In Vivo Kinematics of the Knee Using Both Stationary and Mobile Fluoroscopy

INTRODUCTION: Stationary fluoroscopy has been a viable resource for determining in vivo knee kinematics, but limitations have restricted the use of this technology. Patients can only perform certain normal daily living activities while using stationary fluoroscopy and must conduct the activities at speeds that are slower than normal to avoid ghosting of the images. More recently, a Mobile Tracking Fluoroscopic (MTF) unit has been developed that can track patients in real-time as he/she performs various activities at normal speeds (Figure 1). Therefore, the objective of this study was to compare in vivo kinematics for patient9s evaluated using stationary and mobile fluoroscopy to determine potential advantages and disadvantages for use of these technologies. METHODS: The MTF is a unique mobile robot that can acquire real-time x-ray records of hip, knee, or ankle joint motion while a subject walks/manoeuvres naturally within a laboratory floor area. By virtue of its mechanizations, test protocols can involve many types of manoeuvres such as chair rises, stair climbing/descending, ramp crossing, walking, etc. Because the subjects are performing such actions naturally, the resulting fluoroscope images reflect the full functionality of their musculoskeletal anatomy. Patients in the study were initially fluoroscoped using a stationary unit and then using the MTF unit. RESULTS: Initially, patients were requested to perform gait, stair-rise, stair descent, chair-rise and a deep knee bend under stationary fluoroscopy. Unfortunately, patients were not able to perform most of these activities, under normal conditions, using a stationary fluoroscopy unit. Therefore, only a deep knee bend, step-up and chair-rise were viable activities, but again, performed at slower than normal speeds. While using the MTF unit, all five activities were easily performed and patients received 50% less radiation due to the fact that five activities using the MTF required significantly less time than three activities using a stationary fluoroscopy unit (Figure 2). Also, walking up and down a ramp was also included in the protocol for the MTF analyses, leading to the inclusion of two new activities, not previously evaluated using stationary fluoroscopy. Interestingly, the kinematics for the patients using a stationary fluoroscopy unit produced less overall motion than compared to the MTF analysis, probably due to the slower speed required to perform the activities. While using the MTF unit, kinematic findings such as anteroposterior sliding and reverse axial rotation were more pronounced during each activity. DISCUSSION: This study has revealed that there are distinct advantages for using a MTF unit, compared to a stationary unit. Patient exposure to radiation was significantly reduced and they were able to perform each activity quicker, while only one trial was required for each activity. Patients were able to perform multiple activities at normal speeds and these analyses seemed to produce more overall femorotibial motion than those assessed using a stationary fluoroscopy unit. Also, ghosting of the images, seen when using a stationary fluoroscopy unit were not evident when using the MTF. Future analyses with the MTF will involve more challenging activities, such as stop and start and sequential motion patterns.