In cross-sectional analyses, we firstly demonstrated that subchondral BML size at the medial femorotibial joint compartment in varus knee OA participants was associated with weight-bearing pain severity, and that this association was not explained by non-weight-bearing pain, other OA related MRI features, age, sex or BMI. In longitudinal analyses, increasing or decreasing subchondral BML size at the medial femorotibial joint compartment was associated with increased or decreased weight-bearing pain severity. Our findings support the hypothesis that subchondral BMLs increase OA knee pain due to biomechanical factors acting through the affected subchondral bone.
Greater BML scores at medial femorotibial compartments were significantly associated with greater weight-bearing pain, especially pain on walking and standing, even after adjusting for non-weight-bearing pain and the other OA related MRI features. The increase or decrease in BML score at the medial femorotibial compartment was significantly associated with increased or decreased weight-bearing pain severity. In contrast to these findings, BML scores were not significantly associated with non-weight-bearing pain in our fully adjusted models. Hence, we conclude that subchondral BMLs at weight-bearing components are specifically associated with weight-bearing pain. We previously demonstrated that subchondral bone histopathology characteristic of subchondral BMLs, occurring in middle third of medial tibial plateau (an important weight-bearing area), was associated with knee OA pain, not dependent on chondropathy and synovitis .
In this study, subchondral BMLs at the lateral patellofemoral joint compartment were also associated with weight-bearing pain, especially pain on climbing. The associations of patellar BMLs with any knee pain, and with pain when going up or down stairs have been reported . The patellofemoral joint is one of the most commonly affected compartments in knee OA, and varus knee deformity has been associated with worsening of patellofemoral OA, especially in the lateral facet . We extend these findings to identify that subchondral BMLs at the lateral, and not at medial patellofemoral joint compartments were significantly associated with the severity of weight-bearing pain in people with varus angulation. This again suggests a biomechanical explanation linking subchondral BMLs to OA pain. However, patellofemoral BML score change was not associated with pain change, suggesting that subchondral BML changes at the patellofemoral joint has a smaller impact on changes in weight-bearing pain than does subchondral BML change at the femorotibial joint.
Subchondral BMLs represent regions of subchondral bone characterized as displaying increased bone turnover, and expression of factors, including pro-inflammatory cytokines, that can increase nerve sensitization . We previously showed that nerve growth factor expression within osteochondral channels, and subchondral osteoclast density, each is associated with knee OA pain , and calcitonin gene-related peptide immunoreactive sensory nerves within osteochondral channels are associated with pain in human and rat knee OA . Activation of sensory nerves in subchondral bone might contribute to weight-bearing pain in knee OA. Subchondral BMLs might be an imaging biomarker for pathology which sensitises subchondral nerves, and therefore increases weight-bearing pain in knee OA. Further investigation is needed to clarify the cellular and molecular factors which might mediate the observed association between subchondral BMLs and weight-bearing pain.
Our results suggested that subchondral BMLs might mediate mechanically-induced pain such as during weight-bearing. Biomechanical factors may reciprocally contribute to the pathogenesis of subchondral BMLs. Previous studies  reported that increased mechanical load due to malalignment of the knee joint is a risk factor for incident or enlarging subchondral BMLs in the femorotibial joint. Greater BMI that increases the mechanical load on the knee was also associated with increased BMLs size . History of knee injury (e.g. ACL rupture) has been associated with tibiofemoral BMLs . Biomechanical alterations were persistent even after ACL reconstruction , and therefore the altered biomechanical forces might be involved in incident or enlarging subchondral BMLs. Meniscus and cartilage can be shock absorbers that protect subchondral bone from overloading. Associations between meniscal pathology [19, 20] or cartilage loss [19, 21] and increasing subchondral BML size might be mediated by altered biomechanical forces through the subchondral bone. Some have suggested that strenuous physical activity might increase subchondral BML size by repetitive mechanical load on the knee. Strenuous physical activity was associated with increased subchondral BML size , but, on the other hand, non-strenuous activity was negatively associated with subchondral BML size [19, 22]. Different levels of physical activity may have different influences on subchondral bone.
Biomechanical unloading by patellofemoral bracing reduced subchondral BML volume at patellofemoral joint compartments in patients with painful patellofemoral OA . However, lateral wedge insoles, which might reduce the load in the medial femorotibial joint compartment during walking , did not significantly changes in subchondral BMLs and pain in medial knee OA , and the extent of biomechanical unloading that is required to reverse subchondral BML pathology remains uncertain. Treatments with large effects on mechanical load such as high tibial osteotomy and valgus bracing might be needed to reduce subchondral BMLs at medial femorotibial joint compartments.
Pharmacotherapy also has potential to reduce subchondral BMLs. The bisphosphonate zoledronic acid reduced subchondral BML size and knee pain in people with OA . However, recent meta-analysis  and randomized controlled trial  did not support analgesic effects of bisphosphonates in knee OA. These studies support that biomechanical unloading is more effective than current pharmacotherapy for subchondral BML associated pain. Knee OA pain has multiple sources, and targeting subchondral BMLs would be expected to have greatest benefit in those cases where subchondral BMLs are the predominant driver of weight-bearing pain.
Subchondral BMLs are also found in healthy pain-free young-middle aged adults, with a prevalence of 13-17 % [19, 29, 30], which is less than that in symptomatic knee OA. Previous study using animal OA models reported that the first sign of change in the development of OA was subchondral BML which preceded cartilage degeneration . These findings suggest that subchondral BMLs may be involved in the pathogenesis of knee OA before it becomes clinically apparent.
In this study, medial meniscus extrusion score was significantly associated with non-weight-bearing pain severity after adjusting for weight-bearing pain and the other OA-related MRI changes. A recent study reported that patients with medial submeniscal flap tear complained of pains during sleep, but not during daytime activities . Pain associated with meniscal tears might be caused by increased mechanical load on meniscus not only during knee loading but also during unloaded knee flexion-extension motion. However, change in meniscus extrusion score was not associated with change in non-weight bearing pain, which suggests that meniscus extrusion may have a relative small impact on non-weight-bearing pain.
Not only subchondral BMLs, but also synovitis , effusion-synovitis , cartilage defects  and osteophytes  have previously been associated with knee OA pain. We also demonstrated that subchondral BML, osteophyte and effusion scores were independently associated with total pain score, even after adjusting for the other OA related MRI features (Supplemental Table). Associations of pain with cartilage and Hoffa’s synovitis scores did not retain statistical significance after adjusting for the other OA related MRI features including BML score, suggesting that some of these observed associations may be explained by other closely associated OA features (Supplemental Table).
Our study has several potential limitations. We investigated only 2 time points (baseline and 24 months). A previous study using OAI dataset demonstrated that changes in total subchondral BML volume after 24 months were positively associated with changes in knee pain severity . However, multiple assessments with shorter intervals are needed to evaluate how fluctuations of subchondral BML size relate to changes in weight-bearing pain. Subchondral BML size was assessed semi-quantitatively and, although our study had a large sample size, quantitative BML measurements might have provided additional information. Direct measurement of mechanical loading through joint compartments was not possible in the current study, and interventional studies would be required to confirm our conclusions that subchondral BMLs are biomarkers for pathology which causes biomechanically-induced nociceptive pain.