Development and Evaluation of a Novel Mixed Reality-Based Surgical Navigation System for Distal Locking of Intramedullary Nails

Abstract

Intramedullary nailing (IMN) is the gold standard for fixing mid-shaft fractures of long bones, but distal locking remains a challenging procedure. This study aims to develop and evaluate a novel mixed reality (MR)-based surgical navigation system for distal locking of IMN through phantom experiments. Twelve bone models closely replicating the mechanical properties, anatomy, and density of human tibial bone were utilized. Six orthopedic surgeons participated in the phantom experiments using both MR and traditional electromagnetic (EM) navigation systems. Effectiveness was evaluated using postoperative fluoroscopic imaging and the time taken for distal locking. Compared to the EM navigation system, the MR system significantly reduced distal locking time (81.54 ± 6.06 vs. 132.67 ± 6.45 s per screw) and achieved a higher success rate (23/24 vs. 21/24 screws accurately placed), but the difference in terms of success rate is not statistically significant. The MR-based navigation system for distal locking of IMN is time-efficient, accurate, and shows high potential for enhancing surgical precision in orthopedic procedures.

1. Introduction

Intramedullary nailing (IMN) is the gold standard for the fixation of mid-shaft fractures of long bones. With advancements in nail design and multiple interlocking options, the scope of IMN fixation has significantly expanded [1]. Traditionally, distal locking of IMN has been performed using a free-hand approach under fluoroscopic guidance, where the surgeon relies on real-time X-ray imaging to manually align the nail with the locking holes and guide the insertion of locking screws [2]. However, this technique is technically demanding, requiring high precision, and exposes both the patient and surgical team to prolonged radiation, increasing the risk of radiation-related complications.

To address the limitations of the traditional free-hand approach, various alternative distal locking methods for IMNs have been developed, including modified free-hand techniques with or without the use of radiolucent drills and jigs, mechanical guiding systems that are attached either to the proximal part of the nail or to the image intensifier or even to the surgical table, and computer-assisted navigation systems with or without the application of robotics [3]. However, accurate targeting of the distal holes is problematic due to the inevitable deformation of the nail when it is inserted into the tibia and that is the main cause of failure for many of the aforementioned targeting systems, especially for those that are mounted to the proximal section of the nail [3]. As an alternative solution, several computer-assisted surgical navigation systems for the distal locking of IMN have been developed, many of which utilize electromagnetic (EM) tracking technology [4,5,6]. EM tracking employs a field generator and sensors to monitor the position and orientation of surgical instruments in real time, enabling precise localization of the locking holes and eliminating the need for continuous fluoroscopy [7]. Among these distal aiming systems, commercial solutions such as the SURESHOT Distal Targeting System (Smith & Nephew, Memphis, TN, USA) have demonstrated promising clinical utility. However, the accuracy of EM tracking systems is susceptible to interference from nearby ferromagnetic objects, which can compromise their reliability. Additionally, these systems are limited by proprietary restrictions, as they only support specific nail manufacturers and do not provide access to open-source software for broader clinical adaptation. Another drawback is that surgeons must shift their attention to an external monitor rather than receiving in situ visualizations of the drilling trajectory and IMN, which reduces hand–eye coordination and may serve as a distraction during navigation.

The integration of mixed reality (MR) technology into surgical navigation systems has great potential to overcome these limitations. MR-based navigation systems can be categorized into X-ray image-based [8], integral videography-based [9], and head-mounted display (HMD)-based systems [10]. Among these, HMD-based MR navigation provides the best hand–eye coordination as it overlays critical information directly onto the surgeon’s field of view [11]. This direct integration allows surgeons to access real-time data without diverting their gaze to separate monitors, thereby streamlining the surgical process and reducing cognitive load [12]. Although several HMD-based MR navigation prototypes have been developed, there is still a lack of well-designed experimental studies to validate their feasibility and accuracy. Establishing standardized evaluation methods is essential for assessing the clinical applicability of these systems and improving their reliability in real surgical scenarios.

In this study, we developed an EM tracking-based MR navigation system for distal locking of IMN using Microsoft HoloLens 2, a state-of-the-art optical see-through HMD. We then conducted phantom experiments to evaluate the performance and feasibility of this system.

2. Materials and Methods

2.1. Materials

Synthetic tibial bone models (Tibia Model FZ001, ENOVO Co., Ltd., Beijing, China) were utilized to simulate the distal locking procedures. These bone models have a 17 pounds per cubic foot (PCF) density cancellous foam core, a 10 mm canal, and an overall length of 40.5 cm. Our simulated cortical bone has a density, toughness, strength, modulus, and hardness all similar to cadaveric cortical bone. These models closely replicate the mechanical properties, anatomy, and density of human tibial bone, allowing for controlled, reproducible experimentation in a simulated clinical setting. These models were used primarily to assess the accuracy and efficiency of the navigation system.

The Universal Tibial Nail II B-MIN-01 (Double Medical Technology Co., Ltd., Xiamen, China) was used for phantom experiments. This nail is designed for tibial intramedullary nailing procedures and was selected for its versatility in accommodating a range of tibial fractures. The nail system includes both proximal and distal locking holes, facilitating secure fixation during surgery. The phantom experiments involved the insertion of this nail and the subsequent placement of locking screws, including two mediolateral (ML) and two anteroposterior (AP) distal locking screws, to simulate a typical tibial fracture fixation after fracture reduction.

2.2. The MR Navigation System

As shown in Figure 1, the developed MR navigation system for distal locking of the IMN is a highly integrated software–hardware system designed to enhance precision in surgical procedures. The hardware components consist of several key elements: an HMD device (HoloLens 2, Microsoft Inc., Redmond, WA, USA) for MR visualization, an EM tracking system (Aurora V3, Northern Digital Inc., Waterloo, ON, Canada) for tracking the relative position and orientation between the intramedullary nail and the drill bit, an EM probe (Aurora 6DOF Probe, Part Number 610175, Northern Digital Inc., Waterloo, ON, Canada) for navigation registration, and an in-house 3D-printed registration cube for accurate MR registration.

The software architecture is composed of two primary components: the tracking system, which operates on a workstation, and the MR visualization system, deployed on the HMD device. The tracking system plays a pivotal role in managing communication with the EM tracking components. It ensures the seamless transfer of real-time spatial data, which is critical for the registration and calibration of the surgical navigation process. Through this system, accurate positioning of both the intramedullary nail and the drill bit is obtained in real-time. The MR visualization system, which communicates with the tracking system via a TCP/IP connection, is responsible for creating an MR environment that integrates the physical surgical field with virtual elements. It superimposes the virtual model of the intramedullary nail and the planned screw placement onto the real-world surgical site, facilitating a clear, intuitive visualization of the planned pathway. The system is developed using a combination of open-source software tools, including Unity, the Mixed Reality Toolkit (MRTK 2.5.4), Visualization Toolkit (VTK 7.1), and Qt (5.11.2).

The core principle behind the MR navigation system is based on generating an optimal drilling path derived from the 3D model of the IMN (Figure 2). This drilling path is defined by the central axis of both the ML and AP holes. The system works by aligning landmarks on the virtual IMN model with their corresponding locations on the real IMN, allowing for the construction of a transformation matrix between the IMN model and the EM tracking system. Using the MR registration procedure proposed in [13], the transformation between the EM tracking system and the virtual space is also established. As a result, the virtual IMN model and the planned drilling path can be precisely overlaid onto the real IMN during the procedure. Once the IMN is inserted into the medullary cavity, the surgeon can easily locate the locking hole, facilitating more efficient and accurate distal locking of the IMN.

2.3. System Registration

To achieve the fusion of virtual and real objects and enable dynamic tracking, two extrinsic registration procedures are required for the proposed navigation system, as illustrated in Figure 3. These steps are essential for establishing precise spatial relationships among the physical surgical environment, the EM tracking system, and the MR visualization rendered on the HoloLens 2.

The first registration procedure establishes the transformation between the EM reference coordinate system of the intramedullary nail and the preoperative image coordinate system. Given the slender and symmetric geometry of the IMN, which lacks distinct surface features, we designed a custom cylindrical alignment tool to assist in registering the distal screw hole region. The external marking point $q_i$ was then obtained using a magnetic pointing device. By pairing these points with pre-defined image coordinates $p_i$, the transformation matrix $T_{Image}^{Nail}$ can be calculated through the absolute orientation quaternion method, using the set of corresponding observation points ${(q}_i,p_i)$.

The second registration procedure aligns the coordinate system of the HoloLens 2 with the EM tracking system to enable consistent MR overlay. This was achieved using a pivot calibration technique. A registration cube, designed with a known geometry and equipped with an EM sensor, was used to define spatial correspondence. The tip of the calibration cube was pivoted around a fixed point, enabling the estimation of the sensor’s local transformation within the EM field. Once this is completed, the position of the cube in the world coordinate system can be described as follows:

$$\widehat{m_i}={\left[T_{Cube}^{World}\right]}_{4\times 4}·\widehat{l_i}$$

(1)

For the fusion of virtual and real objects, we preset the virtual point’s position $n_i$ in the HoloLens coordinate system. A transformation matrix $T_{Holo}^{World}$ is then calculated using a landmark-based method to map points from the tracking system to the virtual coordinate system. Since the local position of the intramedullary nail model is the same in both the image and V-model coordinate systems, the matrix chain can be closed, allowing the position and orientation of the V-model coordinate system relative to the HoloLens coordinate system to be determined. This can be represented as follows:

$$T_{Vmodel}^{Holo}={(T_{Holo}^{World})}^{-1}·T_{Nail}^{World}·T_{Image}^{Nail}$$

(2)

During tracking, the virtual model and the intramedullary nail remain aligned by continuously updating the transformation matrix $T_{Vmodel}^{Holo}$ in real time.

2.4. System Integration

Based on the hardware setup described in Section 2.2 and the registration methods detailed in Section 2.3, the proposed MR surgical navigation system is structured as a unified workflow that integrates real-time tracking, coordinate transformation, and MR visualization. The overall system is designed to be modular and compatible with existing surgical workflows in distal interlocking of intramedullary nail.

The process begins with system initialization and calibration. The EM tracking system is first calibrated to ensure accuracy in the presence of potential environmental interference. The MR registration procedure then establishes spatial correspondence between the physical intramedullary nail and its virtual counterpart using landmark-based transformations. Simultaneously, the calibration cube is used to register the HoloLens 2 coordinate system with the global reference frame of the operating space. Once registration is completed, real-time tracking of the IMN and surgical drill is activated. The MR system overlays the 3D model of the IMN and the planned screw path directly into the surgeon’s field of view through the HoloLens 2. This enables intuitive alignment without the need to shift attention to an external monitor.

The MR navigation system is designed to integrate seamlessly into the existing clinical workflow. After fracture reduction and nail insertion, the surgeon activates the MR system, completes registration in two minutes, and proceeds to distal locking guided by the overlaid trajectory. No changes to standard surgical instruments or techniques are required, and no intraoperative fluoroscopy is needed once registration is complete. The system therefore offers an intuitive and minimally disruptive enhancement to current practices, improving efficiency and spatial awareness while maintaining workflow familiarity for the surgeon.

3. Experiments and Results

3.1. Phantom Experiment Design

For the phantom experiments, we designed a comparative study between the MR navigation system and the traditional EM navigation system. Six male orthopedic surgeons, aged between 34 and 40, with 4 to 8 years of surgical experience in orthopedic specialty, participated in the study. All surgeons had prior experience in tibial IMN surgery, using either free-hand techniques or the traditional EM tracking system. However, none of the surgeons had previous experience with MR navigation in orthopedic procedures. Prior to the experiment, a standardized training session was conducted to familiarize the surgeons with the HMD-based MR navigation system. This included a 30 min hands-on session utilizing synthetic bone models, allowing the surgeons to practice using the MR system in a controlled environment. To minimize bias, the six surgeons were randomly assigned into two groups: three surgeons initially performed the procedures using the MR navigation system, while the other three surgeons started with the traditional EM navigation system.

Each surgeon performed procedures on two phantom models: one using MR navigation and the other using traditional EM navigation. In each case, the surgeon was tasked with placing two distal ML locking screws and two distal AP locking screws. In total, 24 screws (12 ML and 12 AP) were placed using the MR navigation system, while another 24 screws (12 ML and 12 AP) were placed using the traditional EM navigation system. This experimental setup allowed for a direct comparison between the two navigation techniques. The MR surgical navigation scene for the phantom experiments is shown in Figure 4.

3.2. Evaluation Metric

The effectiveness and efficiency of the developed MR surgical navigation system were evaluated in comparison to the traditional EM navigation system.

For the effectiveness evaluation, fluoroscopic images in both the AP and ML views were obtained to assess the accuracy of the inserted screws after the phantom experiment. Two orthopedic specialists independently evaluated the screw placement, and the evaluation was considered complete only when both specialists reached a consensus regarding the accuracy of the screw positioning. If the specialists failed to agree, the case was excluded from the analysis. To quantify the system’s performance, the number of cases in which the specialists agreed on the correct placement of the screws was recorded. The success rate was then calculated as the proportion of successful cases (where both specialists agreed) relative to the total number of valid cases. This success rate serves as an objective metric for assessing the MR navigation system’s effectiveness in ensuring precise screw placement during the surgical procedure.

For the efficiency evaluation, the time taken for each screw distal locking was recorded. Specifically, the duration was measured from the initiation of the screw placement procedure (starting from the moment the surgeon begins positioning the drill) to the completion of the insertion (when the screw is fully placed and secured). This time measurement was carried out for each individual hole, including both the ML and AP screw insertions. The total time for each screw placement was recorded for both the MR navigation system and the traditional EM navigation system.

Moreover, immediately after the phantom procedure, each participant was then asked to complete a five-point Likert scale questionnaire that assessed their satisfaction with the MR-based navigation system (MRNS). The questionnaire was designed with advice and feedback from three senior experienced orthopedic surgeons after careful discussion. The score for each question ranges from 1 (completely disagree) to 5 (completely agree). The details of the questionnaire are illustrated in

Table 1. Results of the Likert questionnaire demonstrating the attitude of the surgeons to the novel system.

Statements	Median Score (IQR)
The system setup process is easy and efficient, and needs no special training.	5 (0)
Compared to conventional EM tracking method, this MRNS is more intuitive and helpful.	5 (0)
Compared to conventional EM tracking method, this MRNS is more efficient and less time consuming.	5 (0)
The system does not require intraoperative radiation and could improve the success rate compared to conventional EM tracking method.	5 (0.75)
The problem of hand-eye discordance in the traditional EM navigation system could be solved using the novel MRNS.	5 (0)
You are willing to apply this novel system in clinical practice and recommend it to your colleagues.	5 (0.75)
This system holds great promise for clinical application.	5 (0.75)

Scores range from 1 (completely disagree) to 5 (completely agree).

.

3.3. Phantom Experiment Results

Each of the six orthopedic surgeons performed one entire phantom tibial IMN surgery, including the distal interlocking procedure (two mediolateral (ML) holes and two anteroposterior (AP) holes) under the guide of the MRNS or traditional EM navigation system. Altogether 24 distal locking screws were inserted in each experiment group with no unexpected complications happening during the procedure. The detailed results are presented in

Table 2. Detailed comparison of effectiveness and efficiency between developed MR surgical navigation system and traditional EM navigation system in phantom experiments.

Surgeon ID	MR Surgical Navigation System					Traditional EM Navigation System					p-Value (MR vs. Traditional)
	Time Consumption for Locking (s)				Success Rate	Time Consumption for Locking (s)				Success Rate
	ML 1	ML 2	AP 1	AP 2		ML 1	ML 2	AP 1	AP 2
1	82	84	81	78	4/4	124	132	132	125	4/4	<0.001 (locking time) 0.602 * (success rate)
2	76	79	91	84	3/4	132	139	139	142	3/4
3	85	80	71	89	4/4	128	141	122	141	3/4
4	75	81	69	82	4/4	130	143	134	139	4/4
5	90	81	79	83	4/4	124	129	125	136	4/4
6	75	83	84	95	4/4	131	127	131	138	3/4
Summary	Average: 81.54 ± 6.06				Total: 23/24	Average: 132.67 ± 6.45				Total: 21/24

* Continuity correction of Pearson’s chi-square test.

.

The results show that for the MR surgical navigation system, the mean time for locking (including both drilling and screw insertion) was 81.54 ± 6.06 s for one hole. No intraoperative complications were encountered during the procedures. Postoperative fluoroscopic evaluations, conducted by two orthopedic specialists, reached full agreement in all 24 cases. Of the 24 screws, 23 were accurately inserted into the distal locking hole of the IMN. One AP screw was mispositioned, missing the distal locking hole, resulting in a success rate of 23 out of 24.

In contrast, for the traditional EM navigation system, the mean locking time was 132.67 ± 6.45 s for one hole. There was significant difference in locking time between the two approaches, both for ML screws and AP screws (Figure 5). Moreover, surgeon-level comparisons of time consumption and success rate are presented in Figure 6. The two orthopedic specialists agreed on the screw placement positions in all 24 cases based on postoperative fluoroscopy in each group. However, only 21 of the 24 screws were accurately positioned in the distal locking hole of the IMN. Three AP screws were mispositioned, missing the distal locking hole, yielding a success rate of 21 out of 24. However, there was no significant difference in the success rate between the two approaches (p-value = 0.602, continuity correction of Pearson’s chi-square test).

To evaluate the accuracy of coordinate transformation, the Root Mean Square Error (RMSE) values of the distal screw tip position between the planned and actual locations were calculated. The MR navigation system showed a mean RMSE of 1.95 ± 0.42 mm, while the traditional EM tracking method yielded a mean RMSE of 2.28 ± 0.58 mm. As summarized in

Table 3. Detailed comparison of RMSE (mm) between developed MR surgical navigation system and traditional EM navigation system in phantom experiments.

Surgeon ID	MR Surgical Navigation System				Traditional EM Navigation System				p-Value (MR vs. Traditional)
	ML 1	ML 2	AP 1	AP 2	ML 1	ML 2	AP 1	AP 2
1	1.77	1.54	2.04	2.19	1.53	1.90	2.69	3.26	0.029 *
2	1.62	2.27	2.22	2.36	2.46	1.28	1.66	3.09
3	2.37	1.42	1.83	1.43	1.83	2.98	1.25	2.66
4	1.37	1.12	2.47	2.20	3.28	2.06	2.38	2.24
5	2.02	2.75	2.55	1.54	2.17	1.94	2.60	1.91
6	1.86	1.92	1.86	1.98	2.12	1.87	2.76	2.73
Summary	1.95 ± 0.42				2.28 ± 0.58

* Continuity correction of Pearson’s chi-square test.

, these results demonstrate improved registration accuracy with the MR-based method, with the difference being statistically significant (p = 0.029).

3.4. Likert Scale Questionnaire Results

The results of the questionnaire assessing surgeon satisfaction with the system are displayed in Table 1, which demonstrates high satisfaction to the novel MRNS. They highly agree that the novel MRNS is easy to use and has the advantage of being more efficient, intuitive, not requiring intraoperative radiation, and improving the success rate compared with the conventional EM tracking system. They believe it has great promise in future clinical applications.

4. Discussion and Conclusions

IMN is a widely used surgical technique for stabilizing long bone fractures, particularly femur and tibia fractures [14]. Distal interlocking is a critical step in this procedure, ensuring rotational stability and maintaining the length and alignment of the fractured bone. Although considerable progress has been made in IMN design and computer-aided surgery in recent years, distal interlocking remains challenging, with increased irradiation and time consumption. Once marginal demands for recourses and equipment are taken into account, X-ray-guided free-hand interlocking remains the most acceptable approach [15]. However, it requires 342 s of operative time and 18 s of fluoroscopy time for inserting a single interlocking screw [16], which is time consuming and causes much radiation exposure to the operating team and patients.

In recent years, advancements in targeting devices, navigation systems, and robotic assistance are improving the accuracy and safety of this procedure, with lower radiation exposure. Gugala et al. [17] reported a distal targeting device which could significantly decrease the mean fluoroscopy time necessary to complete distal interlocking versus the free-hand technique. However, surgery time and distal locking time remained unchanged. Anastopoulos et al. [18] reported 63 patients treated with a distal targeting device with failure in two cases and medial cortex penetration in three other cases. Davut and Doğramacı [19] introduced a new technique in which the distal locking step was easier and safer, using an intra-nail endoscopic visualization and illumination method. Chabihi et al. [20] also proposed a novel computer-assisted device for distal locking in interlocked intramedullary nails from a single fluoroscopic image. But both of them have not been tested on cadavers or real patients, which limits the generalizability and the validity of the results.

Among these methods, the commercial EM tracking system tracking the IMN and providing drill positioning for distal interlocking has been suggested by more and more orthopedic surgeons [21]. Han et al. [22] found that the operative time for distal screw insertion and radiation exposure time was significantly reduced using the EM navigation system compared with the free-hand technique. However, the surgeon needs to adapt to a new hand–eye coordinate system, combining the visual frame of reference with that of the navigation system as images are displayed on external monitors. The surgeon is also required to alternate attention between the monitor and the surgical site during the navigation task. Consequently, many surgeons find it difficult to precisely correlate the images on the monitor to the operative site. And it causes distraction to the surgeon and may compromise safety of the surgery.

To address this problem, we developed a clinical prototype navigation system based on MR. A head-mounted display (HMD) is worn, and virtual images are overlaid onto the wearer’s field of view. The wearer sees virtual images that are updated in real time, in synchrony with head movements and those of surrounding objects [23,24]. Core internal structures can also be visualized. The surgeon wearing the HoloLens 2 does not need to switch between the operation scene and the screen. Previously, we developed and tested the prototype MR navigation system for percutaneous sacroiliac screw insertion in a pilot cadaveric study [25]. The results demonstrated that the approach was feasible and accurate for guiding sacroiliac screw placement. In this study, EM tracking was chosen over optical tracking due to its superior adaptability to the clinical environment. Unlike optical systems, EM tracking does not require a clear line of sight and remains stable even when the surgical field is obstructed—an important advantage for distal locking of intramedullary nails, where space is limited and tools often block visual markers. Moreover, EM sensors can be directly integrated into surgical instruments, streamlining the procedure and improving localization efficiency. Although EM systems can be affected by metal interference, proper calibration and setup allowed us to minimize such effects and maintain accurate tracking. Then we proposed a dynamic MR-based navigation system using Microsoft HoloLens 2 to assist surgeons in completing distal interlocking. These results demonstrate that the MR navigation system not only required no intraoperative radiation and reduced the time required for screw placement but also achieved a higher success rate in correctly placing the screws compared to the traditional EM navigation system. The success rate of 23/24 in the phantom experiments for distal hole locking was higher than the success rate of 21/24 in the traditional EM navigation system, but the difference is not statistically significant. These findings support the effectiveness of the MR navigation system, offering high precision and minimal risk of complications. The absence of complications and the accuracy of screw placement further underscore the system’s potential for improving surgical outcomes in tibial IMN procedures. A comparative summary of the three navigation modalities is provided in

Table 4. Comparison of traditional EM tracking-based navigation, optical tracking-based navigation, and our EM- and MR-based navigation for distal interlocking in intramedullary nailing.

Features	Traditional EM Tracking-Based Navigation	Optical Tracking-Based Navigation	Our EM- and MR-Based Navigation
Line of sight dependency	No	Yes	No
Hand–eye coordination mismatch	Yes	Yes	No
Tracking accuracy	Moderate to high	High	Moderate to high
Sensitivity to metal interference	Yes	No	Yes
Intraoperative intuitiveness	Moderate	Moderate	High
Need for external monitor	Yes	Yes	No
Intraoperative radiation required	No	No	No
Integrated 3D visualization in field	No	No	Yes

, outlining their technical features and clinical readiness in the context of distal interlocking.

Moreover, our proposed system allows hand gestures and voice commands throughout the procedure, thus guaranteeing a sterile surgical environment while it is being used. Additionally, aligning the drill tip into the distal target hole is always demanding and time-consuming for 2D screen-based techniques. In our proposed system, this process is replaced by an online correction loop. Surgeons can receive intuitive feedback during the drilling process without having to worry about misalignment caused by lower limb displacement. In fact, the participating orthopedic surgeons involved in the study were very familiar with the EM tracking system for distal interlocking of intramedullary nails in clinical routine. The results of the Likert questionnaire showed high attitudes towards the novel MRNS. They believed that compared to the conventional EM tracking method, this MRNS was more intuitive, helpful, and thus more efficient, and the problem of hand–eye discordance can also be solved.

Our study is not without limitations. The first limitation is the limited number of cases in our study. Therefore, future randomized studies with larger sample sizes comparing our novel MR system with the conventional EM tracking method are required to further assess the approach. However, despite the small sample size, we believe the positive results from our pilot study are encouraging and may spur future studies of MR navigation in orthopedic surgery. The second limitation is that we have not employed tibial fracture models in our study. Complete tibial phantom models were used to simulate the tibial fractures after reduction. The feasibility of MRNS in the reduction process was not evaluated in our pilot study but will be evaluated in our future study.

Despite the promising results, several challenges remain before clinical implementation. First, the integration of the MR navigation system into routine surgical workflows may require additional training for surgeons. Second, potential issues such as system latency and HMD discomfort during prolonged use need to be further addressed and optimized. Finally, large-scale clinical validation is essential to fully assess the safety, reliability, and cost-effectiveness of the system in real-world surgical settings.

It is important to note that the distal interlocking step of IMN, which is the focus of this study, is carried out after fracture reduction and intramedullary nail insertion, when the fracture has already been stabilized. Therefore, the influence of fracture type or reduction strategy is minimal in this stage, and the performance of the MR navigation system is largely independent of the initial fracture complexity. However, we acknowledge that real-world clinical scenarios can be variable and occasionally suboptimal. To further validate the stability and generalizability of the system under clinical conditions, we plan to conduct a prospective in vivo study involving patients with tibial shaft fractures treated with IMN. The study will be carried out in two phases: first, in cadaveric models simulating common fracture types with varied limb positioning and soft tissue coverage; and second, in a clinical trial with patients undergoing tibial IMN surgery. Key outcome measures will include screw placement accuracy, operative time, surgeon workload, and intraoperative workflow integration. This future work aims to assess not only technical performance but also surgical usability in routine orthopedic practice.

To enhance the reliability of EM tracking in the surgical environment, we implemented both environmental optimization and real-time disturbance monitoring. A color-coded Indicator Value (IV), representing the level of EM interference, was displayed within the HoloLens interface to alert the surgeon during the procedure. This allowed for dynamic assessment of field stability and timely response to potential interference. Additionally, the HoloLens 2 demonstrated stable performance under standard surgical lighting and user movement, supported by SLAM-based inside-out tracking. While our system demonstrated reliable performance in a controlled experimental setting, we acknowledge that actual clinical environments are more complex, with variable metallic layouts, lighting conditions, and dynamic team movements that may affect tracking stability. To address this, future studies will evaluate the system’s robustness and usability under real surgical workflows, providing further validation of its clinical applicability.

In conclusion, we proposed an MR-based surgical navigation system to help surgeons complete the distal interlocking of IMN without radiation exposure. Our system allows for real-time fusion of the 3D virtual nail and its physical counterpart. During navigation, the intraoperative warning system works and provides the surgeon with intuitive feedback of deviations and EM disturbances. The experimental results showed that our system has a low time consumption of screw insertion and high success rate and may have great potential in surgical specialties.