Foot plantar pressure and risk of diabetic foot ulcer

Original article:
The forefoot-to-rearfoot plantar pressure ratio is increased in severe diabetic neuropathy and can predict foot ulceration. Caselli A, Pham H, Giurini JM, Armstrong DG, Veves A. Diabetes Care 2002; 25: 1066–71.

Summary
It has previously been demonstrated that high plantar pressures can predict foot ulceration in diabetic patients. The aim of the present study was to evaluate the relationship between forefoot and rearfoot plantar pressure in diabetic patients with different degrees of peripheral neuropathy and their role in ulcer development.
Diabetic patients in a 30-month prospective study were classified according to the neuropathy disability score. The F-Scan® mat system (Tekscan, Boston, MA, USA) was used to measure dynamic plantar pressures. The peak pressures under the forefoot and the rearfoot were selectively measured for each foot, and the forefoot-to-rearfoot ratio (F/R ratio) was calculated.
Foot ulcers developed in 73 (19%) feet. The peak pressures were increased in the forefoot of the severe and moderate neuropathic groups compared with the mild neuropathic and non-neuropathic groups. The rearfoot pressures were also higher in the severe and moderate neuropathic groups compared with the mild neuropathic and non-neuropathic groups. The F/R ratio was increased only in the severe group compared with the moderate and mild neuropathic and non-neuropathic groups. In a logistic regression analysis, both forefoot pressure and the F/R ratio were related to risk of foot ulceration.
The authors conclude that both the rearfoot and forefoot pressures are increased in the diabetic neuropathic foot, whereas the F/R ratio is increased only in severe diabetic neuropathy, indicating an imbalance in pressure distribution with increasing degrees of neuropathy. This may lend further evidence toward the concept that equinus develops in the latest stages of peripheral neuropathy and may play an important role in the etiology of diabetic foot ulceration.

Comment
In this study the authors assessed the risk of foot ulceration in patients with varying degrees of diabetic neuropathy. Several methods were used to assess this risk. In addition to the ‘classic’ physical examination, sensory tests and joint mobility evaluation, they evaluated plantar pressure with the aid of a specific computerized sensor system (Fig. 1).

Fig. 1: Example of a printout of the MatScan® System (from http://www.tekscan.com)

The patient walks barefoot on this system, and the pressures at the plantar side of the foot are recorded. The pressure under the forefoot and rearfoot is measured separately, and the ratio tells us something of the distribution of peak pressure while the patient is walking normally.
Full examinations were carried out in all 248 subjects, most of whom had type 2 diabetes. Moderate and severe degrees of neuropathy were found in 95 and 57 subjects, respectively; these individuals were more frequently males and, as expected, had a longer duration of diabetes. Neuropathy score, cutaneous pressure perception threshold (with a range of Semmes-Weinstein monofilaments from 1 to 100 g) and vibration perception threshold (VPT; assessed by biothesiometry) were more severely disturbed in the subjects with the highest degree of neuropathy. Also, foot mobility and joint movements were more severely impaired.
Both forefoot and rearfoot peak plantar pressures were higher in the moderate and severe neuropathic groups. One of the most important findings was that the F/R ratio was considerably elevated in the patients with the most severe neuropathy. This implies that in this group there is a disproportionate increase of pressure in the forefoot. Although there was an association between plantar pressures and several indices, such as age, duration of diabetes, joint mobility and neuropathy score, all combined factors could account for only 12.6% and 27.1% of the variance measured in the rearfoot and forefoot pressures, respectively.
In a 30-month follow-up most ulcers developed under the forefoot. Peak plantar pressure and especially the F/R ratio were the most powerful predictors of ulcer development. As the authors indicated in the discussion, several factors contributed to this increase in forefoot pressure, amongst them motor impairment; functional changes in tendons such as the Achilles tendon, probably due to glycation of collagenous tissues; and possible rupture of the plantar fascia.
It can be discussed whether pressure measurements are superior predictors of the development of diabetic foot ulcer. The authors performed an extensive analysis to determine the most powerful risk factors for ulcer development. This showed that in univariate regression analysis a pressure >6 kg/cm2 and F/R ratio >2 were strong predictors of future ulceration, with an odds ratio of 3.2 and 2.7, respectively. In multivariate analysis, VPT, Semmes-Weinstein monofilaments, peak plantar pressure and F/R ratio were independent predictors of future ulcers, with the strongest predictive power for VPT and monofilaments (OR 2.7 and 2.6, respectively).
The advanced technique of measuring peak plantar pressure using a sophisticated software analysis program provides us with new insights into the pathophysiology of the development of foot ulcers. In earlier studies the same group had already reported that both high foot pressures (>6 kg/cm2) and neuropathy, assessed by VPT and 10 g Semmes-Weinstein monofilaments, were independently associated with current or previous foot ulcers [1].
The 10 g Semmes-Weinstein monofilament has been widely used to predict neuropathy and the development of ulcers in high-risk patients [2]. It should be emphasized that not all monofilaments are created equally, and that there is a considerable difference in performance between manufacturers [3]. Furthermore, monofilaments tend to become less rigid during use, and therefore may be suitable for use on a maximum of 10 patients per day before requiring a 24-h recovery period [3]. In addition to screening with monofilaments, regular inspection of the feet, education of patients about foot care, and treatment of underlying deformities such as hyperkeratosis and excessive callus formation are necessary to minimize the risk of foot ulcers.