QMT Features: October 2017
Added insight
The key to understanding additive manufacturing is metrology says Baltic Orthoservice


To ensure quality in the production of accurate, patient-specific implants, endoprostheses and surgical guides, Baltic Orthoservice in Kaunas, Lithuania, uses an array of metrology technology. This includes a multi-sensor coordinate measuring machine (CMM) including a laser scanner and a micro-CT (computed tomography) system supplied by Nikon Metrology.

The ALTERA CMM with LC15Dx laser scanner allows fast inspection of 3D-printed implant surfaces and screw holes, while the XT H 225 CT guarantees internal structure quality and accurate geometrical correlation between CAD implant model and manufactured product.

Paulius Lukševicius, Engineer of Mechanics explained, “3D-printing is a complicated technology and there is a big variation in processing parameters, so predicting the quality and geometry of printed objects is quite a challenge.
“Patient-specific implants are a bespoke treatment solution, which means that the surgery must be ‘pre-planned’ virtually so the implant can simply be put in place. To be able to execute the virtual plan, it is vital to be 100% sure that implant geometry is exactly the same as the CAD model and that the holes are machined to high accuracy.

“To fulfil these goals, we use a variety of metrology equipment. The CMM with laser scanner is irreplaceable when we need to perform fast checks after each manufacturing and post-processing stage, especially to check spherical surfaces, bearing surfaces and hole angles.”

Unlike standard modular hip implants used to treat severe clinical conditions, patient-specific alternatives are designed as a single device with anatomically adapted surfaces. It eliminates the risk of instability and adapts the implant to the bone rather than the bone to the implant. A major benefit of the procedure is that, during surgery, there is no need to shape the bone to adapt it to the implant or use bone cement, meshes and augments to fill the bone defect. The implants are designed using virtual anatomical bone models which are obtained from medical CT scans of a patient.

For manufacturing the implants, Baltic Orthoservice uses direct metal laser sintering equipment purchased in 2012. After 3D-printing, implants undergo a variety of post-processing steps, including heat treatment, surface polishing and milling for screw holes. There are a number of medical device standards and regulations that must be met, which is why it is paramount for products to be of ultra-high accuracy.
With fine tolerances and strict standards to be adhered to, the post-processing stages are repeated until the physical implant matches the desired virtual model exactly. Quality assurance measures are taken following every step, during which the LC15Dx laser scanner is able to show quickly how well the physical part matches the virtual model.

Mr Jokymaitytè said, “Laser scanning and micro-CT serve two different purposes and both excel in different areas.

“The CMM with laser scanner is very effective for inspecting features and surfaces of bigger parts such as an acetabular implant, for quality control of standard products like osteosynthesis plates and for assessing standard elements in patient-specific implants, i.e. screw holes or spherical shapes.

“The micro-CT system is a vital tool for non-destructive analysis of the internal structure and geometry of additively manufactured components.”
As the Quality Control Laboratory at Kaunas was a new project to supplement the additive manufacturing facilities, there were no previous systems to be replaced. Baltic Orthoservice knew what was required and compared the best technologies on the market to find the right solution.

Mr Lukševicius said, “The primary requirement was to have the capability to inspect parts made from different types of materials. The size of the working area was also important, but most crucial was accuracy.”

He pointed out that a key advantage of the Nikon Metrology LC15Dx is its ability to scan reflective and multi-material surfaces thanks to ESP (enhanced sensor performance) technology. It maintains accuracy, speed and data quality by intelligently and continuously adapting the laser settings for each measured point. Probing error is comparable to that of tactile inspection (1.9 µm) and data collection is fast at 70,000 points per second. By maintaining such high standards when scanning difficult surfaces, the LC15Dx is an ideal tool for inspecting complex parts in the medical industry.

Paulius concluded, “The Nikon solution offers more in-depth knowledge of what we are manufacturing and gives better precision and understanding of 3D-printing errors and deviations. It means we achieve superior product quality and avoid problems during operations”.
www.nikonmetrology.com

  
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