The importance of position and path repeatability on force at the knee during six-DOF joint motion

Darcy, Shon P.; Gil, Jorge E.; Woo, Savio L-Y.; Debski, Richard E.

doi:10.1016/j.medengphy.2008.11.001

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Volume 31, Issue 5 , Pages 553-557, June 2009

The importance of position and path repeatability on force at the knee during six-DOF joint motion

Shon P. Darcy
- Musculoskeletal Research Center, Department of Bioengineering, University of Pittsburgh, 405 Center for Bioengineering, 300 Technology Drive, Pittsburgh, PA 15219, United States
- McCaig Centre for Joint and Arthritis Research, University of Calgary, Calgary, AB, Canada
Jorge E. Gil
- Musculoskeletal Research Center, Department of Bioengineering, University of Pittsburgh, 405 Center for Bioengineering, 300 Technology Drive, Pittsburgh, PA 15219, United States
Savio L-Y. Woo
- Musculoskeletal Research Center, Department of Bioengineering, University of Pittsburgh, 405 Center for Bioengineering, 300 Technology Drive, Pittsburgh, PA 15219, United States
Richard E. Debski
- Musculoskeletal Research Center, Department of Bioengineering, University of Pittsburgh, 405 Center for Bioengineering, 300 Technology Drive, Pittsburgh, PA 15219, United States
- Corresponding author. Tel.: +1 412 648 1638; fax: +1 412 648 2001.

Received 23 January 2008; received in revised form 3 November 2008; accepted 7 November 2008. published online 07 January 2009.

Abstract

Mechanical devices, such as robotic manipulators have been designed to measure joint and ligament function because of their ability to position a diarthrodial joint in six degrees-of-freedom with fidelity. However, the precision and performance of these testing devices vary. Therefore, the objective of this study was to determine the effect of systematic errors in position and path repeatability of two high-payload robotic manipulators (Manipulators 1 and 2) on the resultant forces at the knee. Using a porcine knee, the position and path repeatability of these manipulators were determined during passive flexion–extension with a coordinate measuring machine. The position repeatability of Manipulator 1 was 0.3mm in position and 0.2° in orientation while Manipulator 2 had a better position repeatability of 0.1mm in position and 0.1° in orientation throughout the range of positions examined. The corresponding variability in the resultant force at the knee for these assigned positions was 32±33N for Manipulator 1 and 4±1N for Manipulator 2. Furthermore, the repeatability of the trajectory of each manipulator while moving between assigned positions (path repeatability) was 0.8mm for Manipulator 1 while the path repeatability for Manipulator 2 was improved (0.1mm). These path discrepancies produced variability in the resultant force at the knee of 44±24 and 21±8N, respectively, for Manipulators 1 and 2 primarily due to contact between the articular surfaces of the tibia and femur. Therefore, improved position and path repeatability yields lower variability in the resultant forces at the knee. Although position repeatability has been the most common criteria for evaluating biomechanical testing devices, the current study has clearly demonstrated that path repeatability can have an even larger effect on the variability in resultant force at the knee. Consequently, the repeatability of the path followed by the joint throughout its prescribed trajectory is as important as the repeatability of the joint at reaching positions making up its trajectory, particularly when joint contact occurs.