High Yield Overview
3D Printing from Imaging: Surgical Planning
—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
Examiner's 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
Mnemonic
PRINT Workflow
P
Pre-operative planning - Identify need for model/guide
Pre-operative planning - Identify need for model/guide
R
Reconstruction - Thin-slice CT imaging
Reconstruction - Thin-slice CT imaging
I
Image processing - DICOM to STL conversion
Image processing - DICOM to STL conversion
N
Numerical control - 3D printer fabrication
Numerical control - 3D printer fabrication
T
Theatre use - Surgical application
Theatre use - Surgical application
Memory Hook:Remember the 5 stages from image to operating theatre
Mnemonic
STERILE Requirements
S
Sterilisation capability - Gamma or autoclave
Sterilisation capability - Gamma or autoclave
T
Thin slices - Less than 1mm for accuracy
Thin slices - Less than 1mm for accuracy
E
Exact segmentation - Precise anatomical modelling
Exact segmentation - Precise anatomical modelling
R
Regulatory approval - TGA/FDA clearance for implants
Regulatory approval - TGA/FDA clearance for implants
I
Image quality - High-resolution CT data
Image quality - High-resolution CT data
L
Layer height - Printer resolution affects surface finish
Layer height - Printer resolution affects surface finish
E
Education value - Teaching and patient communication
Education value - Teaching and patient communication
Memory 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
Clinical Imaging
Imaging Gallery
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Exam 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.
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Workflow Overview
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3D Printing Workflow Steps
| 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
Optimal Imaging Parameters
| 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
Anatomical Model 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
3D Printing Technologies in Orthopaedics
| 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
Quality Considerations
| 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
Current 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.
Evidence Base
Landmark Studies
Clinical Applications of 3D Printing in Orthopaedic Surgery
1
2
3
Australian Evidence
4
Key Points
- Pre-operative planning: Level I evidence supports reduced operative time and improved accuracy
- Patient-specific guides: Custom cutting guides improve precision in deformity correction
- Anatomical models: Enhance surgeon understanding and patient communication
- Regulatory landscape: Evolving framework for point-of-care manufacturing
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
EXAMINER
"You are planning surgery for a complex acetabular fracture. How might 3D printing assist your pre-operative planning?"
EXCEPTIONAL ANSWER
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 POINTS TO SCORE
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 TRAPS
✗Using inadequate CT slice thickness
✗Poor segmentation missing fracture lines
✗Not allowing sufficient time for printing
VIVA SCENARIOStandard
EXAMINER
"A patient requires resection of a pelvic chondrosarcoma with reconstruction. How can 3D printing technology assist?"
EXCEPTIONAL ANSWER
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 POINTS TO SCORE
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 TRAPS
✗Not allowing sufficient lead time
✗Inadequate imaging for tumour margin delineation
✗Using non-approved manufacturers for implants
VIVA SCENARIOStandard
EXAMINER
"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."
EXCEPTIONAL ANSWER
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 POINTS TO SCORE
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 TRAPS
✗Assuming PSI eliminates all technical challenges
✗Not having backup conventional instruments
✗Inadequate soft tissue balancing
3D Printing Quick Reference
High-Yield Exam Summary
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