We analyzed a number of factors that potentially contributed to a history of falls in patients with KOA, including TGS, and clarified which of these had an impact. Only a few studies reported the incidence of falls in individuals with KOA in Japan [36]. To the best of our knowledge, this is the first study to investigate the relationship between history of falls and TGS in older adults with KOA. Of the 5,062 frail older adults in Japan, approximately 30% experienced a fall at least once in a year [37]. Compared to older adults with KOA in Australia (48% fell within 12 months prior to TKA) [4] and in the United Kingdom (24% fell within 3 months prior to TKA) [5], the fall rate 12 months prior to TKA in our study was lower (22.4%). However, according to the publication of vital statistics in Japan, accidental deaths from falls among older adults are on the rise [38]. In addition, a previous study has reported that fall history before performing TKA increased the risk of post-operative falls.4.6 This insight might lead to a better understanding of injury prevention from post-operative falls.
The t-test identified significant differences in TGS and mFES (on the affected and unaffected sides) between the fall and non-fall groups. Adults with KOA with more frequent falls may have a more pronounced fear of falling than those with fewer falls. Tinetti et al. [39] defined fear of falling as "anxiety about falling that causes one to avoid activities of daily living, even though one is capable of performing them”. In their study, fear of falling depended on the history of falls, ranging from 12–65% among community-dwelling older adults without a fall history, and from 29–92% among those with [40]. Therefore, fall history is associated with fear of falling [41]. Our results support those of previous studies [41]. In addition, participants with KOA and a history of falls experienced knee pain, knee instability, and muscle weakness in the lower extremities, which could have also contributed to a greater fear of falling. Post-fall syndrome, in which a loss of self-confidence after a fall leads to a decreased activity level, in turn amplifies the fear of falling.
Multiple logistic regression analysis revealed that TGS on the affected side and mFES were factors related to the presence or absence of falls. A previous study has reported that TGS declines with age [42], resulting in diminished walking ability and static balance, which may be the risk factors for falls. Tsuyuguchi et al. recruited middle-aged adults and provided the age averages of all individuals (62.02) as well as a group subdivision into high and low risk of falls. They found TGS to be an independent risk factor for fall occurrence [43]. However, there is no report on whether reduced TGS in individuals with KOA is associated with the fall itself. Based on the results of our study, we believe that TGS contributes to the challenges faced by older adults with KOA and a fall history. Although a detailed causal relationship is unknown, multiple regression analysis has identified TGS as an independent factor associated with KOA [35]. Conversely, abnormal loading of the knee joint can be caused by changes in the kinematic relationship between the foot and knee [44, 45]. Compared to healthy older adults, those with KOA have lower TGS, and the measured pressure decreases during walking [46, 47]. It is possible that the progression of KOA leads to decreased TGS; conversely, decreased TGS may contribute to KOA progression. However, this causal relationship is unclear; therefore, further studies are required to investigate it. Regardless of the causal pathway, there is interdependence between TGS and KOA which increases the risk of falls. In the future, studies should approach the causal relationship between KOA and TGS from the perspectives of kinesiology and biomechanics, in addition to seeking strategies to prevent falls. Another interesting finding is that an increase in TGS reduced knee pain and knee extensor strength [7], which have been reported to be associated with falls in individuals with KOA. The importance of assessing TGS in addition to existing assessments in individuals with KOA has been emphasized.
In this study, propensity score matching was performed to equalize age before performing logistic regression analysis. As a result, TGS was no longer a relevant factor, and only mFES was extracted as a relevant factor. As expected, age equalization led to the exclusion of TGS from the logistic regression analysis. Age itself was not significantly different between men and women with and without a history of falls. On the other hand, mFES was extracted as an associated factor even after age equalization. Even after adjusting for age, participants with a fall history had fear of falling, while participants without did not.
This study had three major limitations. First, TGS was the only assessment performed on the foot. The degree of flatfoot and range of motion of the foot, which are common problems in individuals with KOA, were not measured. The abnormal foot posture of KOA has excessive first medial tibiofemoral contact force during walking [48]. As these factors are also associated with gait, which is involved in half of all fall scenarios [49], they are likely to contribute to falls. Second, previous studies have reported decreased physical activity [50] as well as hip [51] and ankle [52] weakness as risk factors for falls. Because we did not evaluate these factors and did not include these results in our regression analysis, we could not determine the relative contribution of TGS to physical activity and muscle strength for the above factors in older adults with KOA. Finally, we conducted a cross-sectional observational study; therefore, subsequent cohort and intervention studies should be conducted to better clarify the causal relationships between foot function and falls in older adults with KOA.