The present disclosure relates to implantable orthopedic devices for stabilizing the spine. More particularly, the present disclosure describes an implant that can be reversibly and variably expanded and contracted.
Implantable devices such as cages and spacers are in use for providing support between sequential vertebrae of a human spine. Such devices are selected to provide a precision fitment for the space between the sequential vertebrae. Matching a device precisely can be challenging due to geometric variation. In addition, there may be a need to remove such a device after they are implanted. There is a need for an expandable cage that can be adapted to a variable geometry including the spacing and angular relationship between surfaces of the sequential vertebrae. Also, there is a need for a cage that can be later removed with minimal effect on an implant site.
The disclosure that follows infra describes an implant having an outer cage and a slider mechanism for altering an outer geometry of the outer cage. The cage has proximal and distal ends with respect to a major axis of the cage. The slider mechanism is configured to independently adjust a proximal and distal height of the cage at the proximal and distal ends respectively. The cage in combination with the slider mechanism is configured to provide reversible expansion and contraction of the cage without hysteresis effects by operating within elastic limits of the cage.
In a first aspect of the disclosure, an implant includes a cage and a slider mechanism. The cage refers to the outer housing of the implant, and has a cage length, a cage width, proximal height, and distal height. The cage length is along a major axis X of the cage between a proximal end and a distal end of the cage. The cage width is a long a lateral axis Y. The proximal height is a vertical height of the proximal end of the cage. The distal height is a vertical height of the distal end of the cage. The cage includes an upper support, a lower support, a first serpentine spring, and a second serpentine spring. The first serpentine spring joins the upper support to the lower support along a first path having a first path length that is greater than the cage length. The first path generally lies along a first side of the cage. The second serpentine spring joins the upper support to the lower support along a second path having a second path length that is greater than the cage length. The second path generally lies along a second side of the cage. The slider mechanism includes a proximal slider mechanism and a distal slider mechanism. The proximal slider mechanism is configured to adjust the proximal height of the cage along a vertical axis Z. The distal slider mechanism is configured to adjust the distal height of the cage along the vertical axis Z. During adjustments of proximal and distal height (increasing and decreasing the height), the serpentine springs remain within their elastic strain limits. This allows for complete reversibility of cage expansion and contraction without hysteresis. This in turn allows the implant to be removed from a surgical site with minimal adverse effects upon surrounding tissue and bone. Axes X, Y and Z are mutually orthogonal.
The “serpentine spring” refers to a spring having a serpentine path geometry. A serpentine path geometry according to the present disclosure is defined along the first and second sides of the cage. The first and second sides of the cage are rectangular sides that are each defined along the X and Z axes. The serpentine path defines a plurality of straight segments that are each parallel to the X axis but arranged or disposed along the Z axis. The straight segments have ends that are joined together with curved or U-shaped portions in an alternating manner in order to define a continuous serpentine path along the serpentine spring from the upper support to the lower support. In an illustrative embodiment, the cage has a cage length L. The length of the serpentine spring measured along the serpentine path is preferably about 3×L or 3L. The serpentine spring has a width and thickness that are each preferably less than 0.05L. Thus, the serpentine spring length measured along the serpentine path is at least 60 times the width or thickness of the spring. These dimensional comparisons can vary by design while allowing the implant and serpentine springs to operate within their elastic strain limits.
The first and second serpentine path lengths are each preferably at least two times the cage length. In some embodiments the first and second path lengths each be at least about 2.5 or 2.8 times the cage length. Longer path lengths are advantageous to minimize strain elongation of the serpentine springs as the cage height is varied so that the serpentine springs remain within their elastic limits when the height of the cage is adjusted. In an illustrative embodiment, the first and second serpentine springs each define two U-shaped bends that connect three linear segments. Each linear segment defines a length greater than 80% of the cage length. This geometry is advantageous for keeping strain within the elastic limit.
The “elastic stress or strain limit” is defined as a threshold of stress or strain above which a object is plastically deformed. Above the elastic strain limit, the object exhibits hysteresis in the stress strain curve. The elastic stress limit is also known as the “yield strength”. The design in the present disclosure allows a stress applied by the slider mechanisms to be below the yield strength of the serpentine springs.
In one implementation the proximal slider mechanism has a proximal height range of adjustment. The distal slider mechanism has a distal height range of adjustment. The first and second serpentine springs are each configured to remain within an elastic strain limit throughout the proximal height range of adjustment and the distal height range of adjustment. Thus, the stress applied to the serpentine springs is below the yield strength.
In another implementation the upper support has a lower proximal surface and a lower distal surface. The lower support has an upper proximal surface and an upper distal surface. The lower and upper proximal surfaces define a taper. The proximal slider mechanism includes a proximal threaded bolt coupled to a proximal slide. Rotating the proximal threaded bolt causes sliding engagement between the proximal slide and the taper defined by the upper and lower proximal surfaces which adjusts a vertical distance between the proximal end of the upper and lower supports. In a similar manner, the lower and upper distal surfaces define a taper. Rotating the distal threaded bolt causes sliding engagement between the distal slide and the taper which adjusts a vertical distance between the distal end of the upper and lower supports.
In yet another implementation the (proximal and/or distal) slider mechanism includes a threaded bolt that is threaded to a slide. The slider mechanism includes a linkage that is rotatively coupled between the slide and the upper support. Rotating the threaded bolt translates the slide which in turn rotates the linkage to increase or decrease a distance between the upper support and the lower support (at the proximal and/or distal end of the cage).
In a further implementation a plurality of tapered locking features extend vertically upward and downward from the upper and lower supports respectively.
In a yet further implementation a plurality of bone screws extend through the upper and lower supports.
In another implementation the proximal slider mechanism includes two side-by-side proximal slider mechanisms. The distal slider mechanism includes two side-by-side distal slider mechanisms. Thus there are four slider mechanisms that allow the height of the cage to be adjusted at four quadrants.
In a second aspect of the disclosure, a system is provided including the implant of the first aspect of the disclosure plus an insertion instrument. The insertion instrument is configured to be inserted through the proximal threaded bolt and to simultaneously couple to the proximal threaded bolt and the distal threaded bolt. This allows both independent or simultaneous adjustment of the vertical distance between the upper and lower supports at both the proximal and distal ends with a single coupling of the insertion instrument to the implant.
In a third aspect of the disclosure, a method for inserting and adjusting an implant is provided for the implant of the first aspect of the disclosure. The method includes coupling the implant upon an insertion instrument, manipulating the insertion instrument to position the implant within a surgical site, operating independent proximal and distal drives to independently adjust the proximal and distal heights of the cage, and decoupling the implant from the insertion instrument.
In one implementation the insertion instrument includes a sleeve coupled to a clamp. Coupling the insertion instrument to the implant includes rotating the sleeve in a first direction to close the clamp over the implant. Decoupling the insertion instrument from the implant includes rotating the sleeve in a second direction that is opposite to the first direction to open the clamp.
In another implementation, operating the independent proximal and distal drives includes independently rotating the proximal and distal drives.
In a fourth aspect of the invention, an implant includes a cage, an upper support, a lower support, a first serpentine spring, a second serpentine spring, and at least two independent slider mechanisms configured to at least adjust the proximal height and the distal height of the cage. The cage has a cage length along a major axis X which is the major axis of the cage. The major axis of the cage extends between a proximal and distal end of the cage. The cage has a cage width along a lateral axis Y from a first side of the cage to a second side of the cage. The cage has a proximal height along a vertical axis Z at the proximal end of the cage. The cage has a distal height along a vertical axis Z at the distal end of the cage. The first serpentine spring joins the upper support to the lower support along a first serpentine path having a first path length that is at least two times the cage length. The first serpentine path defines the first side of the cage. The second serpentine spring joins the upper support to the lower support along a second serpentine path having a second path length that is at least two times the cage length. The second serpentine path defines the second side of the cage.
In one implementation the at least two independent slider mechanisms include more than two independent slider mechanisms. The more than two independent slider mechanisms can be four independent slider mechanisms that independently adjust a height of the cage for four quadrants of the cage.
In disclosing an implant and system, certain mutually orthogonal axes X, Y, and Z will be used. The X and Y axes are lateral axes and are generally horizontal. The axis Z is generally vertical. The term “generally” is used because such descriptions are relative to one another rather than absolute. The implant of the present disclosure can be inserted between two sequential vertebrae of a generally vertical spine (mostly but not exactly vertical when a patient is standing). However, it is to be understood that the implant of the present disclosure may find additional applications or implanted orientations. Additionally, the X-axis is generally aligned with a major axis of the implant and can be referred to as the major axis of the implant. The X-axis can also be along the direction the implant is inserted into a patient. The Y-axis can be along a lateral width of the implant. The Z-axis can be along a height of the implant.
The implant 2 also includes a torsion stabilizer 11 at each of the proximal 8 and distal 10 ends of the cage 4. The torsion stabilizers 11 stabilize the cage 4 to prevent twisting of cage 4 about the major axis X.
In some embodiments, the cage 4 is formed from a titanium alloy. In a particular embodiment, the titanium alloy includes 3-5 percent vanadium, 5-7 percent aluminum, small amounts of other elements such as iron, and the balance or about 88-92% titanium. One such alloy is “Ti-6Al-4V” otherwise referred to as “R56400”. Such a titanium alloy is known for high strength and corrosion resistance. Other possible materials can include other titanium alloys, pure titanium, stainless steel, cobalt-chrome alloys, implantable plastics, and other material suitable for medical or spinal implants. Cage 4 can be formed using additive manufacturing (e.g., three-dimensional printing), subtractive processes such as machining (e.g., mechanical milling and/or electrical discharge machining) or even combinations thereof that are known in the art. One additive manufacture method of forming a titanium alloy cage 4 is by “selective laser melting” of titanium alloy powder using a high powered laser in an inert argon environment as is known in the art. Post-treatments such as heat treating and coatings are also known in the art.
The proximal slider mechanism 6 includes a proximal threaded bolt 24 and a proximal slide 26. The proximal threaded bolt 24 is mechanically restrained along the major axis X with respect to the cage 4. The proximal threaded bolt 24 is threaded to the proximal slide 26. Rotation of the proximal threaded bolt 24 induces a translation of the proximal slide 26 along the major axis X. The proximal slide 26 engages surfaces 28 and 30 of the upper 12 and lower 14 supports respectively. Surface 28 is a lower proximal surface 28 of the upper support 12. Surface 30 is an upper proximal surface 30 of the lower support 14. Surfaces 28 and 30 define a taper.
The engagement of proximal slide 26 against the taper formed by surfaces 28 and 30 causes the upper 12 and lower 14 supports to separate along the vertical axis Z as the proximal slide 26 is translated toward the proximal end 8 of cage 4. Throughout the rotational travel of the proximal threaded bolt 24, the serpentine springs 16 are elastically urging the upper 12 and lower 14 supports together. Therefore, there is generally no hysteresis in the effect of rotating the threaded bolt 24 (except perhaps for a small amount of backlash due to thread engagement tolerances and axial retainment of the threaded bolt by the cage 4 along major axis X) back and forth in two angular directions. Clockwise rotation of threaded bolt 24 therefore increases the proximal height of the cage 4 and counterclockwise rotation of the threaded bolt 24 decreases the proximal height of the cage 4.
The distal slider mechanism 7 includes a distal threaded bolt 32 and a distal slide 34. Due to close similarity of operation, the discussion supra for operation of the proximal slider mechanism 6 applies to the distal slide mechanism 7 and vice-versa. The distal threaded bolt 32 is restrained along the major axis X and is threadedly received into the distal slide 34. As the distal threaded bolt 32 is rotated, the effect is to translate the distal slide 34 along X which in turn engages a taper defined by lower distal surface 36 and upper distal surface 38 of the upper 12 and lower 14 supports respectively. Clockwise rotation of threaded bolt 32 therefore increases the distal height of the cage 4 and counterclockwise rotation of the threaded bolt 32 decreases the distal height of the cage 4.
The proximal slider mechanism 6 has a proximal 8 height range of adjustment. The distal slider mechanism 7 has a distal 10 height range of adjustment. The first 16 and second 17 serpentine springs are each configured to remain within an elastic limit throughout the proximal height range of adjustment and the distal height range of adjustment. During the insertion of implant 2 into a patient the proximal 8 and distal 10 height of the cage 4 is increased to provide support between sequential bone segments or vertebrae. Because the serpentine springs 16, 17 remain within elastic limits, the slider mechanisms 6, 7 can later be adjusted to decrease the proximal 8 and distal 10 height of the cage 4 because of a restoring spring force of serpentine springs 16, 17 that urge the proximal 8 and distal 10 ends of the cage toward a more compact or decreased height condition.
The upper 12 and lower 14 supports include tapered and pointed locking features 40 for locking or restraining the cage 4 against the vertebrae or bone segments. In the illustrated embodiment four locking features 40 extend and taper upward from the upper support 12 and four locking features 40 extend and taper downward from the lower support 14.
The insertion instrument 43 also includes a sleeve 45 coupled to jaws of a clamp 47. Clockwise rotation of sleeve 45 relative to handle 48 tightens clamp 47 upon the cage 4. Counterclockwise rotation of sleeve 45 relative to handle 48 relaxes and releases clamp 47 from cage 4.
According to 104, a surgical site for receiving the implant 2 is prepared. According to 106, the insertion instrument 43 is used to position the implant 2 within the surgical site. According to 108 and 110, the distal 46 and proximal 44 drives are rotated relative to the handle 48 to vertically expand the distal 10 and proximal 8 ends of the cage 4 and to lock the cage 4 in place at the surgical site. As a note, the order of steps 108 and 110 can occur in any order and even repeated until the cage 4 is locked in place. This is indicated by a double arrow that connects steps 108 and 110.
According to 112, the insertion instrument 43 is removed from the implant 2. As part of step 112, the sleeve 45 is rotated counterclockwise to release the clamp 47 from the cage 4. Finally, according to 114, the surgical site is closed.
In the embodiment described with respect to
Implant 202 includes cage 204, proximal 206 and distal 207 slider mechanisms, proximal 208 and distal 210 ends of implant 202, upper 212 and lower 214 supports, first 216 and second 217 serpentine springs, proximal 224 and distal 232 threaded bolts, proximal 226 and distal 234 slides, and proximal 228 and distal 236 surfaces engaged by the slides 226 and 234. Thus, clockwise/counterclockwise rotation of proximal 224 and distal 232 threaded bolts vertically expand/contract proximal 208 and distal 210 ends of implant 202 respectively.
Implant 302 includes an outer cage 304, a proximal end 306, and a distal end 308. Cage 304 has a cage length along a major axis X between the proximal end 306 and the distal end 308. Cage 304 has a cage width along a lateral axis Y. Cage 304 has four rectangular quadrants with respect to the X and Y axes (see
To independently adjust the quadrant heights, the implant 302 includes a slider mechanism for each quadrant including a first slider mechanism 318 for adjusting the height of first quadrant 310, a second slider mechanism 320 for adjusting the height of second quadrant 312, a third slider mechanism 322 for adjusting the height of third quadrant 314, and a fourth slider mechanism 324 for adjusting the height of fourth quadrant 316.
The slider mechanism 320 has the same parts, function, and mechanical action as the slider mechanism 322. Rotation of a proximal threaded bolt 338 will adjust the height of the second quadrant 312 in a like manner.
The slider mechanism 324 is coaxially aligned with the slider mechanism 322 and includes a distal threaded bolt 344, a distal slide 346, and a linkage 348. The slider mechanism 324 operates in a manner that is similar to that described with respect to the slider mechanism 322 for adjusting the height of the fourth quadrant 316.
The slider mechanism 318 has the same parts, function, and mechanical action as the slider mechanism 324. Rotation of a distal threaded bolt 344 will adjust the height of the first quadrant 310 in a likewise manner. The slider mechanism 318 is coaxially aligned with the slider mechanism 320.
For each pair of coaxially aligned slider mechanisms (318/320 or 322/324) the bolts are coaxial meaning that for each pair, the proximal threaded bolt 338 is coaxial with the distal threaded bolt 344. This allows an insertion instrument that is similar to insertion instrument 43 to be used to place and lock the implant 302 into a surgical site in a manner similar to that described with respect to
The specific embodiments and applications thereof described above are for illustrative purposes only and do not preclude modifications and variations encompassed by the scope of the following claims. As indicated earlier, clockwise expansion of a cage and counterclockwise contraction can be interchanged with counterclockwise expansion and clockwise contraction for any slider mechanism without departing from the scope of the inventive scope.