Case presented by D. van den Heever, J.H. Muller and C. Scheffer, University of Stellenbosch, South Africa

Every year more than 1 million arthritic patients worldwide undergo knee replacement surgery. Although this is one of the most complex joints, conventional knee implants only exist in standard sizes and shapes predetermined by their manufacturers. In this case study, researchers from Stellenbosch University looked into the possibility of creating custom implants. They analyzed first where the contact pressure points in the knee are situated; second, if a customized approach for total knee arthroplasty prosthesis could be advantageous over the traditional approach; and finally, they did a proof of concept for designing such custom implant. The wide range of tools in the Mimics Innovation Suite proved to be a key technology for the diverse aspects in this research.

Step 1: Contact Pressure Point Analysis

 

FEA prediction of femoral contact areaMimics measurement of femoral contact area

 

Overloading of the knee joint is often the result of a decreased contact area between the articulating surfaces, which leads to elevated contact pressure. In order to simulate this pressure, a patient-specific finite element analysis (FEA), based on 3D models obtained from an MRI or CT scan, is commonly conducted. However, these simulations can only be relied upon when they are validated with experimental data, which introduces some major challenges. Articular cartilage, for example, has the capacity to conform to contact geometry under loading, a phenomenon that is difficult to quantify experimentally. With the help of the Mimics Innovation Suite, the researchers from Stellenbosch University analyzed MRI data of the knee under different loading conditions to validate the contact geometry simulated via FEA.

 

FEA prediction of patellar contact areaMimics measurement of patellar contact area

 

In order to visualize the patellar and femoral cartilage, two MRI scans of the knee were made: one in full extension with the quadriceps relaxed, the other in a 30° flexion angle, loaded using an MRI compatible loading frame. Applying the advanced MRI segmentation tools in Mimics, the researchers converted the stack of 2D images into accurate 3D models of the bone and the cartilage. Next, they used Mimics’ registration tools to place the unloaded cartilage virtually from the extended knee onto the knee in the flexed scan. This resulted in an overlap of the patellar and femoral cartilage, which was extracted using a Boolean intersect. The results indicated a close correspondence between measurement data and the FEA prediction, a conclusion that gave the researchers the necessary confidence to use this technique in their development of a custom knee implant.

 

 

Step 2: Is a Custom Implant Favorable?

In the next phase, the team wanted to determine whether or not the part of the contact area of the femoral condyle that articulates with the tibia had a constant radius and if this radius was consistent across different patients.
 
The team scanned the condyles of the distal femur of several cadaver specimens with a laser scanner and used 3-matic to turn the subsequent point clouds into accurate 3D models. To investigate and compare the condylar curvatures, intersection planes were drawn on each condyle, which approximated the surgical epicondylar view. The resulting intersection curves were exported to analyze the curvature statistically at every point. This analysis showed that the condylar surface differed considerably between individuals and that the radius was never constant. As a result, the researchers hypothesized that a customized approach to a knee prosthesis could optimize its functionality and improve implant longevity.
 
A custom proximal tibial implant could further improve the clinical results by reducing the bone loss during surgery, reducing the duration and complexity of the surgery and avoiding an uneven stress distribution on the sharp edges of the implant. With the aim of removing these obstacles, Stellenbosch University’s team began to design patient- specific knee implants.
 
 
 

Step 3: Designing the Custom Implant

To design a customized implant, the research team started from a CT scan of a knee. With Mimics, they generated 3D models of the distal femur and proximal tibia. The 3D model was exported to 3-matic, where they used the flexible design tools to create a customized, unicompartmental knee replacement directly on patient data, thus guaranteeing a perfect fit. In addition, the tibial trays were designed to provide complete cortical rim coverage. The designs could be used directly for 3D Printing, the production method of choice for patient-specific implants.
 
Positioning of femoral component on bonetibia_dimiretouche-2.jpg

The Standard in ‘Engineering on AnatomyTM

The Mimics Innovation Suite turns 3D image data into high-quality digital models in an accurate and efficient way. Starting from optical scan, CT or MRI data, the Mimics Innovation Suite offers the most advanced image segmentation, the broadest anatomical measurement options, powerful CAD tools for Engineering on Anatomy and 3D Printing, and accurate model preparation for FEA and CFD.
In this case study, the authors used the Mimics Innovation Suite to create a customized knee implant using the following steps:

  • Segment and compare articular cartilage from MRI scans taken under different loading conditions
  • Analyze anatomical variation in articulating surface of the knee between different patients.
  • Design a custom knee implant directly on anatomy

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