Methods for determining pedicle base circumference, pedicle isthmus and center of the pedicle isthmus for pedicle screw or instrument placement in spinal surgery

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 60/761,365 filed on Jan. 24, 2006.

[0002] The present invention relates generally to the field of spinal surgery, to computerized or automated methods for the accurate placement of pedicle screws or instruments in spinal surgery and, more particularly, to methods for determining pedicle base circumference, pedicle isthmus and the center of the pedicle isthmus.

[0003] Placement of screws into the human spine is a common surgical procedure to allow for a multitude of spinal surgeries to be performed. Screws are typically placed into the pedicles of individual vertebra in the lumbar and sacral spine. Given their biomechanical advantages over other modes of fixation, surgeons are expanding the areas of the spine in which pedicle screws are placed. However, adjacent to the spine are numerous vital structures and organs, in particular the cervical and thoracic spine regions, which have very low tolerance for surgically created injuries that may ultimately lead to significant morbidity and/or mortality. For this reason the majority of research focus on placement of pedicle screws is centered on improving accuracy to maintain a screw within a bony (intraosseous) environment.

[0004] Image guided systems are evolving which are increasingly user friendly to assist a surgeon in accurately placing a screw. The critical parameters for placing a pedicle screw into the human spine are diameter, length, trajectory and then actual placement of the screw. To date many of the image guidance systems allow for manual determination of these parameters to improve a surgeon's manual performance in screw placement. Up to the present time, no system is available which will automatically determine ideal pedicle screw diameter, length and trajectory for accurate placement of pedicle screws. The present invention provides this capability akin to a pilot who flies an airplane with computer controlled aviation capabilities, and allows for placement of pedicle screws using either an open or percutaneous technique.

[0005] Patent Application Publication No. US 2004/0240715 A1, published on Dec. 2, 2004, relates to methods and computer systems for determining the placement of pedicle screws in spinal surgery. It discloses a method wherein the minimum pedicle diameter is first established for determining the optimum screw trajectory and then the maximum screw diameter and length using the optimum trajectory for each pedicle. Two dimensional transverse slice data is stacked to form three dimensional data points to determine optimum trajectory by linear least squares solution to fit the data, requiring the solution to go through the overall minimum transverse pedicle widths. A disadvantage of this method is that it allows for eccentric trajectory determination, particularly for distorted pedicle anatomy, with consequent smaller maximum diameter and length screw determinations resulting in biomechanically inferior constructions. In contrast, the new and improved method of the present invention always places the trajectory concentrically through the pedicle by the determination of optimum trajectory by using the center point of the smallest cross sectional area (isthmus) and projecting with a computer a line normal to this circumscribed area in opposite directions, as described more particularly hereinafter. The new and improved methods of the present invention allow for maximum screw diameter and length determinations for intraosseous placement.

[0006] In Patent Application Publication No. 2005/0192575-AL, dated Sep. 1, 2005, relating to methodology for the determination of ideal pedicle screw diameter, length and trajectory there is a description of the transitional interface where the pedicle is joined to the vertebral body. This transitional interface describes the pedicle base circumference (B) which is identified radiographically on anteroposterior radiographic imaging as a round like cortical density seen on the cephalad lateral aspect of the vertebral body. An essential feature of this pedicle base circumference is that it is different from the pedicle isthmus (X, the narrowest region within a pedicle), but can on occasion be the same. The pedicle isthmus is the rate limiting step to maximizing the largest diameter pedicle screw without causing a breach of the cortical wall. To maximize the diameter of the pedicle screw within any given pedicle the pedicle isthmus must be determined. Subsequently, the center of the pedicle isthmus allows determination of the ideal trajectory to allow for concentric pedicle screw placement along the ideal trajectory.

[0007] The present application is directed to new and improved methods for determining the pedicle base circumference, pedicle isthmus and center of the pedicle isthmus.

[0008] In accordance with the methods of the present invention, serial stacked images in any plane are obtained of the vertebral body in any suitable manner. These images are then reconstructed to obtain a dimensionally true three-dimensional rendering of the vertebral body. The pedicle base circumference and pedicle isthmus are depicted in three-dimensional and two-dimensional images.

[0009] Once a true three-dimensional rendering of a vertebral body is obtained, it is then sectioned in a transverse plane to visualize and obtain an outer cortical shell. A series of first lines are then drawn tangentially along the outer cortical surface. A series of second lines are then drawn perpendicular to the tangential lines lying on the outer cortical surface, with the second lines lying within the vertebral body.

[0010] In the area of the pedicle and its transition into the vertebral body, the second lines will define the pedicle base circumference and pedicle isthmus. Specifically, the pedicle base circumference is defined as the region in which the adjacent second lines are at the greatest angle, non-linear or discordant, to one another. The pedicle isthmus is defined as the region in which the opposing second lines are most parallel to one another. Infinitesimal points on the outer cortical surface are utilized for the placement of the first tangential surface lines and their respective second perpendicular lines.

[0011] Once the pedicle isthmus is defined, it is then necessary to define the center of the pedicle isthmus to allow for concentric trajectory determination and pedicle cylinder building. Most pedicles are conceptualized as being cylindrical, although many pedicles have oval or irregular volumes. As such, it is essential to accurately determine the center of these pedicles. The method of the present invention utilizes the cross-sectional area defined by the pedicle isthmus and then identifies the center of this cross-sectional area as that point which lies at the intersection of two lines derived from the centers of infinitesimal orthogonal second perpendicular lines from the outer cortical surface. This methodology allows for pedicle isthmus center determination irrespective of pedicle configuration.

[0022] In accordance with the methods of the present invention, serial stacked images in any plane are obtained of the vertebral body 10 in any suitable manner. These images then are reconstructed to obtain a dimensionally true three-dimensional rendering of the vertebral body 10. The pedicle base circumference B and pedicle isthmus X are depicted in the three-dimensional and two-dimensional images as shown schematically in FIGS. 1A and 1B.

[0023] Once a true three-dimensional rendering of the vertebral body 10 is obtained, it is sectioned in a transverse plane to visualize and obtain an outer cortical shell. A series of first lines T are then drawn tangentially to and along the outer cortical surface 12. A series of second lines P are then drawn perpendicular to the first lines T lying on the outer cortical surface 12 with the second lines P lying within the vertebral body 10. This is illustrated in FIG. 2 with respect to only two first tangential lines T and two second perpendicular lines P as an illustrative example.

[0024] In the area of the pedicle 14 and its transition into the vertebral body 10, the series of second perpendicular lines P will define the pedicle base circumference B and pedicle isthmus X. Specifically, the pedicle base circumference B is defined as that region in which the adjacent second perpendicular lines P are at the greatest angle A, nonlinear or discordant to one another. Conversely, the pedicle isthmus X is the region in which opposing second perpendicular lines P are most parallel to one another. Infinitesimal points on the outer cortical surface are utilized for placement of the first tangential lines T and their respective second perpendicular lines P. This is illustrated schematically in FIG. 3.

[0025] The points from infinite transverse sections (TS1, TS2, TS3 . . . ) defining the pedicle base B and pedicle isthmus X are then collated to determine the anatomical three-dimensional location of the pedicle base circumference B and pedicle isthmus X, as shown in FIGS. 4A, 4B and 4C. FIGS. 4B and 4C show the transverse section TS2 through the center of the pedicle 14 and its corresponding point on a transverse and coronal projection, respectively.

Pedicle Isthmus Center Determination

[0026] Once the pedicle isthmus X is defined, the center of the pedicle isthmus C must be further defined. This is necessary to allow for concentric trajectory determination and pedicle cylinder building. Most pedicles are conceptualized as being cylindrical; however, many pedicles have oval or irregular volumes. As such, it is essential to determine the center of these pedicles. The new and improved method of the present invention utilizes the cross-sectional area defined by the pedicle isthmus X and then identifies the center C of the cross-sectional area as being that point which lies at the intersection of two lines derived from the centers of the infinitesimal orthogonal second perpendicular lines P as illustrated in FIGS. 5A, 5B and 5C. This methodology allows for pedicle isthmus center determination irrespective of different pedicle configurations as shown in FIGS. 5A, 5B and 5C.

[0027] It will be readily seen that the methods of the present invention provide for simple and reliable determination of pedicle base circumference, pedicle isthmus and the center of the isthmus to provide for concentric pedicle screw placement along the ideal trajectory. These methods can be effected in any suitable manner, such as visual imaging through the use of a computer or the like, or manually from two-dimensional sections.

[0028] While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.