According to WHO classification of periapical pathology, 4.81 means:

WHO Classification: Ko 4.4 Acute Apical Periodontitis Ko 4.5 Chronic Apical Periodontitis Ko 4.6 Periapical Abscess with sinus formation 4.60 Sinus to maxillary antrum 4.61 Sinus to nasal floor 4.62 Sinus to oral cavity 4.63 Sinus to outer skin of face Ko 4.7 Periapical Abscess without sinus Ko 4.8 Periapical cyst 4.80 Apical / lateral cyst 4.81 Residual cyst 4.82 Inflammatory paradental cyst.