## 4 Discussion  
In this study, we demonstrated that intramuscular insertion of PDO filaments into the quadriceps using MEST relieves OA-induced pain and mobility limitations in vivo. Through the experiment, we also confirmed that MEST restored the reductions in RFM mass and local amounts of collagen and IGF-1 levels in OA animals without an inflammatory response. However, this recovery was not accompanied by structural changes in the knee OA. These results strongly suggest that the MEST application method for OA is very effective in reducing pain and improving athletic performance. For many years, various types of threads made of PDO have been used for surgery and, more recently, for skin rejuvenation [19]. In particular, cog threads are widely used for cosmetic purposes, because they have barbs that hook and hold the tissue underneath the skin to lift sagging tissue [24; 25]. In this study, we expanded the scope of their current  
Figure 6: MEST does not affect the joint structure. Representative images of the knee joint on day 56 (**A**). The severe joint lesion was obvious in the MIA-injected knee; however, the structural improvement using MEST was inconsistent across individual animals via gross observation. Yellow arrows indicate femoral conodyles and tibial plateau. Representative micro-CT images of the knee joint (**B**) and micro-CT analysis of the proximal tibial epiphysis (**C–G**) on day 56. BV, bone volume; BV/TV, bone volume/total volume, Tb.N, trabecular number; Tb.Sp, trabecular separation; BMD, bone mineral density. **\(p\) < 0.01, ***\(p\) < 0.001. #\(p\) < 0.05, ***\(p\) < 0.001. Data are presented as mean \(\pm\) 95% confidence intervals. applications to muscles and related joint diseases using MEST, which allow safe and simple intramuscular delivery of cog PDO filaments. To hold skeletal muscles effectively, various factors related to the filament (e.g., thickness, length, and degree of flexibility), equipped cogs (e.g., length, angle, spatial distribution, and direction), and delivery methods (e.g., cog protection during insertion, insertion angle, and insertion points) were carefully considered based on preliminary trials (data not shown). For instance, if the cogs are too long or too short, they become too flexible to hook and support muscles and will not be able to hook muscles efficiently. Moreover, it is crucial to keep the cogs in the opposite direction of insertion to avoid ruining during needle insertion using a protective catheter (Figure 1), because only well-preserved bi-directional cogs prevent filaments from moving around by providing firm anchoring to the tissue to avoid thread exposure or other unexpected serious side effects. MEST technique may work against the forces of vectors that generate tension and reconstruct tissue and provide efficient tension to the muscles, as suggested in skin rejuvenation technique using absorbable sutures [26; 27]. Our behavioral tests results showed that a single treatment of MEST reversed MIA-induced mechanical pain in a week and maintained it throughout the entire experiment. In addition, decreased locomotor and exploratory activities and fall latency from rotarod, which reflect the existence of pain [28] and/or motor impairment [29], were successfully reversed. The changes in weight, nucleus density, and fiber thickness of RFM by MEST and no inflammatory response indicate the recovery of knee OA-induced quadriceps atrophy. MEST provides evidence of pain relief and improved mobility in OA. Although MEST administration in quadriceps did not bring a fundamental change in OA joint, the absence of structural improvement of OA joint was not surprising, as pain and loss of function are weakly correlated to the severity of OA by radiography in some cases [30]. Quadriceps weakness, which is commonly observed in patients with knee OA and considered a risk factor for the development of knee OA [5; 6; 7; 8], may reduce the stability of the knee joint and the ability to absorb the shock transmitted to the joint [7; 8; 31; 32].