This study has a double aim. First, to explore the pain inhibitory effect of tens in OAk patients. Second, to assess whether components of central sensitization like temporal summation and diffuse noxious inhibitory control, affect this pain inhibitory effect.
It is believed that tens influences pain through different pathways. One of these pathways is the gate-control theory [9
]. The second goal of our study protocol, i.e. exploring the potential prognostic value of TS and DNIC on the pain inhibitory effect of tens, is based on this rationale. However, opioid pathways that involve peripheral, spinal and supraspinal mechanisms [41
] are also proposed as an explanation for the pain modulation of tens and this pathway may be less vulnerable for an adverse effect of tens in central sensitized OAk patients.
As tens may influence pain through the electrical stimulation of low-threshold A-beta cutaneous fibers, the responsiveness of central pain-signaling neurons of OAk patients who are centrally sensitized is augmented to the input of these electrical stimuli. This would encompass an adverse therapy effect of tens on the pain perception in patients with OAk who are centrally sensitized. Therefore we think that it might be interesting to identify a subgroup of symptomatic OAk patients, i.e., non-sensitized patients, who are likely to benefit from burst tens.
The majority of studies that were published in the past and that focused on treatment effects of tens, embody currents that were administered by a therapist in a practice or hospital setting. As portable tens devices are marketed as small, inexpensive, easy-to-use home units [13
], we choose to use a self-administered protocol. This approach may encompass a positive influence on the patient's participation to the treatment as well as to the cost effectiveness of the treatment, as the machines that are used in (professional) practices are far more expensive than these portable tens devices.
To assess whether dose-effects may influence the treatment outcome, we plan to record the daily duration of the electro stimulation that is applied.
One major concern when using self-administered medical care is treatment adherence. We think that a weekly phone call after the initial start of the study may improve treatment adherence as well as stimulate participants to consistently fill out the diary.
We have chosen a daily treatment duration of 40 minutes continuously. This is based on the findings of Cheing and colleagues [43
]. They found that the cumulative analgesic effect manifested by a tens group that received 40 minutes of tens therapy was significantly greater than those seen in 2 other active tens groups (20 and 60 minutes application) in the follow-up session, i.e. 2 weeks after termination of the 2 week treatment. They conclude that 40 minutes is the optimal treatment duration of tens to be used for the relief of pain in patients with knee osteoarthritis.
The results of this study will not only provide insight into the effect of tens but they may contribute to future studies investigating the identification of patient subgroups that may benefit from tens.