Patent Description:
The present invention generally relates to spinal fixation devices, and more particularly to spinal fasteners having serrated threads.

A technique commonly referred to as spinal fixation is employed for fusing together and/or mechanically immobilizing vertebrae of the spine. Spinal fixation may also be used to alter the alignment of adjacent vertebrae relative to one another so as to change the overall alignment of the spine. Such techniques have been used effectively to treat many degenerative conditions and, in most cases, to relieve pain suffered by the patient.

In some applications, a surgeon will install pedicle screws into the pedicles of adjacent vertebrae (along one or multiple levels of the spine) and thereafter connect the screws with a spinal rod in order to provide immobilization and stabilization of the vertebral column. Whether conducted in conjunction with interbody fusion or across single or multiple levels of the spine, the use of pedicle screws connected by fixation rods is an important treatment method employed by spinal surgeons. Examples of fixation devices are disclosed in <CIT>, <CIT> and <CIT>.

Some surgeons insert pedicle screws via powered screw insertion, while other surgeons prefer manual screw insertion. For the surgeons that opt for manual screw insertion, surgeon fatigue and bone fracturing can be significant problems during surgery. Surgeon fatigue can adversely affect the accuracy of the insertion process and the depth to which the screws are inserted within the pedicle bone.

There remains room for improvement in the design and use of pedicle screws, particularly in the case of manual insertion so that related surgical procedures can be performed with greater efficiency and consistency.

According to the present invention, there is provided a fastener configured for spinal applications as defined in claim <NUM> and in the corresponding depending claims. There is also provided a corresponding kit as defined in the appended claims.

A first aspect of the present disclosure is a fastener having a head including a channel adapted to receive a spinal rod and a shaft extending from the head to a distal tip and including a thread, at least a portion of the thread being serrated.

In other examples according to the first aspect, the shaft has a longitudinal axis and an angle between the longitudinal axis and thread may vary along a length of the shaft. The serrated portion of the thread may include serrations having a width that increases along a portion of a length of the thread toward the distal tip. The shaft may be cannulated. The head may be polyaxially movable with respect to the shaft. The shaft may be tapered. Further, the taper of the shaft measured by a line over a surface of the thread at points on two or more revolutions of the thread may be between <NUM> and <NUM> degrees relative to the longitudinal axis of the shaft. The serrated portion of the thread may include serrations having a width that decreases over a part of the serrated portion toward the distal tip. The thread may include walls disposed between the shaft and a surface of the serrations, the walls angled so that walls adjacent to one another along the longitudinal axis are at a <NUM> to <NUM> degree angle with respect to each other. The head may be monoaxially attached to the shaft.

A second aspect of the disclosure is a fastener having a head including a channel adapted to receive a spinal rod, a shaft coupled with the head, the shaft including a distal tip, a thread extending between the head and the distal tip, and a serration extending along at least a portion of the thread, the serration including peaks and troughs.

In other examples according to the second aspect, the peaks are disposed at a radial distance to the longitudinal axis of the shaft greater than a radial distance to the longitudinal axis of the shaft from the troughs adjacent to the peak, the teeth may have a width measured parallel to the troughs such that the width may be greater at the troughs than at the peaks. The peaks of the teeth may include a first type defined by an edge at an abutment between surfaces connecting the peak with adjacent troughs and a second type defined by a planar surface. The serration may include a progressively increasing pitch from the tip toward the head. The first peak may vary in height along a length of the thread so that a first short peak with a first radius measured from the longitudinal axis of the shaft may be adjacent to a first tall peak with a second radius, which in turn may be adjacent to a second short peak with a third radius, adjacent to a second tall peak with a fourth radius, the first and third radii may be similar and may both be lesser in dimension than the second and fourth radii.

The peaks may extend along helical curves winding around the shaft in a direction opposite to a helical curve along which the thread extends. The peaks may extend along axes that are parallel to or aligned with a longitudinal axis of the shaft. The shaft may include a cutting flute that extends in a linear direction along an axis angled with respect to a longitudinal axis of the shaft. The shaft may include a cutting flute that extends along a helical path from the distal tip of the shaft.

Other examples according to the second aspect, taken alone or in combination, are the following:.

A third aspect of the fastener is a fastener having a head including a channel adapted to receive a spinal rod, a shaft coupled to the head, the shaft including a distal tip, a thread extending between the head and the distal tip, and a serration extending along approximately <NUM> percent of a length of the thread.

In other examples according to the third aspect, the serration may include peaks and troughs. The distal tip may taper such that an angle between an axis measured from a first point on a surface of the thread at a first end of the taper to a second point at the tip of the fastener on the longitudinal axis of the shaft may be approximately <NUM> to <NUM> degrees relative to the longitudinal axis of the shaft.

The present invention relates to a fastener to be used in conjunction with spinal rods during spinal surgery. Those of skill in the art will recognize that the following description is merely illustrative of the principles of the invention, which may be applied in various ways to provide many different alternative embodiments.

<FIG> depict a first embodiment of a fastener <NUM> that is configured for spinal applications, and in particular, for use as a pedicle screw or fastener. Fastener <NUM> includes a screw body <NUM> and a tulip, which has a channel adapted to receive a spinal rod. A spinal rod can be installed into the tulip and held in place by a set screw (not shown), which is threaded into internal threads of the tulip. While the tulip is not shown in <FIG>, tulips are featured in the embodiments shown in <FIG>, described below.

Fastener <NUM> is poly-axial in that screw body <NUM> is separate from the tulip. The tulip and proximal end of the screw body can generally be referred to as a head of fastener <NUM>. Screw body <NUM> includes a shaft <NUM> that extends along a longitudinal axis <NUM> from a proximal portion <NUM> or ahead of fastener <NUM> to a distal tip <NUM>. The tulip is polyaxially movable (i.e., a polyaxial pedicle screw) with respect to proximal portion <NUM> of screw body <NUM>. Proximal end <NUM> of screw body <NUM> forms an interference fit connection with a distal opening of the tulip to create the poly-axial connection. The tulip can swivel about and form different angles with screw body <NUM> to facilitate proper rod placement. In other embodiments, the fastener can be a monolithic structure (i.e., a monoaxial pedicle screw) having the tulip statically connected with the proximal end of the screw body. Both of such embodiments may additionally have retractor blades extending from the tulip, such as those described below in connection with <FIG>.

Shaft <NUM> includes a thread <NUM> extending between proximal portion <NUM> and distal tip <NUM>. Beginning thread <NUM> at distal tip <NUM> allows shaft <NUM> to engage and anchor into the bone immediately upon contact. As seen in <FIG>, thread <NUM> includes along a length thereof a serrated portion <NUM> that has individual serrations <NUM>. Serrated portion <NUM> extends along about <NUM> to <NUM> percent of a length of thread <NUM>. In certain embodiments, serrated portion <NUM> extends along <NUM> percent of the length of thread <NUM>. In other embodiments, the range of serrated portion <NUM> can be about <NUM>-<NUM> percent of a length of thread <NUM>, about <NUM>-<NUM> percent of a length of thread <NUM>, or about <NUM>-<NUM> percent of a length of thread <NUM>. This ratio of the serrated portion to the overall thread length allows for a consistent feel during manual insertion, regardless of the screw length. The inclusion of serrated portion <NUM> provides a solution for the problems of the prior art, as described above. Serrations <NUM> reduce insertion torque, thereby improving ease of insertion, while not compromising pullout strength. Serrated screws also allow for a quicker insertion time. The reduced insertion torque reduces the chance of bone fracturing and breaching. Additionally, serrations <NUM> allow surgeons to retain tactile feedback with minimized energy exertion resulting in greater accuracy during positioning of fastener <NUM>, as compared with manual screw insertion of other non-serrated prior art screws.

Shaft <NUM> is tapered, such that the tapered portion of shaft <NUM> is defined by an angle of between <NUM> and <NUM> degrees measured between longitudinal axis <NUM> of the shaft <NUM> and an axis intersecting outer surfaces of thread <NUM> at two or more revolutions of thread <NUM>. In certain embodiments, the tapered portion of shaft <NUM> extends along about <NUM> percent of the length of thread <NUM>, which can match the length of thread <NUM> along which serrated portion <NUM> extends. In other embodiments, the range of tapered portion can be about <NUM>-<NUM> percent of a length of thread <NUM>, about <NUM>-<NUM> percent of a length of thread <NUM>, or about <NUM>-<NUM> percent of a length of thread <NUM>. This configuration is designed so that once a maximum diameter of the threads is reached, the serrated portion <NUM> ends so that the threads at the maximum diameter do not continue to cut into the bone. Further cutting into the bone by the maximum diameter threads can weaken the engagement between the later-inserted, non-serrated threads and the bone, which reduces tactile feedback to the user. Having the tapered portion of shaft <NUM> and serrated portion <NUM> both extend along the same amount of the length of thread <NUM> (i.e., about <NUM> percent) allows some resistance at all times during insertion of the screw, which is desirable. Other embodiments in accordance with the present disclosure may include a shaft that is not tapered.

In the embodiment of <FIG>, proximal portion <NUM> of screw body <NUM> includes a proximally-facing flat top surface <NUM> and a distally-facing, generally spherical surface <NUM>, which interfaces with the tulip to allow for polyaxial movement between the tulip and screw body <NUM>. As shown in <FIG>, and <FIG>, proximal portion <NUM> includes a projection <NUM> extending from a central portion of flat top surface <NUM>. Proximal portion <NUM> further includes nubs <NUM> extending from flat top surface <NUM> at locations around the periphery thereof and surrounding projection <NUM>. Nubs <NUM> are each smaller in size than projection <NUM>. Although the shape of projections <NUM> and nubs <NUM> can vary depending on the corresponding tulip assembly and the corresponding insertion instruments, in the illustrated embodiment, projections <NUM> and nubs <NUM> are each rounded to have generally spherically-shaped proximal ends. As shown more clearly in <FIG>, in this embodiment, proximal portion <NUM> includes six nubs <NUM>. The number of nubs <NUM> can vary in other embodiments.

Thread <NUM> can have one or more of many cross-sectional areas, such as trapezoidal, square, triangular, rectangular or any other shape known in the art. As shown in <FIG> and <FIG>, thread <NUM> includes sidewalls <NUM> on either side that extend from an inner diameter of shaft <NUM> to an outer diameter of thread <NUM>. Sidewalls <NUM> are angled such that sidewalls <NUM> that face one another along the longitudinal axis <NUM> of shaft <NUM> form an angle α therebetween of <NUM> degrees. Angle α can be about <NUM> degrees, as in the depicted embodiment, while it can range between about <NUM>-<NUM> degrees in other embodiments. In still other embodiments, angle α can range between about <NUM>-<NUM> degrees, and between about <NUM>-<NUM> degrees in other embodiments. Thread <NUM> is configured such that an angle between sidewall <NUM> and longitudinal axis <NUM> of shaft <NUM> varies along a length of the shaft <NUM>. That is, the angle between the sidewall of thread <NUM> and longitudinal axis <NUM> varies along the path of thread <NUM>. In other embodiments, the angle of sidewall <NUM> can be constant. In the embodiment shown in <FIG>, sidewalls <NUM> extend toward longitudinal axis <NUM> until they intersect with a concave, helical path between adjacent passes of thread <NUM>, with the helical path containing the inner diameter of shaft <NUM>. In other embodiments, sidewalls <NUM> may extend to the inner diameter of the shaft by intersecting or by the helical path between adjacent passes of thread <NUM> being flat instead of concave.

As shown in <FIG>, thread <NUM> includes serrated portion <NUM> at a distal end of shaft <NUM> that continuously transitions into a smooth, non-serrated portion at a proximal end of shaft <NUM>. Serrations <NUM> are geometrical cut-outs along serrated portion <NUM> that allow for easier insertion of fastener <NUM> into the pedicle bone by reducing the insertion torque. This reduction in torque limits surgeon fatigue and reduces the chance of fracturing or breaching of the pedicle bone.

Referring to <FIG>, thread <NUM> has a tapered portion that defines a tapered angle β measured between longitudinal axis <NUM> of shaft <NUM> and an axis <NUM> intersecting distal tip <NUM> and an outer surface of thread <NUM> at a proximal end of the tapered portion of thread <NUM>. Angle β is <NUM> degrees in the depicted embodiment. In other embodiments, angle β can be about <NUM> degrees, or between about <NUM> to <NUM> degrees. In still other embodiments, angle β can range between about <NUM>-<NUM> degrees, and between about <NUM>-<NUM> degrees in other embodiments.

Referring to <FIG>, serrated portion <NUM> include peaks <NUM> and troughs <NUM> that alternate along serrated portion <NUM> to define serrations <NUM>. Peaks <NUM> are triangular in shape looking along the longitudinal axis <NUM>. Serrations <NUM> further include respective thicknesses <NUM> measured parallel to longitudinal axis <NUM> of shaft <NUM> such that successive thicknesses <NUM> increase in magnitude along a portion of a length of thread <NUM> toward distal tip <NUM>. Each thickness <NUM> is measured from the distal end to the proximal end of each serration <NUM>. In other embodiments, successive thicknesses can decrease in magnitude along a portion of a length of thread <NUM> toward distal tip <NUM> or can remain constant.

In the embodiment of <FIG>, each peak <NUM> is disposed at a radial distance from longitudinal axis <NUM> of shaft <NUM> that is greater than a radial distance from longitudinal axis <NUM> of shaft <NUM> to an adjacent trough <NUM>. Each peak <NUM> has a thickness measured parallel to longitudinal axis <NUM> of shaft <NUM> that is less than a thickness measured parallel to longitudinal axis <NUM> of shaft <NUM> of an adjacent trough <NUM>. In other words, due to the angle between sidewalls <NUM> of thread <NUM> and/or curvature of surfaces <NUM> of shaft <NUM>, the thickness is greater at the troughs <NUM> than at the adjacent peaks <NUM>.

Serrations <NUM> include respective widths measured perpendicular to longitudinal axis <NUM> of shaft <NUM>, such that successive widths decrease in magnitude along a portion of a length of thread <NUM> toward the distal tip <NUM>. In non-claimed examples, successive widths can increase in magnitude along a portion of a length of thread <NUM> toward distal tip <NUM> or can remain constant.

The pitch of a serration <NUM> is the distance between adjacent troughs <NUM> that define the serration <NUM>, that is, from a first trough <NUM> across a peak <NUM> to an adjacent second trough <NUM>. In the embodiment shown in <FIG>, the pitch of the respective serrations <NUM> progressively and incrementally increases from distal tip <NUM> toward proximal portion <NUM> of shaft <NUM>. Thus, the pitch of a serration <NUM> nearer the distal tip <NUM> is less than the pitch of a serration <NUM> closer to the proximal portion <NUM>. The number of serrations <NUM> per revolution of thread <NUM> is constant, as shown in <FIG> in a view from distal tip <NUM> toward proximal portion <NUM> of shaft <NUM>. This results in the small pitch of a serration <NUM> nearer the distal tip <NUM> because more serrations are fit into a revolution of thread <NUM> that has a generally smaller diameter due to its tapered structure. Different angles of the tapered section of thread <NUM> provide differently shaped serrations <NUM>. In one embodiment, the angle of each face of serration <NUM> measured from a plane through adjacent troughs <NUM> is <NUM> degrees. In other embodiments, the angle of each face of serration <NUM> measured from a plane through adjacent troughs <NUM> is between <NUM>-<NUM> degrees, and between about <NUM>-<NUM> degrees in other embodiments.

Other embodiments of fasteners in accordance with the present disclosure are shown in <FIG>, respectively. Fastener <NUM> is shown in <FIG> with serrations <NUM> running the entire length of thread <NUM> along shaft <NUM>. Fastener <NUM> is shown in <FIG> having a cannulated shaft <NUM> that defines a passage <NUM> along the length of shaft <NUM>. Passage <NUM> can extend along a full length of fastener <NUM> so that it is accessible at both the proximal and distal ends thereof, with the distal opening being shown in <FIG>. The proximal opening corresponds with the central projection at the proximal portion of fastener <NUM>.

<FIG> each depict different embodiments having a varying number of serrations per each thread revolution. In each embodiment, the pitch incrementally increases from the distal tip to the proximal portion. For example, in the fastener <NUM> shown in <FIG>, there are seventy-two (<NUM>) peaks <NUM> per revolution of thread <NUM>. The pitch is smaller at the distal tip <NUM> than on threads <NUM> closer to the proximal portion <NUM> due to the tapered nature of the screw to allow for the same number of peaks per revolution of threads <NUM>.

<FIG> depict fasteners having peaks of one type. For example, in <FIG>, peaks <NUM> are defined by a linear edge <NUM> at an abutment between surfaces connecting peak <NUM> with adjacent troughs. <FIG> depict fasteners having peaks of two types. For example, in <FIG>, certain peaks are defined by a linear edge, while peaks <NUM> are defined by a flat or planar surface at an abutment between surfaces connecting peak <NUM> with adjacent troughs. In some embodiments, successive peaks along the serrated portion can alternate between the linear and planar peaks.

In another embodiment, shown in <FIG>, the threads include two types of peaks that alternate and differ in height, which is the distance from the longitudinal axis of the screw body to the top of the peak. This configuration forms a double-V cut and can vary with a first tall peak, adjacent to a first short peak, the first short peak adjacent to a second tall peak, the pattern (tall-short-tall-short) continuing around the threads. The taller peaks can have the same or different height, though both are preferable greater in magnitude than the heights of the shorter peaks, which can be the same or different.

In another embodiment, shown in <FIG>, the peaks and troughs are rounded. In another embodiment, shown in <FIG>, the threads are square shaped or rectangular in cross-section, having two linear edges defining each peak and two linear edges defining each trough. In another embodiment, shown in <FIG>, the serrations are scalloped such that the peaks are curved with the troughs forming linear edges.

In another embodiment, shown in <FIG> and <FIG>, the serrations can be helical in the opposite direction of the threads of a fastener <NUM>. That is, serrations <NUM> can have peaks <NUM> that extend along helical curves winding around fastener <NUM> in a direction opposite to the helical curve along which thread <NUM> extends. In this way, peaks <NUM> are not aligned with or parallel to a longitudinal axis X of shaft <NUM>.

In another embodiment, shown in <FIG>, a fastener has a cutting flute that extends in a linear direction along an axis angled with respect to the longitudinal axis of the shaft. In another embodiment, shown in <FIG>, a fastener has a cutting flute that extends along a helical path from a distal tip of the shaft. Fasteners according to the present embodiments can include single or dual lead threads and can include one or more cutting flutes. A dual lead provides the fastener with superior pullout strength.

<FIG> depicts an embodiment similar to fastener <NUM> in which a fastener <NUM> is shown with a screw body <NUM> and a tulip <NUM>. <FIG> depicts an embodiment similar to that of <FIG>, but one in which a fastener <NUM> is not cannulated. An embodiment depicted in <FIG> is a fastener <NUM> in which a tulip <NUM> includes retractor blades <NUM> that act as guides during insertion of a spinal rod and can be detached once the spinal rod is anchored to tulip <NUM>. <FIG> depicts an embodiment similar to fastener <NUM> in which a fastener <NUM> has a longer shaft and a tulip <NUM> is angled on its distal surface.

Experimental tests were run with different configurations of screws in accordance with the embodiments of the present disclosure. Each screw has a diameter of <NUM> and a length of <NUM>, and is further configured as follows:.

Screws A-D were tested to determine mean maximum insertion torque. As shown in <FIG>, Screws B-D having serrations in accordance with the present disclosure showed superior performance to Screw A, which does not include serrations. The mean maximum insertion torque is less for all of Screws B-D as compared with Screw A. This evidences the desired result of lowering the insertion torque for a screw by providing serrations, to improve the performance of manual insertion.

In a serrated bone screw according to the present disclosure, the serrated portion can be defined as a function of thread length. Keeping the length of the serrated portion of the thread proportional to the thread length ensures consistent feel irrespective of screw length. By creating a proportional relationship, the end user will have the same experience despite the screw length. Calculating the length of the serrated portion can be done using the following formula: (Serration Length) = (Thread Length) times (X), where X equals a constant. This results in a linear relationship between the length of the serrated portion and the thread length. Thus, kits of screws in accordance with the present disclosure can include screws of different overall lengths having proportional serrated lengths based on a constant value.

Claim 1:
A fastener (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) configured for spinal applications comprising:
a head including a channel adapted to receive a spinal rod; and
a shaft (<NUM>, <NUM>, <NUM>, <NUM>) extending from the head to a distal tip (<NUM>, <NUM>) and having a thread (<NUM>, <NUM>, <NUM>, <NUM>),
characterized in that
at least a portion of the thread is serrated, and in that
the serrated portion of the thread includes serrations (<NUM>, <NUM>) having respective widths measured perpendicular to a longitudinal axis (<NUM>) of the shaft, successive widths decreasing in magnitude along a portion of a length of the thread toward the distal tip.