3D printing is often used to manufacture implants and other prosthetic devices for amputees, but have you ever stopped to consider which 3D printing method, and surface texture, is the best? As we continue to use 3D printing technology to customize textures and geometries to match an amputee’s specific anatomy, it’s important to learn all we can about what works and what doesn’t, especially as more prosthetic devices are transcutaneous osseointegrated implants, as opposed to traditional socket-cup fitting devices. A recent study, co-authored by researchers from the University of North Carolina (UNC) and North Carolina State University (NCSU), evaluated the two most common 3D printing methods used to produce fine and coarse textured titanium implants: electron beam melting (EBM) and direct metal laser sintering (DMLS).
In addition, the study reveals how 3D printed implants can be used to improve the integration of these amputee prosthetic devices with the remaining bone – also known as osseointegration. The researchers compared the strength of bone integration, torque, and interlocking in rats, which had received one or both types of implants in their distal femurs.
While DMLS is able to create either a coarse or a fine textured surface, EBM only produces a coarse textured implant. With this in mind, the research team noted significant differences between the coarse and fine implants, “based on mechanical testing to assess osseointegration and torsional properties, and measures of bone volume fraction and bone-implant contact.”
The research was published in a paper, titled “Osseointegration of Coarse and Fine Textured Implants Manufactured by Electron Beam Melting and Direct Metal Laser Sintering,” in 3D Printing and Additive Manufacturing, the only peer-reviewed journal that’s focused on the field of 3D printing and other related technologies. Study co-authors include David S. Ruppert, Ola L.A. Harrysson, Denis J. Marcellin-Little, Sam Abumoussa, Laurence E. Dahners, and Paul S. Weinhold.
Implants that have integrated with bone can transfer loads from the native bone to a synthetic joint; they can also function transdermally (across the skin) to set up a connection between the prosthesis and the skeleton. This helps get rid of some of the issues that plague socket prostheses, while 3D printing offers a more cost-effective way to produce patient-specific implants.
According to the abstract, “Our objective was to compare the osseointegration strength of two primary additive manufacturing methods of producing textured implants: electron beam melting (EBM) (mean Ra = 23 μm) and direct metal laser sintering (DMLS) (mean Ra = 10 μm). Due to spatial resolution, DMLS can produce surfaces with a roughness comparable to EBM. Two cohorts of Sprague-Dawley rats received bilateral, titanium implants in their distal femurs and were followed for 4 weeks. The first-cohort animals received EBM implants transcortically in one femur and a DMLS implant in the contralateral femur. The second cohort received DMLS implants (either fine textured or coarse textured to mimic EBM) in the intramedullary canal of each femur. Osseointegration was evaluated through mechanical testing and micro-computed tomography (bone volume fraction [BV/TV] and bone-implant contact [BIC]). The fixation strength of coarse textured implants provided superior interlocking relative to fine textured implants without affecting BV/TV or BIC in both cohorts. Coarse EBM implants in a transcortical model demonstrated an 85% increase in removal torque relative to the fine DMLS textured implants. The thrust load in the intramedullary model saw a 35% increase from fine to coarse DMLS implants.”
While titanium implants created through both EBM and DMLS are biocompatible, there have not been many studies that compare the two in terms of osseointegration. An Arcam A2 was used to 3D print the EBM implants for the rats, while the DMLS implants were 3D printed on an EOS M290; all of the implants were 3D printed using grade 5 titanium, and each one was 7 mm long, with a 2 mm diameter. Each pair of femurs was scanned, post-implantation, with micro-computed tomography, and the scans were analyzed using Materialise Mimics software; then the femurs were put through mechanical testing. Then, a second group of rats was tested, this time using implants that had both fine surface texture and coarse.
Ultimately, the researchers determined that implants with coarse textured surfaces offer a higher surface strength for titanium alloy implants, and suggested that further studies are needed in order to determine how rough the implant fixation should be. But the conclusion of the paper explains the most important part of 3D printing studies such as this one:
“As the trend in amputee prosthetic devices moves toward transcutaneous osseointegrated implants instead of socket-cup fitting prosthetic devices, this study is important in showing that additive manufacturing can provide a means of producing well-fitted osseointegrated implants that can be easily customized.”
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