the commitment to achieving high standards of precision and reliability [39,40]. The successful integration of these technologies into the impression reference technique highlights its potential to set new benchmarks in implant rehabilitation.

Moreover, adopting CAD/CAM technology has significantly reduced the overall chair time during treatment, minimizing the need for adjustments and enhancing patient comfort. By eliminating the traditional process of taking postoperative impressions, it also effectively lowers the risk of contamination.

In essence, CAD/CAM technology not only enhances the accuracy but also boosts the efficiency in prosthetic treatments, offering both clinicians and patients a more reliable and streamlined experience.

5. Conclusions

The impression reference technique represents a significant advancement in the field of implant dentistry by bridging the gap between digital and traditional methods. By leveraging the strengths of both approaches, this technique enhances the accuracy, efficiency, and predictability of immediate loading procedures, particularly in challenging clinical scenarios. Future research should focus on further validating the effectiveness of this technique in larger patient cohorts and exploring its potential applications in other areas of implant dentistry.