EOMF Project Author: Joel M Miller Modified: 22.02.2007 Support: NIH/NEI RO1 EY08313 to JLD NIH/NEI R01 EY013443 & EY015314 to JMM Status Ongoing Key Collaborators Danielle E Alexander, BS See Also EOMF Publications Other Labs Your related project here? . . .

Extraocular Muscle Force

Our aim is to determine the innervation-length-tension relationships of extraocular muscles (EOMs) under physiologic conditions. Such data will contibute to the ongoing development of biomechanical models of human ocular statics and strabismus (Robinson, 1975; Clement, 1982; Miller & Robinson, 1984; Clement, 1985; Miller, 1989; Miller, Demer and Rosenbaum, 1993; Simonsz, 1990; Miller, Shamaeva & Pavlovski, 1995) and to studies of the biomechanical properties of abnormal muscles and muscles treated surgically and with neuropharmacologic agents.

In the left panel below is a muscle force transducer (MFT) designed and built in our lab (PDF, 2.2 MB). The MFT can be placed on a muscle during eye muscle surgery under local anesthesia by sliding out the horizontal rod, "tenting" the muscle up through the frame, and replacing the rod.

The right panel shows the data collection set-up. Above the patient's head is a laser pointer attached to a position encoder, which projects a fixation target on the operating room ceiling. As the patient fixates various positions, muscle forces are recorded. The cart holds strain gauge amplifiers, a digital storage scope for viewing muscle force data as it is collected, and an FM tape recorder, for making a permanent record of fixation laser angle, voice annotations, and MFT signals.

Left Panel: Human MFT contains a full Wheatstone Bridge, ie, 4 strain gauges, arranged so that output is compensated for temperature even if top and bottom surfaces are exposed to different temperatures. Right Panels: Human data collection set-up. MFT is installed before muscle is disinserted, with leads projecting forwards. Above the patient's head a laser pointer attached to a position encoder projects a fixation target on the operating room ceiling and records its angular position. A cart holds (a) strain gauge amplifiers, (b) a digital storage scope for viewing muscle force data as it is collected, and (c) an FM tape recorder, for making a permanent record of fixation laser angle, voice annotations, and MFT signals.

Patient MG (below, left) had esotropia of 11.3 deg. LR force in primary position force was relatively high at 18g, and had a normal range of 18g over a field of gaze 40 horizontal and 20 vertical. Patient AR (below, right), diagnosed with a partial 3rd nerve paralysis, had a typical primary position force of 8g, but an unusually small range of 10g.

Figure: LR Force in freely moving human eyes, fixing laser spots at 10 intervals over a 40 horizontal by 20 vertical range.

Below is an example of data we have collected with a similar device in monkeys, showing how tension in the medial rectus (MR) muscle increases as the eye moves toward the nose (AD for adduction) and in the lateral rectus (LR) muscle as it moves toward the temple (AB for abduction). Tension in both muscles is complexly dependent on vertical gaze.

Figure: LR and MR Force in freely moving human eyes, fixing laser spots at 10 intervals over a 30 horizontal by 30 vertical range.

Digit Flexor Tendon Force

As a spin-off of the EOM force study we designed a muscle force transducer for the flexor digitorum superficialis tendon of the human middle finger. This device was designed to measure much higher forces than the EOM device, and to compactly enclose a tendon of round crossection. It has been used to study tendon forces associated with key pressing during carpal tunnel release surgery (see www.hsph.harvard.edu/facres/dennerlein.html).

Figure: The Digit Flexor Tendon Force Transducer is a two-part device, shown assembled on the left. The saddle-shaped piece in the middle pops out for installation, as shown on the right on a cadaveric tendon. When the saddle is replaced under the tendon, the tendon is forced to take a slightly zig-zaged path, so that its tension deforms the transducer frame. A pair of silicon strain gauges is mounted on the top, and another pair on the bottom of the frame. One gauge of each pair measures strain, providing a push-pull signal that sums force aross the tendon, and the other measures temperature, providing compensation separately for the bottom of the device, which is in contact with warm body tissues, and the top, which is in contact with room air.