William R. Klemme

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.

Spinal instrumentation without fusion for progressive scoliosis in young children

Between 1973 and 1993, a heterogeneous group of 67 children with progressive scoliosis entered a program of incremental-distraction spinal instrumentation without fusion supplemented by full-time external orthotic support. Over the course of treatment, curve magnitude improved from an average of 67 degrees at initial instrumentation to 47 degrees at definitive fusion. For all patients, curve response tended to decline with consecutive procedures. The measured growth of the instrumented but unfused spinal segments averaged 3.1 cm over a mean treatment period of 3.1 years. The results of our study suggest that spinal instrumentation without fusion can control progressive scoliosis in a majority of children while allowing normalized growth of instrumented spinal segments. The mean duration of treatment and ultimate gain in spinal length are constrained by progressive structural changes that alter curve response to incremental distraction. Despite these limitations, spinal instrumentation without fusion may provide a reasonable management alternative when individualized among these difficult patients.

Measurement of Thoracic and Lumbar Fracture Kyphosis : Evaluation of Intraobserver, Interobserver, and Technique Variability

Study Design. Statistical analysis of various measurement techniques for thoracolumbar burst fracture kyphosis on lateral radiograph. Objective. To determine the most reliable measurement technique. Summary of Background Data. The treatment of thoracic and lumbar burst fractures involves many factors, including the degree of resultant kyphosis. Although various methods have been described, no study has directly compared these methods for reliability and reproducibility. Methods. Fifty lateral radiographs of thoracic and lumbar burst fractures were randomly selected and measured on two separate occasions by three spine surgeons using five different measurement techniques. Radiograph quality, fracture type, and the center beam location were determined. Statistical analysis included analysis of variance for repeated measures and analysis of variance using a generalized linear model. Results. Intraclass correlation coefficients were most consistent for Method 1 (rho = 0.83–0.94) followed by Method 4 (rho = 0.65–0.89) and Method 5 (rho = 0.73–0.85). Intraobserver agreement (% of repeated measures within 5° of the original measurement) ranged between 72% and 98% for all techniques for all three observers, with Method 1 showing the best agreement (84%–98%). Paired comparisons between observers varied considerably with interobserver reliability correlation coefficients ranging from 0.52 to 0.93. Method 1 showed the highest interobserver reliability coefficient (0.81, range 0.71–0.93) followed by Method 5 (0.71, range 0.68–0.75). Method 1 also had the highest percentage of agreement within categories (90% within 5°). Conclusions. Method 1 (measuring from the superior endplate of the vertebral body one level above the injured vertebral body to the inferior endplate of the vertebral body one level below) showed the best intraobserver and interobserver reliability overall.

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.

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 representimperfect 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 ≥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.

The biomechanical significance of anterior column support in a simulated single-level spinal fusion.

This study examines the biomechanical effects of interbody cages and variations in posterior rod diameter in a simulated single-level spinal fusion. A single-level spinal fusion model composed of polyethylene cylinders, posterior pedicular instrumentation, and variously positioned single or dual interbody cages was used for biomechanical testing. Constructs were tested under compressive flexural load, with measurement of stiffness, rod strain, cage strain, and intracage pressure. A strong linear correlation emerged between the mean construct stiffness and cage positioning within the sagittal plane that was inversely related to posterior rod strain. Two small titanium mesh cages were equivalent to one large cage. In a single-level spine model, the presence of and sagittal position of interbody cages significantly influences overall construct stiffness. Cage strain increased with more anterior positions and was inversely related to rod strain.

Iliotibial band friction syndrome.

Overuse knee injuries are common, but ITBFS is often overlooked as a cause of lateral knee pain in an active population. Iliotibial band friction syndrome is an overuse injury usually seen in long distance runners, cyclists, and military personnel. The exact incidence of the syndrome has been estimated to range from 1.6%-52% depending on the population studied. The diagnosis is often made from a thorough history and clinical examination with an infrequent need for additional studies. Treatment is mostly conservative consisting of rest and anti-inflammatory agents, with only the refractory cases requiring surgical resection of the impinging portion of the ITB.

Osteoporosis and anterior femoral notching in periprosthetic supracondylar femoral fractures: a biomechanical analysis.

Background: This biomechanical study was designed to evaluate the predictive ability of dual-energy x-ray absorptiometry, cortical bone geometry as determined with computed tomography, and radiography in the assessment of torsional load to failure in femora with and without notching.Methods: Thirteen matched pairs of cadaveric femora were randomized into two groups: a notched group, which consisted of femora with a 3-mm anterior cortical defect, and an unnotched group of controls. Each pair then underwent torsional load to failure. The ability of a number of measures to predict femoral torsional load to failure was assessed with use of regression analysis. These measures included dual-energy x-ray absorptiometry scans of the proximal and the distal part of the femur, geometric measures of both anterior and posterior cortical thickness as well as the polar moment of inertia of the distal part of the femur as calculated on computed tomography scans, and the Singh osteoporosis index as determined on radiographs.Results: The torsional load to failure averaged 98.9 N-m for the notched femora and 143.9 N-m for the controls; the difference was significant (p < 0.01). Although several variables correlated with torsional load to failure, distal femoral bone-mineral density demonstrated the highest significant correlation (r = 0.85; p < 0.001). Moreover, multiple regression analysis showed that a combination of distal femoral bone-mineral density and polar moment of inertia calculated with the posterior cortical thickness (adjusted r 2 = 0.79; p < 0.001) had the strongest prediction of torsional load to failure in the notched group. The addition of other measures of cortical bone geometry, proximal femoral bone-mineral density, or radiographic evidence of osteopenia did not significantly increase the models predictive ability.Conclusions: Femoral notching significantly decreases distal femoral torsional load to failure and is best predicted by a combination of the measures of distal femoral bone-mineral density and polar moment of inertia. Together, these values account for the amount of bone mass present and the stability provided by the cortical shell architecture.Clinical Relevance: Femoral notching during total knee arthroplasty decreases distal femoral torsional load to failure. By examination of femoral bone density and distal femoral geometry, the relative decrease in torsional load to failure can be predicted and appropriate precautions taken.

Hemivertebral excision for congenital scoliosis in very young children.

The present study reports the results of a consecutive series of six very young children who underwent single-anesthetic sequential anterior and posterior hemivertebral excision. The children, all less than 34 months old (mean age 19 months), presented with high magnitude or progressive congenital scoliosis related to an unbalanced hemivertebra. Curve correction required hemivertebral excision, which was accomplished during a single operative event using sequential anterior and posterior procedures. The intraoperative curve correction was maintained with plaster immobilization for 3 months. All patients were followed for at least 24 months. Pre-and postoperative spinal radiographs were analyzed for initial and final curve correction. Excellent correction of preoperative deformity was obtained and maintained throughout the follow-up period. The mean postoperative curve correction (67%; range 52%–84%) compared favorably with the average correction at final follow-up (70%; range 50%–85%). Radiographs revealed a consistently solid arthrodesis with no evidence of curve progression. There were no neurologic or other significant complications. In conclusion, single-anesthetic sequential anterior and posterior hemivertebral excision appears to be a safe and efficacious procedure for the management of congenital scoliosis in very young children.

Lumbar sagittal contour after posterior interbody fusion: Threaded devices alone versus vertical cages plus posterior instrumentation

Study Design. An observational radiographic study examining lumbar sagittal contour of patients undergoing posterior interbody arthrodesis. Objectives. To compare operative alterations of lumbar sagittal contour after posterior interbody fusion using threaded interbody devices alone versus vertical cages combined with posterior compression instrumentation. Summary of Background Data. Technique-related alterations of lumbar sagittal contour during interbody arthrodesis have received little attention in the spinal literature. Methods. Standing lumbar radiographs were measured for preoperative and postoperative segmental lordosis at levels undergoing posterior interbody arthrodesis using either stand-alone side-by-side threaded devices or vertical cages combined with posterior transpedicular compression instrumentation. Sagittal plane segmental correction (or loss of correction) was calculated and statistically compared. Results. The radiographs of 30 patients (34 spinal segments) undergoing lumbar or lumbosacral arthrodesis were compared. Seventeen patients (18 segments) had undergone interbody fusion using threaded cages, whereas 13 patients (16 segments) underwent fusion using vertically oriented mesh cages combined with posterior compression instrumentation. Preoperative segmental lordosis averaged 8° for both groups. For patients undergoing fusion with threaded cages, there was a mean lordotic loss of 3°/segment. For patients undergoing fusion with vertically oriented mesh cages combined with posterior compression instrumentation, there was a mean lordotic gain of 5°/segment. This difference in segmental sagittal plane contour was highly significant (P = 0.00). Conclusion. Threaded fusion devices placed under interbody distraction with the endplates parallel fail to preserve or reestablish segmental lordosis. Vertical cages, however, when combined with posterior compression instrumentation, not only maintain segmental lordosis, but also can correct sagittal plane deformity.