Review of the inverse Knapp procedure: indications, effectiveness and results

To evaluate the indications and results of inverse Knapp procedures performed at one institution over a 10 year period between 1987 and 1996.

The records of patients who had undergone inverse Knapp procedures were retrospectively reviewed. Demographic data were collected, pre- and post-operative orthoptic assessments were evaluated, and pre- and post-operative binocular single vision (BSV) charts and Hess charts were scored.

Twenty-one patients were identified and records were available in 17. The main indication for the operation was orbital trauma. The mean vertical deviation in primary position and downgaze improved from 16.06 prism dioptres (PD) to 7.35 PD and 26.45 PD to 6.66 PD respectively. The pre-operative average score for BSV was 42%, increasing to 62% post-operatively. The Hess chart error scores improved on average from 848.8 pre-operatively to 296.4 post-operatively. Further operations were required for 8 patients.

Inverse Knapp procedure is an uncommon strabismus operation but an extremely useful one in selected cases. We recommend it for the treatment of marked inferior rectus weakness, congenital or acquired, for post-traumatic inferior rectus underaction with or without orbital blow-out fracture and for residual large hypertropia in patients with poor binocular functions. The extent of inferior rectus underaction should be assessed very carefully to avoid overcorrecting.