Patellar skin surface temperature by thermography reflects knee osteoarthritis severity

Abstract

Background: Digital infrared thermal imaging is a means of measuring the heat radiated from the skin surface. Our goal was to develop and assess the reproducibility of serial infrared measurements of the knee and to assess the association of knee temperature by region of interest with radiographic severity of knee Osteoarthritis (rOA).

Methods: A total of 30 women (15 Cases with symptomatic knee OA and 15 age-matched Controls without knee pain or knee OA) participated in this study. Infrared imaging was performed with a Meditherm Med2000™ Pro infrared camera. The reproducibility of infrared imaging of the knee was evaluated through determination of intraclass correlation coefficients (ICCs) for temperature measurements from two images performed 6 months apart in Controls whose knee status was not expected to change. The average cutaneous temperature for each of five knee regions of interest was extracted using WinTes software. Knee x-rays were scored for severity of rOA based on the global Kellgren-Lawrence grading scale.

Results: The knee infrared thermal imaging procedure used here demonstrated long-term reproducibility with high ICCs (0.50-0.72 for the various regions of interest) in Controls. Cutaneous temperature of the patella (knee cap) yielded a significant correlation with severity of knee rOA (R = 0.594, P = 0.02).

Conclusion: The skin temperature of the patellar region correlated with x-ray severity of knee OA. This method of infrared knee imaging is reliable and as an objective measure of a sign of inflammation, temperature, indicates an interrelationship of inflammation and structural knee rOA damage.

Keywords: inflammation; infrared imaging; knee; osteoarthritis; thermography.

Figure 1.

Infrared image analysis. To identify regions of interest on the infrared thermal images, it was necessary to localize the patella. For this purpose, two knee images were acquired, one without and subsequently one with a 2 cm anatomic marker (silver sticker) applied to the center of the patella (shown on left). This marker blocked temperature radiation and was readily recognized on the thermal images as a black circle (shown on right). This image facilitated alignment of an acetate diagram centered on the patella in order to quantify mean temperatures for specific regions of interest: P-patellar; M-medial; L-lateral; LM-lower medial; LL-lower lateral; and the overall knee.

Figure 2.

Patellar temperature and radiographic OA. Whisker and box plots depict the mean, 25th and 75th percentiles, minimum and maximum temperatures in the two osteoarthritis subgroups, Kellgren-Lawrence (KL2 and KL3) for the patellar region of interest.