Spinal conditions such as degenerative disc disease or spondylolisthesis can cause signs and symptoms that include back or lower extremity pain, muscle spasms, weakness, dysfunction of bowel and/or bladder, and gait disturbance.
To correct these and other similar conditions of vertebral dislocation, the only effective long-term curative treatment may be achieved by fusion of the affected vertebra to its adjacent neighbor. Vertebral fusion is generally augmented by instrumentation, or fixing apparatus, to and between vertebrae. Transpedicular fixation is a particularly important process in the treatment of spinal conditions that require vertebral fusion. In addition to the stabilization and correction of spondylolisthesis, other spinal conditions may be treated by transpedicular fixation: stabilization of fractures, correction of spinal deformities (scoliosis, kyphosis), stabilization and correction of degenerative spinal lesions, reconstruction after tumor resection, and secondary spinal surgery.
In FIG. 1, a drawing of the human spine shows that the spinal column 1 is comprised of a number of vertebrae, categorized into four sections or types: the lumbar vertebrae 2, the thoracic vertebrae 3, the cervical vertebrae 4 and the sacral vertebrae 5. Starting at the top of the spinal column 1, the cervical vertebrae 4 are labeled 1st cervical vertebra (C1) through 7th cervical vertebra (C7). Just below the 7th cervical vertebra is the first of twelve thoracic vertebrae 3 labeled 1st thoracic vertebra (T1) through 12th thoracic vertebra (T12). Just below the 12th thoracic vertebrae 3, are five lumbar vertebrae 2 labeled 1st lumbar vertebra (L1) through 5th lumbar vertebra (L5), the 5th lumbar vertebra being attached to the sacral vertebrae 5 (S1 to S5), the sacral vertebrae 5 being naturally fused together in the adult.
Spinal fusion surgery typically involves the corrective fusion of lumbar vertebrae 2, of which a representative transverse drawing of such a vertebra is shown in FIG. 2. Representative lumbar vertebra 10 has a number of notable features which are in general shared with the thoracic vertebrae 3 and cervical vertebrae 4, although the feature thicknesses and shapes may alter between the various types of vertebrae. The thick oval segment of bone forming the anterior aspect of the vertebra 10 is the vertebral body 12. Vertebral body 12 is attached to a bony vertebral arch 13 through which the neural elements run. Vertebral arch 13, forming the posterior of vertebra 10, is comprised of two pedicles 14, which are short stout processes that extend from the sides of vertebral body 12, and two laminae 15, the broad flat plates that project from pedicles 14 and join in a triangle to form a hollow archway, the vertebral foramen 16 or spinal canal. The spinous process 17 protrudes from the junction of laminae 15: these are the ridges that can be felt through the skin along the back of the spine. Transverse processes 18 project from the junction of pedicles 14 and laminae 15. The structures of the vertebral arch protect the spinal cord and/or spinal nerves that run through the spinal canal.
The pedicles of typical lumbar vertebra increase in sagittal width from 9 mm to up to 18 mm at L5 (lowest lumbar vertebra). They increase in angulation in the axial plane from 10 degrees at L1 to 30 degrees by L5. Pedicles of the thoracic and cervical vertebra are typically smaller—as small as 4 mm in an adult. Pedicle widths in adolescents are proportionally smaller. The pedicles exhibit good mechanical strength in comparison to the other vertebral features.
Pedicles are used as a portal of entrance into the vertebral body for fixation with pedicle screws for placement of stabilizing rods or plates for fusion as shown in FIGS. 3a and 3b. Pedicles are used due to their strength, size and proximity to ease of entrance through the posterior side of the human body. In FIG. 3a, a transverse view of vertebra 10 is shown including pedicular holes 25 are drilled and tapped with pedicular screws 20 inserted into the vertebral body 12 through pedicles 14.
Typical pedicle screws are made of titanium alloy, are MRI compatible, and are highly resistant to corrosion and fatigue. Pedicle screws have length ranges from 30 mm to 60 mm. The threaded (major) diameter ranges from 4.5 mm to 8.5 mm.
The transpedicular fixation process is accomplished by placing pedicle screws into the pedicular region of adjacent vertebrae, as in FIG. 3a, and attaching rods between the pedicle screws to stabilize the vertebrae with respect to each other. In FIG. 3b, a second vertebral structure, in this case a sacral vertebrae S1 is shown to be connected to L5 vertebra through a set of pedicle screws 23 inserted into L5, a set of pedicle screws 21 inserted into S1; the pedicle screws 23 and 21 being interconnected by a pair of rods 22. Rods 22 may be further connected to each other by a transverse rod 24. The transpedicular fixation is formed by pedicle screws 23, pedicle screws 21, rods 22 and transverse rod 24 which fixes the vertebra L5 and S1 with respect to each other, thereby allowing the vertebrae to fuse in the healing process. If not placed properly, the pedicle screws 23 (or pedicle screws 21) may breach the vertebral walls thereby leading to nerve root damage or pressure on the spinal cord. The mechanical integrity of the screw placement must also be sufficient to support the rod structures and remain intact under mechanical stress. The present invention is an apparatus and method that significantly improves the placement of pedicle holes (and screws) so that vertebral walls are not breached and also promotes mechanical integrity and avoidance of neurological injury.
The method of placement of pedicle screws and fixtures in the prior art is typically done under open surgical operation where the patient's spine is exposed. More recently, percutaneous placement of pedicle screws and fixtures has become more commonplace, causing less tissue damage and allowing for more rapid healing. During each prior art method, a pilot hole, using a pedicle awl or drill, is carefully made through each pedicle and into the vertebral body. The hole is threaded and a pedicle screw inserted.
The position and angle of the pilot hole is crucial to a successful pedicle screw placement and can be difficult to achieve, especially in percutaneous operations. If the pilot hole breaches the pedicle wall, spinal nerve damage may occur accompanied by chronic pain in the patient. Worse yet, if the pedicle wall is breached medially in the cervical or thoracic spine, permanent spinal cord damage may result. Also, a breach will typically weaken the mechanical integrity of the pedicle screw fixture.
In the prior art, the ability to accurately judge the integrity of the pedicle is generally limited to post-operative observation. For example, pedicle screw misplacements are usually detected only when pain or neurological deficit is reported by the patent. Corrective surgery to reposition the malpositioned pedicle screw is expensive, carries inherent medical risks, and may not reverse the neurological deficit or pain.
Radiographic imaging and computer-assisted methods have been developed in the prior art to increase the probability of success in the operative process. For example, fluoroscopic imaging, essential to percutaneous pedicle screw placement, is used to take lateral and anterior-posterior images of the vertebrae and to guide wire placement. After guide wires have been inserted through the pedicle and into the vertebral body, a pilot hole is made using a cannulated pedicle awl. But placement errors still occur. While flouroscopic-assisted pedicle screw guidance decreases the risk of misplacement such equipment is expensive and is not universally available. Hence the methods developed in the prior art have not been entirely successful.
Referring to FIGS. 4a and 4b, it is known in the prior art to stimulate and detect electromyographic (EMG) signals generated from nerve roots that course along the outer surface of the pedicles. In such a prior art method an electrostatic pedicle awl 100 is charged with a current source 105. The current supplied from the current source creates an electric field 125 near the tip of the electrostatic awl which in turn causes charge migrations in nerve ion channels in the vicinity of the tip area shown as the region of nerve excitation 130.
In use, the electrostatic pedicle awl 100 of the prior art is initially positioned by sight or by fluoroscopic imaging onto a pedicle 120 of vertebra 115 and then rotated using a removable handle 110 and lightly tapped with a hammer (not shown) to create the pilot hole. An EMG 150 is taken of muscles that respond to specific nerves in the vicinity of the pedicle wall 122. As electrostatic pedicle awl 100 moves into pedicle 120, EMG 150 is monitored for nerve excitations. If the tip of electrostatic pedicle awl 100 or its associated electric field 125 breaches the pedicle wall 122, nerve channels are excited and the EMG reacts, generating signals that alert the surgeon to the breach. In practice, the surgeon then withdraws electrostatic pedicle awl 100 and either repositions it or abandons the site altogether.
U.S. Pat. No. 6,796,985 to Bolger, et al. essentially describes an electrostatic pedicle awl with special emphasis on the detection of signals from muscles during the drilling process and operational aspects of generating alerts.
One shortcoming of the prior art electrostatic pedicle awl is that the electric field generated is not well contained. The lack of containment of the electric field often results in false readings. For example, in cases where the pedicle has been previously breached and the awl has been redirected, EMG signals may persist despite a corrected trajectory. Inaccurate EMG signals often lead to false placement failure and unnecessary alternative fixation procedures. Unnecessary procedures cause higher patient morbidity and potential liability.
These similar problems exist in other prior art surgical procedures in which avoidance of nerves is critical, including the extreme lateral interbody fusion procedure (“XLIF procedure”). The XLIF procedure is a method of achieving a direct lateral approach to the intervertebral disc space through the psoas muscle. To perform the XLIF procedure, it is necessary to insert a retractor through the muscle, while avoiding the nerves, to provide an operative corridor to the spine and intervertebral disc space. Once access is achieved, pathology may be addressed as well as insertion of interbody implants.
The psoas muscles, crucial for hip flexion, are found in the lumbar region and are anchored on either side of the spine. The muscles extend into the pelvic area and attach to the hip. Critical nerve roots course within the psoas muscle and must be avoided during surgery to avoid damage and resulting pain or paralysis.