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Design and control of a minimally constrained cobot for improving bone cuts in total knee arthroplasty Emrich, Richard

Abstract

Total knee arthroplasty (TKA) requires accurate placement of the prosthetic components to avoid premature failure, and traumatic revision surgery. Accuracy of frontal plane alignment is particularly crucial in ensuring maximized prosthesis longevity. Increasing the accuracy of bone cuts would decrease the need for revision surgery. Errors introduced by radiographic technique, radiographic measurements, and alignment jigs can be reduced by improving frontal plane alignment calculations. A key location necessary for this calculation is the centre of the knee. A point probe could be used to digitize the surface of the knee, which would allow for an accurate calculation of a centre point. This method is susceptible to imperfections in the surface of the knee, which are commonplace in TKA. A "v" probe and flat probe design are presented, and are shown in simulations to be more robust in locating the centre of rounded anatomical features, such as the condyles of the knee. Implementation errors are also major contributors to inaccurate frontal plane alignment. Increasing this accuracy may be possible by harnessing the accuracy of robots. Actively powered robots have failed to be widely accepted in surgical applications because of safety concerns. Collaborative robots, or cobots, reduce these safety concerns by being completely passive. Under computer control, cobots only constrain the user's motions by orienting frictional constraints. A parallel architecture would be desirable for TKA because of it's stiffness. The key component for the cobot is a continuously variable transmission (CVT), but no CVT is currently suitable for use with the parallel manipulator. To address this, a linear CVT was created, and is discussed with respect to its benefits, limitations, and areas requiring future work. The concept of a minimally constrained cobot is also introduced. This type of cobot is particularly well suited to guiding objects to areas with dimensionality greater than one. This makes them well suited to TKA, where the object is to orient a three dimensional cutting plane to guide the bone saw. A control strategy for controlling minimally constrained cobots is presented, along a 3 dimensional simulation of results. The major contribution of this thesis is the development of a framework for minimally constrained parallel cobots for TKA. Future work will be focused on the design, construction and controller implementation of a 6 DOF cobot prototype.

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