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Biomedical subjects

David W Polly

Publications and source records attributed to David W Polly.

At least 19 recordsLinked to original sources
PubMed2004

Computerized tomography evaluation of a resorbable implant after transforaminal lumbar interbody fusion.

OBJECT: Synthetic bioabsorbable implants have recently been introduced in spinal surgery; consequently, the indications, applications, and results are still evolving. The authors used absorbable interbody spacers (Medtronic Sofamor Danek, Memphis, TN) packed with recombinant bone morphogenetic protein (Infuse; Medtronic Sofamor Danek) for single- and multiple-level transforaminal lumbar interbody fusion (TLIF) procedures over a period of 18 months. This is a consecutive case series in which postoperative computerized tomography (CT) scanning was used to assess fusion status. METHODS: There were 22 patients (17 men, five women; 39 fusion levels) whose mean age was 41.6 years (range 23-70 years) and in whom the mean follow-up duration was 12.4 months (range 6-18 months). Bridging bone was noted as early as the 3-month postoperative CT scan when obtained; solid arthrodesis was routinely noted between 6 and 12 months in 38 (97.4%) of 39 fusion levels. In patients who underwent repeated CT scanning, the fusion mass appeared to increase with time, whereas the disc space height remained stable. Although the results are early (mean 12-month follow-up duration), there was only one noted asymptomatic delayed union/nonunion at L5-S1 in a two-level TLIF with associated screw breakage. There were no infections or complications related to the cages. CONCLUSIONS: The bioabsorbable cages appear to be a viable alternative to metal interbody spacers, and may be ideally suited to spinal interbody applications because of their progressive load-bearing properties.

Absorbable Implants↗
PubMed2004

Volumetric spinal canal intrusion: a comparison between thoracic pedicle screws and thoracic hooks.

STUDY DESIGN: A computer-aided design analysis. OBJECTIVES: To introduce the concept of volumetric spinal canal intrusion and report the relative intrusion volumes for thoracic pedicle screws compared to thoracic laminar and pedicle hooks. SUMMARY OF BACKGROUND DATA: Thoracic pedicle screws are being used more frequently; however, there is concern about neurologic risk from medial misplacement. The accepted alternative to screws is hooks. Laminar and pedicle hooks also have significant obligatory spinal canal intrusion. To date, there have been no comparison studies. METHODS: Volumetric analysis of canal intrusion of pedicle screws and hooks was performed by computer-aided design CAM. All implants were of a single product line by a single manufacturer (CD Horizon M8, Medtronic Sofamor Danek). Intrusion of pedicle screws with medial positioning was analyzed in 0.5-mm increments, including a calculation of the "screw shadow," representing additional space not available for the spinal cord between screw threads and lateral to a medially positioned screw with intrusion greater than the screw radius. The length of screw intrusion was determined from postoperative CT scans in patients with thoracic pedicle screw instrumentation. All hook styles were analyzed. The volume of the footplate in line with the dorsal surface of the footplate was considered the intruding volume for laminar hooks, with increasing offset in 0.25-mm increments to represent imperfect fit. Half of the volume of the footplate was considered to be the intruding volume for pedicle hooks since a properly positioned pedicle hook straddles the pedicle. RESULTS: Volumetric intrusion for a 4.5-mm screw ranged from 2.2 mm3 (0.5 mm medial perforation) to 83.4 mm3 (3.0 mm perforation). For a 5.5-mm screw, intrusion volume range was from 1.3 mm3 to 83.2 mm3. Accounting for the "screw shadow," the volumetric intrusion was 9.83 mm3 to 116.3 mm3 and 10.88 mm3 to 134.89 mm3, respectively. Hook volumetric intrusion ranged from 21.15 mm3 for a pediatric narrow-blade ramped pedicle hook to 113.9 mm3 for a wide-blade laminar hook with 1.0 mm of step-off. CONCLUSIONS: A 4.5-mm or 5.5-mm thoracic pedicle screw must have a medial perforation of >or=1.5 mm to have the same volumetric spinal canal intrusion as a pediatric narrow-blade pedicle hook, the smallest hook footplate. Further, the medial violation must be >3 mm to approach the same volumetric intrusion as the largest hook. Accounting for the "screw shadow," a thoracic pedicle screw must have a medial perforation of >2 mm to approach the same intrusion volume as a standard pedicle hook. In the absence of direct neural injury, this explains the clinical finding of medial perforation of up to 4 mm without neurologic compromise.

Animals↗
PubMed2004

Probing for thoracic pedicle screw tract violation(s): is it valid?

BACKGROUND: Preparation of the thoracic pedicle screw tract is a critical step prior to the placement of screws. The ability to detect pedicle wall violation(s) by probing prior to insertion of thoracic pedicles screws, however, has not been studied. The purpose of this study was to evaluate the inter- and intraobserver agreement and the accuracy in detecting thoracic pedicle screw tract violation(s) among surgeons at various levels of training. METHODS: With use of a straightforward trajectory, under direct visualization, 108 thoracic pedicle screw tracts (54 cadaveric thoracic vertebrae) were prepared in a standard fashion, followed by tapping with a 4.5-mm cannulated tap. A deliberate pedicle violation was randomly created by an independent investigator in either the anterior, the medial, or the lateral wall in 65 pedicles. Following this, four blinded, independent surgeons at various levels of training probed the specimens on three separate occasions to determine if a breach was present (1,296 discrete data points). Surgeon findings were then recorded as breach present or absent and, if present, breach location. The Cohen kappa correlation coefficient (kappa a) and 95% confidence interval were used to assess the accuracy of the observers and the inter- and intraobserver agreement. RESULTS: The mean accuracy over three iterations, the validity in detecting the breach location, and the intraobserver agreement varied by level of training and experience, with the most experienced observer (observer 1) scoring the best and the least experienced observer (observer 4) scoring the worst. The three most senior surgeons had good intraobserver agreement. Interobserver agreement was low between the four observers. CONCLUSIONS: An observer's ability to accurately detect the presence or absence of a pedicle tract violation and the breach location, if present, is dependent on the surgeon's level of training. Probing the pedicle tract prior to placement of pedicle screws in the thoracic spine is likely a learned skill that improves with repetition and experience.

Bone Screws↗
PubMed2004

"The visible skeleton series": the art of Laura Ferguson.

Laura Ferguson's The Visible Skeleton Series constitutes an artistic self-representation of scoliosis. Ferguson employs high-tech medical imaging and innovative graphical techniques to suggest a novel view of her skeletal "deformity." This section presents introductions to the series by a historian of medicine and by the artist herself, and offers personal and professional reflections on the work from an orthopedic surgeon, a social worker with lived experience of cleft lip, and a retired pediatric forensic pathologist.

Adaptation, Psychological↗
PubMed2003

Adapting innovative motion-preserving technology to spinal surgical practice: what should we expect to happen?

STUDY DESIGN: A literature-based review of approach-related morbidity and a conjectural analysis of potential complications of disc arthroplasty based on experience with total joint arthroplasty. OBJECTIVE: To describe predictable complications of disc arthroplasty and possible strategies for minimizing or treating these complications. SUMMARY OF BACKGROUND DATA: There is a significant experience with anterior approach-related morbidity in spinal surgery. There is also extensive experience with extremity total joint arthroplasty. The combination of these experiences should predict certain occurrences that will occur with the advent of disc arthroplasty in the spine. METHODS: Review of the medical literature associated with anterior approach to the lumbar spine for spinal fusion was done. Sequential steps for performance of disc arthroplasty and possible problems with each step were evaluated and possible complications identified. Parallel experience in total joint arthroplasty was reviewed for possible predictive experience. RESULTS: There are definable approach-related morbidities that will occur, regardless of prosthesis design and implantation technique. Prosthesis design involves a series of tradeoffs for risks and benefits. Revisions are inevitable; rate of revision and time to revision remain to be determined. CONCLUSIONS: Disc arthroplasty will offer benefits over current fusion techniques. It will come at a cost and certain complications are entirely predictable. There will be deaths from the procedure, due to thromboembolic phenomenon or due to uncontrollable hemorrhage from irreparable vascular injury, especially on repeat operations. There will be prostheses that dislodge. There will be infections that require device removal, a very high-risk procedure. There will be a deterioration of results in the hands of the general medical community as opposed to the hands of the initial investigators, a learning curve if you will. The access surgeon will be critical to minimizing morbidity. Design considerations compete with anatomic constraints. Material choices all have pros and cons. Spine surgeons as a whole are excited about this opportunity, but we must be diligent to minimize these predictable adverse events to make the risk benefit profile the best that it can be for our patients.

Arthroplasty, Replacement↗
PubMed2003

Analysis of patient and parent assessment of deformity in idiopathic scoliosis using the Walter Reed Visual Assessment Scale.

OBJECTIVES: This study evaluates the Walter Reed Visual Assessment Scale (WRVAS) compared with clinical parameters and written descriptions of the deformity from idiopathic scoliosis patients and their parents. SUMMARY OF BACKGROUND DATA: The WRVAS demonstrates seven visible aspects of spinal deformity in an analogue scale. Higher scores reflect worsening deformity. MATERIALS AND METHODS: The WRVAS was administered to 182 idiopathic scoliosis patients at four centers in conjunction with open-ended questions about patients' and their parents' perceptions of their spinal deformity. The open-ended responses were categorized as either "deformity noted" or "no deformity noted." RESULTS: WRVAS scores strongly correlate with curve magnitude (P = 0.01) and clearly differentiates curves of 30 degrees or more from lesser curves. Among treatment groups, patients with surgery recommended had significantly higher scores than that of other patients. The instrument differentiated those noting no deformity from those noting a deformity. The correlation between patients' and parents' scores was high (Spearman's rho = 0.8). When a deformity was noted, parents gave higher scores than did their children for rib prominence, shoulder level, scapular rotation, and the total score, but not for the other dimensions. CONCLUSIONS: Increasing scores of the WRVAS are strongly correlated with curve magnitude lending construct validity to this type of assessment tool. Patients with "surgery recommended" report more visible deformity on the scale than observed, braced, and postoperative patients, supporting the hypothesis that surgery improves the perceived appearance. Parents perceive more deformity of the ribs and shoulders more than did the patients, but other aspects of the deformity are identified equally. WRVAS scores correlate significantly with curve magnitude and treatment. Parents and patients have similar scores, but with parents perceiving more deformity of the ribs and shoulders than patients.

Adolescent↗
PubMed2003

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.

Absorptiometry, Photon↗
PubMed2003

A cost analysis of bone morphogenetic protein versus autogenous iliac crest bone graft in single-level anterior lumbar fusion.

An economic model was developed to compare costs of stand-alone anterior lumbar interbody fusion with recombinant human bone morphogenetic protein 2 on an absorbable collagen sponge versus autogenous iliac crest bone graft in a tapered cylindrical cage or a threaded cortical bone dowel. The economic model was developed from clinical trial data, peer-reviewed literature, and clinical expert opinion. The upfront price of bone morphogenetic protein (3380 dollars) is likely to be offset to a significant extent by reductions in the use of other medical resources, particularly if costs incurred during the 2 year period following the index hospitalization are taken into account.

Bone Morphogenetic Protein 2↗
PubMed2003

Traction versus side-bending radiographs: is the proximal thoracic curve the stiffer curve in double thoracic curves?

This is a consecutive study of patients with adolescent idiopathic scoliosis requiring surgical intervention. The purpose was to determine the best radiographic method of evaluating the flexibility of the proximal thoracic (PT) curve in patients with a double thoracic curve pattern. The PT curve averaged 40 degrees (range, 27 degrees-67 degrees) on posteroanterior (PA) radiographs, 30 degrees (range, 12 degrees60 degrees) on supine side-bending radiographs, and 24 degrees (range, 8 degrees -55 degrees) on traction radiographs. The flexibility index (FI) for the PT curve was 0.72 (range, 0.40-0.96) on side-bending and 0.58 (range, 0.29-0.82) on traction. The main thoracic (MT) curve averaged 62 degrees (range, 45 degrees-102 degrees) on PA, 40 degrees (range, 19 degrees-88 degrees) on supine side-bending, and 35 degrees (range, 17 degrees-76 degrees) on traction radiographs. The FI for the MT curve was 0.62 (range, 0.30-0.89) on side bending and 0.56 (range, 0.32-0.81) on traction. There was a statistically significant difference between the flexibility of the PT curve on supine traction as compared with the side-bending films (P<0.0005). Comparison of the MT curve flexibility with the PT curve demonstrated the PT curve to be less flexible in 11 of 15 cases (P = 0.16). The authors conclude that for double thoracic curve patterns, the PT curve is usually the smaller, yet often the stiffer curve. Furthermore, the supine traction radiograph demonstrates greater flexibility of the PT curve than does the supine side-bending radiograph. Clinically, this may assist scoliosis surgeons in assessing the PT curve and achieving a stable, balanced spine.

Adolescent↗
PubMed2003

Transforaminal lumbar interbody fusion: unilateral versus bilateral disk removal--an in vivo study.

We conducted a study to quantify the attainable cross-sectional area available for fusion and the mean volume of disk removed with a unilateral transforaminal lumbar interbody fusion (TLIF). Starting with the unilateral approach and completing the procedure with a bilateral approach, we removed the maximal amount of disk material. Postoperative computed tomography (CT) images were used to estimate the cross-sectional area/vertebral endplate available for bone grafting and fusion. Results showed that, using the unilateral approach, we removed 11.8 cm3 (range, 7-25 cm3) of disk material--or more than 69% of the total volume of resected disk. Postoperative CT images confirmed that more than 56% of the endplate cross-sectional area is available for fusion using the unilateral approach. We conclude that unilateral TLIF removes sufficient disk material for achieving a solid and stable arthrodesis while minimizing neural retraction and dural exposure.

Adult↗
PubMed2002

Hemimetameric segmental shift: a case series and review.

STUDY DESIGN: A case series of patients with hemimetameric shift. OBJECTIVES: To describe the radiographic and physical findings as well as treatment provided to a subset of patients with congenital scoliosis. SUMMARY OF BACKGROUND DATA: Hemimetameric segmental displacement, or hemimetameric shift, is a clinical entity defined by two contralateral hemivertebrae separated by at least one normal vertebra. Although the entity is briefly described in textbooks, there are no published series to date. METHODS: From 1974 to 2000, 186 cases of congenital scoliosis were identified from two referral centers. Of these, 27 cases (15%) of hemimetameric shift were identified. Medical records and radiographs were reviewed, as well as magnetic resonance imaging when available. RESULTS: There were an average of 2.9 hemivertebrae per patient (range 2-6) with the following distribution: T1-T6 (29), T7-T11 (28), T12-L1 (10), L2-L4 (11), and L5 (1). The average curve magnitude at presentation was 28 degrees (range 9 degrees -55 degrees). Nine patients required surgery-most commonly with involvement of the thoracolumbar or lumbosacral junction. Eleven patients also had associated anomalies to include Klippel-Feil syndrome (3), Goldenhar syndrome (2), imperforate anus (2), tracheoesophageal fistula (2), and a single kidney (2). There was only one patient who had an abnormal magnetic resonance imaging (1 out of 17; 6%). CONCLUSIONS: Hemimetameric shift is a common finding in congenital scoliosis. Hemivertebrae are most commonly found in the thoracic spine; however, surgical intervention is most commonly observed when the caudal hemivertebrae is located from the thoracolumbar to lumbosacral junction. The incidence of abnormal magnetic resonance imaging findings is low (6%).

Adolescent↗
PubMed2002

Economic evaluation of bone morphogenetic protein versus autogenous iliac crest bone graft in single-level anterior lumbar fusion: an evidence-based modeling approach.

STUDY DESIGN: Economic evaluation provides a framework to explicitly measure and compare the value of alternative medical interventions in terms of their clinical, health-related quality-of-life, and economic outcomes. Computerized economic models can help inform the design of future prospective studies by identifying the cost-drivers, the most uncertain parameter estimates, and the parameters with the greatest impact on the results and inferences. OBJECTIVE: An economic analysis of bone morphogenetic protein versus autogenous iliac crest bone graft for single-level anterior lumbar fusion poses several methodologic challenges. This article describes how such an economic evaluation may be framed and designed, while enumerating challenges, offering some solutions, and suggesting an agenda for future research. SUMMARY OF BACKGROUND DATA: An evidence-based modeling approach can incorporate epidemiologic, clinical, and economic data from several sources including randomized clinical trials, peer-reviewed literature, and expert opinion. Sensitivity analyses can be conducted by varying key parameter estimates within a reasonable range to assess the impact on the results and inferences. RESULTS: Preliminary results suggest that from a payer perspective, the upfront price of bone morphogenetic protein is likely to be entirely offset by reductions in the use of other medical resources. That is, bone morphogenetic protein appears to be cost neutral. The cost offsets were attributable largely to prevention of pain and complications associated with autogenous iliac crest bone graft, as well as reduction of the costs associated with fusion failures. CONCLUSIONS: Future research should focus on quantifying the health-related quality-of-life impact of bone morphogenetic protein relative to autogenous iliac crest bone graft, as well as the impact on lost productivity.

Activities of Daily Living↗
PubMed2002

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 (< or = 2 mm of medial or < 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.

Adolescent↗
PubMed2002

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.

Absorptiometry, Photon↗
PubMed2002

The use of interbody cage devices for spinal deformity: a biomechanical perspective.

Spinal instrumentation has revolutionized the treatment of spinal deformities and offers a plethora of techniques and designs to surgically treat deformity conditions. The authors address the biomechanical properties afforded by various posterior and anterior spinal instrumentation mechanisms, with and without intervertebral reconstruction, and the principles associated with optimal reconstruction techniques. The integration of multiple strategies can improve anterior and posterior construct stiffness in the treatment of spinal deformities. Structural interbody support probably is the best method to minimize longitudinal rod and screw-bone interface strain. Moreover, anterior load-bearing structural grafts and interbody devices have been shown to increase construct stiffness, decrease the incidence posterior implant failure, permit the use of smaller diameter longitudinal rods, and may enhance the rate of successful spinal arthrodesis. From a biomechanical standpoint, treatment of medium to high-grade spondylolisthesis with stand-alone interbody or transvertebral cages, in the absence of supplemental posterior fixation, is contraindicated. Collectively, the included studies reinforce the principles of load sharing between the anterior and posterior spinal columns and affirm the biomechanical dominance of anterior column support in circumferential spinal arthrodesis.

Biomechanical Phenomena↗