Novel approach to dynamic knee laxity measurement using capacitive strain gauges.
OBJECTIVE
Knee joint laxities are observed in patients after severe trauma to the joint, resulting in ligament tears. Specifically, injuries to the anterior cruciate ligament may cause a significant instability. The degree of these laxities is essential in diagnostics and may affect which treatment option is suggested.


METHODS
Polydimethylsiloxane (PDMS) strain gauges are proposed as a non-invasive, highly accurate and easy-to-use measurement method to quantify anterolateral and rotational laxities of the knee joint during active and passive motion. In this work, proof-of-concept measurements and a prototype of the proposed device are displayed. The measurements were taken using a knee test rig, which has specifically been designed for this purpose. This apparatus allows the simulation of isolated knee joint instabilities with a motor-controlled model of a human knee.


RESULTS
The absolute sensitivity [Formula: see text] of an exemplary sensor was determined to be 2.038 [Formula: see text]; the relative sensitivity [Formula: see text] was 1.121 [Formula: see text]. Optimal positions of sensors to capture bone-to-bone displacement as projected displacement on the skin were identified.


CONCLUSIONS
PDMS strain gauges are capable of measuring bone-to-bone displacements on the skin. We present an experimental in vitro study using an artificial knee test rig to simulate knee joint laxities and display the feasibility of our novel measurement approach.
Department of Orthopedic Surgery and Traumatology, Freiburg University Hospital, 79106, Freiburg, Germany. martin.zens@imtek.uni-freiburg.de.|Department of Microsystems Engineering (IMTEK), Albert-Ludwigs-Universität Freiburg, Georges-Koehler-Allee 102, 79110, Freiburg, Germany. martin.zens@imtek.uni-freiburg.de.
Martin
M
Zens
Department of Orthopedic Surgery and Traumatology, Freiburg University Hospital, 79106, Freiburg, Germany.
Philipp
P
Niemeyer
Department of Orthopedic Surgery and Traumatology, Freiburg University Hospital, 79106, Freiburg, Germany.
Anke
A
Bernstein
Department of Orthopedic Surgery and Traumatology, Freiburg University Hospital, 79106, Freiburg, Germany.
Matthias J
MJ
Feucht
Department of Orthopedic Surgery and Traumatology, Freiburg University Hospital, 79106, Freiburg, Germany.
Jan
J
Kühle
Department of Orthopedic Surgery and Traumatology, Freiburg University Hospital, 79106, Freiburg, Germany.
Norbert P
NP
Südkamp
Department of Microsystems Engineering (IMTEK), Albert-Ludwigs-Universität Freiburg, Georges-Koehler-Allee 102, 79110, Freiburg, Germany.
Peter
P
Woias
Department of Orthopedic Surgery and Traumatology, Freiburg University Hospital, 79106, Freiburg, Germany.
Herrmann O
HO
Mayr
D053587:Arthrometry, Articular / Q000295:instrumentation* / Q000379:methods; D001696:Biomechanical Phenomena / Q000502:physiology; D004129:Dimethylpolysiloxanes; D006801:Humans; D007593:Joint Instability / Q000175:diagnosis* / Q000503:physiopathology; D007719:Knee Joint / Q000503:physiopathology*; D008954:Models, Biological; D013314:Stress, Mechanical
D004129:Dimethylpolysiloxanes; C013830:baysilon
ACL ruptures; Joint displacement measurement; Knee joint; Laxities; Pivot shift; Strain gauges