Influence of spine morphology on intervertebral disc loads and stresses in asymptomatic adults: implications for the ideal spine

Influence of single-level lumbar degenerative disc disease on the behavior of the adjacent segments--a finite element model study

The current study investigated mechanical predictors for the development of adjacent disc degeneration. A 3-D finite element model of a lumbar spine was modified to simulate two grades of degeneration at the L4-L5 disc. Degeneration was modeled by changes in geometry and material properties. All models were subjected to follower preloads of 800N and moment loads in the three principal directions of motion using a hybrid protocol. Degeneration caused changes in the loading and motion patterns of the segments above and below the degenerated disc. At the level (L3-L4) above the degenerated disc, the motion increased due to moderate degeneration by 21% under lateral bending, 26% under axial rotation and 28% under flexion/extension. At the level (L5-S1) below the degenerated disc, motion increased only during lateral bending by 20% due to moderate degeneration. Both the L3-L4 and L5-S1 segment showed a monotonic increase in both the maximum von Mises stress and shear stress in the annulus as degeneration progressed for all loading directions, expect extension at L3-L4. The most significant increase in stress was observed at the L5-S1 level during axial rotation with nearly a ten-fold increase in the maximum shear stress and 103% increase in the maximum von Mises stress. The L5-S1 segment also showed a progressive increase in facet contact force for all loading directions with degeneration. Nucleus pressure did not increase significantly for any loading direction at either the caudal or cephalic adjacent segment. Results suggest that single-level degeneration can increase the risk for injury at the adjacent levels.

Juvenile idiopathic scoliosis treated with posterior arthrodesis and segmental pedicle screw instrumentation before the age of 9 years: a 5-year follow-up

Study design

Retrospective study.

Objective

To evaluate the radiological results of fusion with segmental pedicle screw fixation in juvenile idiopathic scoliosis with a minimum 5-year follow-up.

Summary of background data

Progression of spinal deformity after posterior instrumentation and fusion in immature patients has been reported by several authors. Segmental pedicle screw fixation has been shown to be effective in controlling both coronal and sagittal plane deformities. However, there is no long term study of fusion with segmental pedicle screw fixation in these group of patients.

Methods

Seven patients with juvenile idiopathic scoliosis treated by segmental pedicle screw fixation and fusion were analyzed. The average age of the patients was 7.4 years (range 5–9 years) at the time of the operation. All the patients were followed up 5 years or more (range 5–8 years) and were all Risser V at the most recent follow up. Three dimensional reconstruction of the radiographs was obtained and 3DStudio Max software was used for combining, evaluating and modifying the technical data derived from both 2d and 3d scan data.

Results

The preoperative thoracic curve of 56 ± 15° was corrected to 24 ± 17° (57% correction) at the latest follow-up. The lumbar curve of 43 ± 14° was corrected to 23 ± 6° (46% correction) at the latest follow-up. The preoperative thoracic kyphosis of 37 ± 13° and the lumbar lordosis of 33 ± 13° were changed to 27 ± 13° and 42 ± 21°, respectively at the latest follow-up. None of the patients showed coronal decompensation at the latest follow-up. Four patients had no evidence of crankshaft phenomenon. In two patients slight increase in Cobb angle at the instrumented segments with a significant increase in AVR suggesting crankshaft phenomenon was seen. One patient had a curve increase in both instrumented and non instrumented segments due to incorrect strategy.

Conclusion

In juvenile idiopathic curves of Risser 0 patients with open triradiate cartilages, routine combined anterior fusion to prevent crankshaft may not be warranted by posterior segmental pedicle screw instrumentation.

Early and intermediate follow-up results after treatment of degenerative disc disease with the Bryan cervical disc prosthesis: single- and multiple-level

STUDY DESIGN

Our clinical study design was prospective, concurrently enrolled and single-center trial of the new functional intervertebral cervical disc prosthesis (Medtronic Sofamor Danek, Memphis, TN) in the treatment of patients with single-level and multiple-level degenerative disc disease of the cervical spine.

OBJECTIVE

The study was designed to investigate the surgical technology skills and clinical effects of Bryan cervical disc prosthesis in Chinese, and to observe the stability and range of movement in the early and immediate postoperative period.

SUMMARY OF BACKGROUND DATA

Recently, motion preservation has come to the forefront of emerging technologies in spine surgery. This is the important background information of the emergence of cervical arthroplasty as an alternative to arthrodesis that offers the promise of restoring normal spinal movement and reduces a kinematic strain on adjacent segments.

METHODS

Nineteen patients (23 discs) had spinal arthroplasty with placement of the Bryan cervical artificial disc prosthesis in our hospital. Clinical and radiologic follow-up was performed. The radiographic parameters evaluated included the treated segments and the overall curvature of the cervical spine.

RESULTS

All the patients were observed up from 8 months to 42 months (average 24 months). According to Odom's scale, all of 19 patients (23 levels) had excellent to good outcome. The range of movement recovered to the preoperative value during the follow up. The treated segment ultimately showed preservation of movement when compared with preoperative levels. No prosthesis subsidence or excursion was identified.

CONCLUSION

Arthroplasty using the Bryan disc seemed to be safe and provided encouraging clinical and radiologic outcome in our study. Although early and intermediate results are promising, this is also a relatively new approach, long-term follow up studies are required to prove its efficacy and its ability to prevent adjacent segment disease.

Long-term outcome after anterior cervical discectomy without fusion

To retrospectively study the long-term outcome of patients after anterior cervical discectomy without fusion (ACD) compared to results published on the long-term outcome after ACD with fusion (ACDF). We reviewed the charts of all patients receiving ACD surgery between 1985 and 2000 to analyze the direct post-operative results as well as complications of the surgery. Moreover, 102 patients, randomly selected, were interviewed with the neck disability index to study possible persisting complaints up to 18 years after ACD surgery. A total of 551 Patients were identified. Two months post-operative follow up at the outpatient clinic revealed that 90.1% of patients were satisfied with the result of ACD surgery. At the time of the survey, this percentage had dropped to 67.6%. In addition, 20.6% and 11.8% had obtained moderate to severe complaints, respectively, in daily-life activities. Complaints were mainly localized in the neck region and occasionally provoked radiating pain in the arm. On the short term, ACD leads to a satisfied outcome. Over the longer term, patients report increasing complaints. The increase in complaints at the time of the survey may be the result of ongoing degenerative effects. Compared to published data on ACDF, there is no superiority of any fusion technique compared to ACD alone.

Gravitational forces and sagittal shape of the spine. Clinical estimation of their relations

The sagittal morphology of the pelvis determines the amount of lordosis needed for each individual. The proper harmony of the sagittal spinal curves allows a stable balance, economical in terms of mechanical effects and muscular energy. A previous barycentremetrical laboratory study allowed us to demonstrate that the axis of gravity of the upper body segment was located behind the lumbar vertebrae and the femoral heads, thus ensuring economy and stability. The determination of the anatomical connection of the individual gravity is thus of primary importance for the evaluation of sagittal balance. Data for 42 patients without spinal pathology, previously evaluated by barycentremetry, were used to establish a predictive equation for the application point of the gravity at the level of the third lumbar vertebra (L3). This equation, using anthropometric and radiographic pelvic and spinal parameters, was integrated into a software program called Similibary. It was applied to the same 42 subjects. These results were compared in order to validate the method. No significant difference was observed between the two techniques. This easy-to-use tool allows a personalised evaluation of the sagittal balance of the spine, both through the evaluation of the harmonious relationship between the spinal curves and the pelvis, and through the location of gravity supported by the vertebral structures in L3.

Thoracic kyphosis affects spinal loads and trunk muscle force

BACKGROUND AND PURPOSE

Patients with increased thoracic curvature often come to physical therapists for management of spinal pain and disorders. Although treatment approaches are aimed at normalizing or minimizing progression of kyphosis, the biomechanical rationales remain unsubstantiated.

SUBJECTS

Forty-four subjects (mean age [+/-SD]=62.3+/-7.1 years) were dichotomized into high kyphosis and low kyphosis groups.

METHODS

Lateral standing radiographs and photographs were captured and then digitized. These data were input into biomechanical models to estimate net segmental loading from T2-L5 as well as trunk muscle forces.

RESULTS

The high kyphosis group demonstrated significantly greater normalized flexion moments and net compression and shear forces. Trunk muscle forces also were significantly greater in the high kyphosis group. A strong relationship existed between thoracic curvature and net segmental loads (r =.85-.93) and between thoracic curvature and muscle forces (r =.70-.82).

DISCUSSION AND CONCLUSION

This study provides biomechanical evidence that increases in thoracic kyphosis are associated with significantly higher multisegmental spinal loads and trunk muscle forces in upright stance. These factors are likely to accelerate degenerative processes in spinal motion segments and contribute to the development of dysfunction and pain.

Mathematical calculation of pedicle subtraction osteotomy size to allow precision correction of fixed sagittal deformity

STUDY DESIGN

This is a retrospective review of 15 consecutive fixed sagittal plane deformity patients who have undergone pedicle subtraction osteotomies. The focus of this article is the application of a trigonometric equation that calculates the degree of correction needed to achieve sagittal balance. The intraoperative predictive accuracy and clinical radiographic results of using this mathematical equation are discussed.

OBJECTIVE

The need for a precise and reproducible planning tool for the correction of sagittal imbalance prompted us to apply a simple trigonometric equation to achieve the desired sagittal alignment of the spine.

SUMMARY OF BACKGROUND DATA

Establishing sagittal balance has been widely recognized as one of the most important parameters in optimizing outcomes for spinal reconstruction patients. Preoperative planning for sagittal plane correction in adult spinal deformity has traditionally been done by estimation or with cumbersome film cutouts. To our knowledge, there has not been a consistent method of calculating the exact number of degrees needed to reestablish spinal balance.

METHODS

Patients' C7 plumb lines are measured on a 36-inch radiograph to assess the degree of sagittal imbalance and determine how many degrees of correction (and subsequent millimeters of wedge resection) are needed. Applying a basic trigonometric formula for the tangent to the sagittal alignment is used to do this.

RESULTS

We have used this technique reliably in a series of 15 consecutive patients to reestablish sagittal balance. The predicted degree of correction was compared to the achieved degree of correction at the site of the osteotomy. This comparison was accurate to within 3 degrees (the standard error of measurement for the method of Cobb) in all cases except 2.

CONCLUSIONS

By using a simple mathematical equation, one can reliably determine the degree of pedicle subtraction osteotomy needed for correction of sagittal deformity. This technique is reproducible and has led to successful clinical outcomes.

Reliabilities of and correlations among five standard methods of assessing the sagittal alignment of the cervical spine

STUDY DESIGN

The reliabilities of and correlations among 5 standard methods of assessing cervical sagittal alignment were evaluated.

OBJECTIVE

To investigate the reliabilities of and correlations among 5 standard methods of assessing cervical sagittal alignment.

SUMMARY OF BACKGROUND DATA

Although various cervical sagittal alignment assessment methods are widely used, their relative reliability and intercorrelation have not been reported.

METHODS

From 442 lateral cervical radiographs, 40 with lordotic, 40 with straight or sigmoid, and 40 with kyphotic alignment were selected. Two orthopedic surgeons independently evaluated the sagittal alignment in each group twice using CCL, C1-C7 Cobb, C2-C7 Cobb, sagittal tangent, and the Ishihara methods. Intraobserver and interobserver reliabilities were confirmed and the correlations among the 5 methods were measured.

RESULTS

Intraobserver and interobserver reliabilities for all 5 methods were good. In the lordotic group, the correlations among all 5 methods were consistently strong (r = 0.731 to 0.922). In the straight or sigmoid group, the correlations were weak to moderate among the CCL, C2-C7 Cobb, sagittal tangent, and Ishihara methods but tended to be weak between these 4 methods and the C1-C7 Cobb method (r = -0.245 to 0.777). In the kyphotic group, the correlations were also weak to moderate among the same 4 methods, and were statistically insignificant between them and the C1-C7 Cobb.

CONCLUSIONS

The correlations among the CCL, C1-C7 Cobb, C2-C7 Cobb, sagittal tangent, and Ishihara methods are strong when lordosis is retained; otherwise, they are moderate to poor. In the kyphotic group, C1-C7 Cobb has no significant correlation with the other 4 methods.

Validation of a computer analysis to determine 3-D rotations and translations of the rib cage in upright posture from three 2-D digital images

Since thoracic cage posture affects lumbar spine coupling and loads on the spinal tissues and extremities, a scientific analysis of upright posture is needed. Common posture analyzers measure human posture as displacements from a plumb line, while the PosturePrint claims to measure head, rib cage, and pelvic postures as rotations and translations. In this study, it was decided to evaluate the validity of the PosturePrint Internet computer system's analysis of thoracic cage postures. In a university biomechanics laboratory, photographs of a mannequin thoracic cage were obtained in different postures on a stand in front of a digital camera. For each mannequin posture, three photographs were obtained (left lateral, right lateral, and AP). The mannequin thoracic cage was placed in 68 different single and combined postures (requiring 204 photographs) in five degrees of freedom: lateral translation (Tx), lateral flexion (Rz), axial rotation (Ry), flexion-extension (Rx), and anterior-posterior translation (Tz). The PosturePrint system requires 13 reflective markers to be placed on the subject (mannequin) during photography and 16 additional "click-on" markers via computer mouse before a set of three photographs is analyzed by the PosturePrint computer system over the Internet. Errors were the differences between the positioned mannequin and the calculated positions from the computer system. Average absolute value errors were obtained by comparing the exact inputted posture to the PosturePrint computed values. Mean and standard deviation of computational errors for sagittal displacements of the thoracic cage were Rx=0.3+/-0.1 degrees , Tz=1.6+/-0.7 mm, and for frontal view displacements were Ry=1.2+/-1.0 degrees , Rz=0.6+/-0.4 degrees , and Tx=1.5+/-0.6 mm. The PosturePrint system is sufficiently accurate in measuring thoracic cage postures in five degrees of freedom on a mannequin indicating the need for a further study on human subjects.