Experimental knee pain evoke spreading hyperalgesia and facilitated temporal summation of pain.

OBJECTIVES This study evaluated the deep-tissue pressure pain sensitivity and temporal summation of pain within and around healthy knees exposed to experimental pain. DESIGN The study was designed as a randomized crossover trial, with each subject tested on 1 day. SETTING All tests were carried out at the Laboratory for Musculoskeletal Pain and Motor Control, Center for Sensory-Motor Interaction, Department of Health Science and Technology at Aalborg University, Denmark. SUBJECTS Seventeen healthy subjects (10 males) participated in this study. INTERVENTIONS Experimental pain model. Pain was induced in the infrapatellar fat pad by injection of hypertonic saline and the contralateral infrapatellar fat pad was injected with isotonic saline as control. OUTCOME MEASURES Pressure pain thresholds, temporal summation of pressure pain, and cutaneous mechanosensitivity were assessed on three occasions: baseline, immediately after the injection, and when pain had vanished. Assessments sites were located in the peripatellar region, vastus lateralis, and tibialis anterior muscles. RESULTS The experimental knee pain model demonstrated 1) hyperalgesia to pressure stimulation on the infrapatellar fat pad during experimental pain, and 2) facilitated temporal summation of pressure pain at the infrapatellar fat pad and knee-related muscles. CONCLUSION The increased sensitivity and temporal summation found in this study were exclusive to deep -tissue with no contralateral decreased pain sensitivity. The study showed that acute knee joint pain leads to hyperalgesia and facilitated temporal summation in the infrapatellar fat pad and in muscles located distant to the injection site, in subjects with no history of knee pain.