Unattached surface probes are commonly used with the O2C analysis system (LEA Medizintechnik, Germany) to monitor microvascular free flap perfusion. This study compared attached and unattached surface probes for extraoral free flaps. The study included 34 patients who underwent extraoral microvascular head and neck reconstruction between 2020 and 2022. Flap perfusion was monitored postoperatively using the O2C analysis system at 0, 12, 24, 36, and 48 h, with an attached surface probe at 3 mm tissue depth and an unattached surface probe at 2 mm and 8 mm tissue depths. Clinical complications, technical errors, and perfusion measurement values were compared. No clinical complications (attachment suture infections) or technical errors (probe detachment) occurred. Flap blood flow values of the probes were partially different (3 mm vs. 2 and 8 mm: p < 0.001; p = 0.308) and moderately correlated (3 mm with 2 and 8 mm: r = 0.670, p < 0.001; r = 0.638, p < 0.001). Hemoglobin concentration and oxygen saturation values were generally different (3 mm vs. 2 and 8 mm: all p < 0.001) and variably correlated (3 mm with 2 and 8 mm: r = 0.756, r = 0.645; r = 0.633, r = 0.307; all p < 0.001). Both probes are comparable in terms of technical feasibility and patient safety, with flap perfusion values dependent on tissue measurement depth.
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