1. Introduction
Knee osteoarthritis (OA), one of the degenerative diseases, is rapidly increasing due to recent increase in human average lifespan and an aging society [
1,
2]. In Korea, knee OA is a common disease that occurs in about one-third of people over the age of 60, with a prevalence rate of 20.2% in men and 50.1% in women [
3]. Biomechanical changes such as increases of knee adduction moment (KAM) and knee varus shear force during walking are known to be involved in the development and severity of knee OA [
4,
5,
6]. Because it is generally difficult to measure joint loads directly, joint moments such as KAM are often used as alternative measures of the force applied to a joint [
7]. Knee moments that occur in real time during the stance phase, when the body weight is supported on the ground while walking, are biomechanical variables calculated by multiplying the magnitude of the three-dimensional ground reaction force and the lever arm distance from the knee joint center. Clinically, KAM is used for diagnosing knee OA and predicting knee pain, knee deformity, and the degree of knee OA progression [
8,
9]. KAM is a coronal kinetic variable. It typically has two distinct peaks during the stance phase of gait. The first peak occurs around the 25% stance phase during walking. The second peak elicits around the 75% stance phase [
3]. The first peak KAM is typically greater than the second one. It is larger for patients with medial knee OA than for age-matched healthy individuals [
10]. Peak KAMs during walking are determined by various factors such as gait pattern, knee pain avoidance strategy, and foot progression angle (FPA). Abnormally increased KAM during gait is closely related to worsening of musculoskeletal diseases such as knee OA [
4,
11,
12].
From a biomechanical perspective, changes in kinematic and kinetic variables of any joints or segments during gait may affect ergonomic variables of all other connected joints and segments of the musculoskeletal system [
13,
14]. Therefore, changes in FPA such as internal rotation or external rotation of the foot in contact with the floor during the stance phase of gait may affect the KAM [
6,
14]. Since a study of Wang et al. [
15], many studies have been conducted on biomechanical effects of lower extremity joints according to changes in FPA during walking. A previous study has demonstrated that the effect of FPA on KAM, in particular, self-selected over 15° of increasing toe-out angle in pain free individuals with knee OA, can significantly decrease the second peak KAM during gait except the first peak [
6]. Additionally, the first KAM in internal rotation of FPA during gait is significantly decreased in patients with medial compartment knee OA [
16]. Although many previous studies have reported that tiptoe walking and tiptoe walking interventions can reduce first and second peak KAMs in patients with medial compartment osteoarthritis of the knee joint, peak KAM change varies significantly between individuals in response to the achievement of the target FPA, anatomical variations, and biomechanical influence of the motor link [
5]. Therefore, limiting exogenous variables that have a negative impact on measurement and evaluating objective and quantitative peak KAMs according to changes in FPA during gait are important for obtaining positive clinical outcomes of patients with knee OA.
Studies verifying effects of foot
rotation modifications during gait have mainly focused on effects of FPA during
gait on biomechanics of the knee joint
[
7,
17]. There is a need to evaluate effects of these modifications on other
lower extremity joints. Most of related studies have mainly examined effects of
gait modification on KAM and biomechanical values of the hip joint
[
7,
18,
19]. However, there have been
few studies on effects of foot modification angle during walking on kinematics
and kinetics of the ankle joint along with its effect on KAM. Although it is
not known how FPA changes affect the ankle joint, increased ankle joint moments
in three dimensional planes of motion may increase biomechanical loads on soft
tissues around the ankle joint and increase the incidence of musculoskeletal
disorders such as ankle OA
[
20]. Therefore, the purpose of this study was to investigate effects of
different FPA conditions during free walking on kinetics of the ankle and knee
joints in pain-free individuals with knee osteoarthritis using a 3D motion
analysis system with two force platforms.
4. Discussion
This study was conducted to determine effects of gait modifications such as toe-in and toe-out walking on knee KAM and 3D moment characteristics occurring at the ankle joint during the stance phase in subjects with knee osteoarthritis. Increased KAM during walking not only increases the loading force imposed on the medial compartment of the knee joint, but also promotes the development of clinical symptoms of knee OA such as pain and inflammation [
24,
25,
26]. Therefore, various gait modification methods such as medial knee thrust, lateral trunk lean, and toe-in or out gait have been applied to patients with knee OA to reduce KAM and alleviate clinical symptoms [
16,
27,
28,
29]. Comparison of effects between gait modifications in previous studies is limited by diverse sample characteristics and difficulties in consistently applying gait modifications [
27,
30]. To complement these methodological limitations of previous studies, this study clearly established inclusion criteria for study participants with knee osteoarthritis through pain severity and clinical evaluation as the Kellgren-Lawrence test. Additionally, guidelines were set for the walking pathway so that three different FPA walking conditions were applied as equally and consistently as possible to all participants.
This study analyzed how changes in FPA conditions affected knee joint and ankle joint moments during walking in 35 patients with medial knee osteoarthritis. Results of this study showed that the first peak value of KAM in the IFP walking condition was significantly decreased by 19.1% compared to that of NFP as the baseline walking condition. These results demonstrate the clinical effectiveness of specific gait modifications in reducing knee joint discomfort during walking, in which participants who have a choice of different gait modifications prefer toe-in gait the most [
27,
29,
31]. In general, the value of the first peak of the KAM that occurred in the early stance phase was greater than the second peak of the KAM. Results of this study showed that toe-in gait modification was closely related to the decrease in the first peak of the KAM. The reason why the FPA condition of toe-in gait significantly reduced the first peak of the KAM was because the loading center moved to the lateral side as the knee joint central axis moved medially [
16]. The second peak of the KAM (-0.50 Nm/kg) of the FPA condition of toe-in gait tended to increase compared to that of the NFP condition (-0.44 Nm/kg), although they showed no significant difference. In a study on the second peak KAM according to the application of FPA in 50 patients with knee osteoarthritis, both the 10° toe-in gait and the baseline gait showed the same moment value (-0.46 Nm/kg) without a significant difference [
7]. In addition, like the current study, there was no significant difference between the two FPA walking conditions on the second peak KAM during toe-in gait and baseline gait in twelve patients with medial compartment knee osteoarthritis [
16]. However, Lynn et al. [
23] reported a significantly greater KAM value of the late stance in internal foot position walking than in normal foot position walking. The reason for such conflicting results of the second peak KAM might be because Lynn et al.'s study was conducted with a small number of healthy individuals without knee OA.
As a result of this study, in contrast to toe-in gait that showed a significant decrease in the first peak KAM, the toe-out gait showed a significant reduction in the second peak KAM compared to the NFP condition in individuals with knee OA. However, there was no significant difference in the first peak KAM value between NFP (-0.42 Nm/kg) and EFP (-0.47 Nm/kg) gait conditions. These results were similar to previous studies reporting reduction of the second peak KAM during toe-out gait in patients with knee OA [
6,
16,
32,
33]. Unlike results of this study, Lin et al. [
34] reported that applying a 30° self-selected toe-out gait to participants caused a significant increase in the first peak KAM. The current study applied a toe-out walking trial, which was increased by 20° compared to NFP angle. Characteristics of the present study subjects were also individuals with knee osteoarthritis, not healthy participants. Peak KAM values of the present study for the FPA toe-out walking condition were somewhat different from previous studies [
35,
36,
37,
38]. These differences might be attributed to different characteristics of study subjects, clinical severity of knee OA, and variations in study design. Specifically, most subjects of the present study were patients with mild knee OA. However, there were studies conducted on patients with moderate knee osteoarthritis of K-L grade IV or higher [
35,
36]. There were also studies conducted on knee OA patients with severe clinical symptoms [
37,
38].
Many studies have determined effects of toe-in or toe-out walking conditions on biomechanical variables of the knee and hip joints in people with medial compartment knee OA. However, very few studies have examined effects of these walking conditions on the moment of the ankle joint. The present study analyzed effects of different walking conditions on 3D ankle moments along with their effects on peak KAM according to FPA during walking in individuals with knee OA. Results showed that there was no significant increase in ankle joint moment value according to changes in FPA walking conditions. Only the peak ankle inversion moment value of the EFP walking condition decreased compared to that of the NFP condition. In a previous study conducted on 15 patients with medial compartment knee OA, there was no change in the peak external ankle inversion moment value (0.02 Nm/kg) of 10° toe-out walking compared to the baseline condition. The peak external ankle inversion moment value (0.01 Nm/kg) of 20° toe-out walking was not changed either [
20]. On the other hand, the toe-in gait condition showed an increase of peak inversion moment value (0.04 Nm/kg) of the ankle joint compared to all other FPA conditions [
20]. As such, ankle moment results of previous studies in the toe-in gait condition did not match results of this study. The reason for such conflicting results was presumed to be due to differences in the number of study subjects, clinical characteristics of subjects, and inter-individual variability in biomechanical variables such as moments.
Although toe-in gait trials showed a significant decrease in the first peak KAM with a significantly decreased second peak KAM in toe-out walking, clinical application of the FPA walking modification for patients with knee osteoarthritis requires a cautious approach. Because the first peak KAM is more closely related to current clinical symptoms, severity, and prognosis of knee OA than the second peak KAM, the decrease in the first peak KAM is considered more important [
16]. However, considering the impact of a 1% increase in KAM on knee OA symptoms [
39], it will not be possible to overlook the tendency to increase the first peak KAM during EFP walking condition and the tendency to increase the second peak KAM during IFP walking. Results showed no significant increase in the ankle joint moment value in the stance phase during walking for both 20° toe-in and 20° toe-out modifications. However, in some participants, the IFP gait modification increased the peak ankle adduction moment when ankle joint contact forces peaked. This indicates that for patients with musculoskeletal deficits of the ankle joint such as ankle OA, the effectiveness of toe-in or toe-out walking trials at the ankle joint should be verified case by case before adopting FPA gait to improve the KAM.
The current study has some strengths. Previous studies related to KAM and FPA modifications have investigated effects of foot internal or external rotation condition on kinematics and kinetics of the knee joint itself during gait [
5,
6,
16,
23,
40]. The study was performed to determine effects on two peak KAMs and 3D moments of the ankle joint using three different FPA modifications in individuals with knee OA through a reliable and objective 3D motion analysis system and force platforms. The present study showed that in participants with mild or moderate knee OA, FPA walking modifications affected peak KAMs without affecting the increase in peak ankle moment. Therefore, clinical application of FPA modification should be approached cautiously through quantitative analysis of the two peak KAMs in patients with knee osteoarthritis. Although abnormally high the peak KAM is a risk factor for medial compartment knee OA, abnormally reduced peak KAM might also be a risk factor for lateral compartment knee OA [
23]. Limitations of this study are as follows. Because most participants had mild knee OA without severe knee pain or clinical symptoms, results of this study cannot be applied to all knee OA patients. In addition, various research methodological techniques were used to apply consistent FPA walking modifications. However, there were difficulties with consistency in some walking trials. Future research will need to verify the impact of clinical application of walking aids such as foot and ankle orthoses or insoles and integrated intervention of gait modification retraining on the first and second peak KAMs and biomechanics of lower extremity joints.