Optimal implant fixation, integrated in an automatic design of patient-specific implants for reconstruction of acetabular defects

Many years of technological development and surgical experience in total hip arthroplasty, have increased the lifetime of implants to 15 years for 80% of young patients (<65) and 94% of older patients (>65). Complications after a primary hip implant often lead to a revision surgery as the only option to reconstruct the functionality of the hip joint. Loosening, failure or removal of implant components regularly involve bone loss, resulting in revision after revision surgery, leading to large acetabular defects and even pelvic dissociations. For these patients, a custom implant based on medical images is a new solution with good short to mid-term results. However, the complexity of the technology and the labor-intensive design process makes these implants very expensive for both the patient and the healthcare system. The goal of this thesis is to use state-of-the-art methods to reduce the implant design time, standardize the design process and improve the quality of the implants to take full advantage of the potential of patient-specific implant solutions.

Before designing an implant, CT-images need to be segmented to make a 3D-model of the pre-operative situation. Next, failed implant components need to be virtually removed. To determine the best treatment, it is important to quantify the bone loss and to consider biomechanical balancing of the contralateral hip joint. In this thesis, a statistical shape model of a healthy hemi-pelvis was constructed and an automatic method to compute a virtual anatomical reconstruction of a defect hemi-pelvis was developed. The reconstruction was later used to improve an existing bone loss quantification method, resulting in a more clinically relevant bone defect quantification. Finally, the shape model was used to determine the anatomical coordinate system of the pelvis, that is crucial for the positioning of the cup component of the implant and the biomechanical balancing compared to the contralateral hip joint center.

The custom hip implant consists of a cup component, connected by three flanges that bridge the bone defect and are anchored in the remaining bone via flange screws. Underneath the flanges and around the cup, a porous structure is designed, filling the defect and allowing for bone ingrowth. Because the cup component is the most crucial component regarding biomechanics of the hip joint and because the rest of the implant is designed around this cup, research was done into objective cup evaluators that allow to compare different cup plannings. These evaluators were then used to develop an automatic cup optimization algorithm.

Because every implant is unique, it is a challenge to guarantee the mechanical integrity of an implant during a normal implant lifetime. For economical reasons, this needs to be assessed without producing a copy of the implant and performing mechanical tests. Therefore, an automatic virtual mechanical test bench was developed using a finite element model, to allow identification and adaptation of weak spots in implants, during the implant design process. Through a validation experiment, it was shown that the virtual test bench gave good indications about the weak spots in the implant design that are prone to fatigue failure.

In this research, a major step was taken in the direction of economically viable custom implants, by speeding up and standardizing the design process and improving the quality of the implants. Therefore, we expect that in the future it will be viable to bring custom implants to the market for patients with less severe bone defects. In that way, suboptimal solutions using standard revision components can be avoided.