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 on the force probe. These changes in resistance are then enlarged by the bridge of an amplifier unit and transferred to the input of the display unit. After analogue-to-digital conversion in the display unit, the amount of strain is displayed. This strain measurement is converted to analogue, and its voltage is finally recorded in the logger.
The force probe (Showa unilateral strain gauge; Showa Measuring Instruments Inc., Tokyo, Japan) is rectangular shaped (width, 1.4 mm; height, 1.4 mm; length, 8 mm) and has a tubular structure with slits entering vertically on one side of its surface (Fig. 2A). When strain is applied to the force probe, the internal strain gauge is distorted, allowing the magnitude of strain to be measured. When the force probe is inserted into the tissue, it may rotate as forces are applied causing the output to be reduced or inverted. To suppress this rotational influence, a tube was attached to the force probe, and both ends were sutured to the tissue to be measured (Fig. 2B).
A performance cube was used to measure the position of the ankle (Fig. 3). The cube is composed of an MPU-9250 motion processing sensor with a nine-axis sensor, an ESP32 microcontroller, and a logger. The MPU-9250 and ESP32 are loaded in the performance cube. The MPU-9250 is a sensor that acquires position information and can acquire values of motion in nine axes in total, each with angular acceleration, and geomagnetism. The MPU-9250 is equipped with hardware called a digital motion processor, which automatically measures at the time of initialization of the sensor and calculates posture. The ESP32 is a microcontroller that calculates data obtained from the MPU-9250 and transmits data to the logger via a WiFi module. This performance cube is synchronized with the MLPP system.
Cadaveric tests using 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. The median age was 64 years (range 46–82 years). These specimens were free of ankle or hind foot deformities, 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.
Experiments on strain patterns of the superficial deltoid ligament
The following procedures were performed in all specimens by a single experienced foot and ankle surgeon. An incision was made in the medial ankle and the superficial layer of the deltoid ligament was exposed. Lines were drawn on ligaments to trace each ligament from its origin to insertion on the bone (Fig. 4A). Ligaments were not isolated to allow investigation as one unit. A force probe was placed in the mid-substance of each ligamentous band of the TNL, TSL, TCL, and SPTTL such that the slit of the force probe was aligned with the long axis of the ligament fibres (Fig. 4B). After introducing the force probe into the ligament, the force probe tube was sutured to the ligament fibres with 3 − 0 nylon thread to prevent the rotation of the force probe.
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 vise to allow for 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 to the calcaneus and a rod (diameter 8 mm) inserted between the second and third metatarsals (Fig. 3). This plate had a 25-cm arm where a 0.5-kg weight could be added at the end, applying a 1.2-N m force to the ankle and subtalar joint complex (0.5 kg × 0.25 m × 9.80665 = 1.2258312 N m). This arm was rotated every 30° on the clock to allow measurement of the strain on each ligamentous band at various ankle positions. The ankle positions were 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 9 o’clock position; 1 and 2 o’clock defined dorsiflexion-adduction, 4 and 5 angle defined supination (plantar flexion adduction), 7 and 8 o’clock defined plantarflexion- abduction, and 10 and 11 o’clock defined pronation (dorsiflexion-abduction).
After the investigation of strain in the designated ankle positions, the strain values of each ligament was also measured in axial motion of the ankle from maximal dorsiflexion to plantar flexion.
The angles of axial, sagittal and horizontal motions were measured by an electronic goniometer (MPU-9250; TDK InvenSense) synchronized to the MLPP system.
The relationship between the foot positions and the tensile forces of each ligamentous band was analysed. The tensile force data from the force probe was obtained by synchronizing to the arm of the clock rotating every 30° and moving the ankle from 15° dorsiflexion to 30° plantar flexion 10 times manually and the strain of each ligamentous band during ankle motion was measured. Individual strain data were aligned to the value at neutral position (0) and to 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.