referring to fig1 a surgical instrument 10 for cutting a flap 11 in the cornea 12 includes a housing 13 that encloses a motor 14 . also within the housing 13 is a power supply 16 , such as a battery , for providing power to the reciprocating motor 14 . a stationary shaft 18 extends distally from the housing 13 to a cutting tip 20 disposed at a distal end thereof . at its distal end , the shaft 18 bifurcates into two prongs 22 a - b separated by a gap 24 . an aplanating member 26 , best seen in fig2 . extends between the prongs 22 a - b and across the gap 24 . in the illustrated embodiment , the aplanating member 26 includes a roller 30 mounted to rotate about an axle 32 that extends across the gap 24 . the coupling between the roller 30 and the axle 32 has a coefficient of rolling friction that is low enough to enable it to roll as it is dragged across the cornea 12 . the aplanating member 26 can also be a stationary rod or runner having a cross section that includes a curved contact surface for sliding across the cornea 12 . the aplanating member 26 is preferably made from a biocompatible material having a low coefficient of sliding friction . the cutting tip 20 includes a bow 28 mounted at the distal tip of a reciprocating member 33 . the motor 14 is coupled to the reciprocating member 33 so as to cause the bow 28 to move from side to side in a direction parallel or essentially parallel to the aplanating member 26 . in some embodiments , the motor 14 is coupled so as to cause the bow 28 to undergo orbital motion by moving along an elliptical path having a major axis parallel to the aplanating member 26 . a cutting blade 35 , which can be a wire or a knife , extends across the opening of the bow 28 in a direction parallel to the aplanating member 26 . referring to fig3 the cutting blade 35 lies in a cutting plane that is vertically offset from a plane defined by the two prongs 22 a - b . the extent of this vertical offset 34 controls the thickness of the flap 11 . the cutting blade 35 is also horizontally offset from the aplanating member 26 either distally or proximally . the extent of this horizontal offset 36 is selected such that the surface of the cornea 12 that is exposed to the cutting blade 35 remains essentially flat even though it no longer lies under the aplanating member 26 . as shown in fig3 the aplanating member 26 is placed on the cornea 12 to flatten the cornea 12 in a region exposed to the reciprocating cutting blade 35 . the surgeon then translates the surgical instrument 10 . in the case in which the cutting blade 35 lies proximal to the roller 30 , the surgeon translates the surgical instrument 10 distally . conversely , when the cutting blade 35 lies distal to the roller 30 , the surgeon translates the surgical instrument 10 proximally . in either case , the direction of motion is selected so that the cutting blade 35 slices into a portion of the cornea 12 that has recently been flattened , or aplanated , by the aplanating member 26 . in another embodiment , shown in fig4 an additional bow 38 is mounted onto or integrally formed with the reciprocating member 33 proximal to the cutting blade 35 . a lifting rod 40 extends across the additional bow 38 to support the flap that is sliced by the cutting blade 35 . because the lifting rod 40 reciprocates with the reciprocating member 33 , the flap is unlikely to adhere to the lifting rod 40 during surgery . using a surgical instrument 10 as described herein , a surgeon can rapidly slice a flap 11 having a uniform and pre - defined thickness . by appropriately selecting the vertical offset 34 between the cutting blade 35 and the aplanating member 26 , the flap 11 can be sliced thinly enough to avoid exposing the stroma .
an apparatus for forming a corneal flap includes a housing from which extend a shaft and a reciprocating member . an aplanating member is mounted at a distal end of the shaft to aplanate a region of the cornea . a cutting blade is mounted to the reciprocating member at a position selected to slice the aplanated region off the cornea .