Which vertebrae should be assessed in diagnosing osteoporosis by plain radiography? Comparative study of radiographic findings and bone mineral density measured by dual energy X-ray absorptiometry

Biomechanical comparison of 4 fixation techniques of sacral pedicle screw in osteoporotic condition

STUDY DESIGN

An in vitro biomechanical cadaver study.

OBJECTIVES

To compare the subsidence displacement after cyclic loading among 4 sacral pedicle screw fixations of bicortical, tricortical, standard polymethylmethacrylate (PMMA) augmentation, and sub-endplate PMMA augmentation in osteoporotic condition.

SUMMARY OF BACKGROUND DATA

Implant failure caused by screw loosening is a clinical problem for lumbosacral fusions, especially in osteoporotic patients. To improve sacral screw anchoring strength, the main fixation techniques need to be evaluated biomechanically.

METHODS

For this study, 11 fresh osteoporotic cadaver sacra were harvested and bone mineral density was measured with dual-energy radiograph absorptiometry. A 7 mm diameter monoaxial pedicle screw (S1) was randomly assigned by side (left vs. right) and placed bicortically or tricortically. The 2 screws, followed 2000 cyclic compression loading of 30 to 250 N, were removed. The screw tracts were filled up with PMMA, then, screws 5 mm shorter than the bicortical or tricortical fixation were reinserted (defined as standard and sub-endplate PMMA augmented sacral screw fixations, respectively). The PMMA augmented screws were then retested as before. Screw subsidence displacement after 2000 cyclic loading was measured and compared.

RESULTS

The average bone mineral density of 11 specimens was 0.71 g/cm, ranged from 0.65 to 0.78 g/cm. No significant difference of subsidence displacement was detected between tricortical and standard PMMA augmented screws (P>0.05), however, the 2 fixations exhibited markedly less subsidence than bicortical screw (P<0.05). Sub-endplate PMMA augmented screw showed the least subsidence among all the screws (P<0.05).

CONCLUSIONS

PMMA augmentation can increase the bonding strength of sacral screw-bone interface and the sub-endplate PMMA augmented sacral screw could obtain the highest stability among the 4 fixation techniques in osteoporotic condition.

Effect of the degree of osteoporosis on the biomechanical anchoring strength of the sacral pedicle screws: an in vitro comparison between unaugmented bicortical screws and polymethylmethacrylate augmented unicortical screws

STUDY DESIGN

An in vitro laboratory study.

OBJECTIVE

(i) To evaluate the effect of osteoporotic degree in determining the strength of sacral screw fixation and (ii) to compare the strength of unaugmented bicortical pedicle screw and polymethylmethacrylate (PMMA) augmented unicortical pedicle screw in sacral fixation.

SUMMARY OF BACKGROUND DATA

Screw loosening is a clinical problem in lumbosacral fusions, especially in osteoporotic patients. To improve the screw anchoring strength of sacrum, bicortical and PMMA augmented sacral pedicle screw fixation techniques are widely used in clinical practice. However, the biomechanical strength of the bicortical and PMMA augmented sacral screw fixations remains undetermined in different degrees of osteoporosis.

METHODS

Twenty-five fresh osteoporotic cadavers were used in this study. According to the value of lumbar bone mineral density (BMD) assessed by DEXA, specimens were divided into 3 groups: group A (N=9): BMD=0.7 to 0.8 g/cm, group B (N=8): BMD=0.6 to 0.7 g/cm, and group C (N=8): BMD<0.6 g/cm. In each specimen, S1 pedicle screw was inserted bicortically on the left side, and S1 pedicle screw with PMMA augmentation was inserted unicortically on the right side of the sacrum. Following a dynamic cyclic loading from 30 to 250 N on the screw head for 2000 cycles, the subsidence displacement and axial pull-out strength of each screw were measured.

RESULTS

No anchoring failure (defined as the subsidence displacement exceeding 2 mm within 2000 loading cycles) occurred in group A and B. However, in group C, 6 cases (75%) in bicortical fixation and 5 cases (63%) in PMMA augmented fixation failed during cyclic loading. In group A, no significant difference between the bicortical and PMMA augmented fixations was detected in terms of the subsidence and maximal pull-out strength. In group B, significantly less subsidence and higher maximal pull-out strength were demonstrated in the PMMA augmented technique than that in the bicortical fixation. Both techniques exhibited lower subsidence of the screw in group A than in group B. The bicortical technique exhibited higher maximum pull-out strength in group A than that in group B. However, statistical difference in terms of PMMA augmentation was not detected between group A and B.

CONCLUSION

For BMD value more than 0.70 g/cm, bicortical sacral pedicle screw fixation could obtain sufficient anchoring strength comparable with the PMMA augmented technique. When BMD value is within 0.6 to 0.7 g/cm, the PMMA augmented technique would be more beneficial in improving the fixation strength than the bicortical fixation. For BMD values less than 0.6 g/cm, early screw loosening may occur in both bicortical and PMMA augmented fixations.

Flattening of sagittal spinal curvature as a predictor of vertebral fracture

In a 12-year prospective study of 100 adult volunteers, incident vertebral fractures were analyzed for potential predictors. Decreased sagittal spinal curvature and pre-existing fractures were identified as independent predictors. The risk of vertebral fracture increased to 8.6-fold with a 1 SD decrease in both thoracic and lumbar curves.

INTRODUCTION

Physiological spinal curvature acts as a shock absorber: however, it has not been demonstrated whether reduced spinal curvature is a risk of vertebral fracture. The aim of this prospective study is to assess the relationship between the extent of sagittal spinal curvature and incidence of vertebral fractures.

METHODS

One hundred community-based volunteers aged 50+ years were followed radiologically for at least 10 years. Entire spine radiograms of volunteers in erect position were used for evaluating sagittal spinal curvatures, grade of osteoporosis, and incident vertebral fractures.

RESULTS

Mean baseline age was 61.9 years and mean follow-up period was 12.0 years. In multivariate analysis, preexisting vertebral fracture (relative risk [RR] 3.70, 95% confidence interval [CI] 1.22-11.20), and a decrease in either thoracic or lumbar curvature by 1-SD (RR 3.06, 95% CI 1.04-9.00) were independent predictors. The fracture risk even increased in the presence of a 1-SD decrease in both thoracic and lumbar curvature (RR 8.64, 95% CI 1.39-53.78).

CONCLUSIONS

Reduction of physiological curvatures in both thoracic and lumbar spine led to the increased risk of vertebral fracture by more than eightfold. The importance of evaluating sagittal spinal alignment should be emphasized, because it might indicate the risk of the first vertebral fracture, which is another strong predictor of subsequent fractures.

A prospective study of de novo scoliosis in a community based cohort

STUDY DESIGN

A 12-year prospective study of de novo scoliosis in a community based cohort. OBJECTIVE.: To investigate factors associated with development of de novo scoliosis.

SUMMARY OF BACKGROUND DATA

De novo scoliosis is becoming one of the most prevalent findings in the aging spine, and this condition is associated not only with severe back or leg symptoms but also with complicated surgical outcomes. Cross-sectional studies were limited in distinguishing de novo scoliosis from preexisting deformities, and there had been controversies over the etiology of degenerative scoliosis.

METHODS

Community based volunteers were recruited, then examined by orthopedic physicians. Radiologic measurements using entire spine radiographs included the angle of scoliosis and sagittal spinal curvatures, sagittal spinal balance, grade of bone atrophy, number of degenerated discs, and vertebral fractures. We defined radiologic parameters, the disc index, and lateral osteophyte difference to evaluate the asymmetrical spinal degeneration.

RESULTS

A total of 60 subjects aged 50-84 years and without scoliosis at baseline were selected and followed for a mean of 12.0 years. De novo scoliosis >or=10 degrees developed in 22 subjects, and logistic regression analysis revealed that the baseline disc index and lateral osteophyte difference values were independent predictors (P < 0.05).

CONCLUSIONS

Incidence of de novo scoliosis was predictable by assessing asymmetric disc degeneration in frontal radiograph. More than 20% decrease in unilateral disc height or more than 5 mm longer osteophyte on one side led to increased incidence of de novo scoliosis, which might also influence long-term results of spinal surgery.