Spatiotemporal gait parameters and plantar pressure distribution during barefoot walking in people with gout and asymptomatic hyperuricemia: comparison with healthy individuals with normal serum urate concentrations

This investigation was a cross-sectional observational study. Ethical approval for the study was obtained from the Auckland University of Technology Ethical Committee (13/100) and Locality Assessment was obtained from the Auckland District Health Board (ADHB) Research Office (A + 5891). All participants provided written informed consent prior to data collection.

Participants with gout were recruited from the ADHB rheumatology clinic and met the 1977 preliminary American Rheumatism Association (ARA) classification criteria for gout [25]. Participants without gout were recruited from the local community and were to have no history of acute arthritis nor meet the ARA criteria. Participants without gout underwent serum urate testing on the day of the study using a Reflotron® Plus capillary test and were stratified into either the asymptomatic hyperuricemia group (serum urate ≥0.41 mmol/L) or the normouricemic control group (serum urate <0.41 mmol/L, 6.8 mg/dL). The three diagnostic groups were age-and sex-matched. Participants were excluded if they were under 20 years of age; had a history of other inflammatory arthritis; were experiencing an acute flare at the time of the clinical visit; had foot and/or ankle surgery in the previous 3 months; had lower limb amputation; or were unable to walk 5 m unaided.

All data were collected during a single session at the Auckland University of Technology Podiatry Clinic (Auckland, New Zealand) by a single researcher (SS). Demographic data were obtained from all participants including age, gender, ethnicity, body mass index (BMI), current medications and medical history. Peripheral sensation was assessed using a 10 g Semmes-Weinstein monofilament at the plantar hallux, first metatarsal head and fifth metatarsal head. A loss of protective sensation for each foot was defined if absent in at least two of the three sites. Additionally, gout disease characteristics were documented for participants with gout including disease duration, flare history, presence of subcutaneous tophi and tophus count.

Spatial and temporal parameters of gait during level barefoot walking were collected using the GAITRite system (CIR Systems, Inc., New Jersey, US). The GAITRite is a 700 cm × 90 cm electronic walkway with an active sensor area of 610 cm long and 60 cm wide. The active area contains 23,040 embedded pressure-activated sensors with a spatial resolution of 1.27 cm and a sampling rate of 120 Hz. All data was processed and stored by an IBM compatible computer using GAITRite® gold, Version 3.2b software. Participants were instructed to walk at their own comfortable walking speed [26] from a point 100 cm before the walkway and finishing 100 cm past its end to ensure that when they reached the walkway they were walking at a normal speed and momentum. Three trials of barefoot walking were recorded for each participant with adequate rest time between trials. Prior to calculation of the gait parameters, the data was reviewed on the monitor screen to ensure that right and left footfalls had been correctly identified, and any footfall not completely on the walkway at either end was removed. For each trial the following temporal and spatial parameters for right and left feet were calculated: velocity (m/s), cadence (steps/min), step length (cm), stride length (cm), support base (cm), step time (s), swing time (s), stance time (s), and single and double support time (s).

Dynamic plantar pressure measurements were captured during level barefoot walking using the TekScan MatScan® system (Boston, MA, USA). The system consists of a 5 mm thick platform (432 × 368 mm), comprising of 2288 resistive sensors (1.4 sensors/cm²) which sample data at a frequency of 40 Hz. Data was collected using the two-step gait initiation protocol [27] which required the participant to step on the platform on their second step. Prior to data acquisition participants were instructed to familiarise themselves with the protocol and line themselves up with the platform to ensure their second step landed in the sensing area. Participants were instructed to walk at their own natural comfortable walking speed and to continue walking past the platform for at least two more steps which ensured that a constant velocity and momentum had been reached and pressure data reflected their normal gait. Three trials were recorded for each foot. The Research Foot® Version 6.61 was used to mask the foot into seven regions representing the heel, midfoot, first metatarsal, second metatarsal, metatarsal three to five, the hallux, and the lesser toes (Fig. 1). This masking method has demonstrated excellent reliability for the calculation of pressure parameters (ICCs 0.96 to 0.99) [28]. Peak plantar pressure (kPa) and pressure time integrals (kPa⋅s) were computed from the software for each masked region.

All continuous outcomes were reviewed for normality using the residuals from a linear model which included relevant demographic covariates and the diagnostic group as the independent variable. Mixed linear regression models were used which accounted for repeated measures taken from right and left feet of each participant in which participant-specific and participant-nested random effects for foot-side were added to the models. This analysis produces results identical to an analysis of measures averaged for each foot-side (if there are no missing values) that allows for a between-foot-side correlation as well as any reweighting required due to missing values. For the spatiotemporal gait parameters the models also accounted for the time-based repeated-measures using a scaled identity repeated covariance structure which assumed the repeated-measures were independent and shared a common variance. For peak plantar pressure and pressure time integrals, which were measured at seven sites on the plantar foot (forming a natural vector of related variables), in addition to the side- and time-based repeated measures, a heterogeneous compound symmetry covariance structure was employed which allowed for separate variances for each site, as well as different covariances (but equal correlations) between each group of sites.

Adjustments for age group, ethnicity and BMI, which were entered into each model simultaneously, were considered only if their level of observed significance achieved at least 10 % on an F-test. These covariates were not expected to behave as confounders due to the frequency matching, but had the potential to decrease residual variance as possible independent variables. Potential covariates were also explored by reviewing box plots of random effects by covariate group.

Two comparisons were considered: gout vs. normouricemic control and asymptomatic hyperuricemic vs. normouricemic control, which were always tested separately. All hypothesis tests (excluding covariate testing) were carried out at a 5 % level of significance against two-sided alternatives. No adjustment for multiplicity was used, but all test-statistics (least-squares (L-S) means), their null distributions and their observed significance levels were reported. Data were analysed using IBM SPSS Statistics version 20 and proc-mixed in SAS version 9.3.

The study was powered to test hypotheses regarding pre-planned analyses comparing normouricemic control parameters with asymptomatic hyperuricemic and gout parameters respectively. With respect to this analysis, the target figures of 21 participants in the gout group, 29 in the asymptomatic hyperuricemia group and 34 in the normouricemic control group are sufficient to detect unadjusted effect sizes on continuous outcomes of 0.7 and 0.8 (moderate to large) between control and each of the asymptomatic hyperuricemia and the gout group, respectively. The achieved sample size improved on these figures.