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Accuracy and Technical Predictability of Computer Guided Bone Harvesting from the Mandible

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11 March 2024
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Accuracy and Technical Predictability of Computer Guided Bone Harvesting from the Mandible: A Cone-Beam CT Analysis in 22 Consecutive Patients

Abstract

This study assesses the accuracy and technical predictability of a computer-guided procedure for harvesting bone from the external oblique ridge using a patient-specific cutting guide. Twenty-two patients needing bone augmentation for implant placement were subjected to mandibular osteotomy employing a case-specific stereolithographic surgical guide generated through computer aided design. Differences between planned and real cut planes were measured comparing preand post-operative Cone Beam Computed Tomography images of the donor site according to six validated angular and displacement indexes. Accuracy and technical predictability were assessed for 119 osteotomy planes over the study population. Three different guide fitting approaches were compared. An average root-mean-square discrepancy of 0.52 (0.30–0.97) mm was detected. The accuracy of apical and medial planes was higher than the mesial and distal planes due to occasional antero-posterior guide shift. Fitting the guide with an extra reference point on the closest tooth performed better than using only the bone surface, with two indexes significantly lower and less disperse. The study showed that the surgical plan was actualized with a 1 mm safety margin, allowing effective nerve preservation and reducing technical variability. When possible, surgical guide design should allow fitting on the closest tooth based on both radiological and/or intra-oral scan data.

Keywords

bone harvesting; computer assisted surgery; accuracy; predictability; cone beam computed tomography

Introduction

During prosthodontic restorations, optimal implant placement in the presence of alveolar crest atrophy or defects is strongly dependent on bone augmentation procedures [1,2]. Autogenous bone is the most predictable material to support new bone formation, allowing for a higher bone survival rate and implant success [3–7]. Harvesting in the intraoral donor sites, such as in the retromolar region of the mandibular ramus, is effective and safe for treating up to medium-size alveolar defects [8], and many freehand bone harvesting approaches have been described in the literature, making use of different tools, such as burs, diamond discs, or piezo surgery [9–11]. Avoiding mandibular canal and nerve damage is essential to the success of these interventions [2,12–14]. For this reason, in order to provide an anatomical localization of the mandibular canal, mental nerve, and dental roots, an accurate pre-procedural planning is usually based on high-resolution cone beam computed tomography (CBCT) images [15]. The current strategy to limit nerve damage with freehand incision is to perform a conservative cutting of the cortical plate in the apical portion of the ramus [12,16].

Intra-operative surgical guidance may overcome the limits of freehand surgery, as successfully shown in the computer-guided implant surgery based on stereolithographic printing of templates able to guide drilling and implant insertion [17–22]. Computer guided bone harvesting may in fact improve the safety of the procedure as well as reduce variability due to skill factors, thus improving the technical predictability of the intervention. Fully digital workflows were explored to support horizontal ridge augmentation with intraoral bone blocks [23]. Computer Assisted Surgery (CAS) based on computer guided bone harvesting has been proposed to guide the in situ autogenous onlay grafting technique to augment horizontal bone defects of the anterior maxilla [24].

Similarly to those technologies, a computer guided approach to harvest the bone from the mandible has been recently presented [25,26]. By this novel approach, a stereolithographic template is designed during the pre-procedural surgical planning on CBCT 3D images and fitted to the harvesting site cortical bone to guide the movement of the cutting tool [27]. In this way, the position, angulation, and depth of the osteotomy is controlled, optimizing the volume of the harvestable bone block while reducing the risk of damage to anatomical structures close to the donor site. The osteotomy lines, in terms of length, direction and depth, are planned in advance using dedicated software, and the case-specific stereolithographic surgical guide is generated through a computer aided design/computer aided manufacturing (CAD/CAM) process [27]. The first published case-report [25] showed that this approach allowed the surgeon to perform the planned osteotomy lines in accordance with the surgical plan with a minimal discrepancy, and an adequate bone volume was effectively collected in relation to the alveolar defect, suggesting that the technique was clinically feasible and safe, as further confirmed by a subsequent short case series [26]. However, to be proposed as a safe and effective procedure, the typical spatial and angular accuracy achievable with this technique should be evaluated and quantified by using relevant metrics and defining clinically relevant indications. A comprehensive framework for accuracy validation has recently been proposed to systematically and automatically quantify the positional and angular errors between planned and real osteotomy planes [28]. A set of metrics were validated on both human cadaver heads and patients who underwent autogenous bone grafting for dental implant placement and proved effective in quantitatively comparing the real outcome of the procedure with the planned osteotomy geometry [28].

The aim of this radiological cone bean computer tomography (CBCT) study was to quantitatively assess the accuracy and technical predictability of this novel computer guided technique that is applied to bone harvesting from the external oblique ridge. The assessment was performed in a cohort of consecutively treated patients providing evidencebased safety margins to be considered during planning. Accordingly, the primary endpoint of the study was the quantification of the spatial and angular displacements between planned and actual cuts obtained with the patient-specific stereolithographic surgical guide generated through a CAD/CAM process. Furthermore, this study investigated the impact, in terms of accuracy, of three different strategies available to position the cutting guide at the donor site. Specifically, the following null hypothesis was challenged: “no difference in accuracy is present whether the antero posterior (i.e., mesial-distal) placement of the surgical guide is obtained exploiting one of the following strategies: using CBCT bone surface data; using bone surface data and a builtin reference point to the closest tooth obtained from CBCT data; using bone surface data and a built-in reference point to the closest tooth obtained from both CBCT and intraoral scan data”.

Materials and Methods

Study Population and Patients’ Groups

The study had an observational retrospective design. Twenty-two consecutive subjects were considered among partially edentulous patients, presenting deficient bone quantity for implant placement and who were treated for an autogenous bone augmentation procedure according to the protocol of De Stavola et al. [25–27] by a single surgeon who also developed the planning and designed the surgical guide. All patients were fully informed about the surgical procedures and treatment alternatives and agreed to the proposed treatment at the time of surgery. Indications for treatment were the presence of a severe bone atrophy of the alveolar ridge in the horizontal and/or vertical plane, and a sufficient bone quantity in the donor area of the mandible (external oblique ridge and/or ramus). Treatment exclusion criteria consisted of bone defects following tumor resection, heavy smoking habits (more than 10 cigarettes per day), severe renal and liver disease, a history of radiotherapy in the head and neck region, chemotherapy for treatment of malignant tumors at the time of the surgical procedures, uncontrolled diabetes, active periodontal disease involving the residual dentition, mucosal disease in the areas to be treated, poor oral hygiene, and non-compliance with autogenous bone augmentation surgery.

According to patients’ clinical characteristics and situation, the surgical guide was planned, exploiting different ways to set the guide in position before the surgery. More specifically, each included subject was treated implementing one of the following three different strategies to allow the mesial-distal guide alignment: the surgical guide was planned using only data of bone surface (group A, seven patients); the surgical guide was planned using only CBCT data of the bone and teeth surface, and realized with a built-in reference point to the closest tooth (group B, eight patients); and the surgical guide was planned using data of both bone (CBCT data) and teeth surface (from both CBCT and intra-oral scan data), and realized with a built-in reference point to the closest tooth (group C, seven patients). 

The observational retrospective design did not require the approval of an ethics committee, as per Italian legislation on clinical investigations at the time of the study. Nevertheless, the investigation was carried out following the rules of the Declaration of Helsinki of 1975, revised in 2013, and performed according to the principles of the ICH Good Clinical Practice.

Computer-Aided Surgical Planning

Pre-operative analysis included a complete medical history, a clinical and radiological examination of the stomatognathic system, and a thorough analysis of the implant recipient site as well as of the bone donor site. Cross sectional images using CBCT (Newtom Giano, Cefla, Charlotte, NC, USA) were obtained preoperatively, with isotropic image reconstruction of 0.3 mm and a field of view of 8 × 11 cm, for assessing the crest dimension and for planning the bone block harvesting. 3D digital dental impressions were also acquired for the patients of group C using an intraoral scanner (Trios 3, 3Shape, Copenhagen Denmark). The planning protocol followed the procedure described in the International Patent N. PCT/IB2014/061624 [27]. Briefly, four steps were realized as described below. The CBCT Digital Imaging and Communication in Medicine (DICOM) datasets were processed with a diagnostic and analysis software (RealGUIDE™ Software Suite, 3Diemme, Cantù, CO, Italy) and the mesial-distal linear defect dimension was measured. Ideal bone cutting planes were defined through each cross-sectional image, keeping a minimum safety margin of 1 mm from the anatomical structures to preserve, such as roots and mandibular canal. Once the cutting planes were established, their projections outside the bone surface were used to define the internal facets of the surgical guide. Each facet was thought to guide the cutting tool direction once this was lent against the surface of the surgical guide. The final guide design was shaped making use of CAD software and then produced in medical polyamide through a CAM process. In those cases where a STereo Lithography interface format (STL) file of the residual dentition was available (from the original DICOM data or from and intraoral scan), a built-in reference point to the closest tooth was designed to facilitate the mesial-distal alignment of the guide.

Surgical Procedure

The surgical interventions were performed according to the protocol described in De Stavola et al. [25,26], and is briefly summarized here. A full thickness flap was elevated evidencing the external oblique ridge and the lateral aspect of the ramus as well as the lateral aspect of the mandibular body. The surgical guide was placed in the donor site, finding its planned position by checking the best fit between the bone surface and the guide shape. When available (group B and C), the tooth-reference point was also exploited to set the guide in position by simply leaning the guide extension on the pre-defined tooth reference point. No mechanical support was obtained by the tooth. In all cases, the surgical guide was securely stabilized to the bone by placing one 1.3 mm-diameter screw in the built-in hole of the surgical guide. Representative pictures of the surgical fields and corresponding 3D rendering of the surgical guides for each study group are shown in Figure 1.

Figure 1. Representative pictures of the surgical fields (top row) and corresponding 3D renderings of the surgical guides (bottom row) for each study group. The surgical guide was designed and positioned according to one of the three following strategies: (a,d), group (A) using CBCT bone surface data; (b,e), group (B) using bone surface data and a built-in reference point to the closest tooth obtained from CBCT data; (c,f), group (C) using bone surface data and a built-in reference point to the closest tooth obtained from both CBCT and intraoral scan data. Before bone incision, the surgical guide was securely stabilized to the bone by placing one 1.3 mm-diameter screw in the built-in hole of the surgical guide (green arrowhead).

The osteotomy cuts were made facing the flat side of the piezoelectric insert to the internal face of the surgical guide. The cutting direction was unequivocally defined by the surgical guide, while the working depth was defined by the volumetric image analysis. The block was then removed by a straight thin elevator without the necessity of hammering.

The flap was sutured with single and/or mattress sutures and the bone block was then grafted at the defect site following Khoury’s bone augmentation approach [11]. Post-operative CBCT data were acquired, applying the same acquisition settings of the pre-operative scans as a standard protocol after the bone augmentation procedure to ensure the good result of the bone augmentation surgery and excluding any complications which could require immediate treatment. According to the specific treatment plan, implants were placed in the reconstructed alveolar crest after 4 months and were prosthetically loaded after an additional four months of healing time.

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