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Scott L. Parker, Anubhav G. Amin, S. Harrison Farber, Matthew J. McGirt, Daniel M. Sciubba, Jean-Paul Wolinsky, Ali Bydon, Ziya L. Gokaslan and Timothy F. Witham

Object

Pedicle screws provide efficient stabilization along all 3 columns of the spine, but they can be technically demanding to place, with malposition rates ranging from 5% to 10%. Intraoperative electromyographic (EMG) monitoring has the capacity to objectively identify a screw breaching the medial pedicle cortex that is in proximity to a nerve root. The purpose of this study is to describe and evaluate the authors' 7-year institutional experience with intraoperative EMG monitoring during placement of lumbar pedicle screws and to determine the clinical utility of intraoperative EMG monitoring.

Methods

The authors retrospectively studied 2450 consecutive lumbar pedicle screws placed in 418 patients from June 2002 through June 2009. All screws were inserted using a free-hand technique and anatomical landmarks, stimulated at 10.0 mA, and evaluated with CT scanning within 48 hours postoperatively. Medial pedicle screw breach was defined as having greater than 25% of the screw diameter extend outside of the pedicle, as confirmed on CT scanning or intraoperatively by a positive EMG response indicating a medial breach. The sensitivity and specificity of intraoperative EMG monitoring in detecting the presence of a medial screw breach was evaluated based on the following definitions: 1) true positive (a positive response to EMG stimulation confirmed as a breach intraoperatively or on postoperative CT scans); 2) false positive (positive response to EMG stimulation confirmed as a correctly positioned screw on postoperative CT scans); 3) true negative (no response to EMG stimulation confirmed as a correctly positioned screw on postoperative CT scans); or 4) false negative (no response to EMG stimulation but confirmed as a breach on postoperative CT scans).

Results

One hundred fifteen pedicle screws (4.7%) showed positive stimulation during intraoperative EMG monitoring. At stimulation thresholds less than 5.0, 5.0–8.0, and > 8.0 mA, the specificity of a positive response was 99.9%, 97.9%, and 95.9%, respectively. The sensitivity of a positive response at these thresholds was only 43.4%, 69.6%, and 69.6%, respectively. At a threshold less than 5.0 mA, 91% of screws with a positive EMG response were confirmed as true medial breaches. However, at thresholds of 5.0–8.0 mA or greater than 8.0 mA, a positive EMG response was associated with 89% and 100% false positives (no breaches), respectively.

Conclusions

When using intraoperative EMG monitoring, a positive response at screw stimulation thresholds less than 5.0 mA was highly specific for a medial pedicle screw breach but was poorly sensitive. A positive response to stimulation thresholds greater 5.0 mA was associated with a very high rate of false positives. The authors' experience suggests that pedicle screws showing positive stimulation below 5.0 mA warrants intraoperative investigation for malpositioning while responses at higher thresholds are less reliable at accurately representing a medial breach.

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Hormuzdiyar H. Dasenbrock, Courtney Pendleton, Matthew J. McGirt, Daniel M. Sciubba, Ziya L. Gokaslan, Alfredo Quiñones-Hinojosa and Ali Bydon

At the beginning of the 20th century, the development of safer anesthesia, antiseptic techniques, and meticulous surgical dissection led to a substantial decrease in operative risk. In turn, the scope of surgery expanded to include elective procedures performed with the intention of improving the quality of life of patients. Between 1908 and 1912, Harvey Cushing performed 3 dorsal rhizotomies to improve the quality of life of 3 patients with debilitating neuralgia: a 54-year-old man with “lightning” radicular pain from tabes dorsalis, a 12-year-old boy cutaneous hyperesthesia and spasticity in his hemiplegic arm, and a 61-year-old man with postamputation neuropathic pain. Symptomatic improvement was seen postoperatively in the first 2 cases, although the third patient continued to have severe pain. Cushing also removed a prominent spinous process from each of 2 patients with debilitating headaches; both patients, however, experienced only minimal postoperative improvement. These cases, which have not been previously published, highlight Cushing's views on the role of surgery and illustrate the broader movement that occurred in surgery at the time, whereby elective procedures for quality of life became performed and accepted.

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Risheng Xu, Matthew J. McGirt, Edward G. Sutter, Daniel M. Sciubba, Jean-Paul Wolinsky, Timothy F. Witham, Ziya L. Gokaslan and Ali Bydon

Object

The aim of this study was to conduct the first in vitro biomechanical comparison of immediate and postcyclical rigidities of C-7 lateral mass versus C-7 pedicle screws in posterior C4–7 constructs.

Methods

Ten human cadaveric spines were treated with C4–6 lateral mass screw and C-7 lateral mass (5 specimens) versus pedicle (5 specimens) screw fixation. Spines were potted in polymethylmethacrylate bone cement and placed on a materials testing machine. Rotation about the axis of bending was measured using passive retroreflective markers and infrared motion capture cameras. The motion of C-4 relative to C-7 in flexion-extension and lateral bending was assessed uninstrumented, immediately after instrumentation, and following 40,000 cycles of 4 Nm of flexion-extension and lateral bending moments at 1 Hz. The effect of instrumentation and cyclical loading on rotational motion across C4–7 was analyzed for significance.

Results

Preinstrumented spines for the 2 cohorts were comparable in bone mineral density and range of motion in both flexion-extension (p = 0.33) and lateral bending (p = 0.16). Lateral mass and pedicle screw constructs significantly reduced motion during flexion-extension (11.3°–0.26° for lateral mass screws, p = 0.002; 10.51°–0.30° for pedicle screws, p = 0.008) and lateral bending (7.38°–0.27° for lateral mass screws, p = 0.003; 11.65°–0.49° for pedicle screws, p = 0.03). After cyclical loading in both cohorts, rotational motion over C4–7 was increased during flexion-extension (0.26°–0.68° for lateral mass screws; 0.30°–1.31° for pedicle screws) and lateral bending (0.27°–0.39° and 0.49°–0.80°, respectively), although the increase was not statistically significant (p > 0.05). There was no statistical difference in postcyclical flexion-extension (p = 0.20) and lateral bending (0.10) between lateral mass and pedicle screws.

Conclusions

Both C-7 lateral mass and C-7 pedicle screws allow equally rigid fixation of subaxial lateral mass constructs ending at C-7. Immediately and within a simulated 6-week postfixation period, C-7 lateral mass screws may be as effective as C-7 pedicle screws in biomechanically stabilizing long subaxial lateral mass constructs.

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Oral Presentations

2010 AANS Annual Meeting Philadelphia, Pennsylvania May 1–5, 2010

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Scott L. Parker, Risheng Xu, Matthew J. McGirt, Timothy F. Witham, Donlin M. Long and Ali Bydon

Object

The most common spinal procedure performed in the US is lumbar discectomy for disc herniation. Longterm disc degeneration and height loss occur in many patients after lumbar discectomy. The incidence of mechanical back pain following discectomy varies widely in the literature, and its associated health care costs are unknown. The authors set out to determine the incidence of and the health care costs associated with mechanical back pain attributed to segmental degeneration or instability at the level of a prior discectomy performed at their institution.

Methods

The authors retrospectively reviewed the data for 111 patients who underwent primary, single-level lumbar hemilaminotomy and discectomy for radiculopathy. All diagnostic modalities, conservative therapies, and operative treatments used for the management of postdiscectomy back pain were recorded. Institutional billing and accounting records were reviewed to determine the billed costs of all diagnostic and therapeutic measures.

Results

At a mean follow-up of 37.3 months after primary discectomy, 75 patients (68%) experienced minimal to no back pain, 26 (23%) had moderate back pain requiring conservative treatment only, and 10 (9%) suffered severe back pain that required a subsequent fusion surgery at the site of the primary discectomy. The mean cost per patient for conservative treatment alone was $4696. The mean cost per patient for operative treatment was $42,554. The estimated cost of treatment for mechanical back pain associated with postoperative same-level degeneration or instability was $493,383 per 100 cases of first-time, single-level lumbar discectomy ($4934 per primary discectomy).

Conclusions

Postoperative mechanical back pain associated with same-level degeneration is not uncommon in patients undergoing single-level lumbar discectomy and is associated with substantial health care costs.

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Giannina L. Garcés-Ambrossi, Matthew J. McGirt, Vivek A. Mehta, Daniel M. Sciubba, Timothy F. Witham, Ali Bydon, Jean-Paul Wolinksy, George I. Jallo and Ziya L. Gokaslan

Object

With the introduction of electrophysiological spinal cord monitoring, surgeons have been able to perform radical resection of intramedullary spinal cord tumors (IMSCTs). However, factors associated with tumor resectability, tumor recurrence, and long-term neurological outcome are poorly understood.

Methods

The authors retrospectively reviewed 101 consecutive cases of IMSCT resection in adults and children at a single institution. Neurological function and MR images were evaluated preoperatively, at discharge, 1 month after surgery, and every 6 months thereafter. Factors associated with gross-total resection (GTR), progression-free survival (PFS), and long-term neurological improvement were assessed using multivariate regression analysis.

Results

The mean age of the patients was 41 ± 18 years and 17 (17%) of the patients were pediatric. Pathological type included ependymoma in 51 cases, hemangioblastoma in 15, pilocytic astrocytoma in 16, WHO Grade II astrocytoma in 10, and malignant astrocytoma in 9. A GTR was achieved in 60 cases (59%). Independent of histological tumor type, an intraoperatively identifiable tumor plane (OR 25.3, p < 0.0001) and decreasing tumor size (OR 1.2, p = 0.05) were associated with GTR. Thirty-four patients (34%) experienced acute neurological decline after surgery (associated with increasing age [OR 1.04, p = 0.02] and with intraoperative change in motor evoked potentials [OR 7.4, p = 0.003]); in 14 (41%) of these patients the change returned to preoperative baseline within 1 month. In 31 patients (31%) tumor progression developed by last follow-up (mean 19 months). Tumor histology (p < 0.0001) and the presence of an intraoperatively identified tumor plane (hazard ratio [HR] 0.44, p = 0.027) correlated with improved PFS. A GTR resulted in improved PFS for hemangioblastoma (HR 0.004, p = 0.04) and ependymoma (HR 0.2, p = 0.02), but not astrocytoma. Fifty-five patients (55%) maintained overall neurological improvement by last follow-up. The presence of an identifiable tumor plane (HR 3.1, p = 0.0004) and improvement in neurological symptoms before discharge (HR 2.3, p = 0.004) were associated with overall neurological improvement by last follow-up (mean 19 months).

Conclusions

Gross-total resection can be safely achieved in the vast majority of IMSCTs when an intraoperative plane is identified, independent of pathological type. The incidence of acute perioperative neurological decline increases with patient age but will improve to baseline in nearly half of patients within 1 month. Long-term improvement in motor, sensory, and bladder dysfunction may be achieved in a slight majority of patients and occurs more frequently in patients in whom a surgical plane can be identified. A GTR should be attempted for ependymoma and hemangioblastoma, but it may not affect PFS for astrocytoma. For all tumors, the intraoperative finding of a clear tumor plane of resection carries positive prognostic significance across all pathological types.

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Giannina L. Garcés-Ambrossi, Matthew J. McGirt, Roger Samuels, Daniel M. Sciubba, Ali Bydon, Ziya L. Gokaslan and George I. Jallo

Object

Although postsurgical neurological outcomes in patients with tethered cord syndrome (TCS) are well known, the rate and development of neurological improvement after first-time tethered cord release is incompletely understood. The authors reviewed their institutional experience with the surgical management of adult TCS to assess the time course of symptomatic improvement, and to identify the patient subgroups most likely to experience improvement of motor symptoms.

Methods

The authors retrospectively reviewed 29 consecutive cases of first-time adult tethered cord release. Clinical symptoms of pain and motor and urinary dysfunction were evaluated at 1 and 3 months after surgery, and then every 6 months thereafter. Rates of improvement in pain and motor or urinary dysfunction over time were identified, and presenting factors associated with improvement of motor symptoms were assessed using a multivariate survival analysis (Cox model).

Results

The mean patient age was 38 ± 13 years. The causes of TCS included lipomyelomeningocele in 3 patients (10%), tight filum in 3 (10%), lumbosacral lipoma in 4 (14%), intradural tumor in 3 (10%), previous lumbosacral surgery in 2 (7%), and previous repair of myelomeningocele in 14 (48%). The mean ± SD duration of symptoms before presentation was 5 ± 7 months. Clinical presentation included diffuse pain/parasthesias in both lower extremities in 13 patients (45%), or perineal distribution in 18 (62%), lower extremity weakness in 17 (59%), gait difficulties in 17 (59%), and bladder dysfunction in 14 (48%). Laminectomy was performed in a mean of 2.5 ± 0.7 levels per patient, and 9 patients (30%) received duraplasty. At 18 months postoperatively, 47% of patients had improved urinary symptoms, 69% had improved lower extremity weakness and gait, and 79% had decreased painful dysesthesias. Median time to symptomatic improvement was least for pain (1 month), then motor (2.3 months), and then urinary symptoms (4.3 months; p = 0.04). In patients demonstrating improvement, 96% improved within 6 months of surgery. Only 4% improved beyond 1-year postoperatively. In a multivariate analysis, the authors found that patients who presented with asymmetrical lower extremity weakness (p = 0.0021, hazard ratio 5.7) or lower extremity hyperreflexia (p = 0.037, hazard ratio = 4.1) were most likely to experience improvement in motor symptoms.

Conclusions

In the authors' experience, pain and motor and urinary dysfunction improve postoperatively in the majority of patients. The rate of symptomatic improvement was greatest for pain resolution, followed by motor, and then urinary improvement. Patients who experienced improvement in any symptom had done so by 6 months after tethered cord release. Patients with asymmetrical motor symptoms or lower extremity hyperreflexia at presentation were most likely to experience improvements in motor symptoms. These findings may help guide patient education and surgical decision-making.

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Risheng Xu, Giannina L. Garcés-Ambrossi, Matthew J. McGirt, Timothy F. Witham, Jean-Paul Wolinsky, Ali Bydon, Ziya L. Gokaslan and Daniel M. Sciubba

Object

Adequate decompression of the thoracic spinal cord often requires a complete vertebrectomy. Such procedures can be performed from an anterior/transthoracic, posterior, or combined approach. In this study, the authors sought to compare the clinical outcomes of patients with spinal metastatic tumors undergoing anterior, posterior, and combined thoracic vertebrectomies to determine the efficacy and operative morbidity of such approaches.

Methods

A retrospective review was conducted of all patients undergoing thoracic vertebrectomies at a single institution over the past 7 years. Characteristics of patients and operative procedures were documented. Neurological status, perioperative variables, and complications were assessed and associations with each approach were analyzed.

Results

Ninety-one patients (mean age 55.5 ± 13.7 years) underwent vertebrectomies via an anterior (22 patients, 24.2%), posterior (45 patients, 49.4%), or combined anterior-posterior approach (24 patients, 26.4%) for metastatic spinal tumors. The patients did not differ significantly preoperatively in terms of neurological assessments on the Nurick and American Spinal Injury Association Impairment scales, ambulatory ability, or other comorbidities. Anterior approaches were associated with less blood loss than posterior approaches (1172 ± 1984 vs 2486 ± 1645 ml, respectively; p = 0.03) or combined approaches (1172 ± 1984 vs 2826 ± 2703 ml, respectively; p = 0.05) but were associated with a similar length of stay compared with the other treatment cohorts (11.5 ± 9.3 [anterior] vs 11.3 ± 8.6 [posterior] vs 14.3 ± 6.7 [combined] days; p = 0.35). The posterior approach was associated with a higher incidence of wound infection compared with the anterior approach cohort (26.7 vs 4.5%, respectively; p = 0.03), and patients in the posterior approach group experienced the highest rates of deep vein thrombosis (15.6% [posterior] vs 0% [other 2 groups]; p = 0.02). However, the posterior approach demonstrated the lowest incidence of pneumothorax (4.4%; p < 0.0001) compared with the other 2 cohorts. Duration of chest tube use was greater in the combined patient group compared with the anterior approach cohort (8.8 ± 6.2 vs 4.7 ± 2.3 days, respectively; p = 0.01), and the combined group also experienced the highest rates of radiographic pleural effusion (83.3%; p = 0.01). Postoperatively, all groups improved neurologically, although functional outcome in patients undergoing the combined approach improved the most compared with the other 2 groups on both the Nurick (p = 0.04) and American Spinal Injury Association Impairment scales (p = 0.03).

Conclusions

Decisions regarding the approach to thoracic vertebrectomy may be complex. This study found that although anterior approaches to the thoracic vertebrae have been historically associated with significant pulmonary complications, in our experience these rates are nevertheless quite comparable to that encountered via a posterior or combined approach. In fact, the posterior approach was found to be associated with a higher risk for some perioperative complications such as wound infection and deep vein thromboses. Finally, the combined anteriorposterior approach may provide greater ambulatory and neurological improvements in properly selected patients.