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_Article_  
Potential Instability and Malfunction of Knee Joints with  
Vastus Medialis Impairment after Total Knee Arthroplasty  
[PERSON] 1,\({}^{\dagger}\), [PERSON] 1,2,\({}^{\dagger}\), [PERSON] 3,4, [PERSON] 5, [PERSON] 6, [PERSON] 3,  
[PERSON] 7,\({}^{\ddagger}\) and [PERSON] 1,4,\({}^{\ast}\),\({}^{\copyright}\)  
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Replacement Registry reported that TKAs were performed for approximately 0.5 million people in the USA in 2016 and will be performed in approximately 3 million people annually worldwide after 2030 [3; 4; 5]. However, TKA continues to exhibit failures generally related to pain, infection, osteolysis, wear, aseptic loosening, periprosthetic fracture, instability, or malalignment [5]. Additionally, muscle impairment (weakness) occasionally influences the quality of life after TKA, resulting in direct or indirect TKA failure with adverse prognosis and satisfaction [6; 7; 8; 9; 10]. In particular, quadriceps impairment is considered the main type of muscle impairment in patients who have undergone TKA [11; 12; 13; 14].  
Many researchers have investigated quadriceps impairment in patients who have undergone TKA [11; 12; 13; 14; 15; 16; 17; 18]. They identified that the extension moment was generally 20% lower in knee joints after TKA than in knee joints that had not undergone TKA. Furthermore, [PERSON] et al. reported that the TKA-related quadriceps impairment produced an asymmetry extension moment, which limited the physical function of the knee joint [3]. [PERSON] et al., [PERSON] et al., and [PERSON] et al. have reported that extensor impairment caused by quadriceps impairment led to a greater risk of posterior instability in knee joints that had undergone TKA [9; 19; 20]. Furthermore, [PERSON] et al. reported that patients with quadriceps impairment had a high risk of hyperextension after TKA because they tended to lock the knee during the extension phase of gait to compensate for a weak extensor mechanism and to avoid instability. This approach led to unreliable results with respect to stability and functional outcomes [8]. Previous clinical studies showed that TKA instability could result from quadriceps impairment. In particular, quadriceps impairment frequently involved the vastus medialis (VM) [21; 22]. Quadriceps impairment presumably induces undesirable joint reaction forces and moments due to changes in the force direction and magnitude of the quadriceps in knee joints that have undergone TKA [23; 24]. To our knowledge, there is minimal information concerning the mechanisms of instability and malfunction due to VM impairment in knee joints that have undergone TKA.  
Changes in the force direction and magnitude of quadriceps by VM impairment may induce changes in the vectors of the joint reaction force and moment, resulting in instability and malfunction within knee joints that have undergone TKA. Thus, the current study was performed to elucidate the mechanisms of potential instability and malfunction due to VM impairment in knee joints that have undergone TKA through analysis of the characteristics of joint reaction forces and moments in knee joints with VM impairment after TKA.