A mechanical stimulus, evoking pain from pressure, is a naturally occurring stimulus well suited to nociceptive threshold testing (NTT) in animals. It is widely used for NTT, both as an alternative to, but also in conjunction with a thermal stimulus to cover both modalities – a closer representation of naturally occurring pain.
Many methods for mechanical NT (MNT) testing are described in the literature, making it difficult to compare data from different studies. Results are affected by the following:
- Probe size & shape
- The rate and method of force application (hand-held or fixed on the subject)
- The interval between tests
- The characteristics of the tissue that is stimulated
- The testing environment and level of distraction – as for any NTT
The size of the probe tip also influences the type of tissue most affected by the stimulus. Larger probes (probably larger than 2mm in humans) elicit deep muscle pain while smaller probes elicit cutaneous pain. Studies have shown that the application of local anaesthetic cream to the skin increases MNT very little with the larger probes, showing that the pain is largely from deep muscle.
Tissue types are not equally compressible and MNT is affected by their physical properties. Nociceptor stimulation is related to tissue distortion and smaller probes (2mm versus 11.3mm diameter) have proved better for evaluating bone (periosteal) pain as they produce more compression here at lower applied force.
Tissue distortion is decreased when muscles are hard (under tension) as transmission of the force to the deeper tissues is reduced, leading to higher MNT. Conversely, adipose tissue is easily compressed and, although a normal amount of subcutaneous fat does not affect MNT significantly, the probe simply has to travel further before the pressure rises. In really obese subjects, the MNT may be increased as the forces are not transmitted to the deeper tissues
Topcat’s innovative rolling diaphragm pneumatic actuator is silent, lightweight and unaffected by the angle of application or hand tremor.
All the mechanical systems use this actuator either via a hand held algometer (ProdPlus) or via animal-mounted actuators (ProdPro), usually on a limb.
A wireless mechanical system is available for large animals, and is controlled remotely in the same way as our wireless thermal systems