Association between knee osteoarthritis and foot deformities: epidemiological analysis of hallux valgus and flatfoot

Abstract

Objective

To investigate the association between knee osteoarthritis (KOA) and the presence of hallux valgus and flat feet.

Method

This retrospective study analyzed KOA patients from January 2022 to March 2024, investigating the correlation between KOA and hallux valgus and flatfoot. The diagnosis of KOA and assessment of its symptoms were conducted using the diagnostic criteria and grading system established by the American College of Rheumatology, specifically the Kellgren-Lawrence (K-L) grading system and measurement of the femorotibial angle (FTA). Physical examination, weight-bearing X-ray examination and MRI evaluation of the knee were performed. Foot examination included weight-bearing and lateral X-ray imaging to determine the presence of hallux valgus and flat feet.

Results

A total of 168 patients met KOA criteria: 162 with genu varum, 6 with genu valgum and 39 with pes anserine tendinitis. Among those with genu varum, 145 had hallux valgus, 136 had flatfoot and 39 had pes anserine tendinitis. Gender distribution: 37 male (22.02%) and 131 female (77.98%). The mean age was 73.84 ± 4.96 years. Spearman correlation analysis showed significant associations between the severity of KOA (K-L grade) and hallux valgus (r = 0.681, p < 0.05) and flatfoot (r = 0.689, p < 0.05).

Conclusion

There was a significant correlation between KOA, hallux valgus and flatfoot. However, due to the study’s cross-sectional design, causality cannot be established. There was also a significant link between knee varus and pes anserine bursitis.

Introduction

KOA is a prevalent musculoskeletal disorder characterized by degenerative changes that adversely affect soft tissues, degrade cartilage and induce sclerosis in the underlying bone. The manifestation of severe KOA is often accompanied by significant joint discomfort, amplifying the risk of falling [1,2]. The management of KOA presents a multifaceted challenge. In cases of mild KOA, therapeutic interventions involve oral medications, local joint injections and localized physical therapy [3]. In instances of severe cases, surgical intervention is contemplated, such as high tibial osteotomy, unicondylar replacement and total knee arthroplasty. However, the result is unpredictable and cannot ensure a total remedy, leading to less than ideal postoperative results [4,5].

A comprehensive and unequivocal understanding of the variables, mechanisms and potential pathologies contributing to KOA remains elusive. Contentious discussions persist regarding various risk factors associated with the condition, encompassing factors like advancing age, gender, genetic susceptibility, higher body mass index occupation and heightened physical activity [6,7]. Simultaneously, more attention has been paid to the relationship between leg posture and KOA. When leg posture changes, the normal lower limb alignment is altered, leading to abnormal pressure distribution within the knee joint and changed mechanical stress on the lower limb joints, thereby raising the incidence and risk of KOA [8]. Overloading the knee joint can result from factors that elevate compression and/or shear stress within the tibiofemoral or patellofemoral compartments. The alignment of the hip joint, knee joint and ankle joint does indeed affect the load distribution on the knee joint: inward rotation (pronation) increases the medial load [9]. Patients with flat feet typically have concurrent hallux valgus and may also have knee joint pain and degeneration [10]. The biomechanical parameters linked with the start of KOA are not restricted to the knee joint alone, but are also closely related to the joints that surround it. Several investigations on the link between hallux valgus, flatfoot and KOA have been undertaken. However, there has been little research in this field, and there are many disagreements. Flat feet are linked to frequent knee pain and medial tibiofemoral cartilage injury [11], with studies indicating a higher likelihood of developing hallux valgus in individuals with KOA and flat feet [12]. Hallux valgus, in turn, may exert excessive strain on the knee’s medial compartment [13]. Gait analysis reveals increased adduction moments in KOA patients, a factor associated with the condition. The anatomical structure of the foot is implicated in KOA development, but the biomechanical connection between hallux valgus, flat feet and KOA remains a subject of debate.

Although previous studies have suggested an association between foot deformities and the onset of KOA, the precise biomechanical mechanisms remain unclear. This study aimed to explore the correlation between foot deformities (such as hallux valgus and flatfoot) and the severity of KOA. Our hypothesis was that foot deformities might alter lower limb biomechanics, increasing the load on the knee and correlating with KOA severity. This study investigated the relationship between knee varus and KOA severity through retrospective analysis. The study found a significant association between KOA and foot deformities, particularly hallux valgus and flatfoot.

Methods

The study was approved by the Ethics Committee of The First Affiliated Hospital of Zhejiang Chinese Medical University (approval number: 2024-KL-055-01).

Basic information

From January 2022 to March 2024, a retrospective study was conducted on KOA patients, with an average age of 73.84 ± 4.96 years. The sample size calculation was performed, and the necessary statistical power was ensured.

The inclusion criteria: (i) The diagnosis of KOA was based on the American Rheumatism Association’s diagnostic criteria [14]; and (ii) radiographic changes in the tibiofemoral joint space, and Kellgren-Lawrence (KL) grade 2 or above.

The Exclusion criteria: (i) Patients with a history of pelvic and lower limb trauma leading to changes in lower limb force lines; (ii) Undergone lower limb surgery; (iii) Individuals with severe cardiovascular and cerebrovascular, liver, kidney and haematopoietic system diseases, as well as mental illness; (iv) Neurological diseases causing muscle dysfunction; (v) Traumatic KOA; and (vi) Rheumatic autoimmune diseases.

Evaluation of OA-related knee symptoms

Each participant requires three examinations: (i) Physical examination to determine whether there is flatfoot or hallux valgus, including pain of the pes anserinus tendinitis; (ii) Full-length standing weight-bearing X-ray of both lower limbs and lateral X-ray of the knee joint; and (iii) Magnetic Resonance Imaging (MRI) of the knee joint.

For evaluation, the Kellgren-Lawrence (K-L) radiological scale was used[15]. A K-L grade of 0 to 1 indicates that there is no osteoarthritis (OA), patients included in this study had to have either a KL score of 2 to 4 or be receiving treatment for KOA.

Foot assessment

Including full-length weight-bearing radiographs of the lower limbs and weight-bearing lateral radiographs, were assessed by two attendings to determine if they fit the hallux valgus and flatfoot study criteria [16,17].

Diagnosis criteria for knee varus and valgus

The diagnostic criteria for genu varum (knee varus) and genu valgum (knee valgus) are based on the angle between the sagittal axis of the knee joint and the axis of the lower limb. A diagnosis of knee varus or valgus is made when the angle exceeds 5 degrees [18].

Pain assessment

Pain was assessed using the Visual Analog Scale (VAS) by trained clinical evaluators. The VAS ranges from 0 to 10, where 0 represents no pain and 10 represents extreme pain. All patients underwent pain evaluation at the time of enrolment, ensuring that no external pressure was applied during the assessment.

Reducing bias

We have implemented measures to minimize bias in the data collection and analysis process, including having all evaluations conducted by two attending physicians, with double-checking and independent assessments. In cases of discrepancies between the opinions of the two doctors during the evaluation, the final judgement was made by the chief physician.

Statistical analysis

The statistical analysis of case data from the information collecting table was conducted using SPSS 25.0. Initial analysis utilized the Chi-square test (p < 0.05). The chi-square test examined KOA severity in relation to hallux valgus and flatfoot, and Spearman correlation analysis assessed correlations. A Z-test for independence determined statistical significance.

Power analysis

Power analysis was conducted using G*Power 3.1.9.7 with an effect size of 0.681, which was based on the correlation between KOA severity and foot deformities. The significance level was set to 0.05, and the desired statistical power was 0.80. The minimum required sample size was calculated to be 14. With a sample size of 168, the actual statistical power was calculated to be 0.818, exceeding the standard threshold of 0.80. This suggests that the study has sufficient statistical power to reliably detect significant correlations between knee osteoarthritis and foot deformities.

Results

Initially, 210 individuals were enrolled, among whom 24 cases did not meet the inclusion criteria, 14 cases lacked foot characteristic data and 4 cases lacked knee imaging data. Finally, 168 patients were included in this study. Among the 168 patients: 37 males and 131 females, with an average age of 73.84 ± 4.96 years. There were 162 cases of genu varum, 6 cases of genu valgum and 39 cases of pes anserine tendinitis. Among the patients with genu varum, 145 had hallux valgus, 136 had flatfoot and 38 had pes anserine tendinitis. In patients with genu valgum, no cases of flatfoot, hallux valgus were observed (Table 1).

Table 1.

Basic patient information (n = 168).

Category	Case information	Number of patients (percentage)
Gender	Male	131 (77.98%)
	Female	37 (22.02%)
Age	73.84 ± 4.96
Hallux valgus	Without	25 (14.88%)
	With	143 (85.12%)
Flatfoot	Without	32 (19.05%)
	With	136 (80.95%)
Genu valgum	Without	162 (96.43%)
	With	6 (3.57%)
Varus knee	Without	6 (3.57%)
	With	162 (96.43%)
Medial compartment knee osteoarthritis	With	148 (88.10%)
Severity of knee OA (K-L grade)
	Grade2	30 (17.86%)
	Grade 3	89 (52.98%)
	Grade 4	49 (29.17%)
Pes anserinus tendinitis	Without	129 (76.79%)
	With	39 (23.21%)
Medial meniscus injury	With	138 (82.14%)

Considering the age group of the participants (73.84 ± 4.96 years), this population exhibits a high prevalence of KOA and foot deformities. The reason for choosing this group was to explore the role of hallux valgus and flatfoot in high-risk populations, providing a basis for early intervention and prevention in elderly individuals.

The statistical analysis revealed that among patients with hallux valgus or flat feet, higher K-L grades were associated with significant differences (p < 0.05). The prevalence of hallux valgus increased markedly in patients with higher K-L grades. Similarly, a chi-square test demonstrated a significant relationship (p < 0.05) between the severity of KOA (K-L grade) and the presence of flat feet, with flat feet being more prevalent in patients with higher K-L grades (Table 2). The study can only assess the correlation between KOA and hallux valgus or flatfoot, and cannot evaluate causality.

Table 2.

The distribution of hallux valgus and flatfoot in different K-L grades of knee osteoarthritis.

	With Hallux valgus		With flat foot		With hallux valgus and flat foot
	Yes	No	Yes	No
Grade 2	18	12	16	14	8
Grade 3	79	10	76	13	51
Grade 4	46	3	44	5	32
Total	143	25	136	32	91

The analysis of the relationship between K-L grade and hallux valgus shows a p-value <0.01 and an r-value = 0.335, indicating a statistically significant positive correlation. The analysis of the relationship between K-L grade and flat foot shows a p-value <0.01 and an r-value = 0.332, indicating a statistically significant positive correlation.

Discussion

Varus knee and medial compartment KOA

Patients with KOA exhibit a substantial increase in the coronal plane angle, exacerbating medial loading [19]. This reinforces the viewpoint that knee internal rotation increases the risk of medial compartment KOA. Moreover, poor knee alignment is a robust independent risk factor for continuous OA progression [20,21]. Visualizing the load axis from the femoral head centre to the ankle joint centre as a weight-bearing axis is an effective approach. In the case of knee internal rotation alignment, this axis intersects from the knee’s medial side to the centre, reinforcing force through the medial tibiofemoral compartment. This method provides insight into why individuals with varus knee are more prone to developing medial compartment KOA. Additionally, for both internal and external rotation, there are biomechanical stress distribution changes within the knee joint. However, the alignment associated with internal rotation appears to more easily promote tibiofemoral arthritis [18]. This can be attributed to the inward movement of the weight-bearing axis towards the knee’s centre in knee internal rotation alignment. This creates a lever arm, enhancing the strength of the medial compartment and reducing lateral loads. In contrast, external rotation alignment has the opposite effect. The studies mentioned above collectively illustrate how varus knee disrupts the even distribution of forces across the joint, leading to degeneration of articular cartilage and menisci in the medial region, perpetuating the progression of KOA. This study revealed that 162 cases out of the total participants had varus knee, constituting 96.43%. When considering the K-L grading system and MRI findings, it was found that among the cases of varus knee, 148 cases (91.36%) were diagnosed with medial compartment KOA based on K-L grading. MRI findings further confirmed the presence of medial meniscus damage in 138 cases (85.19%) with varus knee, supporting the correlation between varus alignment and the development of medial compartment KOA, with detailed data available in Figure 1. These results reinforce the association between varus knee deformity and the progression of medial KOA in our patient population. In addition, all participants had a K-L grade of ≥2, indicating that all participants were in the moderate or severe stage of KOA, beyond the early stages as defined by the K-L grading system. The K-L score was set to 2 or higher because we selected KOA patients, and only those with a K-L score of 2 or above meet the diagnostic criteria for KOA. Furthermore, patients with K-L grades of 0 or 1 typically exhibit mild or no symptoms of osteoarthritis, making it less likely for them to seek treatment, and thus it is relatively uncommon to collect these patients in an outpatient setting.

Figure 1.

The quantity and proportion of meniscus injuries at different K-L grades.

As a precursor to varus knee: flat feet and hallux valgus

In an anatomical study, severe ankle degeneration was consistently found to coexist with knee degeneration [22]. This aligns with our observations, where a higher proportion of knee varus and elevated K-L grades were found in patients with hallux valgus and flat feet. There were 91 patients with both conditions.

The results of the chi-square analysis indicate that patients with flat feet have a higher K-L grade compared to patients without foot deformities. Pearson correlation analysis also demonstrates a significant association between medial compartment KOA and flat feet. Flat feet are characterized by a flattened arch, altering force distribution during weight-bearing activities. The loss of a natural arch weakens the foot’s ability to absorb shock and adapt to uneven surfaces [23,24]. Biomechanical models suggest that reduced arch height in flat feet inhibits tibial rotation, exacerbates rotational stresses on the tibiofemoral joint and ultimately leads to cartilage damage in the knee joint [25,26]. This biomechanical shift results in excessive bending – an inward rolling of the foot during gait. Excessive internal rotation, in turn, disrupts lower limb alignment, exerting a cascading effect on the knee joint.

Similar to flatfoot, patients with hallux valgus also exhibit a higher K-L grade compared to those without hallux valgus, and a correlation exists between hallux valgus and medial compartment KOA. Hallux valgus is characterized by the deviation of the big toe towards the other toes. When the big toe deviates laterally, it disrupts the normal toe-off phase during walking [27]. This deviation alters propulsion forces, affecting the biomechanics of the entire lower limb. The compensatory mechanism aimed at reducing joint loads inadvertently increases the internal varus moment at the knee joint, a critical factor in predicting the occurrence and progression of medial KOA. Researchers consistently associate an increase in the internal varus moment with an elevated risk of developing KOA [28,29]. This underscores the importance of carefully examining internal varus moments as they intricately impact the medial aspect of the knee, exacerbating medial loads [30,31].

The interplay of hallux valgus and flat feet exhibits a mutually reinforcing dynamic [10,32,33], which promotes the onset and progression of KOA. These findings are consistent with our statistical results. Compared to patients with isolated hallux valgus or flat feet, those with both conditions show a significantly higher proportion in the higher K-L grades (Figure 2). Hallux valgus can cause instability in the foot during the push out stage [34], exacerbating internal rotation associated with flat feet. The relationship between these two becomes apparent through the interconnection of the lower limb motor chain. Abnormalities in forefoot mechanics propagate upward, affecting knee alignment (Figure 3). The combined effect of flat feet and hallux valgus collaboratively creates a biomechanical environment conducive to the development of knee varus.

Figure 2.

Distribution of different K-L grades for isolated hallux valgus, isolated flatfoot, and hallux valgus combined with flatfoot.

Figure 3.

Flatfoot, hallux valgus, and knee varus result in a mechanical diagram of medial compartment knee osteoarthritis.

Varus knee and pes anserinus

The pes anserinus is an important structure that connects the knee joint and tibia, helping to maintain medial knee stability and playing a key role in preserving the arch structure of the foot and regulating foot stability [35]. In patients with knee varus, flat feet and hallux valgus, tibial external rotation increases the tension and friction on the pes anserinus tendon, resulting in misalignment of the knee joint and elevated intra-articular pressure [11,21]. This misalignment exacerbates friction between the pes anserinus tendon and adjacent structures. Prolonged mechanical stress not only heightens the risk of tendon injury but also triggers inflammatory responses. Sustained loading and friction can activate local inflammatory pathways, such as the release of cytokines, which ultimately worsen the pathological changes in the tendon [36]. Additionally, Laxity of the medial collateral ligament can exacerbate the overuse of the pes anserinus tendon, weakening knee joint stability and increasing load and pressure on the medial side, leading to greater tension on the tendon. Prolonged overuse not only worsens tendon damage but may also accelerate joint degeneration, triggering inflammation and tissue damage [37,38]. Factors such as degenerative changes in the joint that lead to increased medial loading on the knee require further investigation. Among a group of 162 patients with internal rotation of the knee, 38 patients (23.46%) exhibited coexisting pes anserinus pathology, suggesting a significant association between these two conditions. This finding is consistent with previous studies. Research by Abicalaf et al. demonstrated that, based on ultrasound diagnosis, patients with KOA have a high prevalence of pes anserinus tendinitis [39].

This study found a significant positive correlation between KOA and hallux valgus (r = 0.681, p < 0.05), as well as between KOA and flat feet (r = 0.689, p < 0.05). These results suggest a strong link between the presence of hallux valgus and flat feet with the development and severity of KOA. Based on these findings, this study links foot abnormalities (such as flat feet and hallux valgus) to knee varus and KOA, and notes that knee varus may increase the risk of pes anserinus tendinitis, which has not been noted in prior studies. Regarding the relationship between medial compartment KOA, hallux valgus and flatfoot, this paper suggests that hallux valgus and flatfoot are more likely to lead to the development of medial compartment KOA. This assertion is supported by the presence of flatfoot and hallux valgus in children and adolescents, who do not exhibit related knee pain or symptoms and imaging evidence of cartilage damage [40,41]. The precise nature of this relationship requires further research and exploration.

This study has several limitations. First, the distribution of participants across K-L grades was imbalanced, with the K-L grade 3 group being significantly larger than the K-L grade 2 and K-L grade 4 groups. This imbalance may have influenced the results, particularly in the correlation analysis, as the larger K-L grade 3 sample could exaggerate the observed associations. Second, the cross-sectional design of the study limits our ability to establish causality, as it only reveals correlations between variables. While we found significant correlations between KOA and foot deformities (such as hallux valgus and flat feet), we cannot determine the direction of causality. Additionally, the relatively small sample size and the focus on an East Asian population may limit the generalizability of the findings. The imbalance in K-L grading and the exclusion of asymptomatic individuals may further exaggerate the association between foot deformities and severe KOA. Future research should include larger, more diverse populations, balance sample sizes across different K-L grades and explore the biomechanical relationships between foot abnormalities and KOA to provide a more comprehensive understanding.

Conclusions

This study found a significant association between KOA and foot deformities, particularly hallux valgus and flatfoot. However, due to the cross-sectional design, the study cannot establish causality and does not confirm that these foot deformities directly cause KOA. Additionally, knee varus is associated with pes anserine bursitis. This study suggests that there are some correlation between knee varus, hallux valgus and flat feet, which could collectively influence the onset and progression of KOA.

Funding Statement

There was no funding source.

Ethic approval and consent to participation

This study was approved by the Ethics Committee of The First Affiliated Hospital of Zhejiang Chinese Medical University (approval number: 2024-KL-055-01). All methods were performed in accordance with the relevant guidelines and regulations, including the principles outlined in the Declaration of Helsinki. All patients provided informed consent to participate in the study.

Consent for publication

All patients confirmed consent to publish these details.

Informed consent

Written informed consent was obtained from all participants included in the study.

Disclosure statement

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Source of research data

The participant data for this study was obtained from The First Affiliated Hospital of Zhejiang Chinese Medical University.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.