Research Papers

A Reconfigurable Multiplanar In Vitro Simulator for Real-Time Absolute Motion With External and Musculotendon Forces

[+] Author and Article Information
Joshua T. Green

Department of Metallurgical,
Materials and Biomedical Engineering,
College of Engineering,
The University of Texas at El Paso,
500 West University Avenue,
El Paso, TX 79968
e-mail: jtgreen2@miners.utep.edu

Rena F. Hale

Orthopedic Biomechanics Laboratory,
Mayo Clinic,
200 1st Street Southwest,
Rochester, MN 55905
e-mail: hale.rena@mayo.edu

Jerome Hausselle

Mechanical and Aerospace Engineering,
College of Engineering,
Architecture and Technology,
Oklahoma State University,
218 Engineering North,
Stillwater, OK 74078
e-mail: jerome.hausselle@okstate.edu

Roger V. Gonzalez

Department of Engineering Education and Leadership,
College of Engineering,
The University of Texas at El Paso,
500 West University Avenue,
El Paso, TX 79968
e-mail: rvgonzalez@utep.edu

1Corresponding author.

Manuscript received October 13, 2016; final manuscript received August 31, 2017; published online September 28, 2017. Editor: Beth A. Winkelstein.

J Biomech Eng 139(12), 121001 (Sep 28, 2017) (11 pages) Paper No: BIO-16-1403; doi: 10.1115/1.4037853 History: Received October 13, 2016; Revised August 31, 2017

Advancements in computational musculoskeletal biomechanics are constrained by a lack of experimental measurement under real-time physiological loading conditions. This paper presents the design, configuration, capabilities, accuracy, and repeatability of The University of Texas at El Paso Joint Load Simulator (UTJLS) by testing four cadaver knee specimens with 47 real-time tests including heel and toe squat maneuvers with and without musculotendon forces. The UTJLS is a musculoskeletal simulator consisting of two robotic manipulators and eight musculotendon actuators. Sensors include eight tension load cells, two force/torque systems, nine absolute encoders, and eight incremental encoders. A custom control system determines command output for position, force, and hybrid control and collects data at 2000 Hz. Controller configuration performed forward-dynamic control for all knee degrees-of-freedom (DOFs) except knee flexion. Actuator placement and specimen potting techniques uniquely replicate muscle paths. Accuracy and repeatability standard deviations across specimen during squat simulations were equal or less than 8 N and 5 N for musculotendon actuators, 30 N and 13 N for ground reaction forces (GRFs), and 4.4 N·m and 1.9 N·m for ground reaction moments. The UTJLS is the first of its design type. Controller flexibility and physical design support axis constraints to match traditional testing rigs, absolute motion, and synchronous real-time simulation of multiplanar kinematics, GRFs, and musculotendon forces. System DOFs, range of motion, and speed support future testing of faster maneuvers, various joints, and kinetic chains of two connected joints.

Copyright © 2017 by ASME
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Grahic Jump Location
Fig. 1

(a) Computer rendering of the UTJLS. Transparent components of the lower gantry system have been designed and manufactured but were not implemented for this study. (b) Diagram identifies UTJLS variables and corresponding orientations relative to knee specimen and UTJLS coordinate system. Colors of axes correspond to component colors in (a).

Grahic Jump Location
Fig. 2

(a) Instrumented knee specimen mounted in the UTJLS. Tension load cells are covered in a protective plastic cover, and tendon connections are wrapped in black pipe insulation to assist with freezing. (b) Illustration of musculotendon paths including bone orientation, custom casting molds, and UTJLS interfacing brackets.

Grahic Jump Location
Fig. 3

Block diagram of UTJLS control structure. Specimen instrumentation is not included in the diagram but is collected in parallel by another computer and data-acquisition system.

Grahic Jump Location
Fig. 4

Simulator response during heel squat tests with 50% musculotendon forces. Each series is the average of three trials for each specimen. In addition to musculotendon forces, (a) independent variables include position control of four rotations (θxH, θyH, θzA, and θxA) and hybrid control of five GRFs (FxA, FyA, FzA, MyA, and MzA). (b) Dependent variables measured by the UTJLS include three hip positions relative to ankle center (Hx, Hy, and Hz), two tibial rotations (θzA and θxA), and one GRF moment (MxA). Note that θyH and θzA follow offset profiles in accordance with specimen-specific, alignment parameters identified during static testing.

Grahic Jump Location
Fig. 5

Average musculotendon forces of specimen B during heel squat simulation, which are indicative of a typical trial. The root-mean-square (RMS) error shown is calculated from heel squat simulation data with musculotendon forces collected from four specimens that underwent three trials each. Controller accuracy is compared to desired path, and specimen repeatability is compared to specimen average.

Grahic Jump Location
Fig. 6

Average anatomical rotations of specimen U during heel and toe squat tests performed on the UTJLS. The positive direction of vertical axis indicates increasing flexion, varus, and internal rotation on respective graphs.

Grahic Jump Location
Fig. 7

Center of pressure (COP) on tibial plateau of specimen U during squat simulation



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