Abstract
The present application relates to a spherical bead (30) to be interposed between a locking bone screw head (2) and a receiving hole (34) in a bone plate (10) and to an implant construct comprising such a bead.
Claims
1. Spherical bead (30) with a conical hole (34) to be interposed between a locking bone screw head (2) and a receiving hole (14) in a bone plate (10).
2. The spherical bead of claim 1, wherein the hole (34) is tapered with a self-locking angle (33).
3. The spherical bead of claim 2, wherein the angle (33) is between about 2.7 degrees and about 8 degrees, preferably about 5.7 degrees, corresponding to the taper of 1:10.
4. An implant construct comprising a bead (30) of claim 1, and a locking screw (40), and optionally a locking plate (10).
5. The implant construct of claim 4, wherein the total angle (42) of the conical screw head (41) is larger than the angle (33) of the hole (34) in the bead (30) by about 0.2 to about 0.4 degrees, preferably by about 0.3 degrees.
6. The implant construct of claim 4, wherein the total angle (15) of the conical hole (14) in the plate (10) is about the same as the total angle (33) of the conical hole (34) in the bead (30).
7. The implant construct of claim 4, comprising a bone locking plate (10), a bone locking screw (40) and a spherical bead (30), wherein the bead (30) includes a conical hole (34) and is to be interposed between a conical head (41) of the bone locking screw (40) and a receiving hole (14) in the bone locking plate (10), characterized in that the receiving hole (14) in the bone locking plate (10) is conical.
8. The implant construct of claim 7, comprising a bone locking plate (10), a bone locking screw (40) and a spherical bead (30); wherein the bead (30) includes a conical hole (34) and is to be interposed between a conical head (41) of the bone locking screw (40) and a receiving hole (14) in the bone locking plate (10), characterized in that the receiving hole (14) in the bone locking plate (10) is conical, wherein the total angle (15) of the conical receiving hole (14) in the plate (10) is about the same as the total angle (33) of the conical hole (34) in the bead (30).
9. The implant construct of claim 7, wherein the total angle (42) of the conical screw head (41) is larger than the total angle (33) of the conical hole (34) in the bead (30) by about 0.2 to about 0.4 degrees preferably by about 0.3 degrees.
10. The implant construct of claim 7, wherein the conical bead hole (34) is tapered with a self-locking angle (33).
11. The implant construction of claim 7, wherein the conical bead hole angle (33) is between about 2.7 degrees and about 8 degrees, preferably about 5.7 degrees, corresponding to the taper of 1:10.
12. A method for fixing a bone plate to a bone comprising the steps: (i) drilling at least one hole into the bone, (ii) providing a bone plate comprising at least one conical hole, (iii) providing a spherical bead with a conical hole wherein the bead is adapted to fit into the conical hole of the bone plate, (iv) inserting the bead into the conical hole of the bone plate, and (v) inserting a bone screw having a conical self-locking head through the conical hole of the bead into the bone.
Description
LIST OF FIGURES
[0028] FIG. 1. Locking bone screw with Morse taper-type head.
[0029] FIG. 2. Transverse and longitudinal cross-sections of the bone plate with a conical hole.
[0030] FIG. 3. Perspective views of a bone plate section with a conical hole.
[0031] FIG. 4. Locking screw inserted in the plate in transverse and in longitudinal cross-sections.
[0032] FIG. 5. Bead for the conical locking screw head.
[0033] FIG. 6. A bone screw inserted through a bead angulated in the transverse plane.
[0034] FIG. 7. A bone screw inserted through a bead angulated in the longitudinal plane.
[0035] FIG. 8. Drill sleeve seated in the conical hole, angulated in the longitudinal direction before drilling a hole in the bone is performed.
[0036] FIG. 9. Use of the beads to allow for angulation of the bone screws.
[0037] FIG. 10. Offset screw placement for interfragmentary compression using a locking screw augmented with a bead.
DETAILED DESCRIPTION
[0038] FIG. 1 shows a bone screw 1 of the present invention with a conical head 2, tapered with a total cone angle 3. Angle 3 is smaller than required for the self-locking function of the Morse taper. The range of the angle 3 is usually limited on the lower end by the machining tolerances and is about 2.9 degrees corresponding to the taper of 1:20. On the upper end, an angle of 8 degrees would call for a high coefficient of friction not easily provided inside the body with lipids covering all implant surfaces. The compromise value of 5.7 degrees corresponding to the taper of 1:10, has been proven satisfactory. However, if the receiving holes in the plates are made with this angle, the compression between the head and the plate will be concentrated at the lower end of the screw head with a solid core and thus of higher stiffness. To reduce this stress concentration, the angle of the screw head should be larger than the angle of the receiving hole—a value of 6 degrees has been found to provide a satisfactory stress distribution with a screwdriver recess 4 of hexalobular type (Torx™ type). As the screw is inserted into the hole of the plate made with an angle of 5.7 degrees, the first contact occurs at the top of the hole and with the deformation of the head over the screwdriver recess with higher compliance, the contact widens to the bottom of the hole and the lower end of the screw head. This reduces the risk of cold welding. The threads 5 of the screw engage the bone following the threads cut in the bone by the cutting flutes 6 of the screw.
[0039] FIG. 2 shows a transverse (a) and a longitudinal (b) cross-section of the bone plate 10 with a hole 14 for receiving the tapered head of the bone screw. The plate is of width 12 and height 13, with a facet 11 along the upper edge to facilitate soft tissue cover. The hole 14 is tapered towards the upper surface with a total angle 15 slightly smaller than the angle of the screw head. For the preferred self-locking combination with a nominal taper of 1:10, the angle of the hole is about 5.7 degrees and the head angle is about 6 degrees. On the bone-facing surface, the hole 14 is surrounded by a recess 16, which combined with a transverse cut 17 reduces the potential contact to the bone to small areas 18. On the lower side of the plate, the hole 14 is provided with longitudinally oriented cylindrical undercuts 19 which allow angulation of the screws.
[0040] FIG. 3 shows perspective views of a section of the plate 10 with a tapered hole 14. View (a) of the top 20 of the plate 10, shows the facet 11 and the undercut 19. View (b) of the bottom side of the plate shows the recess 16 surrounding the hole 14, the transverse cuts 17, the potential bone-contacting areas 18, and the cylindrical undercuts 19.
[0041] FIG. 4 shows a transverse (a) and a longitudinal (b) cross-section of the bone plate 10 with a locking screw 1 inserted in the plate hole at 90 degrees. The head 2 of the screw 1 is fully seated and thus locked in the plate 10 in all degrees of freedom, forming a construct capable of transferring all loads between the plate and the bone. The plate and the screws become essentially a single implant unit. This is important not only for mechanical reasons for load transfer but also for avoidance of any movement between the plate and the screws that can lead to fretting corrosion and hence release of very fine metal particles and ions. Tissue response to such debris can have serious medical sequelae, locally but also systemically. Cylindrical undercuts 19 do reduce the contact area between the screw heads and the plate holes, but there is still sufficient contact on the sides of the holes to provide for safe locking.
[0042] FIG. 5 shows the bead head 30, the central item of this invention. The outside shape of the bead is a section of a sphere with the diameter 31. The height 32 of the bead is sufficient to cover most of the conical head of the screw inserted in the conical hole 34 with a total angle 33. For a nominal taper of 1:10, the angle 33 is about 5.7 degrees, i.e. about the same as the angle in the bone plate holes.
[0043] FIG. 6 shows a transverse cross-section of the bone screw 40 inserted in the bone plate 10 through a bead 30. The diameter 31 of the bead is chosen and machined to a precise tolerance so that the depth 35 to which the bead is recessed in the hole 14 is equal to approximately a half, e.g. about 40% to about 60%, particularly about 50% of the hole height. In this cross-section the screw is slightly inclined with respect to the plate—the range usually called for is +−5 degrees. The diameter of the screw 40 is smaller than the diameter of a nominal locking screw that fits in the hole 14 when inserted at 90 degrees.
[0044] For practical reasons of producing the beads and the selection of bone screws, the difference of the two screw diameters is about 0.9 mm to about 1.1 mm, particularly about 1 mm. The total angle 42 of the conical head 41 of the screw 40 is the same as the total angle 3 of the screw head 2 of the screw 1, FIG. 1, i.e. preferably about 6 degrees.
[0045] FIG. 7 shows the screw 40 and the plate 10 in a longitudinal cross-section with the screw inclined to the maximum angle of about 30 degrees. This is made possible with the cylindrical undercuts 19 in the plate 10. The bead 30 is still engaging the conical hole of the plate 10 sufficiently to lock, but the contact is only a line contact and thus less resistant to bending loads. However, the screws are less likely to go loose from the bone by unscrewing and the potential for fretting is also substantially reduced compared to conventional screws inserted in conventional, non-locking plates.
[0046] FIG. 8 shows the drill sleeve 50 with a spherical tip 52 of the same diameter as the beads to be used in this hole of the plate 10. The sleeve is placed in the hole straight down and then inclined as needed. The nose 53 of the sleeve matches the outside diameter of the bone screw to be inserted at an angulation. The drill 51 matches the core diameter of the screw. Once a hole is drilled in the bone, the drill is retracted; the sleeve is turned to about 90 degrees to the plate and removed from the plate. Careful inspection of the geometry shows that the sleeve could not be removed from the plate hole while the drill is still in the bone without sliding the plate over the bone in a longitudinal direction.
[0047] FIG. 9 shows the sequence of insertion of the angulated screw. The bead 30 is held on a bead holder 54, cross-section (a) that can be inserted in the direction of the pre-drilled hole 56 in the bone 55. By a slight pressure on the bead 30 via the bead holder 54 the bead can be positioned in the plate hole where it will remain as the bead holder is removed. The bead holder is made from plastic and its tip is slotted, holding the bead just enough for handling.
[0048] Once the bead 30 is in place, cross-section (b), the screw 40 can be inserted through the bead and screwed into the bone 55, locking its head into the bead and the bead into the plate.
[0049] FIG. 10 shows the use of a screw 40 augmented by the bead 30 for generating so-called dynamic compression. Some of the holes in the bone plate 10 are made as compression holes by extending the conical hole 14 into a hole 61, offset from the axis of the hole 14 by a distance 60. If the hole in the bone is drilled with a dedicated sleeve that can be centered in the hole 61, and the screw 40 with the bead 30 pre-placed on its head is inserted and screwed down into the bone 55 through the plate 10, the plate will be shifted longitudinally as shown by the arrow 62.
[0050] The invention of the screw head beads disclosed herein makes it possible to use, in addition to perpendicular, nominally sized locking screws with Morse taper heads, inclined screws of a smaller diameter in the bone plates with conical holes. It is also possible to use bead-augmented screws to create dynamic compression if needed. On a practical side, only one type screw—locking with a tapered head—can provide for all applications of bone plates. The use of beads in most cases is optional—only in peri-articular fractures and in rare cases where dynamic compression is possible and called for. An important advantage of using the beads, when indicated, is also the elimination of fretting between the screws and the plates, which is the main risk of detrimental tissue response to implants.
[0051] The following items of the specification further characterize the invention: [0052] 1. An implant construct comprising a bone locking plate (10), a bone locking screw (40) and a spherical bead (30), wherein the bead (30) includes a conical hole (34) and is to be interposed between a conical head (41) of the bone locking screw (40) and a receiving hole (14) in the bone locking plate (10), characterized in that the receiving hole (14) in the bone locking plate (10) is conical. [0053] 2. An implant construct comprising a bone locking plate (10), a bone locking screw (40) and a spherical bead (30); wherein the bead (30) includes a conical hole (34) and is to be interposed between a conical head (41) of the bone locking screw (40) and a receiving hole (14) in the bone locking plate (10), characterized in that the receiving hole (14) in the bone locking plate (10) is conical, wherein the total angle (15) of the conical receiving hole (14) in the plate (10) is about the same as the total angle (33) of the conical hole (34) in the bead (30). [0054] 3. The implant construct of item 1 or 2, wherein the total angle (42) of the conical screw head (41) is larger than the total angle (33) of the conical hole (34) in the bead (30) by about 0.2 to about 0.4 degrees preferably by about 0.3 degrees. [0055] 4. The implant construct of any one of items 1-3, wherein the conical bead hole (34) is tapered with a self-locking angle (33). [0056] 5. The implant construction of any one of items 1-4, wherein the conical bead hole angle (33) is between about 2.7 degrees and about 8 degrees, preferably about 5.7 degrees, corresponding to the taper of 1:10.
