US20110054544A1

US20110054544A1 - System with integral locking mechanism

Info

Publication number: US20110054544A1
Application number: US12/551,182
Authority: US
Inventors: Foley Kevin; Newt Metcalf; Greg Marik
Original assignee: Warsaw Orthopedic Inc
Current assignee: Warsaw Orthopedic Inc
Priority date: 2009-08-31
Filing date: 2009-08-31
Publication date: 2011-03-03
Also published as: WO2011026032A3; US20110054542A1; US20110054543A1; US8591555B2; WO2011026032A2

Abstract

A stratum to be affixed to bone is disclosed. The stratum has a first surface, a second surface, and at least one hole extending between the first surface and the second surface, wherein the second surface is configured to engage at least a portion of the bone, and wherein the stratum further has a slot in the stratum that extends from the at least one hole to another location on the stratum, and the stratum is further configured to deflect, allowing a fastener to pass at least partially through the hole.

Description

FIELD OF INVENTION

The present invention is directed to systems for affixing a stratum to bone.

BACKGROUND

The present disclosure is related to commonly owned and copending U.S. applications ______ (having Attorney Docket No. P35832.00) and ______ (having Attorney Docket No. P35833.00), each of which has a filing date that is the same as the present disclosure, and both of which are hereby incorporated herein by reference in their entireties.
The present disclosure relates to locking mechanisms, and more particularly, systems for affixing a stratum to bone.

SUMMARY OF THE INVENTION

A stratum to be affixed to bone is disclosed. The stratum has a first surface, a second surface, and at least one hole extending between the first surface and the second surface, wherein the second surface is configured to engage at least a portion of the bone, and wherein the stratum further has a slot in the stratum that extends from the at least one hole to another location on the stratum, and the stratum is further configured to deflect, allowing a fastener to pass at least partially through the hole.
Additional aspects and features of the present disclosure will be apparent from the detailed description and claims as set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a system for affixing a stratum to bone;

FIG. 1A is a cross-sectional view of the system of FIG. 1, depicting deflection of the stratum;

FIG. 2 is a side view of the fastener of the system of FIG. 1;

FIG. 3 is a cross-sectional view of another system for affixing a stratum to bone.

FIG. 4 is an isometric view of a system with integral locking mechanism;

FIG. 5 is an isometric view of another system with integral locking mechanism;

FIG. 6 is an isometric view of another system with integral locking mechanism; and

FIG. 7 is the system of FIG. 5 in cooperation with a portion of vertebral column.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments, or examples, illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates.
FIG. 1 shows a cross-sectional view of a system 100 for affixing a stratum 20 to bone 50. The system 100 has a stratum 20 having a first surface 20 a, a second surface 20 b, and at least one hole 30 extending between the first surface 20 a and the second surface 20 b, wherein the second surface 20 b is configured to engage at least a portion of the bone 50. The system 100 further has a fastener 10 configured to pass at least partially through the at least one hole 30 and engage at least a portion of the bone 50, wherein the stratum 20 is further configured to deflect, allowing the fastener 10 to pass at least partially through the hole 30. In the context where the stratum 20 may be a spinal plate, for example, the stratum may be used to fuse adjacent vertebrae together in a relatively fixed relationship.
In the system 100 of FIG. 1, the fastener 10 has a length and a width, the length being greater than the width, and a central longitudinal axis. A side view of the fastener 10 of FIG. 1 is shown in FIG. 2, which is drawn to scale. The fastener 10 further has a head portion 12, an intermediate portion 18, and a distal portion 15. The distal portion 15 has a proximal end 16 that is proximate the intermediate portion 18, and a distal end 14 located at the tip of the fastener 10.
In the context of spinal plates, the fastener 10 may be, for example, a screw. In fact, a screw is shown as the fastener 10 in FIGS. 1 and 2. In an embodiment where the fastener 10 is a screw, the head portion 12 is a head of the screw, the distal portion 15 contains threads of the screw, and the intermediate portion 18 has no threads. The cross section of the fastener 10 may be substantially circular, as is common with screws. As shown in FIGS. 1 and 2, the head portion 12 has a width (or diameter in the case of a fastener having a circular cross section) that is the largest of the fastener 10, while the intermediate portion 18 has the smallest width (or diameter). The distal portion 15 of the fastener 10 has a proximal end 16 having a first width (or diameter) and a distal end 14 having a second width (or diameter), wherein the first width is greater than the second width. As used herein, the width of each respective section is substantially perpendicular to the central longitudinal axis X-X′ of the fastener 10.
In the embodiment of FIG. 1, the system 100 shows a stratum 20 that is substantially non-rigid, such as, for example, polyetheretherketone (“PEEK”). Such a stratum may be composed solely of PEEK, or contain enough PEEK so as to be non-rigid. Other suitable non-rigid materials may include, but are not limited to polyetherketoneketone (“PEKK”), ultra high molecular weight polyethylene (“UHMWPE”), polyethylene, shape memory metals and other polymers. The term “substantially” as used herein may be applied to modify any quantitative representation which could permissibly vary without resulting in a change in the basic function to which it is related. For example, a stratum 20 may be considered substantially non-rigid if can deflect (at the location of the hole 130) upon the insertion of a fastener 10 through hole 30, but rebound to the position or approximate position prior to insertion of the fastener 10. Specifically, system 100 is designed so that the proximal end 16 of the distal portion 15 of the fastener 10 causes the stratum 20 to deflect as the proximal end 16 of the fastener 10 moves across the at least one hole 30 in the stratum 20 in the direction towards the second surface 20 b of the stratum 20. FIG. 1A shows system 100 as the proximal end 16 of the fastener 10 moves across the at least one hole 30 in the stratum 20 in the direction towards the second surface 20 b of the stratum 20, thereby deflecting the stratum 20.
FIG. 1A illustrates a deflection mechanism that is a type of deflection due to, for example, the characteristics of the material of the stratum 20. In addition to the deflection mechanism illustrated in FIG. 1A, the stratum 20 may deflect in a radial direction so as to enlarge the hole 30. That is, the stratum 20 adjacent to the hole 30 or portions of the stratum 20 adjacent the hole 30 may move in a direction away from the fastener 10, yet remain in the same plane of the stratum 20. In such a mechanism, the stratum 20 deflects in a direction away from and substantially perpendicular to the fastener 10. Such a mechanism is a type of deflection due, for example, the geometry of the stratum 20.
Further, the distal portion 15 of the fastener 10 has a lip 16L that allows passage of the proximal end 16 through the at least one hole 30 in the stratum 20, and prevents inadvertent backing out of the fastener 10, i.e., moving back out of the at least one hole 30 in a direction away from the bone 50. As shown in FIGS. 1, 1A and 2, the lip 16L is situated at the proximal-most location of the distal portion 15 of the fastener 10, and also has the largest width (or diameter) over the distal portion of the fastener 10. Also, when the fastener 10 is inserted through the hole 30 on the stratum 20, the stratum 20 starts to deflect when the surface 16 a of the proximal end 16 of the distal portion 15 contacts the stratum 20, whereas the stratum 20 prevents inadvertent backing out of the fastener 10 by means of the contact between the second surface 20 b of the stratum 20 and surface 16 b of the proximal end 16 of the distal portion 15. In the embodiments of FIGS. 1, 1A and 2, the fastener 10 is made of a material that allows this function to be accomplished. For example, the fastener 10 may be made of a material (metal or non-metal) that is able to cause the stratum 20 to deflect and rebound, as described above. Some suitable materials include, but are not limited to, Titanium Alloys, commercially available Titanium, stainless steel, PEEK, cobalt chrome (“CoCr”), and shape memory metals. Further, as shown in FIG. 1, the stratum 20 has a recess 24 surrounding the hole 30 that helps accommodate at least a portion of the head portion 12 of the fastener 10.
FIG. 3 shows a cross-sectional view of a system 200 for affixing a stratum 120 to bone 150. The system 200 has a stratum 120 having a first surface 120 a, a second surface 120 b, and at least one hole 130 extending between the first surface 120 a and the second surface 120 b, wherein the second surface 120 b is configured to engage at least a portion of the bone 150. The system 200 further has a fastener 110 configured to pass at least partially through the at least one hole 130 and engage at least a portion of the bone 150, wherein the stratum 120 is further configured to deflect, allowing the fastener 110 to pass at least partially through the hole 130.
In system 200 of FIG. 3, the fastener 110 has a length and a width, the length being greater than the width, and a central longitudinal axis. The fastener 110 further has a head portion 112, an intermediate portion 118, and a distal portion 115. The distal portion 115 has a proximal end 116 that is proximate the intermediate portion 118, and a distal end 114 located at the tip of the fastener 110.
In the context of spinal plates, the fastener 110 may be, for example, a screw. In fact, a screw is shown as the fastener 110 in FIG. 3. In an embodiment where the fastener 110 is a screw, the head portion 112 is a head of the screw, the distal portion 115 contains threads of the screw, and the intermediate portion 118 has no threads. The cross section of the fastener 110 may be substantially circular, as is common with screws. As shown in FIG. 3, the head portion 112 has a width (or diameter) that is the largest of the fastener 10, while the intermediate portion 118 has the smallest width (or diameter). The distal portion 115 of the fastener 110 has a proximal end 116 having a first width (or diameter) and a distal end 114 having a second width (or diameter), wherein the first width is greater than the second width. As in the embodiments of FIGS. 1, 1A and 2, as used herein, the width of each respective section is substantially perpendicular to the central longitudinal axis of the fastener 110.
In the embodiment of FIG. 3, the system 100 shows a stratum 120 that is substantially non-rigid, such as, for example, polyetheretherketone (“PEEK”). Such a stratum may be composed solely of PEEK, or contain enough PEEK so as to be non-rigid. Other suitable materials for the stratum 120 of FIG. 3 are similar to those that are suitable for the stratum 20 of FIGS. 1 and 1A. A stratum 120 may be considered substantially non-rigid if can deflect (at the location of the hole 30) upon the insertion of a fastener 110 through hole 130, but rebound to the position or approximate position prior to insertion of the fastener 110. Specifically, system 200 is designed so that the proximal end 116 of the distal portion 115 of the fastener 110 causes the stratum 120 to deflect as the proximal end 116 of the fastener 110 moves across the at least one hole 130 in the stratum 120 in the direction towards the second surface 120 b of the stratum 120.
Further, the distal portion 115 of the fastener 110 has a lip 116L that allows passage of the proximal end 116 through the at least one hole 130 in the stratum 120, and prevents inadvertent backing out of the fastener 110, i.e., moving back out of the at least one hole 130 in a direction away from the bone 150. As shown in FIG. 3, the lip 116L is situated at the proximal-most location of the distal portion 115 of the fastener 110, and also has the largest width (or diameter) over the distal portion of the fastener 110. Also, when the fastener 110 is inserted through the hole 130 on the stratum 120, the stratum 120 starts to deflect when the surface 116 a of the proximal end 116 of the distal portion 115 contacts the stratum 120, whereas the stratum 120 prevents inadvertent backing out of the fastener 110 by means of the contact between the second surface 120 b of the stratum 120 and surface 116 b of the proximal end 116 of the distal portion 115. In the embodiment of FIG. 3, as with those illustrated in the previous Figures, the fastener 110 is made of a material that allows this function to be accomplished. For example, the fastener 110 may be made of a material (metal or non-metal) that is able to cause the stratum 120 to deflect and rebound, as described above. As with system 100 of the previous figures, with the system 200 of FIG. 3, the stratum 120 and the lip 116L of the fastener 110 are configured and work in conjunction to allow deflection of the stratum 120 in the direction toward the bone 150, while at the same time, work to not allow deflection in the opposite direction, i.e., to prevent inadvertent backing out of the fastener. Further, as shown in FIG. 3, the stratum 120 has a recess 124 surrounding the hole 130 that helps accommodate at least a portion of the head portion 112 of the fastener 110.
FIG. 4 shows an isometric view of a system 500 with integral locking mechanism. System 500 for affixing a stratum 520 to bone has a stratum 520 having a first surface 520 a, a second surface 520 b, and at least one hole 530 extending between the first surface 520 a and the second surface 520 b, wherein the second surface 520 b is configured to engage at least a portion of the bone (shown, for example, as bone 50 in FIGS. 1, 1A and 3). The system 500 further has a fastener (shown, for example, as fastener 10 in FIGS. 1, 1A and 2) configured to pass at least partially through the at least one hole 530 and engage at least a portion of the bone, wherein the stratum 520 is further configured to deflect, allowing the fastener to pass at least partially through the hole 530.
In the context of spinal plates, plates may be used to fuse adjacent vertebrae together in a relatively fixed relationship. In particular, stratum 520 has a first hole 530 and a second hole 530 a. For example, each hole may be positioned over different, yet adjacent, vertebral bodies. Stratum 520 further has a slot 522 in the stratum 520 that extends from the at least one hole 530 to another location on the stratum 520. Specifically, slot 522 extends from the first hole 530 to the second hole 530 a. The slot 522 facilitates deflection of the stratum 520 that allows the fastener to pass at least partially through the first hole 530, as well as another fastener to pass through the second hole 530 a. On stratum 520, the first hole 530 and the second hole 530 a further have recesses 524 and 524 a, respectively, and the slot 522 intersects the respective recesses 524 and 524 a. Stratum 520 further has secondary slots 526 a and 526 b. As shown on stratum 520, the secondary slots 526 a and 526 b may be curved or have the shape of an arc. Slots 526 a and 526 b further facilitate deflection of the stratum 520 that allows the fastener to pass at least partially through the first hole 530, as well as another fastener to pass through the second hole 530 a.
In particular, the slots 522, 526 a and 526 b of system 500 are provided to allow the stratum 520 surrounding holes 530 and 530 a to deflect in the radial direction. That is, the slots 522, 526 a and 526 b provide a geometrical design that allows the holes 530 and 530 a to enlarge, increasing their respective radii, when the portions of the stratum 520 adjacent to the holes 530 and 530 a move in a direction away from the holes, but remain substantially in the plane of the stratum 520. In doing so, the portions of slot 522 adjacent to the holes 530 and 530 a widen as the fastener passes through the holes 530 and 530 a. After a fastener is in its intended implanted position, the corresponding slots move back to their original positions, as shown in FIG. 4. In one embodiment of system 500, where stratum 520 has a thickness of 2.0 mm., the hole 530 in such a stratum 520 may have a diameter of 3.4 mm., and the slot 522 extending from such a hole 520 may be 0.25 mm. wide.
FIG. 5 shows an isometric view of a system 600 with integral locking mechanism. System 600 for affixing a stratum 620 to bone has a stratum 620 having a first surface 620 a, a second surface 620 b, and at least one hole 630 extending between the first surface 620 a and the second surface 620 b, wherein the second surface 620 b is configured to engage at least a portion of the bone (shown, for example, as bone 50 in FIGS. 1, 1A and 3). The system 600 further has a fastener 610 (shown, for example, as fastener 10 in FIGS. 1, 1A and 2) configured to pass at least partially through the at least one hole 630 and engage at least a portion of the bone, wherein the stratum 620 is further configured to deflect, allowing the fastener to pass at least partially through the hole 630.
In the context of spinal plates, plates may be used to fuse adjacent vertebrae together in a relatively fixed relationship. In particular, stratum 620 has a first hole 630 and a second hole 630 a. For example, the first hole 630 and the second hole 630 a may be positioned over different, yet adjacent, vertebral bodies. Stratum 620 further has a third hole 640, which may be positioned over a spacer or other intra-discal device located between adjacent vertebrae. Stratum 620 further has a first slot 622 a in the stratum 620 that extends from the at least one hole 630 to another location on the stratum 620. Specifically, slot 622 a extends from the first hole 630 to the third hole 640. Stratum 620 further has a second slot 622 b in the stratum 620 that extends from the second hole 630 a to another location on the stratum 620. Specifically, slot 630 b extends from the second hole 630 a to the third hole 640. The first slot 622 a facilitates deflection of the stratum 620 that allows the fastener to pass at least partially through the first hole 630. Similarly, the second slot 622 b facilitates deflection of the stratum 620 that allows another fastener to pass through the second hole 630 a. On stratum 620, the first hole 630 and the second hole 630 a further have recesses 624 and 624 a, respectively, which the respective slots 622 a and 622 b intersect. Further, the slots 622 and 622 a each intersect a recess 644 of the third hole 644. Note that the mechanism of deflection of system 600 is similar to that of system 500, i.e., slots 622 a and 622 b are provided to allow the stratum 620 surrounding holes 630 and 630 a to deflect in the radial direction.
FIG. 6 shows an isometric view of a system 700 with integral locking mechanism. System 700 for affixing a stratum 720 to bone has a stratum 720 having a first surface 720 a, a second surface 720 b, and at least one hole 730 extending between the first surface 720 a and the second surface 720 b, wherein the second surface 720 b is configured to engage at least a portion of the bone (shown, for example, as bone 50 in FIGS. 1, 1A and 3). The system 700 further has a fastener 710 (shown, for example, as fastener 10 in FIGS. 1, 1A and 2) configured to pass at least partially through the at least one hole 730 and engage at least a portion of the bone, wherein the stratum 720 is further configured to deflect, allowing the fastener to pass at least partially through the hole 730.
In the context of spinal plates, plates may be used to fuse adjacent vertebrae together in a relatively fixed relationship. In particular, stratum 720 has a first hole 730 and a second hole 730 a. For example, the first hole 730 and the second hole 730 a may be positioned over different, yet adjacent, vertebral bodies. Stratum 720 further has a third hole 740, which may be positioned over a spacer or other intra-discal device located between adjacent vertebrae. Stratum 720 further has a first slot 722 a in the stratum 720 that extends from the at least one hole 730 to another location on the stratum 720. Stratum 720 further has a second slot 722 b in the stratum 720 that extends from the second hole 730 a to another location on the stratum 720, a location that is a different location than the location where the first slot 722 a terminates. The first slot 722 a facilitates deflection of the stratum 720 that allows the fastener to pass at least partially through the first hole 730. Similarly, the second slot 722 b facilitates deflection of the stratum 720 that allows another fastener to pass through the second hole 730 a. On stratum 720, the first hole 730 and the second hole 730 a further have recesses 724 and 724 a, respectively, which the respective slots 722 a and 722 b intersect. Note that the mechanism of deflection of system 700 is similar to that of systems 500 and 600, i.e., slots 722 a and 722 b are provided to allow the stratum 720 surrounding holes 730 and 730 a to deflect in the radial direction.
Note that any of the slots described above may have different shapes than those shown and described. For example, any of the slots may be linear, curved, or take on a variety of other shapes.
FIG. 7 shows an isometric view of system 600 in cooperation with a portion of vertebral column. In particular, FIG. 7 depicts the stratum 620 positioned over two vertebral bodies V1 and V2 and an intradiscal device 680 (such as spacer) that is located in the disc space between the two vertebral bodies V1 and V2. Specifically, the first hole 630 of the stratum 620 is positioned over vertebral body V1, the second hole 630 a is positioned over vertebral body V2, and the third hole 640 is positioned over the intradiscal device 680. The stratum is affixed to the vertebral bodies V1 and V2 by means of fasteners 610 a and 610 b, respectively, which may, for example, be screws (such as screw 10 shown and described with reference to FIGS. 1, 1A, 2 and 3). Specifically, as shown in FIG. 7, fastener 610 a is positioned through hole 630, and fastener 610 b is positioned through hole 630 a. When fastener 610 a is placed through the first hole 630 and into the vertebral body V1, the first slot 622 a facilitates deflection of the stratum 620. Similarly, the second slot 622 b facilitates deflection of the stratum 620 when fastener 610 b is placed through the second hole 630 a and into the vertebral body V2. Further, a fastener 641 such as a screw may be placed through the third hole 640 and into the intradiscal device 680.
Note that all of the previous systems provide a monolithic stratum having the capability for self locking. That is, in the exemplary context of spinal plates, the systems 100, 200, 500, 600 and 800 provide monolithic plates (or stratums) with integral locking mechanisms that do not require an additional locking element, i.e., something in addition to the stratum, to prevent inadvertent backing out of a fastener.
All adjustments and alternatives described above are intended to be included within the scope of the invention, as defined exclusively in the following claims. Those skilled in the art also should realize that such modifications and equivalent constructions or methods do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. Furthermore, as used herein, the terms components and modules may be interchanged. It is understood that all spatial references, such as “superior,” “inferior,” “anterior,” “posterior,” “outer,” “inner,” and “perimeter” are for illustrative purposes only and can be varied within the scope of the disclosure.

Claims

1. A stratum to be affixed to bone, the stratum comprising:

a first surface, a second surface, and at least one hole extending between the first surface and the second surface, wherein the second surface is configured to engage at least a portion of the bone, and wherein the stratum further has a slot in the stratum that extends from the at least one hole to another location on the stratum, and the stratum is further configured to deflect, allowing a fastener to pass at least partially through the hole.

2. A system for affixing the stratum of claim 1 to the bone, wherein the fastener comprises:

a length and a width, the length being greater than the width, and a central longitudinal axis;

a head portion;

an intermediate portion; and

a distal portion, wherein the distal portion has a proximal end that is proximate the intermediate portion, and the proximal end has a width substantially perpendicular to the central longitudinal axis so that the proximal end causes the stratum to deflect as the proximal end of the fastener moves across the at least one hole in the stratum in the direction towards the second surface of the stratum.

3. The system of claim 2, wherein the distal portion of the fastener has a lip that allows passage of the proximal end through the at least one hole in the stratum, and prevents inadvertent backing out of the fastener.

4. The stratum of claim 1, wherein the fastener is configured to pass at least partially through the at least one hole and engage at least a portion of the bone.

5. The stratum of claim 4, wherein the fastener is a screw that is substantially circular in cross section, and the width is a diameter.

6. The stratum of claim 1, wherein the bone is spine, and wherein the stratum is a spinal plate.

7. The stratum of claim 1, wherein the stratum comprises polyetheretherketone.

8. The stratum of claim 1, wherein the stratum consists essentially of polyetheretherketone.

9. The system of claim 3, wherein the width of the proximate end of the distal portion is a first width and the intermediate portion has a second width, wherein the second width is smaller than the first width.

10. A system for affixing a stratum to bone, the system comprising:

a stratum having a first surface, a second surface, and at least one hole extending between the first surface and the second surface, wherein the second surface is configured to engage at least a portion of the bone and wherein the stratum further has a slot in the stratum that extends from the at least one hole to another location on the stratum; and

a fastener configured to pass at least partially through the at least one hole and engage at least a portion of the bone, wherein the fastener has a length and a width, the length being greater than the width, and a central longitudinal axis, the fastener further comprising:

a head portion;

an intermediate portion; and

a distal portion, wherein the distal portion has a proximal end that is proximate the intermediate portion, and the proximal end has a width substantially perpendicular to the central longitudinal axis, wherein the proximal end causes the stratum to deflect as the proximal end of the fastener moves across the at least one hole in the stratum in the direction towards the second surface of the stratum.

11. The system of claim 10, wherein the distal portion of the fastener has a lip that allows passage of the proximal end through the at least one hole in the stratum, and prevents inadvertent backing out of the fastener.

12. The system of claim 10, wherein the fastener is a screw.

13. The system of claim 12, wherein the screw is substantially circular in cross section, and the width is a diameter.

14. The system of claim 10, wherein the bone is spine, and wherein the stratum is a spinal plate.

15. The system of claim 10, wherein the stratum comprises polyetheretherketone.

16. The system of claim 10, wherein the stratum consists essentially of polyetheretherketone.

17. The system of claim 12, wherein the width of the proximate end of the distal portion is a first width and the intermediate portion has a second width, wherein the second width is smaller than the first width.

18. A system for affixing a stratum to bone, the system comprising:

a stratum having a first surface, a second surface, and at least a first hole and a second hole, each hole extending between the first surface and the second surface, wherein the second surface is configured to engage at least a portion of the bone and wherein the stratum further has a slot in the stratum that extends from the first hole to another location on the stratum;

a first fastener configured to pass at least partially through the first hole and engage at least a portion of the bone, wherein the stratum is further configured to deflect, allowing the first fastener to pass at least partially through the hole;

a second fastener configured to pass at least partially through the second hole and engage at least a portion of the bone, wherein the stratum is further configured to deflect, allowing the second fastener to pass at least partially through the hole.

19. The system of claim 18, wherein the slot extends from the first hole to the second hole.

20. The system of claim 18, wherein the slot is a first slot, and the stratum further has a second slot in the stratum that extends from the second hole to another location on the stratum.