Hip replacement is a very common surgery, one typically used to treat osteoarthritis. In these cases, the implant is an off-the-shelf acetabulum cup—a simple, half-dome shape. But there is another group of hip replacement patients with more complex needs that the off-the-shelf devices do not solve. Many have had hip replacements already, and often have extensive damage to the hip joint that requires a bespoke, patient-specific implant.
Complex hip replacements involve considerable surgical challenges. In such cases, scar tissue and multiple previous operations distort the anatomy. There may be fragile bone, dead bone or cancerous tumors. Unique bone preparation is required for the bespoke implant, which only fits into one shape with one orientation. Finally, the nature of how these surgeries are performed—through a relatively small incision, through multiple layers of tissue, carefully avoiding a network of nerves and blood vessels—makes it difficult to see if the implant is seated correctly.
For all of these reasons, it is critical that the implant be designed and manufactured accurately. It is a process that encompasses a number of steps and software applications from the first patient scan to the actual surgery, a process that was covered recently during a webinar featuring physicians from the Royal National Orthopedic Hospital (RNOH) in the UK.
Starting with accurate imaging
The patient-specific hip implant starts with CT or MRI imaging of the patient for analysis of bone quality and morphology, which helps the team establish where the implant will be positioned and secured. In many cases, these images are augmented with standing x-ray images that provide weight-bearing imaging. This is critical for a 3D understanding of pelvic orientation.
The next step in the process is taking these 2D medical images and turning them into a 3D model. The software involved here is Simpleware, a high-end image processing application that is specifically designed to convert CT or MRI images into 3D models. It imports the image data, then enables a combination of manual and automated, AI-driven segmentation to define the anatomical structures in the image data. It then generates the 3D model so implant designers can reconstruct the surfaces of the hip and take accurate measurements.
At this point, the 3D model must be exported into a design application, in this case Geomagic Freeform. The export process from Simpleware to Freeform is quick and easy. The resulting model in Freeform can include all the labels from the various planes as well as a colour map of the bone density.
From 3D modelling to printable design
Freeform is a modelling system that produces manufacturable, engineered objects that incorporate complex and organic forms. In other words, it helps design objects that fit the human body, including patient-specific hip implants.
Traditional CAD programs often struggle with these types of objects because they represent shapes with boundary representational surfaces (B-reps), which define the volume of an object using a combination of edges, faces and vertices. While this approach excels at objects that incorporate straight lines and smooth edges, it gets extremely complicated when the shapes are organic.
Freeform, on the other hand, uses voxels, which are 3D pixels that can be combined like grains of sand. Because of this, it is very easy for Freeform to represent even highly complex organic shapes and structures.
Importantly, Freeform includes a full set of interactive tools designed to handle complex models and data. In the case of the hip implants, the design team leading up these efforts was implantcast Gmbh, a privately owned medical company headquartered outside Hamburg, Germany. Specifically, the work was head up by a department within implantcast called C-Fit 3D, which specialises in bespoke, patient-specific implants and handles more than 1,000 of these cases every year.
According to the C-Fit 3D team, Geomagic Freeform software provides a very broad range of features and capabilities, only a fraction of which are involved in the design of the patient-specific hip implants. In addition to the clay and meshing features, team leader Christopher Bünning, Head of C-Fit 3D®, called out the software’s haptic device, which is used instead of a traditional mouse. This device resembles a pen that is held in mid-air to shape the model, providing real-time physical feedback as if it were touching an actual block of modelling clay or other material.
Collaborating with the surgical team
The C-Fit 3D team noted the importance of close collaboration between the implant design team and the surgical team. Discussing the 3D model of the implant helps both sides understand the character of the hip defect and how it may affect bone preparation. The goal is to maximise contact surface between the bone and the implant, which helps improve osteointegration. The RNOH team noted that recent improvements in the design process have yielded meaningful improvement in bone-implant contact surface. While the previous approach had an average contact surface of 12%, the more recent approach has achieved an average contact surface of 69%.
Once the design is finalised, the implant is typically 3D printed in titanium and then the surgery can proceed. After a successful implantation, the surgical team must evaluate patient outcomes over time, using a combination of clinical and radiological analysis to assess functional scores, rates of complication (such as dislocation, infection or fracture), loosening, implant failure, and migration.
The Geomagic Freeform team will continue to collaborate with Simpleware and implantcast to optimise these workflows and pave the way for even more accurate and effective patient-specific implants. For even more detail on the process, watch the full webinar.
