A patient in Argentina required a particularly large cranial implant after stroke-related surgery, placing stringent requirements on the manufacture of the prosthetic. Naturally it needed to fit precisely, but in this case it also had to be permeable to allow brain fluid to pass through. Minimal heat conduction to the cerebral tissue was important, especially in a sunny climate. Additionally, biocompatibility was needed to allow the bone to grow into the edges of the implant.
A titanium alloy lattice structure secured by screws directly into the skull was deemed to be ideal. It was additively manufactured layer by layer from metal powder in a machine produced by German firm, EOS, whose UK subsidiary is on the Warwick Technology Park.
The one-and-a-half-hour surgical procedure was carried out successfully in May last year (2014). The patient left hospital after two days and the wound healed within three weeks. Since that time there have been no complications and the patient has been able to lead a normal life.
Several stringent mechanical requirements had to be met to ensure a successful result and technological advances in additive manufacturing allowed them to be achieved. The pores in the implant are approximately 1 mm across, while the links are about 0.2 mm thick, resulting in 95 percent porosity. To achieve such a fine mesh in a rigid structure to tight dimensional and profile tolerances would be impracticable using conventional, subtractive production techniques.
Time was of the essence in producing the implant. The process was started by Novax DMA in Buenos Aires, which specialises in developing and supplying medical implants for traumatology, orthopaedics and craniofacial surgery. For the 3D design of the implant, software was employed from UK company, Within, which allowed the basic form and porous structure to be defined quickly.
As soon as the CAD work was completed, Alphaform AG, near Munich, manufactured the implant in a matter of hours in an EOSINT M 280 metal additive manufacturing machine from EOS. The implant was in the operating theatre less than three weeks later, with transportation consuming one-third of that time.
Christoph Erhardt, Director of Additive Manufacturing at Alphaform, commented, “We had already successfully completed many additively manufactured products in the EOS system.
“However, we are particularly proud of this implant, not only because of the precise realisation of the form, but also because we were able to optimise the porous structure and the difficult process of cleaning the small interior spaces.
“We developed a multi-step process of abrasive and mechanical cleaning, rinsing and ultrasonics to arrive at the required level of medical purity, which is vital as particles can dislodge with the slightest movement, leading to the possibility of infections or rejection.”
The level of cleanliness was verified by extensive tests, including particle and cytotoxicity testing. Gas-chromatography analysis was also performed. Other tests confirmed that the implant fulfilled the necessary requirements to stabilise and protect the patient's skull.
Daniel Fiz, CEO of Novax DMA, added, “We have been supplying medical implants since 1995. Additive manufacturing represents a new milestone for patients. It offers optimal biomedical characteristics together with the highest levels of compatibility, thereby having a lasting effect on improved quality of life.
“For these reasons, we have applied the technology with success to other areas of the body. Alphaform has also manufactured jaw implants for us, as well as a hip joint and a spinal implant. For the latter, we are currently considering series production using additive manufacturing.”
Christoph Erhardt concluded, “For us, additive manufacturing and EOS are synonymous. We have already produced an enormous number of parts for many companies. Both we and our customers are continually amazed by the application possibilities and the high-quality production that can be achieved.
“That was once again the case here. We were able to help a person to live a normal life, on an ongoing basis, despite their having suffered a very serious injury.”