To the best of our knowledge, this is the first study to investigate the effect of PEMF on subchondral bone degeneration at the early stage of TMJOA and explored its possible molecular mechanism.
OA is one of the most common joint disorders worldwide[27]. Mechanical stresses on TMJ that originated from daily functional activity such as talking, chewing, clenching and grinding, are thought to be significant for remodeling of the mandibular condyles[28]. Joint tissues are exquisitely sensitive to their mechanical environment, and mechanical loading may be the most important external factor regulating the development and long-term maintenance of joint tissues[29]. Aberrant biomechanical stimulation from abnormal dental occlusion plays an important role in TMJOA process[30]. In the present study, diffuse subchondral bone loss, enlargement of bone marrow cavity, rough and uneven surface of subchondral bone were observed in UAC group, accompanied with decreased BV/TV and Tb.Th, and increased BS/BV and Tb.Sp, especially at 6W time-point after modeling, which were consistent with our previous study[30]. Therefore, it is an effective animal model to evaluate the effect of different intervention factors on the prognosis of the early subchondral bone resorption during TMJOA process.
In literature, substantial studies have demonstrated that PEMF exposure has the potential to be a promising therapeutic method to promote fresh/delayed fracture healing and inhibit bone loss caused by disuse or estrogen deficiency with no observable side effects[31]. To examine the effects of PEMF therapy on symptoms and quality of life in patients with OA, a meta-analysis on 15 clinical studies was conducted by Yang XT et al. in 2020, which concluded that there was a beneficial effect of PEMF therapy on pain, stiffness, and physical function in patients with OA[32]. Appropriate PEMF exposure (75 Hz, 1.6 mT, 2 h/d, 4 W) can preserve the structural integrity of subchondral bone in rats knee OA induced by low-dose monosodium iodoacetate (MIA) injection by activating OPG/RANKL/RANK signaling[33]. In addition, appropriate PEMF exposure (8 Hz, 3.8 mT, 40 min/d, 4 d) can substantially reduce the number of osteoclast-like cells in the culture with M-CSF + RANKL, and subsequent bone resorption, at least partially, through NFATc1, CAII and RANK[34]. However, it is still unknown whether appropriate PEMF exposure could effectively reverse condylar subchondral bone loss during TMJOA process. In the present study, the abnormal bone resorption, the increased number of osteoclasts, the decreased expression of osteogenesis-related factors, the increased expression of osteoclast-related factors and OPG/RANKL ratio induced by UAC, were reversed by PEMF exposure (2mT, 15Hz, 2h/d, 3 or 6 W) to the normal level similar to that of CON group. It is suggested that abnormal subchondral bone remodeling at the early stage of TMJOA could also be reversed effectively by PEMF, similar to other large joints of limbs, although TMJ is different from them in anatomy. The present study offers an exciting new avenue to interfere with the early progress of TMJOA.
It is well known that estrogen play an important role in maintaining bone homeostasis, and estrogen supplement may be beneficial to patients with early OA or postmenopausal osteoporotic osteoarthritis[35]. In the present study, similar to the effect of PEMF exposure, estrogen supplement can also reverse the abnormal bone resorption, the increased number of osteoclasts, the decreased expression of osteogenesis-related factors, the increased expression of osteoclast-related factors and OPG/RANKL ratio induced by UAC, which was consistent with our previous study[25]. It is indicated that the effect of PEMF exposure is similar to that of high physiological concentration of estrogen. Unfortunately, estrogen replacement therapy is reported to be associated with a higher risk for breast, endometrial and ovarian cancer as well as cardiovascular disease and stroke[36].Taken this side effect into consideration, appropriate PEMF exposure is a better way to reverse abnormal subchondral bone remodeling at the early stage of TMJOA. Interestingly, in the present study, the combination of E2 supplement and PEMF exposure not only reversed the effect of estrogen on the body weight of rats at the early stage of TMJOA, but also further increased the ratio of OPG/RANKL, the expression of some osteogenesis-related factors and Tp.Sp of micro-CT at 6 W. It is suggested that E2 supplement and PEMF exposure may have a synergistic effect to some extent, which can further improve the osteogenic potential of subchondral bone in TMJOA.
Considerable evidence supports the crucial role of canonical Wnt signaling pathway (β-Catenin-dependent) in regulating bone mass and architecture and maintaining bone homeostasis[18][37]. Wnt/β-Catenin signal promotes osteogenic activity by stimulating the proliferation and differentiation of osteoblasts[38]. In 2016, Zhai M et al. found that PEMF exposure (2 mT, 15.38 Hz, 2 h/day) can increase expressions of ALP and OCN at the differentiation phase, then upregulated expressions of COL1A1, Runx2 and Wnt/β-Catenin signaling (Wnt1, LRP6, and β-Catenin) at proliferation and differentiation phases in osteoblasts[20]. Similarly, Fathi E et al. found that ZnSO4, in the presence of PEMF exposure (20mT, 50Hz, 30min/day, 21 days), resulted in an increase in the expression of osteogenic genes (OCN, Runx2 and BMP2), ALP activity and calcium levels during osteogenic differentiation of adipose tissue-derived mesenchymal stem cells (ADSCs), accompanied with increased expressions of β-Catenin, Wnt1, Wnt3a, LRP5 and DKK1[39]. In the present study in vivo and in vitro, we found that PEMF can also up-regulate Wnt/β-Catenin signal pathway, promoting the entry of β-Catenin into the nucleus and inhibiting GSK3β activity. It is suggested that Wnt/β-Catenin signal pathway is involvled in the effect of PEMF exposure in reversing abnormal subchondral bone remodeling at the early stage of TMJOA.