|Year : 2020 | Volume
| Issue : 1 | Page : 114-117
Larger screw on the concave side of apex pedicle: Friend or foe? Report on a rare cause of neurological deficit in scoliosis surgery
Bhavuk Garg1, Nishank Mehta1, Ashok Jaryal2
1 Department of Orthopaedics, All India Institute of Medical Sciences, New Delhi, India
2 Department of Physiology, All India Institute of Medical Sciences, New Delhi, India
|Date of Submission||06-Feb-2019|
|Date of Decision||27-May-2019|
|Date of Acceptance||12-Aug-2019|
|Date of Web Publication||05-Feb-2020|
Dr. Nishank Mehta
Dr. Nishank Mehta, Department of Orthopaedics, All India Institute of Medical Sciences, Ansari Nagar, New Delhi 110029.
Source of Support: None, Conflict of Interest: None
Pedicle screw instrumentation is currently the “gold standard” in scoliosis surgery. However, placement of pedicle screws in thoracic spine is considered challenging. Previous studies have described morphometric changes in the pedicle when a larger screw is inserted, with pedicle expansion preceding screw cutout and pedicle fracture. We report an unusual case of neurological deficit due to cord compression by an expanded pedicle following pedicle screw insertion on the concave side of the apical vertebra in a 14-year-old patient with Lenke Type 3C(-) adolescent idiopathic scoliosis. Identification of the expanded pedicle as the cause of neurological symptoms, prompt action, and deferring the corrective surgery while accepting less correction helped us in negotiating the problem without causing permanent neurological deficit. Pedicle expansion in an immature spine can cause neurological complications. The screw diameter at the apical vertebrae on the concave side should be carefully selected.
Keywords: Neuromonitoring, pedicle expansion, pedicle screws, scoliosis
|How to cite this article:|
Garg B, Mehta N, Jaryal A. Larger screw on the concave side of apex pedicle: Friend or foe? Report on a rare cause of neurological deficit in scoliosis surgery. Indian Spine J 2020;3:114-7
|How to cite this URL:|
Garg B, Mehta N, Jaryal A. Larger screw on the concave side of apex pedicle: Friend or foe? Report on a rare cause of neurological deficit in scoliosis surgery. Indian Spine J [serial online] 2020 [cited 2020 Feb 27];3:114-7. Available from: http://www.isjonline.com/text.asp?2020/3/1/114/277812
| Introduction|| |
Segmental pedicle screw fixation has numerous advantages over other fixation methods in scoliosis surgery: better screw pullout strength, greater three-dimensional correction, and decreased need for an anterior release. The placement of pedicle screws in the thoracic spine is considered challenging and controversial in view of the smaller pedicle size, complex anatomy, and proximity of visceral, vascular, and neural structures.,
Increasing a screw’s diameter can improve the pullout strength. Morphometric changes in the form of pedicle expansion, pedicle cutout by screw threads, and pedicle fracture have been described when larger screw sizes were used. Viscoelastic expansion of the pediatric pedicle up to 196% of its transverse internal diameter before failure has been reported. We report an unusual case of neurological deficit due to cord compression by an expanded thoracic pedicle following screw insertion.
| Case Presentation|| |
Our patient was a 14-year-old male with Lenke Type 3C(-) adolescent idiopathic scoliosis curve with a Cobb’s angle of 75° [Figure 1] without any neurological deficit. Magnetic resonance imaging showed no cord anomaly.
|Figure 1: Preoperative radiographs of the patient showing a Lenke Type 3C(-) curve with Cobb’s angle = 75°|
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The patient was operated in a prone position with the use of intraoperative neuromonitoring. The implant used for the surgery was MESA Deformity Spinal System (K2M Inc., Leesburg, VA). After exposure from T3 to L5 through a midline approach, baseline neuromonitoring was recorded. Our freehand technique includes a preoperative assessment of individual pedicle morphometry on computed tomography (CT) scans. During insertion, the inferior 3–5mm of the inferior facet was osteotomized and articular cartilage on the dorsal side of the superior facet was completely removed. The ventral projection of the pedicle was closely related to the base of the superior articular process. The “funnel” technique described by Gaines was used to visualize the pedicle’s cancellous bone. Pedicle screws were inserted first on the right (convex) side and later on the left (concave) side. Evoked potentials at the culmination of pedicle screw insertion on each side were normal. Peri-apical Ponte’s osteotomies (D5-D9) were performed, cobalt-chrome rods were pre-contoured to match the thoracic kyphosis, and lumbar lordosis was prepared after the application of MESA crickets.
On applying the left (concave) rod, there was a significant drop in the neuromonitoring signals on the left side. The rod was immediately removed—only for the signals to return again. A single 100-mg dose of hydrocortisone was given and after 15min, the rod was reinserted, again leading to a dip in signals. A similar drop in signals from the left side was encountered when we attempted to insert the rod on the right (convex) side, which recovered when the rod was removed. As Ponte’s osteotomy increases the segmental mobility and considering that the patient had a drop in the neuromonitoring signals, we felt that in situ temporary stabilization across the osteotomies was the best option. Hence, we spanned the osteotomies with two short rods and deferred the deformity correction [Figure 2].
|Figure 2: Postoperative radiographs after first surgical procedure showing two short rods spanning the osteotomy sites|
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Initial postoperative assessment revealed no neurological deficit. On giving skull traction using Crutchfield tongs, there was a clinically documented fall of power in left lower limb, which improved with the removal of traction. An urgent CT scan revealed no screw malpositioning except dubious Grade-1 medial cortical perforation at the concave side of the apical vertebrae (D8 and D9) [Figure 3]. In view of the recurring neurological deficit, we took the patient for emergency surgery wherein the D6, D8, and D9 pedicle screws on the concave side were removed and their medial pedicle walls were confirmed to be intact on probing. Laminectomy was conducted at the concave side of the apex (D8-D9) to assess the suspected medial wall perforation. Although the medial wall was intact, it was significantly expanded—visualized as bulging of the pedicle indenting the spinal cord. Apex concave pedicle was drilled and removed. Rod application and deformity correction were performed in the same sitting with no further drop of neuromonitoring signals. We accepted less correction though the coronal and sagittal balance of the spine was restored [Figure 4]. The patient was mobilized with a brace on the third postoperative day and discharged a week later. Follow-up assessments revealed significant correction of the deformity.
|Figure 3: Postoperative computed tomography scan images showing dubious Grade-1 medial cortical breech in D8, D9 pedicle screws|
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|Figure 4: Postoperative radiographs and clinical photograph after the second and final surgical procedure|
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| Discussion|| |
Surgeons are tempted to insert larger screws at apex for better purchase and correction of deformity. Pedicles in mid-thoracic region in young patients, in particular, can undergo significant expansion to accommodate a larger screw. Pedicle screw placement produces strain and plastic deformation in the pedicle cortex. Misenhimer et al. noted expansion in 97.3% of the pedicles examined when the screw size exceeded 80% of the pedicle’s outer cortical diameter. Pedicle expansion preceded screw cutout and wall fracture in all cases. The lateral and medial cortices responded differently to screw insertion with the lateral cortex generating higher strains. Prior insertion and removal of a smaller screw decreased the plastic deformation, suggesting that pilot hole preparation affects pedicle biomechanics.
The transverse pedicle diameter of the mid-thoracic pedicles (T4-T9) is characteristically the smallest—varying from 4.7 to 6.1mm with the concave pedicle in scoliotic spines being smaller. Many pedicles have a tear-drop shape; thus, superolateral screw placement runs the risk of causing pedicle expansion or pedicle fracture. Cadaveric studies have established that the spinal cord directly abuts the medial aspect of the pedicle. Although screw malposition is not uncommon, neurological complications are rare even with medial transgression of up to 4mm. However, an expanded pedicle may act as a tether to the abutting spinal cord, rendering it more susceptible to injury during deformity correction maneuvers.
Major and permanent neurological injuries have only been reported in anecdotal case reports.,, We document a previously unreported cause of neurological deficit during scoliosis correction surgery. The apical vertebrae on the concave side of deformity are most likely to undergo pedicle expansion. The recommendation by Fujimoto et al. of limiting the screw diameter to at least 0.5mm smaller than the outer pedicle width for safe insertion seems reasonable. Considering the safety margin medially of 2–4mm, one might be drawn to insert a larger screw in the pedicle of the apical vertebrae. However, as this case shows, pedicle expansion in an immature spine can occur to the extent of causing neurological complications. Because of the metallic artefact/blush of the screw, the expansion could not be appreciated on postoperative CT. In fact, we assumed that there was a Grade-1 medial cortical breech and hence decided to reexplore. However, during reexploration, after removal of the screw, probing revealed the medial wall to be intact. After performing laminotomy and decompressing the cord, we directly observed the medial pedicle wall to be intact, albeit expanded and indenting on the cord. Prompt action, identification of the problem, and deferring the corrective surgery while accepting less correction helped us negotiate the problem without causing permanent deficit.
Informed consent was obtained from the patient for inclusion in this study.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]