3D Printed Titanium Alloy Hip: How the Part Gets Made

• 1
  Why the Polish Case Points to Titanium, Not Steel
• 2
  What Powder Bed Fusion Actually Does to the Metal
• 3
  Where CNC Takes Over From the Printer
• 4
  When Printing Is the Wrong Answer
• 5
  Additive vs. Subtractive for a Titanium Hip Component

The Lublin case in April 2023 was not a routine revision. A patient had a steel prosthesis in place, developed complications, and the surgical team needed a replacement fast. They used additive manufacturing to build a titanium-based composite hip prosthesis and implanted it. The detail that matters to engineers is not the headline. It is the material switch.

Titanium alloys sit in the body differently from stainless steel. Ti-6Al-4V (TC4) has an elastic modulus far closer to cortical bone than 316L or cobalt chrome. That gap, not the surgery itself, drives most long-term loosening. When a stiff stem carries load and the surrounding bone does not, the bone resorbs. Stress shielding is a mechanical problem, and material choice is the first lever against it.

The second lever is geometry. A solid machined stem has one shape for every patient. A 3D printed titanium alloy hip can be built with a lattice or porous region where bone wants to grow in, and solid material only where load demands it. That is the real reason a hospital would print instead of order from a catalog. It buys stiffness control, not just a custom fit.

Laser powder bed fusion spreads a thin layer of Ti-6Al-4V powder, typically 30–50 μm, and melts the cross-section with a laser. Layer by layer, the part grows. The melt pool cools fast, on the order of 10⁴–10⁶ K/s. That cooling rate is the whole story. It produces a fine acicular martensitic alpha-prime structure with high residual stress locked into the part.

That as-built state is hard and brittle compared to wrought Ti-6Al-4V. Yield strength can run high, ductility low. Nobody implants an as-built part. It goes into a vacuum or argon furnace for stress relief, usually 600–800 °C for a few hours, then often a hot isostatic press around 900–950 °C at roughly 100 MPa to close internal porosity. After that, the microstructure is closer to what a mill product would give you.

Support structures are the other cost nobody plans for. Overhangs past about 45° from vertical need support, and that support has to be cut off without nicking the part. On a hip stem, the support contacts often land on surfaces that will be machined anyway, so the design should push supports toward stock allowance. If supports land on a finished porous surface, you have created a rework problem.

Printing gets you near net shape. It does not get you a taper. The femoral head taper, the stem taper, and any mating cone need tolerances and surface finish that no powder bed process holds as-built. Typical as-built Ra on an upward-facing surface lands around Ra 10–15 μm. A taper that locks with a ceramic head needs far less, and it needs a controlled angle.

This is the handoff point. GreatLight machines the printed blank on 5-axis centers, using the printed datum features to locate the part. We hold ±0.005 mm (±0.0002 in) on critical diameters and can take finishes to Ra 0.2–0.8 μm where a bearing or locking surface needs it. A machined finish of Ra 0.8–1.6 μm covers most non-articulating interfaces.

The trap is tolerance stack between print and machine. If the printed blank shifts 0.3 mm, and your taper has 0.05 mm of stock, you cut into the lattice or scrap the part. The fix is generous stock on machined features, typically 0.4–0.8 mm per side, plus printed datum pads that the machine can pick up repeatably. Print cheap, machine precise.

Additive is slow and expensive per part. A hip stem can take many hours of machine time, plus support removal, heat treatment, and finishing. If the geometry is a simple cylinder with a taper, a bar of wrought Ti-6Al-4V turned on a lathe is cheaper, faster, and has better and more predictable fatigue properties. Wrought material has no porosity question at all.

Fatigue is the honest limit. As-built and even HIPed additive Ti-6Al-4V can carry lower fatigue strength than wrought, especially at the surface. Surface roughness acts as a stress riser. If the part sees cyclic load and you cannot machine or peen the critical surface, think hard before printing. For a porous bone ingrowth region, that trade is deliberate. For a load-bearing stem with a rough as-built flank, it is usually a mistake.

There is also the validation load. A printed implant needs powder traceability, build parameter records, and CT or micro-CT checks for internal voids. That is real work. If your program cannot carry that documentation, the geometry advantage does not pay for itself. Printing earns its place when shape freedom, stiffness grading, or porous structure are the point.