Three-dimensional kinematics at the ankle joint complex in rheumatoid arthritis patients with painful valgus deformity of the rearfoot

J. Woodburn1,, P. S. Helliwell1,2 and S. Barker2

1 Rheumatology and Rehabilitation Research Unit, University of Leeds, Leeds and
2 Rheumatology Unit, Bradford NHS Trust, St Luke's Hospital, Bradford, UK


    Abstract
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Objective. To evaluate the three-dimensional (3D) kinematics of the ankle joint complex (AJC) in rheumatoid arthritis (RA) patients with painful valgus deformity of the rearfoot.

Methods. Fifty patients with RA underwent gait analysis using electromagnetic tracking techniques to measure 3D kinematics at the AJC under barefoot and shod walking conditions. Three axial rotations (dorsiflexion/plantarflexion, inversion/eversion and internal/external rotation) were measured during the gait cycle, and the angular positions at key gait events, range of motion and the motion: time integral were measured. Descriptive and analytical comparisons were made with normative data derived from a sex- and age-matched population (n=45).

Results. AJC dysfunction in RA was characterized by excessive eversion motion (within an eversion range) and no inversion motion through the neutral joint position for the subtalar component of the joint complex, in both barefoot and shod walking conditions. Motion was coupled such that internal rotation of the leg relative to the rearfoot was greater than normal. AJC motion was different for all rotations between barefoot and shod conditions, but in both situations there were statistically significant between-group differences in the motion:time integral for inversion/eversion (barefoot, P<0.0001; shod, P<0.0001) and external/internal rotation (barefoot, P<0.0001; shod, P<0.0001). There were no statistically significant differences between RA and normative data for dorsiflexion/plantarflexion motion under barefoot (P=0.16) and shod (P=0.50) walking conditions.

Conclusion. Painful valgus deformity of the rearfoot is associated with changes in the 3D kinematics affecting eversion at the AJC and internal rotation of the leg, both when walking barefoot and in shoes.


    Introduction
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Chronic synovial inflammation and progressive erosion of cartilage and bone have been described for the tibiotalar, subtalar and midtarsal joints in rheumatoid arthritis (RA) [15]. Structurally, we recognize the clinical manifestations as valgus deformation of the rearfoot, usually accompanied by medial longitudinal arch collapse, and this is estimated to occur in 67% of patients with RA [68]. Progressive deformity is associated with increased disease duration, radiological pathology in the ankle and tarsus, and peri-talar swelling, tenderness and pain [9, 10]. Platto et al. [11] demonstrated significant gait impairment in RA patients with rearfoot pain and valgus deformity.

The appearance of valgus heel deformity, medial longitudinal arch depression and bulging of the talonavicular complex are often sufficient clinically to diagnose a hyperpronated foot [12]. For detailed, non-invasive study, simple observation of gross foot morphology lacks the necessary precision and sensitivity. Hand-held goniometry, whilst introducing quantification, has been shown to be unreliable for rearfoot assessment [13, 14]. Motion analysis techniques often describe tibiotalar and subtalar joint motion simultaneously through the ankle joint complex (AJC) because there are no suitable surface landmarks on the talus to attach a motion sensor needed to separate the joints out. Analysis is therefore undertaken using sensors or markers placed on the calcaneus and tibia, making the assumptions that sagittal plane rotations (dorsiflexion/plantarflexion) occur largely through the tibiotalar joint component, frontal plane rotations (inversion/eversion) occur through the subtalar joint component and transverse rotations (internal/external rotation) of the tibia occur relative to the calcaneus. Previous gait studies of AJC function in small cohorts of RA patients, usually with long-standing disease, have shown kinematic dysfunction characterized by increased eversion from heel-strike through midstance, both in terms of magnitude and duration, and decreased inversion motion during the propulsive stage of the stance phase [6, 15, 16]. The purpose of this study was to enhance this data by undertaking measurement in a cohort of earlier RA patients with active peritalar disease [17, 18].


    Patients and methods
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Patients
Patients were eligible for inclusion if they met the American College of Rheumatology criteria for the diagnosis of RA [19], had bilateral clinical features of active disease in the peritalar region and had valgus deformity of the calcaneus demonstrable on weight-bearing. Patients with a history of foot surgery to the ankle, rearfoot or midfoot region and those currently using orthopaedic footwear or foot orthoses were excluded. Normative data were gathered from an age- and sex-matched cohort of healthy adult volunteers with no significant history of musculoskeletal disease or injury to the lower limb or foot. Approval was granted for this project by the Huddersfield NHS Trust ethics committee and written informed consent was obtained from all subjects.

Measurement of ankle joint complex kinematics
An electromagnetic tracking (EMT) technique was developed to measure the 3D kinematics at the AJC. The physical principles and technical specifications of this method have been described in detail elsewhere [17, 18, 20, 21]. Briefly, we used the 6D Research System (Skill Technologies, Phoenix, AZ, USA) with Fastrak® (Polhemus, Colchester, VT, USA) EMT sensors. Sensors were attached on the skin overlying the medial tibial surface between the midline of the knee and ankle joints and the posterior surface of the calcaneus [18]. A ‘bore-sighting’ procedure was undertaken prior to data capture to align the two sensors relative to a neutral subtalar joint standing position. When comfortable with the set-up, patients were requested to walk at normal speed over an 8 m distance, passing through a transmitter-generated low-strength electromagnetic field. Synchronous temporal gait parameters were captured using foot pressure switches [18]. Computer software was used to detect the position and orientation of the EMT sensors through the electromagnetic field. After filtering raw data with a 6 Hz low-pass digital Butterworth filter, further software routines were undertaken to calculate joint coordinate system angles as defined for the AJC by Allard et al. [22].

Motion was recorded simultaneously for the left and right AJC from an average of five trials. Data were captured under barefoot and shod walking conditions. By creating an open window in the heel area of standard stock shoes, the difficulty of accommodating the skin-mounted calcaneal EMT sensor was overcome [18]. Furthermore, this technique eliminated any variability that might have arisen had patients been measured in their own shoes with shoe-mounted sensors. Normal daily walking aids were permitted during data capture. Validation studies of this technique in our own unit, involving repeated measurements in RA patients and healthy normal subjects, indicate a high degree of precision and accuracy [17].

Statistical analyses
6DNorm software (M. Cornwall, Northern Arizona University, Flagstaff, AZ, USA) was used to generate motion:time curves, normalized to 100% of the gait cycle, for each axis of rotation (tibiotalar dorsiflexion/plantarflexion in the sagittal plane, subtalar joint inversion/eversion in the frontal plane and leg-to-calcaneus internal/external rotation in the transverse plane). Data from the RA patients were compared descriptively with reference values derived from the sex- and age-matched normal population, with reference to angular position at key stages during the gait cycle, including heel-strike, foot flat, midstance, heel-lift and toe-off, and to minimum and maximum joint angles and the range of motion.

Data reduction was performed to derive a single useful variable—the integral of the motion:time curve, for statistical analysis between groups. The trapezium rule was used to calculate the integral, and negative and positive areas about the neutral joint position were summed. This approach was valid because the underlying shape of the joint motion curves was largely the same in most subjects and abnormal motion was largely characterized in preliminary studies by a shift of the motion curves on the ordinate (relative to the neutral joint position). Between-group differences in the mean motion:time integrals were analysed using Student's t-test. Differences in the means were considered statistically significant for P values <0.05.


    Results
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
Study patients
Fifty RA patients were recruited and their demographic and disease characteristics are presented in Table 1Go. Disease duration ranged from <1 to 10 yr, and the patients presented predominantly with active disease, pain and disability and erosions in their hands and feet. All patients reported foot pain and disability and the mean Foot Function Index score was 41.1 mm (S.D. 20.3). Forty-five subjects formed the normative data group and they were well matched with the patient group by age, sex, ethnic origin and body mass.


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TABLE 1. Demography and disease characteristics of the study participants

 

Descriptive comparison of AJC 3D kinematics
Under barefoot walking conditions there were differences in AJC kinematics between the study groups for two axes of rotation (Table 2Go and Fig. 1Go). For inversion/eversion motion, both groups had the same curve shape, i.e. during the stance phase eversion motion followed heel-strike and continued to the heel-lift period, followed by rapid inversion motion through toe-off. However, in the RA group motion occurred within an eversion envelope and at no period did the joint move through the neutral position. The inversion/eversion ROM was similar but in the RA group the angular positions at each stage of the gait cycle were excessively everted or everted when normally inverted when compared with the normative data.


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TABLE 2. Summary kinematic data for barefoot and shod walking conditions

 


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FIG. 1. Angular rotations in (A) the normative group barefoot, (B) the RA group barefoot, (C) the normative group shod, and (D) the RA group shod. (-{blacksquare}-{blacksquare}-{blacksquare}-) Dorsiflexion (+)/plantarflexion (-); (-{blacklozenge}-{blacklozenge}-{blacklozenge}-) inversion (+)/eversion (-) and (-•-•-•-) internal (+)/external rotation (-). Gait events: HS, heel strike; FF, foot flat; MS, mid-stance; HL, heel lift; TO, toe off. Bars represent the 95% confidence interval around the mean.

 
In the RA group the internal/external rotation motion curve was shifted positively on the ordinate in comparison with normal data, although the range of motion (ROM) was the same for the two groups. Motion was contained within an internally rotated envelope for the RA group and at no period during the stance phase was external leg rotation past the neutral position realized. Comparison of the absolute angular positions by gait event showed the AJC in the RA group to be (a) excessively internally rotated, and (b) internally rotated at late stance when external rotation is expected.

The overall curve shape for AJC dorsiflexion/plantarflexion was similar for the two groups and the mean angular positions at key gait events during stance were almost the same. The normative group showed more plantarflexion motion (normative data, -13.7°; RA, -9.8°) and the ROM was slightly reduced in the RA group (normative, 20.8°; RA, 18.7°).

Footwear changed AJC motion in both study groups (Table 2Go and Fig. 1Go). The dorsiflexion/plantarflexion motion curve shape was the same for the two groups but shifted negatively (more plantarflexed) on the ordinate, with a small increase in the ROM. The relative difference between the inversion/eversion curves remained similar between barefoot and shod walking conditions in both groups, although the motion was shifted positively, i.e. in an inversion direction on the ordinate in shod gait. Most notably, this permitted two periods of joint motion through the neutral position, at heel-strike and during toe-off, in the RA group. Footwear shifted the internal/external rotation slightly negatively on the ordinate for both groups but had no effect on curve shape or ROM.

Motion:time integrals
Under barefoot walking conditions the mean motion:time integral for dorsiflexion/plantarflexion motion in the RA group had a small positive value (69.5 area units) in comparison with a small negative mean value for the normative group (-36.2 area units), but the difference was not significant (P=0.16) (Table 3Go). The AJC demonstrated predominantly eversion motion in the RA group; thus, the mean motion:time integral showed a large negative value (-563.4 area units), and this was significantly lower than for the normative data group (87.3 area units; P<0.0001). Positive mean motion:time integrals were observed for internal/external rotation for both study groups but the mean value was significantly greater in the RA group (610.1 area units) in comparison with the normative group (65.7 area units; P<0.0001).


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TABLE 3. Analysis of motion:time integrals for barefoot and shod walking conditions

 
Under shod walking conditions, both groups had large negative motion:time integrals for dorsiflexion/plantarflexion motion, reflecting the increased plantarflexion motion, but mean values were not significantly different (RA, -523.3 area units; normative, -573.0 area units; P=0.50). A negative mean motion:time integral for inversion/eversion motion was observed in the RA group (-264.1 area units) in comparison with the normative group (247.8 area units) and the difference was statistically significant (P<0.0001). For internal/external rotation both motion:time integrals were positive, but the mean value was significantly greater in the RA group (507.6 area units) in comparison with the normative group (12.6 area units; P<0.0001).


    Discussion
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 
In this study, painful valgus deformity of the rearfoot in RA was associated with altered 3D kinematics at the AJC, notably for subtalar inversion/eversion and leg-to-rearfoot internal/external rotation. To our knowledge this is the first time this has been reported in patients with average disease duration <5 yr (median 3 yr). For all axes of rotation, the range of motion was small: approximately 20° for dorsiflexion/plantarflexion and typically<10° for frontal and transverse plane rotations. These values are much smaller than clinical values from passive or active joint ROM tests. However, we are sufficiently confident from our validation studies that these small rotations can be captured precisely and accurately using the EMT technique described [17, 18]. Clinically differentiating normal and abnormal heel alignment from static observation poses a greater challenge. Indeed, asymptomatic valgus rearfoot deformity may be regarded as an anatomical variation in the normal population, although there are no precise figures. Furthermore, there may be disassociation between the static observed rearfoot alignment and the underlying dynamic motion characteristics, and this has been observed for other lower limb joints and for the AJC itself [23]. These are unresolved challenges, but in practical terms they are some of the reasons why the intrasubject variability reported here and elsewhere is high [2426].

Tibiotalar joint dysfunction in RA is thought to arise largely from secondary stresses from other joints, particularly when the subtalar joint is aligned abnormally [27]. The frequency of tibiotalar joint involvement in RA is less than subtalar or midtarsal joint involvement, but reports of decreased ROM, particularly dorsiflexion, have been published [3, 9, 15, 27, 28]. Having demonstrated abnormal frontal plane alignment for the subtalar component, we found an associated 10% reduction in tibiotalar dorsiflexion/plantarflexion motion in comparison with normal, in agreement therefore with previous observations. Tibiotalar dorsiflexion was increased and plantarflexion motion decreased; however, as the talus itself is not used as the distal marker site this observation may reflect motion captured in the calcaneus undergoing sagittal rotation as a component of abnormal motion (as identified) in the subtalar and distal joints.

In RA, structures that guide and stabilize subtalar and talonavicular motion, especially the ligaments, are vulnerable to stretching and attenuation in the presence of persistent synovitis [6, 9, 17]. The initial direction of motion in the subtalar joint is eversion during the loading response, where forces are high, so ligaments that are not taut cannot control joint motion and, in the case of the peritalar structures, unrestricted joint displacement in the direction of eversion occurs [6, 8, 9]. Ligamentous instability has yet to be quantified experimentally, but in a small case–control study Keenan et al. [6] described three subtalar motion abnormalities for 10 patients with a mean disease duration of 25 yr: abnormal eversion of the calcaneus at heel-strike; everted subtalar alignment through the entire stance phase of gait; and insufficient inversion motion during propulsion to establish a neutral or inverted subtalar joint alignment. Active peritalar disease is often thought to occur late in the disease, but we found strong evidence to support its early development (at <5 yr of disease duration). The inversion/eversion motion patterns were consistent with the observations of Keenan et al. and others, but with less severe deformity. As subtalar erosions have been found to occur with less frequency than more distal joints, and given that true ankylosis is rare, the potential for progressive deformity in our patients exists. Indeed, the available motion that was necessary for this to occur was present in the subtalar component, because, in agreement with other published data, we found no evidence of a reduced ROM in our RA patients [5, 6, 15, 29, 30].

Coupled AJC motion has been described previously and refers to the simultaneous rotation of the tibia internally with subtalar joint eversion and externally with subtalar inversion [3133]. Excessive eversion can be coupled with excessive internal tibial rotation and this has been reported in the pronated foot type in the normal population and postulated as an injury mechanism at the knee in runners [3133]. We report here for the first time the same effect in RA. Indeed, there was associated failure of the subtalar joint to reach a neutral or inversion alignment and of leg rotation to reach a neutral or externally rotated alignment, under the barefoot walking condition. The RA group showed no significant reduction in the ROM for leg internal/external rotation, and we speculate that the subtalar joint motion in its unstable lax state drives the dysfunction. Valgus rearfoot (distal) and valgus knee (proximal) deformities are frequently observed together in RA and a distal–proximal causal relationship has been proposed but never studied seriously [6, 27]. The observations made here merit further attention in a controlled study.

We tested the 3D kinematics in a standard shoe in an attempt to capture rearfoot motion under realistic walking conditions. Although not formally reported, variability between five repeated trials was consistently less in the RA patients in-shoe in comparison with barefoot walking, and we noted appreciably different spatial and temporal gait parameters on observation, all related to reduced foot pain. Shod 3D kinematics has never been described previously for the AJC in RA and we observed changes for all three axes of rotation. The standard shoe had a heel height of 4 cm and served to increase the anterior–posterior pitch of the shoe, so that plantarflexion motion was increased for both study groups. The range of motion was not changed appreciably and a small decrease in dorsiflexion was noted for both groups. Most interestingly, the stiff medial counter in the test shoe served to invert the subtalar joint and rotate the leg externally, bringing about partial correction of the abnormalities described previously. The motion:time curves were not restored to normal values, as statistically significant differences remained between the RA and normative data for both these axes of rotation. Nevertheless, the data demonstrate that permanent joint changes have not occurred and that, in motion terms, the deformity is reversible, even though the motion envelope has shifted towards eversion and internal rotation. In otherwise healthy populations, small and subject-specific reductions of excessive eversion and internal tibial rotation with modified shoes and orthoses have been reported [3436]. No assumptions can be made here on the closeness of match between the test shoe and those worn day-to-day by patients. Nevertheless, the observed subtalar control has some interesting implications for the future study of the potential therapeutic benefits of footwear.

Pain and deformity impairments in the RA forefoot are associated with delayed and reduced forefoot loading [37]. We were unable to control for forefoot symptoms in our study because they were present almost universally. Furthermore, proximal limb pain and deformity and gait-compensatory strategies, such as slowing the walking speed, reducing stride length and increasing double-support time, all may have affected joint loading forces and thus movement patterns [29]. Further studies will refine data analysis to account for these confounding factors.

In summary, we found that RA patients with clinically observed valgus heel deformity and peritalar inflammation had abnormal 3D kinematics of the AJC characterized by excessive subtalar eversion and internal leg rotation. AJC mechanical dysfunction may be one factor that localizes persistent synovitis to the peritalar region of the foot. More aggressive management for early rearfoot disease has been called for, notably from within orthopaedics and rehabilitation medicine, to reduce the burden of disease locally in the rearfoot, but also as a mechanism to protect the forefoot, the knee and knee implants following arthroplasty [6, 38, 39]. The gait analysis technique described may provide an accurate and precise means of quantifying rearfoot joint function to help facilitate and evaluate early appropriate mechanical treatment before irreversible joint changes occur.


    Acknowledgments
 
JW is the recipient of a UK Medical Research Council Clinical Training Fellowship. The study was also supported by project grants from the Arthritis Research Campaign of Great Britain and Northern and Yorkshire NHS Research and Development.


    Notes
 
Correspondence to: J. Woodburn, Rheumatology and Rehabilitation Research Unit, University of Leeds, 36 Clarendon Road, Leeds LS2 9NZ, UK. Back


    References
 Top
 Abstract
 Introduction
 Patients and methods
 Results
 Discussion
 References
 

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Submitted 19 April 2002; Accepted 21 May 2002