Personalized Prosthesis Using 3D Printing for Complex Hip Revision

• 1
  When a Personalized Prosthesis Using 3D Printing Beats a Catalog Implant
• 2
  From CT Scan to Printable Geometry
• 3
  Printing, Heat Treatment, and Machining the Interfaces
• 4
  Material Choice and the Tests That Close the Loop
• 5
  Catalog Implant or Personalized Prosthesis: Which Case Fits Which

A primary hip replacement works because the surgeon can pick a cup and stem from a size chart. The bone is intact enough to hold them. A revision after one or two failed implants is a different problem. The acetabulum may have lost its medial wall, the columns may be cracked, and the femur may be missing the metaphyseal bone that a standard stem needs for fixation.

When the defect gets large, no catalog part matches the remaining bone. The surgeon either rebuilds the defect with bone graft and cages, or removes the guesswork and fits an implant to the anatomy that is actually left. That second route is where a personalized prosthesis using 3D printing earns its place.

The decision is not about novelty. It is about contact area and load path. A printed cup with a patient-matched flange can sit on the ilium and ischium where bone still exists. A standard hemispherical cup needs a rim to press into. If the rim is gone, the cup has nothing to grip.

Size is the second gate. Revision stems are offered in a limited range of lengths and diameters. When the canal is wider than the largest available stem, or the offset needed is outside the catalog, the fit becomes a compromise. A printed body can follow the canal and place the joint center where the soft tissues want it.

• 1
  Large uncontained defectsBone loss beyond what a hemispherical cup can bridge.
• 2
  Pelvic discontinuityColumns cracked or separated; the implant must span the gap.
• 3
  Canal wider than the largest stemStandard stems cannot achieve a scratch fit.
• 4
  Joint center off by more than 10 mmLeg length and abductor tension need a patient-matched offset.

Everything starts with imaging. A thin-slice CT scan, usually 0.5–1 mm slices, is segmented to separate bone from soft tissue and from metal artifact left by the old implant. Metal scatter is the first real obstacle. Without artifact reduction, the bone surface near the failed cup looks like a cloud, and any surface built on it will be wrong.

The segmented bone becomes a surface model. The engineer then mirrors the healthy side to estimate where the original joint center sat, and overlays that target on the damaged side. This step sets the offset, the version, and the leg length. Get it wrong here and the whole implant is wrong, no matter how well it is printed.

Next comes the implant body itself. It is not a solid block. Solid titanium is stiff, and a stiff implant next to relatively flexible bone takes almost all the load. That leads to stress shielding and bone resorption around the implant. The fix is a lattice or porous region at the bone interface, with a solid core only where screws and the joint bearing need material.

Finally the design is checked against the surgical plan. Screw corridors must pass through bone, not through the bladder or the great vessels. Flanges must not impinge on the sciatic nerve. This review is a joint effort between the surgeon and the manufacturing engineer, and it usually runs two or three rounds before the file is locked.

• 1
  Slice thickness0.5–1 mm; thicker slices blur the acetabular rim.
• 2
  Metal artifact reductionRequired when the old implant is still in place.
• 3
  MirroringUses the contralateral side to restore the joint center.
• 4
  Screw corridor checkEvery hole must land in bone with a safe margin.

The build runs on a laser powder bed fusion machine using Ti-6Al-4V (TC4) powder. Layer thickness sits between 30 and 60 μm. Thinner layers give a smoother surface and better resolution on fine lattice struts, but they slow the build. For a cup-sized part, a 30 μm layer is common; for a large revision cage, 50–60 μm keeps the build time practical.

Laser power, scan speed, and hatch spacing are tied together. The goal is full density with no lack-of-fusion pores. Pores at the surface become crack initiation sites under cyclic load, and a hip implant sees millions of cycles. Melt pool monitoring during the build flags any layer where the signal drifts outside the qualified window.

After the build, the part is stress relieved and then hot isostatically pressed. HIP closes internal porosity and improves fatigue life. The part is then cut from the build plate and the support structures are removed. Support removal on a lattice region is delicate work. Cutting too deep removes struts; leaving too much leaves rough material that cannot be cleaned.

Printed surfaces are not final surfaces. The bearing taper, the screw holes, and any mating face are machined. On our 5-axis centers we hold ±0.005 mm (±0.0002 in) on those features and finish them to Ra 0.8–1.6 μm. The porous bone-contact region is left as-printed, because that roughness is what bone grows into. Blasting or polishing it would destroy the function.

Cleaning follows. Loose powder trapped in lattice channels has to come out. The part goes through ultrasonic baths and, where the geometry demands it, pressure washing from multiple angles. Powder left inside a lattice can migrate after implantation and cause third-body wear at the bearing. This is a documented step, not a rinse.

• 1
  Layer thickness30–60 μm depending on feature size and build time.
• 2
  Build atmosphereArgon, with oxygen kept low to avoid oxidation.
• 3
  HIPCloses porosity and lifts fatigue strength.
• 4
  Machined featuresTaper and screw holes held to ±0.005 mm.

Titanium is the default for load-bearing revision implants. Ti-6Al-4V (TC4) has the fatigue strength and the elastic modulus that make a porous lattice viable. Commercially pure TA1 and TA2 are softer and are used for non-load-bearing plates and covers rather than for a cup that carries body weight every step.

The powder itself is a controlled input. Particle size distribution, chemistry, and oxygen content are checked on receipt, and powder is reused only within a validated number of cycles. Oxygen pickup during reuse raises brittleness. A supplier that tracks reuse without a limit is a risk, no matter how good the printer is.

Testing runs at three levels. Powder and build coupons verify the process. Coupons are printed with the part and tested for density, chemistry, and tensile properties. Then the finished part is checked dimensionally against the design model, usually on a coordinate measuring machine for the machined features and by CT for internal lattice geometry.

Traceability ties it together. The build file, the machine, the powder lot, the heat treatment batch, and the inspection records all attach to one device history record. For a medical device this is not optional. If a question comes up two years later, the answer has to be findable.

One limit worth stating plainly. A printed implant solves fit and fixation. It does not solve infection. If there is active infection at the site, the revision is usually staged, with a spacer first and the definitive implant later. Printing a custom part into an infected bed is the wrong use of the technology.

• 1
  Ti-6Al-4V (TC4)Load-bearing cups, cages, and stems.
• 2
  TA1 / TA2Non-load-bearing covers and plates.
• 3
  Powder reuse limitValidated cycle count, oxygen monitored.
• 4
  Device history recordBuild, lot, heat treat, and inspection linked.