Foot pain is a common complaint, affecting almost one in four adults aged over 45 years [1
]. Obesity is a risk factor for the development of foot pain [2
], and an elevated body mass index (BMI) is strongly associated with both chronic plantar heel pain and non-specific foot pain [3
]. Moreover, the feet of people with obesity are structurally and functionally different to their non-obese counterparts, manifesting as thicker, wider and larger, along with flatter-foot postures, reduced joint range of motion and increased peak plantar pressures [4
]. It is therefore conceivable that foot pain in people with obesity is related to these mechanical adaptations, particularly the flattening of the foot arches and the increase in plantar pressures.
Studies have found that people with obesity display increases in plantar pressures that are not uniform, with the areas of highest pressure being the midfoot and forefoot, when compared to non-obese people [5
]. Given that obesity is strongly associated with plantar heel pain [8
], increased plantar pressure elsewhere is discordant if pain was strongly related to excessive pressure. Paradoxically, people with chronic plantar heel pain display reduced
loading under the heel when compared to controls [10
]. Indeed, pain may persist even when gait patterns change to offload a painful region of the foot. Thus, chronic foot pain in people with obesity may be more than mechanical overload, involving a complex interplay between mechanical, metabolic and psychological factors.
Musculoskeletal pain has a bidirectional relationship with both obesity [11
] and depression [12
], while depression and obesity also amplify each other [13
]. These relationships, however, are not limited to weight-bearing joints with a known association between elevated BMI and symptomatic hand osteoarthritis [14
], suggesting that metabolic mechanisms, including systemic inflammation [15
], may underpin the relationship between obesity and joint pain [16
]. Whilst these relationships exist in the general population, it is particularly pertinent in bariatric surgery candidates, who are over-represented amongst those complaining of musculoskeletal pain [17
]; with foot and ankle pain prevalence cited as 34–50% [18
]. There is evidence that spatiotemporal gait patterns, such as increased limb swing and decreased double-limb support time [20
] improve following bariatric surgery, but currently only limited investigations regarding associations between weight loss in a bariatric cohort and changes in foot pain, foot function and foot posture. In order to effectively develop and understand treatment methods, it is important to determine whether weight loss has a direct influence on foot structure and function that could be linked with pain, given the high prevalence of obesity across the community [21
Despite a high prevalence of foot pain, depression and elevated plantar pressures, there is little evidence that mechanical and non-mechanical factors relate to foot pain before and after bariatric surgery. Therefore, the aims of this study were to investigate changes in foot pain, posture and function after bariatric surgery compared to a group remaining on the waiting-list, acting as controls, and to determine the factors related to changes in foot pain post-surgery.
This is the first study to comprehensively examine the effect of weight loss following bariatric surgery on foot pain, and to explore the mechanical and non-mechanical factors associated with foot pain severity. Depressive symptoms were associated with foot pain severity at baseline, after accounting for age, gender, BMI, foot posture and plantar pressure. At follow-up, foot pain severity was associated with depressive symptoms in those who had undergone bariatric surgery. The change in plantar pressure, walking speed or ankle joint dorsiflexion was not associated with a change in foot pain following bariatric surgery, but weight loss following bariatric surgery resulted in a significant reduction in foot pain severity at 6-months. Therefore, in this cohort, both baseline foot pain and change in foot pain appear more strongly related to non-mechanical or non-local factors.
Previous studies examining the association between the change in weight and change in plantar pressure have largely focused on the effect of weight gain. The effect of weight gain on plantar pressures has been frequently performed on asymptomatic participants, using weighted backpacks as a proxy for the increase in weight [8
]. This method, while practical, measures the instantaneous effect of a change in weight and does so in asymptomatic feet, and therefore may not accurately reflect how pressures change over time in symptomatic feet. This method is also impractical in assessing weight loss. Investigations analysing the effects of weight loss are limited, although a randomised controlled trial investigated the effects of non-surgical weight loss on plantar pressures, albeit in asymptomatic participants, and found that even a small amount of weight loss can significantly reduce mean peak plantar pressure across multiple regions of the foot [40
]. This study found that larger weight loss following bariatric surgery results in widespread reductions in plantar pressures in symptomatic feet, with the largest reduction in plantar pressure found in the midfoot.
Interestingly, the reduction in midfoot pressure in the treatment group occurred without a significant change in radiographic foot posture and may therefore be related to soft tissue changes. A previous cross-sectional study found that people with obesity have increased three-dimensional foot circumference at multiple sites when compared to people with a healthy weight [41
], these differences are likely soft tissue related. Moreover, the change in BMI and midfoot pressure is concordant with a previous study investigating weight gain and plantar pressure [42
], and suggests that the midfoot may be a region that is the most responsive to a change in weight. The reduction of force in the midfoot in our study may have resulted in a larger reduction in peak pressure, if it were not for the significant simultaneous reduction in contact area of the midfoot. Furthermore, a study investigating contact area and body composition found a positive association between total body fat mass, but not fat-free mass, and the midfoot contact area only [43
]. Together, these findings suggest that people with higher fat mass may deposit fat mass in the midfoot and, following bariatric surgery, there may be a loss of this fat mass that could appear to elevate the longitudinal arch of the foot. This may have implications for the fit of footwear or orthoses following soft tissue adaptations after bariatric surgery, and in patients with significant weight loss.
The association between foot pain severity and depressive symptoms has previously been established in a community cohort [44
], suggesting that foot pain may be a manifestation of either widespread or reduced threshold for pain that extends beyond localised discomfort. The results of our study are concordant with this premise, although ours are unique given they are exclusively from a bariatric cohort and we were able to adjust for local foot measures, including foot posture and plantar pressure at baseline and follow-up. Given the high prevalence of depression in bariatric surgery candidates, and the improvement in depressive symptoms following surgery [45
], it is possible that foot pain severity is mediated by depressive symptoms, rather than by body weight alone. There is evidence that while depressive symptoms improve in the short-term following bariatric surgery, there may be attenuation of this improvement in the longer term, and indeed some people have increased depressive symptoms following surgery, often with concomitant weight regain [46
]. Whether this causes an exacerbation of musculoskeletal pain is not known, but this may be worth exploring.
This study should be considered in light of some limitations. Firstly, the small sample size limited the number of variables we could include in our models and may have may resulted in type II errors for the variables we did include. Secondly, the cohort was recruited via convenience sampling and consisted of mainly women which may limit the generalisability (and the ability to analyse between gender comparisons) of the findings, however, this is consistent with the demographics that present for bariatric surgery thus findings are applicable to that context [47
]. Thirdly, the spatial resolution of the MatScan® plantar pressure system is relatively low and thus the sensitivity to detect all changes in plantar pressure may have been compromised. Furthermore, the importance of the size of the sensors used in plantar pressure systems has also been well described [48
], and this may have impacted on measuring contact area, particularly for the lesser toe region, which is prone to measurement error [49
]. Whilst there are limitations regarding sensor size and spatial resolution, the detection of subtle changes in plantar pressure was less important given the gross changes in body mass (and pressure) that occurs following bariatric surgery. Fourthly, the duration of foot pain was not recorded, so there may be variation of pain duration prior to participant enrolment. Finally, the change in foot pain and plantar pressure was measured over a six-months, and may not reflect changes seen over longer periods.
Nonetheless, this study has a number of strengths. It is the first to examine foot pain, foot posture and plantar pressures in bariatric candidates, reporting the effect of bariatric surgery on all variables. This study also considered relationships between mechanical and non-mechanical factors and foot pain severity at baseline and prospectively following bariatric surgery.
The results of this study provide proof of concept that weight loss improves foot pain, and future studies in this area, including non-surgical weight loss strategies and less obese cohorts, may be warranted. Deeper analysis of gait characteristics before and following weight loss in people with foot pain, may also determine if changes occurring beyond peak plantar pressures are important to consider in this cohort, and even footwear choices may be relevant.