Application of 3D printing in spine care

Patient specific tangible, 3D printed physical models can improve surgical performance and outcome, compared to the sole on-screen inspection of the virtual models [120]. The first step in the medical image processing for the 3D printing is the segmentation method. The accuracy of this procedure is influenced by the resolution and the slice thickness of the 2D CT image series used for the segmentation [95]. In our institution the minimum criteria for the printing process is 512x512 pixel matrix resolution, and a maximum slice

automatic or semi-automatic processes are not always adequate and therefore manual editing is inevitable. A solution to this issue is offered by the possibility of quantifying the segmentation accuracy using the inter-investigator DSI. According to the literature a DSI value of >0.85 is preferable [121],[91]. In this study the DSI value was 0.96, indicating that the segmented geometry of the LIV geometry represents accurately the anatomy of the vertebrae. Before printing, additional steps of the image processing are necessary to obtain high quality models. The surface mesh quality of the segmented geometry must be inspected for e.g. irregularities, holes, overlapping edges. In order to minimize the geometrical distortions, the following remeshing and optimization must take the preservation of the contour into consideration .

Once an accurate model of the vertebral geometry is achieved in STL format we propose a strategy of choosing any of the available 3D printing services, that the resources of a hospital permit. Our reasoning is that without an optimal, continuous utilization (not feasible in a hospital) of an in-house printing facility, its maintenance cost is a financial burden for the healthcare providers. Moreover, the technical parameters of a chosen in-house machine might not be adequate for all purposes, and therefore could potentially limit or define the projects or patients who can benefit from these technologies. In contrast, our strategy of choosing an available service, based on the predefined expectation on the geometrical accuracy, permits the most cost-effective choice for each case individually. Our comparison of an entry level, low cost (FDM) and a high category, expensive and highly precise (DLP) technologies provides evidence that a cost-effective technology can be more than suitable for patient specific 3D printed spine physical models. Final vertebral model (FVM) printing parameters with FDM technology: printing time: 343 min, total cost 198 € (euro), printing material cost 1 €/cm³. Spine TXI-LIII model printing parameters with FDM technology: printing time: 660 min, total cost 336 €, printing material cost 1 €/cm³. FVM printing parameters with DLP technology: printing time: 294 min, total cost 355 € (euro), printing material cost 3.2 €/cm³. Spine TXI-LIII model printing parameters with DLP technology: printing time: 353 min, total cost 605 €, printing material cost 3.2 €/cm³. 3D printing machines. The size of surface irregularities, even though somewhat larger for the

during surgical planning, therefore the advantage of superior printing precision of the more expensive DLP models is lost.

5.3.2. PART V. Affordable 3D printed patient-specific surgical navigation template The concept of a patient-specific navigation template was first introduced by Radermacher et al. [122], who proposed a new navigation solution for the lumbar spine, hip and knee joint surgical procedures [122],[123]. The method used milling machine to manufacture the templates from polycarbonate. Due to the advances in 3D printing and 3D modelling technologies, the use of the individualised templates became more widespread [64], [124],[125], and it made an accurate and precise choice for navigational challenges [126].

The present study demonstrates the accuracy and applicability of the developed workflow which allows the creation of an affordable, metal individualized navigational template by integration FEA in the design and surgical planning process. The integration of FEA in the pedicle screw intraoperative navigation was investigated by Abbeele M. et al.

[127], however the application of FEA in the design process of a navigational template in spine surgery by integrating the patient bone mineral density related material properties is new. The results of the simulations showed that the convergent S1 insertion is significantly stiffer than the divergent ALA insertion. This finding is supported by cadaveric experimental studies [128], [129] and clinical experience as well [130]. The biomechanical difference of the convergent and divergent insertions relies on the differences in the local bone mineral densities [131].

The combination of the 3D printing technology and cobalt-chrome casting makes the manufacturing process more affordable. Investment casting of cobalt-chrome is a widely used technology in dental laboratories [132]. 3D printed patterns for casting is an accepted method in dentistry [132], [133]; however, its application in spine surgery navigational templates is novel. The production of individualized metal navigational templates for screw

based alloys [134] , but at a higher cost and lower accessibility compared to dental casting.

Metal templates are robust, resistant to damage and can also be easily autoclaved [134].

It is widely accepted in the literature to use cadavers for testing, evaluating the fitting accuracy of a navigational template [135]. FDM technology can produce geometrically accurate spine physical models [136] and the different designs can be tested as well as the drilling accuracy can be evaluated. The use of FDM models for design process evaluation and development is advantageous due to the possibility to include retrospective patient imaging data with complex anatomical/geometrical variation (deformities, tumours, etc.) which is extremely difficult to control and integrate in the case of cadaveric specimen studies.

According to the Gertzbein-Robbins scale [137] the template theoretically allows an accurate (grade A) screw insertion (Figure 46). The suggested screw insertion surgical technique uses the philosophy of the minimally invasive pedicle screw insertion techniques (MIS) by using a Kirschner wire, cannulated tap and a pedicle screw. This technique can easily be performed by any spine surgeon familiar with MIS pedicle screw insertion.

Limitations of this study include the fact that the developed template is presented using a single case, however the workflow can be applied for different parts of the spine with different geometrical difficulties/pathologies. The presented FEA models’ loading conditions are simplified as well as the material property assignments; more complex FEA investigations would be desirable. In the future, a randomized study of specific subtypes of spinal pathologies (tumours, deformities, etc.) with a larger sample size would be preferred to demonstrate the clinical efficacy and cost-effectiveness of the developed methodology.