| Achilles tendon injury is a common musculoskeletal disorder with a high incidence,a high disability rate,and long course.Achilles tendon injuries mostly cause somatosensory and motor system disorders,including Achilles tendon pain,gait abnormalities,and walking difficulties,which seriously affect the quality of patients’ life.In recent years,low level laser therapy have been widely used in treatment of tendon injuries with good efficacy,including anti-inflammatory,analgesic,promote tissue repair,etc.In our previous researches have confirmed that the thalamus-mediated endogenous descending facilitation and descending inhibition of pain have plastic changes in pathological pain.By focusing on Achilles tendon injury,this study used low level laser to treat Achilles tendon was induced by means of blunt impact,the changes of endogenous descending modulation of thalamus-mediated in Achilles tendon injury were explored,and the peripheral and central related mechanisms lies in neuroregulatoryion mechanisms involved in the analgesic effect of low level laser were futher investigated.Purposes: 1.To observe the plasticity of endogenous modulation of pain following Achilles tendon injury.2.To explore the efficacy of low level laser therapy at different wavelengths for Achilles tendon injury induced pain,and effect on endogenous descending modulation of pain.3.To explore the peripheral and central mechanisms lies in neuromodulation of low level laser therapy for Achilles tendon pain.Methods: 1.Achilles tendon injury model was established by striking the middle of the left Achilles tendon in female rats by percussion,hematoxylin-eosin staining of Achilles tendon tissue,and the bilateral paw withdrawal mechanical thresholds(PWMT)and the paw withdrawal thermal latencies(PWTL)were observed before and 7-21 days after the modeling.2.A period of 3 min of low level laser at different wavelengths at 650 nm,810nm,915 nm and 980 nm with output power of 0.5W were used to irradiate the left Achilles tendon tissue in naive rats.The bilateral PWMT and PWTL were measured before,and 1-4h and 1-5 days after the laser therapy.3.During the 7th day after the Achilles tendon injury modeling,low level laser at wavelengths of 650 nm,810nm,915 nm,980nm with output power of 0.5W were used at the injured or uninjured Achilles tendon in rats with Achilles tendon injury for 3 min.The bilateral PWMT and PWTL were observed before,1-4h and 1-5 days after laser therapy.4.During the 7th day after the Achilles tendon injury modeling,low level laser with wavelengths of 650 nm,810nm,915 nm,980nm and output power of 0.5W were used to treat the injured Achilles tendon for 3min/day for three consecutive days.The bilateral PWMT and PWTL were observed before,1-4h and 1-7 days after laser therapy.5.During the 7th day after the Achilles tendon injury modeling,the Achilles tendon injury site was treated by laser at wavelength of 980 nm with 0.5W for 3 min.30 min after the laser exposure,1-10μg naloxone was injected at the treatment site.30 min after the injection of naloxone,1-5nmol μ-opioid receptor antagonist β-FNA was injected into the thalamic VM nucleus.The bilateral PWMT and PWTL were observed before,30 min and 1-4h after drug administration.Results: 1.After the establishment of unilateral Achilles tendon impact injury modeling,the bilateral PWMT decreased significantly,and the PWTL prolonged significantly(P < 0.05).The hematoxylin-eosin staining of injured Achilles tendon showed that the collagen fibers were in a chaotic and wavy arrangement,and sporadic tendon cells were interspersed between the collagen fibers.2.PWMT and PWTL at laser exposure side were significantly increased 1h after low level laser at 650 nm on normal Achilles tendon tissue(P < 0.05),no significant changes in PWMT and PWTL were found on the side contralateral to laser exposure(P > 0.05).In rats with lasers exposure at 810 nm,915nm and 980 nm,bilateral significantly increased PWMT and PWTL were found 1-4h after laser(P < 0.05).3.No significant changes in the bilateral PWMT and PWTL were found following low level laser at 650 nm on injured Achilles tendon.Bilateral increased PWMT and PWTL were observed 1-4h after 810 nm,915nm and 980 nm lasers treatment(P < 0.05),and returned to baseline level 24 h after lasers therapy for injured Achilles tendon.Bilateral increased PWMT and PWTL were observed 1-2h after laser(810nm,915 nm,980nm)on uninjured Achilles tendon in rats with Achilles tendon injury(P < 0.05).Low level laser at different wavelengths for injured Achilles tendon once per day for 3 consecutive days,and each treatment had the same effect on the mechanical/thermal paw withdrawal reflex.4.Topical injection with different doses of naloxone at injured site can dose-dependently depress the increased PWMT and PWTL induced by low level laser at 980 nm,0.5W on the injured Achilles tendon(P < 0.05).Microinjection of β-FNA into the thalamic VM nucleus dose-dependently depressed the increased PWTL on the uninjured side caused by laser therapy for injured Achilles tendon(P < 0.05).Conclusions: 1.Achilles tendon injury models were successfully established by impact on the Achilles tendon in rats,characterized by weakened endogenous descending inhibition and enhanced facilitation in rats with Achilles tendon injury.2.Laser stimulation at lower energy can significantly relieve pain induced by Achilles tendon injury,and the analgesic efficacy of near-infrared(810nm-980nm)laser is better.3.Consecutive low level laser cause the same analgesic effect following every treatment,and no tolerance can be induced by laser therapy.4.Peripheral opioid receptors and μ-opioid receptors in the thalamic VM nucleus are involved in low level laser induced analgesia for Achilles tendon injury. |
