Ageing of the bony orbit is a major cause of age-related intraorbital fat herniation.

We evaluated the relationship between infraorbital fat herniation and age-related changes in the bony orbit and orbital fat density using computed tomography. Two hundred and sixty-five patients were enrolled (60 patients were evaluated for changes in the bony orbit and 205 for changes in orbital fat density). Five measurements using parasagittal sections and one measurement using three-dimensional images were obtained. Intraorbital fat herniation length was positively correlated with orbital rim inclination. Lowering of the inferior orbital rim, which is connected to the orbicularis retaining ligament, tear trough ligament and orbital septum, can cause mechanical stretching of the lower eyelid and may contribute to infraorbital fat herniation. A strong and significant negative correlation was observed between orbital fat density and age, indicating that existing orbital fat can accommodate an enlargement in bony orbit volume without orbital fat hyperplasia/hypertrophy. In other words, an increased orbital fat volume may be a by-product of the adaptation of orbital fat to changes in bony orbit volume. Mechanical stretching of the lower eyelid due to ageing of the bony orbit and weakening of the lower eyelid due to age-related factors such as dermal/fat/muscle atrophy and loss of muscle tone can together result in anterior drooping of the lower eyelid. Therefore, age-related changes in the bony orbit contribute to intraorbital fat herniation.