Background

The rehabilitation period following total-knee arthroplasty (TKA) is characterized by pain, edema, and limited range of motion (ROM). Infrared Thermal Imaging is a noninvasive technology used here for measuring spatial distribution of skin temperature.

Objective

This study assesses the correlation between thermal imaging and clinical outcomes following TKA.

Methods

This was a cross-sectional study of 81 patients who underwent thermal imaging at 4 time points from TKA (ethics-approval-#0251-20-HMO): (1) prior to TKA ([pre-TKA], n=20), (2) 2-weeks post-TKA (n=20), (3) 6 weeks post-TKA (n=20), and (4) 1 year post-TKA (n=21). Thermal images were captured and skin temperature measured with a FLIR-One-Pro thermal camera and software (FLIR Systems, Wilsonville, OR). Normal thermal distribution over the knees was measured in non-operated controls (n=13). A difference of =1°C between knees was considered pathological. Knee ROM on extension and flexion were measured by digital goniometry. Edema was assessed by measuring leg circumference around the patella (?circumference). Skin temperature by thermal imaging was correlated with edema and range of motion.

Results

A pathological increase in skin temperature of the operated-vs-non-operated knee was detected in all patients at 2 and 6 weeks following TKA, but only in 4 of 21 (19%) patients 1 year after TKA. In controls and pre-TKA patients, pathological increases in temperature were detected in only 1 of 20 (5%) and 1 of 13 (8%) respectively (p<0.001, by Chi square contingency). The difference in skin temperature between knees was significant across all patients but was significantly higher at 2 and 6 weeks post-TKA compared to pre-TKA and 1y post-TKA (Pre-TKA=0.3±0.5°C; 2-weeks post-TKA=2.5±0.7°C; 6-weeks post-TKA=2.4±0.7°C; 1y post-TKA=0.6±0.7°C, p<0.05 by mixed-design ANOVA with Bonferroni). Edema, extension, and flexion correlated positively with the skin temperature (r=0.66, r=0.62, and r=0.52 respectively, p<0.001 by linear regression).

Conclusions

Thermal imaging of knees correlates significantly with extent of edema and limitation of ROM after TKA. Further studies using advanced thermal properties are in progress to refine the assessment of these and other clinical outcomes.