3D Printing in Orthopaedic Oncology and Trauma

The intersection of advanced imaging, computer-aided design (CAD), and additive manufacturing (3D printing) has catalyzed a paradigm shift in orthopaedic surgery. We have moved from the era of "making the patient fit the implant" to "making the implant fit the patient." This revolution is most palpable in the high-stakes fields of orthopaedic oncology and complex trauma, where standard off-the-shelf solutions often fall short.

This article provides a comprehensive overview of the workflow, clinical applications, material science, and regulatory landscape of 3D printing in these demanding subspecialties.

The Digital Workflow: From Pixel to Part

Understanding the process is crucial for any surgeon wishing to incorporate this technology into their practice. It is not as simple as pressing "print."

1. Image Acquisition

High-quality input is the foundation. Thin-slice CT scans (typically <1mm slice thickness) are mandatory for bony architecture. Metal artifact reduction sequences (MARS) MRI are essential when hardware is present or when soft tissue tumor margins must be fused with bony models.

2. Segmentation

This is the process of converting DICOM data (slices) into a 3D surface model (STL file). Radiologists or biomedical engineers threshold the bone density to isolate the anatomy of interest, manually cleaning up artifacts.

Visual Element: A split-screen showing a raw CT slice alongside the segmented colorful 3D model of a pelvic sarcoma.

3. Surgical Planning & Design

The surgeon and engineer collaborate virtually.

• Oncology: Resection planes are defined based on MRI margins. Safe margins are strictly adhered to.
• Trauma: Fracture fragments are virtually reduced. The contralateral side can be mirrored to provide a template for reconstruction.
• Implant Design: Porous lattices are generated for bone interface, and screw trajectories are optimized to avoid neurovascular structures.

4. Manufacturing (The Print)

Using technologies like Electron Beam Melting (EBM) or Selective Laser Sintering (SLS), the physical object is built layer by layer from titanium or polyamide powder.

5. Post-Processing

Heat treatment (annealing) relieves thermal stresses. Surface finishing reduces roughness on articular surfaces, while porous coatings are preserved. Finally, rigorous cleaning and sterilization/packaging.

Applications in Orthopaedic Oncology

Limb salvage surgery has been the primary driver of custom implant technology.

Patient-Specific Instrumentation (PSI) for Resection

Achieving negative margins is the gold standard. Freehand osteotomies in the pelvis or sacrum are notoriously difficult.

• Cutting Guides: PSI guides fit uniquely onto the patient's bony landmarks. They contain slits that direct the oscillating saw blade at the exact pre-planned angle.
• Evidence: Studies consistently show that PSI improves the accuracy of resection margins, reducing the rate of intralesional cuts compared to freehand techniques.

Massive Endoprostheses

When a tumor resection creates a massive defect (e.g., hemipelvectomy, total femoral replacement), standard modular implants may not bridge the gap or provide adequate fixation.

• Custom Flanges: Implants can be designed with flanges that grasp the remaining host bone (e.g., ilium), secured with multiple screws in non-standard trajectories to maximize pull-out strength.
• Soft Tissue Integration: A major failure mode in limb salvage is soft tissue failure (dislocation, infection). 3D printed meshes on the implant surface allow for the suture of muscles and tendons directly to the metal, promoting a stable soft-tissue envelope.

Visual Element: Detailed diagram of a custom hemipelvis implant showing the porous iliac interface, screw trajectories, and acetabular cup positioning.

3D Printed Spacers

For two-stage revisions in infected tumor prostheses, custom antibiotic-loaded cement spacers can be printed (using molds) to maintain limb length and joint stability while delivering high-dose local antibiotics.

Applications in Complex Trauma

While acute trauma often does not allow time for custom implants (which can take weeks), 3D printing is invaluable for sequelae and complex acute planning.

Malunion Correction

Corrective osteotomies for malunited fractures (e.g., distal radius, tibia) benefit immensely from PSI.

• The Workflow: The contralateral normal bone is imaged and mirrored. A cutting guide is designed to osteotomize the malunion at the exact point of deformity. A second guide or custom plate then holds the bone in the corrected position.
• Clinical Pearl: Always check the "fit" of the guide on the bone first. If the guide rocks or doesn't sit perfectly, soft tissue may be interposed, or the segmentation was inaccurate. Do not cut unless the fit is perfect.

Complex Articular Fractures

In acetabular or tibial plateau fractures, a 3D printed "biomodel" allows the surgeon to:

1. Pre-contour Plates: Bend plates on the model before surgery, saving tourniquet time.
2. Understand the Fracture: Tactile interaction with the fracture pattern is superior to viewing 2D screens.
3. Simulate Reduction: Plan the sequence of screw insertion and fragment manipulation.

Custom Plates/Implants

In rare cases of massive bone loss (e.g., blast injuries, high-energy open fractures) where bone transport is not feasible, custom titanium trusses/cages can be printed to span the defect, packed with graft to induce Masquelet-like induced membrane healing or primary fusion.

Materials Science: The Magic of the Lattice

The true power of 3D printing lies in the control of Microstructure.

Titanium Alloy (Ti6Al4V)

The workhorse of orthopaedics.

• Modulus Matching: Solid titanium is much stiffer than bone (Young's modulus ~110 GPa vs. ~15-20 GPa for cortical bone). This mismatch causes stress shielding and bone resorption.
• Lattice Structures: By printing titanium as a porous lattice (scaffold) rather than a solid block, the effective modulus can be lowered to match that of bone. This promotes load transfer and preserves bone stock.
• Osseointegration: The pore size (typically 300-600 microns) and interconnectedness are optimized to invite capillary ingrowth and osteoblast migration, resulting in biological fixation superior to plasma-spray coatings.

PEEK and Polymers

Used for cutting guides (polyamide) or radiolucent implants (PEEK).

• Carbon-Fiber PEEK: Printed PEEK reinforced with carbon fiber offers strength approaching metal but with radiolucency, allowing for easier postoperative monitoring of tumor recurrence.

Challenges and Limitations

1. Cost and Economics

Custom implants are significantly more expensive than off-the-shelf alternatives. The design time, engineering hours, and machine time contribute to high costs. However, health economic studies are beginning to show savings through reduced operative time and lower revision rates.

2. Time Delays

The "design-to-implantation" window is typically 3-6 weeks.

• Trauma: Often unusable for acute fractures.
• Oncology: Acceptable for most sarcomas where neoadjuvant chemotherapy provides a window, but challenging for rapidly progressing tumors.

3. Regulatory Hurdles

Custom implants often fall under "Compassionate Use" or "Custom Device" exemptions (e.g., TGA Custom Made Medical Device). As volumes increase, regulators are demanding more rigorous quality assurance and clinical evidence, moving towards "Patient-Matched" classifications which require standard approvals.

4. Intraoperative Reality vs. Digital Plan

Bone is not static. Tumor progression during the 4-week lead time can render a cutting guide inaccurate. Soft tissue stripping required to fit bulky guides can devascularize bone.

Future Directions

The field is evolving rapidly.

• Point-of-Care Printing: Hospitals establishing in-house 3D printing labs to produce guides and models within 24 hours.
• Bioprinting: The holy grail—printing viable bone constructs with live cells and growth factors to replace defects without metal.
• Smart Implants: Printing sensors directly into the implant to monitor infection (pH change), loosening (micro-motion), or load bearing.

Conclusion

3D printing in orthopaedic oncology and trauma is no longer a novelty; it is a vital tool in the reconstructive ladder. It empowers surgeons to tackle pathologies that were previously deemed unresectable or unfixable. While barriers of cost and time remain, the trajectory is clear: personalized, precision surgery is the future standard of care.

Clinical Trap: Never rely solely on the PSI guide. Always have a backup plan (standard instrumentation) and verify the guide's position with fluoroscopy or navigation. A printed guide is only as good as the segmentation it came from.

References & Further Reading

1. Wong, K. C., et al. (2012). "Computer-assisted pelvic tumor resection and reconstruction with a custom-made patient-specific implant." Orthopedics.
2. Cartiaux, O., et al. (2018). "Accuracy of patient-specific cutting guides for bone tumor resection." BJJ.
3. Mobed, D., et al. (2024). "Osseointegration of 3D printed porous titanium implants: A systematic review." Acta Orthopaedica.