3D-Printed Patient-Specific Instruments-Combined AI Virtual Preoperative Planning-Assisted Medial Fixed-Bearing Unicompartmental Knee Arthroplasty

Summary

Here, we present a protocol to enhance the precision of fixed-bearing unicompartmental knee arthroplasty. It uses a digital workflow, combining 3D-printed patient-specific instruments with preoperative simulation to improve alignment accuracy, increase surgical safety, and enable personalized, reproducible outcomes.

Abstract

Unicompartmental knee arthroplasty (UKA) is widely recognized as an effective treatment for advanced knee osteoarthritis. However, its dependency on surgical experience and two-dimensional (2D) radiographs introduces significant challenges for less-experienced surgeons and for patients with femoral or tibial bone deformities. To enhance surgical precision and reduce prosthesis malposition-related complications, our team integrated three-dimensional (3D) printing technology for patient-specific instrumentation (PSI) with artificial intelligence (AI)-based preoperative simulation planning, thereby addressing the limitations of traditional approaches. This study presents a comprehensive digital workflow for fixed-bearing UKA that aims to achieve precise prosthetic alignment, improve surgical efficiency and safety, reduce operative time, and enable personalized prosthesis positioning with consistent and reproducible outcomes. The protocol consists of five critical steps: 1. Strict patient selection based on fixed-bearing UKA indications; 2. Comprehensive imaging acquisition, including CT scans of the affected knee, anteroposterior and lateral radiographs, and full-length weight-bearing X-rays in DICOM format; 3. AI-assisted preoperative planning with detailed surgical simulation reports; 4. Fabrication of customized 3D-printed cutting guides; 5. Precise osteotomy execution with intraoperative verification. By implementing this standardized digital approach, we demonstrate how integrating AI and 3D printing can optimize UKA outcomes through enhanced reproducibility, accuracy, and patient-specific customization.

Introduction

In recent years, the deep integration of digital technologies with orthopedic science has driven a paradigm shift in clinical practice-transitioning from traditional empirical and generalized methods to precision-based, personalized, and fully digital diagnostic and therapeutic strategies¹. Within this transformative landscape, 3D printing technology, as a flagship application of digital innovation, has emerged as a pivotal enabler for achieving patient-specific customization and highly precise surgical interventions in diverse orthopedic procedures^2,3. Osteoarthritis (OA), recognized as the most common degenerative joint disease worldwide, exhibits a strong correlation between its prevalence and population aging, with epidemiological studies demonstrating that accelerating global demographic aging has established OA as a major cause of chronic pain and locomotor dysfunction in middle-aged and elderly populations⁴. Regarding treatment options for medial compartment osteoarthritis (MOA), UKA presents distinct clinical advantages over total knee arthroplasty (TKA), including better preservation of native knee anatomy and physiological function, a lower incidence of postoperative complications, and significantly shorter rehabilitation periods.

The successful implementation of medial UKA is critically dependent on precise lower limb alignment reconstruction, where key challenges include variability in osteotomy execution, soft tissue balancing, and component alignment. Technically demanding aspects such as prosthesis selection and osteotomy positioning remain predominantly dependent on the surgeon's visual estimation and manual techniques, resulting in substantial operator-dependent variability, with performance discrepancies exceeding 30% among surgeons with different levels of experience⁵. Furthermore, patient-specific anatomical variations coupled with the surgeon's learning curve for new prosthesis systems frequently contribute to inconsistent clinical outcomes in UKA procedures, highlighting the need for greater standardization in surgical techniques⁶. The integration of AI planning and 3D-printed PSI directly addresses these challenges by enabling patient-specific alignment planning, automated guide design, and submillimeter osteotomy accuracy, thereby reducing dependence on surgical experience and improving reproducibility. Recent advances in 3D-printed PSI for UKA have further demonstrated these advantages⁷. Extensive research in recent years has shown that 3D digital imaging design combined with 3D printing technology enables the development of personalized surgical navigation systems for knee arthroplasty, where computed tomography-magnetic resonance imaging (CT-MRI) multimodal image fusion-based 3D-printed PSI significantly enhances the accuracy of limb alignment correction, osteotomy volume control, tibial plateau sizing, and femoral condylar prosthesis positioning^8,9,10.

To address the inherent limitations of conventional UKA techniques, our research team has developed an innovative approach by integrating 3D-printed PSI with AI-based virtual preoperative planning to assist in medial fixed-bearing UKA. By establishing a fully digitized surgical workflow for fixed-platform UKA, critical technological advancements have been achieved, including (1) preoperative biomechanical simulation using AI-driven dynamic modeling and (2) submillimeter osteotomy precision guided by 3D-printed navigation modules.

Protocol

This research received ethical approval from the First Hospital of Hebei Medical University Research Ethics Committee. Informed consent was obtained from all individual participants included in the study.

1. Patient selection and preoperative imaging

1. Select patients with anteromedial compartment osteoarthritis or medial femoral condyle osteonecrosis.
2. Confirm an intact anterior cruciate ligament (ACL), competent medial collateral ligament (MCL), and lateral compartment cartilage with Outerbridge grade ≤ II changes.
3. Exclude patients with inflammatory arthritis, varus deformity varus deformity >15°, flexion contracture >15°, or knee flexion <90°.
4. Acquire a thin-slice CT scan (≤1.0 mm) extending from 15 cm proximal to the femoral condyles to 15 cm distal to the tibial plateau.
5. Obtain complementary full-length, weight-bearing radiographs and dedicated knee views for comprehensive preoperative evaluation.

2. CT data segmentation

1. Open Mimics software v2.1 and import the CT DICOM data of the femur, patella, tibia, and fibula.
2. Use the Threshold command to adjust the standardized threshold range for bone (minimum value: 175) and filter out non-bone voxels from the image (Figure 1A).
3. Use the Region Grow command to extract bones in separate regions (Figure 1B).
4. Use the Edit Masks tool to add and fill any unselected skeletal areas (Figure 1C).
5. Convert the Masks into mesh (grid) data (Figure 1D).
6. Export the STL bone models for all relevant structures and save as backup files (Figure 1E).

3. AI-guided virtual prosthesis templating and surgical simulation

1. Open Creo Parametric software and import the STL model data of the femur, patella, tibia, and fibula.
2. Analyze the femoral anatomy by identifying the condylar line, distal femoral surface, posterior condylar surface, mechanical axis, and anatomical axis (Figure 2A-B).
3. Determine the patient-specific femoral angles (e.g., femoral varus and femoral external rotation) based on individual anatomical characteristics (Figure 2A-B).
4. Determine the patient-specific tibial angles (e.g., tibial varus, tibial retroversion, and tibial external rotation) according to the 3D anatomical model (Figure 2C-D).
5. Based on each patient's bone morphology, select the appropriate prosthesis components: femoral component (size 1-6) positioned 2-3 mm from the femoral cortical edge; tibial component (size 1-6) avoiding edge overhang or prosthesis conflict; and polyethylene insert (size 9-13 mm) (Figure 2E-F).
6. Verify implant alignment and gap balance to achieve 0° flexion and 90° extension gaps within a tolerance of ±1 mm.
7. Generate a preoperative planning report summarizing component sizing, alignment parameters, and virtual simulation results.
   NOTE: The virtual simulation for this preoperative plan was completed in an average duration of less than 2 h on a standard clinical workstation. This process typically involved 3 to 5 iterations to refine component positioning and achieve optimal soft-tissue balance, ensuring a reproducible and efficient workflow.

4. Design and production of patient-specific instruments

1. Use AI algorithms to automatically generate cutting guide designs from the finalized 3D plan. The patient-specific instrument (PSI) designs are initially generated using an automated AI-driven workflow.
   ​NOTE: The core algorithms, based on Artificia Neural Networks (ANNs) for anatomical feature recognition, analyze the 3D bone model, predefined osteotomy planes, and key anatomical landmarks¹¹. This system automatically generates the core functional elements of the guides, including the osteotomy slot positions, the bone-contacting surface ("fit-to-bone" geometry), and the overall external support structure, optimizing for parameters such as resection depth, guide stability, and contact area.
2. Perform a mandatory manual validation and editing step following the AI generation. Inspect the proposed designs meticulously in the planning software. If any issues are identified, such as insufficient contact with the cortical bone or potential soft tissue impingement, the relevant parameters (e.g., contact surface offset, pin trajectory) must be manually adjusted.
3. Design a tibial cutting guide for coronal and axial resections, incorporating three fixation pinpoints.
4. Design femoral guide I for distal resection and femoral guide II for posterior cuts and peg-hole drilling (Figure 3).
5. Fabricate all guides using medical-grade polyamide 12 via additive manufacturing using Selective Laser Sintering (SLS) technology. The printing orientation was optimized to minimize the need for support structures on critical bone-contacting surfaces, typically orienting the guide's base at a 15^\circ15 angle relative to the build platform.
6. Post-processing:
   1. Unpacking and depowdering: Remove the parts carefully from the build chamber
   2. Shot blasting: Perform shot blasting to achieve a smooth surface finish
   3. Quality inspection: Verify dimensional accuracy against the digital model
   4. Ultrasonic cleaning: Clean the guidesin deionized water to remove any residual powder
   5. Drying: Dry the guides and pack them for terminal sterilization via autoclaving at 132°C for 4 min prior to surgery.

5. Surgical procedure

1. Verify patient identity and procedure with the patient and against medical records in the preoperative holding area. Confirm signed informed consent.
2. Induce general endotracheal anesthesia. Administer prophylactic intravenous antibiotics (cefuroxime 1.5 g) 30 min prior to skin incision. Additionally, administer 1 g intravenous tranexamic acid prior to tourniquet inflation to minimize surgical blood loss.
3. Position the patient supine on the operating table. Place a nonsterile tourniquet high on the proximal thigh. Secure a lateral post at the level of the tourniquet to stabilize the femur during flexion. Pad and secure the contralateral leg. Prep and drape the operative leg in the standard sterile fashion, then, cover the foot and ankle with a sterile stockinette.
4. Perform a medial parapatellar arthrotomy while preserving the medial meniscotibial ligament.
5. Remove osteophytes that may impair guide apposition using rongeurs and osteotomes. Perform all bone resections (tibial and femoral) using an oscillating saw. The removal of osteophytes and the bone cuts are performed under direct visualization without intraoperative imaging and are guided by the preoperative plan and anatomical landmarks to ensure precise component positioning and soft tissue balance.
6. Position and fix the tibial cutting guide using 2.0 mm Kirschner wires (K-wires). Insert the K-wires under direct visualization with their trajectory aligned with the orientation of the guide pin holes.
   NOTE: Fluoroscopy is not used for guide alignment; placement is based on direct anatomical exposure and adherence to preoperative planning landmarks.
7. Perform the vertical tibial cut using a posteriorly inclined oscillating saw blade.
8. Perform the horizontal tibial cut according to the preoperatively planned posterior slope.
9. Position and fix femoral guide I, then execute the distal femoral resection along the preplanned resection plane.
10. Position and fix femoral guide II, then execute the posterior chamfer cuts and drill the fixation peg holes.
11. Trial the prosthesis components and assess the implant positioning, joint stability, and ligament balance through the range of motion.

6. Intraoperative verification and validation

1. Protect the medial collateral ligament (MCL) with a Hohmann retractor throughout the procedure.
2. Using digit calipers, verify each osteotomy surface by comparing it with the preoperative 3D-printed bone model to confirm geometric accuracy.
   NOTE: The geometry and dimensions of each osteotomy surface are verified by direct comparison with the preoperative 3D-printed bone model. Digital calipers were utilized to perform precise quantitative measurements (e.g., resection thickness, condylar dimensions) to a precision of 0.1 mm, ensuring the achieved bone cuts matched the preoperative plan.
3. Measure flexion and extension gaps under physiological loading conditions, ensuring the difference is <1 mm.
4. Assess patellar tracking dynamically through the full flexion-extension arc to confirm central tracking and prevent malalignment.
5. Document the final implant position using intraoperative radiographs and confirm absence of cortical overhang and restoration of the joint line.

Results

A 69-year-old male patient presented with bilateral knee osteoarthritis, who had previously undergone mobile-bearing unicompartmental knee arthroplasty on the right side two years ago with satisfactory postoperative recovery. Due to an intramedullary nailing procedure performed for femoral shaft fracture on the left knee two decades ago, conventional surgical instrumentation could not be properly inserted owing to the altered femoral medullary canal anatomy, thereby posing significant ris...

Discussion

The successful implementation of this digital UKA protocol hinges on several critical steps. First, high-quality, thin-slice (≤1.0 mm) CT data acquisition is paramount, as the fidelity of the subsequent 3D bone models and the precision of the PSI guides are directly dependent on it^8,12. Second, the AI-guided virtual planning stage is crucial for determining patient-specific alignment. This involves accurately identifying anatomical axes and landmarks (e.g.,...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Creo Parametric 8.0.0.0	PTC	https://www.ptc.com/ en/products/creo	Surgical simulation guide output
3D-Printed Patient-Specific Instruments	Sikon Medical Co., Ltd. China	D250610	Anatomic Conformity & Precision Positioning; Optimized Limb Alignment;Surgical Efficiency; Minimally Invasive Approach?Enhanced Reproducibility
Femoral Component	Beijing Leadcom Biomedical Co., Ltd.	A5101-05lM	Resurfaces the arthritic femoral condyle (medial/lateral).
Mimics Research 21.0	Materialise	https://www.materialise. com/en/software/mimics-research-suite	image processing
Polyethylene Insert	Beijing Leadcom Biomedical Co., Ltd.	A5301-0410	Acts as an articulating surface between femoral and tibial components
Tibial Componen	Beijing Leadcom Biomedical Co., Ltd.	A5201-04LM	Replaces the worn tibial plateau and provides a base for  the polyethylene insert.