1. Introduction
Knee osteoarthritis (OA) is one of the most common age-related diseases featured by cartilage loss and subchondral bone damage [
1,
2]. Even if these latter are typically used to quantify disease severity, it is known that knee OA affects almost all of the other components in the knee joint including the ligaments, the menisci, the nerves and the muscles acting on the affected joint [
3,
4,
5,
6]. It has been postulated for a long time that the degeneration of knee joint proprioception increases as osteoarthritis worsens [
4,
5,
6,
7,
8]. Sensory receptors located in the knee joint components are damaged resulting in the generation of abnormal proprioceptive sensory signals informing the brain on joint position and joint movement in an incorrect manner [
4,
9]. The abnormalities in proprioception contribute to a decline in balance control, also defined as postural stability, and as a consequence to an increase in the risk of falls [
10,
11,
12,
13,
14]. In addition to proprioception impairment, other variables affecting balance have been claimed: muscle strength, radiograph visual indications of the severity of knee OA, pain, and knee alignment [
12,
15]. A clear relationship between the impairment of balance and proprioception and the degree of functional impairment as measured by means of common scoring systems has not yet been demonstrated [
8].
Literature has reported impairments in standing balance and proprioception in people with knee OA both through clinical tests [
8,
15,
16,
17] and laboratory-based measurements [
17,
18,
19,
20]. These studies clearly show that the lack or the inappropriate proprioceptive information arising from the knee joint has a negative effect on static and dynamic balance control. However, most of these previous researches were based on the instrumental assessment of postural stability in bipodalic stance. Considering that single limb stance is very common during basic movements of daily life such as walking (where single stance lasts about 80% of the gait cycle) or climbing a stair, testing single stance stability seems to be even more of relevance for people showing bipodalic balance impairment. In particular, assessing postural stability in single stance with closed eyes, excluding the visual control, allows indirect evaluation of the effectiveness of the proprioceptive control [
21].
Based on these considerations, the aim of this study was to assess postural stability and proprioception in patients with knee osteoarthritis during a double-limb and a single-limb standing postural stability task, and the relation between postural stability and proprioception impairments with the radiological severity of knee OA, and impairments in functional performance as measured by clinical scores.
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.
Results from trials with closed eyes are also worse in the women patients compared to the reference data, but with results showing a trend to statistical significance only for the contralateral limb (
p = 0.05), and without significant changes in Autonomy. This result was quite unexpected. Based on the premise that Stability on a single limb eliminating visual control should enlighten the quote of proprioception control, data provided from the present study support this assumption, and only in the female patients. Also, no between-limb differences were found. Considering that patients included in the study were scheduled for a unilateral knee prosthesis, a possible explanation of this symmetry is that knee OA usually affects both knees [
34]. Another possible explanation of this result, is represented by the low scores of the Stability Index and the Autonomy of healthy subjects. It is likely to think that performing a single-limb stance task with eyes-closed leads to the recording of low scores for postural stability for patients with knee OA, but also for healthy adults and elderly, in particular if they are sedentary and not involved in balance exercise programs [
35,
36]. This means that adults/elderly individuals mostly rely on other afferent sensitive information, such as tactile information, more than sensitive information arising from other proprioceptors, when asked to maintain postural stability on a single limb and with eyes closed.
Findings in the male patients also need careful interpretation. Although values reported by these patients were always below the reference values both for SI and Autonomy, and both in the OE and CE trials, evidence from statistical analysis did not demonstrate differences. Some explanation should be hypothesized, such as greater muscle strength in the lower limb, which allows a good compensation to the lack of postural stability in single support, as discussed below.
In general, clinical-functional scores of patients in the present study are not different from values available in other studies on knee OA patients for KSS [
37,
38,
39] and for SPPB [
40,
41,
42]. Also, knee OA as radiologically measured by means of the Kellgren–Lawrence score resulted in correlation to the average Stability Index in the open eyes trial, suggesting that a worse functional performance is present in more severe radiological OA. This finding confirms previous evidence of impaired postural stability in patients with osteoarthritis of the knee, especially those diagnosed with grade III or IV degeneration [
6,
17,
20]. The worst performance of women in the clinical-functional scores, namely the KSS-F and SPPB scores, confirm a strong relationship between clinical and instrumental tests, in accordance with previous literature reporting higher levels of disability in females compared to males with knee OA [
43]. It has also been reported that females usually have a more severe pathology than males and higher levels of pain [
44,
45], but, in our findings, females did not show significant differences with men in the radiological level of knee OA, and in the KSS-C, which includes pain assessment. Other factors, such as muscle strength, or previous and current physical activity level, might give important information for the interpretation of this result. In particular, quadriceps strength, one of the main features of knee OA [
46] which has been shown to be more reduced in women than in men with knee OA [
47,
48], has a significant correlation with static and dynamic balance [
23,
49].
In light of the impairments in postural stability and proprioception found in this study, it is essential to address clinical practice towards ongoing assessments of postural stability and proprioception in patients with knee OA, in particular in those patients who chose a conservative treatment rather than the surgical intervention for TKA, and in particular in female patients. The observation that the measures of postural stability and proprioception are related to clinical and functional scores further support these observations. It seems thus that the implementation of specific rehabilitation interventions should be paramount in order to improve postural stability and proprioception [
50], and assess their effectiveness on clinical and functional scores.
Some limitations need to be mentioned for this study. The sample of patients included in the study is small. A high proportion of the patients who were asked to participate in the study at the pre-admission clinic refused to participate. Furthermore, a high number of the patients did not meet the inclusion criteria and thus they were excluded from the study since they had other medical conditions potentially affecting postural control and proprioception. While this gives the sample greater homogeneity, on the other hand the findings could be biased. Furthermore, it was not possible to carry out a priori a power analysis since, for the proprioceptive test used, mean and standard deviations values useful for performing a statistical analysis were not available. In fact, the device used in this study for postural control and proprioception assessments, to the best of our knowledge, has never been used in patients with knee OA. It should be noted however that the number of patients in this study is in the range of samples included in other studies [
3,
7,
16,
18]. Also, the contralateral limb was not assessed for OA. In most of the cases OA affects both limbs. Thus, the lack of difference between the affected and the contralateral limb reported in this study should be considered in light of this limitation. Future studies should consider OA assessment in both limbs. Another limitation of the study was the lack of a matched healthy control group assessed comparatively with OA patients, which we tried to overcome comparing data from literature on the device [
21]. It was not possible to make further comparisons with previous studies, since participants in other studies using the same device were different from patients with knee OA, as they were affected by other orthopedic conditions [
29,
30] or were in other age ranges [
27].