The present study demonstrated that the knee valgus angle at initial foot contact can be reduced by the medial arch support of foot orthoses during cutting manoeuvres. In addition, a higher medial arch support resulted in an increased reduction of the knee valgus angle at initial contact. Motion analysis of cutting manoeuvres may be useful in a study of ACL injury mechanisms. To our knowledge, however, there are no previous studies in the literature that have attempted to clarify these injury mechanisms.
Many studies have attempted to improve knee kinematics with the use of foot orthoses. For medial osteoarthritis, the use of a lateral (valgus) wedge orthosis has been reported to reduce the load on the medial compartment and help reduce pain in the medial knee joint [37–39]. Similarly, the use of a medial wedge orthosis can improve symptoms for cases with lateral osteoarthritis [40]. In this study, we speculated that by increasing the longitudinal arch of the foot using the arch support of foot orthoses, the hindfoot valgus angle during cutting manoeuvres would decrease, and the knee valgus angle would decrease accordingly. The results of this study showed that the angle of knee valgus during initial foot contact decreased with the use of foot orthoses with a high medial arch. This may potentially reduce the risk of ACL injury in sports activities caused by knee valgus and reduce the risk of recurrence after postoperative ACL rehabilitation.
The present study demonstrated that foot orthoses reduced the knee valgus angle only at initial foot contact and did not influence the kinematics of the knee joint when the ground reaction force was at its maximum. Therefore, unlike at the moment of initial foot contact, the medial support of foot orthoses may not be effective enough with larger loads. On the other hand, the period between initial foot contact and maximum ground reaction force was around 40 milliseconds in almost all cases in our study. In a study by Koga et al. [9] on the mechanism of ACL injury, ACL tears were reported to occur within 40 milliseconds after initial foot contact. Taking this mechanism into consideration, it is important to understand how to reduce the knee valgus up to 40 milliseconds, and the reduction of knee valgus angle at initial foot contact may be effective in reducing the risk of ACL injury.
To our knowledge, few studies have demonstrated a significant difference of knee kinematics between various types of foot orthoses. Although there have been several studies that examine how foot orthoses affects knee kinematics, it is difficult to detect significant differences in their effects on knee kinematics. Wahmkow et al. [41] evaluated the kinematics of the knee and lower leg when wearing foot orthoses during walking. A comparison of kinematics with and without foot orthoses showed no significant difference between them. The authors suggested that the differences in kinematics per movement between patients were large compared to the small differences in whether a foot orthosis is used, which made it difficult to detect significant differences. Elsewhere, Christopher et al. [42] analysed how knee joint kinematics changed with the use of foot orthoses during landing movements. There was also no significant difference in knee kinematics between those with and without foot orthoses in this study. The authors assessed that several factors were likely to have accounted for the results, including the limitation of a static measure to predict dynamic movement, inter-subject variability, and the physical characteristics of the orthotic device. The absolute value of the knee valgus angle is smaller than that of flexion-extension in a general motion analysis study, and there is a great deal of variation from case to case. Imwalle et al. [43] reported that kinematics of the knee in a cutting manoeuvre showed a large standard deviation of varus and valgus angle. As there is only a small variation in terms of arch support for foot orthoses, finding a significant difference between commercially available foot orthoses would be difficult; therefore, the present study included a foot orthosis with normal height in addition to an overcorrected double-height orthosis.
The results of this study show that the difference in the angle of knee valgus at initial foot contact between orthoses with low and mid medial arch support height is miniscule (0.2 degrees on average). On the other hand, the mean difference between the mid and high arch support height is as large as 1.7 degrees. Based on this result, we found that the foot orthosis with a normal arch support height caused a small reduction in the knee valgus angle, while the foot orthosis with a greater arch support height caused a larger reduction in the knee valgus angle. In general, the foot has a physiological longitudinal arch, and foot orthoses with an arch support of comparable height to physiological height do not have a significant effect on knee kinematics. We believe that foot orthoses with a higher arch support than the physiological height will have a greater impact on the kinematics of the knee.
In the present study, a higher medial arch resulted in a significant reduction of knee valgus angle at initial foot contact. However, foot orthoses with double the normal height were quite uncomfortable for the feet. In fact, most of the subjects complained of discomfort in the medial part of the foot. Su et al. [44] stated that the selection of suitable materials and support designs for orthopaedic foot orthoses may improve the correction of foot arch height but can also result in excessive stress on the joint and ligaments. Foot orthoses should improve the correction of arch height and simultaneously reduce the stress on tissues of the foot; therefore, we believe that orthoses with higher arch support can be worn for a controlled period of time during post-operative ACL rehabilitation but may cause pain and disability in the foot when used for actual sports activities. It remains to be seen what level of arch support will enable the correction of the knee valgus angle without causing foot problems.
Neuromuscular and strength training have been reported to be effective in reducing the risk of ACL injuries [12–18]. While this requires a lot of time and effort, reducing the risk of ACL injury with foot orthoses is a simple, inexpensive, quick, and accessible prophylactic strategy. If foot orthoses are used in conjunction with other treatments such as neuromuscular training, it may be more effective in reducing the risk of ACL injury. Based on our findings, the use of foot orthoses with the highest possible arch support without causing discomfort to the feet may be the optimal solution.
A high valgus knee moment and a small flexion angle at initial foot contact are reported to be risks for ACL injury [11, 45–49]. It has also been reported that a greater distance between the trunk and the centre of the ankle joint (COM distance) at initial foot contact can generate a greater risk [22, 50]. In the current study, these were also assessed at the same time, but there were no significant differences between foot orthoses. These values also have a large inter-individual error with a large variation from one movement to another; therefore, it is difficult to detect significant differences and will require further study in the future.
This study has several limitations. Firstly, the effect of medial arch support on the foot was not evaluated since a motion analysis was not performed around the foot in this study. Because the subjects wore shoes during this study, the presence of shoes made the accurate assessment of the foot difficult to achieve in a motion capture system which we used in this study. Therefore, the kinematics and kinetics of the foot were not assessed. Secondly, the hip joint was not evaluated. As it is not an adjacent joint to the ankle joint, it was determined to be less directly affected by foot orthoses than the knee joint. Thirdly, the subjects of the current study were limited to young female athletes because non-contact ACL injuries are more common in young females. As females have lower muscle density and softer tissues than males, they are expected to be more affected by the arch height of foot orthoses [19, 51–53]. It is unclear whether these results will hold true for male athletes. Fourth, this study did not assess the height of the arches of individual subjects. In this study, foot orthoses of the same arch height were used for all subjects. It is unclear whether it is effective for people with a high or low arch. Although the effect of arch height of subjects warrants further investigation, the findings of this study suggest that medial arch support affects knee joint kinematics during a cutting manoeuvre. Further studies on other motion tasks, male athletes, and individual arch height should be carried out to evaluate the effect of medial arch support of foot orthoses on knee motion.