Structures of the MLPP system
The MLPP system is composed of a strain gauge (force probe), amplifier unit, display unit, and logger (Figure 1). The system is capable of detecting small changes in resistance via a force probe. These changes in resistance are then enlarged by the bridge of the amplifier unit and then transferred to display unit output, where analog-to-digital conversion takes place. Subsequently, degree of strain is displayed. This strain measurement is then converted to an analog value, and its voltage is recorded in the logger.
The force probe (Showa Unilateral Strain Gauge; Showa Measuring Instruments Inc., Tokyo, Japan) is rectangular (width, 2 mm; height, 1.5 mm; length, 8 mm) and has a tubular structure, with slits that extend vertically on one side of its surface (Figure 2A and B). In the force probe, the internal strain gauge is distorted by applying force in a certain direction, thus allowing strain to be measured. When the force probe is inserted within tissue, it may rotate as forces are applied, which may reduce or invert output (Figure 3A and B). 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), microcontroller (ESP32), and logger. The MPU-9250 and ESP32 were loaded within the performance cube. The MPU-9250 is a sensor that records position information and measures values of nine axes in total. It also records angular acceleration and geomagnetism. The MPU-9250 is equipped with a digital motion processor, which automatically measures motion at the time of sensor initialization and determines posture. 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). Three specimens were from male and three were from female cadavers. The median age was 64 years (range, 46–82 years). The specimens were free of ankle or hindfoot deformity, did not undergo surgery or dissection, and did not have any history of trauma or other pathology that may alter anatomy. All cadaveric studies were performed at the University of Barcelona in Catalonia, Spain. All methods in this study were reviewed and approved by the institutional review board of the same institution. Consent for the storage and use of the bodies for research purposes was given by all body donors prior to death or by their next of kin.
Investigating AFTL strain patterns
Procedures described in this section were performed on all specimens by an experienced foot and ankle surgeon. An incision was made in the lateral ankle, and the ATFL was exposed. A force probe was placed in a force probe tube in the midsubstance of the ATFL to align the slit of the force probe with the long axis of the ligament fiber. After placing the force probe into the ligament, the force probe tube was sutured to ligament fibers with a 3-0 nylon thread 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 for the localization of distal upper and proximal lower portions of specimens. A round metal disk (clock, diameter 150 mm) with a 6-mm-diameter hole every 30° 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 screw (diameter, 6 mm) and inserted into the calcaneus, and a rod (diameter, 8 mm) was inserted between the second and third metatarsals (Figure 5A). This plate had a 25-cm arm, and a 0.5-kg weight was added at its end, thus an approximate 1.2-Nm force was applied to the ankle and subtalar joint complex (0.5 kg × 0.25 m × 9.80665 = 1.23 Nm). The arm of the plate rotated every 30° on the clock and allowed for the measurement of AFTL strain each of its ends (Figure 5B). Dorsiflexion, plantar flexion, inversion and eversion ankle positions were designated when the plate arm was at the 12 o’clock, 6 o’clock position, 3 o’clock, and 9 o’clock position, respectively. The angles of axial motion, dorsiflexion, and plantar flexion were measured using an electronic goniometer (MPU-9250; TDK InvenSense), which was synchronized with the MLPP system. After all measurements were made in intact specimens, the ATFL was cut at fibular attachment points to free each force probe.
The relationship between foot position and AFTL tensile force was analyzed. Tensile force data from the force probe were obtained by synchronizing the probe with the arm of the clock after each 30°. The ankle was moved from 15° dorsiflexion to 30° plantar flexion 10 times manually, and the strain of the ATFL during ankle motion was measured. Individual strain data were aligned with the value at the neutral position was 0 and the maximum value was 100. The average values at each position were connected with a line, and the ligament tension patterns of specimens were compared.