To analyse the correlation between varus deformity of knee osteoarthritis (KOA) and area of low-density region under tibial plateau on the basis of thermography.

Imaging and clinical data of 406 patients (594 knees) diagnosed with knee osteoarthritis, from June 2020 to April 2021, in our hospital were retrospectively analysed. The inclusion criteria were: those who met the diagnostic criteria in X-ray images and clinical manifestations of KOA; age ≥50 years old; X-ray showed a normal mechanical alignment or otherwise a deformity of genu varum. Exclusion criteria were: genu valgus deformity, incomplete imaging data, those had been taking drugs that affected bone metabolism in last six months and alignment of lower limbs was affected by non-articular deformity. Alignments of lower limbs were measured according to full-length lower limb X-ray in weight-bearing position. The patients were divided into three groups according to the lower limb alignments. The patients were assigned in the normal group while the alignment of lower limb was found across the tibial intercondylar eminence, those in the mild varus group had the alignment of lower limb across the medial tibial plateau and those in the severe varus group were found that the alignment of lower limb extended beyond the medial tibial plateau. A retrospective analysis was performed to compare the age, body mass index (BMI) and other indicators among the three groups. Anteroposterior weight-bearing X-ray images of the knees in three groups were imported into E-3D software to extract the contour of the low-density region under tibial plateau. Area within the contour was then measured and calculated by Image-Pro Plus 6.0 software. The contours were then mapped with the standard knee models and the locations and areas of the low-density regions under tibial plateaux between the three groups were compared by one-way ANOVA.

Of the 406 patients (594 knees), there were 12 males (16 knees) and 394 females (578 knees), including 188 bilateral knees and 218 single knees. The age of the patients was (69±8) years. There were 66 knees in the normal group, 351 knees in the mild varus group and 177 knees in the severe varus group. Comparison of general data of the patients showed that there was no statistically significant difference in age and BMI among the three groups (all P>0.05). Thermographs confirmed that, in the normal group, the low-density region under tibial plateau was located in the middle under the tibial plateau with a smallest size of low-density region in the three groups. Low-density region in the severe varus group was closer to the medial tibial cortical bone and with the largest size than the other two groups. The average area in the normal group was found at (6 568±519) mm², (7 266±577) mm² in the mild varus group and (8 240±485) mm² in the severe varus group. The differences between the three groups were statistically significant (F=293.140, P<0.001).

Low-density region under tibial plateau shown by an X-ray image is an important risk factor of the severity of genu varus deformity in patients with knee osteoarthritis. The region of low-density under tibial plateau is positively correlated with the severity of genu varus deformity in patients with knee osteoarthritis.