PurposeThis prospective study tested a simple formula for selecting an implant size for patients undergoing enucleation, evisceration, and secondary implantation. The formula axial length−2 mm=implant diameter (subtract 1 mm from implant diameter for evisceration and for hyperopia) was tested by the outcome measures, superior sulcus deformity, enophthalmos, and volume of the prosthesis.MethodsFifty-four patients undergoing primary or secondary implant surgery after enucleation or evisceration received implants based on the above formula. The volume of the eye, volume of the implant, volume of the prosthesis, and the total percent volume replacement were recorded for each patient. Outcome measures considered clinically acceptable were <2 mm enophthalmos and less than grade 1 superior sulcus deformity, which is defined as barely perceptible deepening of the medial superior sulcus.ResultsThe average volume replacement was 101%; average prosthetic volume was 2.1 mL; average grade of superior sulcus deformity was 0.6; and average enophthalmos was 1.2 mm.ConclusionsThis formula allows 100% replacement of the volume removed, leaves space for a prosthesis 1.5 to 2.5 mL, and eliminates clinically unacceptable superior sulcus deformity and enophthalmos in 85% of patients. Patients with a history of infection, radiation, buphthalmos, or large orbital fractures (15%) had residual superior sulcus deformity greater than grade 1 and enophthalmos ≥2 mm despite 100% volume replacement. Further study will elucidate the histopathologic processes responsible for residual superior sulcus deformity and enophthalmos in the latter subgroup of patients.