Spine Stabilization Device and Method

The present invention relates to devices and methods for stabilizing the spine. It more particularly relates to anterior stabilization of a spinal column (backbone) that has been rendered unstable by any disease such as a tumor, infection, or congenital anomalies affecting the vertebral bodies or associated supporting structures.

Preservation of neurologic function and relief of pain are the primary goals of treatment of diseases affecting the vertebral bodies. Oftentimes such diseases themselves destroy the vertebral body or the surgical treatment necessitates its resection. As a result of the loss of bony support, the vertebral column is unstable causing pain, progressive neurologic deterioration, and physical deformity of the back. In such cases stabilization of the spinal column by an anterior surgical approach may be required.

Each of the various procedures described in the past, however, has its individual shortcomings. No entirely satisfactory method until the present invention has been developed for effectively stabilizing the spine from the anterior surgical approach.

The most commonly used method of anterior spine stabilization is to fill the space of the resected vertebral bodies with methylmethacrylate or other similar plastic or plastic-like polymers. Difficulties encountered with this method include: (1) the inability to bond securely the plastic material to tissue or bone, (2) the potentially injurious heat released into the surrounding tissue because of the exothermic chemical reaction required for polymerization of the plastic, (3) potential stress fractures of the plastic caused by mechanical forces related to weight bearing, and (4) the need to mold the plastic to the appropriate size which is often technically cumbersome and difficult. Also, to secure the methylmethacrylate adequately, metallic pins or struts must be incorporated into the molded methylmethacrylate after their impaction or penetration into the adjacent vertebral bodies (vide infra).

Another common method of anterior spine stabilization utilizes bone to act as a replacement for the vertebral body after its removal because of disease. Bone of the iliac crest, fibula, or tissue-banked cadaver bone has been used. However, several limitations of this technique exist: (1) Immediate stabilization is not possible because adequate bony fusion requires three to six months. As a result, such individuals must wear an external spinal stabilization device for weeks or months postoperatively. Patient tolerance of such externally worn devices is often poor. (2) Oftentimes, especially when radiation therapy and/or chemotherapy are required to treat the primary disease process affecting the spine, the implanted bone graft does not survive and this results in further spinal instability secondary to resorption of the bone graft. This shortcoming has attempted to be overcome by a surgical procedure in which a bone graft on a vascularized pedicle is used since it is felt that most failures are due to inadequate blood supply to the graft. However, this procedure is technically difficult and therefore not commonly used. Even under the best of circumstances, the bone graft is partially resorbed and collapse of the vertebral space can lead to compression of the neural tissues in the spinal canal.

Attempts at spinal stabilization have also been performed utilizing a variety of metal pins and struts as mentioned previously. One common method is to penetrate adjacent vertebral bodies with Steinmann pins and then surround the pins with methylmethacrylate. Many technical problems are associated with this procedure including (1) those difficulties specially related to methylmethacrylate; (2) the inability to maintain distraction adequately while polymerization of the plastic is taking place; (3) the inability to adequately place the pins because of space restrictions; and (4) the fracture of the pins resulting in loss of stability and possible movements of the methylmethacrylate.

Another method for anterior spinal stabilization utilizes one or two metal rods attached by screws to the lateral aspect of the adjacent vertebral bodies. This procedure was first described by Dunn in 1980. There are two primary limitations of this procedure. First, it cannot be used in certain areas of the spine because of the vital structures adjacent to the lateral aspect of the vertebral body. Second, the device is inherently weak since mechanical stresses are maximally exerted at the site of attachment of the device to the vertebral body by small metal screws.

Metal rod distraction utilizing Harrington or Knodt rods, with or without methylmethacrylate, is another method to stabilize the spine via the anterior approach. This procedure was described by Harrington in 1976. One or more rods are typically attached via hooks to the anterior portion of the vertebral body and they can then be covered with methylmethacrylate to further secure them in place. The major disadvantages of this technique is the slippage of the stabilizing rods secondary to the mechanical forces generated by spine movement. These forces are greatest at the site of hook insertion and may cause fracture of the vertebral body to which it is attached. Moreover, involvement of the adjacent vertebral bodies with disease may weaken the bone at the site of hook insertion and lead to migration of the rods with resultant collapse of the spinal column.

Accordingly, the principal object of the present invention is to provide a novel device and method for stabilizing the spinal column but without the shortcomings and disadvantages of the prior methods as set forth above.

Other objects and advantages of the present invention will become more apparent to those persons having ordinary skill in the art to which the present invention pertains from the foregoing description taken in conjunction with the accompanying drawings.

FIG. 1 is a perspective view of a spine stabilization device of the present invention shown secured to the spinal column.

FIG. 2 is an enlarged cross-sectional view of the device of FIG. 1 taken along line 2--2.

FIG. 3 is a top cross-sectional view of the device of FIG. 1 with the anchoring pins illustrated in their retracted position.

FIG. 4 is a view similar to FIG. 3 with the anchoring pins illustrated in their fully extended anchoring position.

FIG. 5 is an end cross-sectional view of the device of FIG. 1 illustrating the wrench in its posterior engaging position.

FIG. 6 is a view similar to FIG. 5 with the wrench in its anterior engaging position.

FIG. 7 is a perspective exploded view of the device of FIG. 1.

FIG. 8 is a perspective exploded view of a second embodiment of the gear mechanism illustrated in isolation of the device of FIG. 1.

FIG. 9 is an exploded perspective view of a spine stabilization device according to the present invention that incorporates spacers for replacing multiple consecutive vertebral bodies.

A spine stabilization device according to the present invention is illustrated generally at 10 in FIGS. 1, 2 and 7. It is seen to be comprised of a vertebral body prosthesis 12 made of a bio-compatible material such as metal or plastic and approximating the dimensions of one human vertebral body. As will be described later, prosthesis 12 has the ability to be elongated when necessary to accommodate different dimensions between the vertebral bodies 14, 16. Contained within prosthesis 12 are two pairs of biocompatible metal anchoring pins 18, 20 which are advanced from their retracted position, as best illustrated in FIG. 3, to their fully extended position as illustrated in FIGS. 4 and 2 with their pointed ends penetrating the abutting vertebral bodies.

Neighboring, but separated vertebral bodies are first distracted as by temporarily attaching conventional distraction devices (not shown) and prosthesis 12 then anteriorly interposed between the distracted vertebral bodies. Pins 18, 20 are in their retracted position, as shown in FIG. 3, when prosthesis 12 is inserted into the separation between neighboring vertebral bodies 14, 16. After being inserted into proper anatomical position, the anchoring pins 18, 20 are extended via gear mechanisms shown generally at 22 and 23 out through their axial prosthesis channels 24, 25 into the adjacent vertebral bodies 14, 16 to secure prosthesis 12 therebetween.

Gear mechanisms 22, 23 include rack gear teeth 26 provided on the interior sides of each of the anchoring pins 18, 20. Rack gear teeth 26 communicate with wrench opening 28 when the pins are in their retracted position. Wrench opening 28 extends anteroposteriorly through prosthesis 12 to posterior wrench abutment plate 30. When it is desired to advance the pins, a pinion wrench 32 is inserted anteriorly into wrench opening 28, as best illustrated in FIG. 7. Referring to FIGS. 5 and 6, pinion wrench shown generally at 32 includes a cylindrical gear head 34 having protruding gear teeth, a gear shaft 35 secured to gear head 34 and a handle 36 perpendicularly attached to the outermost end of shaft 35. Wrench 32 is held by handle 36 and gear head 34 inserted into wrench opening 28 until it is in its further inserted position and in engagement with rack gear teeth 26 of the posterior pair 18 of oppositely disposed pins, as best shown in FIG. 5. Then wrench 32 is rotated via handle 36 until the pins are fully extended as shown in FIG. 4. Wrench 32 is then removed slightly out through wrench opening 28 until gear head 34 is in engagement with the anterior pair 20 of oppositely disposed pins as best shown in FIG. 6. Handle 36 is turned in the opposite direction and anterior pins 20 are advanced in their channels 25 from their retracted position to their fully extended position similar to the motion of the posterior pins but in opposite directions. It is also within the scope of the present invention to extend the anterior anchoring pins before the posterior anchoring pins. Anchoring pins 18, 20 thereby fix prosthesis 12 in place between neighboring vertebral bodies 14 and 16 and thus stabilize the vertebral bodies one to the other.

In lieu of the gearing arrangements of FIGS. 5 and 6, an alternative gear mechanism can be used as shown generally at 38 in FIG. 8. Referring thereto, it is seen that two pinion gears 40, 42 are fitted into wrench hole 28 with a cylindrical spacer 44 between them within prosthesis 12. Each pinion gear has a square opening 46, 48, respectively, extending therethrough. A wrench similar to pinion wrench 32 but having a square gear head 50 is then temporarily inserted into posterior gear head opening 46. The sides of square gear head 50 engage the surfaces of the square opening 46. When the wrench is turned gear head 40 turns and the posterior pins 18 are extended similar to the first embodiment. After the posterior pins 18 have been advanced to their fully extended position, wrench 32 is pulled slightly out of wrench opening 28 until wrench gear head 50 is in engagement with the anterior pinion gear 42. Wrench 32 is then twisted in the other direction via handle 36 and the anterior pair of pins 20 are advanced to their fully extended position. Wrench 32 is then completely removed from prosthesis 12 out through wrench opening 28 and pinion gears 40, 42 remain within prosthesis 12.

Referring to FIGS. 3 and 4 it is seen that each of the anchoring pins 18, 20 has associated with it two spring-loaded detent pins 52, 54, which are screwed into the side openings 56 of prosthesis 12 and are in engagement with the side of anchoring pins as shown in FIG. 3. Detent pins 52, 54 bias the anchoring pins towards wrench opening 28 and against pinion gear head 34 when inserted providing an interlocking between the gear rack teeth. Detent pins 18, 20 also have a track 58 extending longitudinally on their outside surface opposite the tooth gear 26 and anchoring pins 52, 54 ride in tracks 58 as the pins are extended into the adjacent vertebral bodies. When the pins are in their fully extended position, detent pins 52, 54 then engage into a pair of spaced holes 60, 62 in anchoring pins 18, 20 thereby locking the pins in place and thus preventing their movement back into prosthesis 12. Hole 60 which is nearest the pointed pin tip is provided with a chamfered shoulder 64, which allows the innermost retaining pin 52 to ride freely up the shoulder and to continue riding until it is engaged and held in hole 62.

Prior to extending anchoring pins 18, 20 but after prosthesis 12 has been positioned between the vertebral bodies 14, 16, a biocompatible metal retaining plate 66 which overlaps the two abutting vertebral bodies is attached to the bodies 14, 16 as well as to the prosthesis 12. Plate 66 secures prosthesis 12 in place and prevents its posterior migration. Plate 66 is secured via machine screws 68 screwed into aligned holes in the plate and the prosthesis, as best shown in FIG. 7. Plate 66 is directly fastened to the bony structure of the vertebral bodies, as shown in FIG. 2, by bone screws 69 through holes in the overlapping ends of plate 66.

The present invention also provides that spine stabilization device 10 can be elongated to accommodate greater dimensions between separated vertebral bodies. This can be done in one of two ways or a combination thereof, as illustrated in FIG. 9. First, when the additional elongated distance needed is not great a spacer member 70 having the same general cross-sectional configuration as prosthesis 12 can be positioned between the end surface of the prosthesis 12 and the first vertebral body 14. To provide rigidity spacer member 70 can be first screwed to prosthesis 12. Its holes 72 are aligned with channels 24, 25 of prosthesis 12. A longer retaining plate is secured in place and pins 18, 20 are extended via the gear mechanism as previously described. The pins will freely pass through holes 72 of spacer member 70. Spacer member 70 can be provided in a variety of different widths to accommodate different spinal configurations with each spacer measuring various fractions of a vertebral body and/or vertebral interspace. Second, in addition to or in lieu of spacer member 70, a longer prosthesis spacer 74 can be used for spanning multiple vertebral bodies, as shown in FIG. 9. For this embodiment a second generally indentical prosthesis 76 is provided and prosthesis spacer 74 is placed between two vertebral body prosthesis 12, 76. Prosthesis 12, 76 and the spacer 74 are then connected with a connecting plate 78 as shown in FIG. 9 and secured thereto by machine screws 68 passing through aligned openings with the wrench openings 28 and 82 in connecting plate 78 being aligned with the openings 84, 86 in the two vertebral body prosthesis. Then using the stabilizing pin setting wrench 32 the anchoring pins of prosthesis 12 are advanced until they extend into the first vertebral body 14 on the one side and on the other side into prosthesis spacer 74. Similarly the anchoring pins (not shown) in the second vertebral body prosthesis 76 are advanced until they extend into the second vertebral body 16 at one end and into the prosthesis spacer 74 at the other end. The ends of connecting plate 78 that overlap abutting vertebral bodies 14, 16 are fixed to those vertebral bodies by means of bone screws 69 in the fashion depicted in FIGS. 2 and 7.

From the foregoing detailed description it will be evident that there are a number of changes, adaptations and modifications of the present invention which come within the province of those persons having ordinary skill in the art to which the aforementioned invention pertains. However, it is intended that all such variations not departing from the spirit of the invention be considered as within the scope thereof as limited solely by the appended claims.