This is the first longitudinal study of proprioceptive acuity as a risk factor for the initiation and progression of pain and structural damage in knee OA. Our baseline findings confirm previous cross-sectional studies reporting a relationship between poor proprioceptive acuity and worse pain and physical functioning. Although our longitudinal findings showed that people with greater proprioceptive deficits were at increased risk of deteriorating physical function and more severe pain, these effects were modest and their clinical relevance unclear. We did not find that proprioceptive deficits affect the risk of new OA or new knee pain. Our data therefore suggests that proprioceptive deficits at least as assessed by a person's ability to reproduce a knee flexion angle are not a major risk factor for OA.
The major strengths of this study are its size and longitudinal design. Our failure to find that proprioceptive acuity had a major effect on knee OA outcomes is unlikely to be a consequence of inadequate power, since this study is a large study, the largest yet undertaken and confidence intervals were narrow especially for baseline analyses and even for x-ray progression. When we looked at the average inaccuracy in joint position sense across all 10 trials in individuals, we found similar inaccuracy (3.9°) as reported in other studies ((24
) This strengthens confidence in the generalizability of our inferences.
However, several limitations of the study may mask any effect of proprioceptive deficit. The most important limitations are the difficulty in measuring proprioception and the insensitivity of the tools used to measure the outcomes. Accurate proprioception is essential for planning, executing and monitoring safe, efficient movement. To ensure that the body has an accurate “real time” picture of what is happening, it collects collates and assimilates sensory information from several overlapping physiological systems. If one physiological system is compromised, others compensate to minimize sensory deficits. This makes measuring proprioception problematic(15
) and could prevent us detecting associations between proprioceptive deficits.
There are several ways of measuring proprioceptive acuity. One often used is the threshold detection of passive movement, but passive movements do not reflect real life movement or function. We estimated people's ability to replicate limb position using active movement as this maximizes sensory input to the central processing systems and replicates normal movement which is almost always active, and as such is a closer measure of “real life” proprioceptive acuity. The technique does require concentration and cognitive skills by the subjects if these skills are compromised, this will interfere with the accurate estimation of proprioception. Other investigators have suggested there is a poor correlation between different approaches to proprioception assessment (27
Similarly, X-rays are an insensitive measure of structural joint damage, so that the poor correlation between proprioception and joint damage might be partially due to insensitive outcome measures. Since x-ray damage is weakly related to function loss and pain in OA, one might argue that any factor related to pain and function in OA might not be expected to correlate with x-ray OA. In addition, since OA is a slowly progressive condition despite this study involving a relatively long follow-up it may still be too short to see changes in structural damage and proprioception.
Many factors may combine to worsen a patient's course, and proprioceptive deficits may be only one of them, one which may not have a unique independent effect. People may accommodate for (sub)conscious proprioceptive decline by adapting their behavior. Thus. impaired proprioception may explain why people with symptomatic OA walk more slowly and with longer doublTe limb stance to avoid risk of joint injury and prevent worsening disease (4
) These compensatory mechanisms could also explain the lack of association between poor proprioception and progression of OA. The modest association of joint position sense and function loss and pain worsening may not relate to OA progression at all but rather to poor motor control and muscle function in those with impaired proprioception and the contribution of these with pain and function.
Subjective assessment of pain and physical function, as measured by the WOMAC, is influenced by many psychological factors, traits and emotions. Therefore, a weak relationship between structural articular damage, pain and impaired proprioception is not be surprising, and reflects the well described lack of association between structural damage, pain and disability. Including an objective measure of physical function might have provided a better comparison to assess the effect of proprioception on physical function.
The severity of the condition may affect the association between proprioception and outcomes measures (3
). From the baseline pain (mean WOMAC score about 3 from a possible range of 0-best to 20-worst), physical functioning (WOMAC score of approximately 14 from a possible range of 0-best to 64-worst) and percentage of people with radiological damage suggest the cohort recruited into the MOST study from the community had mild disease. In early stages of disease when overlapping protective strategies function well, proprioception deficits may not emerge as identifiable risk factors for disease. Perhaps it is only when disease is farther advanced and all protective strategies are impaired that proprioception deficits play a critical role.
In summary, poor proprioceptive acuity as assessed by joint position sense is related cross-sectionally with the presence of knee pain and its severity and worse physical functional limitations. However, it is not strongly related to risk of later outcomes---we found no significant relations with new knee pain or x-ray worsening. We did find that, compared to those with good proprioceptive acuity, persons with poor acuity had more deterioration in physical function and worse pain over time but the associations were modest.