Objective      To explore the histological findings of the focal subchondral bone marrow lesions (BMLs) of the knee joint detected by MRI.       Methods      From September 2013 to February 2015

the knee osteoarthritis patients (aged 40~70) referred for total knee arthroplasty were examined with sagittal short inversion time inversion-recovery (STIR) and T1- and T2-weighted turbo spin-echo MRI imaging before surgery. MRI detected the subchondral BMLs (hyperintense on STIR images and T2-weighted MR images

and hypointense on T1-weighted MR images) at the non-weight-bearing area in 10 patients (3 males and 7 females). Five specimens were obtained from 5 patients (2 males and 3 females) with the mean age of 61±6.1 (ranged from 53 to 67) in the surgery. All the sections were stained with HE and Safranin O/fast green for observing the pathological features. The percentage of various tissue areas and the trabecular bone volume (BV/TV) were measured by Image Pro Plus 6.0 imaging system.       Results      Five BMLs included 2 cases involved in the patella

2 in anterior femoral condyle and 1 in posterior femoral condyle. All the acetabular components were well fixed by the radiograph imaging.Ⅰ~Ⅳ cartilage lesions were found at the BMLs. Cystic change was found in 1 case of patella and the others were cancellous bone. Histological appearance contained 1 case of bone cyst

and the rest 4 cases were consisted of normal tissues (76.9%) and several abnormalities (23.1%). Trabecular bone volume (BV/TV) of the 4 cases was 26.7%. Compared with the control regions

significant differences were found in the bone marrow necrosis (t=-4.0

P=0.029)

trabeculae abnormalities (t=-4.0

P=0.028) and BV/TV (t=6.4

P=0.008).       Conclusion      Focal subchondral BMLs at the non-weight-bearing area of the knee joint represent a number of noncharacteristic histological abnormalities

and have some common features

especially the cartilage lesions and decrease of trabeculae.