High-yield overview
Additive Manufacturing in Orthopaedic Surgery
—Source Imaging
—CT (most common)
—File Format
—STL, DICOM
—Slice Thickness
1Less than mm optimal
—Applications
—Models, guides, implants
3D Printing Applications
Anatomical Models: Pre-operative planning, education
Patient-Specific Instruments: Cutting guides, drill guides
Custom Implants: Tumour reconstruction, revision arthroplasty
Key: 3D printing transforms 2D imaging into tangible surgical tools
Critical Must-Knows
- CT provides best data for 3D printing (bone segmentation)
- Thin-slice (less than 1mm) CT required for accuracy
- Patient-specific instruments improve implant positioning
- Custom implants for complex reconstruction
- Regulatory requirements for implantable devices
Clinical Pearls
- “DICOM to STL conversion is the key processing step
- “Segmentation quality determines print accuracy
- “Anatomical models reduce operative time in complex cases
- “PSI can improve component alignment in arthroplasty
Clinical Warning
3D printing is an emerging topic with increasing exam relevance. Understand the workflow from imaging to print, key applications (models, PSI, custom implants), and imaging requirements (thin-slice CT). Know the advantages and limitations of this technology.
Mnemonic
PRINTPRINT Framework
P
Problem
Define the surgical question before segmenting anything
R
Resolution
Thin-slice source imaging determines print quality
I
Isolation
Segment the anatomy accurately and remove artefact
N
Need
Print only when it changes planning, teaching, or templating
T
Test
Verify the model against source images before use
Hook:Good orthopaedic printing starts with the imaging dataset, not the printer.
Overview
Three-dimensional printing is a downstream use of imaging, not a separate imaging modality. The quality of the printed model depends first on acquisition, then segmentation, then the clinical relevance of the model that is produced.
For orthopaedics, the strongest indications are complex articular trauma, deformity planning, implant templating, and patient-specific education where spatial understanding genuinely changes decision-making.
Clinical Imaging
Imaging Atlas
Mnemonic
PRINTPRINT Workflow
P
Pre-operative planning - Identify need for model/guide
R
Reconstruction - Thin-slice CT imaging
I
Image processing - DICOM to STL conversion
N
Numerical control - 3D printer fabrication
T
Theatre use - Surgical application
Hook:Remember the 5 stages from image to operating theatre
Mnemonic
STERILESTERILE Requirements
S
Sterilisation capability - Gamma or autoclave
T
Thin slices - Less than 1mm for accuracy
E
Exact segmentation - Precise anatomical modelling
R
Regulatory approval - FDA/EU MDR clearance for implants
I
Image quality - High-resolution CT data
L
Layer height - Printer resolution affects surface finish
E
Education value - Teaching and patient communication
Hook:STERILE workflow ensures safe clinical application
Systematic Approach to 3D Printing
The Five-Stage Workflow
Stage 1: Imaging Acquisition
- Thin-slice CT (less than 1mm) provides optimal data
- MRI for soft tissue assessment in complex cases
- DICOM format required for processing
Stage 2: Segmentation and Modelling
- DICOM to STL conversion
- Threshold-based bone segmentation
- Manual refinement for accuracy
- Quality control checks
Stage 3: Design and Planning
- Virtual surgical planning
- Guide design (cutting planes, drill trajectories)
- Implant templating
- Sterilisation considerations
Stage 4: Manufacturing
- Material selection (PLA, nylon, titanium)
- Printer calibration and quality control
- Surface finishing for theatre use
- Sterilisation validation
Stage 5: Clinical Application
- Pre-operative team briefing with model
- Intra-operative guide placement
- Post-operative validation
Critical Requirements
- Sterilisation: Gamma irradiation or autoclave compatible materials
- Accuracy: Less than 1mm deviation for implantable devices
- Documentation: Full traceability from imaging to implant
Workflow Overview
| Step | Process | Key Considerations |
|---|---|---|
| 1. Image Acquisition | CT/MRI scan | Thin slices (less than 1mm), minimal artefact |
| 2. DICOM Transfer | Send to processing software | Anonymisation, secure transfer |
| 3. Segmentation | Separate structures of interest | Quality determines accuracy |
| 4. 3D Model Creation | Generate surface mesh | STL file format |
| 5. Design/Modification | Add guides, cutting slots | Engineering input for instruments |
| 6. Printing | Additive manufacturing | Material selection for purpose |
| 7. Post-Processing | Cleaning, sterilisation | Regulatory compliance for implants |
Segmentation
Segmentation is the process of identifying and separating specific anatomical structures from the imaging data. For bone, thresholding based on Hounsfield units (typically greater than 150-200 HU) can automatically segment cortical bone. Manual refinement is often needed for pathological areas, fracture fragments, or tumour boundaries. Segmentation quality directly affects the accuracy of the final print.
Imaging Requirements
| Parameter | Recommendation | Rationale |
|---|---|---|
| Modality | CT preferred for bone | Superior bone-soft tissue contrast |
| Slice thickness | Less than 1mm (ideally 0.5-0.625mm) | Reduces stair-stepping artefact |
| Field of view | Include relevant anatomy | Sufficient margins for planning |
| Metal artefact | MARS protocol if hardware present | Improves segmentation accuracy |
| Contrast | Usually unnecessary for bone | May help tumour delineation |
Clinical Applications
| Application | Benefit | Examples |
|---|---|---|
| Pre-operative planning | 3D visualisation of pathology | Complex fractures, tumour resection |
| Template/trialling | Pre-bend plates, trial implants | Pelvic fractures, spine deformity |
| Patient education | Tangible explanation of surgery | Joint replacement, deformity correction |
| Surgical training | Practice complex procedures | Resident education, rare procedures |
| Intraoperative reference | Anatomical guide in OR | Tumour margins, fracture reduction |
Cost-Benefit
Studies show anatomical models can reduce operative time by 20-30% in complex cases through improved pre-operative planning. The cost of printing is often offset by reduced theatre time. Models are particularly valuable for pelvic and acetabular trauma, complex spine deformity, and tumour surgery.
Printing Technologies
| Technology | Mechanism | Applications |
|---|---|---|
| FDM (Fused Deposition) | Extruded thermoplastic | Anatomical models, low-cost |
| SLA (Stereolithography) | UV-cured resin | High detail models, surgical guides |
| SLS (Selective Laser Sintering) | Laser-fused powder | Durable guides, nylon models |
| DMLS (Direct Metal Laser Sintering) | Metal powder laser fusion | Titanium implants |
| EBM (Electron Beam Melting) | Metal powder electron beam | Porous metal implants |
Material Selection
Material choice depends on application: PLA/ABS plastics for anatomical models, biocompatible resins for surgical guides, and titanium alloys (Ti6Al4V) for implantable devices. Porous titanium structures can be designed to match bone modulus and promote osseointegration.
Quality Assurance
| Aspect | Requirement | Verification |
|---|---|---|
| Dimensional accuracy | Less than 1mm deviation | Calliper measurement, CT comparison |
| Anatomical fidelity | Matches patient anatomy | Overlay on source CT |
| Sterilisation | Appropriate for OR use | Validated sterilisation process |
| Material biocompatibility | Non-toxic, implant grade | Material certification |
| Structural integrity | Withstands intended use | Mechanical testing |
Accuracy Validation
Studies show 3D printed anatomical models typically achieve dimensional accuracy within 0.5-1mm of the source CT data. Accuracy depends on segmentation quality, printer resolution, and post-processing. For surgical guides and implants, verification against the original imaging data is essential before clinical use.
Limitations
| Limitation | Explanation | Mitigation |
|---|---|---|
| Time | Days to weeks for complex prints | Early planning, in-house printing |
| Cost | Equipment, materials, expertise | Case selection, shared services |
| Imaging artefact | Metal hardware degrades segmentation | MARS protocols, manual editing |
| Regulatory complexity | Especially for custom implants | Partner with approved manufacturers |
| Learning curve | Segmentation and design skills | Training, dedicated staff |
Point-of-Care Printing
Hospital-based (point-of-care) 3D printing allows rapid turnaround and lower costs for anatomical models. However, it requires significant infrastructure, expertise, and quality management. For implantable devices, regulatory requirements generally necessitate partnership with certified manufacturers.
Choosing the Right Technology
Examiners often probe whether a candidate can select the appropriate technology rather than reflexively reaching for a 3D print. The table below contrasts 3D printing with the alternatives it competes with for the same clinical problems.
| Feature | 3D-printed model/guide | On-screen virtual planning | Intra-op CT navigation | Robotic assistance |
|---|---|---|---|---|
| Tactile model in hand | Yes | No | No | No |
| Pre-bend plates / trial reduction | Yes | No | No | No |
| Intra-operative adjustability | Low (fixed at print) | n/a (planning only) | High | High |
| Real-time accuracy feedback | No | No | Yes | Yes |
| Lead time before surgery | Days to weeks | Hours | None | None |
| Capital cost | Low to moderate | Low | High | Very high |
| Best-suited problem | Complex articular/pelvic trauma, deformity, tumour | Rapid templating, teaching | Pedicle screws, tumour margins | Arthroplasty alignment |
Decision rule
Print when a physical model or a custom cut/drill trajectory genuinely changes the operative plan and there is time to manufacture. For routine arthroplasty alignment, navigation or robotics outperform a static guide because they allow intra-operative correction. For most standard cases, no advanced technology is needed at all.
Controversies and Areas of Uncertainty
| Question | Current position | Uncertainty |
|---|---|---|
| Does PSI improve TKA outcomes? | No clinically relevant benefit in routine primary TKA (Level I meta-analysis) | May still help in extra-articular deformity or retained hardware where canals cannot be referenced |
| Do 3D models help complex trauma? | Reduce operative time, blood loss, fluoroscopy in acetabular fractures | Mostly small single-centre series; few multicentre RCTs; outcome (not process) benefit less proven |
| Point-of-care vs industry printing | Models/guides increasingly printed in-house | Implants almost always require certified manufacturers; in-hospital QMS burden is high |
| Custom tumour/revision implants | Allow reconstruction otherwise impossible | Long-term survivorship data immature; failure modes and registry capture incomplete |
| Accuracy standard | Sub-millimetre dimensional fidelity achievable | No universal validation/verification standard; accuracy degrades with metal artefact and poor segmentation |
Clinical Warning
The single highest-yield controversy: PSI does NOT improve outcomes in routine total knee arthroplasty. Quote the Level I meta-analysis and reserve PSI/3D guides for cases where conventional referencing fails (severe extra-articular deformity, retained hardware, tumour margins).
Guidelines, Registries & Global Practice
| Body / Region | Framework | Practical implication |
|---|---|---|
| FDA (US) | Guidance on technical considerations for additively manufactured devices; point-of-care framework | Anatomical models and guides have clear pathways; patient-matched implants need validated processes |
| EU MDR (Europe) | Custom-made device provisions under MDR 2017/745; notified-body oversight for higher-risk implants | Documentation and conformity assessment scale with device risk class |
| ISO 13485 | Quality management system for medical devices | Required for any site manufacturing implantable or patient-contacting printed devices |
| AO Foundation / EFORT | Education and consensus on computer-assisted and patient-specific planning | Endorse case selection over routine use; emphasise verification against source imaging |
| RSNA-ACR / radiology bodies | Appropriateness and quality criteria for clinical 3D printing in radiology | Standardise indications, segmentation QA, and reporting for point-of-care labs |
Evidence Base
Landmark and Supporting Studies
Evidence
Evidence
Evidence
Evidence
Evidence
Evidence
Key Points
- Trauma planning: Comparative evidence supports reduced operative time, blood loss, and fluoroscopy with pre-operative 3D models in complex acetabular fractures.
- Deformity guides: Meta-analytic Level I evidence shows 3D-printed drill guides improve pedicle screw accuracy and lower radiation exposure.
- PSI in routine TKA: High-quality Level I evidence shows no clinically relevant outcome benefit — value is confined to selected, atypical anatomy.
- Custom implants: Geometry and stiffness matching are real advantages, offset by cost, multi-week lead time, and loss of intraoperative flexibility.
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
Viva scenarioStandard
Clinical prompt
“You are planning surgery for a complex acetabular fracture. How might 3D printing assist your pre-operative planning?”
Practical approach
Workflow: (1) Obtain thin-slice CT (less than 1mm) of the pelvis including the contralateral hip for reference. (2) Transfer DICOM data to segmentation software. (3) Segment the fractured acetabulum and separate fracture fragments. (4) Generate STL file of the bone model. (5) Print in appropriate material (PLA or resin). Benefits for acetabular fracture: (6) Visualise fracture pattern in 3D - understand fragment displacement. (7) Pre-contour reconstruction plates on the model - saves intraoperative time. (8) Plan surgical approach - identify which fragments are accessible. (9) Trial reduction - practice fragment manipulation. (10) Mirror the contralateral normal hip if needed for template. Studies show reduced operative time and blood loss with 3D printed planning for complex acetabular fractures.
Key clinical points
Thin-slice CT (less than 1mm) essential for accuracy
Segmentation quality determines model accuracy
Pre-contour plates on printed model
Practice reduction technique
Reduces operative time 20-30%
Common pitfalls
Using inadequate CT slice thickness
Poor segmentation missing fracture lines
Not allowing sufficient time for printing
Further questions
“How would you validate the accuracy of the printed model? What other complex fractures benefit from 3D printing?”
Viva scenarioStandard
Clinical prompt
“A patient requires resection of a pelvic chondrosarcoma with reconstruction. How can 3D printing technology assist?”
Practical approach
Multiple applications: (1) Anatomical model - precise 3D visualisation of tumour extent, relationship to vital structures, and planned resection margins. (2) Cutting guides - patient-specific guides to achieve planned resection margins accurately. These are designed with slots that match the planned bone cuts. (3) Custom implant - design a reconstruction implant that matches the exact geometry of the resection defect. Can incorporate porous structure for bone ingrowth and attachment points for soft tissue. (4) Template - pre-contour plates or plan screw trajectories on the model. (5) Patient education - explain the surgery using the model. Workflow: CT/MRI for planning, multi-disciplinary review, design cutting guides and implant, manufacture (weeks lead time), sterilise, intraoperative use. Custom implants require regulatory-approved manufacturing partners.
Key clinical points
Anatomical model shows tumour-structure relationships
Cutting guides ensure accurate margins
Custom implant matches resection geometry
Requires weeks of lead time for design and manufacture
Implants need regulatory-approved manufacturing
Common pitfalls
Not allowing sufficient lead time
Inadequate imaging for tumour margin delineation
Using non-approved manufacturers for implants
Further questions
“What regulatory requirements apply to custom implantable devices (e.g. FDA patient-matched device pathway, EU MDR custom-made device provisions)? How do you verify the accuracy of cutting guides?”
Viva scenarioStandard
Clinical prompt
“You are considering patient-specific instruments (PSI) for a complex total knee replacement in a patient with severe extra-articular deformity from a malunited tibial fracture.”
Practical approach
PSI for complex TKA: In this case with extra-articular deformity, conventional intramedullary instruments cannot be used due to the malunited fracture. PSI advantages: (1) Reference off the bone surface rather than the medullary canal. (2) Pre-operatively planned cuts based on CT reconstruction. (3) Account for the deformity in the planning software. (4) Guides with specific cut angles and depths. (5) May avoid simultaneous corrective osteotomy in some cases. Process: CT scan (specific protocol), planning with software, design guides for femur and tibia, manufacture, sterilise. Limitations: (6) Additional cost and lead time. (7) Relies on accurate CT and planning. (8) Cannot adjust intraoperatively as easily as conventional instruments. (9) Soft tissue balancing still required. For severe extra-articular deformity, PSI is particularly valuable as conventional referencing is unreliable.
Key clinical points
PSI references bone surface, not medullary canal
Useful when conventional instruments cannot be used
Pre-planned cut angles account for deformity
Cannot adjust easily intraoperatively
Soft tissue balancing still required
Common pitfalls
Assuming PSI eliminates all technical challenges
Not having backup conventional instruments
Inadequate soft tissue balancing
Further questions
“What evidence exists for PSI improving alignment in standard TKA? How do you manage the surgical approach with severe deformity?”
Imaging Atlas
Exam day cheat sheet
3D Printing Quick Reference
Workflow
- CT acquisition (less than 1mm slices)
- DICOM to segmentation software
- Generate STL file
- Print and post-process
- Sterilise if for OR use
Applications
- Anatomical models (planning, education)
- PSI (cutting guides, drill guides)
- Custom implants (tumour, revision)
- Pre-contoured plates
Imaging Requirements
- CT preferred for bone
- Slice thickness less than 1mm
- MARS protocol if metal present
- Include relevant anatomy margins
Limitations
- Time (days to weeks)
- Cost (equipment, materials)
- Regulatory for implants
- Requires expertise