Device development: hardware
A support was designed and built in medium density fiberboard (MDF) wood to hold the lower limb. For high reliability on readings three Kirschner wires (3 mm diameter) were inserted in the tibia and fixed to the MDF structure in order to immobilize it, avoiding any rotation of the lower limb, and allowing only the movement of the ankle joint (Fig. 2). This support holds the leg in a 45 degrees angle with the horizontal plane.
The measurement system comprised a Mpu-6050 GY-521, which is a 6 degrees of freedom inertial measurement unit (IMU) with three-axis accelerometer and a three-axis gyroscope to obtain the movement angles. The IMU must be fixed to the talus. Two Kirschner wires (3 mm diameter) were inserted in the talus neck in an anterior-posterior direction, intersecting its longitudinal axis and the IMU’s were fixed to the wire making both the talus and the IMU rigidly connected. The IMU sensor is controlled by an Arduino Mega 2560 board, which was used as the microcontroller. The calibration of the system assured that the gyroscopes will measure the rotation angles (yaw, pitch and roll) between the starting position and final movement position after each step of the testing protocol. This way, it was possible to do a precise evaluation of the tibiotalar movements without interference of hind-foot, mid-foot or fore-foot movements. Only the rotations of the talus were being measured with respect to its three own cardinal planes (axial, coronal, and sagittal).
Device development: software
A sensor fusion algorithm was used for the acquisition and interpretation of the raw data received from the IMU. The programming code is based on open source libraries available on internet, performing the initialization, calibration (including offset values) and filtering of the sensor values. This process ensures more accurate values of the relative angles. A new dedicated software was developed, which enabled us to obtain the angular values relative to the difference between the initial orientation of the ankle’s body fixed axes and the final orientation originated by extrinsic rotations. The software allows real time analysis of the angular displacement of the talus in three planes, simultaneously, using Tait–Bryan angles.
The software interface has some features that can be very helpful with the device such as real-time angle measurement, entries for the name and observations to save the data in an excel document and 3D visualization of the movement. This user interface intends to help the user with a graphical context and is divided in seven regions (Fig. 3). It was developed in “Processing” (an open source programming language based on graphical representation, https://www.processing.org/) to communicate with Arduino. The user can observe the angle values change in real time (region 1) along with its visual movements (region 4), perform a set of tests (region 6) and save the results (region 3) in a “.tsv” file (tab separated values) to be read into a spreadsheet Excel file for data analysis. Each time the user presses the “Enter” key the values shown on region 1 are copied to region 6. In region 7 the user can switch the laser on/off.
Cadaveric tests
Twelve specimens were obtained under to the body donation program from the Institution where the cadaveric tests were performed. It complies with all bioethical requirement according the rules of this donation program approved by our University. It comprised 7 female and 5 male patients with ages comprehended between 57 to 81 years old (average 72).
All specimens were frozen no more than 1 year and were defrost to be used in this study according to the guidelines of the pre-existent local program. The twelve cadaveric ankles were free of any abnormalities. The tibia and fibula were systematically sectioned below the knee joint. The skin and fat pat over the distal third of the leg and ankle were removed. The proximal and distal extensor tendon retinaculum were also removed allowing the access to the anterior joint line, whereas the peroneal tendon’s retinaculum was opened longitudinally. The ankle joint was opened in the mid-line by detaching the anterior capsule from the distal tibia, while preserving the medial and lateral capsule-ligamentous complex. No tissue was removed around the anterior talofibular and the calcaneofibular ligaments, because the anterolateral capsule complex might have some mechanical resistance.
The tibiotalar displacement was evaluated through the prototyping device described above. For each specimen our research started by calibrating the system in neutral ankle dorsiflexion by holding the foot at 90° to the tibia, which means a square ankle, and the tibia oriented 45° in relation to the floor/table where it stood. That was our zero reference and initial position (Fig. 4). The 90° angle is assured by using a set square, and the 45° angle is imposed by the wood support of the device. This tibia orientation is similar to the leg position during regular clinical examinations, when the patient is lying on a bench and the hanging foot and ankle is held by the medical examiner (Figs. 5 and 6).
We would then let the gravitational forces act on the hanging foot and record its final orientation (Fig. 5). It should be noticed that the starting position is the same as the one used in the clinical objective examination, as shown if Fig. 6b. Four different measurements were taken. On each specimen, the first measurement was done with all ligaments intact and the followings after progressive sectioning each of the lateral ankle ligaments (ATFL, CFL and PTFL). The second measurement was done after a complete section of the ATFL. This was done through a complete section of the capsule and ligament from the anterior facet of the lateral malleolus. The third measurement was done after a complete section of the CFL from the inferior aspect of the lateral malleolus. The fourth measurement was done after complete section of the PTFL. This section was done from anterior to posterior, through the ATFL section window into the PTFL, as well as behind the lateral malleolus while reflecting the peroneal tendons posteriorly. At each measurement point the value of the angular rotations was registered in the three planes of movement: sagittal (flexion), axial (rotation) and coronal (varus).
Statistical analysis
All statistical analyses were conducted using the IBM SPSS Statistics V.24.0. A significance level of 0.05 was considered for all statistic tests. Continuous variables were described with described using mean and standard deviation (SD). The variables were tested for outliers and normality (Kolmogorov-Smirnov test). Mean differences (angular displacement) with the respective 95% confidence intervals (CI) were calculated as the difference of intact and ligament sectioning condition. The Friedman test was used to test the overall changes and the Bonferroni correction was used to compute the adjusted significance between pairwise comparisons of progressive sectioning. The Kendal W was computed to evaluate the consistency of changes. The intraclass correlation was used to test the variability of the different measures based on a single measure, absolute-agreement, 2-way mixed-effects model. Values less than 0.5 are indicative of high variability, values between 0.5 and 0.75 indicate moderate variability, values between 0.75 and 0.9 indicate low variability, and values greater than 0.90 indicate very low variability [13]. The standardized response mean (SRM) and effect size (ES) were used to measure the responsiveness of progressive sectioning. The SRM was defined as the mean change between the two conditions divided by the standard deviation (SD) of this change. The ES was defined as the mean change between intact and sectioning conditions divided by the SD of the intact condition. The ES and SRM were considered large if greater than 0.80, moderate if between 0.51 and 0.80 and small if lower than 0.50 [3].