Philip J. Belmont

In vivo accuracy of thoracic pedicle screws

Study Design. A retrospective observational study of 279 transpedicular thoracic screws using postoperative computed tomography (CT). Objective. To determine the accuracy of transpedicular thoracic screws. Summary of Background Data. Previous studies have reported the importance of properly placed transpedicular thoracic screws. To our knowledge, the in vivo accuracy of pedicle screw placement throughout the entire thoracic spine by CT is unknown. Methods. The accuracy of thoracic screw placement within the pedicle and vertebral body and the resultant transverse screw angle (TSA) were assessed by postoperative CT. Cortical perforations of the pedicle were graded in 2-mm increments. Screws were regionally grouped for analysis. Results. Forty consecutive patients underwent instrumented posterior spinal fusion using 279 titanium thoracic pedicle screws of various diameters (4.5–6.5 mm). The regional distribution of the screws was 39 screws at T1–T4, 77 screws at T5–T8, and 163 screws at T9–T12. Fifty-seven percent of screws were totally confined within the pedicle. Although medial perforation of the pedicle wall occurred in 14% of screws, in <1% there was >2 mm of canal intrusion. Lateral pedicular perforation occurred in 68% of perforating screws and was significantly more common than medial perforation (P < 0.0005). Seventeen screws penetrated the anterior vertebral cortex by an average of 1.7 mm. Screws inserted between T1 and T4 had a decreased incidence of full containment within the pedicle (P < 0.0005) and vertebral body (P = 0.039) compared with T9–T12. The mean TSA for screws localized within the pedicle was 14.6° and was significantly different from screws with either medial (mean 18.0°) or lateral (mean 11.5°) pedicle perforation (P < 0.0005). Anterior vertebral penetration was associated with a smaller mean TSA of 10.1° (P = 0.01) and with lateral pedicle perforation (P < 0.0005). There were no neurologic or vascular complications. Conclusions. Ninety-nine percent of screws were fully contained or were inserted with either ≤2 mm of medial cortical perforation or an acceptable lateral breech using the “in-out-in” technique. Anterior cortical penetration occurred significantly more often with lateral pedicle perforation and with a smaller mean TSA. The incidence of fully contained screws was directly correlated with the region of instrumented thoracic spine.

Straight-Forward Versus Anatomic Trajectory Technique of Thoracic Pedicle Screw Fixation : A Biomechanical Analysis

Study Design. A biomechanical study on cadaveric thoracic vertebrae using pullout strength, insertional torque, and bone mineral density to determine the optimal sagittal trajectory of thoracic pedicle screws. Objective. To perform a biomechanical study on cadaveric thoracic vertebrae using insertional torque, pullout strength, and bone mineral density to determine the optimal biomechanical sagittal trajectory for placement thoracic pedicle screws. We compared the straight-forward (paralleling the vertebral endplate) with anatomic trajectory (directed along the true anatomic axis of the pedicle). Methods. Thirty cadaveric thoracic vertebrae were harvested and evaluated with dual-energy x-ray absorptiometry to assess bone mineral density. Matched, fixed-head pedicle screws were then randomly assigned by side and placed using the straight-forward or anatomic technique under fluoroscopic visualization while recording the maximum insertional torque. Pullout strength testing was then performed. Results. The maximum insertional torque for the straight-forward technique was 2.58 ± 0.14 (SE) in pounds, whereas the anatomic technique averaged 1.86 ± 0.14 (SE) in pounds (P = 0.0005). The maximum insertional torque at the neurocentral junction for the straight-forward technique averaged 1.89 ± 0.17 (SE) in-lbs. (73% of maximum insertional torque), whereas the anatomic trajectory averaged 1.39 ± 0.11 (SE) in pounds (75% of maximum insertional torque) (P = 0.007). The average pullout strength using a straight-forward trajectory was 611 ± 50 (SE) N compared to the anatomic trajectory, which averaged 481 ± 54 (SE) N (P = 0.034). The pullout strength correlated with mean bone mineral density for both the straight-forward (r = 0.461, P = 0.027) and anatomic (r = 0.598, P = 0.004) techniques. Conclusions. The straight-forward technique results in a 39% increase in maximum insertional torque and a 27% increase in pullout strength compared to the anatomic technique. The maximum insertional torque at the neurocentral junction resulted in a 36% increase using the straight-forward technique versus the anatomic trajectory. Bone mineral density directly correlates with pullout strength for both techniques.

Accuracy of thoracic pedicle screws in patients with and without coronal plane spinal deformities.

Study Design. This retrospective observational study evaluated 399 transpedicular thoracic screws using postoperative computed tomography (CT). Objectives. To examine the in vivo accuracy of transpedicular thoracic screws in patients with and without coronal plane spinal deformities. Summary of Background Data. There are no comparative studies regarding the safety and accuracy of thoracic pedicle screws in patients with and without coronal plane spinal deformities. Methods. Curve magnitude and segmental vertebral rotation were determined from preoperative radiographs. Postoperative CT was used to assess the placement accuracy of titanium thoracic pedicle screws. Results. Forty-seven patients underwent instrumented posterior spinal fusion using 399 titanium thoracic pedicle screws. Fully contained screw accuracy in patients with coronal plane spinal deformities was less than in patients without coronal plane spinal deformities at T9-T12 (59%vs. 73%, P = 0.04) and overall (42%vs. 62%, P = 0.001). There was no difference between the overall percentages of acceptably positioned screws (≤ 2 mm of medial or ≤ 6 mm of lateral pedicle perforation) in patients with coronal plane spinal deformities (98%) versus patients without coronal plane spinal deformities (99%) (P = 0.69). Penetration of the anterior vertebral cortex was more frequent in patients with coronal plane spinal deformities than in those without coronal plane spinal deformities (8.0%vs. 1.0%, P = 0.008). There was no correlation between the accuracy of screw placement and the degree of segmental rotation, screw proximity to the curve apex, or screw position relative to the curve concavity or convexity (P > 0.12). There were no neurologic or vascular complications. Conclusions. The overall percentage of acceptably positioned screws was 98% in patients with coronal plane spinal deformities and 99% in patients without coronal plane spinal deformities. In patients with coronal plane spinal deformities, penetration of the pedicle wall and the anterior vertebral cortex was increased at T9-T12 and overall.

Risk Factors for 30-Day Postoperative Complications and Mortality after Below-Knee Amputation: A Study of 2,911 Patients from the National Surgical Quality Improvement Program

BACKGROUNDnThis investigation sought to evaluate risk factors for morbidity and mortality from a large series of below-knee amputees prospectively entered in a national database.nnnSTUDY DESIGNnAll patients undergoing below-knee amputations in the years 2005-2008 were identified in the database of the National Surgical Quality Improvement Program (NSQIP). Demographic data, medical comorbidities, and medical history were obtained. Mortality and postoperative complications within 30 days of the below-knee amputation were also documented. Chi-square test, univariate, and multivariate logistic regression analyses were used to assess the effect of specific risk factors on mortality, as well as the likelihood of developing major, minor, or any complications developing.nnnRESULTSnBelow-knee amputations were performed in 2,911 patients registered in the NSQIP database between 2005 and 2008. The average age of patients was 65.8 years old and 64.3% were male. There was a 7.0% 30-day mortality rate and 1,627 complications occurred in 1,013 patients (34.4%). Multivariate logistic regression analysis identified renal insufficiency, cardiac issues, history of sepsis, steroid use, COPD, and increased patient age as independent predictors of mortality. The most common major complications were return to the operating room (15.6%), wound infection (9.3%), and postoperative sepsis (9.3%). History of sepsis, alcohol use, steroid use, cardiac issues, renal insufficiency, and contaminated/infected wounds were independent predictors of one or more complications developing.nnnCONCLUSIONSnRenal disease, cardiac issues, history of sepsis, steroid use, COPD, and increased patient age were identified as predictors of mortality after below-knee amputation. Renal disease, cardiac issues, history of sepsis, steroid use, contaminated/infected wounds, and alcohol use were also found to be predictors of postoperative complications.

Advantage of pedicle screw fixation directed into the apex of the sacral promontory over bicortical fixation: a biomechanical analysis.

Study Design. A biomechanical study of human cadaveric sacra using insertional torque and bone mineral density was conducted to determine the optimal sagittal trajectory of S1 pedicle screws. Objective. To measure the maximal insertional torque of sacral promontory versus bicortical pedicle screw fixation. Summary of Background Data. Fixation of instrumentation to the sacrum is commonly accomplished using S1 pedicle screws, with previous studies reporting biomechanical advantages of bicortical over unicortical S1 screws. The biomechanical effect of bicortical screws (paralleling the endplate) versus screws directed into the apex of the sacral promontory is unknown. Methods. For this study, 10 fresh frozen cadaver sacra were harvested and evaluated with dual-energy radiograph absorptiometry to assess bone mineral density. Matched 7.5-mm monoaxial stainless steel pedicle screws then were randomly assigned by side (left versus right) and placed bicortically or into the apex of the sacral promontory under direct visualization. Maximum insertional torque was recorded for each screw revolution with a digital torque wrench (TQJE1500, Snap-On Tools, Kenosha, WI). Results. Maximum bicortical S1 screw insertional torque averaged 5.22 ± 0.83 inch-pounds, as compared with the maximum sacral promontory S1 screw insertional torque of 10.34 ± 1.94 inch-pounds. This resulted in a 99% increase in maximum insertional torque (P = 0.005) using the “tricortical” technique, with the screw directed into the sacral promontory. Mean bone mineral density was 940 ± 0.25 mg/cm2 (range, 507–1428 mg/cm2). The bone mineral density correlated with maximal insertional torque for the sacral promontory technique (r = 0.806;P = 0.005), but not for the bicortical technique (r = 0.48;P = 0.16). Conclusions. The screws directed into the apex of the sacral promontory of the S1 pedicle resulted in an average 99% increase in peak insertional torque (P = 0.005), as compared with bicortical S1 pedicle screw fixation. Tricortical pedicle screw fixation correlates directly with bone mineral density.

The role of imaging and in situ biomechanical testing in assessing pedicle screw pull-out strength

Study Design This study determined the predictive ability of quantitative computed tomography, dual energy x‐ray absorptiometry, pedicular geometry, and mechanical testing in assessing the strength of pedicle screw fixation in an in vitro mechanical test of intrapedicular screw fixation in the human cadaveric lumbar spine. Objective To test several hypotheses regarding the relative predictive value of densitometry, pedicular geometry, and mechanical testing in describing pedicle screw pull‐out. Summary of Background Data Previous investigations have suggested that mechanical testing, geometry, and densitometry, determined by quantitative computed tomography or dual energy x‐ray absorptiometry, predict the strength of the screw‐bone system. However, no study has compared the relative predictive value of these techniques. Methods Forty‐nine pedicle screw cyclic‐combined flexion‐extension moment‐axial pull‐out tests were performed on human cadaveric lumbar vertebrae. The predictive ability of quantitative computed tomography, dual energy x‐ray absorptiometry, insertional torque, in situ stiffness, and pedicular geometry was assessed using multiple regression. Results Several variables correlated to force at failure. However, multiple regression analysis showed that bone mineral density of the pedicle determined by quantitative computed tomography, insertional torque, and in situ stiffness when used in combination resulted in the strongest prediction of pull‐out force. No other measures provided additional predictive ability in the presence of these measures. Conclusions Pedicle density determined by quantitative computed tomography when used with insertional torque and in situ stiffness provides the strongest predictive ability of screw pull‐out. Geometric measures of the pedicle and density determined by dual energy x‐ray absorptiometry do not provide additional predictive ability in the presence of these measures.

Morbidity and Mortality in the Thirty-Day Period Following Total Hip Arthroplasty: Risk Factors and Incidence

The study sought to ascertain the incidence rates and risk factors for 30-day post-operative complications after primary total hip arthroplasty (THA). Complications were categorized as systemic or local and subcategorized as major or minor. There were 17,640 individuals who received primary THA identified from the 2006-2011 ACS NSQIP. The mortality rate was 0.35% and complications occurred in 4.9%. Age groups ≥ 80 years (P <0.001) and 70-79 years old (P = 0.003), and renal insufficiency (P = 0.02) best predicted mortality. Age ≥80 years (P <0.001) and cardiac disease (P = 0.01) were the strongest predictors of developing any postoperative complication. Morbid obesity (P <0.001) and operative time > 141 minutes (P <0.001) were strongly associated with the development of major local complications.

Thirty-day morbidity and mortality after elective total shoulder arthroplasty: patient-based and surgical risk factors

BACKGROUNDnTotal shoulder arthroplasty (TSA) is an effective treatment for painful glenohumeral arthritis, but its morbidity has not been thoroughly documented.nnnMETHODSnThe National Surgical Quality Improvement Program database was queried to identify all patients undergoing primary TSA between 2006 and 2011, with extraction of selected patient-based or surgical variables and 30-day clinical course. Postoperative complications were stratified as major systemic, minor systemic, major local, and minor local, and mortality was recorded. Odds ratios (ORs) with 95% confidence intervals (95% CIs) were derived from bivariate and multivariable analysis to express the association between risk factors and clinical outcomes.nnnRESULTSnAmong the 2004 patients identified, the average age was 69 years, and 57% were women. Obesity was present in 46%, and 48% had an American Society of Anesthesiologists classification of ≥3. The 30-day mortality and total complication rates were 0.25% and 3.64%, respectively. Comorbid cardiac disease (OR, 85.31; 95% CI, 8.15, 892.84) and increasing chronologic age (OR, 1.19; 95% CI, 1.06, 1.33) were independent predictors of mortality, whereas peripheral vascular disease was associated with statistically significant increase in any complication (OR, 6.25; 95% CI, 1.24, 31.40). Operative time >174 minutes was an independent predictor for development of a major local complication (OR, 4.05; 95% CI, 1.45, 11.30). Obesity was not associated with any specified complication after controlling for other variables.nnnCONCLUSIONSnWhereas TSA has low short-term rates of perioperative complications and mortality, careful perioperative medical optimization and efficient surgical technique should be emphasized to decrease morbidity and mortality.

Intraspinal anomalies associated with isolated congenital hemivertebra: the role of routine magnetic resonance imaging.

BACKGROUNDnPrevious studies have demonstrated high rates of intraspinal anomalies in patients with congenital scoliosis; however, various authors have not considered the presence of an isolated hemivertebra to be sufficient reason for further evaluation with magnetic resonance imaging. Consequently, the rate of magnetic resonance imaging-detected intraspinal anomalies and subsequent neurosurgical intervention in patients with a single hemivertebra is unknown. Therefore, we studied all patients with a hemivertebra, after eliminating patients with a myelomeningocele, to compare those who had a single hemivertebra with those who had a complex hemivertebral pattern.nnnMETHODSnA retrospective review of the history, physical examination findings, and magnetic resonance imaging findings for patients who had presented with at least one hemivertebra, excluding those who had a myelomeningocele, was conducted to identify the prevalence of intraspinal anomalies as seen on magnetic resonance imaging and the rate of subsequent neurosurgical intervention. Additionally, the diagnostic value of the history and the physical examination in predicting the presence of intraspinal anomalies was determined.nnnRESULTSnOne hundred and sixteen patients with congenital scoliosis and a curve that included at least one hemivertebra were identified. Seventy-six of these patients had had magnetic resonance imaging and were included in the present study. The mean age of these patients at the time of presentation was 4.9 years, and the mean duration of follow-up was 7.7 years. Twenty-nine patients had an isolated hemivertebra, and forty-seven patients had a complex hemivertebral pattern. Eight (28%) of the twenty-nine patients with an isolated hemivertebra and ten (21%) of the forty-seven patients with a complex hemivertebral pattern had an intraspinal anomaly that was detected with magnetic resonance imaging. Overall, an abnormal finding on the history or physical examination demonstrated an accuracy of 71%, a sensitivity of 56%, a specificity of 76%, a positive predictive value of 42%, and a negative predictive value of 85% for the diagnosis of an intraspinal anomaly. Three patients with an isolated hemivertebra and five patients with a complex hemivertebral pattern underwent neurosurgical intervention. All eight patients who underwent neurosurgical intervention had had detection of an intraspinal anomaly with magnetic resonance imaging, whereas only four of these patients (two of whom had an isolated hemivertebra and two of whom had a complex hemivertebral pattern) had had an abnormal finding on either the history or the physical examination.nnnCONCLUSIONSnPatients who have an isolated hemivertebra and those who have a complex hemivertebral pattern have similar rates of intraspinal anomalies that are detected with magnetic resonance imaging and similar rates of subsequent neurosurgical intervention. The history and physical examination findings are not predictive of intraspinal anomalies. Therefore, a magnetic resonance imaging evaluation of the entire spine should be considered for all patients with congenital scoliosis, including those with an isolated hemivertebra.

The effects of hook pattern and kyphotic angulation on mechanical strength and apical rod strain in a long-segment posterior construct using a synthetic model

Study Design. Synthetic spine models were used to compare the effects of hook pattern and kyphotic angulation on stiffness and rod strain in long-segment posterior spinal constructs. Objectives. To examine the biomechanical effects of hook patterns and kyphotic angulation on long-segment posterior spinal constructs. Summary of Background Data. Kyphotic deformities managed by increasing rod diameter and hence construct stiffness have shown decreased postoperative loss of correction and hardware complications. The biomechanical effects of hook pattern and kyphosis are unknown. Methods. Spine models of 0°, 27°, and 54° sagittal contour, composed of polypropylene vertebral blocks and isoprene elastomer intervertebral spacers, representing T3–T12, were used for biomechanical testing of long-segment posterior spinal constructs. Models were instrumented with 6.35-mm titanium rods and one of the following hook configurations: 20-hook compression, 16-hook compression, 16-hook claw apex-empty, 16-hook claw apex-full, or 8-hook claw. Construct stiffness and rod strain during axial compression were determined. Results. The compression-hook patterns provided at least a 45% increase in construct stiffness (P = 0.013) and a 22% decrease in rod strain (P < 0.0001) compared with those obtained with the claw-hook pattern with the best biomechanical performance. When analyzing all five hook patterns, there was a 19% decrease in construct stiffness and 27% increase in rod strain when progressing from straight alignment to 27° of sagittal contour (P < 0.0001). Progressing from straight alignment to 54° decreased construct stiffness by 48% and increased rod strain by 55% (P < 0.0001). Construct stiffness was inversely correlated to rod strain in all five hook patterns (R2 = 0.82–0.98, P < 0.001). Conclusions. Using compressive-hook patterns and decreasing the kyphotic deformity significantly increases construct stiffness and decreases rod strain.