BME has previously been noted in the musculoskeletal system in osteoarthrosis and other conditions [1, 4–11, 13, 14, 20]. Felson and colleagues have examined BME and its relation to progression of knee osteoarthrosis and noted that BME is a potent risk factor for structural deterioration in knee osteoarthrosis [1, 7, 17]. Its relation to progression is explained in part by its association with limb alignment. In two of his studies, medial bone marrow lesions were seen mostly in patients with varus limbs and lateral lesions were seen mostly in those with valgus limbs [7, 17].
Link and colleagues  reviewed MR findings in osteoarthrosis and noted that cartilage lesions, BME patterns, and meniscal and ligamentous lesions were frequently demonstrated as MR changes in patients with advanced osteoarthrosis. However, in this study, there was no significant correlation between MR and clinical findings.
A recent study conducted by Kornaat et al.  examined multiple imaging findings and their association with clinical symptoms. Their results suggest that only findings of a large-joint effusion or the presence of an osteophyte in the patellofemoral compartment were associated with pain and/or stiffness. They found no association between BME and symptoms of pain or stiffness.
Three studies have reviewed the MR appearances with histopathology findings [6, 9, 10]. Bergman and colleagues in their study found that subchondral bone marrow changes were present in seven of nine patients undergoing total knee replacement . Histopathologically, those regions showed focal areas where fibrous tissue replaced fatty marrow in the subchondral trabecular space. Zanetti and colleagues reviewed the histopathological findings in 16 patients who had MR before total knee replacement. The BME pattern consisted of normal tissue and a smaller proportion of several abnormalities including bone marrow necrosis, bone marrow fibrosis, abnormal trabeculae, BME, and bone marrow bleeding. They concluded that a BME pattern in knees with osteoarthrosis represents a number of non-characteristic histological abnormalities .
Nolte–Ernstein and colleagues examined the correlation between MR and histological findings of degenerative bone marrow lesions in experimental osteoartrhosis models in canine knee joints . In these experimental lesions, the histopathology revealed 21 osteosclerotic lesions and 5 intraosseous cysts. Histopathological findings showed different degrees of osteosclerosis associated with bone marrow degeneration. Cystic lesions were of two types: subchondral epiphyseal cysts and synovial cysts within a large tibial osteophyte. High signal intensity on T2-weighted images and decreased signal intensity on T1-weighted images indicated high fluid content.
None of these prior studies specifically looked at the BME pattern on MR. Our classification of the patterns into global, focal, cystic, and absence of edema is an attempt to subdivide the presence or absence of edema in osteoarthrosis. However, this attempt is limited by the absence of histopathological findings.
We were surprised by the significantly increased risk of knee joint replacement with the global pattern of BME in relation to the other patterns. It appears that the more extensive and intense the BME, the more likely it is for the patient to have symptoms. The global pattern of BME was the best predictor of risk of TKA within 3 years, as those subjects with the global pattern were over five times as likely to receive a TKA when compared to those with any of the other patterns and over 13 times as likely to have a TKA when compared with subjects with no BME, after accounting for the age difference.
We were also surprised by the lack of association between cartilage loss and the likelihood of total knee replacement. Intuitively, one would think that the greater the cartilage loss, the more likely the possibility of total knee replacement. However, Link and colleagues  noted that clinical findings showed no significant correlation with the extent of cartilage loss on MR imaging. In this study, the lowest scores for pain and function loss were found in patients without cartilage lesions. However, the highest scores for pain, stiffness, and function loss were found in patients with less than 50% cartilage loss. The lowest scores for stiffness were found in patients with more than 50% cartilage loss and full-thickness lesions. These findings further support the theory that cartilage loss may not be the primary source of pain in patients with OA of the knee.
We did find, in the medial compartment only, that subjects with high grades of cartilage loss or advanced degenerative changes on radiographs tended to have some degree of BME. The lack of association between cartilage loss and the likelihood of TKA in our study could be a reflection of the limitations of the Noyes classification system. However, other investigators have also found that radiographic signs of cartilage loss may not relate to the degree of clinical symptoms. As mentioned above, Link and colleagues found no correlation with the extent of cartilage loss on MR and clinical findings. In their study, they also used a modified Noyes classification system . Kornaat et al. also found no correlation between the extent of cartilage damage seen on MR imaging and clinical symptoms of pain and stiffness. They used a grading scale based on the maximum diameter of cartilage defects and the depth of the lesion based on the percentage of cartilage loss .
In clinical practice, OA is associated with the radiographic findings of joint space loss and subchondral sclerosis, presumably secondary to cartilage loss that results in pain. However, as stated earlier, there are no pain fibers in hyaline cartilage . Furthermore, many patients have pain out of proportion to their radiographic findings. Our study may provide an insight into another possible mechanism for pain production in OA of the knee. While not every patient with OA warrants an MR scan, those patients without the classic presentation may benefit.
We did find that patients who had a TKA were 12.6 years older than those who did not have a TKA. Patients who are older are more likely to have OA and, in particular, more severe OA. Surgical replacement of the knee is more likely to occur in an older patient with OA than a younger patient with the same severity of OA. After further statistical analysis, we however found that our BME results were still significant despite the differences in age.
This paper has several limitations. Most patients who have a TKA do not get a pre-operative MR, and therefore, we may be dealing with a pre-selected population. What we have called “BME” on MR may not be true edema but may relate to other histologic findings as noted earlier. Our numbers are also small, and we had to group some of our subsets together for statistical analysis. Only one musculoskeletal radiologist was involved in the review of the radiology. There was also no arthroscopic correlation for cartilage defects or presence of meniscal tears. Our study is limited by the fact that it is retrospective. To generate a group of MR scans to review, we had to use a keyword search to identify potential subjects. We would have obviously missed all scans where the specific terminology “bone edema” or knee “osteoarthrosis/osteoarthritis” was not utilized. However, we feel that this is a relatively minor limitation, as we were able to generate 381 cases for review. Another potential limitation in the retrospective study design is the possibility of missing subjects who received follow-up at a different institution or those whose symptoms resolved without treatment.
In summary, we reviewed a series of knees in patients with osteoarthrosis and evaluated the pattern of BME, cartilage loss, radiographic findings when available, and the incidence of total knee joint replacement within a 3-year follow-up. Subjects with any bone marrow edema pattern were more likely to have a total knee joint replacement compared to subjects with no bone marrow edema. The worst prognostic pattern was the global pattern of bone marrow edema. Subjects who had a total knee replacement were also older than those who did not. However, even allowing for age, the global pattern of BME remained the variable with the highest statistically significant association with the incidence of total knee replacement.