Design Innovation Paper

A New Sensor for Measurement of Dynamic Contact Stress in the Hip

[+] Author and Article Information
M. J. Rudert

Department of Orthopaedics and Rehabilitation,
University of Iowa,
Iowa City, IA 52242

B. J. Ellis

Department of Bioengineering and
Scientific Computing and Imaging Institute,
University of Utah,
Salt Lake City, UT 84112

C. R. Henak

Department of Bioengineering,
University of Utah,
Salt Lake City, UT 84112

N. J. Stroud, D. R. Pederson, T. D. Brown

Departments of Orthopaedics and Rehabilitation
and Biomedical Engineering,
University of Iowa,
Iowa City, IA 52242

J. A. Weiss

Departments of Bioengineering and Orthopaedics and
Scientific Computing and Imaging Institute,
University of Utah,
Salt Lake City, UT 84112

1Current address: Exactech, Inc., Gainesville, FL 32653.

Contributed by the Bioengineering Division of ASME for publication in the Journal of Biomechanical Engineering. Manuscript received October 2, 2012; final manuscript received October 25, 2013; accepted manuscript posted November 27, 2013; published online February 13, 2014. Assoc. Editor: Richard Neptune.

J Biomech Eng 136(3), 035001 (Feb 13, 2014) (8 pages) Paper No: BIO-12-1459; doi: 10.1115/1.4026103 History: Received October 02, 2012; Revised October 25, 2013; Accepted November 27, 2013

Various techniques exist for quantifying articular contact stress distributions, an important class of measurements in the field of orthopaedic biomechanics. In situations where the need for dynamic recording has been paramount, the approach of preference has involved thin-sheet multiplexed grid-array transducers. To date, these sensors have been used to study contact stresses in the knee, shoulder, ankle, wrist, and spinal facet joints. Until now, however, no such sensor had been available for the human hip joint due to difficulties posed by the deep, bi-curvilinear geometry of the acetabulum. We report here the design and development of a novel sensor capable of measuring dynamic contact stress in human cadaveric hip joints (maximum contact stress of 20 MPa and maximum sampling rate 100 readings/s). Particular emphasis is placed on issues concerning calibration, and on the effect of joint curvature on the sensor's performance. The active pressure-sensing regions of the sensors have the shape of a segment of an annulus with a 150-deg circumferential span, and employ a polar/circumferential “ring-and-spoke” sensel grid layout. There are two sensor sizes, having outside radii of 44 and 48 mm, respectively. The new design was evaluated in human cadaver hip joints using two methods. The stress magnitudes and spatial distribution measured by the sensor were compared to contact stresses measured by pressure sensitive film during static loading conditions that simulated heel strike during walking and stair climbing. Additionally, the forces obtained by spatial integration of the sensor contact stresses were compared to the forces measured by load cells during the static simulations and for loading applied by a dynamic hip simulator. Stress magnitudes and spatial distribution patterns obtained from the sensor versus from pressure sensitive film exhibited good agreement. The joint forces obtained during both static and dynamic loading were within ±10% and ±26%, respectively, of the forces measured by the load cells. These results provide confidence in the measurements obtained by the sensor. The new sensor's real-time output and dynamic measurement capabilities hold significant advantages over static measurements from pressure sensitive film.

Copyright © 2014 by ASME
Topics: Sensors , Stress , Calibration
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Fig. 1

A hip joint contact stress sensor showing the ring-and-spoke configuration. The spoke leads, on the front (“UP”) of the sensor, and the ring leads, on the back, converge at the active region, where they are separated by an intervening layer of piezoresistive ink to form a grid of stress-sensing sensels. Eight 1-mm diameter holes on the periphery provide for suture tie-down to the acetabular labrum.

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Fig. 2

Hip contact stress sensor placement within a cadaver hip joint. (a) An oblique view of the femur showing the sensor positioned on the femoral head. The outer perimeter of the sensor active region rests approximately at the equator of the head. (b) An anterior view, showing the femoral head within the acetabulum. The periphery of the active region is still visible at the edge of the labrum. (c) A mediolateral view of the acetabulum only; arrows indicate where the sensor has been sutured to the labrum.

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Fig. 3

The hip sensor flat- and conical-calibration devices and representative contact patches. In (a), the flat upper platen spanned 12 spokes and 21 rings (not visible) of the sensor; (b) graphically indicates sensor raw output at an applied contact stress of approximately 10 MPa. Four repositionings of the platen were sufficient to calibrate the full surface of the sensor. (c) Conical upper platen with a sensor in place; raw output at 10 MPa is shown in (d). Two further rotations of the cone are sufficient to calibrate the entire sensor.

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Fig. 4

A hip motion simulator was used to drive a human cadaver hemi-pelvis through a user-programmed three-dimensional motion/force protocol. A PMMA-potted femur (a) is supported on a 2-DOF translational stage (b) (at 45 deg flexion in the case shown); the hemi-pelvis (c) is mounted to the inner yoke (d), which applies flexion/extension. The outer yoke (e) applies abduction/adduction while the Bionix axial-rotary actuator (f) delivers endorotation/exorotation and axial force. Hip joint forces and moments are transduced via a 6-DOF load cell (g).

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Fig. 5

Sensor calibration curves obtained with three different platen configurations. Comparison of the flat-platen curves, rigid versus 90A polyurethane (PU), indicated that the PU lining attenuated sensor output by approximately a factor of 4, to simulate in situ cartilage response. The flat- and conical-platen results, when both platens were lined with PU, were virtually identical, which indicated that sensor behavior was unaffected by substrate curvature.

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Fig. 6

A comparison of hip sensor measured contact stress (top) versus pressure-sensitive film measured stress (bottom) for a statically loaded cadaver hip joint. Stress patterns exhibited good qualitative agreement. (a) and (b) Heel strike during walking and heel strike during stair climbing, respectively. Applied loads and the percentage of applied load recovered by the sensor are listed in Table 3.

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Fig. 7

Contact stress dynamic results: Nine representative (of 80 total) frames recorded at 5 frames per second during a hip-simulator applied loading protocol. (a)–(c) were recorded during the load uptake phase; (d)–(f) were recorded during the joint flexion phase; and (g)–(i) were recorded during the load removal phase. Applied loads for each frame and the percentage of that load recovered by the sensor are listed in Table 3.




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