Sensory responses to mechanically and chemically induced tendon pain in healthy subjects

This investigation aimed to quantify and compare sensory responses to hypertonic saline-induced pain in the tendoachilles and the common extensor tendon of the elbow. Healthy subjects (n=14; seven males) received in randomised order, injections of sterile saline (0.5ml, 5.8% hypertonic or 0.9% isotonic saline) at each tendon bilaterally at two sessions separated by one week. Mechanical sensitivity (pressure pain threshold), muscle pain intensity (visual analogue scale; VAS area-under-curve, pain duration, peak pain) and pain distribution were assessed pre, immediately after and post saline injection. Hypertonic saline-induced pain intensity (VAS area-under-curve, duration and peak) was significantly greater compared with control injections (P<0.001) and induced significantly greater VAS area (P<0.01) and longer pain duration (P<0.001) in tendoachilles compared with the common extensor tendon. Regardless of saline type and compared with pre and post injection, mechanical sensitivity increased significantly (P<0.01) immediately after injections at all injected tendon sites. Hypertonic saline-induced referred pain was infrequent (tendoachilles: n=3 and common extensor tendon: n=4). Significant maximal force attenuation occurred immediately after hypertonic saline injections in both tendons (P<0.001) compared with control injections. The greater induced deep tissue pain and hyperalgesia demonstrated at tendoachilles compared with the common extensor tendon may relate to anatomical differences such as higher nociceptor density or increased vascular perfusion at the injection site. This translational tendon pain model may contribute to the further understanding of pain mechanisms in tendinopathic conditions.