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Design Innovation Paper

Design and Cadaveric Validation of a Novel Device to Quantify Knee Stability During Total Knee Arthroplasty

[+] Author and Article Information
Robert A. Siston1

Department of Mechanical and Aerospace Engineering, Department of Orthopaedics,  The Ohio State University, Columbus, OH 43210siston.1@osu.edu

Thomas L. Maack, Erin E. Hutter

Department of Mechanical and Aerospace Engineering,  The Ohio State University, Columbus, OH 43210

Matthew D. Beal

Department of Orthopaedics,  The Ohio State University, Columbus, OH 43210

Ajit M. W. Chaudhari

Department of Mechanical and Aerospace Engineering, Department of Orthopaedics,  The Ohio State University, Columbus, OH 43210

1

Corresponding author. Present address: Department of Mechanical and Aerospace Engineering, E305 Scott Laboratory, 201 W. 19th Avenue, Columbus, OH 43210.

J Biomech Eng 134(11), 115001 (Oct 26, 2012) (7 pages) doi:10.1115/1.4007822 History: Received February 23, 2011; Revised September 06, 2012; Posted October 10, 2012; Published October 26, 2012; Online October 26, 2012

The success of total knee arthroplasty depends, in part, on the ability of the surgeon to properly manage the soft tissues surrounding the joint, but an objective definition as to what constitutes acceptable postoperative joint stability does not exist. Such a definition may not exist due to lack of suitable instrumentation, as joint stability is currently assessed by visual inspection while the surgeon manipulates the joint. Having the ability to accurately and precisely measure knee stability at the time of surgery represents a key requirement in the process of objectively defining acceptable joint stability. Therefore, we created a novel sterilizable device to allow surgeons to measure varus-valgus, internal-external, or anterior-posterior stability of the knee during a total knee arthroplasty. The device can be quickly adjusted between 0 deg and 90 deg of knee flexion. The device interfaces with a custom surgical navigation system, which records the resultant rotations or translations of the knee while the surgeon applies known loads to a patient’s limb with a handle instrumented with a load cell. We validated the performance of the device by having volunteers use it to apply loads to a mechanical linkage that simulated a knee joint; we then compared the joint moments calculated by our stability device against those recorded by a load cell in the simulated knee joint. Validation of the device showed low mean errors (less than 0.21 ± 1.38 Nm and 0.98 ± 3.93 N) and low RMS errors (less than 1.5 Nm and 5 N). Preliminary studies from total knee arthroplasties performed on ten cadaveric specimens also demonstrate the utility of our new device. Eventually, the use of this device may help determine how intra-operative knee stability relates to postoperative function and could lead to an objective definition of knee stability and more efficacious surgical techniques.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Device overview showing the surgical boot, boot peg, instrumented handle, varus-valgus cart, internal-external wrench, and anterior-posterior bracket

Grahic Jump Location
Figure 2

The varus-valgus low friction slide, which is shown disconnected from the support frame for clarity. The surgeon applies loads to the cart with the instrumented handle, causing it to translate on the rail. As the cart translates, the bearings on the fork interact with the peg of the boot that is resting on the fork, driving the leg into varus or valgus. Blocks at both ends of the cart have attachment points for the instrumented handle, allowing the stability test to be performed from both the medial and lateral sides of the patient. The fork is allowed to rotate with respect to the cart.

Grahic Jump Location
Figure 3

Internal-external stability test wrench with a 12.0 in. (30.48 cm) lever arm. Rotation between the wrench and the boot peg is constrained by a 4 mm (5/32 in.) wide rectangular channel running along the anterior surface of the boot peg and a rectangular key. During testing, the handle of the wrench is located directly anterior to the fork to prevent the application of unwanted varus-valgus moments during tests of internal-external stability.

Grahic Jump Location
Figure 4

Boot coordinate system is found by identifying locator points (shown circled). The position of the peg is first adjusted so that it is collinear with the mechanical axis of the tibia. The vector connecting the two points on the boot peg forms the superior-inferior axis or the z-axis, corresponding to the tibial mechanical axis. The cross product of a vector connecting the proximal boot peg point and the anterior point with the z-axis forms the medial-lateral axis or x-axis. Finally, the cross product of the z-axis with the x-axis forms the y-axis. The origin of the boot coordinate system is unimportant for later calculations but is arbitrarily set as the proximal boot peg point.

Grahic Jump Location
Figure 5

The slide cart coordinate system is found by identifying three locator points on the cart and one point on the fork (shown as solid circles and connected by the dotted lines). A vector connecting the anterior-medial and anterior-lateral points on the distal side of the slide cart form the medial-lateral or x-axis. The cross product of the x-axis with the vector connecting the anterior-medial and posterior-medial points forms the superior/inferior axis or z-axis. The cross product of the z-axis with the x-axis forms the y-axis.

Grahic Jump Location
Figure 6

Use of the device to collect varus-valgus (a), internal-external (b), and anterior-posterior (c) stability data on a cadaver specimen with the knee in full extension in our laboratory

Grahic Jump Location
Figure 7

Representative varus-valgus (a), internal-external (b), and anterior-posterior (c) stability data from one representative cadaver specimen with the knee in full extension. Mean values from three cycles of testing are plotted along with ± one standard deviation. The novel device is able to collect repeatable data that is able to elucidate differences in stability before and after a total knee arthroplasty.

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