Thermography in persistent postoperative pain after knee surgery

AIMS AND OBJECTIVES: Background: Postoperative persistent pain occurs in approximately 10% of surgically treated patients. It incapacitates and reduces quality of life in those affected. The aetiology is poorly understood, predictive factors are currently unknown, diagnosis and therapy are difficult. OBJECTIVES: To evaluate the benefit of dermal thermography in the diagnosis and therapy of persistent postoperative pain. STUDY DESIGN: retrospective, Level of evidence IV MATERIALS AND METHODS: A single surgeon performed highly standardized ACL reconstructions, arthroscopies and total endoprotheses of the knee. Patients were seen six weeks postoperative for clinical and thermographic evaluation. Regions of interest were defined for the affected knee, a control region defined for the contralateral knee. The thermographic image was used to pinpoint a site for subcutaneous local anaesthesia. The clinical evaluation was then repeated. RESULTS: Patients in total (n=133), arthroscopy (n=21), total endoprothesis (n=42), ACL (n=50). If the temperature difference (TD) between the site of pathology and the average ipsilateral knee temperature was negative, pathology was classified as hypothermic (n=19), otherwise hyperthermic (n=94). Arthroscopically treated patients showed a TD of 0,88 ± 0,39 °C in hyperthermic (n=12) and -0,811 ± 0,623 °C in hypothermic (n=9) knees. Prosthetically treated patients showed a TD of 1,29 ± 0,51 °C in hyperthermic (n=37) and -0,88 ± 0,33 °C in hypothermic (n=5) knees. ACL treated patients showed a TD of 1,20 ± 0,48 °C in hyperthermic (n=45) and -0,62 ± 0,41 °C in hypothermic (n=5) knees. The difference in temperature between the site of pathology and the average temperature of the ipsilateral knee was significant for all subgroups: arthroscopy-hypothermic p < 0,001, arthroscopy-hypothermic p < 0,005; endoprothesis-hyperthermic p < 0,001, endoprothesis-hypothermic p < 0,004; ACL-hyperthermic p < 0,001, ACL-hypothermic p < 0,027. In contrast to the hyperthermic groups (for all p < 0,001), there was no significant difference between the site of pathology and the contralateral control region in the hypothermic groups. All patients reported a reduction of pain and better mobility after subcutaneous infiltration with Scandicain 2% at the thermographically defined site of pathology. CONCLUSION: Thermography is useful to pinpoint sites of pathology in persistent postoperative pain. Two types of thermal abnormality could be identified at the site of pathology: hypo- and hyperthermic. Local anaesthesia at the site of pathology resulted in pain reduction and improved mobility. Further studies are necessary to understand the postoperative thermal changes in order to devise a suitable therapy.