3D Printing from Imaging: Surgical Planning

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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.

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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

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.

The anchor review (Wong et al, cited below) names four core orthopaedic uses of additive manufacturing, but the body develops only three - models, patient-specific instruments and custom implants. The fourth is bone/cartilage tissue engineering, where 3D printing is the fabrication route:

• Printed scaffolds. A biodegradable scaffold with a controlled porous architecture is printed from polymers (such as PCL or PLGA), ceramics (such as tricalcium phosphate or hydroxyapatite) or bioactive glass, then seeded with cells (e.g. mesenchymal stem cells) and/or growth factors to regenerate bone or cartilage.
• Bioprinting goes further, depositing living cells suspended in a bio-ink (a hydrogel such as alginate, gelatin or collagen) layer-by-layer to build a living construct.
• This remains largely experimental / pre-clinical: the barriers are vascularising large constructs, maintaining cell viability, achieving load-bearing strength and gaining regulatory approval. The biology of tissue engineering is covered in its own topic; here the point is that 3D printing is the manufacturing method that makes patient-specific scaffolds possible.

The topic notes that 'porous titanium structures can be designed to match bone modulus and promote osseointegration' but never explains how or why - and the Wong review (below) states printed implants can carry scaffold lattices that reduce stiffness and favour osteointegration.

• Design freedom. Metal additive manufacturing (laser sintering or electron-beam melting) can build a porous or trabecular lattice inside a titanium implant that a solid cast or machined implant cannot - and the porosity can be varied regionally within one part.
• Reducing stress shielding. A lattice lowers the implant's effective elastic modulus toward that of bone (solid titanium is around 110 GPa versus roughly 15 to 20 GPa for cortical bone), so more load is transferred to the surrounding bone and less is shielded - reducing peri-implant bone resorption.
• Promoting osseointegration. The interconnected pores allow bone to grow into the implant (optimal pore size roughly 300-700 micrometres), giving durable biological fixation - the same principle as a trabecular-metal ingrowth surface.

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Landmark and Supporting Studies

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.