Avaliação da eficácia do uso dos monofilamentos de Semmes-Weinstein para avaliação da sensibilidade abdominal Evaluation of the effectiveness of Semmes-Weinstein monofilaments in assessing abdominal sensitivity

Objetivo: O objetivo deste estudo é avaliar se o método de Semmes-Weinstein tem resultados consistentes e verificar se existem diferenças de sensibilidade entre as regiões da parede abdominal. Métodos: Os monofilamentos de Semmes-Weinstein foram utilizados para avaliar a sensibilidade da pele à pressão abdominal em 20 voluntárias. A pele abdominal foi dividida em nove regiões. A avaliação da sensibilidade cutânea à pressão foi realizada três vezes em cada área, com um intervalo de uma semana entre as medidas. Resultados: Analisando-se as três medidas em cada área, isoladamente, não houve diferenças significativas no limiar pressórico. No entanto, o método de Semmes-Weinstein mostrou diferença estatisticamente significativa entre os valores pressóricos obtidos de cada região abdominal. Conclusões: Os monofilamentos de Semmes-Weinstein são consistentes para avaliar a sensibilidade da pele abdominal à pressão. Notou-se que o método de Semmes-Weinstein mostrou diferença estatisticamente significativa entre as diferentes áreas da região abdominal. Utilizando-se a metodologia do presente estudo, foi possível estabelecer um protocolo para reduzir o aspecto subjetivo, medindo a sensibilidade à pressão.


INTRODUCTION
Plastic surgery procedures involving flaps and grafts may cause a reduction or loss of skin sensitivity.Although there are several methods to assess sensitivity changes after surgery, Semmes-Weinstein monofilaments are one of the most commonly used tools in evaluating skin sensitivity to pressure.Few studies have been devoted to evaluating the methods of sensitivity measurement.Therefore, it is possible that some of the tests used in previous studies may cause physicians to incorrectly inform patients about a possible loss of or reduced sensitivity, and about the type of sensitivity loss they may experience.As, after plastic surgery, patients may be exposed to different types of trauma, it is important that they are properly informed about the type and extent of sensitivity loss, both for legal reasons and to help them avoid injury 1 .
Traditionally, Semmes-Weinstein monofilaments were used to evaluate pressure thresholds 2 .However, several studies demonstrate the difficulties of obtaining reliable measurements with this method 3,4 .

OBJECTIVE
In this study, we aim to evaluate the abdominal sensitivity to pressure by using Semmes-Weinstein monofilaments.

METHODS
The studied group included 20 volunteers, all females, healthy, aged between 18 and 24 years (average age, 20.95 years), with a body mass index (BMI) between 18.83 and 24.91 (average BMI, 21.52), who were submitted to the evaluation of abdominal sensitivity to pressure.From this group, we excluded those presenting any kind of prior incision in the abdomen, as well as previous or current cutaneous or systemic pathologies that could interfere with the sensitivity tests.

Sensitivity evaluation
All patients were evaluated by the same examiner in a quiet environment, with the patient in a supine position on a comfortable table and blindfolded.
Sensitivity to pressure was evaluated in the geometric center of each area, calculated by using the intersection of the diagonals of the rectangles.Each volunteer was evaluated three times by using the Semmes-Weinstein technique, and each test was performed with 1 week intervals.In all tests, the data were considered positive when the patient felt the stimulus, or negative if the patient was unable to feel it.The duration of the sensory stimulation was 5 s.The geometric center of each area was stimulated three times with the same technique.

Semmes-Weinstein monofilament technique
To evaluate the degree of pressure sensitivity, were used six Semmes-Weinstein monofilaments (SORRI Bauru) varying between 50.32 kPa to 2.14 MPa.Each filament was attached to a stick, placed on the skin of each analyzed region, and force was applied to bend the filament.The monofilament was maintained for a period of 5 s.The response was recorded as positive if the patient showed sensitivity.Each area was evaluated three times with 1 week intervals, and the threshold pressures of each area were recorded.

Statistical analysis
To analyze the results, the following statistical tests were applied: 1. Friedman's analysis of variance 5 to separately analyze the three measurements carried out in each region with the method described.2. Friedman's analysis of variance 5 to compare the averages of the three measurements obtained in each region with the method studied.The level of rejection of the null hypothesis was set between 0.05% and 5%.

RESULTS
We obtained 540 values of pressure thresholds.The pressure thresholds obtained in each region were compared separately (Table 1).No statistically significant difference was observed between the three measurements that were carried out in each area (Table 1), except for region H.The coefficient of variation was separately obtained for each area, and the Semmes-Weinstein technique showed a significant variation (Table 1).
When the average values obtained in each area were compared between the regions of the abdomen evaluated, the use of Semmes-Weinstein monofilaments showed a statistically significant difference between these areas (Table 2).Regions A and B were more sensitive to pressure than regions G and I; region H proved to be more sensitive to pressure than region G; and regions F and H were more sensitive than region I (Figure 2).The results are shown in Tables 1 and 2.
No statistically significant difference was observed between the average values of symmetrical areas (A × C; D × E; G × I) (Figure 3).

DISCUSSION
The literature presents few studies on methods for measuring skin sensitivity to pressure.In 1898, von Frey 6 introduced a method for evaluating sensitivity to pressure that consists of monofilaments of different thicknesses and hardness.The filament was slowly located on the place until it bends.The

Table 1. Pressure thresholds obtained for each abdominal region (A × B × C × D × E × F × G × H × I) and at repetitions (T1 × T2 × T3
).The result of Friedman's analysis of variance (X2) is expressed by comparing T1 × T2 × T3.The average (X), median (Mi), and standard deviation (SD) are expressed in kPa.given to the patients for them to ring as soon as they sense the pressure limit; then, the value of pressure threshold is recorded in g/mm 2 .Fels et al. 15 used the PSSD test to assess skin sensitivity to abdominal pressure after abdominoplasty procedures.The results of this study showed an average pressure threshold between 1 and 1.2 g/mm 2 , with no statistically significant difference between abdominal areas.

COMPARISON OF ABDOMINAL SENSITIVITY BETWEEN DIFFERENT AREAS BY USING FRIEDMAN'S ANALYSIS OF VARIANCE (Region A × region B × region C × region D × region E × region F × region G × region H × region I
With the method of this study, it was possible to establish a protocol to reduce subjectivity and measure skin sensitivity to pressure, at three different times with 1 week intervals, thus reducing the influence of short-term memory and synaptic fatigue.
In the future, a study comparing the different methods for evaluating skin sensitivity to pressure, e.g., PSSD and the Farah test, should be performed.Therefore, implementation of a method for evaluating abdominal sensitivity to pressure that is capable of setting normal pressure limits would be necessary.

CONCLUSION
In conclusion, the Semmes-Weinstein monofilament technique is a reliable method to assess abdominal skin sensitivity to pressure.By using this method, different sensitivities to pressure, depending on the evaluated abdominal area, were obtained.It is important to establish accurate methods to measure skin sensitivity that would allow the surgeon to better inform patients who will be submitted to abdominal surgery.This may also aid physicians in forensic cases.

Figure 1 .
Figure 1.Schematic representation of the evaluated abdominal areas.

Figure 2 .
Figure 2. Comparison of sensitivities between different areas by using a gray scale.

Figure 3 .
Figure 3.Comparison of skin sensitivity between abdominal symmetrical areas by using a gray scale.

Table 2 .
Friedman's analysis of variance (region A × region B × region C × region D × region E × region F × region G × region H × region I).The average (X), median (Mi), and standard deviation (SD) are expressed in kPa.