3D printed titanium alloy prosthesis to treat finger bone defects

A finger bone implant is not a scaled-down hip stem. The phalanges are roughly 25 to 45 mm long and only a few millimetres thick, so the implant has to carry bending and torsion through a very small cross-section. The medullary canal is narrow, often under 3 mm in the distal phalanx. Any stem that fits inside it must taper hard or the surgeon cannot seat it.

Load matters too. Pinch force at the fingertip can exceed 100 N, and that force concentrates at the joint face and the stem-bone interface. A solid stem that is stiff enough to survive the load will also be stiff enough to stress-shield the surrounding bone. That trade-off is why porous or lattice structures appear in this application.

Defect geometry drives everything else. After tumor resection the remaining bone ends are irregular, and the gap length is patient-specific. A catalog implant rarely fits. Patient-matched geometry, built from CT or MRI data, is the usual route for defects longer than a simple joint replacement.

Sterilization and cleaning also constrain the design. Steam autoclave cycles reach 134 °C. Any internal lattice must be open enough to let steam in and debris out, or the part will not pass a cleaning validation.

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  Canal diameterStems below 3 mm leave little room for a lattice; solid or thin-walled is safer.
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  Load pathKeep the porous region near the bone interface, not in the middle of the stem.
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  Gap lengthDefects over about 10 mm usually need patient-matched geometry.
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  CleanabilityOpen lattice cells and no closed internal channels.

Ti-6Al-4V (TC4, UNS R56400) is the standard choice for load-bearing implants. Its elastic modulus sits around 110 GPa, far below the 200 GPa of 316L stainless, and closer to cortical bone at roughly 10 to 20 GPa. That gap still exists, but it is smaller than with steel, and a lattice can lower the effective stiffness further.

The alloy also tolerates the body environment. A passive oxide layer forms within milliseconds and re-forms if scratched. Corrosion rates in saline are negligible over the service life of an implant. For a 3D printed titanium alloy part, the same chemistry applies; what changes is the microstructure.

Additive manufacturing leaves a martensitic or fine acicular alpha-prime structure in as-built Ti-6Al-4V, with higher yield strength but lower ductility than wrought material. A stress-relief or hot isostatic pressing cycle is normally required before the part goes near a patient. HIP also closes internal porosity, which matters for fatigue at the stem.

Machined wrought Ti-6Al-4V remains the reference for mechanical properties. If your design is a simple cylinder or a plate, there is no reason to print it. Printing earns its place when the geometry is porous, hollow or patient-matched.

The joint face and the stem taper are the two features that decide whether the implant seats correctly. Neither prints to tolerance. A printed surface on a taper typically lands at ±0.1 mm or worse, and that is enough to cause a loose fit or a stress riser.

The working answer is hybrid. Print the body with the lattice and the patient-matched contour, leaving 0.3 to 0.5 mm of stock on the taper and the articulating face. Then machine those faces on a 5-axis center. We hold ±0.005 mm on turned and milled features, with Ra 0.2–0.8 μm available on the bearing surface.

Fixturing is the hard part. A printed lattice body is not rigid, and clamping it directly will crush struts. We build a sacrificial boss into the print, grip that boss in a soft jaw, and cut it off in the last operation. The boss adds mass to the build but saves the part.

Inspection follows the same split. The lattice is checked visually and by mass or CT where the customer requires it. The machined faces are checked with a CMM and a surface profilometer. Reports are available on request, and every part is inspected before shipment.

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  Stock allowance0.3–0.5 mm on tapers, joint faces and any press-fit bore.
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  Datum strategyPick datums on printed surfaces that will not be machined away.
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  Heat treat orderHIP and stress relief before final machining, or the part will move.

As-built titanium has a rough, partially melted surface with attached powder particles. For a bone-contacting implant that is a problem on two counts: loose particles and unpredictable fatigue life. Support removal is the first step, usually by hand with a carbide tool at this size.

Bead blasting with a fine media removes most attached particles and brings the surface to a uniform matte. Acid etching or electropolishing goes further and reaches into lattice struts that a blaster cannot. Electropolishing also rounds sharp edges, which reduces the chance of a crack initiating at a strut junction.

Cleaning validation is a separate exercise. Ultrasonic baths in a sequence of solvents and DI water, followed by a particle count on the final rinse, is the usual approach. The acceptance criteria come from the customer's regulatory pathway, not from us. We run the process and provide the records.

Laser marking is used for traceability where the customer allows it. Minimum character height is 1.5 mm, which is large on a 30 mm phalanx. Most customers put the UDI on the packaging instead and leave the implant unmarked.

Printing is expensive per part and slow compared to turning. A solid phalangeal stem with a simple taper is faster and cheaper on a lathe, and the wrought material has better fatigue data behind it. If the design has no lattice, no internal channel and no patient-specific curve, print it only if you need the geometry for another reason.

Small features are another limit. Struts below 0.2 mm are difficult to build reliably and nearly impossible to inspect. Pores that are too small for bone ingrowth, usually under 100 μm, add cost without adding function. Design the lattice around the cell size the surgeon actually wants, typically 300 to 800 μm.

Batch size changes the math. At one to ten patient-matched parts per year, printing wins. At several hundred identical parts, a machined design with a pressed or sintered porous coating is usually cheaper and has a longer regulatory history.

We quote both routes when a design is borderline. The DFM analysis we return within 12 hours will say which one we would pick and why, based on the geometry in front of us rather than a general rule.