## 4 Discussion  
Proprioception is supposed to be compromised with progressive osteoarthritis due to impairment of joint structures and muscles, thus affecting receptors and paths conducting afferent signals. Furthermore, proprioception may be impaired due to the symptoms of osteoarthritis, such as pain and joint oedema [6; 31]. Together with visual and vestibular sensory inputs, proprioception is one of the most important factors affecting balance ability. The evaluation of balance and, in particular, proprioception in patients with knee osteoarthritis is thus essential to understand possible effects on functional performance during the activity of daily life, risk of fall and, possibly, to have objective tools for the assessment of rehabilitation and surgical therapeutic strategies outcome. A recent review [32] reported that although different devices have been used across time to assess balance and proprioception in patients before and after total knee replacement, no standardized comprehensive evaluation protocol presently exists. In the present study an innovative device for postural stability and proprioception assessment, the Delos Proprioceptive System, was used. It demonstrated providing reliable parameters as already reported in the literature [21; 29; 30]. In particular, the Delos system allows estimating the proprioceptive component of postural stability when standing on single stance with closed eyes. High values of Stability Index in the trial with closed eyes are in fact assumed to be indicative of a more refined control, as expression of effective proprioceptive reflexes that are able to stabilize the patient rapidly before the vestibular responses can be activated [21]. To our knowledge, this is the first time that proprioception assessment is obtained through an objective instrumental measure; most of the research on proprioception evaluation is carried out by means of clinical or instrumental test based on the knee position sense, or sensations of passive, slow knee motion [8]. Findings in the present study on postural stability in double stance confirm previous data reported in the literature in patients with knee OA of a worst performance with respect to healthy people both in the trials with open eyes and closed eyes, when postural stability is assessed via posturography [19]. Usually, when posturography is performed, the velocity and the displacement of the center of pressure are recorded to assess postural stability [19]. Our study differs from previous research since an accelerometer was placed over the trunk of the patients to measure trunk oscillation and displacement. In addition, the device used in our study allowed quantifying the autonomy of the patients in maintaining a steady stance without the need of additional external sensitive information, which in our case was represented by touching a bar. With regards to single stance tests, only the group of females showed a reduced Postural Stability with respect to the reference data, both when standing on the KOA limb and on the contralateral only in the trial with open eyes. This worst performance is consistent with reduced Autonomy (which means that women spent more time during the test holding the bar placed in front of the patient for hand support, thus avoiding a fall). Since the trials were conducted in an open eyes condition, it should be speculated that these results should be related to a combination of impaired proprioception and impaired balance, but also to a lower muscle strength and worse functional performance abilities, when compared to males. Studies have reported high levels of functional impairments in females' patients with knee OA [33]. Previous research has also reported high pain in females [33], however, in our study, KSS results, which account also for knee joint pain, showed no differences between males and females and thus knee pain should not be the explanation of our results.