WO2018051228A1

WO2018051228A1 - Limited variable diameter bone screw

Info

Publication number: WO2018051228A1
Application number: PCT/IB2017/055494
Authority: WO
Inventors: Mark Levy; Eran ISHAY; Ohn LEVY; Jaffar Hleihil
Original assignee: Expanding Orthopedics Inc
Current assignee: Expanding Orthopedics Inc
Priority date: 2016-09-14
Filing date: 2017-09-12
Publication date: 2018-03-22
Anticipated expiration: 2019-03-14

Abstract

A bone fastener (10) includes a shank (12) that has a proximal spherical head (14) and is formed with a longitudinal bore (20). The shank (12) includes three threaded portions, a distal portion (22), a middle portion (24) and a proximal portion (26). The middle portion (24) and proximal portion (26) each include two or more expandable struts (28). The longitudinal bore (20) in the proximal portion (26) is larger in diameter than in the distal portion (22) and the longitudinal bore (20) in the spherical head (14) is larger in diameter than in the proximal portion (26) and terminates distally in the spherical head (14) as a shoulder (32). A wedge element (34) is movably disposed in the longitudinal bore (20). The wedge element (34) includes a distal shank (36) and a proximal head (38), the proximal head (38) being arranged to abut against the shoulder (32).

Description

LIMITED VARIABLE DIAMETER BONE SCREW

FIELD OF THE INVENTION

The present invention generally relates to bone fasteners, such as pedicle screws, and particular to an expanding bone screw with superior pullout strength.

BACKGROUND OF THE INVENTION

Pedicle screws have been the gold standard for internal fixation for the vast majority of surgical procedures in the thoracic and lumbar portions of the spine for more than two decades, and are generally considered a safe and effective treatment modality.

Implantation of short-term and long-term stable constructs for spine fixation is critical, especially when fusing more than one level. Typically, bilateral pedicle screws are implanted in each vertebra since the pedicle isthmus is the strongest point of attachment to the bone, in addition to crossing through all three vertebral columns. A strong bone-screw interface is of cardinal importance for screw-loosening prevention, particularly when dynamic loading is increased due to various clinical factors or when shorter fusion segments are selected. The typical indications for pedicle screw systems include fracture fixation, treatment of spondylolisthesis, fixation after tumor resection, revision cases, correction of scoliosis and other congenital, idiopathic, developmental and/or degenerative deformations, and stabilization after spinal stenosis decompression and osteotomies.

It is generally accepted that the outer diameter of the screw has the largest influence on the pullout strength and resistance to loosening under cyclic loading. Other factors that affect fixation strength to a lesser degree include screw length, inner diameter, pitch, core shape, use of single or double threads, coating and/or surface finish.

Screw fixation also depends on screw placement. Optimal fixation is achieved by use of convergent screws in a parallel transversal orientation, using the largest and longest screw possible without damaging the dorsal cortex or pedicle breach. The surgical instruments used to prepare the pilot hole should be less than or equal to the inner screw diameter in order to preserve their maximum pullout force.

There is a need for a screw design that balances the desire to maximize pullout strength while minimizing screw diameter at insertion.

SUMMARY OF THE INVENTION

The present invention seeks to provide an expanding bone screw with superior pullout strength, as is described further in detail below. The screw diameter is minimal at insertion to facilitate screw placement, using the smallest pilot hole possible, and the screw diameter is then expanded in place after insertion. The diameter is expanded at the proximal end of the screw, within a limited range until a maximal diameter, where it will have the greatest effect as it is performed in the region of the bone with the highest density, i.e. the pedicle. This is in contrast with prior art screws that expand at the distal end, i.e., far from the screw head, which have had inferior pullout strength.

The screw can be made of any medical compatible material for bone utilization, such as but not limited to, titanium, titanium-aluminum-vanadium alloy, CoCr alloy, PEEK (polyetheretherketone), carbon fibers reinforced polymer, polymers, or any other metal, natural or synthetic material including resorbable materials. The surfaces of such devices may be regular or irregular, including surfaces that may enhace attachment to bone.

There is thus provided in accordance with a non-limiting embodiment of the present invention a bone screw including a shank having a proximal spherical head and formed with a longitudinal bore that extends an entire length of the shank, the shank including three threaded portions, a distal portion, a middle portion and a proximal portion, the distal portion extending at least a third of the entire length of the shank, the middle portion and the proximal portion each including two or more expandable struts, and the struts of the middle portion being joined to the struts of the proximal portion by a bending region, wherein the longitudinal bore in the proximal portion is larger in diameter than in the distal portion and the longitudinal bore in the spherical head is larger in diameter than in the proximal portion and terminates distally in the spherical head as a shoulder, and a wedge element movably disposed in the longitudinal bore in the proximal portion and in the middle portion, the wedge element including a distal shank and a proximal head, the proximal head being arranged to abut against the shoulder to expand the struts outwards, wherein abutment of the proximal head against the shoulder limits expansion of the struts. This structure not only limits the expansion, but also ensures that the bone screw cannot be expanded beyond a certain diameter, whichacts as a safety feature.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawing in which: Figs. 1A and IB are simplified illustrations of a bone screw, constructed and operative in accordance with a non-limiting embodiment of the present invention, respectively in contracted and expanded orientations.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is now made to Figs. 1A-1B, which illustrate a bone screw 10, in accordance with a non-limiting embodiment of the present invention.

The bone screw 10 may be used in any bone including the vertebra, such as but not limited to, a pedicle screw. In the illustrated embodiment, screw 10 has a threaded shank 12 with a proximal spherical head 14 on which is mounted a polyaxial head 16. Polyaxial head 16 may have a threaded opening 18. Shank 12 may be hollow (cannulated), that is, formed with a longitudinal bore 20 (e.g., central bore) that extends the entire length of the shank 12 of screw 10.

Shank 12 of screw 10 has three threaded portions, a distal portion 22, a middle portion 24 and a proximal portion 26. The distal portion 22 preferably does not expand, but other embodiments are contemplated in which the distal portion 22 is allowed to expand. Distal portion 22 preferably extends at least a third of the entire length of shank 12 (the length of shank 12 being defined as the distance from the distal tip of the screw to the proximal face of spherical head 14). The middle portion 24 and proximal portion 26 may each include two or more expandable struts 28, such as but not limited to, three struts 28 that are equally spaced (120° apart) circumferentially about the perimeter of shank 12. (Two struts would be spaced 180° apart; four struts would be spaced 90° apart, and so forth.) The struts 28 of middle portion 24 may be joined to the struts 28 of proximal portion 26 by a bending region 30, such as a live hinge or pivot element. The bending region 30 may mean the struts bend about a bending line (in which case, the struts are basically straight on either side of the bending line) or the bending region may be a curved region (in which case, the struts curve or bow outwards and are not necessarily straight after bending).

The longitudinal bore 20 in proximal portion 26 is larger in diameter than in distal portion 22 and middle portion 24. The distal end of longitudinal bore 20 in proximal portion 26 may be conical. The longitudinal bore 20 in spherical head 14 is larger in diameter than in proximal portion 26 and terminates distally in spherical head 14 as a shoulder 32.

A wedge element 34 is initially disposed in longitudinal bore 20 in proximal portion 26 and middle portion 24. The wedge element 34 includes a distal shank 36 (with a chamfered distal end) and a proximal head 38. Like screw 10, wedge element 34 may be cannulated. Wedge element 34 may slide in bore 20, or alternatively, the longitudinal bore 20 in spherical head 14 may be tapped (internally threaded) and head 38 may be threaded externally so that turning head 38 (such as with a screwdriver) either advances or retracts wedge element 34 in longitudinal bore 20 (that is, longitudinal bore 20 threadingly mates with a portion of wedge element 34). Optionally, as seen in Fig. IB, wedge element 34 may move along a guide element 40, such as a rod 40 positioned in portions of middle portion 24 and proximal portion 26.

Fig. 1A shows screw 10 in its retracted state, before distal advancement of wedge element 34. Fig. IB shows screw 10 in its expanded state, after distal advancement of wedge element 34.

Screw 10 has limited expansion, because wedge element 34 can advance distally only until its head 38 abuts against shoulder 32. Screw 10 can return from the expanded state to the retracted state by simply moving wedge element 34 proximally backwards (the struts 28 having undergone elastic deformation, which is fully reversible). This will allow the easy removal of the screw if needed. This limited expansion ensures that the bone screw cannot be expanded beyond a certain diameter, thus providing a safety feature. In another embodiment, the struts may have a certain degree of plastic deformation, producing some hysteresis so the reversibility may not be complete but small enough to allow the removal of the screws after moving the wedge element .

Without limitation, screw 10 may expand by a maximum of 1 mm in diameter (0.5mm in radius). For example, a 5.5 mm diameter screw is capable of increasing its diameter up to 6.5 mm (and 6.5 mm up to 7.5 mm). The slits or gaps separating struts 28 from each other may be, without limitation, 0.4 mm in width or less and about 20 mm in length. Upon maximum expansion, the gap width may increase without limitation from 0.4 to 1.4 mm, while the length may remain substantially unchanged.

Although in this non-limiting example the maximum expansion is 1 mm, however, the user may choose to stop turning the wedge element 34 at any point to arrive at any desired partial expansion.

Claims

CLAIMS What is claimed is:

1. A bone fastener (10) comprising:

a shank (12) having a proximal spherical head (14) and formed with a longitudinal bore (20) that extends an entire length of said shank (12);

said shank (12) comprising three threaded portions, a distal portion (22), a middle portion (24) and a proximal portion (26), said distal portion (22) extending at least a third of the entire length of said shank (12), said middle portion (24) and said proximal portion (26) each comprising two or more expandable struts (28), and said struts (28) of said middle portion (24) being joined to said struts (28) of said proximal portion (26) by a bending region (30);

wherein said longitudinal bore (20) in said proximal portion (26) is larger in diameter than in said distal portion (22) and said longitudinal bore (20) in said spherical head (14) is larger in diameter than in said proximal portion (26) and terminates distally in said spherical head (14) as a shoulder (32); and

a wedge element (34) movably disposed in said longitudinal bore (20) in said proximal portion (26) and in said middle portion (24), said wedge element (34) comprising a distal shank (36) and a proximal head (38), said proximal head (38) being arranged to abut against said shoulder (32) to expand said struts (28) outwards, wherein abutment of said proximal head (38) against said shoulder (32) limits expansion of said struts (28).

2. The bone fastener (10) according to claim 1, wherein said wedge element (34) is cannulated.

3. The bone fastener (10) according to claim 1, wherein said distal portion (22) does not expand.

4. The bone fastener (10) according to claim 1, wherein said longitudinal bore (20) in said spherical head (14) is internally threaded and threadingly mates with a portion of said wedge element (34).

5. The bone fastener (10) according to claim 1, wherein said wedge element (34) is arranged to move along a guide element (40).

6. The bone fastener (10) according to claim 1, wherein a distal end of said longitudinal bore (20) in said proximal portion (26) is conical.

7. The bone fastener (10) according to claim 1, wherein a polyaxial head (16) is mounted on said spherical head (14).