Concept description and accuracy evaluation of a moldable surgical targeting system

Abstract

Purpose: We explain our concept for customization of a guidance instrument, present a prototype, and describe a set of experiments to evaluate its positioning and drilling accuracy. Methods: Our concept is characterized by the use of bone cement, which enables fixation of a specific configuration for each individual surgical template. This well-established medical product was selected to ensure future intraoperative fabrication of the template under sterile conditions. For customization, a manually operated alignment device is proposed that temporary defines the planned trajectory until the bone cement is hardened. Experiments ( $n=10$ ) with half-skull phantoms were performed. Analysis of accuracy comprises targeting validations and experiments including drilling in bone substitutes. Results: The resulting mean positioning error was found to be $0.41\pm 0.30\mathrm{mm}$ at the level of the target point whereas drilling was possible with a mean accuracy of $0.35\pm 0.30\mathrm{mm}$ . Conclusion: We proposed a cost-effective, easy-to-use approach for accurate instrument guidance that enables template fabrication under sterile conditions. The utilization of bone cement was proven to fulfill the demands of an easy, quick, and prospectively intraoperatively doable customization. We could demonstrate sufficient accuracy for many surgical applications, e.g., in neurosurgery. The system in this early development stage already outperforms conventional stereotactic frames and image-guided surgery systems in terms of targeting accuracy.

Keywords: cochlear implantation; image-guided surgery; micro-stereotactic frame; minimally invasive surgery; surgical template.

Fig. 1

(a) The reference frame (1, Trifix) is positioned behind the pinna (2) considering the expected drill path. (b) It is bone-anchored through small skin incisions using three bone screws (3). (c) The MCI, consisting of the planar top face of the frame and two dowel pins (4, 5), enables mounting of the surgical template. It is also used to the define the MCS ${\{\mathrm{CS}\}}_{3\text{fix}}$ with the origin in the center of the larger dowel pin, the planar top face being the $xy$-plane, and the $x$ axis being oriented toward the smaller dowel pin. For image-to-patient registration, the Trifix is equipped with four titanium spheres (6).

Fig. 2

Schematic drawing of the moldable surgical targeting system. Base plate (1), subcarrier (2), and drill bushing holder (3) forms the individual component while the bone-attached reference frame (4, Trifix) is the reusable part. Dowel pins and a screw (5) secure the connection.

Fig. 3

The Jig Maker. The pose setting device consists of six prismatic joints (1) which connect a ground plate (2) with a moving platform (3) using magnetic ball joints (4). Each strut has an interlock mechanism for rough length setting (5) and a micrometer screw (6) with Vernier scale (7) for fine adjustment. A central pillar (8) ends in an alignment pin (9) representing the planned trajectory $a_{\text{pin}}$. A mounting table (10) on top of the moving platform features the same MCI and therefore referred to as ${\{\mathrm{CS}\}}_{3\text{fix}}$ as well.

Fig. 4

A virtual CAD assembly model of the pose setting device was used to determine the strut lengths based on the planned trajectory. Coordinates of the trajectory ${\overrightarrow{t}}_{\text{plan}}$ are stored in an Excel spreadsheet that drives the parametric Jig Maker CAD model. Driven dimensions between the centers of the ball joints provide lengths of the corresponding struts. Exemplarily ${\lambda }_A$ and ${\lambda }_C$ are visualized with the corresponding center points $G_{A,0}$, $G_{A,1}$, $G_{C,0}$, and $G_{C,1}$.

Fig. 5

Ring-shaped target phantom. The RTP was screwed on the skull phantom after removing material in the region of the petrous apex to place the RTP at the natural position of the inner ear. Attached titanium spheres improve visibility in CT images and serve for registration purposes. The inward-looking tips of the RTP were used to determine the position of the bore hole by triangulation (see inset with exemplary measurements).

Fig. 6

(a), (c) Two surface renderings and (b) one slice of CBCT imaging with additional rendered objects visualizes the half-skull phantom with silicone skin (1), the reference frame (2), the RTP (3), and spherical registration markers (4). The base plate (5) is visualized to show the available workspace during trajectory planning. Target point was planned at the level of the RTP (3).

Fig. 7

Moldable surgical targeting system. (a) Base plate (1), subcarrier (2), and drill bushing holder (3) with headed drill bushing (4) on top of the mounting table (5) of the Jig Maker before applying the bone cement (6). Alignment pin (7). Security screw (8). (b) Surgical template after customization on top of the skull-mounted reference frame (9, Trikfix). (c) The surgical targeting system before starting the drilling along the rigidly determined trajectory.