Biomechanical Effect of Valgus Knee Braces on the Treatment of Medial Gonarthrosis: A Systematic Review

Background Valgus braces are prescribed as a common conservative treatment option for patients with medial gonarthrosis to improve their quality of life. Many studies had reviewed the effects of the valgus braces on patients with medial gonarthrosis, while they mainly focused on the knee adduction moment (KAM), with less attention paid to other parameters such as spatiotemporal and morphological parameters. Objectives The purpose of this study was to review the effects of valgus braces on the spatiotemporal, kinematic/kinetic, morphological, and muscle parameters. Methods Based on the selected keywords, a survey of literatures was performed in Web of Science, PubMed, Scopus, and Google Scholar using the PRISMA methods, and the search period was established from January 2000 to March 2022. Results Thirty-four articles were included. According to the conclusion of these articles, the valgus brace can be used to relieve the symptoms of patients with medial gonarthrosis by decreasing the varus angle, decreasing the KAM, and redistributing the knee compartment loads. However, the effects of valgus braces on other biomechanical parameters (e.g., walking speed, cadence, joint angle, and joint space) had not reached a consensus. Conclusions The valgus knee brace can effectively relieve the symptoms of medial gonarthrosis through multiple mechanisms, while there is still some confusion about the effectiveness of the valgus brace on the other biomechanical parameters.


Introduction
Gonarthrosis is a widespread degenerative musculoskeletal disease that typically occurs in elderly people, obese people, and those who have suffered from a lower limb injury [1,2]. Due to the physiological geometry of the tibial plane, the internal forces of the knee show nonuniform distribution in the tibial plane during motion [3]. Generally, 60-80% of the compressive load transmitted throughout the knee is applied on the medial compartment, which means that the loads on medial compartment are approximately 2.2 times higher than the lateral compartment [4]. Therefore, there will be more severe effects of gonarthrosis on the medial compartment compared with the lateral compartment [5].
Varus alignment may be a cause or result of gonarthrosis, which will result in the overload of medial compartment and cartilage wear [6,7]. If the intervention and therapy are not taken, the total knee arthroplasty (TKA) will be the only option due to the deterioration of gonarthrosis. Unfortunately, doctors only advise TKA to patients with severe gonarthrosis due to the invasive nature of the surgery, the high costs, and the risk of complications [4]. Approximately 19% of TKA patients are unsatisfied with their treatment, especially those under the age of 70, who met a higher risk of renovation surgery [8].
Currently, the valgus knee brace, a common and effective conservative treatment option, has been shown to be effective in delaying the progression of the medial gonarthrosis. The valgus knee brace improves the knee joint alignment by applying an auxiliary force in the coronal plane, and the load will be transferred to the lateral compartment. Then, the contact forces on the medial compartment were decreased, and the suffering of the patients could be relieved [9,10].
In the past few decades, the treatment effects of valgus braces on medial gonarthrosis have been reviewed in several related studies. Ramsey and Russell [11] summarized the effects of valgus braces on kinematic/kinetic and perception parameters, and the joint space, joint moment, and joint load were analyzed. Alfatafta H et al. [12] and Khosravi et al. [13] reviewed the effects of valgus braces on pain and functional activity levels of patients with medial gonarthrosis, while the biomechanical effects of valgus braces were excluded in this paper. Moyer et al. [14] assessed the biomechanical effects of braces on medial gonarthrosis, which focused on the knee joint moment, joint space, and muscle cocontraction. The effects of valgus braces on spatiotemporal and kinematic parameters were not involved. Petersen et al. [15] evaluated the biomechanical effects of valgus braces on medial gonarthrosis, which included the effects on the knee adduction moment (KAM). Steadman et al. [16] reported the effects of valgus braces on clinical application and biomechanical parameters such as joint load, joint space, and varus angle. Overall, most studies have concentrated on the effects of braces on KAM and pain index, with little attention paid to other parameters. The KAM and ache index may reflect the effect of braces on patients with medial gonarthrosis, while some studies have proven that the real situation of the knee compartment can be reflected by the KAM partially [17][18][19], so other parameters such as spatiotemporal, kinematic/kinetic, morphological, and muscle parameters should be considered to further understand the effects of valgus braces on medial gonarthrosis. Therefore, the purpose of this review article is to critically evaluate the biomechanical effects of the valgus knee braces on patients with medial gonarthrosis such as spatiotemporal, kinematic/kinetic, and morphological parameters. This review would help understand the biomechanical effects of valgus braces and possibly improve their design and effectiveness.

Methods
This review followed the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines [20].
2.1. Search Strategy. In this paper, three online databases (Web of Science, PubMed, and Scopus) were searched by 3 independent reviewers in triplicate for relevant articles from January 1, 2000, to March 1, 2022. Search terms such as "unloader brace", "valgus brace", "medial compartment knee osteoarthritis", "medial gonarthrosis", and "knee orthosis" were used. A manual search of included papers' references and abstracts from recent conferences, as well as Google Scholar, was conducted to find any further related research.
2.2. Assessment of Study Eligibility. The inclusion and exclusion criteria were determined ahead of time. The following criteria were used to determine eligibility: (1) the treatment of medial gonarthrosis, (2) at least one knee brace, and (3) biomechanical evaluation of knee braces. Exclusion criteria included (1) nonhuman studies, (2) other types of orthoses instead of knee valgus brace, (3) other pathological conditions, (4) studies involving the effect of orthotic devices on other knee disorders, (5) pain or activity level outcome using Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC), 36-item Short-form Health Survey (SF-36), Knee Injury and Osteoarthritis Outcome Score (KOOS), Visual Analog Scales (VAS), etc., and (6) systematic reviews or meta-analyses. If an article contained both biomechanical and clinical results, the article would be retained, and only the biomechanical results were analyzed.
2.3. Study Screening. Three reviewers (YZY, LZ, and RTG) independently screened all titles, abstracts, and full-text articles. Any discrepancies at the title and abstract stages were tolerated, and the articles were forwarded to the next step of screening to ensure that relevant articles were not overlooked. The three reviewers discussed their disagreements at the full-text stage. When they could not reach a consensus, the opinion of the senior reviewer (GL) was considered to determine the eligibility of the article.

Results
Following the selection procedure, 34 studies that evaluated the effects of valgus braces on medial gonarthrosis were matched the inclusion criteria in this review. The search process is demonstrated using the diagram in Figure 1, and the biomechanical parameters summarized in this review are shown in Figure 2 3.1. Brace Condition. Valgus knee brace, as a common and effective conservative treatment, applies bending moment at the knee joint, modifies the knee joint alignment in the coronal plane, shifts the load from the medial compartment to the lateral compartment, and reduces the forces in the medial compartment. The use of different types of valgus braces has been reported to delay the deterioration of the medial gonarthrosis and reduce knee pain. The traditional valgus brace currently on the market could be divided into two types, single-hinged and double-hinged, as shown in Figures 3(a) and 3(b). In recent years, researchers optimized the braces to make patients feel comfortable, mainly by adding the air cushion (Figure 3(c)), increasing the freedom of the brace (Figure 3(d)), and introducing the control systems ( Figure 3(e)). The 34 articles included in this article contained 19 different types of knee valgus braces, including 7 types of single-hinged braces, 6 types of double-hinged braces, and 6 types of modified braces (3 with air cushions, 2 with structural optimization, and 2 with control system). Table 1, the spatiotemporal parameters reviewed in this review included walking speed, cadence, step length and stride length, stride width, and foot progression angle (FPA). The variations of spatiotemporal parameters between the braced and unbraced conditions are shown in Table 2.  [26, 30-32, 35, 36, 38, 39]. However, seven studies reported no significant difference in walking speed when patients wore the brace [21, 27-29, 33, 34, 37], and two of those studies even showed negative effects of valgus braces on walking speed [29,33].

3.2.2.
Cadence. The number of steps taken per minute is called the cadence, also known as the step rate [40]. Four studies discussed the effects of valgus braces on cadence. Two studies found that cadence was significantly raised after wearing the brace, with an increase of 2.80% [38] and 4.40% [39], respectively. However, two studies showed no statistical difference between braced and unbraced conditions [27,33].

3.2.3.
Step Length and Stride Length. The distance in one step is called the step length, and the distance between two consecutive heel contacts of the same foot is called the stride length [40], as described in Figure 4. Three studies found that the valgus braces were effective in improving step length and stride length in the arthritic limb, with a maximum increase of 16.90% and a minimum increase of 5.66% [26,30,38], as shown in Table 2, whereas six studies showed that the braces had no significant improvement in step length and stride length [27,33,34,39] and even had a negative effect on step length due to the restriction of range of motion (ROM) in the sagittal plane [21,31].

3.2.4.
Step Width. As shown in Figure 4, the distance between the centers of heels of two consecutive feet touching each other is called the step width [40]. Despite two studies Spatiotemporal Walking speed candence Step/stride length stride width  3 Applied Bionics and Biomechanics reported the change in step length, there was no statistical difference in either study [21,36], as shown in Table 2. But Laroche et al. [36] discovered that the stride width of the two limbs tended to be constant after wearing a brace.
3.2.5. FPA. The angle generated between the longitudinal axis of the foot and the forward line of progression when walking is known as the FPA [40,41], which has been associated with the KAM and might be used to reduce joint loading and pain [42][43][44][45]. Laroche et al. [36] revealed that the FPA increased by an average of 36.76% after wearing a brace. However, Brand et al. and Gaasbeek et al. found no significant difference in FPA compared to the unbraced condition [27,31].

Kinematic and Kinetic Parameters.
Currently, researchers have focused on the biomechanical effects of valgus braces, especially whether the kinematic and kinetic parameters changed after the patients wearing the brace, as shown in Table 3. 3.3.1. Knee Joint Angle. The knee joint angle could be divided into the flexion-extension angle (sagittal plane) and the adduction-abduction angle (frontal plane), as shown in Figure 5(a). The effects of valgus braces on knee joint angle were investigated in five studies. Laroche et al. [36] discovered that the knee extension angle at heel strike (HS) and midstance (MS) was significantly increased after wearing the brace. Brand et al. [27] indicated that the knee adduction angle in braced condition was decreased by 84.31% and 55.31% at HS and push-off (PO), respectively. Toriyama et al. [39] found that only the contralateral knee adduction angle was significantly increased by an average of 0.32°during 46%-55% of the stance phase in the braced condition. Two studies showed that the flexion-extension angle did not significantly change during the gait [29,49].

Knee Range of Motion.
Knee ROM is mainly represented as the range of flexion/extension angles during knee movement, as shown in Figure 5(a). Four studies investigated the effects of valgus braces on the knee ROM. Arazpour et al. [26] discovered that knee ROM was decreased by 11.36% after wearing the brace. Fesharaki et al. [30] found a maximum reduction of 25.68% in knee ROM with wearing the brace. Gaasbeek et al. [31] reported that the brace significantly decreased the ROM by 5.45%. However, one study [33] found no significant difference in knee ROM between braced and unbraced conditions.
Other variables were not significantly changed. ). Three studies found no significant difference between braced and unbraced conditions [27,33,52]. However, Schmalz et al. [38] argued that the horizontal force on the arthritic limb was increased by 16.4% BW in brace condition. One study observed the changes in foot pressure. Kim et al. [50] found that the lateral-side foot pressure was significantly reduced during the stance phase.  [21,22,26,28,29,32,37,46,54], while nine studies reported the change of the first and second peak KAM separately [24,27,31,35,36,39,47,48,54]. Most studies showed that the braces significantly reduced the KAM, with a maximum reduction of 48% and a minimum reduction of 3.63%. But two studies found no significant difference in the KAM [22,46], as shown in Table 4. Meanwhile, two studies showed a significant trend of improvement in the KFM [21,39].
3.3.5. Knee Adduction Moment Impulse (KAI). KAI, which considers both the magnitude and duration of stance, is more sensitive at differentiating between disease severities and may provide a more comprehensive description of medial knee joint load [56,57], as shown in Figure 5(d). Three studies indicated that braces were effective in reducing KAI. Fantini et al. [47,48] discovered a maximum reduction in KAI of 36% when the brace set to 8°valgus mode. Lamberg et al. [35] demonstrated that the KAI in the second half of the stance phase was significantly decreased by 36%. However, Laroche et al. [36] held an opposite view, and they reported that KAI did not show any significant difference.
3.3.6. Knee Contact Forces and Stress. The KAM was considered to be a surrogate measure of knee contact force (KCF), while researchers discovered a moderate correlation between KAM and KCF [58][59][60]. To better understand the effects of valgus brace on the knee compartment, the researchers calculated KCF and cartilage stresses through musculoskeletal models, and the typical results are shown in Figure 5(e). Six studies reported the changes in KCF. Four studies reported that the medial knee compartment contact force (MKCF) was significantly reduced in brace condition [22,34,37,49], indicating Step width FPA Figure 4: Spatial description of gait. FPA: foot progression angle. 6 Applied Bionics and Biomechanics 7 Applied Bionics and Biomechanics that the load was shifted to lateral compartment. Two studies discovered no significant difference in the peak KCF between the braced and unbraced conditions [21,33]. Three publications mentioned the change of contact stress by using the finite element method. Two studies confirmed that the brace significantly reduced the peak contact stress in the medial compartment while increased in the lateral compartment [51,55]. One study [53] showed that the mean contact stress and contact area of the medial compartment were not changed when the knee joint was at 5-10°and 15-20°flexion states.
3.3.7. Interactive Force. The interactive force could be divided into the valgus force, brace abduction moment (BAM), and shear force. The shear force is commonly generated at the interface between the brace and the thigh. Six publications evaluated the interactivity force between the brace and the human body. One study reported a maximum valgus force of 60 N, which was maintained constant throughout the stance phase [54]. Three studies described the BAM, which was around 10% of the KAM, and suggested that BAM was associated with valgus angulations or strap tensions [25,37,38]. Two studies mentioned that the 2 degree-of-freedom (DOF) brace decreased the external shear force by 41:31 ± 8:34 N compared to the 1-DOF brace when knee joint at 90°flexion states [24,30], as shown in Figure 5(f).

Morphological Parameters.
Lower limb malalignment, dynamic knee joint space, and medial cartilage crosssectional area (MCCA) are the morphological parameters of interest to researchers, as shown in Table 5.

Lower Limb Malalignment.
Lower limb malalignment is presumed to cause and/or accelerate gonarthrosis, which is usually denoted by the knee varus angle (Figure 6(a)) [7,[65][66][67]. Two studies used anterior/posterior X-rays to observe lower limb malalignment under static standing conditions. Arazpour et al. [23] found that the varus angle was decreased by 6°when the patient wore the brace. Draganich et al. [29] reported that the knee varus angle was decreased by an average of 1.5°after wearing the bespoke brace. The total contact force, contact area, and contact pressure of the medial and lateral compartment were significantly changed.
Toriyama et al. [39] Single-hinged brace To understand the effects of valgus braces on the knee joint space during walking, the researchers used biplane radiographs or three-dimensional (3D) fluoroscopy methods to analyze the dynamic changes in the knee joint space [68,69], as shown in Figure 6(b). Four studies revealed the effects of valgus braces on the treatment of gonarthrosis. Dennis et al. [61] reported that the medial compartment separation increased by an average of 1.3 mm during the gait cycle. Dessinger et al. [62] found that 80% of patients experienced a medial joint space increase of more than 1.0 mm at heel strike, while 65% had a similar change during midstance. Nagai et al. [52] showed that the dynamic space of

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Applied Bionics and Biomechanics the medial compartment was significantly increased by 0.3 mm after wearing the brace. In addition, one study [63] found no significant difference in the separation of the medial and lateral compartments between the braced and unbraced conditions.

MCCA.
The articular cartilage response to loading is dependent on the magnitude and rate of the load [70,71], so observing the deformation of MCCA by ultrasonic testing techniques can be used to evaluate the effects of valgus braces, as shown in Figure 6(c). In 2019, Pfeiffer et al. [64] showed no significant difference in percent change of MCCA between the braced and unbraced conditions.

Muscle Parameters.
Only five (of 34) studies examined the effects of valgus braces on muscle parameters such as muscle activation, muscle strength, relative contribution of muscle, and cocontraction ratios (CCRs), as shown in Table 6. Brandon et al. [22] reported that only the electromyography (EMG) of the biceps femoris was significantly reduced during walking after wearing the brace, as shown in Figure 7(a). Fantini et al. [72] found that the muscle activation and CCRs were significantly reduced in all muscle groups (rectus femoris, lateral hamstring, and gastrocnemius lateralis), but only the 4°valgus mode caused differences in CCRs between the muscle groups.
Hall et al. [21] found that the peak medial-to-lateral muscle CCRs were reduced and the peak extensor-to-flexor muscle CCRs increased at midlate stance, as shown in Figure 7(b). Total muscle activation and relative contribution of muscles to medial compartment load were not significantly changed compared with the unbraced condition, as shown in Figures 7(c) and 7(d). But the relative contribution of muscles to the lateral compartment was increased by 2.35% after wearing a brace. Thigh girth measurement was used as a biomarker of quadriceps strength by Johnson et al. [32], and patients who satisfied the criteria showed an average increase of 1.90 cm in thigh girth measurement after three months of treatment. Ebert et al. [46] found no significant difference in total muscle activation and the mediolateral-directed CCRs with and without wearing the brace, which might be attributable to the fact that the experimental subjects were healthy individuals.

Discussion and Conclusions
This systematic review was conducted to evaluate the spatiotemporal, kinematic/kinetic, morphological, and muscle effects of valgus braces on patients with medial gonarthrosis, suggesting that the valgus brace could significantly change the biomechanics of the knee joint during daily activities through a multitude of mechanisms. Studies found that the potential mechanisms of valgus braces included applying valgus moments at the knee to directly oppose KAM [21,23,26,28], altering the alignment of the lower limbs in the frontal plane [23,29], increasing medial joint space during gait [52,61,62], and increasing knee stability to reduce muscle cocontraction [21,72].
This systematic review showed that the biomechanical effects of valgus braces on patients with gonarthrosis were still contradictory. The contradictory results between the studies might be associated with the differences in the type of braces and the duration of treatment [22,35,40,61] or might also be related to the physiological conditions of the subjects [46,62]. Most researchers showed that the knee braces could reduce the KAM [26,29,32,35,36,47,48] and KCF [21,22,33,34], while other biomechanical parameters were not significantly changed. Even though KAM and KCF were closely related to the progression of gonarthrosis, they did not cause gonarthrosis alone. Therefore, the effects of valgus braces on biomechanical parameters should be fully considered in future studies.  [22]. (b) Cocontraction ratios (CCRs) with/without brace [21]. (c) Muscle activation with/ without brace [21]. (d) Relative contributions of muscle to medial and lateral compartment tibiofemoral contact force with/without brace [21]. M+L: medial and lateral directed; F+E: flexion and extension directed.

Applied Bionics and Biomechanics
Some limitations of the current studies must be noted. First, the knee ROM is significantly recued, which might lead to a reduction in foot clearance and a shorter step length [26,30,31]. Second, few studies evaluate the patient compliance. Studies reported that patient compliance reduced as the duration of treatment increased, which might affect the real treatment effects of valgus braces [73,74]. Third, the quantitative relationship between the valgus angle and the KCF is not established. The valgus force was empirically adjusted to the level of comfort accepted by the patients, which may lead to different treatment effects on different patients. Fourth, the long-term biomechanical effects of valgus braces are not studied. The valgus braces must be used for several years as a conservative treatment for gonarthrosis. The load transferred to the lateral compartment by the valgus braces might aggravate the wear of lateral cartilage, which might have negative effects in the long term [16,22].
This review still has some limitations. First, only studies published in English were included, which created a language bias in the selection of articles. Second, the review findings are limited to the studies identified by the set search strategy. Third, we were unable to pool data in the form of a meta-analysis because of the heterogeneity of the outcome measure and the various comparison groups used in the included studies.
In conclusion, this review showed that the valgus knee brace can effectively improve the symptoms of medial gonarthrosis through multiple mechanisms, primarily by decreasing the varus angle, reducing the KAM, and redistributing the knee compartment loads. However, the current studies suggested that the effects of valgus braces on other biomechanical parameters were still controversial.

Conflicts of Interest
The authors have no conflicts of interest to declare.