Use of fluoroscopy to evaluate iliac screw position.

Iliac screw fixation is often used for long fusions to the sacropelvis. Maximum iliac screw purchase is obtained both by placing the screws within 1.5 cm of the greater sciatic notch and by extending them anterior to the axis of rotation in flexion-extension. Screw insertion is "blinded" or dependent on tactile feedback, and hence extreme care is necessary to avoid incorrect placement and damage to vital neurovascular structures in the pelvis and sciatic notch. Long screws may violate the hip joint while medial placement may injure the lumbosacral plexus and the nearby vessels. To explore the best intraoperative fluoroscopic method of determining optimal iliac screw placement, we used a synthetic pelvis model to investigate screw placement conditions: (1) optimal anatomic placement, (2) violation of the sciatic notch, (3) hip joint violation, (4) medial wall violation, and (5) lateral wall violation. Each condition was examined utilizing fluoroscopy with posteroanterior, inlet, outlet, lateral, iliac oblique, and obturator oblique Judet views to simulate operative conditions. These views were obtained to evaluate critical malposition of iliac screws. We found that, for a sciatic notch violation, the obturator oblique view best demonstrated the cortical breech, while for a hip joint violation, the inlet and outlet views were best. For a medial wall violation, the iliac oblique view best showed the violation. For a lateral wall violation, we were unable to demonstrate the cortical breech using these fluoroscopic views. Fluoroscopy is an effective method to determine sciatic notch, hip joint, and medial wall violations after iliac screw placement; however, it is not effective in identifying a lateral wall violation.

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 model's 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.

Braided hamstring tendons for reconstruction of the anterior cruciate ligament. A biomechanical analysis.

BACKGROUND: In an effort to improve the strength and stiffness of anterior cruciate ligament grafts, several authors have advocated alterations of graft structure and orientation, including braiding the tendons in hamstring tendon grafts. HYPOTHESIS: Braiding hamstring tendons does not increase graft strength and stiffness. STUDY DESIGN: Controlled laboratory study. METHODS: Sixteen hamstring tendon and 21 bone-patellar tendon-bone grafts were harvested from 12 cadavers and divided into three groups: 1) braided four-strand hamstring tendon, 2) unbraided four-strand hamstring tendon, and 3) bone-patellar tendon-bone. All grafts were placed under a 50-N preload on a servohydraulic testing device and were tensioned to failure. RESULTS: The strength and stiffness of the tested specimens averaged 427 +/- 36 N and 76 +/- 10 N/mm, respectively, for braided specimens, 532 +/- 44 N and 139 +/- 18 N/mm for unbraided specimens, and 574 +/- 46 N and 158 +/- 15 N/mm for patellar tendon specimens. There was a 20% decrement in hamstring tendon graft tensile strength and a 45% decrease in stiffness after braiding because of the suboptimal multidirectional orientation of individual tendons within the braided grafts. CONCLUSIONS: In vitro braided hamstring tendon grafts demonstrated mechanically inferior strength and stiffness characteristics compared with unbraided hamstring tendon grafts and patellar tendon grafts. CLINICAL RELEVANCE: Braiding of hamstring tendon grafts provides no mechanical advantage in anterior cruciate ligament reconstruction.

Anterior thoracic scoliosis constructs: effect of rod diameter and intervertebral cages on multi-segmental construct stability.

BACKGROUND CONTEXT: Many studies have reported on the use of anterior instrumentation for thoracolumbar scoliosis and more recently thoracic scoliosis. However, the optimal construct design remains an issue of debate. PURPOSE: To optimize construct design and enhance implant survival until a successful spinal arthrodesis is achieved. STUDY DESIGN: This study evaluated the effect of rod diameter and intervertebral cages on construct stiffness and rod strain using a long-segment, anterior thoracic scoliosis model with varying levels of intervertebral reconstruction. METHODS: Sixteen fresh-frozen calf spine specimens (T1 to L1) were divided into two groups based on rod diameter reconstruction (4 mm and 5 mm). Testing included axial compression, anterior flexion, extension and lateral bending with variations in the number and level of intervertebral cage reconstructions: apical disc (one), end discs (two), apical and end discs (three), all seven levels (seven). Multisegmental construct stiffness and rod strain were determined and normalized to the intact specimen for analysis. RESULTS: The seven-level intervertebral cage construct showed significantly greater stiffness in axial compression for both the 4-mm (366% increased stiffness) and 5-mm (607% increased stiffness) rod groups (p<.001). The remaining constructs were not significantly different from each other (p>.05). In flexion, similar results were obtained for the 4-mm construct (p<.001) but not the 5-mm construct, because the reconstruction-alone, one-, two- and three-cage constructs were all significantly stiffer than the intact specimen (p<.05). Multisegmental construct stiffness under extension loading, as well as right and left lateral bending, also exhibited significant differences between the seven-level interbody cage reconstructions and the remaining constructs. Apical rod strain for both the 4-mm-rod and 5-mm-rod groups were significantly higher for the two cage constructs (a cage at either end but not the apex where the strain gauges were located) as compared with the other constructs (p<.05). These differences were more pronounced in the 4-mm-rod group. Similar results were obtained in anterior flexion, extension and lateral bending. CONCLUSIONS: Intervertebral cages at every level significantly improved construct stiffness compared with increasing rod diameter alone. Moreover, cages markedly decreased rod strain, and when structural interbody supports were not used, axial compression created the greatest rod strain.

The effects of tibial malrotation on the biomechanics of the tibiotalar joint.

The effects of tibial malrotation on the biomechanics of the tibiotalar joint were studied using a cadaveric model loaded in an Instron 8521 materials testing device and a TEKScan I-Scan thin-film resistive ink pressure measuring system. Testing of 23 legs was performed using rotational conditions of 10 and 20 degrees internal and external rotation as well as neutral rotation. All rotational conditions were found to decrease joint contact area. Peak pressures were significantly greater with 20 degrees internal rotation as well as 20 degrees external rotation. Total load across the joint was significantly lower for both 10 and 20 degrees of external rotation. In conclusion, rotational deformity across the tibiotalar joint results in significant alteration of overall joint biomechanics and should be minimized whenever possible.

Thoracic pedicle screws: comparison of start points and trajectories.

STUDY DESIGN: Experimental design using cadaveric computerized tomography (CT) scans and a computer-assisted image guidance system to compare various thoracic pedicle screw start points and trajectories. OBJECTIVE: To compare described thoracic pedicle screw start points and trajectories to determine which allows strictly intrapedicular screw placement with the most margin of error. SUMMARY OF BACKGROUND DATA: Thoracic pedicle screws are being used in a variety of spinal conditions to include fracture, tumor, and deformity. Optimal thoracic pedicle screw start points have received increasing attention in the literature. Optimal thoracic pedicle trajectory is still undetermined. METHODS: Using fine cut CT scans of 3 cadaveric male specimens (aged 65-70 years) loaded onto a computer-assisted image guidance system, 966 pedicle screws, were virtually inserted. The effective pedicle diameter (EPD) and maximum insertional arc (MIA) was assessed using 3 different trajectories and start points: (1) straight ahead, (2) straight forward, and (3) anatomic. EPD was measured by placing a maximum-sized virtual screw, using a specific trajectory, without cortical violation of the pedicle and/or the vertebral body. The MIA was assessed by measurement of the angle formed by the most superiorly and inferiorly directed 0.1-mm virtual screw through a given start point without violation of the pedicle cortex and obtaining at least 50% vertebral body purchase. RESULTS: Mean EPD in the sagittal plane was 7.6 +/- 0.3 (SEM) mm for the straight forward trajectory and 9.1 +/- 0.3 (SEM) mm for the anatomic trajectory, a 20% increase (P < 0.0005). Mean EPD in the axial plane was 4.1 +/- 0.2 (SEM) mm for the straight ahead trajectory and 5.0 +/- 0.2 (SEM) mm for the anatomic trajectory, a 22% increase (P < 0.0005). EPD was found to be statistically different based on the trajectory used for placement in both the axial and sagittal planes in the upper (T1-T4), middle (T5-T8), and lower (T9-T12) thoracic spine. Mean MIA in the sagittal plane was 18.7 +/- 1.1 (SEM) for straight ahead start points, 25.8 degrees +/- 0.8 degrees (SEM) for straight forward start points, and 30.2 degrees +/- 0.8 degrees (SEM) for anatomic start points, a 38% increase (P < 0.0005) in MIA compared with straight ahead and a 17% increase (P < 0.0005) in MIA compared with straight forward. Mean MIA in the axial plane was 17.8 degrees +/- 0.6 degrees (SEM) for straight ahead and anatomic start points, and 18.6 degrees +/- 0.6 degrees (SEM) for straight forward start points. This difference was not statistically significant (P = 0.086). MIA was found to be statistically different based on start points used in the sagittal, but not the axial plane, in the upper, middle, and lower thoracic spine. CONCLUSION: EPD and MIA are trajectory (EPD) and start point (MIA) dependent. In the axial plane, anatomic EPD was greater than straight ahead EPD. In the sagittal plane, anatomic EPD was greater than straight forward EPD. Using anatomic start points in the sagittal plane, a greater MIA is achievable. These data suggest that in the diminutive thoracic pedicle or when a larger screw is needed, an anatomic trajectory using anatomic start points may allow a larger bone channel for intrapedicular placement of instrumentation.

Comparison of sagittal contour and posterior disc height following interbody fusion: threaded cylindrical cages versus structural allograft versus vertical cages.

OBJECTIVE: Segmental restoration of sagittal contour is recognized as critical for improved long-term success following instrumented lumbar fusions. As such, the use of wedged implants has become more popular. Few studies exist to assess the postoperative lordotic and disc height changes following these varied techniques in spinal fusion. An observational radiographic study examining lumbar sagittal contour and posterior intervertebral disc space height following posterior lumbar interbody fusion (PLIF) or transforaminal lumbar interbody fusion (TLIF) was conducted using vertical cages (VCs), wedged structural allograft (WSA), and threaded cylindrical cages (TCCs). METHODS: Forty-nine consecutive patients (59 spinal segments) were evaluated following single- or two-level interbody fusion with either stand-alone TCCs (n = 18 levels), WSA with posterior transpedicular compression instrumentation (n = 25 levels), or VCs with posterior transpedicular compression instrumentation (n = 16 levels). Standing lumbar radiographs were measured by two independent observers preoperatively, immediately postoperatively (within 1 week), at 6-week follow-up (range 4-8 weeks), and postoperatively (at 1-year follow-up) for segmental lordosis at each level undergoing posterior interbody arthrodesis and posterior intervertebral disc space height to assess indirect nerve root decompression. RESULTS: At the 1-year follow-up, postoperative lordosis was improved in the VC group (+5.3 degrees ; P < 0.005), whereas it decreased in the WSA group (-0.9 degrees ; P = 0.407) and TCC group (-3.5 degrees ; P < 0.005). The posterior disc space height decreased in the VC group (-0.5 mm; P = 0.109), whereas it increased for both the WSA group (+1.2 mm; P = 0.05) and the TCC group (+0.8 mm; P = 0.219). CONCLUSIONS: PLIF with stand-alone TCC and PLIF (or TLIF) with WSA and posterior transpedicular instrumentation results in an increased posterior disc height and thus improved indirect nerve root decompression. PLIF (or TLIF) with VC and posterior transpedicular instrumentation results in an overall decrease in posterior disc height. However, TCC and WSA resulted in a loss of lumbar lordosis, whereas VC resulted in an increase in lumbar lordosis.

Microradiographic and histopathologic findings in a human cage explant after two-level corpectomy: a case report.

STUDY DESIGN: A case involving microradiographic and histopathologic analysis of an explanted human corpectomy mesh cage is reported. OBJECTIVE: To describe the clinical circumstance, the radiographic appearance, and the histopathologic assessment of a titanium mesh device explanted from a two-level corpectomy. SUMMARY OF BACKGROUND DATA: To the authors' knowledge, no published microradiographic or histopathologic reports have described a retrieved human corpectomy cage. METHODS: The explanted device was stained using Osteochrome Villanueva bone stain and underwent routine decalcified histologic processing and embedding in polymethylmethacrylate. Midsagittal sections were prepared and polished to 100 microm for histologic and microradiographic analysis. RESULTS: Microscopic analysis demonstrated normal-appearing lamellar and woven trabecular bone in close contact with the titanium implant interface. Further analysis of serial sections indicated that, on the average, 35% (range, 30-40%) of the inner device region contained trabecular bone. CONCLUSION: Osteosynthesis and bone remodeling can occur within titanium corpectomy cages. METHODS: This study involved one titanium mesh device (Harms cage), 20 mm in diameter and 45 mm long, explanted from a two-level corpectomy clinical case. This device was retrieved, processed, and analyzed after informed patient consent and approval from the authors' institutional review board.

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.

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.