# Effect of an Innovative Biofeedback Insole on Patient Rehabilitation after Total Knee Arthroplasty
## 2 Materials and Methods  
This study was conducted according to the guidelines of the Declaration of Helsinki (7 th revision; 64 th WMA General Assembly, Brazil, October 2013) and in observance of existing Italian legislation on privacy and sensitive data processing. Prior informed consent was obtained from all subjects involved in the study. From November 2019 to February 2020, we enrolled 30 patients diagnosed with gon-narthosis (stages C and D according to the International Knee Documentation Committee, IKDC) and who were scheduled to receive a total joint replacement. It was not possible to blindly collect the results with a control arm due to the obvious presence of the insole and collection device. However, in order for any possible confounding factors (i.e., bias) to be reduced, the patients were operated on by the same surgeon (G.C.) using the same surgical technique, which consists of an extra medullary guide (EMAS) already described in previously published papers [15; 16], the same approach, and the same prosthesis (Genus mobile bearing knee with LS insert, Adler Ortho SpA, Cormano, Italy) [15; 16; 17; 18]. In all cases the Posterior Cruciate Ligament (PCL) was spared. All prosthetic components were cemented, and the patella was never resurfaced. Of the 30 enrolled patients (20 females,10 males), the mean age was 69.5 years (min. : 60, max. : 83). Eight patients had their left knee replaced, and the remaining 22 had their right knee replaced; the mean bodyweight was 69.28 \(\pm\) 12.9 kg. Patient selection was performed according to the following inclusion criteria: age < 85 years, no neurological or orthopedic pathologies other than knee arthrosis, and no comorbidities or other issues that could lead to low compliance with the insole and the rehabilitation protocol. A total of 23 patients (77% of the cohort) had pre-operative vars deformity > 5\({}^{\circ}\), 3 patients (10% of the cohort) had a varus deformity between 3\({}^{\circ}\) and 5\({}^{\circ}\), and 4 patients (13% of the cohort) had a valgus deformity between 3\({}^{\circ}\) and 5\({}^{\circ}\). Only one patient was excluded halfway through the study due to the development of a severe neurological deficit in active hip flexion; this cruralgia relapse was secondary to a pre-existing lumbar disc pathology and was non-responsive to pharmacological therapy. Another patient had minor problems using the insole because of an ankle sprain that occurred in the immediate pre-operative period, which significatively reduced the patient's compliance. The feedback system was the Blu Insole (FGP srl, Dossobuono 37062, VR, Italy); Figure 1). The device is made of thin (<5 mm), flexible polyester available in different shoe sizes (European sizes 35-48) and two sides (left and right) that can be easily cut and shaped to fit each patient's foot and any kind of shoe. Compared to previous commercial designs [9,10] that enable the use of a maximum 13 FSRs, the Blu Insole allows the use of 214 FSRs, improving the overall quality of the measurement. FSRs are designed to measure load at a frequency of 10 Hz and response time of about 0.2 s. This extremely low response time makes it is possible to train patients to autonomously correct their weight bearing deficits or excesses. The insole is connected to a capture device using a flexible connector; the capture device is fixed to the same patient's ankle with an adjustable ankle band (Figures 1 and 2). The device is CE mark, and it has been developed and produced according to the EU standards 93/42/CEE. Using an appropriate software provided by the manufacturer and connecting the insole to the operator PC by means of an USB cable, one can program the unit to provide patients audible and tactile (vibration) feedback on two different thresholds: one when they succeed in gaining the minimum requested load, and one when they exceed the maximum weight allowed. In practical terms, the operator enters the patient body weight and then sets up the maximum percentage of it allowed before the patient will feel the vibrating feedback.