Implementation of a new navigated parallel drill guide for femoral neck fractures

Abstract

Trauma navigation modules often use conventional mechanical surgical tools for basic simulation of drilling procedures or single screw placement. However, the precise parallel placement or specific angles that may be required are not displayed on the navigation screen. A more complex mechanical tool, a parallel drill guide (PDG) for femoral neck fractures, was integrated into a navigation module as part of this study, thus combining the advantages of mechanical tools with the benefits of fluoroscopic navigation. To implement a conventional PDG with a conventional navigation system, the tool was equipped with a non-detachable reflective marker array. Navigation engineers adapted the software to enable the navigated PDG to be displayed. We evaluated the conventional technique in comparison with the navigated technique using plastic bone models and a fresh frozen cadaver. Implementation of the navigated PDG did not pose any problems, in terms of either the software or the surgical procedure itself. The total operation time was extended by 30%, but the radiation time for the navigated group was reduced by 50%. No software or hardware-related failures occurred. Complete integration of a cannulated parallel drill guide into a navigation system has been successfully implemented. Continuous display of the mechanical guide with two parallel trajectories on the navigation screen enables safe, efficient screw placement within mechanical guidelines, without the need for additional radiographic control once initial registration has been completed.

• Navigated drill guide
• femoral neck fracture
• fluoroscopic navigation
• tool implementation

Introduction

During surgical navigation procedures, the use of conventional surgical instruments is currently the preferred approach. The computer generates models of these instruments, and displays them in relation to the bone structures shown in the fluoroscopic images. To facilitate localization by the navigation system, the instruments are equipped with reflective marker spheres. The system integrates the localization data for the simulation and execution of procedures such as drilling, screw placement, and implant positioning. However, some surgical procedures require specific anatomical guides. These ensure, for example, the exact parallel placement of two or more screws, and thus enable correct and stable implant positioning. Common applications include fractures of the dens axis, the femoral neck, or the talus and tibial plateau.

Surgical treatment of femoral neck fractures with minimally invasive screws has increased in importance over the last few years, offering an alternative to hip arthroplasty. Reported failure rates after primary misplacement or secondary dislocation range from 10 to 50%, often as a result of inadequate screw placement [1], [2]. Only the exact parallel placement of two or sometimes three screws guarantees sufficiently secure biomechanical fixation [1], [3]. Some of the available complex mechanical tools, such as the Manninger parallel drill guide (PDG) for femoral neck fractures, allow minimally invasive screw placement under radiographic control, without the aid of navigation. However, screw misplacement has been known to occur when those devices are used [3–6]. The use of fluoroscopic navigation resulted in both improved accuracy and a significant reduction in radiation exposure time for certain traumatological indications [7–9]. These included drilling procedures, specific surgical tool placement, or reduction procedures.

Applications and benefits for hip fractures have been demonstrated and evaluated in preclinical and clinical studies [7], [10]. In many cases, simple adaptations were enhanced by attaching temporary reference marker arrays to conventional tools for procedures such as k-wire and drill bit navigation or single screw placement. Consequently, operative requirements such as parallel screw placement or the representation of specific angles were not displayed or included in the software. In this paper, we describe the development and implementation of a new navigated PDG for the treatment of femoral neck fractures. This approach combines the mechanical advantages of the existing PDG with the benefits of fluoroscopic navigation, and enables exact parallel screw placement. Initial tests with the new and fully integrated navigated PDG were performed on intact plastic hip models and a cadaver, and comparisons made to the conventional technique.

Materials and methods

Initially, a conventional mechanical parallel drill guide (PDG) for femoral neck fractures from the Manninger® screw system (DePuy, Germany) was implemented and modified. Once the requirements for the new navigated PDG were defined, a prototype was constructed by professional mechanical engineers. This prototype was equipped with a non-detachable reflective marker array, and adapts easily for bilateral use (Figure 1). Software engineers at BrainLAB then integrated the tool into a special fluoroscopic navigation system (VectorVision, BrainLAB, Heimstetten, Germany). The requirements for adequate parallel screw placement, such as exact indication of distance and screw trajectory, were primary factors in the development of the software (Figure 2). Other factors were continuous display on the screen of the navigated PDG and any additional tools that might be required (Figure 3). In addition to the commercial trauma navigation module, this software enables illustration of the navigated PDG itself, precalibration of the PDG, planning of two parallel screw trajectories, extended parallel screw trajectories, visualization of the navigated drill in combination with the PDG, and recording of first and second screw placements.

Figure 1. A customized prototype of the navigated parallel drill guide (PDG), equipped with a stable, integrated marker array. The reference markers can easily be adjusted for bilateral use.

Figure 2. Visualization of the parallel drill guide displayed in the software. An aiming device and parallel screw planning allow for exact parallel screw placement within the radiographic landmarks. This trial was done on a plastic bone model. The upper right “AutoPilot” view visualizes the aiming of the actual drill or PDG (green broken line) according to the planned trajectory (red line), and shows discrepancies by deviations of the red dot from the center.

Figure 3. Navigation screen during the use of the navigated PDG on the cadaver trial. The three X-ray-based images show in axial and AP views the initially planned trajectory of the first caudal screw (red line). The two green lines visualize the potential aiming device, including the projected position of the caudal (green broken line) and cranial (green dotted line) drill canals through the PDG. The navigated drill bit positioned in the cranial screw canal is shown in brown with a larger diameter and displayed in combination with the PDG.

Initial feasibility tests comparing the conventional PDG and the navigated PDG were performed on 10 intact plastic femur bone models (Synbone, Switzerland). These included five complete insertion procedures for both femoral neck screws in accordance with the manufacturer's recommended technique. Finally, additional tests were conducted on a fresh frozen cadaver, with the conventional and navigated techniques performed bilaterally. Cylindrical foam was used to simulate soft tissue on the plastic femurs. The conventional technique was tested using two conventional C-arms (Exposcope 8000, Ziehm, Germany) and a predetermined operational setup. The navigated technique used one C-arm (Exposcope 8000, Ziehm, Germany). The dedicated trauma navigation software was used in the Vector Vision navigation system. The conventional and navigated techniques described below were repeated with the exact same procedure on all plastic bones and the cadaver. During the performance of both techniques, the total radiation exposure times (in seconds) and the operating times (in minutes) were recorded and compared.

Conventional technique

Initial k-wire (2.5 mm) placement was carried out under continuous fluoroscopic control, using anterior-posterior (AP) and axial views. The conventional PDG was used to perform step-by-step insertion of two k-wires (for two cannulated screws), drill sleeves, and, finally, two 9-mm parallel sliding femoral neck screws (DePuy). Final results were evaluated using AP and axial radiographs.

Navigated technique

A Vector Vision navigation system (BrainLAB) was used. In all cases, the reference array for the femoral bone was attached 20 cm distal to the trochanter major. Two initial C-arm radiographs (AP and axial) were taken for image registration. Trajectories were planned for two parallel screws and saved on the screen in all cases. Navigated drilling of a 3.6-mm hole was then performed instead of k-wire placement. The drill sleeves and the two parallel sliding screws were then inserted without radiographic control. Instead, continuous display of the navigated PDG was used as a guide (Figures 3 and 4). Individual steps from the conventional insertion procedure were adapted to the requirements of navigation.

Results

No problems were encountered with the software integration of the navigated PDG. There were no complications with the navigated tool, and handling of the tool itself did not differ from that of the conventional PDG. After slight modification of the operating setup, and positioning of the C-arm monitor and the navigation system, the implementation of the navigated PDG was successful (Figures 4 and 5). Both the handling and the radiographic results of the navigated technique could be reproduced. The average total operation time with the conventional technique was 16 min, while the use of navigation extended the average time by 30% to 21 min per case. In contrast, the measured radiation exposure time for the navigated group was reduced by more than 50%, from an average of 48 seconds conventionally to 18 seconds with navigation. All results were based on average measurements obtained with both plastic bones and the cadaver trial.

Figure 4. Combination of the navigated PDG with drill sleeves and the navigated drill bit in a cadaver trial during navigated fixation; both the navigated drill and the parallel drill guide (PDG) are displayed on the screen. The reference array on the bone is placed 20 cm distal to the surgical field.

Figure 5. A dedicated set-up of all necessary systems facilitates cost-effective computer-assisted navigation. The set-up varies from that used with the conventional technique.

Discussion

Until now, a combination of mechanical and navigated tools to achieve the required screw placement and stability in femoral neck fractures has not been available. An integration of the Manninger cannulated femoral neck screw system with the corresponding PDG has been developed and described in this paper. Continuous display of the mechanical drill guide and the two parallel trajectories on the navigation screen enables safe, efficient placement of two or more parallel screws without additional radiographic control. Although radiographs are still necessary for registration and final assessment, continuous fluoroscopy throughout the procedure is not required. We do, however, strongly recommend acquisition of a control X-ray before completion of final screw placement. Other surgical indications also require exact parallel placement of screws for osteosynthesis. Fractures and the bone morphology in various anatomic regions require exact parallel placement of screws to achieve stable fixation [1], [3]. Examples include screw fixation of fractures of the dens axis, tibial plateau, and hip, and arthrodesis of the ankle and other foot joints. A randomized cadaver study is necessary to confirm the ease of use of the reported system, and clinical studies will be required to demonstrate the intraoperative benefits of the navigated PDG as compared to the conventional technique.