Caudal or cranial partial tenotomy of the horizontal rectus muscles in A and V pattern strabismus. 
When performing a recession with vertical transposition of the insertions of the horizontal muscles in patients with A or V pattern strabismus, there is a risk of overcorrection in those with a relatively small angle of strabismus and almost-straight eyes in either upgaze or downgaze.  To determine whether a caudal or cranial partial tenotomy of the horizontal rectus would be sufficient to reduce the horizontal angle in gaze ahead and minimise the risk of overcorrection in gaze direction, either up or down, with the smallest horizontal deviation.  A retrospective evaluation was performed of patients who had a caudal or cranial partial tenotomy of the horizontal rectus between January 1996 and January 2006.  Patients were excluded if they had undergone previous surgery and or required additional oblique-muscle surgery.  The reduction in the horizontal angle of strabismus in gaze ahead and in 25 degrees upgaze and downgaze was evaluated.  Fifty-two patients were included, 16 with A-eso pattern, 12 with V-eso pattern, 7 with A-exo pattern, and 17 with V-exo pattern.  Their age at operation ranged from 2 to 80 years (median 16).  The mean (SD) reduction in the horizontal angle was 8.1 (4.5) degrees in the working direction of the transposition, either upgaze or downgaze, 6.2 (4.5) degrees in gaze ahead, and 3.3 (4.4) degrees out of the working direction.  Four patients had to be reoperated in the short term, one because of overcorrection and three because of undercorrection.  In patients with A or V pattern strabismus and an almost-straight eye position in either upgaze or downgaze, a partial tenotomy of the horizontal rectus is an effective treatment, with minimal risk of overcorrection.