title: The prism bar--Prentice and frontal positions.
abstract: The prism cover test has been in use for many years for measuring ocular alignment. The measurements obtained are used for both calculating the amount of correction needed in strabismus surgery, and monitoring any change in ocular alignment with recovery from muscle imbalance. Variability may arise from its use in either the frontal position or the Prentice position. Most prism bars used in the United Kingdom are calibrated for use in the Prentice position. Inaccurate results arise when a prism bar is used in a position for which it is not calibrated. The theoretical calculation for adjusting measurements obtained in the frontal position with prisms calibrated for use in the Prentice position was assessed clinically. A new modified equation is proposed incorporating practical aspects of performing the prism cover test. A table of values is included to assist in determining the necessary adjustment, so allowing for a more reliable assessment.
authors.affiliation: St. Paul's Eye Hospital, Liverpool.
authors.forename: S B
authors.initials: SB
authors.lastname: Kaye
authors.forename: A M
authors.initials: AM
authors.lastname: Ansons
authors.forename: J R
authors.initials: JR
authors.lastname: Green
authors.forename: J
authors.initials: J
authors.lastname: Wylie
mesh_terms: D000293:Adolescent; D000328:Adult; D002648:Child; D002675:Child, Preschool; D005403:Fixation, Ocular; D006801:Humans; D009988:Orthoptics / Q000379:methods; D013285:Strabismus / Q000503:physiopathology* / Q000601:surgery