Mechanisms of posterior lens dislocation during phacoemulsification have been suggested, but the actual mechanism involved has not been demonstrated. Chart reviews of cataract surgeries performed at Indiana University from 1991 until the present identified nine cases of posterior lens dislocation that occurred during phacoemulsification. In four, the procedure was converted early to a can-opener capsulotomy. Cadaver eye studies showed the role of an imperfect capsulorhexis in extending tears while a trap door was being formed for posterior nuclear dislocation during phacoemulsification.