Knapp procedure

The Knapp procedure is a strabismus operation for monocular elevation deficiency, the condition previously termed double elevator palsy. The surgeon transfers the entire tendons of the medial and lateral rectus muscles to the medial and lateral edges of the superior rectus insertion.^1,2 Monocular elevation deficiency is an apparent paralysis of both elevators of one eye, the superior rectus and inferior oblique, and produces a hypotropia that increases on upgaze.¹

Surgery is indicated only for hypotropia of the involved eye in primary position, chin elevation, or both, and its aim is single binocular vision in primary position.² Forced ductions come first, to confirm any mechanical restriction.¹ A contracted inferior rectus must also be recessed, and the recession is often done as a separate first stage before the transposition is judged necessary.^1,2 After the transposition, vertical rotations improve remarkably and horizontal rotations are impaired only slightly.¹ The operation sacrifices four anterior ciliary arteries, and surgery on a third rectus muscle is best delayed 6 months to let the blood supply adapt.¹