ABSTRACT
Progression of medial knee osteoarthritis (OA) has been associated with walking biomechanics, specifically with the knee adduction (KAM) and flexion (KFM) moments. Lower medial stiffness shoes (LMSS), that are shoes with the sole made of softer material medially than laterally, were proposed for disease management through KAM reductions. This study primarily tested the hypothesis that larger pKAM reductions can be achieved with stiffness ratios selected individually than with the smallest ratios of the LMSS. Secondarily, the proportions of individuals reducing the KAM or reducing the KAM without increasing the KFM were compared between the individualized and smallest ratios conditions. The two LMSS conditions were also compared with lateral wedge insoles. Walking biomechanics were recorded for 15 OA patients (8 males; 62.3 ± 9.6 years old) and 14 asymptomatic individuals (5 males; 53.6 ± 3.6 years old) wearing LMSS with various stiffness ratios and wedges. Larger decreases in KAM were obtained with individualized stiffness ratios (14.0%–16.4%) than with the two other interventions (6.7%–12.5%) (p < 0.001). The percentage of participants reducing the KAM without increasing the KFM was larger with individualized ratios compared to the smallest ratios in the OA group (14 vs. 6; p = 0.001) and compared to wedges in the asymptomatic group (13 vs. 6; p = 0.015). This exploratory study showed the potential of individualizing the stiffness ratios, particularly in terms of KAM reduction amplitude, percentage of individuals achieving specific modifications, and possibility to aim for more complex kinetic changes. Further work is necessary to assess the effect of individualized stiffness ratios on clinical outcomes.
Keywords: gait, kinematics and kinetics, knee, osteoarthritis, rehabilitation
1. Introduction
Knee osteoarthritis (OA) is a chronic degenerative joint disease affecting millions of individuals worldwide and representing a significant burden on both patients and healthcare systems [1, 2]. It is characterized by a progressive deterioration of the knee joint, leading to pain, functional limitations, and reduced quality of life [3]. While the pathomechanism of knee OA is not fully understood, ambulatory mechanics have been identified as a critical factor in disease progression particularly in relation to the repetitive loading of the joint [4, 5].
The knee adduction moment (KAM) during walking, which reflects the medial‐lateral loading distribution at the knee [6], has long been associated with medial knee OA, the most common form of the disease [7]. Specifically, the peak amplitude of the KAM during the first half of stance (pKAM) has been linked to disease progression, severity, and pain [8, 9, 10, 11]. This association has spurred the development of various conservative interventions aimed at reducing the pKAM to relieve symptoms and ideally slow down disease progression [12].
Footwear interventions are particularly appealing to reduce the pKAM in medial knee OA, due to their ease‐of‐use and noninvasive nature which represent important factors for compliance and long‐term success. Mostly two approaches have been considered and studied so far: lateral wedge insoles to be added in the patients' shoes and lower medial stiffness shoes [12]. These later shoes are characterized by a medial‐to‐lateral stiffness ratio smaller than 1.0, meaning that the midsole is made of softer material medially than laterally. Although promising, literature has shown that these two approaches do not consistently reduce the pKAM in all individuals [13, 14], and their clinical effectiveness has been reported to be variable [15, 16]. Interestingly, a later study by Felson and colleagues showed clinical improvements in patients for whom lateral wedge insoles actually decreased the pKAM [17]. This is an important work in support of therapeutic footwear for medial knee OA, stressing the necessity of interventions effectively decreasing the pKAM. These findings are consistent with gait retraining research where encouraging effects on clinical outcomes have also been reported with a reduction of the pKAM [18, 19, 20, 21]. Consequently, there is a body of evidence in support of a reduction of the pKAM in patients with medial knee OA. In the case of footwear interventions, the literature also recommends the development of personalized solution as biomechanical responses may vary among patients.
There is a paucity of data concerning the influence of lower medial stiffness shoe design on the pKAM [22]. Further research is particularly needed regarding the role of the ratio between the stiffness in the medial and lateral sides of the insole. For instance, there is an interest in assessing if larger pKAM reductions are systematically obtained with smaller stiffness ratios or if greater reductions could be achieved with ratios that are not the smallest possible. Such understanding would be important to guide the design of future footwear solutions, particularly concerning the necessity and benefits of individualizing the midsole properties.
While the pKAM received most of the attention in gait modification for medial knee OA, considerations are growing regarding the peak knee flexion moment during the first half of stance (pKFM). In particular, a larger pKFM has been associated with faster disease progression [8] and larger loading at the knee joint [23, 24]. In consequence, researchers have started to aim for more specific gait modifications combining a decrease in pKAM with no increase in pKFM [25, 26]. While possible, a decrease in the percentage of individuals achieving the gait modification was reported when there was a pKFM objective in addition to a pKAM objective. This is another situation where lower medial stiffness shoes with individualized stiffness ratio could show valuable. Indeed, in addition to larger pKAM reductions, they could allow more patients to achieve gait modifications combining pKAM and pKFM objectives.
This exploratory study primarily aimed to test the hypothesis that larger pKAM reductions can be achieved with stiffness ratios selected individually than with the smallest ratios of the lower medial stiffness shoes. This hypothesis was tested in asymptomatic individuals and patients with medial knee OA, because differences in ambulatory kinetics have been reported between these populations [27, 28]. Secondarily, the study aimed to compare the individualized and smallest ratios conditions in terms of the percentage of participants achieving a gait modification consisting of either reducing the pKAM or of reducing the pKAM without increasing the pKFM. Additionally, to facilitate the interpretation of the results with respect to the literature which mostly used lateral wedge insoles, this study aimed to compare the two conditions of lower medial stiffness shoes with lateral wedge insoles.
2. Methods
2.1. Participants
For this study, two groups of participants aged between 45 and 80 years old were recruited: patients suffering from medial knee OA and asymptomatic individuals. The inclusion criterion for the OA group was a diagnosis of medial knee OA by a physician unilaterally in the left knee or bilaterally with left predominance. A focus on the left knee was motivated by the experimental lower medial stiffness shoes used in this study that allow varying the stiffness ratio only for the left shoes. Inclusion criteria for the asymptomatic group were an absence of OA symptoms at lower limbs and spine, and an absence of pain and difficulty during ambulation. For both groups, exclusion criteria were any sort of neurological disorders, history of lower limbs surgery, or frequent use of mechanical walking aids, such as braces, orthotics or walking sticks. Severe obesity (BMI above 35 kg/m2) was also an exclusion criterion for both groups, because it has been shown to affect walking mechanics [29]. A sample size calculation for an analysis of variance aiming to identify larger pKAM reductions with individualized stiffness ratios indicated a minimum of 12 participants per group, considering differences of at least large effect size (Cohen's d ≥ 0.80) [30, 31], a power of 80%, and an α level of 5% [32]. This study was approved by the Local Ethics Committee and all participants provided informed consent before participation.
2.2. Experimental Procedure
The participants were equipped with reflective markers following a common protocol [33] and performed multiple walking trials at normal self‐selected speed with different footwear conditions in a gait lab. Footwear conditions, detailed below, were tested in random order. For each footwear condition, the participants were given practice time and when they felt used to the footwear, five successful walking trials were collected using a motion capture system (Vicon, UK) and a ground‐embedded force plate (Kistler AG, CH) recording at 120 and 1200 Hz, respectively. A trial was considered successful when the left foot fully stepped on the force plate. The peak knee adduction moment (pKAM) and the peak knee flexion moment (pKFM) during the first half of stance were computed using inverse dynamics following standard procedures and normalized to percent bodyweight and height (%BW*Ht) [33]. Gait parameters, including walking speed, were averaged over the five trials. All biomechanical processing was done using the software application BioMove (Stanford, USA).
2.3. Footwear Conditions
This study used custom‐made shoes with modifiable stiffness ratios in the left shoes and a neutral ratio in the right shoes. The midsole of the left shoes was stiff on the lateral side and had a modifiable stiffness on the medial side. Specifically, the lateral side of the sole was made of polyamid plastic and had therefore a hard stiffness. The stiffness of the softer materials in the medial side was quantified using the Shore A scale which ranges from 0 (e.g., soft gels) to 100 (e.g., semi‐rigid plastics). On the medial side, 4 different stiffnesses lower or equal to the lateral one were used: very soft (Shore A 30), soft (Shore A 35), medium (Shore A 50), and hard (polyamid plastic), resulting in very small (VS), small (S), medium (M), and neutral (N) medial‐lateral stiffness ratios, respectively. The medial stiffness was modifiable at both the rearfoot and the forefoot, resulting in 16 combinations of stiffness ratios (4 rearfoot ratios × 4 forefoot ratios). For convenience, in this article, the medial‐lateral stiffness ratios are referred as follow: “rearfoot ratio” − “forefoot ratio.” For instance, the sole with the very small ratio at the rearfoot and the neutral ratio at the forefoot is referred as VS–N. In addition to the 16 stiffness combinations, a lateral wedge insole condition was also tested by inserting a full‐length 5° lateral wedge insole in the shoe below the comfort insole, and on top of the N–N condition.
2.4. Statistical Analysis
The lower medial stiffness shoes with the N–N ratios were defined as the control condition and the changes in gait parameters with respect to this condition were calculated independently for each participant and all other footwear conditions. Shapiro–Wilk tests indicated that the changes were not normally distributed for some parameters and conditions. Consequently, the changes were reported and analyzed using nonparametric statistics.
The changes in pKAM obtained with the following three footwear conditions were compared: the lower medial stiffness shoes with an individualized ratios combination, the lower medial stiffness shoes with the smallest ratios (i.e., VS–VS), and the lateral wedges. A Friedman test and post hoc Wilcoxon signed‐rank tests with Bonferroni correction (effective p < 0.017) were done to compare the changes among conditions, separately for the OA and asymptomatic groups. Wilcoxon signed‐rank tests were also performed independently for each group and condition to detect changes significantly different from zero. The individualized lower medial stiffness shoes condition was selected independently for each participant among the 15 stiffness ratio combinations as the combination decreasing the pKAM the most. For a more complete characterization of the three conditions, the analysis described above for the changes in pKAM was repeated for the changes in pKFM and walking speed. The additional pKFM analysis was motivated by the complementarity of the pKFM to the pKAM with respect to knee loading [23, 24] and OA progression [8]. Specifically, it was important assessing whether the pKFM increased, because this could compromise the benefits of decreasing the pKAM. It was also important analyzing walking speed, because both the pKAM and the pKFM have been shown to be speed dependent [34]. Indeed, slower walking could decrease the pKAM and the pKFM, and thus complexify the interpretation of the kinetic responses to the footwear interventions.
Furthermore, the participants achieving a reduction of the pKAM as well as those achieving a reduction of the pKAM without increase of the pKFM were identified, and their percentages were compared among the three footwear conditions using χ 2 tests. For the individualized lower medial stiffness shoes condition, a kinetic change was considered achieved if at least one of the stiffness ratio combinations induced the change of interest. Data processing and statistical analyses were done with MATLAB version R2021a (MathWorks, MA, USA), considering a significant level set a priori at 5%.
3. Results
The OA group was composed of 7 females and 8 males, with mean ± standard deviation (SD) age, height, and weight of 62.3 ± 9.6 years old, 1.70 ± 0.11 m, and 73.5 ± 11.3 kg, respectively (details in Table 1). They had mean ± SD Ahlbäck grade of 1.50 ± 0.53, VAS scores for pain and stiffness of 2.2 ± 1.6 and 1.4 ± 1.4, respectively, and KOOS overall score of 70.9 ± 23.6. The Ahlbäck scale is a classification system for OA severity, ranging from 0 (normal) to 5 (severe bone attrition) [7]. The asymptomatic group was composed of 9 females and 5 males, who were of mean ± SD age, height, and weight of 53.6 ± 3.6 years old, 1.69 ± 0.12 m, and 68.1 ± 14.7 kg, respectively.
Table 1.
Demographics and health status for the osteoarthritic and the asymptomatic groups.
| Osteoarthritic group | Asymptomatic group | |
|---|---|---|
| n = 15 | n = 14 | |
| Gender, number | W: 7, M: 8 | W: 9, M: 5 |
| Age, years | 62.3 ± 9.6 | 53.6 ± 3.6 |
| Height, m | 1.70 ± 0.11 | 1.69 ± 0.12 |
| Weight, kg | 73.5 ± 11.3 | 68.1 ± 14.7 |
| Walking speed, m/s | 1.30 ± 0.17 | 1.40 ± 0.16 |
| Ahlbäck grade, range 1–5 | 1.50 ± 0.53 | n/a |
| Visual Analog Scale (VAS), range 0–10 | ||
| Pain | 2.2 ± 1.6 | 0.1 ± 0.3 |
| Stiffness | 1.4 ± 1.4 | 0.0 ± 0.1 |
| Knee Injury and Osteoarthritis Outcome Score (KOOS), range 0–100 | ||
| Overall | 70.9 ± 23.6 | 99.1 ± 1.7 |
| Pain | 78.9 ± 13.1 | 98.4 ± 2.4 |
| Symptoms | 76.1 ± 15.3 | 99.1 ± 2.2 |
| Function, daily living | 83.0 ± 26.1 | 100.0 ± 0.0 |
| Function, sports, and recreational activities | 60.4 ± 26.0 | 98.2 ± 4.2 |
| Quality of life | 57.1 ± 21.0 | 99.6 ± 1.6 |
In the control footwear condition (i.e., lower medial stiffness shoes with N–N ratios), the OA patients walked at a mean ± SD velocity of 1.30 ± 0.17 m/s, with pKAM of 3.49 ± 0.79%BW*Ht, and pKFM of 2.45 ± 1.37%BW*Ht. Respectively, the asymptomatic participants walked at 1.40 ± 0.16 m/s, with pKAM of 3.45 ± 0.52%BW*Ht, and pKFM of 2.94 ± 1.25%BW*Ht. For none of the footwear conditions and participant groups was the speed change significantly different from zero (all p ≥ 0.23) (Table 2). Furthermore, there was no significant difference in speed changes among the three footwear conditions, neither in the OA (p = 0.17) nor in the asymptomatic (p = 0.95) groups.
Table 2.
Changes in walking speed and peak knee adduction (pKAM) and flexion (pKFM) moments induced by the lower medial stiffness shoes with an individualized stiffness ratios combination, by the lower medial stiffness shoes with the smallest stiffness ratios combination (VS–VS), and by the lateral wedge insoles.
| Lower medial stiffness shoes with individualized ratios | Lower medial stiffness shoes with the smallest ratios | Lateral wedge insoles | |
|---|---|---|---|
| Osteoarthritic group, n = 15 | |||
| pKAM | −0.46 [−0.77; −0.36] * , *** | −0.36 [−0.49; −0.30] ** | −0.25 [−0.36; −0.14] ** |
| pKFM | −0.04 [−0.16; +0.05] | +0.06 [−0.05; +0.32] | −0.06 [−0.11; +0.18] |
| Speed | +0.01 [−0.05; +0.06] | +0.02 [−0.01; +0.06] | −0.00 [−0.04; +0.02] |
| Asymptomatic group, n = 14 | |||
| pKAM | −0.54 [−0.68; −0.42] * , *** | −0.47 [−0.60; −0.35] ** , *** | −0.32 [−0.49; −0.27] * , ** |
| pKFM | −0.20 [−0.42; −0.13] | −0.18 [−0.29; +0.11] | +0.06 [−0.27; +0.20] |
| Speed | −0.01 [−0.03; +0.02] | −0.01 [−0.03; +0.02] | −0.01 [−0.03; +0.02] |
Note: Changes are reported in %BW*Ht for the pKAM and pKFM, in m/s for the speed (compared to the control footwear condition), and are presented as median [1st quartile; 3rd quartile]. Bold values indicate changes statistically significantly different from zero (p < 0.017).
Statistically significantly different from the changes obtained using the lower medial stiffness shoe with the smallest ratios (p < 0.017).
Statistically significantly different from the changes obtained using the lower medial stiffness shoe with an individualized ratios combination (p < 0.017).
Statistically significantly different from the changes obtained using lateral wedge insoles (p < 0.017).
Figure 1 presents the pKAM changes of the 29 participants with the 15 different combinations of stiffness ratios in the lower medial stiffness shoes as well as with the lateral wedge insoles. It notably illustrates the range of pKAM changes that were obtained by varying the stiffness ratios of the lower medial stiffness shoes. These ranges were on average of 17.3% ± 5.1% in the OA group and of 15.0% ± 3.4% in the asymptomatic group.
Figure 1.
Changes in peak knee adduction moment (pKAM) for each osteoarthritic patient (top) and asymptomatic individual (bottom) with the different footwear conditions. The blue rectangles correspond to the range of changes achieved with the different combinations of stiffness ratios in the lower medial stiffness shoes. Black circles indicate the changes with the smallest ratios combination in the lower medial stiffness shoes, and red diamonds indicate the changes with the lateral wedge insoles. To facilitate reading, this figure reports relative changes (i.e., as percents of the values obtained with the control condition).
Seven combinations of stiffness ratios induced the largest pKAM reductions with the lower medial stiffness shoes in the OA group (VS–VS, VS–S, VS–M, VS–N, S–VS, S–S, M–S). They were eight to do so in the asymptomatic group (VS–S, VS–M, VS–N, S–VS, S–S, S–M, S–N, M–N). These combinations are presented in Tables 3 and 4.
Table 3.
Individually selected stiffness ratios for the osteoarthritic group when aiming for a maximum decrease of the peak knee adduction moment (pKAM).
| Forefoot medial‐lateral stiffness ratio | ||||||
|---|---|---|---|---|---|---|
| Very small | Small | Medium | Neutral | Number of participants | ||
| Rearfoot medial‐lateral stiffness ratio | Very small | #5, #7, #10 | #9 | #12, #13, #15 | #6, #8, #14 | 10 |
| Small | #2 | #1, #3, #11 | — | — | 4 | |
| Medium | #4 | — | — | — | 1 | |
| Neutral | — | — | — | — | 0 | |
| Number of participants | 5 | 4 | 3 | 3 | 15 | |
Table 4.
Individually selected stiffness ratios for the asymptomatic group when aiming for a maximum decrease of the peak knee adduction moment (pKAM).
| Forefoot medial‐lateral stiffness ratio | ||||||
|---|---|---|---|---|---|---|
| Very small | Small | Medium | Neutral | Number of participants | ||
| Rearfoot medial‐lateral stiffness ratio | Very small | — | #4 | #6, #8, #10, #11 | #7, #13 | 7 |
| Small | #12, #14 | #3, #5 | #2 | #1 | 6 | |
| Medium | — | — | — | #9 | 1 | |
| Neutral | — | — | — | — | 0 | |
| Number of participants | 2 | 3 | 5 | 4 | 14 | |
Analyzing the changes in pKAM using the lower medial stiffness shoes with an individualized ratios combination, the lower medial stiffness shoes with the smallest ratios combination, and the lateral wedges showed statistically significant decreases for all three footwear conditions in both groups (Table 2, all p < 0.001). Friedman tests revealed a significant effect of the footwear conditions on the pKAM changes in both groups (p < 0.001). Specifically, in the OA group, post hoc analyses indicated larger pKAM decreases in the individualized stiffness ratios condition (median [1st quartile; 3rd quartile] relative change of −14.0 [−23.5; −11.0]%) compared to both the smallest stiffness ratios (−10.4 [−19.1; −7.6]%) and the lateral wedges (−6.7 [−14.6; −3.3]%) conditions (both p ≤ 0.001), while there was no difference between the pKAM changes in the smallest stiffness ratios and the lateral wedges condition (p = 0.12). Similarly, in the asymptomatic group, pKAM reductions were significantly larger in the individualized stiffness ratios condition (relative change of −16.4 [−18.7; −12.7]%) compared to both the smallest stiffness ratios (−12.5 [−16.0; −9.9]%) and the lateral wedges (−9.8 [−12.4; −6.7]%) conditions (both p ≤ 0.001). In this group, pKAM reductions were also significantly larger in the smallest stiffness ratios condition compared to the lateral wedge condition (p = 0.007). Regarding the pKFM, a statistically significant change was only observed in the asymptomatic group with the individualized stiffness ratios condition (relative change of −6.9 [−15.8; −3.5]%; p = 0.01).
All 15 patients and 14 asymptomatic individuals (100.0% of them) reduced the pKAM with the lower medial stiffness shoes with individualization of the stiffness ratios condition (Table 5). In comparison, two fewer patients reduced the pKAM with the smallest stiffness ratios condition (86.7%) and one fewer asymptomatic individual reduced the pKAM with the lateral wedges (92.9%). Additionally, the individualized stiffness ratios condition allowed larger pKAM decreases compared to the smallest stiffness ratios condition in 73.3% of the OA participants (11 out of 15) and in 92.9% of the asymptomatic participants (13 out of 14) (Figure 1). Similarly, the individualized stiffness ratios condition allowed larger pKAM decreases compared to the lateral wedges condition in 86.7% of the OA patients (13 out of 15) and in 92.9% of the asymptomatic participants (13 out of 14).
Table 5.
Participants who reduced the peak knee adduction moment (pKAM) alone or in combination with no increase of the peak knee flexion moment (pKFM) when walking with the lower medial stiffness shoes with an individualized stiffness ratios combination, the lower medial stiffness shoes with the smallest stiffness ratios combination, or with lateral wedge insoles.
| Participants | ||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Osteoarthritic group | #1 | #2 | #3 | #4 | #5 | #6 | #7 | #8 | #9 | #10 | #11 | #12 | #13 | #14 | #15 | Total |
| pKAM reduction | ||||||||||||||||
| Lower medial stiffness shoes with individualized ratios | o | o | o | o | o | o | o | o | o | o | o | o | o | o | o | 15 |
| Lower medial stiffness shoes with the smallest ratios | o | o | o | o | o | o | o | o | X | o | o | X | o | o | o | 13 |
| Lateral wedge insoles | o | o | o | o | o | o | o | o | o | o | o | o | o | o | o | 15 |
| pKAM reduction without pKFM increase | ||||||||||||||||
| Lower medial stiffness shoes with individualized ratios | o | o | o | o | o | o | o | o | X | o | o | o | o | o | o | 14* |
| Lower medial stiffness shoes with the smallest ratios | o | o | X | X | o | X | o | X | X | o | o | X | X | X | X | 6** |
| Lateral wedge insoles | o | X | o | X | o | X | X | X | o | X | o | o | o | o | o | 9 |
| Asymptomatic group | #1 | #2 | #3 | #4 | #5 | #6 | #7 | #8 | #9 | #10 | #11 | #12 | #13 | #14 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| pKAM reduction | |||||||||||||||
| Lower medial stiffness shoes with individualized ratios | o | o | o | o | o | o | o | o | o | o | o | o | o | o | |
| Lower medial stiffness shoes with the smallest ratios | o | o | o | o | o | o | o | o | o | o | o | o | o | o | 14 |
| Lateral wedge insoles | o | o | X | o | o | o | o | o | o | o | o | o | o | o | 13 |
| pKAM reduction without pKFM increase | |||||||||||||||
| Lower medial stiffness shoes with individualized ratios | o | o | o | o | o | o | o | o | o | X | o | o | o | o | 13*** |
| Lower medial stiffness shoes with the smallest ratios | o | o | o | o | X | o | o | o | X | X | o | o | X | o | 10 |
| Lateral wedge insoles | o | X | X | o | X | X | X | o | X | X | o | o | X | o | 6** |
Note: The green circles indicate participants for which the kinetic change was achieved with the corresponding footwear condition. The red crosses indicate the participants who did not achieve the kinetic change with the corresponding footwear condition. The last column of the table indicates the total number of participants achieving the kinetic change with the corresponding footwear condition.
Statistically significantly different percentage than using the lower medial stiffness shoe with the smallest stiffness ratios combination (p < 0.017).
Statistically significantly different percentage than using the lower medial stiffness shoe with an individualized stiffness ratios combination (p < 0.017).
Statistically significantly different percentage than using lateral wedge insoles (p < 0.017).
The percentages of OA patients and asymptomatic individuals reducing the pKAM without increasing the pKFM with the lower medial stiffness shoes with an individualized stiffness ratios combination were 93.3% (14 out of 15) and 92.9% (13 out of 14), respectively (Table 5). The stiffness ratios combinations allowing these dual changes are presented in Tables 6 and 7. Comparatively, this dual change was achieved by 40.0% of the OA patients (6 out of 15) and 71.4% of the asymptomatic participants (10 out of 14) with the smallest stiffness ratios condition, while they were 60.0% of the OA patients (9 out of 15) and 42.9% of the asymptomatic participants (6 out of 14) to achieve it with the lateral wedges (Table 5). The percentage of participants achieving the dual change was statistically significantly larger with the individualized stiffness ratios condition compared to the smallest stiffness ratios condition in the OA group (p = 0.001) and compared to the lateral wedges condition in the asymptomatic group (p = 0.015).
Table 6.
Percentage of osteoarthritic patients decreasing the peak knee adduction moment (pKAM) without increasing the peak knee flexion moment (pKFM) per stiffness ratios combination.
| Forefoot medial‐lateral stiffness ratio | ||||||
|---|---|---|---|---|---|---|
| Very small | Small | Medium | Neutral | Total | ||
| Rearfoot medial‐lateral stiffness ratio | Very small | 40.0 | 40.0 | 53.3 | 46.7 | 80.0 |
| Small | 60.0 | 33.3 | 60.0 | 46.7 | 73.3 | |
| Medium | 53.3 | 53.3 | 46.7 | 40.0 | 80.0 | |
| Neutral | 40.0 | 60.0 | 33.3 | — | 80.0 | |
| Total | 86.7 | 73.3 | 73.3 | 66.7 | 93.3 | |
Table 7.
Percentage of asymptomatic participants decreasing the peak knee adduction moment (pKAM) without increasing the peak knee flexion moment (pKFM) per stiffness ratios combination.
| Forefoot medial‐lateral stiffness ratio | ||||||
|---|---|---|---|---|---|---|
| Very small | Small | Medium | Neutral | Total | ||
| Rearfoot medial‐lateral stiffness ratio | Very small | 71.4 | 71.4 | 71.4 | 85.7 | 85.7 |
| Small | 71.4 | 78.6 | 71.4 | 78.6 | 92.9 | |
| Medium | 64.3 | 71.4 | 57.1 | 78.6 | 92.9 | |
| Neutral | 57.1 | 78.6 | 42.9 | — | 78.6 | |
| Total | 85.7 | 92.9 | 85.7 | 92.9 | 92.9 | |
4. Discussion
This exploratory study showed that individualizing the stiffness ratios in lower medial stiffness shoes for patients suffering from medial knee OA could be beneficial. Indeed, doing so allowed obtaining pKAM reductions of larger amplitude than when the smallest stiffness ratios combination was systematically used. Furthermore, equal or larger percentages of participants could achieve single pKAM or dual pKAM and pKFM reductions with individualization of the ratios than with the smallest ratios. These results are important because they suggest that individualizing the stiffness ratios could induce greater unloading of the knee joint [23], which could increase the therapeutic effect both with respect to symptoms and disease progression [17, 18]. The outreach of these findings could exceed lower medial stiffness shoes. Indeed, individualization could be a key action to unlock greater therapeutic potential in footwear interventions for medial knee OA management.
It is interesting to note that the stiffness ratios inducing the largest pKAM decreases varied substantially among participants. In fact, from all the combinations available in this study, about half of them were selected for a participant or another. The combination the most frequently selected (VS–M) suited four asymptomatic participants and three OA participants, which corresponded to < 30% of the participants. The variability was particularly pronounced in the forefoot, with an almost equal distribution among the four stiffness ratios. The situation was different for the rearfoot, with the very small stiffness ratio selected for more than half of the participants. Although further analysis will be necessary to fully understand the mechanisms underlying these patterns, the greater variability observed in the forefoot stiffness ratio could be related to the larger variability in foot rollover dynamics previously reported for the forefoot [35]. Although incompletely understood, this variability already highlights the importance of individualizing the stiffness ratios. This observation aligns well with prior research on lateral wedge insoles, where interindividual variation was also reported, and recommendations were made of individualizing the wedge angle based on a gait analysis [36]. Similarly, studies on gait retraining have consistently demonstrated variability between individuals in the gait modifications needed to achieve specific pKAM reductions [19, 25, 37]. These findings confirm the necessity of developing customizable interventions, including in the case of therapeutic footwear, to accommodate the biomechanical variability among patients.
The study also showed that larger amplitudes of pKAM reduction could be achieved with the lower medial stiffness shoes with individualized stiffness ratios combination than with standard lateral wedge insoles in similar or larger percentages of participants. These results are encouraging as clinical improvement has been reported in patients reducing their pKAM with the wear of lateral wedge insoles [17]. Given that the lower medial stiffness shoes in the present study induced larger pKAM reductions than lateral wedges, it could be expected that these shoes could also lead to clinical improvements, especially with an individualized stiffness ratios combination. The potential of achieving a pKAM reduction in a larger proportion of patients than lateral wedge insoles is another aspect supporting the therapeutic value of individualized lower medial stiffness shoes.
In addition to allowing larger reductions in pKAM, the lower medial stiffness shoes with an individualized stiffness ratios combination showed valuable to target more complex kinetic changes than single pKAM reduction, as it has been the case so far with footwear interventions. Indeed, the individualization of the lower medial stiffness shoes allowed to have a pKAM decrease without pKFM increase in a larger percentage of participants than the nonindividualized shoe and the lateral wedge insoles. This additional advantage of individualized lower medial stiffness shoes is particularly relevant with respect to the increasing interest for combined pKAM and pKFM modifications [8, 23, 25]. While the optimal kinetic modification remains unknown, the present results showed that individualized therapeutic footwear can achieve more complex kinetic changes than solely pKAM reductions, and, in this respect as well, the present work may be an important step forward to more effective management of knee OA. Furthermore, customizable footwear like the ones used in this study will be pivotal in future research aiming at determining target kinetic changes individually.
The present study has some limitations worth discussing. First, the individualized lower medial stiffness shoes condition was selected independently for each participant as the combination of ratios decreasing the pKAM the most. Although adequate with respect to the study objectives, doing so introduced a selection bias which implies caution when interpreting the results. Specifically, as said above, by observing more interesting kinetic changes with this condition, the present study supported the idea that individualizing the stiffness ratios could be beneficial. However, in the future, studies dissociating the individualization from the evaluation will be necessary to confirm the benefices. To assess the benefices in terms of clinical outcomes, longitudinal studies will likely be needed as well. Second, participants were tested only one time, and further studies will be necessary to determine whether the stiffness ratios individually selected may differ from a day to another. Third, while adapted to the objectives, the sample size remained limited. Evaluating larger populations in future works could allow to detect additional statistically significant differences and better understand the variability among individuals. This appears particularly relevant given that some parameters were not normally distributed. Forth, when determining the proportions of participants achieving particular kinetic changes, the magnitude of the changes was not considered. While appropriate in this study for the comparison of footwear conditions, in the future, when our understanding will have sufficiently grown, it will be important using individual objectives of gait changes. Finally, for individualized lower medial stiffness shoes to be used in routine clinical practice, further research will be necessary to determine the individual stiffness ratios with simpler instrumentation than the gait lab used in this study, such as forces plates, color cameras, or inertial sensors [38, 39, 40].
5. Conclusion
This exploratory study showed that individualizing the stiffness ratios in lower medial stiffness shoes could be beneficial. Indeed, compared to the standard alternatives of lower medial stiffness shoes with fixed stiffness ratios and lateral wedge insoles, this solution showed promising in terms of pKAM reduction amplitudes, percentage of individuals achieving specific modifications, and possibility to aim for kinetic changes more complex than only a reduction in pKAM. Further works will be needed to confirm the findings with experimental setups dissociating the individualization from the evaluation procedures and extending the analysis to clinical outcomes.
Ethics Statement
This study was approved by the Local Ethics Committee.
Consent
All participants provided informed consent before participation.
Conflicts of Interest
The authors have submitted a patent application for a footwear design allowing adjusting midsole stiffness.
Acknowledgments
This study was partially funded by the Swiss National Science Foundation (SNSF Grant #32003B‐166433) and the Lausanne Orthopedic Research Foundation. The study sponsors were neither involved in the study design, in the collection, analysis, and interpretation of data, in the writing of the manuscript, nor in the decision to submit the manuscript for publication. The authors thank Dr. Charles Baur and Mr. José Luis Rivera Gutiérrez for their contribution to the construction of the study shoes. B.M.J. and J.F. supervised this study and should be considered as last authors. Open access publishing facilitated by Universite de Lausanne, as part of the Wiley ‐ Universite de Lausanne agreement via the Consortium Of Swiss Academic Libraries.
Ulrich‐Ischer B., Cantaloube A., Hoffmann L., Jolles B. M., and Favre J., “Therapeutic Footwear for Medial Knee Osteoarthritis: Individualizing Midsole Stiffness Enhances Intended Kinetic Changes,” Journal of Orthopaedic Research 0 (2025): 1–9, 10.1002/jor.70110.
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