Objectives: Von Frey monofilaments (VFM) and Electronic von Frey are used as quantitative sensory testing to explore mechanical hyperalgesia. The aim of the study was to determine VFM and Electronic von Frey reproducibility and the time required for testing undamaged areas in volunteers and surgical in-hospital patients.
Methods: Prospective study in 2 groups of individuals without neurological disorders and not receiving treatments that alter sensorial response. Group 1 was formed by 30 healthy volunteers and group 2 by 28 patients after knee arthroplasty who received unilateral femoral block as postoperative analgesia. Testing was performed by 2 trained investigators in the forearm and abdominal wall and repeated 24 hours later. Pain threshold was determined as the VMF or the pressure rate applied with the Electronic von Frey referred to as unpleasant. The time used in each test was measured. Intraobserver and interobserver reproducibility were analyzed by means of Kappa and Lin coefficients.
Results: Intraobserver and interobserver reproducibility for VFM was only fair or moderate in both groups (Kappa<0.6). Intraobserver reproducibility for Electronic von Frey was good in Group 1 (Lin 0.61 to 0.8) and almost perfect in Group 2 (Lin>0.8). No differences were found between the 2 tested areas. Time used was 6.03 and 3.16 minutes for VFM and Electronic von Frey, respectively.
Discussion: Exploration of mechanical allodynia in postoperative patients requires rapid and reliable quantitative sensory testing. Electronic von Frey was more reliable and rapid than VFM in exploring mechanical pain thresholds in undamaged areas in volunteers and patients. Further studies are required to confirm whether these results can be extrapolated to areas affected by surgery.