Ligament balancing can be difficult to perfect in total knee arthoplasty (TKA), where current surgical practice is subjective and highly dependent on the individual surgeon. Proper ligament balancing contributes to postoperative stability, prosthetic alignment, and proprioception. Conversely, imbalance is linked to increased wear rates of the polyethylene component within the implant and, in turn, early surgical revision. With the end goal of quantification of joint compartmental pressures, pressure sensor arrays have been designed to quantify contact stresses within the knee during TKA. Flexible, capacitive pressure sensors are designed as simple parallel plates, enabling a robust solid state design. Modification of cleanroom microfabrication processes enable realization of these arrays on polyimide (common in microdevices), and polyethylene (common in joint replacements). Readout circuitry implements an Analog Devices capacitance to digital chip and output is compared to direct LCR meter data. Testing verifies the highly linear response of the sensors with applied normal loads corresponding to pressure magnitudes present in passive (intraoperative) knee flexion. Spatial resolution of the arrays is 0.5 mm, with a critical dimension of 25 micrometers, allowing the magnitude and location of forces to be accurately recorded. The MEMS pressure sensors are mounted on a tibial trial, with the body of the trial housing all circuitry. The sensors are read sequentially, and the data undergoes analog to digital conversion prior to wireless data transmission at 2.4 GHz. An Instron machine is used for compressive loading for laboratory calibration and testing. This paper outlines device fabrication, readout circuit implementation, and preliminary results.
Force profiles across the foot yield information on abnormal kinematics and may be used to indicate pathological changes in the lower limb. However, current technology is limited to tethered systems using wired sensors. This paper outlines a wireless prototype that allows force profile measurement and through an in-shoe monitoring device utilizing custom high-accuracy sensors. Direct measurement of the ground reaction force using a force plate is common practice for use in kinematic studies and is used as an input for mathematical models to predict forces across joints of interest during various activities. Force plates are reasonably accurate but are bulky and only allow one net force measurement at a single location and are not portable. Thus natural patient motion may be modified, intentionally or unintentionally, in order for heelstrike to occur on the force plate. In addition to force magnitude, it is useful to record force location to correlate with kinematics; abnormal kinematics will cause weight-bearing forces to shift across the foot. Current in-shoe pressure measurement devices on the market are plagued by errors up to 30% and require a cumbersome cable out of the shoe to read sensor data. By eliminating all wires, our device enables in-shoe monitoring in a research or clinical environment. The device uses microelectromechanical system (MEMS) capacitive pressure sensors fabricated in a flexible array that attaches to a shoe insole or orthotic. The sensors are concentrated at the heel and forefoot in the prototype design and they exhibit a highly linear response to loading, eliminating the need for constant recalibration. Electronics embedded in the shoe read the entire array of 256 sensors at a rate of 60 Hz. The data is transmitted via Bluetooth at 2.4 GHz to the receiving computer for visualization and analysis. The paper assesses current technology in in-shoe sensing, outlines the device design, and reports initial stages of testing. The prototype developed in this study shows promise for wireless monitoring of ground reaction forces for biomechanics analysis without restricting activity or impeding natural motion.
