Funnel hole for intramedullary nail

1. An intramedullary nail comprising:

a shank with a centerline defined along a length thereof; and

said shank having a channel with a channel axis perpendicular to the centerline, the channel having an obround shape in a first cross-sectional plane perpendicular to the channel axis at the centerline and a tapered profile in a second cross-sectional plane containing the channel axis, such that a portion of a proximal edge of the channel within the second cross-sectional plane forms a first angle relative to the channel axis and a distal edge of the channel within the second cross-sectional plane forms a second angle relative to the channel axis that is different from the first angle, wherein the channel is internally threaded.

2. The intramedullary nail of claim 1 , wherein opposed proximal and distal ends of the channel are each defined by a round surface extending along respective non-parallel axes, and opposed sides of the channel are planar and extend between the two round surfaces to define the obround shape.

3. The intramedullary nail of claim 2 , wherein an extension portion of the channel defines a circular opening at a surface of the shank.

4. The intramedullary nail of claim 2 , wherein the two non-parallel axes each intersect the centerline.

5. The intramedullary nail of claim 2 , wherein the first angle is non-zero and the second angle is zero.

6. The intramedullary nail of claim 2 , wherein the nail includes an obround opening in the shank that feeds into the channel, and at least part of a perimeter of the obround opening is chamfered.

7. The intramedullary nail of claim 2 , wherein an angle between the two non-parallel axes is between 3° and 7°.

8. The intramedullary nail of claim 7 , wherein the angle between the two non-parallel axes is 6.5°.

9. The intramedullary nail of claim 2 , wherein:

a portion of the round surface of the proximal end of the channel defines a half of a cylinder centered on the proximal axis, and

a portion of the round surface of the distal end of the channel defines a half of a cylinder centered on the distal axis.

10. The intramedullary nail of claim 1 , wherein:

the shank is capable of elastic deformation across a range of deflection on the second cross-sectional plane, a ratio of a distance travelled by a distal tip of the shank between opposite ends of the range of deflection to a total length of the nail being at least 1:30; and

a rectangular area exists on the second cross-sectional plane that does not intersect a proximal or distal edge of the channel at any position of the shank within the range of deflection.

11. The intramedullary nail of claim 10 , wherein the ratio of the range of deflection of the distal tip of the shank to the total length of the nail is 1:24.

12. The intramedullary nail of claim 1 , wherein the shank defines a bore extending along the centerline and dividing the channel into two aligned passages.

13. A method for treating injury to a bone, comprising:

inserting an intramedullary nail into an intramedullary canal of the bone generally along the anatomical axis of the bone, wherein the nail includes:

a shank with a centerline defined along a length thereof; and

said shank having a channel with a channel axis perpendicular to the centerline, the channel having an obround shape in a first cross-sectional plane perpendicular to the channel axis at the centerline and a tapered profile on a second cross-sectional plane containing the channel axis at the centerline, wherein the channel is internally threaded; and

inserting a screw through a predetermined location of the bone such that the screw threadingly engages at least a portion of the channel.

14. The method of claim 13 , wherein the nail includes an obround opening in the shank that feeds into the channel, and the nail is implanted in the bone with the obround opening oriented in an anterior direction relative to the bone.

15. The method of claim 14 , wherein the nail is implanted in the bone such that the tapered profile narrows toward a posterior direction relative to the bone.

16. The method of claim 15 , further comprising drilling through the bone and the channel at a predetermined location of the bone from the anterior direction toward the posterior direction.

17. The method of claim 13 , wherein the screw engages at least a portion of the channel to accommodate flexion of the intramedullary nail that occurs during implantation.

18. A method of designing an intramedullary nail, comprising:

estimating an anticipated range of flexion of a shank of an intramedullary nail on a flexion plane corresponding to flexion of the nail during a procedure for implanting the nail in a bone;

defining a fixed position for a cylindrical fixation element relative to a head of the nail, the head located at a proximal end of the nail, the fixed position being such that the cylindrical fixation element would extend transverse to and intersect the shank at any position of the shank within the anticipated range of flexion; and

selecting respective locations and angles for a proximal extremity of a channel disposed through a distal half of the shank and a distal extremity of the channel such that the cylindrical fixation element located at the fixed position would extend through the channel without crossing the proximal extremity or the distal extremity of the channel at any degree of flexion of the shank within the anticipated range of flexion.

19. The method of claim 18 , further comprising:

defining an additional fixed position for an additional cylindrical fixation element; and

selecting respective locations and angles for proximal and distal extremities of an additional channel disposed through the distal half of the shank such that the additional cylindrical fixation element located at the additional fixed position would extend through the additional channel without crossing the proximal or distal extremity of the additional channel at any degree of flexion of the shank within the anticipated range of flexion.

20. A method for treating injury to a bone using an intramedullary nail designed according to the method of claim 18 , the method comprising:

inserting the nail into the bone generally along the anatomical axis of the bone while allowing flexion of the nail within the bone; and

after the nail is inserted into the bone, inserting a screw through the bone and channel at a location and angle relative to the head of the nail corresponding to the fixed position of the cylindrical fixation element.