Patient-Specific Instrumentation for Multiligament Knee
How 3D-printed patient-specific guides reduce femoral tunnel convergence in multiligament knee reconstruction, grounded in a 2026 OJSM controlled laboratory study.
Multiligament knee reconstruction is one of the most technically demanding procedures in orthopedic sports surgery. When a knee dislocation tears several ligaments at once, the surgeon may need to drill four, five, or six bone tunnels through a small volume of the distal femur. Each tunnel has an ideal anatomic entry point and trajectory. Getting them all correct, without any two tunnels colliding, is a spatial problem that is difficult to solve freehand inside the intercondylar notch.
This is the femoral tunnel convergence problem. This article explains why it happens, what the published anatomic guidelines recommend, and how 3D-printed patient-specific instrumentation (PSI) planned from a patient's own CT can help. It is anchored on a 2026 controlled laboratory study published in the Orthopaedic Journal of Sports Medicine (OJSM), authored by our team.
What is femoral tunnel convergence?
When multiple ligaments are reconstructed anatomically, each graft is fixed in a bone tunnel drilled at the native footprint of the ligament it replaces. On the femur, the footprints of the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), fibular collateral ligament (FCL), popliteus tendon, superficial medial collateral ligament (sMCL), and posterior oblique ligament (POL) sit close together. Because the femur is a curved, finite volume, tunnels drilled at anatomic entry points can intersect below the surface.
Tunnel convergence is not a cosmetic issue. Published anatomic work notes that colliding tunnels can damage a graft, compromise fixation, leave tunnels too short for secure fixation, or leave insufficient bone stock for graft integration (Moatshe et al., AJSM 2017). In a single-ligament ACL reconstruction this is rarely a concern. In a multiligament knee, where several tunnels share the same block of bone, the risk compounds with each additional tunnel.
Why freehand drilling is hard here
The published guidelines to avoid convergence are precise and directional. Moatshe and colleagues calculated intertunnel relationships across ACL, PCL, FCL, popliteus, sMCL, and POL reconstructions and recommended, for example, that FCL and popliteus tunnels be aimed roughly 35 degrees anteriorly to avoid the ACL tunnel, and that medial-side tunnels be angled proximally and anteriorly to avoid the PCL tunnels (Moatshe et al., AJSM 2017; Moatshe et al., Annals of Joint).
Translating those numbers to a specific patient in the operating room is the hard part. The guidelines are population averages. Every femur is a slightly different shape, and the surgeon has to reproduce a three-dimensional drilling angle by feel, often through a limited arthroscopic or open field, while managing multiple grafts. The procedure carries a steep learning curve, and small errors in entry point or trajectory accumulate across tunnels.
How patient-specific instrumentation addresses it
Patient-specific instrumentation reframes the problem from "reproduce an angle by feel" to "seat a guide that only fits one way." The workflow, at a high level, is:
- Acquire a CT of the patient's knee.
- Segment the femur into a 3D bone model.
- Plan each tunnel's entry point and trajectory on that model, checking for convergence in software before any bone is drilled.
- Manufacture a 3D-printed guide whose contact surface is the negative of that specific patient's bone anatomy, so it registers to one location and directs the drill along the planned path.
Because the plan is checked in three dimensions ahead of time, convergence can be designed out before surgery rather than discovered during it. The guide carries that plan into the operating room physically, which reduces reliance on freehand angle estimation. This is closely related to the CT-based bone segmentation and 3D modeling steps that underpin modern surgical planning.
The OJSM 2026 controlled laboratory study
Our group tested this directly. In 3D-Printed Patient-Specific Guides Reduce Femoral Tunnel Convergence in Anatomic Knee Multiligament Reconstruction: Controlled Laboratory Study (Cirdi, Serteser, Mavi, Ergun, Akgun), we compared patient-specific 3D-printed guides against conventional freehand technique in a simulated multiligament reconstruction (Cirdi et al., OJSM 2026, DOI 10.1177/23259671261417360).
The study found that patient-specific guides significantly reduced femoral tunnel convergence and improved tunnel entry-point precision compared with the freehand approach. A companion deep dive on the OJSM PSI study walks through the design and results in more detail.
Two honesty points matter here, and we state them plainly:
- This is a controlled laboratory study, measuring tunnel convergence and entry-point accuracy in a simulated setting. It is a bench-level engineering result, not a clinical outcomes trial. It does not measure graft survival, patient-reported outcomes, revision rates, or return to sport.
- A laboratory reduction in convergence is a proxy for a plausible clinical benefit, not proof of one. Whether reduced convergence translates to better patient outcomes requires clinical study.
That said, our findings are consistent with an independent body of work reporting that 3D-printed patient-specific instrumentation reproduces femoral bone tunnels in multiligament knee injuries accurately, in some reports more accurately than freehand technique (three-dimensional-printed PSI for femoral bone tunnels, International Orthopaedics 2023). Convergence in a controlled model is a well-defined, measurable engineering endpoint, which is what makes it a useful thing to study first.
Where Salnus fits
Salnus is software-only, browser-based orthopedic surgical planning. For the PSI workflow, our tools support CT-based 3D bone segmentation and tunnel planning that feed the guide-design step. Two design choices are worth calling out:
- Client-side by default. DICOM images are parsed and inference runs on the surgeon's own device (Cornerstone3D for rendering, ONNX Runtime Web for on-device inference), so scans are not uploaded to a server to be planned. In practice this supports a scan-to-plan-in-minutes workflow on the machine in front of you, without the round trip of a cloud upload. We frame that as a client-side speed and data-locality property, not a cleared clinical benchmark.
- Implant-agnostic and vendor-neutral. The planning geometry is not tied to a single implant system or manufacturer, which is one reason the approach is relevant to OEMs and to surgeons who use different fixation systems.
A note on regulatory status: Salnus is currently designated Research Use Only (RUO) and is in pilot. It is not a cleared medical device. Nothing here should be read as a clearance claim.
PSI versus freehand: a plain comparison
| Dimension | Conventional freehand | Patient-specific instrumentation |
|---|---|---|
| Tunnel planning | Intraoperative, by feel and landmarks | Preoperative, on a 3D model of the patient's bone |
| Convergence check | During drilling | Before drilling, in software |
| Angle reproduction | Estimated in the OR | Directed by a guide that fits one way |
| Anatomy handled | Population-average guidelines | This patient's specific femur |
| Evidence in this article | Baseline comparator | Reduced convergence in OJSM 2026 lab study (not clinical outcomes) |
Related planning problems
Tunnel convergence is one instance of a broader theme in orthopedic AI: turning a spatial judgment call into a measured, pre-checked plan. The same logic applies to coronal-plane alignment with LDFA, MPTA, and HKA in arthroplasty planning, and to the PCL, a ligament whose reconstruction is under-studied relative to the ACL and where AI-assisted research on the PCL is still early. In each case the goal is not to replace the surgeon's decision but to give it a quantified, reproducible substrate.
FAQ
What is femoral tunnel convergence and why does it matter? It is when two or more bone tunnels drilled in the femur intersect below the surface. In multiligament reconstruction it can damage a graft, weaken fixation, shorten a tunnel, or reduce bone stock available for graft integration. It becomes more likely as more ligaments are reconstructed in the same block of bone.
How do 3D-printed patient-specific guides reduce convergence? Each tunnel's entry point and trajectory is planned on a 3D model of the patient's own femur, and convergence is checked in software before surgery. A guide is then printed whose contact surface matches that patient's bone, so it seats in one place and directs the drill along the planned path, reducing reliance on freehand angle estimation.
Does the OJSM study prove better patient outcomes? No. The 2026 OJSM paper is a controlled laboratory study. It shows reduced tunnel convergence and improved entry-point precision in a simulated setting. It does not measure clinical outcomes such as graft survival, revision, or return to sport, and those questions require separate clinical study.
Is Salnus an FDA or CE cleared device? No. Salnus is currently Research Use Only (RUO) and in pilot. It is not a cleared medical device. Regulatory status should be verified independently for your jurisdiction.
What does client-side planning mean here? DICOM images are parsed and the AI inference runs on the surgeon's own device rather than being uploaded to a server. This supports a fast, local scan-to-plan workflow and keeps images on the machine. It is described as a data-locality and speed property, not a clinical performance claim.
Is the planning tied to a specific implant brand? No. The planning geometry is implant-agnostic and vendor-neutral, which is why it is relevant across different fixation systems and to OEM partners.
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Disclaimer: This article is for educational and research purposes only. Salnus tools are designated for Research Use Only (RUO) and are not cleared medical devices. Mention of third-party products is for educational context only and does not imply endorsement or comparison of clinical equivalence. Clinical decisions should be made by qualified physicians, and regulatory status should be independently verified for your jurisdiction.
References:
- Cirdi YU, Serteser B, Mavi A, Ergun S, Akgun U. 3D-Printed Patient-Specific Guides Reduce Femoral Tunnel Convergence in Anatomic Knee Multiligament Reconstruction: Controlled Laboratory Study. Orthopaedic Journal of Sports Medicine, 2026. DOI 10.1177/23259671261417360.
- Moatshe G, Brady AW, Slette EL, Chahla J, Turnbull TL, Engebretsen L, LaPrade RF. Multiple Ligament Reconstruction Femoral Tunnels: Intertunnel Relationships and Guidelines to Avoid Convergence. American Journal of Sports Medicine, 2017.
- Moatshe G, et al. How to avoid tunnel convergence in a multiligament injured knee. Annals of Joint.
- Three-dimensional-printed patient-specific instrumentation is an accurate tool to reproduce femoral bone tunnels in multiple-ligament knee injuries. International Orthopaedics, 2023.
- MacDessi SJ, Griffiths-Jones W, Harris IA, Bellemans J, Chen DB. Coronal Plane Alignment of the Knee (CPAK) classification. The Bone & Joint Journal, 2021.
Reviewed by the Salnus biomedical engineering team.