Hertel Curve: Orbital Volume Increment and Proptosis in a Cadaver Model

PURPOSE: This study examined the relation between effective orbital volume increment and proptosis induced by an inflatable orbital implant in a human cadaver orbit. METHODS: A 25-ml inflatable latex balloon was inserted between the periorbita and orbital floor in 15 human cadavers. Hertel measurements were taken for each milliliter over a total 7-ml volume increment. Five trials per orbit for 15 cadavers resulted in 525 data points. RESULTS: Average exophthalmos per milliliter of volume increment was plotted over 7 ml, using 1-ml volume increments. The resultant curve, which was termed a Hertel curve, was linear over 7 ml (R > 99%) and had a slope of 0.62 mm per milliliter of volume increment. CONCLUSIONS: An inflatable orbital implant is an effective tool for introducing a specified amount of volume in the cadaver orbit. The predictable relation between proptosis and volume increment (Hertel curve) may be a useful tool for surgeons in planning the size of an implant required to surgically correct enophthalmos.