StraightTrack: Towards Mixed Reality Navigation System for Percutaneous K-wire Insertion

TL;DR

This work targets reliable MR-guided percutaneous K-wire insertion by addressing wire bending through a cannula-based trajectory-preserving device integrated with an on-device OST-HMD tracking system. The method combines a rigid cannula body to stabilize the wire with real-time 3D navigation and a user interface featuring an Error Indicator and Surface Marker to facilitate accurate placement without external trackers. In phantom tests with two experienced surgeons, the approach achieved an end-to-end accuracy of $2.89\pm0.97$ mm and a mean rotation of $2.88^\circ$, with the cannula-based design yielding the best overall performance and reduced wire deviation, while some surface-marker guidance results were inconsistent. The study demonstrates the potential of MR navigation to improve internal fracture fixation procedures and outlines necessary next steps, including larger-scale validation and cadaver studies, to move toward clinical translation.

Abstract

In percutaneous pelvic trauma surgery, accurate placement of Kirschner wires (K-wires) is crucial to ensure effective fracture fixation and avoid complications due to breaching the cortical bone along an unsuitable trajectory. Surgical navigation via mixed reality (MR) can help achieve precise wire placement in a low-profile form factor. Current approaches in this domain are as yet unsuitable for real-world deployment because they fall short of guaranteeing accurate visual feedback due to uncontrolled bending of the wire. To ensure accurate feedback, we introduce StraightTrack, an MR navigation system designed for percutaneous wire placement in complex anatomy. StraightTrack features a marker body equipped with a rigid access cannula that mitigates wire bending due to interactions with soft tissue and a covered bony surface. Integrated with an Optical See-Through Head-Mounted Display (OST HMD) capable of tracking the cannula body, StraightTrack offers real-time 3D visualization and guidance without external trackers, which are prone to losing line-of-sight. In phantom experiments with two experienced orthopedic surgeons, StraightTrack improves wire placement accuracy, achieving the ideal trajectory within $5.26 \pm 2.29$ mm and $2.88 \pm 1.49$ degree, compared to over 12.08 mm and 4.07 degree for comparable methods. As MR navigation systems continue to mature, StraightTrack realizes their potential for internal fracture fixation and other percutaneous orthopedic procedures.

Figures (7)

• Figure 1: (a) A real-world failure mode of a conventional mixed reality (MR) navigation system for K-wire insertion, in which the K-wire bends in an uncontrolled manner. (b) Our StraightTrack navigation system visualizes the real-time K-wire trajectory error based on head-mounted tracking of the cannula body. The user aligns the cannula body by minimizing the radius of two circles, which visualize the entry- and endpoint error of the current trajectory. The hatch mark indicates the direction of the error. (c) The cannula body features a rigid steel sheath that ensures accurate tracking of the actual K-wire trajectory up to the cortical bone. This is necessary to overcome two real-world causes of K-wire bending, namely interaction with tough soft tissue and "skating" of the K-wire tip on hard bone.
• Figure 2: The transformation chain of the system. (a) Pre-operative trajectory planning phase: The trajectory points $\mathbf{P}_{\rm l_{entry}}$ and $\mathbf{P}_{\rm l_{exit}}$(shown in green points) were annotated from a CT image (shown with a blue arrow). The CT registration $F^{\text{P}}_{\text{I}}$ (shown with a yellow dotted arrow) is computed by using an optical tracker that tracks both the CT machine $F^{\text{T}}_{\text{M}}$ and the patient $F^{\text{T}}_{\text{P}}$ (shown with a red arrow). (b) Intra-operative guidance phase: The headset tracks both the patient $F^{\text{H}}_{\text{P}}$ and the cannula $F^{\text{H}}_{\text{C}}$(shown with a red arrow), and provides navigation to align $\mathbf{n}_{tip}$ with desired trajectory(shown in green dotted line) in world coordinate. The annotated entry and exit points in the world coordinate are computed using $F^{\text{I}}_{\text{W}}$(shown in a dotted orange arrow).
• Figure 3: Navigation interface features for K-wire placement: (a) The error indicator: the user aligns the tracked insertion path(shown in blue line) with the desired trajectory(shown in green line). The circle and lines located at the entry point and end point serve as visual cues. Users need to manipulate the cannula body to minimize the size of green circles and lines. (b) The surface marker: the user positions the cannula body tip to the red cross mark, which is the insertion point on the patient's surface. The insertion point was generated by finding the closest point on the patient's surface to the desired trajectory. (c) The user perspective of error indicator in a phantom study. (d) The user perspective of surface marker and error indicator in a phantom study.
• Figure 4: System end-to-end accuracy evaluation: (a) The transformation chain used in the study, consisting of the World (T), HoloLens (H), Patient (P), CT Image (I), and Pointer (tip). 'F' represents the transformation from one element (arrowback) to another (arrowhead). The transformation was obtained using optical tracking (shown in red), manual labeling (shown in blue), pre-computed values (shown in yellow), or spatial mapping (shown in green). (b) A test user points the tooltip (shown as a red cross mark) at a landmark on a pelvic phantom, with the printed tip position shown nearby.
• Figure 5: Phantom study: (a) Experimental setup for inserting 2.8 mm k-wires under MR guidance with different marker mounts.(b) Components of the phantom, include a silicone rubber soft tissue phantom, a 3D-printed bone structure phantom, and a holder with a patient array marker.