Recent research has demonstrated the importance of alignment of the components in total knee arthroplasties (TKA). It has been documented in clinical trials that malalignment is an issue that occurs during TKA. It has also been determined that this malalignment causes altered loading and could lead to early failure of the implant. Therefore, researchers are working to find a solution to this issue so that it may be avoided in the future.

Additionally, numerous studies have shown that the knee flexes about a fixed axis. It is this single axis of rotation of the knee that lead Stryker Orthopaedics to design the Scorpio implant designed with a single rotation axis. A question that remains is how to properly align the implant rotation axis to the natural rotation axis of the knee.

The goal o f this project was to establish a method of aligning the components o f a TKA to the single rotation axis of the knee. Two implantation axes were evaluated in this study;​ a helical axis and a cone-fit axis. The helical axis was calculated using proprietary methods, and the cone-fit axis was determined by fitting a truncated cone to the condyles of the femur. The cone-fit axis, or the natural flexion axis of the knee, was identified in cadaveric knees using 3D models of CT data and was verified by observing kinematics values through flexion/extension of the knee. Five specimens were implanted such that the axis of the Scorpio implant was collinear with the cone-fit axis, and five were aligned to the helical axis. Finally, the results were evaluated for success in restoring natural kinematics postoperatively.

Results were not conclusive for the cone-fit or helical knees. Overall, the data from each group showed restoration of kinematics was not achieved, and the level of restoration success was comparable between the two. However, the data revealed possible faults in the helical axis and indicated a repeat test of the cone-fit implantations could prove beneficial.

One major issue involved in the implantation of the cone-fit knees was performing the non-traditional surgeries using an implant and instrumentation designed for the traditional approach. This issue limited the ability to accurately position the implant components in the cone-fit knees. In addition, the calculations of the implant positioning goal needed improvement, which could be found with an improved fitting technique. There are indications that, with adjustments to the protocol, the cone-fit kinematics restoration could be improved.

The implantation o f the helical knees was only altered slightly from the traditional approach making it much easier to use the implant and instrumentation to achieve target placement. However, even with the position accuracy, the kinematics were not restored, which indicated a possible inherent error in the approach.