Components of the MLPP system
The MLPP system is composed of a commercially available strain gauge (force probe), amplifier unit, display unit, and logger (Figure 1), allowing the system to detect and report small changes in resistance. These resistance changes are enlarged by the bridge of the amplifier unit and transferred to the display unit output, where analog-to-digital conversion takes place and the degree of strain is displayed. This strain measurement is also converted to an analog value, and its voltage is recorded in the logger.
The force probe (Showa Unilateral Strain Gauge; Showa Measuring Instruments, Tokyo, Japan) is rectangular (width, 2 mm; height, 1.5 mm; length, 8 mm) and has a tubular structure, with slits that extend vertically along one side of its surface (Figures 2A and 2B). In the force probe, the internal strain gauge is distorted by applying force in a certain direction, allowing the strain to be measured. When the force probe is inserted into tissue, it may rotate as forces are applied, which may reduce or invert the output (Figures 3A and 3B). To avoid this rotational influence, a tube for preventing rotation was attached to the force probe, and both ends were sutured to the target tissue (Figure 4).
A performance cube was used to measure the position of the ankle (Figure 5). The cube is composed of a nine-axis sensor (MPU-9250; TDK InvenSense, San Jose, CA, USA), microcontroller (ESP32), and logger. The MPU-9250 and ESP32 were loaded into the performance cube. The MPU-9250 is a sensor that records positional information and measures the values of nine axes, in total; it also records angular acceleration and geomagnetism. The MPU-9250 is equipped with a digital motion processor that automatically measures motion at the time of sensor initialization and determines posture. The ESP32 is a microcontroller that records data obtained from MPU-9250 and transmits it to the logger using a wireless module. The performance cube was synchronized with the MLPP system.
Six fresh-frozen, through-the-knee, lower extremity cadaveric specimens were used for this study (three right and three left legs). Three specimens were from male and three were from female cadavers. The median age of each cadaver, at the time of death, was 64 years (range, 46–82 years). The specimens were free of ankle or hindfoot deformities, had not undergone surgery or dissection, and did not have histories of trauma or other anatomy-altering pathologies. All cadaveric studies were performed at the University of Barcelona (Catalonia, Spain), and all methods used in this study were reviewed and approved by the university’s institutional review board. Consent for the storage and use of the bodies for research purposes was given by the donors prior to their deaths or by their next of kin.
Investigating AFTL strain patterns
The procedures described in this section were performed by an experienced foot and ankle surgeon. An incision was made in the lateral ankle of each specimen, and the ATFL was exposed. A force probe, in a force probe tube, was placed into the midsubstance of the ATFL, and the slit in the force probe was aligned with the long axis of the ligament fiber. After placing the force probe into the ligament, the force probe tube was sutured to the ligament fibers using 3-0 nylon sutures to prevent force probe rotation (Figure 4).
An Ilizarov ring-shaped external fixator was placed on the lower leg, and the lower limb was fixed vertically, relative to the measurement desk, using a vice to allow localization of the distal upper and proximal lower portions of each specimen. A round metal disk (a “clock”, diameter 150 mm), with 6-mm-diameter holes placed every 30° around its circumference, was affixed to an acrylic plate (width, 120 mm; length, 280 mm; thickness, 10 mm). The plate was fixed to the plantar aspect of the foot with a 6-mm-diameter screw inserted into the calcaneus; an 8-mm-diameter rod was also inserted between the second and third metatarsals (Figure 5A). The plate had a 25-cm arm, and a 0.5-kg weight was added to its end to approximate the application of a 1.2-Nm force (0.5 kg × 0.25 m × 9.80665 = 1.23 Nm) to the ankle and subtalar joint complex. The arm of the plate was rotated to each position (in 30° increments) on the clock, allowing for the measurement of AFTL strain at each of its ends (Figure 5B). The ankle positions corresponding to dorsiflexion, plantar flexion, inversion, and eversion were achieved when the plate arm was at the 12-, 6-, 3-, and 9-o’clock positions, respectively. The axial motion angles for dorsiflexion and plantar flexion were measured using the electronic goniometer (MPU-9250), which was synchronized with the MLPP system. After all measurements in the intact specimens were completed, the ATFL was cut at the fibular attachment points to free the force probe.
The relationships between the foot positions and the AFTL tensile forces were analyzed. The tensile force data from the force probe were obtained by synchronizing the probe with the clock’s arm at each 30° stop. The ankle was moved from 15° dorsiflexion to 30° plantar flexion 10 times, manually, and the strain on the ATFL during the ankle motion was measured. Individual strain data points were aligned with the values at the neutral (0) position; the maximum value was 100. The values at each position were connected with a line, and the ligament tension patterns of the specimens were compared. All strain data are presented as means ± standard deviations (SD).