Fiberoptic measurement of tendon forces

From Biomch-W

Operating Principles

A minimally invasive method to measure tendon forces relies on the fiberoptic technology. The technique is an indirect measurement of tendon or ligament forces, and suitable for in situ applications. A fiberoptic cable (usually 0.5 mm in diameter) is passed through the tissue of interest perpendicular to the loading axis. Axial loading of the tendon pinches the fiberoptic cable, causing microbending. The intensity of the light reaching the receiver decreases due to this mechanical process. A calibration protocol can provide the relationship between tendon force and change in light intensity. Implementation is easy and inexpensive.

The methodology was first introduced by Komi et al, Eur J Appl Physiol Occup Physiol 72: 278-280, 1996 and has been used in vivo to measure Achilles tendon and patellar ligament forces (Finni et al, Eur J Appl Physiol 85: 170-176, 2001), and in vitro to record plantar aponeurosis loading (Erdemir et al, J Bone Joint Surg Am 86-A:546-552). The output of the sensor was found to be sensitive to loading rate and possible cable migration (Erdemir et al, J Biomech 35: 857-862, 2002). Distortion of transducer output was also observed due to skin movement in case the cable is passed through the skin and tendon (Erdemir et al, J Biomech 36: 449-455, 2003).

How to build and use?

Components

• unjacketed plastic fiberoptic cable (0.5 mm in diameter; PGR500 Series, Toray Ind. Inc., alternatively, J02-532, Edmund Scientific Co., Inc.)
• transmitter and receiver (HFBR 1414T and 2416T; Agilent Tech., Inc.)
• ST style connectors (stainless steel ferrule, fiber size 0.125 mm, part #: F1-0061; Fiber Instrument Sales, Inc.). Custom drilled using size 76 drill (0.5080 mm in diameter, part #: Y-HSD-76; Small Parts Inc.)

Components: (A) fiberoptic cable, (B) transmitter and receiver, and (C) connectors

A circuit board can operate the transmitter and receiver. The intensity of the transmitted light can be measured as a differential analog output. The electronics and necessary connections can be built in a box which serves as the transmitter-receiver unit.

Electronics: (A) circuit, (B) after construction

Accessories

• anaerobic adhesive (part #: 680; Fiber Instrument Sales, Inc.)
• primer (part #: 7471; Fiber Instrument Sales, Inc.)
• polishing film (0.012 mm, part #: F1-0102-12; 0.003 mm, part #: F1-0102-3; 0.0003 mm, part #: F1-0102-03; Fiber Instrument Sales, Inc.)
• polishing disc for ST style connectors (part #: 503304-1; Fiber Instrument Sales, Inc.)
• syringe
• 20 gauge hypodermic needle
• alcohol pads
• canned air

Data acquisition can be performed by using the following components:

• DAQCard-1200 (National Instruments Corp.)
• PR50-50F DAQ cable (National Instruments Corp.)
• Custom made interface box
• Custom made cables to connect the transmitter-receiver unit to the interface box
• 12V adapter to power the transmitter-receiver unit

Application

Application: (A)-(C) insertion of the fiberoptic cable into an Achilles tendon, (D)-(F) installation of a connector

Connector preparation: Polishing connector tips using (A) 0.012 mm and (B) 0.003 mm polishing papers on a soft pad, (C) 0.003 mm and (D) 0.0003 mm polishing papers on a glass plate

1. cut approximately 1 m of fiberoptic cable.
2. pass the hypodermic needle through the tendon transverse to its loading axis.
3. pass the fiberoptic cable through the hypodermic needle.
4. remove the needle. The fiberoptic cable should remain in the tendon.
5. sterilize the fiberoptic cable end using disinfectant.
6. pass the cable through the strain relief boot supplied in the connector package.
7. clean the cable end with alcohol pads and blow dry with canned air.
8. clean the connector with alcohol pads and blow dry with canned air.
9. apply primer to the end of the fiberoptic cable.
10. inject adhesive into the large opening of the connector using a syringe.
11. insert the fiberoptic cable into the large opening of the connector. The tip of the cable should appear at the other side of the connector.
12. install the strain relief boot to the connector.
13. let the adhesive cure for 2-3 minutes.
14. cut the excess of the fiberoptic cable at the tip.
15. polish the connector end using polishing disc and polishing films. First, on a 0.012 mm polishing film placed on a soft pad, then on a 0.003 mm. Final polishing is done on a glass plate using 0.003 mm and 0.0003 mm polishing films.
16. clean the connector with alcohol pads and blow dry with canned air.
17. repeat steps 5-16 for the other end of the fiberoptic cable.
18. connect the fiberoptic cable to the transmitter--receiver unit.
19. connect the transmitter--receiver unit to the interface box.
20. power the trasmitter-receiver unit with the adapter and turn the switch on. LED should be lit up.
21. System is ready for measurements.

LabVIEW (National Instruments Corp.) can be used for data collection. Note that, after the experiments, the tendon should be tested with known forces in order to obtain calibration constants which will be used to calculate forces experienced during the experiments.