Fig. 33.1
(a) Transverse pedicle diameters. (b) Superoinferior pedicle diameters. (c) Anteroposterior pedicle angles. (d) Horizontal pedicle angles
In children, the pedicles are smaller, but their relative dimensions and orientations are similar to those found in adults. As the spinal canal reaches YU% of adult size at the time of birth and reaches the adult size by the age of 2 [12], pedicle screw fixation may be carried out without dangers of causing iatrogenic spinal stenosis after this age. Though the pedicles are very small in pediatric patients, their bone is very plastic and the pedicles usually receive a screw larger than the outer diameter of the specific pedicle by plastic deformation of the pedicle cortex when the screw is inserted slowly through the center of the pedicle provided that the pedicular cortex is not violated [13, 14].
Biomechanics
Among the many advantages the greatest offered by the pedicle screw is rigid fixation, far superior to non-pedicle fixations [4–6] However, obtaining such a reliable, rigid fixation is possible only through a sound technique, with a thorough understanding of the variables which may affect the strength of the fixation. The influence of various screw related and insertion technique related parameters on the rigidity of the fixation offered by a pedicle screw has been extensively studied by nondestructive biomechanical studies and studies measuring the pullout strength of pedicle screws.
Pedicle Screw Diameter
Generally, the pullout strength of a pedicle screw increase with increasing major diameter of the screw as long as the integrity of the pedicle is not violated [15, 16]. The pedicles usually accepted screws with diameters less than 86 % of the isthmic outer diameter of the pedicle without significant structural alterations [10]. A screw of a larger diameter may cause a linear fracture of the pedicles, but the pullout strength is not significantly affected. The pedicles are capable of receiving a screw up to 116 % of the isthmic diameter without significant change in the pullout strength. Though some speculate that a screw with a diameter of 3.5–4 s is sufficient to resist any pullout force generated in the human body, we believe it is much more reliable to use a screw with a diameter of approximately 80 % of the pedicle diameter; this offers maximum pullout strength.
Screw Length
Theoretically, the pullout strength of a pedicle screw increases with increasing depth of insertion as the surface area of contact increases. Some even advocate penetration of the anterior vertebral cortex to obtain a bicortical fixation to increase the pullout strength [17]. However, there have been contradictory studies stating that the pullout strength is not significantly affected by the depth of insertion when the screw passes deeper than the posterior one half of the vertebral body [16, 18, 19]. The authors agree with the latter view and advocate using screws that penetrate .0-.030 of the anteroposterior vertebral diameter. This apparent paradox is attributed to the presence of a dense sheet of cortical bone, the so called neurocentral junction [19], situated in the posterior one third of the vertebral body at the site of former neurocentral synchondrosis, that makes biomechanical bicortical fixation possible (Fig. 33.2). The neurocentral junction marks the site of union between the centrum and the two neural arches which develop from separate primary ossification centers and unite at the age of 3–6 years.
Fig. 33.2
Neurocentral junction
Screw Direction
Except in the sacrum where the pedicles are large enough to allow a significant alteration in the directions of the pedicle screws to engage the medial anterior cortex, the upper sacral end plate or the ala, the optimal direction of a pedicle screw seems to be along the axis of the pedicle. Though this does not significantly affect the screw pullout strength in most situations, it reduces the chance of pedicular cortical perforations.
Screw Hole Preparations
Pedicle screws offer greatest pullout strength when the diameter of the screw holes are as large as the minor diameter of the screws inserted, approximately 60 % of the pedicular isthmic outer diameter [14]. When the diameter of the holes are smaller, screw insertion is not only difficult but may also cause pedicle fractures. Making a hole of a larger diameter in the pedicle may cause breakage of the pedicle cortex during the hole preparation procedure and reduces the pullout strength [14, 20]. As to the method of making the screw holes, there was no significant difference in the pullout strength between holes prepared by drilling and probing [20, 21]. However, probing seems to reduce the chance of damage to the pedicular cortex.
Number of Screws
The rigidity of fixation increases with increasing number of screws, thus being most rigid in segmental instrumentation where screws are inserted in every segment fused [5, 22]. In a nondestructive study using porcine vertebral columns, the segmental screw fixation construct was significantly stiffer than the nonsegmental screw construct in all the parameters of flexion, extension, lateral bending, and torsion [23] Though there had been enthusiasm to reduce the instrumentation to one side (unilateral instrumentation technique), the authors advocate bilateral instrumentation to enhance resistance to torsional forces.
Transverse Links
The audition of transverse links to bilateral segmental constructs significantly increases the stability in axial rotation [24–26]. A transverse link is more effective if placed in the proximal part of the construct than if placed distally. Two transverse links show increased torsional stiffness than one transverse link especially in longer constructs.
Methods of Increasing the Stiffness of the Pedicle Screw Construct
When posterior instrumentation with pedicle screws is performed on an unstable spine with an anterior column defect, the lack of normal anterior support significantly reduces the flexion/extension and torsional stiffness of the pedicle screw construct even when a very rigid implant is used [27, 28]. In this situation the stability of the pedicle screw construct may be enhanced by restoration of the anterior defect, by expanding the level of instrumentation, or by the use of more rigid external immobilization postoperatively
Treatment Considerations and Indications
The ultimate goals of treatment for a spinal fracture are restoration of the neurologic and the mechanical stability of the injured spine with minimal sacrifice of motion segments. This same principle applies to the pedicle screw fixation. When restoration of the spinal stability with pedicle screw fixation is contemplated for a thoracic or thoracolumbar burst fracture, both the neurologic and the mechanical aspects need to be considered before finally deciding on the combination of available surgical techniques to successfully accomplish the goals.
As regards biomechanical aspects, the degree of vertebral body comminution and the magnitude of local kyphotic deformity are the major determinants. Though modern biomechanics understand the intact spine as a tricolumnar structure composed of anterior, middle, and the posterior column, the biomechanics concerning the surgical reconstruction of a burst fracture considers the spine basically as a bicolumnar structure, composed of an anterior weight-bearing column and the posterior tension column, whose stability is essentially dependent on the competence of the anterior column. This means that when there is residual, unattended incompetence of the anterior column, the spine will continue to be unstable and is ultimately doomed to fail under repeated loading. This fact is to be always kept in mind in all posterior instrumentation and fusions for thoracic and thoracolumbar burst fractures, including pedicle screw fixation, which inherent y restores or reinforces the posterior column but leaves the essential anterior column untouched, relying for its restoration on the bony healing of the fractured vertebral body The essence of the advantage offered by rigid fixation with pedicle screw fixation lies in the fact that the fractured vertebral body is more effectively protected from the detrimental forces until the fracture union occurs and becomes stable under physiologic loads. By the same token, when the anterior column incompetence is so severe that it is not expected to become a competent weight-bearing structure by itself, a deliberate restoration of the anterior column is warranted. When a significant local kyphosis or comminution of the vertebral body is present, a mere posterior reduction and stabilization with pedicle screw fixation will create an anterior unsupported gap. In the presence of such a gap, the posterior constructs are destined to fail, however stiff they may be, not being able to withstand the flexion moment concentrated at the fracture site devoid of the anterior load sharing structural support [27, 28]. in these situations, elimination of the anterior defect either by restoration of the anterior column or by shortening of the posterior column is warranted. Anterior column reconstruction may be done by an interbody fusion via an anterior or a posterior route, or transpedicular bone grafts.
Posterior Pedicle Screw Fixation
Pedicle screw fixation is applicable and is indicated in all thoracic and thoracolumbar burst fractures in which a posterior fixation and fusion is under consideration. Since it offers a rigid fixation with enhanced segmental control, it is more advantageous than hooks and other nonpedicle fixation devices in the aspect that it offers a better restoration of the spinal sagittal contour and makes a shorter fusion feasible, saving more motion segments [29]. Moreover, as pedicle screw fixation offers a more reliable fixation in the osteoporotic spine and in vertebra with previous laminectomy, they are particularly advantageous in these situations.
Though there are still questions about the safety of pedicle screws in the thoracic and thoracolumbar region, when correctly placed in the pedicles, the screws stay out of the spinal canal and remain insulated from the neural elements by the surrounding pedicular bone, precluding neurologic derangement by the device. The authors have been using pedicle screws in the thoracic spine since 1988 in thousands of patients with various conditions including the most severe deformities and revision surgeries, inserting probably more than 20,000 screws. Yet, there was not a single major neurologic or visceral complication attributable to the pedicle screw fixation per se and the authors believe the procedure to be perfectly safe when performed with a sound technique [5, 30–32]
The indications of a simple posterior stabilization with pedicle screw fixations in thoracic and thoracolumbar burst fractures are:
Cord level fractures with complete paraplegia: Since saving the motion segments is not a crucial problem in this situation, a lengthy fusion with pedicle screws which increases the stiffness of the construct is a suitable choice as it allows an early mobilization without an external support.
Unstable burst fractures without neurologic compromise when they satisfy all of the following
Spinal canal encroachment <60 % on axial CT or MRI.
Local kyphosis <5O degrees.
Without significant comminution of the vertebral body
Fracture less than 72 h old.
As these fractures will heal eventually to offer an anterior support, a mere posterior ligamentotaxis and protection from the detrimental forces will suffice (Fig. 33.3a–g).
Fig. 33.3
(a, b) A 50-year-old female with L1 unstable burst fracture. There was a 20° kyphotic deformity at the thoracolumbar junction. (c) Preoperative CT show 40 % canal encroachment. (d, e) She was treated by posterior fusion with pedicle screw fixation from T11 to L2. Postoperatively kyphosis was corrected to 10°. (f) Postoperative CT show canal encroachment reduced to 10 %. (g) Ligamentotaxis
Fractures with neurologic compromise when they satisfy all of the following.
Spinal canal encroachment <40 % on axial CT or MRI.
Local kyphosis <50°.
Without significant comminution of the vertebral body.
Fracture less than 72 h old.
In these fractures, a posterior ligamentotaxis may be tried. When there is no neurologic improvement, an additional direct decompression of the neural element is necessary (Fig. 33.4a–h).
Fig. 33.4
(a, b) A 32-year-old female with L1 unstable burst fracture at vertebra. Preoperative neurology was intact except for bowel and bladder control. There was a 30° kyphotic deformity at the thoracolumbar junction. (c) Preoperative CT show 60 % canal encroachment. (d, e) She was treated by the posterior fusion with pedicle screw fixation from T11 to L2. Following surgery, kyphosis was corrected to 12°. (f) Postoperative CT show canal encroachment reduced to 20 %, but she had no neurologic improvement. (g–h) One week later after the posterior surgery, anterior decompression was carried out. Following the anterior surgery, her bladder control improved. Postoperative 1-year-follow-up radiographs show satisfactory maintenance of reduction
Established post-burst fracture kyphosis of less than 50°without significant neurologic compromise: Though the correction of kyphotic deformity is negligible, most patients do well without further progression of the deformity.
Senile burst fractures with pain and progressive deformity without significant neurologic compromise: These osteoporotic burst fractures are best stabilized with a pedicle screw fixation as the pedicles are affected less by the osteoporosis than the laminae. As most of these fractures are without significant canal compromise, an in situ stabilization is usually sufficient.
Posterior Pedicle Screw Fixation with an Anterior Column Reconstruction
In pedicle screw fixation, an additional anterior column reconstruction is indicated when there is significant anterior column incompetence which is not expected to heal sufficiently to function as a stable, weight-bearing structure as in fractures with significant kyphosis, severe vertebral body comminution, and following a corpectomy procedure for direct decompression of the neural elements. Though some favor an anterior stabilization and fusion in these situations, we believe a combined anterior and posterior column reconstruction with posterior pedicle screw fixation has several advantages over the anterior instrumentation/fusion only [33]. They are: 1) Better restoration of spinal alignment. When the kyphosis is severe, it is easier to restore a physiologic sagittal profile with a posterior pedicle screw fixation than with an anterior instrument. 2) More reliable rigid fixation. In osteoporotic patients, the pedicle screw fixation offers more reliable fixation than the anterior instruments holding the osteoporotic vertebral bodies. 3) increased versatility Pedicle screw fixation offers a rigid fixation in locations where anterior instrumentation is difficult or needs extensive dissection (e.g., cervicothoracic junction). Though an anterior column reconstruction is more commonly performed through an anterior approach, a reliable anterior column reconstruction is also feasible from the posterior, by transpedicular bone grafting [34], by interbody fusion using the costotransversectomy approach [35], or using a modified egg-shell procedure [36]. In the former, cancellous chip grafts are added into the fractured vertebral body and the damaged disc space through the pedicles of the fractured vertebra using a narrow funnel. In the latter, the anterior column may be supported by a structural graft or by autogenous cancellous graft tightly packed into the void in the anterior column. When local kyphosis is severe, a type of posterior closing wedge osteotomy may be performed through the posterior approach to obliterate the anterior unsupported gap. The advantage of the anterior column reconstruction front the posterior in conjunction with pedicle screw fixation is the feasibility of a global fusion through a single approach, saving the operative time and reducing the morbidity of an anterior thoracotomy or thoracolumbotomy [33]. It also permits a direct exploration and repair of the associated dural tear or nerve root entrapment. Its disadvantage is destruction of the relatively intact posterior column, reducing the posterior fusion base. However, this drawback may be overcome by one of the following methods; 1) Saving the lamina and pedicle on one side. 2) By adding a bridging bone graft. 3) By shortening the entire posterior column to achieve a bony contact between the two laminae above and below the laminectomy.
The indications for posterior stabilization with anterior column reconstruction are;
Presence of a neurologic deficit necessitating a formal decompression of the neural tissue:
Spinal canal encroachment more than 60 %.
Failure of the indirect decompression by ligamentotaxis
Fractures more these 72 h old.
Local kyphosis greater than 50°
Significant comminution of the vertebral body.
Established post-burst fracture kyphosis greater than 50° (Posterior or anterior fusion alone in this situation is more prone to failure and should be treated by a global fusion.) (Fig. 33.5a–f).
Fig. 33.5
(a, b) A 68-year-old male with a post-traumatic kyphosis and cauda equina syndrome. One and half years prior to the visit, he sustained a burst fracture which was treated by posterior decompression and fusion. (c) Preoperative MRI show local kyphosis with canal encroachment. (d, e) He was treated by the posterior vertebral column resection at L1 and fused from T11 to L2. Postoperative one-year-follow-up radiographs show satisfactory maintenance of sagittal balance. (f) Posterior vertebral column resection
Surgical Techniques
Presurgical Considerations
Anesthesia
For a posterior stabilization with pedicle screw fixation, a general anesthesia with a full monitoring of vital signs including the arterial and the central venous pressure is prefreed. As an anterior column reconstruction is often accompanied by a substantial amount of bleeding from the epidural vein and the cancellous bone of the fractured vertebral body, securing a large-bore central venous channel is highly recommended. Hypotensive anesthesia is especially helpful in cases where anterior column reconstruction is considered. When intraoperative-evoked potential monitoring is contemplated, intravenous anesthesia with fentanyl and propopol is preferable to inhalation anesthesia as they affect monitoring less severely than the latter: However intravenous anesthetics require a substantially longer time for recovery than inhalation anesthetics and may even require several hours of ventilator care in the recovery room.
Intraoperative Monitoring
As we have never had a major neurologic complication related to the pedicle screw placement and are perfectly confident of the safety, we do mat use intraoperative neurophysiologic monitoring except in cases in which the spinal cord is directly exposed. However, intraoperative monitoring of the neurologic function r5 a valuable aid to ensure the safety of the procedure and for keeping a record for possible medicolegal problems [37, 38]. Although the wake-up test is a reliable monitoring method, we prefer motor-evoked potential or somatosensary-evoked potential, which can be more conveniently performed without extending the operative time [39].
Positioning
The ideal position for a pedicle screw fixation is the standard prone position with the abdomen hung free by means of a pad, four posters, or a special surgical frame to reduce venous bleeding. The operating table should be X-ray penetrable to allow the intraoperative postero-anterior roentgenogram. When posterior column shortening is contemplated, care should to taken to place the osteotomy site over the hinge of the operating table so that closure of the posterior gap created by the osteotomy may be aided by extension of the operating table. For this procedure, we prefer to use roll pads which are flexible enough to allow extension of the spinal column with the table extension.
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