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Investigating Micromotion in Total Knee Joint Replacements with NI LabVIEW and Data Acquistion

Author(s):

Mark R. DiSilvestro, DePuy, a Johnson & Johnson Company; Donald E. McNulty, DePuy, a Johnson & Johnson Company; Stephen W. Swope, DePuy, a Johnson & Johnson Company

Industry:

Life Science

Product:

Data Acquisition, LabVIEW, PXI/CompactPCI

The Challenge:

Implementing a user-friendly and reliable data acquisition system that operates unattended for days at a time, monitors knee simulator signals, keeps accurate count of simulator cycles, saves both control and sensor signals at user-prescribed cycle numbers, and stops after the last prescribed save cycle.

The Solution:

Using software written in LabVIEW to deliver user-friendly and reliable performance of all required system tasks using a single PCI-6033E multifunction DAQ card housed in a Dolch portable computer.


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Measuring Micromotion in Knee Simulators
The Materials Research Department at DePuy Orthopaedics in Warsaw, IN uses knee simulators (AMTI; Watertown, MA). We use these simulators to apply known motions and loads to prosthetic knee joints and count the number of simulated cycles of use, so we can assess the wear of the polyethylene as a function of use.

Micromotion between the polyethylene tibial insert and the metal tibial tray has been hypothesized to result in an additional wear interface. To measure micromotion and relate it to additional wear, we integrated miniature differential variable reluctance transducers (DVRTs) with signal conditioning into one knee simulator. We used a data acquisition (DAQ) system for the upgraded simulator.

The DAQ system had the following requirements:

  • Maintain a user-friendly and easily accessible interface near the simulator
  • Read in calibration information from a file
  • Enable users to prescribe a loading cycle offset
  • Sample and display 26 signals continuously – not necessary during file save
  • Display system time continuously
  • Display and keep count of simulator outputs synchronized with the simulator counters at all times
  • Sample and save finite data at predefined cycle numbers
  • Stop data acquisition automatically after the last prescribed file save



We used a Windows-based departmental computer for ruggedness, cost reduction, and ease of use. Reliability in cycle counting and data saving also was a key concern because cycle counts needed to be less than 1 percent different than those of the simulator, and we cold not miss a prescribed file save.

Developing a Reliable Data Acquisition System
We mount the computer on the simulator for accessibility and proximity to the device. We mount sets of three DVRTs on each knee station.

In the DAQ system, four control signals generated by the simulator’s controller control each of the banks. As a result, micromotion may occur between the polyethylene tibial insert and the metal tibial tray of the prosthetic joint. The DVRTs sense this motion. We pull the control signals for each bank and the counter signal from the simulator and wire the signals to the SCB-100 along with the 18 conditioned DVRT signals. The connector block connects to a PCI-6033E multifunction DAQ board housed in the computer.

With a LabVIEW 6.1 user interface, we can set all DAQ channels, initial cycle offset, cycle numbers at which file save is to occur, file save location and prefix, and location of the calibration data file. The UI also displays sampled data in organized waveform charts. The interface displays loaded calibration factors and offsets, current cycle count, system time, and a file save status LED. Users can press the stop button to stop the application.

The application utilizes three parallel processes. The first updates the cycle count every 50 ms, and adjusts the count for the initial offset. We have measured this count within 1 percent of the simulator’s counter. The cycle count indicator does not always advance in steps of one because there is a prescribed delay between updates. Therefore, we append the original user-prescribed array of cycle counts that specify the rate to save with integer values surrounding each prescribed value in a range of +/- 5. This ensures that we will not miss any data. Also, because the sampling time is fixed at 10 s for data save, two saves will never occur for a single user-prescribed save. The second process begins by searching the appended array of cycle numbers specified to save data for the current cycle count. If it finds the current cycle count in the appended array, it saves data at 1000 Hz for 10 s. During this time, the indicator LED turns on, and the waveform charts are not updated. However, if the current cycle count is not found in the appended array, the system performs buffered acquisition of all analog signals at 1000 Hz for 2 s. A pause of 100 ms occurs between each buffered acquisition and the restart of the same process. The third process compares the current cycle count to the maximum user-prescribed cycle count at which data saves. If the current value exceeds this value, the application automatically stops. This process also updates the current system time on the UI. Because this process is the least critical to the application, it is updated only every 1000 ms.

Delivering a Reliable, Efficient Solution
With the DAQ-based system, we have a fully-automated data acquisition system that operates continuously. We now can monitor all signals simultaneously, and our data is consistently saved after a specified number of cycles. The standard user interface makes it easy to set up data acquisition applications, so data acquisition occurs at the same sampling rate for the same sampling time.

The users wanted a DAQ system controlled in a Windows environment using hardware that was compatible with an existing computer. The resulting system developed using National Instruments hardware and LabVIEW meets all of the needs and multiple functions of the users. The cost of the system components and the development time were minimal in comparison with the benefits of the resulting DAQ system.

For more information, contact:
Mark R DiSilvestro
DePuy, a Johnson & Johnson Company
700 Orthopaedk Dr.
Warsaw, IN 46581
Tel: 574-372-7055
Fax: 574-372-7373