Intra-operative applications

		The measurement system: the developed force-sensing-device (center), the signal processing unit (left) and the data display on a notebook (right) The force-sensing device in a knee joint model equipped with spring-ligaments

Sensor for optimal ligament balancing of artificial knee joints

The knee sensor for optimal ligament balancing is an example, how intelligent intra-operative micro-devices can provide valuable assistance to the surgeon, while leaving him in full control.

In a cooperative project between the EPFL (Laboratoire de Systèmes Robotiques LSRO, Laboratoire de Production Microtechnique LPM) and the University of Bern (M.E. Müller Research Center) a force-sensing device for knee arthroplasty is being developed. Balancing of ligament tensions and alignment of the tibiofemoral mechanical axis are crucial clinical parameters of total knee arthroplasty. Ensuring an optimized patient-specific ligament balance can increase the prosthesis lifetime. The developed sensor allows precise real-time measurements of the condyle contact forces and the useful biomechanical parameters are displayed on the computer screen after data processing. The system makes therefore important quantitative information accessible to the surgeon.

The device consists of two sensitive plates, a tibial base plate and a set of lateral and medial spacers, which allow varying the tibio-femoral gap. Due to its small thickness, the device entirely fits inside the knee joint in the tibio-femoral gap after an initial tibial precut with the patella in its anatomical place. The contact force and the contact location of each condyle are measured using three deformable bridges instrumented with thick-film piezoresistive sensors. These measurements allow the computation of the net varus-valgus moment of the contact forces, which is considered as the parameter describing the ligament imbalance.
Control experiments validated the device measurement principle and the first in-situ trials showed that the information provided by the system is in agreement with the surgeon's perception. Currently, the device-assisted and the manual ligament balancing approach are being quantitatively compared through a series of in-vitro experiments.