Pressure pain thresholds: Subject factors and the meaning of peak pressures

Background The assessment of pressure pain has become an integral part in pain research. The distribution of pressure under a plunger can be uneven. However, measurements based on conventional devices show the applied force or mean pressure, failing to take local pressure peaks into account. Our main question was whether peak pressures under the probe are responsible for pain onset. Methods A force-controlled algometer was fitted with a newly developed pressure-indicating film. Pressure pain thresholds (PPTs) of 100 healthy subjects (57 men, age 18-66 years) were assessed at 29 sites across the body. Each site was measured three times, nonconsecutively and presented in randomized order. Forty subjects were manual labourers. Results Pressure distributions on hard tissue (bone) were more heterogeneous and showed more prominent peaks beneath the probe when reaching the PPT. Soft tissue (e.g. muscle) created a distinct distribution, with higher pressure especially around the corners of the probe. A high variability of PPTs between subjects and different measurement sites was observed. Men as well as manual labourers had comparatively higher adjusted pressure pain thresholds (force and pressure). Conclusions Peak pressures could be relevant for pain onset and should be accounted for in mechanical pain studies. The probe, indentation depth and tissue properties have a major impact on pressure distributions and may therefore affect the perception of pressure pain. Due to higher intra-individual differences regarding peak pressures at the spinous processes, breastbone, forehead and abdomen caution are needed when interpreting those sites. Significance This study adds some important considerations for the use of pressure algometers. It was found that during pressure pain thresholds readings distinct peak pressure profiles could arise, which may influence the perception of pain. Peak pressure could be another contributing factor, which may explain some of the high variability in pressure pain readings.