Bone anchors for use in attaching soft tissue to a bone

[0001] 1. The Field of the Invention

[0002] The present invention relates to soft tissue repair surgery, such as rotator cuff repair surgery. More specifically, the present invention relates to a bone anchor used to attach soft tissue to bone using a suture.

[0003] 2. Related Technology

[0004] Soft tissue injuries, especially rotator cuff injuries, can occur from repeated stress or acute trauma. The rotator cuff is a group of muscles and tendons in the shoulder that attach to the humerus bone of the arm. The rotator cuff allows a person to rotate the arm and raise it above the head. A common injury to the rotator cuff occurs when repeated stress or acute trauma causes the rotator cuff to partially or complete tear away from the humerus bone. These and similar types of injuries may require surgery to correctly reattach the soft tissue to the bone.

[0005] Various devices have been used to reattach soft tissue to bone. Known methods include staples, wedges, plugs, screws, and sutures alone. Threaded suture anchors, such as bone screws, have recently been developed to provide a particularly firm location where a suture can be anchored to bone. In these systems, a suture is tied between the bone anchor and soft tissue. Providing a firm attachment point for the suture is important because of the relatively strong forces that are experienced in a flexing muscle.

[0006] Despite recent advances in bone anchors, existing bone anchors and rotator cuff repairs can fail and have other disadvantages. Typically, a rotator cuff repair fails either because the bone anchor dislodges or the suture tears through the soft tissue. As force is applied to the suture, the suture can cut through the soft tissue like a cheese wire, leaving the rotator cuff detached from the humerus bone. When one suture fails, it can place more stress on the surrounding sutures, thus increasing the likelihood that other sutures will fail in like manner.

[0007] Using a greater number of sutures per unit area of soft tissue can minimize suture attachment failure. However, the number of sutures is limited by the space available for inserting bone anchors. Alternatively, additional sutures can be connected to a single bone anchor. Double and triple loading of bone anchors, however, increases the forces applied to the bone anchor and increases the likelihood that the bone anchor will fail by being pulled out of the bone into which it is secured.

[0008] Bone anchors can fail for various reasons. One reason is that existing bone anchors are not threaded to the proximal end of the anchor where the anchor meets the surface of the bone in the hard cortical bone region. In existing bone screws, the proximal end is not threaded because the driver tool used to insert the bone anchor fits over a hex shaped protrusion. The hex protrusion cannot extend above the bone surface so the screw is driven into the bone until the protrusion is below the surface. Since the protrusion has no threads, the bone anchor does not engage the bone near the surface, but only the soft cancellous bone beneath the cortical bone layer. This feature of existing bone anchors is very problematic because it prevents a practitioner from placing the threads of the bone anchor in the harder cortical bone, which is near the bone surface.

[0009] Some existing anchors engage the sutures above the anchor threads. Such screws have an eyelet formed in the protrusion used to drive the screw. Sutures are looped in the eyelet and fed through the driver tool. Because the attachment point for the sutures is above the threads, the threaded portion of the bone anchor experiences a high torque moment when a force is applied by the suture to the attachment point. Consequently, existing bone anchors are prone to experience a certain degree of movement within the soft bone layer. In some cases, the bone anchor can retreat from the bone and rub against soft tissues overlaying the bone anchor. This may result in the irritation or damage of such tissues.

[0010] Recently, bone screws have been used that place the attachment point for the sutures within the body of the screw, specifically within the bore of the screw. In these screws an attachment site within the bore is created using a small piece of suture. The ends of the suture are fed through holes near the proximal end of the screw to form a small suture loop within the bore. Knots are tied in the end of the suture to prevent the ends from passing back though the holes. The suture loop within the bore provides an anchoring point for threading sutures used to secure tissue in a surgical procedure.

[0011] One problem with these screws is that the knots that hold the suture loop can come untied and/or break, which releases the sutures anchored to the suture loop. This problem is particularly difficult to fix once the screw has been placed in a patient because it would require the surgeon to remove the screw from the patient. An additional problem with using suture loops as an attachment mechanism is that the suture material can fray as the anchored sutures slide on it. Furthermore, when loading multiple sutures on the suture loop, the suture loop can flex and cause the sutures to bunch together. When a practitioner is using a suture the practitioner needs the suture to freely slide in the bone anchor. The friction and/or pinching that a suture loop can cause between lengths of suture can cause suture capture, which is undesirable.

[0012] Therefore, what is needed is a bone anchor that better engages its surrounding bone tissue and allows greater forces to be applied to the bone anchor without becoming dislodged or failing. In addition, a bone anchor is needed that can reduce suture capture.

[0013] The bone anchors of the present invention overcome the disadvantages of the prior art discussed above by providing improved suture attachment sites and improved threads for engaging bone tissue. In an exemplary embodiment, the bone anchor of the present invention has an anchor body extending between a proximal end and a distal end. A continuous thread extends around the anchor body and is configured to engage both cortical and cancellous bone tissue. The anchor body includes an interior surface that defines a bore that opens at the proximal end of the anchor body. One or more transverse pins are placed in and across the bore interior to provide one or more locations within the interior of the anchor body for looping one or more sutures.

[0014] According to one embodiment, the pins are placed deep within the bore of the anchor body. This feature allows room for a socket to be formed in the proximal end of the anchor body where a driver tool can be inserted for driving the bone anchor into a bone. Because the driver tool is placed on the interior of the bone anchor, the anchor body can be threaded to the proximal end. Threading the proximal end of the anchor body provides the bone anchor with the ability to better engage the cortical bone near the surface of the bone.

[0015] The placement of the pins also provides a more central location for anchoring sutures. The more centrally anchored sutures exert a force on the anchor body that is more evenly distributed and more aligned with the axis of the bone anchor. Consequently, the bone anchor can better distribute the exerted force to the surrounding bone tissue without causing the bone tissue to weaken so as to cause loosening or withdrawal of the bone anchor.

[0016] The pins disposed within the bore of the anchor body also provide a better attachment mechanism for attaching a suture than exists in the prior art. The pins are securely fixed to the anchor body. Unlike the bone anchors in the prior art, the bone anchors of the present invention do not rely on knots to secure sutures within the bore. In addition, the pins are made of a material that does not fray and is less likely to break.

[0017] Double and triple loading of the bone anchor with more than one suture is more easily accomplished with a pin because the pins provide individual attachment locations for each additional suture. In one exemplary embodiment, two pins are disposed in the bore at right angles to each other. Two sutures are loaded on the lower pin with the upper pin separating the two sutures. In another embodiment, the upper pin is also loaded with a suture. This latter configuration properly spaces three sutures and minimizes friction and suture capture among the three sutures.

[0018] In an exemplary embodiment, the anchor body has a non-threaded portion at the distal end that forms a stabilizing extension. The extension provides additional stability to the bone anchor by reducing the tendency of the anchor body to move or rotate laterally. Bone anchors are often placed in a bone at an angle. Much like a longer tent stake is less likely to give out, the extension of the bone anchor of the present invention helps prevent the bone anchor from becoming dislodged. Stabilizing the bone anchor reduces the likelihood that the bone anchor will fail and allows the bone anchor to be safely loaded with more sutures.

[0019] In yet another embodiment, the anchor body is a screw that includes a portion of fine threads and a portion of coarse threads. The fine threads are configured to engage hard bone tissue, such as cortical bone, and the coarse threads are configured to engage soft bone tissue, such as cancellous bone. A portion of the threads are made finer by increasing the root diameter and increasing the surface angle of the thread. The pitch of the coarse threads and fine threads is kept the same such that the proximal threads can follow the impression created by the distal threads as the bone anchor is driven into a bone.

[0020] Optimizing the thread pattern for engaging different types of bone tissue allows the bone anchor to better engage adjacent bone tissue. Because the bone anchor can better engage adjacent bone tissue, the bone anchor can be loaded with additional sutures without compromising stability of the bone anchor. The additional sutures per anchor reduce the stress placed on each individual suture through the soft tissue, which helps prevent the sutures from cutting through the soft tissue.

[0021] These and other objects and features of the present invention will become more fully apparent from the following description and appended claims.

[0031] With reference to FIG. 1, exemplary embodiments of the present invention are directed to an improved bone anchor 10 for affixing soft tissue to bone, such as in a rotator cuff repair surgery. Bone anchor 10 has an anchor body 12, which extends between a proximal end 14 and a distal end 16. Distal end 16 of anchor body 12 has a non-threaded portion that forms a stabilizing extension 18. Stabilizing extension 18 helps prevent lateral movement of anchor body 12 within bone tissue during use. Anchor body 12 further comprises a threaded portion, which includes threads 20 for engaging bone tissue.

[0032] Proximal end 14 includes an opening 30, which provides access to a hollow interior bone 30 of anchor body 12. A hex socket 22 is formed in bore 30 of anchor body 12, which allows bone anchor 10 to be driven into a bone using a hex driver. It will be appreciated that bore 30 of anchor body 12 can have nay other desired shape, such as triangular, square, pentagonal, star-shaped, oval, etc. Transverse pins 23a and 23b are disposed through anchor body 12 and provide attachment points for looping sutures thereon.

[0033] FIG. 2 illustrates a cross-sectional view of bone anchor 10. In an exemplary embodiment, anchor body 12 has a length of about 8 to about 15 mm a major diameter 26 of about 5 mm, and a root diameter 24 of about 3.5 mm. Anchor body 12 can have sizes other than these; however, the size of anchor body 12 is limited by the size of the bone where the bone anchor 10 is to be placed. For example, in rotator cuff repair surgery, increasing the diameter of anchor body 12 can reduce the number of bone anchors 10 that can be positioned at the repair site.

[0034] Bone anchor 10 has threads 20 that wrap continuously around anchor body 12 in a desired (e.g., clockwise) direction. The pattern of threads 20 determines in part how bone anchor 10 engages surrounding bone tissue. In one embodiment, threads 20 make about six turns around anchor body 12 and extend toward proximal end 16.

[0035] As shown in FIG. 2, major diameter 26 and root diameter 24 may taper slightly inward toward distal end 16. The slight taper causes the threads 20 to engage bone tissue more tightly as the bone anchor 10 is driven further into a bone. Of course, bone anchor 10 can have threads that taper more or less depending on a practitioner's preference and/or the needs of a patient.

[0036] Anchor body 12 also has a non-threaded portion at distal end 16, which forms stabilizing extension 18. Stabilizing extension 18 is generally cylindrical and typically has a width less than root diameter 26. In one embodiment, the length of extension 18 is more than about 20% of the length of the threaded portion of anchor body 10. In another embodiment, extension 18 is more than 100% the length of the threaded portion and in yet another embodiment, more than about 200% of the length of the threaded portion.

[0037] In one embodiment, extension 18 is configured to be inserted into a pilot hole drilled into the bone where bone anchor 10 is to be placed. Preparing a pilot hole reduces the risk that insertion of the screw in a bone will cause damage to bone anchor and/or fracture bone tissue as the bone anchor 10 is inserted into the bone. Preventing damage to bone anchor 10 and surrounding bone tissue reduces the chances that bone anchor 10 will become loosened or fail during use. In another embodiment, the bone anchor can have a self drilling distal end.

[0038] Anchor body 12 is advantageously made from a strong biocompatible material, such as a titanium alloy or stainless steel. Alternatively, anchor body 12 can be made from a biodegradable material, such as poly-l-lactic acid (PLLA) that can be absorbed into adjacent bone tissue over time as the repair site heals. Other biocompatible and/or biodegradable materials suitable for use in bone anchors are known to those skilled in the art.

[0039] Bore 30 opens at proximal end 14 to provide access to the interior of anchor body 12. In an exemplary embodiment, a proximal portion of bore 30 is hexagonally shaped to form a hex socket 22 for receiving a hex driver. This female type driver tool engagement mechanism eliminates the need to have a protruding proximal end 16. Instead, proximal end 16 is substantially flat and can be placed at or just below a bone surface, as discussed more fully below.

[0040] As shown in FIGS. 2 and 3, hex socket 22 can be wider than the distal portion of bore 30. For example, a ridge 34 can be formed at the distal end of hex socket 22. Ridge 34 acts as a stop to limit how deep a hex driver can be inserted therein (e.g., to protect post 23b and/or a suture attached thereto). Alternatively, the insertion depth of a hex driver can be controlled by placing a ridge of material on the hex driver. In this case, insertion of the driver would be stopped when the ridge on the driver engages the proximal end 16 of the anchor body 12.

[0041] The distal portion of bore 30 also contains one or more transverse pins for looping sutures thereon. Pins 23a and 23b are formed or inserted in anchor body 12 lying across bore 30. The diameter of pins 23a and 23b are selected such that there is sufficient space between pin 23a and pin 23b and interior surface 28 for passing a suture around the pins 23a and 23b. In addition to providing a location for looping sutures, pins 23a and 23b provide additional structural support (i.e., an endoskeleton) to anchor body 12.

[0042] Pins 23a and 23b are disposed in bore 30 substantially non-parallel to each other. As illustrated in FIG. 3, in one embodiment, pins 23a and 23b are at right angles to each other. Offsetting pins 23a and 23b exposes portions of pin 23a that would otherwise be occluded from the top by pin 23b. Such a configuration allows a suture to be loaded on pin 23a on either side of pin 24b, as shown in FIG. 4.

[0043] Bone anchor 10, in FIG. 4, is illustrated loaded with three sutures 36a, 36b, and 36c (collectively sutures 36). Pin 23a is shown loaded with sutures 36a and 36b extending on either side of pin 23b. Pin 23b has a single suture 36c loaded thereon. Each suture has its own space to slide on its respective pin 23a or 23b. The proper spacing of sutures 36 in bone anchor 10 prevents sutures 36 from rubbing extensively or getting caught on one another.

[0044] Another advantageous feature of pins 23a and 23b is their position distal to hex socket 22 and within bore 30. Pins 23a and 23b are placed within bore 30 such that the forces applied by sutures 36 are transferred to a more central location within anchor body 12. Forces applied to bone anchor 10 below the surface of a bone are less likely to cause bone anchor 10 to become loosened or dislodged. Pins 23a and 23b are placed below hex socket 22 so a hex driver can be inserted without hitting the pins. This placement also allows sutures to be threaded through a hole in a driver tool (not shown) so that bone anchor 10 can be installed pre-loaded with sutures.

[0045] While bone anchor 10 has been illustrated with two pins (i.e., pins 23a and 23b), bone anchor 10 may have more or fewer pins depending on the required number of sutures and/or the space available within bore 30 for placing more sutures. For instance, in another embodiment, the anchor body 10 may have a single post with one or more sutures loaded thereon. Furthermore, a second pin can be disposed in bore 30 even where sutures are to be placed only on one pin, with the second pin being used solely to separate sutures. In addition, a second pin can be placed in anchor body 12 for structural support only.

[0046] Pins 23a and 23b are made from a strong metal or synthetic fiber so as to provide a ridged attachment point for sutures 36. In an exemplary embodiment, pins 23a and 23b are cylindrical to provide a smooth surface for sutures 36 to slide against. While pins 23a and 23b are illustrated as straight, pins 23a and 23b can be bent or have shapes other than cylindrical. Straight pins, however, can be more easily placed in anchor body 12 and therefore can reduce the cost and complexity of manufacturing bone anchor 10. The foregoing and similar attachment devices are examples of rigid attachment means for attaching a suture to an anchor.

[0047] In one embodiment, one or both of pins 23a and 23b are made from a radioopaque material such as titanium or stainless steel. A radioopaque pin can be used with a radiotransparent anchor body, such as an anchor body made from a biodegradable material such as PLLA. This configuration of materials allows a practitioner to identify and locate bone anchor 10 in a radiograph when bone anchor 10 is made mostly of biodegradable materials.

[0048] The bone anchor according to the present invention need not be formed as a threaded device, but can also be formed as a tap-in type anchor. Also, the measurements, angles and ratios between the dimensions of the bone anchor can be varied from those described above and in the following alternative embodiment so as to be suitable for the conditions and applications in which the bone anchor is to be used.

[0049] FIG. 5A shows an alternative embodiment of the bone anchor of the present invention having a section of finer threads 38 at the proximal end 16 and a section of coarser threads 40 distal thereto. Fine threads 38 and coarse threads 40 have the same major diameter 42 which has a slight taper illustrated by lines 44a and 44b. Fine thread 38 and coarse threads 40, have different root diameters 46 and 48 respectively. Root diameters 46 and 48 have a slight taper similar to major diameter the taper shown by lines 44a and 44b.

[0050] Fine threads 38 are finer because they have a wider root diameter 46. Root diameter 46 of fine thread 38 is wider than root diameter 48 of coarse threads even after subtracting out the increase in width due to the overall taper of anchor body 12 as illustrated by lines 44a and 44b. Fine threads 38 have root diameter 46 and a major diameter 42 that are configured to engage harder bone. Coarse threads 40 have a root diameter 48 and major diameter 42 that are configured to engage soft bone. In an exemplary embodiment, major diameter 42 is about 5.3 mm, root diameter 46 is about 4.8 mm and root diameter 48 is about 3.3 mm. Fine threads 38 can have a similar shape as coarse threads 40 or a different shape as desired. For example, fine threads 38 can have a larger or smaller thread angle.

[0051] In one embodiment, fine threads 38 have the same pitch as coarse threads 40. By keeping the pitch the same between thread sections, the finer threads 38 will be able to use the same impression cut by coarse threads 40.

[0052] FIG. 5B shows yet another alternative embodiment of the present invention where fine threads 38 form a double flute. A first flute 39a follows the thread pattern of coarse threads 40 such that first flute 39a follows the grooves created by coarse threads 40 as the bone anchor 10 is driven into a bone. In an exemplary embodiment, second flute 39b has the same pitch as first flute 39a. Second flute 39b can have a similar shape as flute 39a or a different shape as desired. For example, threads 39b can have a larger or smaller thread angle and/or major diameter.

[0053] FIG. 6 shows bone anchor 10 disposed in a typical bone 50 having a cortical bone region 52 and a cancellous bone region 54. Cortical bone region 52 comprises dense bone, while cancellous bone region 54 comprises bone that is soft or spongy. When bone anchor 10 is properly inserted into bone 50, fine threads 38 engage the hard cortical bone region 52 and coarse threads 40 engage the softer cancellous bone region 54.

[0054] In manufacturing bone anchor 10, in accordance with the present invention, anchor body 12 and posts 23 can be cast and formed in a die. Alternatively anchor body 12 can be cast or formed and posts 23a and 23b inserted later. For instance, anchor body 12 can be cast and formed from PLLA. Anchor body 12 can then be drilled to prepare holes for stainless steel pins 23a and 23b.

[0055] The suture anchors according to the present invent can be distributed to practitioners with one or more of sutures 36 threaded through bore 30 and looped to pins 23a and/or 23b. In one method of manufacturing bone anchor 10, sutures 36 are looped on pins 23a and 23b as pins 23a and 23b are inserted into anchor body 12.

[0056] An example of a type of suture suitable for use in conjunction with the bone anchor of the present invention is #2 braided polyester. If more than one strand of sutures 36 is used, the sutures can be a different color such that a practitioner can more easily pair the ends of the sutures during a surgical operation.

[0057] FIG. 7 illustrates the use of bone anchor 10 in a rotator cuff repair surgery. Bone anchor 10 is placed in humerus bone 54, and sutures 36 are passed through rotator cuff 56 and tied. Before bone anchor 10 is inserted in humerus bone 54 a pilot hole may be drilled. Bone anchor 10 is inserted into the pilot hole using a driver tool until proximal end 14 is substantially flush with the outer surface of humerus bone 54. Bone anchor 10 is advantageously placed in humerus bone 54 at an angle to the tangent of the humerus bone, also known as the “dead man's angle.”

[0058] Because bone anchor 10 is placed in humerus bone 54 at an angle, extension 18 provides a mechanical advantage against bone anchor 10 moving laterally and opening the angle to the tangent. By preventing lateral movement, extension 18 prevents sutures 36 from loosening once sutures 36 have been properly tied. In addition, if bone anchor 10 were to move within bone 54, bone anchor 10 can become dislodged and fail. Extension 18 does not have threads thereon, which reduce friction as bone anchor 10 is driven into a pilot hole.

[0059] Proximal end 14 of bone anchor 10 is substantially flat or non-protruding such that bone anchor 10 can be placed at or just below the surface of bone 54. Threads extend to proximal end 14 such that bone anchor 10 has maximum engagement with bone 54. The opening at proximal end 14 also allows for sutures 36 to exit the bore. The opening of the bore is smooth such that sutures 36 can easily slide thereon.

[0060] Sutures 36 exit bone anchor 10 at proximal end 14 and are drawn through the soft tissue of rotator cuff 56. Sutures 36 can be spaced to more evenly distribute the load exerted by rotator cuff 56. As shown in FIGS. 5 and 7, sutures 36 exert a force on bone anchor 10 on the pins in the bone and at proximal end 14. Because proximal end 14 is at or below the surface of bone 54, less torsion is applied to bone anchor 10. Instead, the force of rotator cuff 56 is distributed vertically along the anchor body through the pins. Consequently, bone anchor 10 is less likely to be dislodged and fail.

[0061] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.