Mobile Fluoroscopy: Determination of Natural, Unrestricted Knee Motion

Introduction

Over the past two decades, fluoroscopy has been used as a tool to perform in vivo analysis of postoperative total joint replacement (TJR) results, primarily knees. The fluoroscope digital images frames when computationally combined with three-dimensional (3D) graphical models of the joint implants allow the 3D kinematics of the implanted joint motion to be revealed. Furthermore, the concept of inverse dynamics can then be used to calculate the actual forces occurring (eg, between the femur and tibia in the case of a knee joint). Such information is useful in understanding the detailed in vivo kinematics of a TJR surgical result, including the resulting force system, which has particular relevance to the performance of the joint interface materials (eg, polyethylene wear).

Conventional C-arm fluoroscopes have been used to capture in vivo skeletal movement during activities, such as deep knee bends, chair rises, and walking on treadmills. C-arm fluoroscopes are static, or nonmoving devices, such that the recorded images are susceptible to blurring because of the relative motion of the joint features with respect to the field of view of the imaging sensor. In addition, because the imaging sensor is fixed, the field of view is limited to the sensor's size and often is not able to record all of the desired aspects of a particular activity. In some cases human operators have been used on either side of the C-arm to physically maneuver the unit such that the field of view approximately follows the subject's motion.

The concept of mobile fluoroscopy is about relieving the constraints associated with fixed C-arms by creating a robotic-type of fluoroscope that is capable of “tracking” a human subject during natural activities. Each person's musculoskeletal system is unique and displays unique motion details. A key aspect of mobile fluoroscopy is the capabilities to evaluate implant kinematics and kinetics while a subject is engaging their full musculoskeletal system through natural motion activities.

This chapter discusses the goals and requirements one can envision for the ideal mobile fluoroscope capable of imaging hip, knee, and ankle joints. Other research in mobile fluoroscopy from around the world will be touched upon, but with the specific capabilities and clinical results obtained with the tracking fluoroscope system (TFS) at the University of Tennessee, Knoxville, being the focal point. Potential future directions in mobile fluoroscopy are summarized.

Mobile Fluoroscopy Concepts