Laboratory Stresses and Tractional Forces on the TMJ Disc Surface

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Laboratory Stresses and Tractional Forces on the TMJ Disc Surface

J.C. Nickel¹^,2^,*, L.R. Iwasaki¹^,2, M.W. Beatty²^,3, and D.B. Marx⁴

¹ University of Nebraska Medical Center College of Dentistry, Departments of Growth and Development,
² Oral Biology, and
³ Adult Restorative Dentistry, PO Box 683740, Lincoln, NE 68583-0755, USA; and
⁴ University of Nebraska, Department of Biometry, 103 Miller Hall, Lincoln, NE 68583-0712, USA;

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Figure 1. Laboratory equipment used to produce compressive and tractional loads. (A) Schematic diagram of equipment and data acquisition system. (a) Pressure-sensitive array: An array of gauges lined the inside the loading tray and measured pressure along the mediolateral axis of the disc. (b) An acrylic indenter (major radius, 125 mm; minor radius, 31 mm) polished loading surfaces and milled holes to reduce the effect of mass on tractional force measurements. The indenter was connected to (c) a pendulum by an instrumented steel strut. Strain gauges attached to the surfaces of the strut (Fig. 1B

) measured bending of the strut during movement of the indenter over the surface of the cartilage. Calibration of gauge output voltages for given loads made it possible for us to measure tractional forces in real time. (d) Electromagnetic force generator: A computer and custom-built software controlled the position and velocity of force generator displacement. (e) Mass platform on (f) loading beam: a hinged beam that facilitated placement of a static load at one end of the beam (e), and which caused the acrylic indenter to load the TMJ disc at the other end of the beam. We used linear voltage differential transformers (g) to measure real-time horizontal position of the indenter relative to the disc, while (h) was used to measure cartilage thickness during translation of the indenter over the surface of the disc. (i) Power supply. (j) 16-channel amplifier.

(B) Detail of (a) pressure transducer array and (b) acrylic indenter. During experiments, the disc was supported by a curved acrylic base and tray. The most medial portion of the disc was positioned over Pressure Gauge 1 (PG1). The acrylic indenter (b) placed a reactive compressive force on the TMJ disc in response to the 10-N load produced by the loading beam, and a tractional ploughing force produced by horizontal displacement by the electromagnetic force generator (Fig. 1A).

(C) Variation in indenter position, velocity, and tractional force with time. Normalized indenter position (•), velocity ( ${diamondsuit}$ ), and tractional force ( ${blacksquare}$ ) are plotted on the vertical axis vs. time on the horizontal axis. Large positive values indicate that the indenter was over the lateral portions of the disc. Velocity of translation of the stress-field was zero when the indenter movement stopped at the most medial or lateral position on the disc. (+) velocities occurred when the indenter was moving medially to laterally. Note that peak velocities occurred over the center of the disc.

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Figure 2. Effects of load duration and cycle number on peak tractional force. Cycle number is plotted on the horizontal axis, while mean peak tractional forces (N) are plotted on the vertical axis for static loading durations of 10 sec ( ${blacksquare}$ , n = 50) and 60 sec ( ${blacksquare}$ , n = 50). Significant differences between 10- and 60-second load-duration results for a given cycle number are indicated by * (p < 0.001). Lower-case letters represent comparisons between 10-second load-duration results for different cycle numbers, where different letters indicated significant differences (p < 0.001). Capital letters represent comparisons between 60-second load-duration results for different cycle numbers, where different letters indicate significant differences (p < 0.01). Error bars represent standard error.

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Figure 3. Peak tractional force vs. velocity of stress-field translation. The data were derived from the second and third cycles of movement over 20 discs following 10 sec of static loading. Criteria for inclusion of the data were synchronization of peak tractional force with peak velocity, and smooth transition of the tractional forces as the velocity of translation decreased to zero and then accelerated to peak velocity in the opposite direction (Fig. 1C

). A second-order polynomial was fitted to the data (R² = 0.73).

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Figure 4. Surface compressive stresses during stress-field translation. (A) Pressure distribution during stress-field translation following 60 sec of static loading. Data shown were recorded following 60 sec of loading of one disc. Time (sec) and Pressure Gauge Number are plotted on the horizontal axes. Pressure Gauge 1 was located under the most medial portion of the disc. Pressure (kPa) is plotted on the vertical axis. Following static loading, stress-field translation was initiated when the indenter moved toward the lateral portion of the disc (0.25–0.3 sec). On the return phase, the indenter moved toward the medial part of the disc (0.30–0.45 sec).

(B) Effect of load duration on maximum compressive stress. Cycle number is plotted on the horizontal axis, while mean maximum compressive stress is plotted on the vertical axis for static load durations of 10 sec ( ${blacksquare}$ , n = 50) and 60 sec ( ${blacksquare}$ , n = 50). Significant differences are indicated by * (p < 0.001). Error bars indicate standard error.

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