## 2 Materials and Methods
### Assessments and Data Analysis  
The functional assessments were performed by one investigator ([PERSON]) who was unaware of the knee undergoing surgical intervention. The radiological severity of knee OA was quantified by visual inspection of the knee radiography, assigning a 0 to 4 score according to the Kellgren-Lawrence scale [23] where 0 = no sign of OA, 1 = doubtful, 2 = minimal OA, 3 = moderate OA, and 4 = severe OA. Two assessors ([PERSON] and [PERSON]) assigned the score, and in case of disagreement a third assessor ([PERSON]) was consulted. The Short Physical Performance Battery (SPPB) [24] was used to clinically assess the function of the lower limbs in terms of balance, walking ability and chair standing. For balance assessment, the test consisted in maintaining a side-by-side, a semi-tandem and a tandem stance for 10 s. Walking ability was assessed measuring the time employed to walk for 4 m. The chair stands required participants to stand-up from a chair for five repetitions,holding arms across the chest. Each test has a score ranging from 0 to 4 where 0 represents the inability to perform the task and 4 represents the best performance according to the test requirements. Final maximal score of the test is thus 12, which represents the highest level of functional ability. The score of each participant was calculated and used for further analysis. The Knee Society Score (KSS) (KSS) (KSS) (KSS-F) was used to assess knee function, pain and mobility. It is composed of two parts: a function score, assessing patient's mobility in terms of walking, stair climbing and the use of walking aids; and a clinical score assessing knee pain, range of motion, joint alignment and stability. The function score (KSS-F) and clinical score (KSS-C) were calculated and used for further analysis. The score of each part ranges from 0 to 100, with 100 representing the highest level of function. Balance and proprioception were assessed using the Delos Postural Proprioceptive System (Delos S.r.l., Turin, Italy) ([PERSON], 2016; KSS-F). The Delos system is composed of a vertical controller, a horizontal bar and a software for assessment and data analysis. The vertical controller is applied on a patient's sternum with elastic bandaging; following the calibration by means of a specific software (DPPS 6.0), it defines and records the trunk inclination in the frontal (x) and sagittal (y) plane by means of a two-dimensional accelerometer unit. The horizontal bar, instrumented with an infrared sensor, is placed in front of the patient for hand support (Figure 2). The bar measures the number and the duration of hands contact on it. According to [PERSON] and [PERSON] (2018), postural assessment is assessed during the performance of two tasks: a stabilometric static bipodialic test (double stance test), and a static monopodialic test (single stance test). The double-stance test consists of two trials: the first with open eyes (OE) and the second with closed eyes (CE), while standing on a flat wooden surface with the arms resting at sides for 20 s. The patient is asked to stay still and to minimize body sway. No feedback on postural stability is provided during the tests. Participants are allowed to rest for 15 s after each trial. The variable considered for this test was the Postural Instability value, measured in degrees, both in the OE and CE trials ([PERSON], 2016). The single-stance test consists of four trials of 20 s each, standing on the wooden flat surface with the arms aligned at the sides. Two trials are performed with OE, one in single stance support on the right lower limbs, and one on the left limb. Figure 1: Flow chart showing patients’ recruitment. The limb not touching the ground is maintained with knee flexed. The same trials are repeated with CE. The variables considered for this test were: the Stability Index (SI), a score (0-100%) based on two components: autonomy and average postural instability (Plxy, cone of instability).