Patent Application
20230142246
In general, the present invention relates to the bone fixation systems that are used to adjust and/or stabilize the vertebrae of the spine. More particularly, the present invention relates to the structure of pedicle screws and alignment rods used in bone fixation systems.
It is not uncommon for a person to be born with some form of a skeletal irregularity or to develop some skeletal irregularity as they age. Most irregularities are minor and do not adversely affect a person's wellbeing. However, some skeletal irregularities, especially spinal irregularities, can cause pain and can limit range of motion. These irregularities can result from trauma, age, disc degeneration, and disease. Often, the treatment for such irregularities is the repositioning and/or immobilizing of a portion of the spine. This treatment commonly involves affixing a plurality of pedicle screws to the vertebrae and interconnecting the pedicle screws with one or more elongated rods. The problem that there are differences in the size and location of vertebrae and the degree to which vertebrae need to be adjusted in each person.
Consider the spinal irregularities caused by scoliosis. Scoliosis may cause deviations in all three directions or planes of the spine, i.e. frontal (coronal), lateral (sagittal) and transversal (axial). The list of clinical problems associated with scoliosis is extensive. It includes alteration of normal gait associated with pelvic obliquity, distortion of abdominal and chest organs, and the associated alteration of functional capabilities of the organs.
For idiopathic types of scoliosis, there is no congenital anomalies of the vertebrae. Therefore, evaluation of treatment is complicated and often empirical. Historically, the treatment is the surgical correction of the spine. The principles of surgical correction include two basic steps. The first step comprises the acute correction of spinal deformity during the surgery and the insertion of a holding device. The second step comprises the solid fusion of vertebral bodies in the position of gained correction, by insertion of bone graft during the same surgical procedure. Idiopathic scoliosis is not an acute illness, and with time, vertebrae become secondarily deformed. Surgeons who treat scoliosis know about-deformed shape of scoliotic vertebrae. For correction of such scoliotic deformity, acute manual correction of deformity during surgery is used. Positioning is maintained using pedicle screws that attach holding rods or plates to the vertebral body or vertebral prominences. When a surgeon manipulates the pedicle screws, rods and plates, the degree of fine adjustment is limited. Accordingly, there are pedicle screws, rods and plates that have been designed to be finely adjusted after they are surgically implanted. In this manner, a surgeon can make fine adjustments after all the hardware is anchored in place and the effects of the hardware can be observed.
Traditionally, pedicle screws are used to mechanically engage the vertebrae. The pedicle screws have anchor heads that are shaped to engage a rod or plate. In the prior art, there are pedicle screws that can be adjusted in length within the body. Such prior art pedicle screws are exemplified by U.S. Pat. No. 9,089,371 to Faulhaber. These prior art pedicle screws have different sections that are internally threaded together. As a result, if one section is rotated relative to a second section, the pedicle screw will either elongate or retract. A problem associated with such prior art pedicle screws is that some of the internal threading used for length adjustment is exposed on the exterior of the pedicle screw as it is elongated. The pitch of the exposed internal threading does not match the pitch of the threading on the exterior of the pedicle screw. As a result, when the pedicle screw is rotated into bone, or removed from bone, the bore in the bone is exposed to threading having two different pitches. This causes the bone bore to become double threaded as the pedicle screw turns. This makes the pedicle screw more difficult to install and/or remove. Furthermore, the double threading causes the lesser of the threads to strip through the bone as the more dominant threading turns. This widens the bone bore and degrades the mechanical connection. Another problem associated with adjustable pedicle screws in the prior art is that the pedicle screws are difficult to adjust so that the screws engage the cortical bone on opposite sides of the vertebral body. Rather, many prior art pedicle screw anchor into the porous cancellous bone within the vertebral body, which creates far weaker bond than would connection with the stringer cortical bone.
In addition to adjusting the length of the pedicle screws, the engagement between the rods and the pedicle screws must also be adjustable. In the prior art, there are many rods that are machined or textured in some manner to mechanically interconnect with the head of a pedicle screw in a manner that is adjustable. Such prior art systems are exemplified by U.S. Pat. No. 8,088,149 to White. A problem associated with such adjustment systems is that the head of the pedicle screw only engages the adjustment rod when tightened. When correcting a curvature in the spine, a surgeon must reposition the vertebrae. This requires a surgeon to maintain forces on the vertebrae as the alignment rod is anchored in place. As such, a surgeon must position and maintain tension on the adjustment rod while tightening the adjustment rod to the pedicle screws. This is a complicated task. Accordingly, a need exists in the art for both an adjustable pedicle screw that better engages the cortical bone on a vertebral body. A need also exists for a pedicle screw/adjustment rod system that makes it easier to maintain the adjustment rod in tension prior to being firmly anchored into a fixed position. These needs are met by the present invention as described and claimed below.
The present invention is a bone fixation system where an alignment rod is engaged by pedicle screws. The alignment rod has a top surface upon which sections of buttress thread are formed.
Each pedicle screw has a threaded shaft that supports a slotted receptacle. The slotted receptacle defines a slot that is sized to receive an alignment rod. The slot has opposing walls that are internally threaded. A set screw that threads into the opposing walls, wherein the set screw can be selectively advanced into the slot to an adjustment position and a deeper locked position.
A locking plate is interposed within the slot between the set screw and the alignment rod. The locking plate contains protrusions that engage the sections of buttress thread on the alignment rod when the protrusions are biased against the alignment rod by the set screw. The locking plate prevents the alignment rod from moving in more than one direction through the slot when the set screw is in its adjustment position. Furthermore, the locking plate prevents the alignment rod from moving through the slot in any direction when the set screw is in its locked position.
The threaded shaft of the pedicle screw has an upper section and a lower section. A threaded connecting rod interconnects the upper section and the lower section. The threaded connecting rod is exposed to different degrees when the threaded shaft is adjusted in length and the lower section is turned relative to the upper section. The upper section, lower section and threaded connecting rod all have external threads of the same thread pitch.
For a better understanding of the present invention, reference is made to the following description of an exemplary embodiment thereof, considered in conjunction with the accompanying drawings, in which:
Although the present invention system can be embodied in many ways, only one exemplary embodiment is illustrated. The exemplary embodiment is being shown for the purposes of explanation and description. The exemplary embodiment is selected in order to set forth one of the best modes contemplated for the invention. The illustrated embodiment, however, is merely exemplary and should not be considered a limitation when interpreting the scope of the claims.
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A locking plate 42 is provided as part of the anchor head assembly 18. The locking plate 42 is sized to fit into the slot 38 between the opposing walls 34, 36 atop the alignment rod 14. The locking plate 42 has a top surface 41 and an underlying bottom surface 43. Buttress protrusions 44 are disposed on the bottom surface 43. The buttress protrusions 44 are sized and spaced to engage the buttress threads 24 on the top surface 20 of the alignment rod 14. A set screw 46 is also provided. The set screw 46 screws into the threading 35 on the opposing walls 34, 36. As the set screw 46 is tightened, the set screw 46 presses against the top surface 41 of the locking plate 42. This biases the locking plate 42 against the alignment rod 14.
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The first open end 56 of the upper section 52 is affixed to the slotted receptacle 30. The first open end 56 is accessible through the bottom 40 of the slotted receptacle 30. In this manner, an Allen key or similar tool can be inserted into the interior of the upper section 52 though the bottom of the slotted receptacle 30.
The second lower section 54 of the shaft assembly 16 is solid. The lower section 54 has a first end 74 and an opposite second end 76. The threaded connection rod 70 extends from the first end 74 of the lower section 54. The threaded connection rod 70 has a smaller diameter than does the lower section 54. As such, there is a stepped transition 75 at the point of attachment. The threaded connection rod 70 has external threads 72. The external threads 72 match the pitch, thread angle and diameter of the interior threads 64 on the interior surface 60 of the upper section 52. In this manner, the upper section 52 of the shaft assembly 16 threads onto the threaded connection rod 70. A keyed depression 80 is formed in the threaded connection rod 70. This keyed depression 80 can be accessed through the slotted receptacle 30. It will therefore be understood that a surgeon can advance a tool through the slotted receptacle 30 and the upper section 52. The tool can be used to turn the threaded connection rod 70 and the lower section 54 relative the upper section 52. As the lower section 54 turns relative to the upper section 52, the external threads 72 on the threaded connection rod 70 engage the interior threads 64 within the upper section 52. Accordingly, the turning of the threaded connection rod 70 and the lower section 54 will cause the lower section 54 and the upper section 52 to either move together or spread apart, depending upon the direction of rotation. As the lower section 54 and the upper section 52 move apart, the threaded connection rod 70 becomes more exposed and the overall length of the shaft assembly 16 increases. Conversely, as the lower section 54 and the upper section 52 thread together, the exposed length of the threaded connection rod 70 decreases as does the overall length of the shaft assembly 16.
The lower section 54 of the shaft assembly 16 has exterior threads 82. The exterior threads 82 match the pitch, thread angle and diameter of the exterior threads 66 on the upper section 52. Furthermore, when the lower section 54 is fully seated into the upper section 52, the exterior threads 82 of the lower section 54 seamlessly align with the exterior threads 66 on the upper section 52, as though the two sections 52, 54 were threaded as a single piece.
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As the length of the pedicel screw 12 is increased, the threaded connection rod 70 becomes increasingly exposed within the vertebral body 55. However, the threaded connection rod 70 has the same thread pitch as is used on the exteriors of both the lower section 54 and the upper section 52. Accordingly, all exposed threading follows a single thread path through the vertebral body 55. Over time, bone grows against the pedicle screw 12. By utilizing a single pitch thread across all exposed threading, the pedicle screw 12 can be removed from the bone with far less damage to the bone since all threading follows a single thread path.
Once a pedicle screw 12 is implanted, the slotted receptacle 30 is oriented and adjusted to the proper elevation, via length adjustments to the pedicle screw 12. The surgeon selects the proper alignment rod 14 and sets the alignment rod 14 into the slotted receptacle 30 of the pedicle screw 12. The locking plate 42 and set screw 46 are installed above the alignment rod 14. The set screw 46 is advanced to an adjustment position where the locking plate 42 only engages the alignment rod 14 in one direction. The surgeon can therefore pull and tension the alignment rod 14 with one hand. Once the alignment rod 14 is properly tensioned, the alignment rod 14 can be released by the surgeon. The surgeon can then fully advance the set screw 46 form the adjustment position to a locked position, wherein the alignment rod 14 is locked in place.
It will be understood that the embodiment of the present invention that is illustrated and described is merely exemplary and that a person skilled in the art can make many variations to that embodiment. All such embodiments are intended to be included within the scope of the present invention as defined by the claims.
