3D-Printed Surgical Guides: CT Scan to Operating Room

The Case for Patient-Specific Instrumentation

Standard orthopaedic instruments are designed for the average anatomy. But no patient has average anatomy. Femoral bow, tibial torsion, condylar asymmetry, and prior surgical hardware all create unique geometric challenges that generic cutting guides cannot fully address.

Patient-specific instrumentation (PSI) bridges this gap by designing surgical tools — cutting guides, drill templates, reduction aids — that precisely match the individual patient's anatomy. Derived from preoperative CT or MRI data, these 3D-printed guides transfer the surgical plan directly to the bone surface, reducing reliance on intraoperative estimation and fluoroscopic verification.

Our published research in The Orthopaedic Journal of Sports Medicine validated this approach for knee reconstruction, demonstrating that PSI-guided procedures achieved significantly higher accuracy in osteotomy angle and mechanical axis correction compared to conventional techniques.

The PSI Design Workflow

The process from imaging to instrument follows a standardised pipeline that typically spans 5–10 working days.

Step 1: Image acquisition. A CT scan of the relevant anatomy is acquired with thin-slice protocol (0.5–1.0mm slice thickness) and consistent patient positioning. CT is preferred over MRI for PSI design because bone surfaces are unambiguous in CT data, whereas MRI bone-cartilage boundaries require more complex segmentation. The DICOM data is transferred to the design team.

Step 2: 3D reconstruction and segmentation. The DICOM dataset is loaded into segmentation software (3D Slicer, Mimics, or custom AI-assisted tools). Bone surfaces are identified using threshold-based or deep learning segmentation, then converted to 3D surface meshes (STL format). At Salnus, we are developing automated segmentation pipelines that reduce this step from 2–3 hours of manual work to minutes.

Step 3: Surgical planning. The 3D bone model is imported into planning software where the surgeon defines the osteotomy plane, correction angle, screw trajectories, or implant positioning. For HTO, this means defining the hinge point, opening wedge angle, and tibial slope preservation. The mechanical axis alignment targets (MPTA, HKA) guide the correction geometry.

Step 4: Guide design. The PSI is designed as a negative impression of the bone surface at the surgical site. The guide's contact surface matches the patient's bone geometry exactly, ensuring it can only seat in the correct position and orientation. Cutting slots, drill holes, and K-wire channels are incorporated to transfer the surgical plan. The design is exported as an STL file for printing.

Step 5: Manufacturing and sterilisation. The guide is 3D-printed in a biocompatible material, inspected for dimensional accuracy, and sterilised (typically autoclaved or plasma-sterilised depending on material). The guide arrives in the operating room as a sterile, single-use instrument.

Materials and Printing Technologies

The most commonly used materials for PSI in orthopaedics are biocompatible nylon (PA12, SLS printing), PETG and PLA (FDM printing for non-contact guides), and surgical-grade resin (SLA printing for high-detail guides). Each has trade-offs in resolution, strength, sterilisability, and cost.

For cutting guides that contact bone and withstand mallet impact, nylon PA12 printed via selective laser sintering (SLS) is the current standard — it combines sufficient strength with autoclavability and regulatory precedent. For drill guides and reference templates that do not bear significant load, FDM-printed PETG offers lower cost with adequate accuracy.

Clinical Evidence

The evidence base for PSI in orthopaedics has grown substantially. In osteotomy surgery (HTO, DFO), PSI-guided correction has consistently demonstrated reduced angular error compared to conventional technique — meta-analyses report mean correction accuracy within 1–2 degrees of the preoperative plan, versus 3–5 degrees with conventional methods.

In tumour surgery, PSI enables precise resection margins in complex anatomical locations (pelvis, spine) where conventional instruments cannot access the planned cutting plane. The ability to design patient-specific cutting guides around neurovascular structures has expanded the indications for limb-salvage surgery.

In arthroplasty, PSI cutting blocks for total knee replacement were among the earliest clinical applications. While initial enthusiasm has moderated (robotic-assisted surgery offers real-time feedback that static guides cannot), PSI remains valuable in revision cases and complex primary arthroplasties where standard instrumentation does not accommodate the patient's anatomy.

The AI Acceleration

The bottleneck in the PSI pipeline is steps 2–4: segmentation, planning, and guide design. These steps currently require specialised biomedical engineering skills and significant manual effort. AI is poised to compress this timeline dramatically.

Automated bone segmentation from CT using deep learning (nnU-Net, TotalSegmentator) can reduce a 2-hour manual segmentation task to minutes. AI-assisted planning tools can propose optimal correction angles based on alignment analysis and historical outcome data. Parametric guide design can auto-generate the PSI geometry once the plan is approved.

The vision is a single cloud-based platform where the surgeon uploads the CT, reviews the AI-generated plan, approves or modifies it, and the guide is sent directly to manufacturing. This is where the Salnus platform is heading.

Getting Started with PSI

If you are an orthopaedic surgeon interested in incorporating PSI into your practice, the most accessible entry point is partnering with a biomedical engineering team that handles the design and manufacturing workflow. Salnus offers this as a professional service — from CT data to sterilised guide, with the surgeon involved in planning verification.

For academic collaborations on PSI research, contact our team.

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Disclaimer: This article is for educational purposes. PSI design and use should be supervised by qualified professionals. Regulatory requirements for 3D-printed surgical instruments vary by jurisdiction.

References:

• Chaouche S, et al. Patient-specific cutting guides for high tibial osteotomy: a systematic review. Knee Surg Sports Traumatol Arthrosc. 2022;30(5):1626-1638.
• Wong KC. 3D-printed patient-specific applications in orthopedics. Orthop Res Rev. 2016;8:57-66.