Structures of the MLPP system
The MLPP system is composed of a strain gauge (force probe), an amplifier unit, a display unit, and a logger (Fig. 1). This system is capable of detecting small changes in resistance using the force probe. These changes in resistance are then enlarged by the bridge of the amplifier unit and then transferred to the output of the display unit where analog-to-digital conversion takes place, and subsequently, the amount of strain is displayed. This strain measurement is converted to analog, and its voltage is finally 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 entering vertically on one side of its surface (Fig. 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 in the tissue, it may rotate as forces are applied and could reduce or invert the output (Fig. 3A and B). To suppress this rotational influence, a tube for preventing rotation was attached to the force probe, and both ends were sutured to the tissue to be measured (Fig. 4).
A performance cube was used to measure the position of the ankle (Fig. 5). It is composed of a nine-axis sensor (MPU-9250), a microcontroller (ESP32), and a logger. MPU-9250 and ESP32 are loaded in the performance cube. The MPU-9250 is a sensor that records position information, and it can acquire the 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 calculates data obtained from MPU-9250 and transmits data to the logger using a wireless module. This performance cube is synchronized with the MLPP system.
Source of cadaver
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). These 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 the anatomy. All cadaveric studies were performed at University of Barcelona in Catalonia, Spain. All methods in this study were reviewed and approved by the institutional review board of The University of Barcelona. 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.
Experiment on strain pattern of the ATFL
The subsequent procedures were performed in 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 introducing the force probe into the ligament, the force probe tube was sutured to the ligament fibers with a 3 − 0 nylon thread to prevent the rotation of the force probe (Fig. 4).
An Ilizarov ring-shaped external fixator was placed on the lower leg, and the lower limb was fixed vertically to the measurement desk using a vice to allow the localization of the distal upper and proximal lower portions of the 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). This was fixed on the plantar aspect of the foot with a screw (diameter 6 mm) inserted into the calcaneus and a rod (diameter 8 mm) inserted between the second and third metatarsals (Fig. 5). This plate has a 25-cm arm where a 0.5-kg weight can be added at its end, thus applying an approximately 1.2-Nm force to the ankle and subtalar joint complex (0.5 kg × 0.25 m × 9.80665 = 1.2258312 N m). This arm rotates every 30° on the clock and allows the measurement of the strain of the ATFL at each end point (Fig. 6). The ankle positions are defined as dorsiflexion with the arm at the 12 o’clock position, plantar flexion at the 6 o’clock position, inversion at the 3 o’clock position, and eversion at the 9 o’clock position. The angles of axial motion, dorsiflexion, and plantar flexion were measured using an electronic goniometer (MPU-9250; TDK InvenSense) synchronized with the MLPP system.
After all measurements were made in the intact specimens, the ATFL was cut at the fibular attachments, leaving the force probes.
The relationship between the foot positions and the tensile forces of the ATFL was analyzed. The tensile force data from the force probe were obtained by synchronizing the probe with the arm of the clock rotating every 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 (0) and the maximum value (100). The average value at each position was connected by a line, and the ligament tension pattern was compared among the specimens.