“Transformative” claims about manufacturing dissipate easily enough in the light of day but in the operating room, with a patient’s mobility and quality of life literally at stake, additive manufacturing succeeds in every respect. Recently, a partnership between PTC, Hexagon, and the Sourasky Medical Center in Tel Aviv was brought about in the creation of a customized titanium scapula implant for a 16-year-old patient suffering from Ewing’s sarcoma.
The scapula is involved in complex shoulder motion as it provides attachment points for many muscles responsible for arm movement. For the particular patient, the malignancy had weakened the bone, leading to issues with pain, inflammation, and reduced functionality. “Our goal was to keep the same shape and general size of the bone, but replace it with something we engineered,” stated Lee Goodwin, a solution consultant with PTC. While titanium was the obvious material for its compatibility with the body’s biological environment, it is three times more dense than bone with four times more rigidity, requiring a design solution to prevent it from stressing other tissue too heavily.
They started with high-resolution MRI scans of the normal left scapula, taking full advantage of the tissue contrast provided by the MRI without the use of ionizing radiation. The image was flipped to represent the right-sided geometry and imported into the PTC Creo CAD environment. Material was removed selectively to lighten the structure and a lattice structure added not only for lightness but also for the promotion of tissue growth into the structure as a result of a study showing that structures modeled after interconnected pores led to greater osseointegration. Muscle reattachment points were also added, and loading paths were analyzed in Creo.
However, a sophisticated CAD design may not translate properly to manufacturing. “All the work that has been done to match the weight, the center of gravity, the dynamics of the implant, that doesn’t guarantee it’s going to print okay,” pointed out Mathieu Pérennou, a director of AM solutions at Hexagon AM. Pérennou and his team analyzed part orientation and support, followed by a digital twin analysis of a simulation print cycle. By inserting data on material properties and process variables, shrinkage was predicted and corrected before committing to a material and a laser.
This allows for better alignment with optimal methodologies in patient custom implant manufacturing, which are seeing an increasing number of regulatory guidelines centered around verification of a design before its physical production. The FDA/RSNA 3D Printing Special Interest Group recommends a robust simulation verification process in order to verify size accuracy, especially when working with complex shapes obtained through medical imaging. In this way, the simulations previewed a printing process that could have been completed in a matter of days with a resulting part that could have been produced with absolute tolerance without any rework.
Once printed, ongoing post-processing of the titanium scapula was meticulous. Contact areas were polished, followed by CT analysis using VGStudioMax to evaluate for porosity or trapped powder in the scapula. These non-destructive analyses reflect current protocols used for craniofacial and orthopedic procedures that involve reconstruction for which differences of as little as 0.18% might impact fit, prior to clearance for actual surgery after assessment of inspection results by the surgeon.
One very interesting aspect about permanent titanium implants is the potential problem of “stress shielding,” in which bone tissue surrounding an implant may become weakened if it is too strong. The weight-optimization structure provided a benefit in this area by controlling the mechanical strength while providing physiological suitability. The lattice structure in this case offered benefits in both weight reduction and mechanical support.
Time was of essence. “What we managed to do in four days,” Pérennou said, “is scan the patient, design an implant, print it, and implement it.” In procedures such as orbital fracture repair, research has demonstrated the reduction of operating times by over 40 minutes and the elimination of revision surgeries when rapid prototyping and intraoperative imaging are employed. Although the patient with the scapula problem did not undergo intraoperative imaging, reducing the time available for the progression of a tumor was certainly beneficial.
“This project exemplifies the level of maturity that the additive manufacturing process chain has achieved to address critical applications within the field of medicine, such as precision imaging, model generation, lattice structure optimization for biocompatibility, simulation for print processing, and analysis post-processing for final inspection,” said Srinivasan. “For design engineers and experts within the field of AM, it’s a success story about how knowledge from diverse fields, ranging from kinematics to verifiable validation, comes together to create one life-changing outcome,” Srinivasan added.