Assessing the accuracy of computer-planned osteotomy guided by stereolithographic template: A methodological framework applied to the mandibular bone harvesting

https://doi.org/10.1016/j.compbiomed.2019.103435Get rights and content

Highlights

  • A framework for accuracy quantification of computer-planned osteotomy techniques.

  • The framework is non-invasive, requiring only post-procedural tomographic imaging.

  • The accuracy is measured by clinically significant displacement and angle errors.

  • The accuracy assessment may define a tolerance for intervention planning.

Abstract

Intraoral autologous bone grafting represents a preferential choice for alveolar reconstruction prior to dental implant placement. Bone block harvesting guided by a computer-planned lithographic template is a novel and promising technique for optimizing the volume of harvested material, while controlling the osteotomy 3D position with respect to delicate anatomical structures. We provide a quantitative framework to non-invasively estimate the accuracy of this technique. In the proposed framework, the planned osteotomy geometry was compared to the real outcome of the procedure, obtained by segmentation of post-procedural cone beam computed tomography data. The comparison required the rigid registration between pre and post-procedural mandibular models, which was automatically accomplished by minimizing the sum of squared distances via a stochastic multi-trial iterative closest point algorithm. Bone harvesting accuracy was quantified by calculating a set of angular and displacement errors between the planned and real planes which characterized the excision block. The application of the framework to four cases showed its capability to quantify the tolerance associated with computer-guided bone harvesting techniques with submillimetric accuracy (<0.4 mm), within the limits of native image resolution. The validation methodology proved suitable for defining the safety margins of osteotomy surgical planning.

Introduction

The functional and aesthetic success of prosthodontic restorations in presence of atrophy of the alveolar crest is highly dependent on bone augmentation procedures [1,2], which allow implant placement in the optimal position. Autologous bone grafting is currently considered the “gold standard” among different bone reconstruction techniques, as it offers higher bone survival rate and implant success, thanks to the non-immunogenic, osteo-inductive and osteo-conductive properties of the augmentation material [[3], [4], [5], [6]]. Intraoral donor sites are particularly advantageous in terms of accessibility and proximity to recipient sites, morbidity, operative time, and costs [7]. In fact, bone harvesting in the retromolar region of the mandibular ramus has proved to be an effective and safe method to treat up to medium-size alveolar defects [8].

In spite of the benefits, intraoral sites may offer a limited available bone volume and may be associated with post-procedural complications, including mandibular and neurologic damage, which can affect patient acceptance in the long term [2,[9], [10], [11]]. The use of high-resolution cone beam computed tomography (CBCT) has introduced key advantages in implant dentistry [12], allowing a pre-procedural 3D planning of the intervention. Although pre-procedural imaging can also inform the harvesting procedure, the intra-procedural use of this anatomical information is limited due to the lack of reference points for accurately identifying the position of the mandibular canal, mental nerve, and dental roots. Conservative cutting of the cortical plate in the ramus apical portion is the current strategy to contain the risk of nerve damage [9,13], while damage to other anatomical structures cannot be fully avoided due to the limited control on freehand incisions. On the other hand, computer-guided implant surgery is becoming increasingly popular in dental rehabilitation, even if evidence of a clear superiority with respect to conventional methods is still inconclusive [14,15]. In particular, an established static computer-assisted procedure consists in the preoperative virtual planning of a template fitted to the local anatomy, the stereolithographic printing of the template, and its use during the intervention as a guide for drilling and implant insertion. Computer-designed stereolithographic surgical guides may offer an advantage with respect to freehand methods once the surgeon is aware of the possible accuracy deviations [[16], [17], [18], [19], [20], [21]].

Drawing from this technology, an analogous approach has been recently investigated for guiding the intraoral harvesting of autologous bone [[22], [23], [24]]. Applying this novel technique, the surgeon can take full advantage of pre-procedural, CBCT based, surgical planning using a stereolithographic template, which is fitted to the harvesting site cortical bone and guides the movement of the cutting tool. This allows to control the position, angulation and depth of osteotomy lines, maximizing the volume of the harvestable bone block at minimal risk of damage for nearby delicate anatomical structures. Given their recent introduction, computer-guided stereolithographic bone block harvesting systems still lack a proper accuracy evaluation to define their safety and precision with respect to other harvesting techniques. In this paper we propose a framework for estimating the accuracy of these novel systems, which systematically quantifies the discrepancy between the planned and real harvesting procedures in terms of positional and angular deviations of the osteotomy planes. The framework was preliminarily tested on a limited number of cases to assess the feasibility and the suitability of the method, with the ultimate goal to use it for validating the harvesting technique on a wider cohort of patients.

Section snippets

Dataset

The validation method was tested on four cases, two obtained from a human cadaver head (case 1 and 2), and two from anonymized images of two patients (case 3 and 4) who underwent autogenous bone grafting for the application of dental implants [24]. The use of medical images abided by the local statutory requirements. The validation used both the pre-procedural tomographic dataset, acquired for virtual planning and production of the surgical guides, and the follow-up post-procedural dataset

Segmentation

The segmented models of the mandible from pre and post-procedural images are shown in Fig. 4, including the cadaver skull (first row) and the two patients (second and third rows). Differences between the pre (Mpre) and post (Mpost) models were present in the harvesting site and in smaller details, as a consequence of the star artefacts produced by dental restoration materials or of alterations caused by dental interventions occurred between data acquisitions. The osteotomy surfaces resulted

Mandibular guided bone harvesting

The major limitation of conventional freehand mandibular osteotomy is the safety margin required in order to avoid damage to anatomical structures, reducing the potential volume of harvestable bone. Conservative cut patterns with limited depth have been suggested to avoid interference with the alveolar nerve and other underlying structures [9,13], promoting a fracture of the bone block instead. This practice however entails further risks and intraoperative patient discomfort. The introduction

Acknowledgments

The study has been developed under the Healthcare Research and Innovation Program (IRCS-FBK-PAT), funded by the Autonomous Province of Trento, Italy.

The study uses anonymized patient CT images collected during standard procedures of diagnosis and follow up. Informed consent was given for the use of the images, in agreement with the local statutory requirements.

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