Tissue mechanics during temporal summation of sequentially cuff pressure-induced pain in healthy volunteers and patients with painful osteoarthritis.

BACKGROUND: The phenomenon of temporal summation to repetitive pressure pain stimuli is an important central neural mechanism for pain intensity encoding. This study evaluated the time-dependent behaviour of mechanical characteristics of soft tissue during repeated cuff stimulation used for eliciting temporal summation of cuff pressure-evoked pain. Such information of tissue mechanics is important for the interpretation of the pain response evoked during sequential stimulations. METHODS: Temporal summation was assessed in 16 subjects separated into two groups (healthy controls and severe knee osteoarthritis patients) using a visual analogue scale during 10 repetitive painful cuff stimuli (1-s duration, 1-s break) of the lower leg. The geometry of the lower leg was constructed based on magnetic resonance image (MRI) data. The loading boundary condition of the finite element model was defined according to the parabolic pattern of the interface pressure around the limb and the time-dependent profile of the cuff pressure during repetitive stimuli. RESULTS: The pain intensity significantly increased with an increasing number of stimuli (p < 0.001), and facilitated temporal summation of pain was observed in patients compared with healthy controls (p < 0.001). The maximal deep tissue stress and strain during stimuli 1-4 varied 43% and 9%, respectively. No variation was observed for stimuli 5-10. CONCLUSIONS: The study concludes that the temporal summation of pain response during sequential cuff pressure is not explicable by a specific time-dependent behaviour of stress and strain in the activated deep tissue and hence not due to changes in tissue biomechanics. SIGNIFICANCE: The temporal summation of pain during sequential cuff stimulation is inexplicable by the time-dependent response of mechanical stress and strain in soft tissue.

Deformation and pressure propagation in deep somatic tissue during painful cuff algometry.

BACKGROUND: Cuff algometry is used for quantitative pain assessment although it is not clarified which tissues are actually challenged by the stimulation. This study investigated the mechanical stress and strain distribution in superficial and deep tissues during cuff algometry applied on the lower leg at three different intensities (mild pressure, pain threshold and supra pain threshold). METHODS: A computational three-dimensional finite element model of the lower leg with three different layers of soft tissue was developed based on magnetic resonance imaging (MRI) recorded during cuff stimulation. Tissue indentation maps were extracted from the MRI scans and transferred into the model as displacement of boundary condition. In all stimulation conditions, the mean stress of subcutaneous adipose and muscle tissue below the cuff decreased compared with the skin while the mean strain peaked in subcutaneous adipose and decreased in other tissues. RESULTS: At pain threshold stimulation intensity, the mean muscle stress was 2.9% of mean skin stress and the mean muscle strain was 55.1% of mean strain in adipose layer. The mean stress and strain increased by 30.4% and 27.1%, respectively, in muscle tissue from painful to supra pain threshold stimulation. The stress and strain was mainly focused around the bones and superficially under cuff. CONCLUSIONS: This study shows the better capability of cuff algometry for stimulation of deep somatic tissue in terms of generation of mechanical stress and strain in contrast to the more superficial muscle tissue previously demonstrated to be strained by single-point pressure algometry.