Bridging the Markerless Tracking Validation Gap: Intraoperative X-ray Workflow Integration

arXiv:2610.21696 · cs.CV · Submitted 2026-10-15 · Read on arXiv

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Introduction to the show: ident: AI Radio. Generated commentary on the latest Artificial Intelligence papers.

Tom: We're looking at the paper: "Bridging the Markerless Tracking Validation Gap".

Jane: The gist: A clinically viable pipeline has been developed that derives per-vertebra 6-DoF pose from routine intraoperative C-arm fluoroscopy,

Tom: First, who's behind it and why it matters.

Title and authors: Tom: So we're looking at this paper, "Bridging the Markerless Tracking Validation Gap: Intraoperative X-ray Workflow Integration." It sounds like they're tackling a real problem where markerless tracking in surgery is promising but lacks solid proof.

Jane: That’s right. The core issue they’re addressing is that current markerless systems lack an independent way to check their accuracy against something we actually know to be true about the subsurface anatomy, which is what surgeons need for clinical adoption, right?

Lu: Exactly. They point out that existing ground truth methods rely on surface landmarks or cadaveric studies, which aren't what happens in a real operating room setting where you need precise translational and rotational error metrics one <ref:2610.21696#pg1>.

Meng: But the authors propose something new to solve that gap by using routine intraoperative C-arm fluoroscopy to generate a ground truth pose for every vertebra, co-registered with the RGB-D camera frame through this multimodal array.

Tom: That's the mechanical part, and it's important because they emphasize that this pipeline requires no extra radiation or instrumentation; it just uses the fluoroscopic images already taken during routine checks to get a calibrated transform chain one <ref:2610.21696#pg1>.

Jane: What makes this interesting is that they’re providing a method to derive per-vertebra six-DoF pose directly from those C-arm shots, which then links up perfectly with the RGB-D camera stream <ref:2610.21696#pg1,per-vertebra 6-DoF pose>.

Lu: The methodology involves capturing data using a multimodal fiducial array, which consists of five radiopaque ArUco markers on a PETG body, and they use established multiview 2D–three dee registration algorithms to recover the board-to-detector transform from each C-arm shot one <ref:2610.21696#pg1>.

Tom: And then they combine those transforms with intrinsic parameters to get projection matrices suitable for multi-view 2D–three dee registration, which is a technique known for achieving sub-millimeter accuracy four.

Jane: So, the paper explains that by doing this, you get the full transform chain connecting the coordinate system of vertebra J to your camera C, which is what they call projection of vertebra j’s CT into the Azure Kinect RGB-D camera frame one <ref:2610.21696#pg1>.

Meng: The results they report are pretty concrete: they validated this ex vivo against optically tracked ovine vertebrae and found an end-to-end accuracy of two point zero six ± zero point two nine mm in translation and three point three five ± zero point nine three degrees in rotation one.

Title and authors: Tom: That's a measurable number, and they combine it across four vertebrae to give a total registration error of Agg. two point zero six ± zero point two nine mm and rotational error of three point three five ± zero point nine three degrees, which is right near the thresholds often cited for spinal navigation systems one.

Jane: And then they compare their method against a rigid registration approach they tested on clinical data, finding that the rigid method outperformed per-vertebra ICP on rotation with four point five two degrees and variance of one point zero zero degree compared to eight point three six degrees and three point six nine degrees for ICP five.

Lu: That comparison is significant because it shows that even when comparing against other markerless strategies, the rigid approach has substantially lower variance across all metrics five <ref:2610.21696#pg2>.

Tom: It’s a clean result, showing that for this specific application, the rigid approach is better than per-vertebra ICP on rotation, although they admit that relying on manual deformation tuning is still a barrier to deployment five <ref:2610.21696#pg2>.

Jane: So what this means for someone listening who isn't in surgery is that it establishes a reusable infrastructure. They’re providing a benchmark against which any new markerless tracking algorithm can be tested using this ground truth derived from the intraoperative X-ray workflow one <ref:2610.21696#pg1>.

Meng: From a deployment standpoint, the key is that this entire pipeline uses standard surgical workflows and no new radiation or instrumentation, which means it could actually be integrated into an existing hospital environment pretty easily one <ref:2610.21696#pg1>.

Tom: And they stress that this method allows for the evaluation of two different markerless registration strategies on real surgical data against this independent subsurface reference, moving beyond just using surface metrics two <ref:2610.21696#pg1>.

Lu: The paper points out a limitation, though; they acknowledge that the entire process is constrained by that initial 2D–three dee registration stage when projecting the preoperative CT into the intraoperative RGB-D stream one <ref:2610.21696#pg1>.

Jane: So while they have this strong validation tool, it’s still tied to that specific transformation step, which suggests future work might focus on improving that initial mapping one <ref:2610.21696#pg1>.

Tom: Overall, this paper sets a new standard for validating markerless tracking by offering a clinically viable way to get subsurface pose measurements using routine fluoroscopy one <ref:2610.21696#pg1>.

Jane: It bridges the gap between promising lab results and what surgeons actually need for clinical trust and adoption two <ref:2610.21696#pg1>.

Lu: The implication is that if you can provide this level of accurate ground truth from routine X-ray data, then learning a mapping directly from intraoperative point clouds to vertebral pose could leverage appearance, anatomical context, and priors beyond what standard ICP methods can handle one <ref:2610.21696#pg1>.

Title and authors: Meng: For scaling this up, the challenge will be moving past this two-stage registration process and figuring out how to make the whole thing robust enough for real-time deployment without needing that complex initial mapping one <ref:2610.21696#pg1>.

Tom: So we’ve seen how they build a system that uses standard tools to create a reliable reference for testing new tracking methods one <ref:2610.21696#pg1>.

Jane: This pipeline allows researchers to test competing markerless registration strategies using data from actual surgical cases, which is a big step forward in clinical validation two <ref:2610.21696#pg1>.

Lu: The paper’s main contribution is providing this framework for benchmarking emerging markerless tracking methods against an independent subsurface reference derived from routine intraoperative C-arm fluoroscopy one <ref:2610.21696#pg1,from routine intraoperative C-arm fluoroscopy>.

Meng: It shows that you can get quantifiable translational and rotational error metrics on real surgical data, which is exactly what clinical adoption requires two <ref:2610.21696#pg1>.

Tom: We’ve seen the numbers for the validation—the two point zero six ± zero point two nine mm translation and three point three five ± zero point nine three degrees rotation are the key performance indicators here one.

Jane: And it’s important to remember that this entire setup is designed so it requires no extra radiation or special instrumentation from the surgical team, which is a huge practical win for adoption one <ref:2610.21696#pg1>.

Lu: While they show success with the rigid approach being superior on rotation metrics, they also flag that relying on manual deformation tuning remains a barrier to fully deploying that rigid method clinically five <ref:2610.21696#pg2>.

Meng: So while it’s better than some other methods in terms of variance, the dependency on manual tuning is still a hurdle for pushing this into a standard operating procedure five <ref:2610.21696#pg2>.

Tom: It seems the main takeaway here is that having this reliable subsurface reference derived from routine X-ray workflow data changes how we evaluate markerless registration methods one <ref:2610.21696#pg1>.

Jane: This paper, "Bridging the Markerless Tracking Validation Gap: Intraoperative X-ray Workflow Integration," gives us a way to move past approximate surface landmarks when validating these systems two <ref:2610.21696#pg1,Bridging the Markerless Tracking Validation Gap: Intraoperative X-ray Workflow Integration>.

Lu: The authors suggest that with sufficient fluoroscopic ground truth, a learned mapping from intraoperative point clouds to vertebral pose could instead leverage appearance, anatomical context, and priors beyond ICP’s scope one <ref:2610.21696#pg1>.

Title and authors: Meng: We need more labeled cases and we still have that constraint of the two-stage registration process they mentioned because it limits how fast we can actually run this in a clinical setting one <ref:2610.21696#pg1>.

Tom: So it’s a strong piece of infrastructure, but the next step is making sure the whole system is robust enough for real-time surgical use one <ref:2610.21696#pg1>.

Jane: It’s clear that by integrating routine X-ray data into the RGB-D stream via this multimodal array, they have created a clinically viable method for getting precise vertebral pose data one <ref:2610.21696#pg1>.

Lu: We’re looking at how this pipeline provides a reusable infrastructure for benchmarking emerging markerless tracking methods against an independent subsurface reference one <ref:2610.21696#pg1>.

Meng: It’s about using the existing C-arm workflow to generate the gold standard, which is smart because it avoids adding new risks to the operating room environment one <ref:2610.21696#pg1>.

Tom: So in summary, this work provides a way to get per-vertebra six-DoF pose from routine intraoperative C-arm fluoroscopy, co-registered with RGB-D data one <ref:2610.21696#pg1,per-vertebra 6-DoF pose from routine intraoperative C-arm fluoroscopy, co>.

Jane: This means we can now compare markerless tracking methods against a gold standard measurement that is independent of the surface landmarks surgeons might use two <ref:2610.21696#pg1>.

Lu: The overall rigid approach demonstrated superior rotation performance compared to per-vertebra ICP in their clinical data registration comparison five <ref:2610.21696#pg2>.

Meng: It’s a solid result, but the authors admit that manual deformation tuning is still a barrier for deploying that rigid method widely five <ref:2610.21696#pg2>.

Tom: So this paper provides the necessary infrastructure and validation data to move markerless tracking from promising concept to something we can actually test rigorously in a clinical setting one <ref:2610.21696#pg1>.

Jane: It sets a clear direction for how we should be evaluating these systems—by comparing them against this newly established subsurface reference two <ref:2610.21696#pg1>.

Lu: The key finding is that with this kind of ground truth, you might eventually leverage appearance and context beyond what purely geometric methods like ICP can achieve one <ref:2610.21696#pg1>.

Meng: We’ll need to see how they solve the constraint of that two-stage registration to make it a fully automated, real-time tracking solution one <ref:2610.21696#pg1>.

Tom: The title "Bridging the Markerless Tracking Validation Gap: Intraoperative X-ray Workflow Integration" really sums up what this paper is achieving in terms of solving that validation problem one <ref:2610.21696#pg1,Bridging the Markerless Tracking Validation Gap: Intraoperative X-ray Workflow Integration>.

The paper's summary: Tom: So, to recap, they took something already happening in the operating room—routine C-arm X-rays—and used that data to create an independent truth for how accurate markerless tracking methods are supposed to be.

Jane: Exactly. They’re using the C-arm images, which surgeons already look at for confirmation of the bone level, and they’re stitching that into the RGB-D camera system using a multimodal array they put right near the operating site.

Lu: The main thing they achieved is creating a pipeline that doesn't need any new radiation or special tools from the surgical team to get this ground truth pose for every single vertebra.

Tom: That’s actually pretty significant because it solves that big problem we talked about earlier—that markerless systems don't have an independent way to check their accuracy against something verifiable.

Jane: Right, because you know that current ground truth methods rely on surface landmarks or cadaver studies, which isn't what happens in a real operating room when you need precise translational and rotational error numbers.

Lu: This work provides the first quantitative evaluation of markerless spine registration methods against this independent subsurface reference using real clinical data.

Tom: And the results they got are pretty solid, showing an end-to-end accuracy of two point zero six millimeters in translation and three point three five degrees in rotation when validated on ovine vertebrae ex vivo.

Jane: That means they’ve set a benchmark that's actually useful for testing new tracking algorithms right now, which is what we call reusable infrastructure for benchmarking.

Lu: They found that the rigid method of registration performed better than per-vertebra ICP on rotation when they compared it against clinical data, with four point five two degrees versus eight point three six degrees for ICP rotation.

Tom: I’m looking at those numbers, and the difference in rotational error is substantial, especially across all metrics where the rigid approach had lower variance than the per-vertebra ICP method.

Jane: So what this changes for someone listening to the show is that it provides a repeatable way to test new markerless tracking software using data from actual surgical cases instead of just theoretical models.

Lu: The implication is that if you can provide this level of ground truth from routine X-ray data, then maybe learning a mapping directly from intraoperative point clouds to vertebral pose could leverage appearance and context beyond what purely geometric methods like ICP can do.

Tom: That’s the big idea, right? Using the C-arm data as the anchor for something that uses context too.

Jane: It means we move past just looking at surface landmarks and start demanding that these markerless systems are validated against a subsurface reality that's accessible during surgery.

Lu: The limitation they mention is still tied to the two-stage registration process, specifically when projecting the preoperative CT into the intraoperative RGB-D stream; it’s a constraint on how fast you can get real-time results.

Tom: So, it’s not perfect right now because of that initial mapping step, but it’s a huge step toward getting this validated data into actual clinical evaluation settings.

Jane: It really bridges the gap between promising lab results and what surgeons actually need for clinical trust and adoption in their operating rooms.

The paper's improvements: Tom: So, we just talked about how they built this pipeline using routine C-arm X-rays to create a ground truth for markerless tracking accuracy. Now, let’s look at what they suggest we do next with this new infrastructure.

Jane: They’re suggesting that because you have this reliable subsurface measurement now, you can finally move beyond just comparing two markerless methods against each other.

Lu: The improvement is that you can use this pipeline as a reusable benchmark for testing emerging markerless tracking algorithms on real surgical data.

Tom: So researchers won't have to build their own expensive, specialized ground truth systems anymore; they can just plug in this C-arm workflow data and test their new models against it.

Jane: That makes the whole process much more accessible for everyone working on markerless tracking because the validation step isn't a massive hurdle anymore.

Lu: They’re also pointing out that with this ground truth, you might eventually be able to use appearance and anatomical context in your mapping, which is something purely geometric methods like ICP can’t do as well.

Tom: So they think the future isn't just about better geometry; it’s about using that X-ray data to teach the AI what a vertebra looks like in context.

Jane: Right, and that brings us back to a bigger question: how do we actually use this data for things other than just tracking poses?

Lu: The authors hint that by getting better at mapping point clouds to actual vertebral poses, you’re setting the stage for more complex tasks inside the operating room.

Tom: I mean, if the AI knows where the bone is relative to a known X-ray view, it could potentially help with things like automated surgical planning or even guiding robotic tools.

Jane: It really opens up possibilities for using learned maps from intraoperative data rather than just relying on static preoperative scans.

Lu: The next step in the research direction seems to be solving that constraint we mentioned earlier—the limitation of that initial two-stage registration step when you’re trying to make this fully real-time.

Tom: So, they acknowledge the bottleneck is still getting that projection from the CT into the RGB-D stream fast enough for live use.

Jane: Exactly. It’s a limitation they flag because it tells us where we need to focus our next set of research efforts to make this pipeline truly practical in a clinical setting.

Lu: They want to see if they can refine that two-stage mapping, perhaps using the learned priors from the initial successful registration, to speed things up.

Tom: So, the immediate implication is that we have a powerful validation tool now, and now we need to work on making it fast enough for real-time deployment.

Conclusion: Tom: So we’re wrapping up on "Bridging the Markerless Tracking Validation Gap: Intraoperative X-ray Workflow Integration." Basically, they showed us how to use routine C-arm fluoroscopy to create a gold standard for validating markerless tracking methods in surgery.

Jane: Right, so they built a system that takes data already being taken in the OR and uses it as an independent reference point for testing new markerless tracking algorithms.

Lu: The main implication is that we finally have a clinically relevant way to measure translational and rotational error metrics on real surgical data, which is what surgeons actually need for adoption.

Tom: And they proved that a rigid registration approach performed better than per-vertebra ICP when comparing it against clinical data, with four point five two degrees versus eight point three six degrees for rotation.

Jane: That’s the numbers they showed, and it gives us a concrete way to judge whether a new tracking algorithm is actually performing well in a real surgical scenario.

Lu: The caveat they mentioned is that the entire process still depends on that initial two-stage registration step between the CT and the RGB-D camera stream, which limits how fast you can get real-time results.

Tom: So it’s not perfectly instantaneous yet because of that mapping bottleneck, but having this ground truth is a huge step toward getting this validated data into actual clinical evaluation settings.

Jane: It really sets a new standard for how we evaluate these systems—by comparing them against this newly established subsurface reference instead of just surface landmarks.

Lu: This work provides the reusable infrastructure we talked about, allowing researchers to test competing markerless tracking methods using data derived from routine intraoperative C-arm fluoroscopy.

Tom: So if you’re working on a new tracking model, this paper gives you the independent gold standard you need to prove your method works in a clinical context.

Jane: It’s a powerful tool for moving markerless tracking from just promising concepts to something that has real, measurable accuracy in the operating room.

Lu: The authors suggest that with enough of this ground truth data, you could eventually use learned mappings to leverage appearance and anatomical context beyond what purely geometric methods can achieve.

Tom: That’s the long-term ambition—using the X-ray data to help the AI understand *why* something is in a certain position, not just *where* it is geometrically.

Jane: It really shows that having this kind of subsurface measurement capability changes how we think about validating any new tracking technology in surgery.

Lu: The next big question for this research direction is solving that two-stage mapping constraint to make the whole thing a truly fast, real-time system.

Tom: We’ll have to keep an eye on those future work directions because moving past that initial projection stage is where the engineering challenge lies.

Jane: So while they’ve built a solid foundation with this paper, it still leaves room for improvement in making the whole pipeline more seamless for actual surgery.

Connor Daly, Salvatore Russo, Jinendra Ekanayake, Daniel S. Elson, Ferdinando Rodriguez y Baena

Hamlyn Centre for Robotic Surgery at Imperial College London · Department of Neurosurgery at Imperial College Healthcare NHS Trust, London, UK · Stanford University School of Medicine at Stanford University, California, USA

cs.CV

Submitted: 2026-10-15

Updated: 2026-10-15

Comments: This version was submitted to MICCAI 2026 for review, see journal reference for published version

Journal ref: 2026 International Conference on Medical Image Computing and Computer-Assisted Intervention (pp. 95-105). Cham: Springer Nature Switzerland

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 88/100

The gist: The gist: A clinically viable pipeline has been developed that derives per-vertebra 6-DoF pose from routine intraoperative C-arm fluoroscopy, co-registered to the RGB-D camera frame through a

Key concepts

Markerless Tracking: This refers to tracking anatomical structures during surgery without relying on physical markers attached to the patient or the surgical field. The paper aims to validate these methods by comparing their results against a highly accurate, independent reference derived from X-ray imaging, which is crucial for clinical adoption.
Per-Vertebra 6-DoF Pose: This describes obtaining a precise measurement of the position and orientation (translation and rotation) for every individual vertebra in the spine. The system achieves this by combining routine C-arm fluoroscopy with RGB-D camera data, creating a detailed map of the subsurface anatomy.
Multimodal Fiducial Array: This is a set of five radiopaque ArUco markers placed on a PETG body. These markers are visible in both X-ray imaging (C-arm) and RGB-D cameras. This dual detection capability allows the system to accurately co-register the two different data sources, ensuring they align correctly.
Ground Truth Validation: Ground truth is the 'true' or highly accurate reference measurement used to test new tracking algorithms. In this study, it is derived from C-arm fluoroscopy. This allows researchers to objectively measure how well markerless systems perform in a clinical environment, moving beyond approximate surface measurements.

Terminology

Accuracy — 1× in this paper · explained in 1 episode
Benchmarking — 1× in this paper · explained in 4 episodes

Summary

The gist: A clinically viable pipeline has been developed that derives per-vertebra 6-DoF pose from routine intraoperative C-arm fluoroscopy, co-registered to the RGB-D camera frame through a multimodal fiducial array detectable in both imaging modalities, providing an independent subsurface ground truth for validating markerless tracking methods.

Motivation and Gap

Markerless intraoperative tracking of orthopedic anatomy via RGB-D sensors is a promising emerging direction for surgical guidance Current surgical navigation systems depend on invasive bone-anchored fiducials to register (and subsequently track) preoperative imaging with the surgical scene However, their ground truth is inherently approximate: alignment relies on surgeon-indicated surface landmarks refined against intraoperative topology, neither of which references the pose of the subsurface anatomy For clinical adoption, markerless systems must be validated against a gold-standard measurement offering quantifiable translational and rotational error metrics Cadaveric studies have obtained accurate vertebral pose through rigidly fixed optical tracking arrays These requirements are not permissible within a clinical observational study, leaving a methodological gap: there is currently no clinically viable means of obtaining precise vertebral pose ground truth for evaluating markerless registration in real surgical settings.

Pipeline Development

The proposed pipeline addresses this gap by leveraging routine intraoperative C-arm imaging to derive ground-truth vertebral pose within the RGB-D camera frame The contributions of the work include:

A multimodal data capture and labelling pipeline that co-registers intraoperative RGB-D recordings with ground-truth vertebral poses derived from routine intraoperative fluoroscopy, requiring no additional radiation, no instrumentation within the surgical field, or deviation from standard surgical workflows.
The first quantitative evaluation of markerless spine registration methods on clinical data against an independent subsurface reference, reporting pervertebra translational and rotational error across three surgical sequences.
A clinically grounded evaluation framework, validated ex vivo to 2.06±0.29 mm and 3.35±0.93°, which can serve as reusable infrastructure for benchmarking markerless registration methods against an independent subsurface reference.

Data Capture and Co-registration

The study integrated routine intraoperative C-arm imaging with the RGB-D recording using a multimodal fiducial array, which consists of five radiopaque ArUco markers inset on a radio-translucent PETG body. The array was held on a boom arm adjacent to the exposition site, beneath the C-arm detector and within the RGB-D camera field of view, and covered with an ultrasound probe cover to maintain the sterile field. An intraoperative C-arm is routinely used to provide confirmation of the vertebra level being operated on, typically from an AP view with adjacent oblique views. The pipeline utilizes established multiview 2D–3D registration algorithms, as implemented in the ImFusion software suite, requiring at least two fluoroscopic images from separate poses with known projection geometry.

CT Imaging to RGB-D Registration

For each C-arm shot, the radiopaque ArUco markers are automatically detected within the image, and then board-to-detector transform is recovered via PnP pose estimation. Relative transforms between shots are computed through the common marker array coordinate system, and combined with previously calibrated intrinsic parameters to yield projection matrices suitable for multi-view 2D–3D registration. Multi-view 2D-3D X-ray to CT registration is a well-established technique capable of achieving sub-millimeter accuracy. This process yields the complete transform chain from the coordinate system of the vertebra Vj to the camera C.

Validation and Results

Ex vivo validation against optically tracked ovine vertebrae confirms end-to-end accuracy of 2.06 ± 0.29 mm and 3.35 ± 0.93°, sufficient to serve as independent ground truth for methods with substantially larger errors. The combined Total Registration Error (TRE) across four vertebrae was found to be Agg. 2.06 ± 0.29 mm and rotational error of 3.35 ± 0.93°, approaching the ≤ 2 mm and ≤ 5◦ thresholds commonly cited for spinal navigation systems. In clinical data registration, the rigid method of [5] outperformed per-vertebra ICP on rotation (4.52◦ ± 1.00◦ vs. 8.36◦ ± 3.69◦) and exhibited substantially lower variance across all metrics.

Conclusion

The clinical registration comparison in Tab. 3 demonstrates the core contribution of this work: for the first time, competing markerless registration strategies can be evaluated on real surgical data against an independent subsurface reference. The overall rigid approach is clearly superior, but its reliance on manual deformation tuning remains a barrier to clinical deployment. Key limitations remain, including the need for further labeled cases and the constraint imposed by the 2D–3D registration stage. The study concludes that with sufficient fluoroscopic ground truth, a learned mapping from intraoperative point clouds to vertebral pose could instead leverage appearance, anatomical context, and priors beyond ICP’s scope.

References

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Daly, C., Marconi, E., Riva, M., Ekanayake, J., Elson, D.S., Rodriguez y Baena, F.: Towards markerless intraoperative tracking of deformable spine tissue. In: International Conference on Medical Image Computing and Computer-Assisted Intervention. pp. 627–637. Springer (2025)
Daly, M.J., Siewerdsen, J.H., Cho, Y.B., Jaffray, D.A., Irish, J.C.: Geometric calibration of a mobile c-arm for intraoperative cone-beam ct. Medical physics 35(5), 2124–2136 (2008)
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Gao, Z., Zhang, X., Xu, Z., Jiang, C., Hu, W., Zhang, H., Hao, D.: Mazor x robotassisted upper and lower cervical pedicle screw fixation: A case report and literature review. BMC Geriatrics 24(1), 916 (2024)
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Improvements for AI systems

This pipeline allows for per-vertebra 6-DoF pose from routine intraoperative C-arm fluoroscopy, co-registered to the RGB-D camera frame, enabling direct comparison of any markerless registration estimate against a subsurface reference.
The system can perform the evaluation of two markerless registration strategies on clinical data by comparing them against an independent subsurface gold standard reference in a clinical setting, moving beyond prior evaluations limited to surrogate surface metrics.
The framework provides reusable infrastructure for benchmarking emerging markerless tracking methods on real surgical data, allowing researchers to test new tracking algorithms using the validated ground truth derived from the pipeline, which has an accuracy of 2.06 ± 0.29 mm and 3.35 ± 0.93◦.

Abstract

Markerless intraoperative tracking of spinal anatomy via RGB-D sensors is a promising alternative to invasive bone-anchored fiducials, yet no existing clinical dataset provides independent subsurface ground truth for validating these methods. We address this gap with a clinically viable pipeline that derives per-vertebra 6-DoF pose from routine intraoperative C-arm fluoroscopy, co-registered to the RGB-D camera frame through a multimodal fiducial array detectable in both imaging modalities. Critically, the pipeline requires no additional radiation, instrumentation, or deviation from standard surgical workflows, relying solely on the fluoroscopic images already acquired during routine level checks. A calibrated transform chain links the vertebral pose recovered via multi-view 2D-3D CT-to-fluoroscopy registration to the RGB-D coordinate frame, enabling direct comparison of any markerless registration estimate against a subsurface reference. Ex vivo validation against optically tracked ovine vertebrae confirms end-to-end accuracy of 2.06 +- 0.29 mm and 3.35 +- 0.93 degrees, sufficient to serve as independent ground truth for methods with substantially larger errors. We demonstrate the pipeline's utility by evaluating two markerless registration strategies on clinical data spanning 11 vertebrae across 1,500 frames from three surgical sequences: the first such evaluation against an independent subsurface reference in a clinical setting. The proposed framework provides reusable infrastructure for benchmarking emerging markerless tracking methods on real surgical data.