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Rheumatology 2006;45:212–214 Advance Access publication 4 October 2005 doi:10.1093/rheumatology/kei137 Concise Report Plantar sensitivity, foot loading and walking pain in rheumatoid arthritis D. Rosenbaum, A. Schmiegel, M. Meermeier and M. Gaubitz1 Objective. The aim of the present study was to investigate the tactile sensitivity of the plantar surface in rheumatoid feet and its relationship to walking pain and plantar foot loading characteristics. Methods. In 25 patients with rheumatoid arthritis (RA) and 21 healthy controls, Semmes-Weinstein monofilaments were used to assess tactile sensitivity in six foot regions. Walking pain was examined clinically. Pedography was used to analyse foot loading parameters during barefoot walking. Results. In RA patients, plantar sensitivity was significantly decreased under all foot regions examined compared with the control group (P_0.05). A loss of protective sensation was found in a total of 10 regions in seven patients but not in the control group. In the RA patients, foot loading was reduced in the hindfoot (P_0.05) but was slightly increased in the forefoot (not significant). Average walking pain was 3.8 2.1 on a scale from 0 to 10 but did not correlate with the sensitivity levels. Conclusion. In patients with RA, no direct relationship between pain intensity and plantar foot loading was found. The decreased tactile sensitivity may be indicative of a disturbed sensation for high plantar pressures. Therefore, pedography can be useful as an additional tool in the detection of excessive forefoot loading before complications are manifested. KEY WORDS: Rheumatoid arthritis, Foot pain, Plantar sensitivity, Pedography. RA is one of the most frequent reasons for limited functional capacity [1] and limited mobility is often due to the burden of foot problems [2]. Pedography is an established tool for measuring dynamic foot loading characteristics and is therefore able to reveal foot deformities of rheumatoid feet [3; A. Schmiegel, M. Gaubitz, A. Schorat, A. Hilker and D. Rosenbaum, manuscript in preparation]. Until now, the relationship between foot loading and walking pain has not been fully clarified [4]. There is still a lack of understanding of the aetiology of pain in rheumatoid feet [5]. RA patients with longer disease durations are known to suffer from greater foot pain in inflamed joints but pain sensitivity is increased in non-inflamed tissues as well [6, 7]. High pain intensity may decrease the tactile sensitivity due to an inhibitory effect of nociceptive input [7–9]. Neuropathy can affect somatosensory functions of RA patients, leading to reduced or decelerated sensory conduction [10, 11]. Reports concerning the prevalence of compression neuropathies are inconsistent and vary between 32% [12] and 42% [10], but it appears that neuropathies increase with higher age [13]. The only report on the relationship between neurological abnormalities and foot pressure investigated vibration perception in RA patients compared with diabetic patients [3]. To date, no study has examined the impairment of tactile sensitivity on the plantar surface of patients with RA compared with controls. Tactile sensation is mediated by mechanoreceptors that conduct action potentials via A-fibres [14] and can be detected with monofilaments, which are easy to use and reliable for quantifying plantar sensitivity [15, 16]. Therefore, the primary aim of this study was to examine the tactile sensitivity in rheumatoid feet compared with healthy control feet. A secondary aim was to investigate the relationship between tactile sensitivity, self-assessed walking pain and plantar foot loading characteristics. Methods Twenty-five patients (23 females, 2 males) with RA who met the 1987 American College of Rheumatology criteria for RA were examined. The average age of the patients was 55.0 9.9 yr and their body mass index was 26.8 6.6 kg/m2. Further inclusion criteria were: (i) independent walking ability; (ii) bilateral foot pain during walking; (iii) no other joint diseases; (iv) no other systemic disease with relevant effects on walking ability and pain sensation; and (v) no previous foot and ankle surgery. All patients continued their long-term anti-rheumatic medication but were off medications that might have altered their mental state. Average duration of disease was 9.6 7.0 yr and the HAQ score averaged 1.4 0.7 (on a scale of 0–3). Furthermore, 21 subjects (20 females, 1 male) with an average age of 50.8 9.3 yr and a body mass index of 24.1 3.5 kg/m2 served as a control group. These subjects were free of foot problems and any other orthopaedic impairment or systemic disease that could have affected their gait patterns. Walking pain of the right foot of every patient was evaluated with multiple-item scales that represent the degree of pain in Movement Analysis Laboratory, Orthopaedic Department, University Hospital Münster and 1Department of Rheumatology, University Hospital Münster, Münster, Germany. Received 31 May 2005; revised version accepted 19 August 2005. Correspondence to: D. Rosenbaum, Funktionsbereich Bewegungsanalytik Klinik und Poliklinik für Allgemeine Orthopädie, Universitätsklinikum Münster, Domagkstr. 3D-48129 Münster, Germany. E-mail: [email protected] 212 ß The Author 2005. Published by Oxford University Press on behalf of the British Society for Rheumatology. All rights reserved. For Permissions, please email: [email protected] Plantar sensitivity, foot loading and walking pain in RA Results The RA patients did not differ from controls with respect to age or body mass index. Average walking pain was 3.8 2.1 on a scale from 0 to 10. Sensitivity was significantly reduced in the RA patients under all foot regions investigated (P50.05; Table 1). In seven RA patients (27%) loss of protective sensation was found in a total of 10 regions located under the heel (one patient), midfoot (one), first metatarsal head (one), third metatarsal head (three), fifth metatarsal head (three) and hallux (one). Dynamic pedographic measurements revealed several differences between groups. The significantly longer total contact time in the RA patients (797 146 ms) indicated that the gait velocity was lower than in control group (716 66 ms, P50.03). TABLE 1. Results of the sensitivity assessment (because of the discrete nature of the data, the median and first and third quartiles are reported) Sensitivity threshold Heel Midfoot First metatarsal Third metatarsal Fifth metatarsal Hallux RA patients 4.56 4.17 4.17 4.31 4.31 4.17 (4.17, (3.61, (4.17, (4.17, (4.17, (4.17, Controls 4.74) 4.17) 4.56) 4.61) 4.56) 4.56) Controls Average Pressure [kPa] 4.24 3.84 4.08 4.17 4.17 4.17 (4.13, (3.42, (4.08, (4.17, (4.17, (3.84, P-value 4.31) 3.90) 4.17) 4.31) 4.31) 4.17) 0.027 0.018 0.032 0.032 0.015 0.035 RA patients 600 500 400 300 * * 200 100 45 T3 T2 H 45 M3 M2 M1 LM MM LH 0 MH chronic diseases [17]. We did not include both feet to avoid falsepositive statistical findings [18]. The RA patients were asked to fill in the Health-Assessment Questionnaire (HAQ) [19]. Furthermore, a foot function index was used to assess the footspecific impairment [20]. Tactile sensitivity under the plantar aspect of the right foot was assessed by measuring the perception threshold to light touch [7] with Semmes–Weinstein monofilaments (North Coast Medical, Morgan Hill, CA, USA). A set of these filaments consists of 20 pens with different force levels. The detection of sensory thresholds was conducted in a random order in six plantar regions: heel, midfoot, first, third and fifth metatarsal head, and hallux. For each filament, a total of 10 stimuli were applied: seven real stimuli and three null stimuli were randomly included. If three failures were observed the level was classified as failed. We used a modified 4–2–1 step algorithm that has been described before [15, 21]. Beginning with the filament at an intermediate level (4.56), the stimulus was increased (if the first level was not felt) or decreased (if felt) by four steps. After the first turnaround (felt at the higher level when not felt at lower levels or vice versa) stepping was changed to steps of two. After the second turnaround the final stepping was reduced to a single level step. The last level that was correctly felt was used as the tactile threshold. For the present study, a filament index above 5.07 was considered as an indication of loss of protective sensation [22]. Therefore, we noted every region under the foot with reduced sensitivity and registered every subject with at least one region above this threshold. For pedographic measurements, patients and controls were asked to walk barefoot over a capacitive platform (EMED ST4; Novel, München, Germany) embedded flush in the floor. The platform consisted of 2736 sensors with a spatial resolution of four sensors per cm2 and a measurement frequency of 50 Hz. The subjects were asked to walk barefoot across the platform with normal step length and self-selected walking speed. Measurements were repeated until five steps of the right foot had been recorded. Pedographic pressure patterns were subdivided into 10 regions: medial (MH) and lateral heel (LH), medial (MM) and lateral midfoot (LM), first metatarsal (M1), second metatarsal (M2), lateral metatarsals (M345), hallux (H), second toe (T2) and lateral toes (T345) (Database Pro-M 11.26; Novel). The average pressure was calculated for each foot region [8]. The contact time during the roll-over process was measured as an indication of the patient’s gait velocity [23]. Õ Data were analysed using StatView 5.0 (SAS Institute, Cary, NJ, USA). The Kruskal–Wallis test for non-parametric data was used to reveal the significance of differences between subject groups ( level 0.05). Correlation analysis was performed with Spearman’s correlation coefficient. Informed consent was obtained from all patients and subjects. This study was approved by the local ethics committee. 213 FIG. 1. Pedographic results for average pressure. *P50.05. The average pressure was reduced under the medial and lateral hindfoot (P50.05; Fig. 1) and the lesser toes (not significant) but slightly increased under the midfoot, forefoot and hallux (not significant). Only one significant correlation coefficient was found between sensitivity level (i.e. filament indices) and average pressure in the same foot region, i.e. under the third metatarsal head (r ¼ 0.564). This indicated that patients tended to produce higher pressures under less sensitive areas. No significant correlation was found between walking pain and sensitivity level. Discussion The present study revealed that plantar sensitivity may be impaired in patients with rheumatoid feet. While not all patients showed a loss of protective sensation in at least one of the foot regions investigated, the patients were on the average significantly less sensitive in all regions compared with the control group. This result may help to explain previous findings that revealed significantly increased pressures under the central and lateral forefoot in less severely affected patients with lower walking pain, as opposed to lower pressures in more severely affected patients with higher walking pain [A. Schmiegel, M. Gaubitz, A. Schorat, A. Hilker and D. Rosenbaum, manuscript in preparation]. However, we found only one significant correlation coefficient between sensitivity and pressure under the central forefoot (r ¼ 0.564), which explains only 32% of the variance of the pressure changes. Furthermore, no correlation was found between sensitivity level and pain, which indicates that other factors in addition to the local sensitivity may play a role in the relationship between pain and pressure. Plantar pressures may be affected by insufficient ligaments in the forefoot [24, 25], rearfoot pronation [26], and pain-induced gait modifications [27–30]. In conclusion, plantar sensitivity is decreased in RA patients compared with healthy controls. Decreased tactile sensitivity may indicate disturbed sensation for high peak pressures that may ultimately cause mechanical damage in the foot. Therefore, in RA patients—especially in those with impaired sensation— pedography may be a useful tool in the detection of excessive 214 D. Rosenbaum et al. forefoot loading before complications are manifested. However, walking pain in RA patients is caused by multiple factors and depends on inflammation and deformity of the foot joints and impairment of pain sensation as well as pain-induced changes in gait patterns. Acknowledgements Financial support from the German Ministry of Economy is gratefully acknowledged (BMWA, PROINNO Project KF 0497301KWM3). The authors have declared no conflicts of interest. References 1. Westhoff G, Zink A. Hilfesituation und Hilfeperspektive von erheblich beeinträchtigten Kranken mit rheumatoider Arthritis. 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