Effectiveness of transfixation and length of instrumentation on titanium and stainless steel transpedicular spine implants

Biomechanical Effects of a Cross Connector in Sacral Fractures - A Finite Element Analysis

Background: Spinopelvic fractures and approaches of operative stabilization have been a source of controversial discussion. Biomechanical data support the benefit of a spinopelvic stabilization and minimally invasive procedures help to reduce the dissatisfying complication rate. The role of a cross connector within spinopelvic devices remains inconclusive. We aimed to analyze the effect of a cross connector in a finite element model (FE model). Study Design: A FE model of the L1-L5 spine segment with pelvis and a spinopelvic stabilization was reconstructed from patient-specific CT images. The biomechanical relevance of a cross connector in a Denis zone I (AO: 61-B2) sacrum fracture was assessed in the FE model by applying bending and twisting forces with and without a cross connector. Biomechanical outcomes from the numerical model were investigated also considering uncertainties in material properties and levels of osseointegration. Results: The designed FE model showed comparable values in range-of-motion (ROM) and stresses with reference to the literature. The superiority of the spinopelvic stabilization (L5/Os ilium) ± cross connector compared to a non-operative procedure was confirmed in all analyzed loading conditions by reduced ROM and principal stresses in the disk L5/S1, vertebral body L5 and the fracture area. By considering the combination of all loading cases, the presence of a cross connector reduced the maximum stresses in the fracture area of around 10%. This difference has been statistically validated (p < 0.0001). Conclusion: The implementation of a spinopelvic stabilization (L5/Os ilium) in sacrum fractures sustained the fracture and led to enhanced biomechanical properties compared to a non-reductive procedure. While the additional cross connector did not alter the resulting ROM in L4/L5 or L5/sacrum, the reduction of the maximum stresses in the fracture area was significant.

Cross-links in posterior pedicle screw-rod instrumentation of the spine: a systematic review on mechanical, biomechanical, numerical and clinical studies

Purpose

Dorsal screw-rod instrumentations are used for a variety of spinal disorders. Cross-links (CL) can be added to such constructs, however, no clear recommendations exist. This study aims to provide an overview of the available evidence on the effectiveness of CL, potentially allowing to formulate recommendations on their use.

Methods

A systematic literature review was performed on PubMed and 37 original articles were included and grouped into mechanical, biomechanical, finite element and clinical studies. The change in range of motion (ROM) was analyzed in mechanical and biomechanical studies, ROM, stiffness and stress distribution were evaluated in finite element studies and clinical outcome parameters were analyzed in clinical studies.

Results

A relative consistent reduction in ROM in axial rotation with CL-augmentation was reported, while minor and less consistent effects were observed in flexion–extension and lateral bending. The use of CLs was clinical beneficial in C1/2 fusion, while the limited clinical studies on other anatomic regions show no significant benefit for CL-augmentation.

Conclusion

While CL provides some additional axial rotation stability in most situations, lateral bending and flexion–extension are less affected. Based on clinical data, CL-augmentation can only be recommended for C1/2 instrumentations, while for other cases, further clinical studies are needed to allow for evidence-based recommendations.

Biomechanical Analysis of a Pedicle Screw-Rod System with a Novel Cross-Link Configuration

Study Design

The strength effects of a pedicle screw-rod system supplemented with a novel cross-link configuration were biomechanically evaluated in porcine spines.

Purpose

To assess the biomechanical differences between a conventional cross-link pedicle screw-rod system versus a novel cross-link instrumentation, and to determine the effect of the cross-links.

Overview of Literature

Transverse cross-link systems affect torsional rigidity, but are thought to have little impact on the sagittal motion of spinal constructs. We tested the strength effects in pullout and flexion-compression tests of novel cross-link pedicle screw constructs using porcine thoracic and lumbar vertebrae.

Methods

Five matched thoracic and lumbar vertebral segments from 15 porcine spines were instrumented with 5.0-mm pedicle screws, which were then connected with 6.0-mm rods after partial corpectomy in the middle vertebral body. The forces required for construct failure in pullout and flexion-compression tests were examined in a randomized manner for three different cross-link configurations: un-cross-link control, conventional cross-link, and cross-link passing through the base of the spinous process. Statistical comparisons of strength data were analyzed using Student's t-tests.

Results

The spinous process group required a significantly greater pullout force for construct failure than the control group (p=0.036). No difference was found between the control and cross-link groups, or the cross-link and spinous process groups in pullout testing. In flexion-compression testing, the spinous processes group required significantly greater forces for construct failure than the control and cross-link groups (p<0.001 and p=0.003, respectively). However, there was no difference between the control and cross-link groups.

Conclusions

A novel cross-link configuration that features cross-link devices passing through the base of the spinous processes increased the mechanical resistance in pullout and flexion-compression testing compared to un-cross-link constructs. This configuration provided more resistance to middle-column damage under flexion-compression testing than conventional cross-link configuration.

A survey of current controversies in scoliosis surgery in the United Kingdom

STUDY DESIGN

A 10-point questionnaire was constructed to identify the philosophy of surgeons on various aspects of scoliosis surgery, such as choice of implant, bone graft, autologous blood transfusion, cord monitoring, and computer-assisted surgery. Comparisons were then made with recommendations published in the spinal literature.

OBJECTIVE

To determine certain aspects of the current practice of scoliosis surgery in the United Kingdom.

SUMMARY OF BACKGROUND DATA

Guidelines for good clinical practice in spinal deformity surgery are available in the United Kingdom but do not cover a number of controversial issues.

METHODS

Consultants and fellows attended the 2009 British Scoliosis Society meeting. Fifty questionnaires were completed by 45 consultants and 5 fellows.

RESULTS

All pedicle screw constructs favored by 25 of 50, hybrid 24 of 50 (1 undecided). Posterior construct of fewer than 10 levels, 20 of 50 would not cross-link, 11 of 50 used 1, and 19 of 20 used 2 or more. More than 10 levels 17 of 50 considered cross-links unnecessary, 4 of 50 used 1 and 29 of 50 used 2 or more. Eighty-eight percent preferred titanium alloy implants, whereas others used a mixture of stainless steel and cobalt chrome. When using bone graft, respondents used bone substitutes (24), iliac crest graft (14), allograft (12) and demineralized bone matrix (9) in addition to local bone. Ten of 50 would use recombinant bone morphogenetic protein (3 for revision cases only). Thirty-nine of 50 routinely used intraoperative cell salvage and 4 of 50 never used autologous blood. All used cord monitoring: sensory (19 of 50), motor (2 of 50), and combined (29 of 50). None used computer-aided surgery. Twenty-six operated alone, 12 operated in pairs, and 12 varied depending on type of case.

CONCLUSION

This survey shows interesting variations in scoliosis surgery in the United Kingdom. It may reflect the conflicting evidence in the literature.

Video-assisted thoracoscopic spinal fusion compared with posterior spinal fusion with thoracic pedicle screws for thoracic adolescent idiopathic scoliosis

BACKGROUND

Although the gold standard for the surgical treatment of thoracic adolescent idiopathic scoliosis has been posterior spinal fusion, video-assisted thoracoscopic surgery recently has become a viable alternative. In the treatment of structural thoracic curves, video-assisted thoracoscopic surgery has demonstrated outcomes equivalent to those of posterior spinal fusion with use of an all-hook or hybrid pedicle screw-hook construct. No study to date, however, has compared this technique with posterior spinal fusion with thoracic pedicle screws, which has become the current standard of care.

METHODS

A matched-pair analysis of thirty-four consecutive patients (seventeen pairs) undergoing either video-assisted thoracoscopic surgery or posterior spinal fusion with thoracic pedicle screws for the treatment of structural scoliosis was performed; the study included eight male and twenty-six female patients with an average age of 15.0 years. Pairs were matched according to curve type and magnitude, patient age, and sex. Clinical data, the results of the Scoliosis Research Society questionnaire, and radiographic data were collected preoperatively and at a minimum of two years postoperatively and were compared between the groups.

RESULTS

Video-assisted thoracoscopic surgery was associated with significantly increased operative times (mean, 326 compared with 246 minutes; p = 0.033) and reduced blood loss (mean, 371 compared with 1018 mL; p = 0.001), but there were no differences between the groups in terms of the transfusion rate (18% compared with 29%; p = 0.69) or the length of stay. The percentage correction of the major curve was 57.3% for the video-assisted thoracoscopic surgery group and 63.8% for the posterior spinal fusion group (p = 0.08). With the numbers available, no differences were detected in terms of the cephalad thoracic curve, caudad compensatory lumbar curve, coronal balance, thoracic kyphosis, lumbar lordosis, sagittal balance, end vertebra tilt angle, or angle of trunk rotation measurements preoperatively or at the time of the latest follow-up. The average number of fused levels was 5.9 in the video-assisted thoracoscopic surgery group and 8.9 in the posterior spinal fusion group (p < 0.001). Relative to the Cobb end vertebra, the most caudad instrumented vertebra was 0.81 level more cephalad in the video-assisted thoracoscopic surgery group as compared with the posterior spinal fusion group (p = 0.004). No significant differences were detected in any of the questionnaire outcomes at any time point. Although both groups experienced similar improvement from baseline in terms of pulmonary function at two years, the posterior spinal fusion group had significantly improved peak flow measurements (p = 0.04) in comparison with the video-assisted thoracoscopic surgery group.

CONCLUSIONS

For single thoracic curves of <70 degrees in patients with a normal or hypokyphotic thoracic spine, video-assisted thoracoscopic surgery can produce equivalent radiographic results, patient-based clinical outcomes, and complication rates in comparison with posterior spinal fusion with thoracic pedicle screws, with the exception that posterior spinal fusion with thoracic pedicle screws may result in better major curve correction. The potential advantages of video-assisted thoracoscopic surgery over posterior spinal fusion with thoracic pedicle screws include reduced blood loss, fewer total levels fused, and the preservation of nearly one caudad fusion level, whereas the disadvantages include increased operative times and slightly less improvement in pulmonary function.

Cost-effectiveness of the treatment of traumatic thoracolumbar spine fractures: Nonsurgical or surgical therapy?

BACKGROUND

Spinal fractures can be an important cause for disabling back pain. Therefore, in judging the cost-effectiveness of nonsurgical or surgical therapy, not only direct costs but also the indirect costs should be calculated. In this prospective randomized study, the costs incurred by nonsurgically and surgically treated patients with a traumatic thoracolumbar spine fracture without neurological involvement were analysed.

MATERIALS AND METHODS

32 patients with a traumatic thoracolumbar spine fracture were prospectively randomized for operative or nonsurgical treatment. Patients were sent a questionnaire every three months to inquire about work-status, additional health costs and doctor visits. The patients who have minimum followup of two years were included.

RESULTS

Of thirty-two patients, 30 met the criterion of the followup period of at least two years. Fourteen patients received nonsurgical therapy, while 16 received surgical treatment. Direct costs of the treatment of nonsurgically treated patients were €10,608 ($12,730). For the operatively treated group, these costs were €18,769 ($22,523). Indirect costs resulted in a total of €219,187 ($263,025) per nonoperatively treated patient. In the operatively treated group, these costs were €66,004 ($79,206).

CONCLUSION

In the treatment of traumatic thoracolumbar spine fractures, the indirect costs exceed the direct costs by far and make up 95.4% of the total costs for treatment in nonsurgically treated patients and 71.6% of the total costs in the operative group. In view of cost-effectiveness, the operative therapy of traumatic thoracolumbar spine fractures is to be preferred.

Treatment of traumatic thoracolumbar spine fractures: a multicenter prospective randomized study of operative versus nonsurgical treatment

STUDY DESIGN

Multicenter prospective randomized trial.

OBJECTIVE

To test the hypotheses that thoracolumbar AO Type A spine fractures without neurologic deficit, managed with short-segment posterior stabilization will show an improved radiographic outcome and at least the same functional outcome as compared with nonsurgically treated thoracolumbar fractures.

SUMMARY OF BACKGROUND DATA

There are various opinions regarding the ideal management of thoracolumbar Type A spine fractures without neurologic deficit. Both operative and nonsurgical approaches are advocated.

METHODS

Patients were randomized for operative or nonsurgical treatment. Data sampling involved demographics, fracture classifications, radiographic evaluation, and functional outcome.

RESULTS

Sixteen patients received nonsurgical therapy, and 18 received surgical treatment. Follow-up was completed for 32 (94%) of the patients after a mean of 4.3 years. At the end of follow-up, both local and regional kyphotic deformity was significantly less in the operatively treated group. All functional outcome scores (VAS Pain, VAS Spine Score, and RMDQ-24) showed significantly better results in the operative group. The percentage of patients returning to their original jobs was found to be significantly higher in the operative treated group.

CONCLUSIONS

Patients with a Type A3 thoracolumbar spine fracture without neurologic deficit should be treated by short-segment posterior stabilization.

Pedicle screw fixation of the thoracic spine: an in vitro biomechanical study on different configurations

STUDY DESIGN

An in vitro biomechanical study of different pedicle screw configuration usage on the thoracic spine using a cadaveric model.

OBJECTIVES

To investigate the degree of motion afforded different pedicle screw configurations in the thoracic spine using a cadaveric model with 2 different degrees of intrinsic stability.

SUMMARY OF BACKGROUND DATA

Recently, thoracic pedicle screws have become an alternative to hook and wire fixation, and have gained popularity. Clinically, pedicle screw use has ranged from application to every segment, to skipping every other level. There exists no clear consensus as to which strategy is most appropriate.

METHODS

The load-displacement behavior of 6 different constructs was determined on 8 fresh frozen cadaver spine specimens (T4-T12). Each construct was evaluated on 2 destabilization models, including minimum destabilization (bilateral facetectomy) and maximum destabilization (facetectomy and annulotomy). Pure moments were applied, and the resultant range of motion for each scenario was determined.

RESULTS

Facetectomy did not significantly destabilize the thoracic spine. Annulotomy and facetectomy created gross instability that rendered testing of this destabilization model impossible. All constructs significantly reduced the range of motion compared to intact or facetectomized specimens (P < or = 0.001). When different constructs were compared to each other, a pattern of continuously increasing stability emerged, with the "maximum" construct being the most stable and "minimum" configuration being the least, with varying degrees of statistical significance.

CONCLUSIONS

Our results suggest that the most important factor for the acute postoperative stability of spinal fixation is the degree of preoperative or iatrogenic destabilization. The minimum amount of pedicle screws provides adequate stability when there is minimal destabilization of the spine. On the other hand, when anterior column release has been performed or instability exists before surgery, segmental pedicle screw fixation may be necessary to achieve adequate stability.

Torsional stability of cross-link configurations: a biomechanical analysis

BACKGROUND CONTEXT

Cross-link systems have been used to augment segmental spinal instrumentation since the earliest introduction of these fixation systems. Although transverse cross-links have little impact on sagittal motion of spinal constructs, cross-linkage does affect torsional rigidity. Despite the wide variety of cross-link designs, almost all have been configured as transverse devices. The relative mechanical benefit of different cross-link configurations is not known.

PURPOSE

The purpose of this study was to compare the torsional stability of three different cross-link configurations.

STUDY DESIGN

Biomechanical analysis of segmental instrumentation constructs using porcine spines.

METHODS

Thoracic porcine spines (T4 to T10) were instrumented with 6.5-mm conical pedicle screws and 7.0-mm connecting rods from T5 to T9. Terminal vertebrae were embedded in polymethylmethacrylate (PMMA) after a T7 corpectomy. Four cross-link configurations were tested in a randomized manner: Un-cross-linked Control (CONT); Transverse Rod-Rod (RR); Transverse Screw-Screw (SS); and Diagonal Screw-Screw (DX) Cross-links. The specimens were rotated to 3 Nm at a rate of 0.2 degrees/s and cycled six times with data acquisition over the final two cycles. Stiffness, rotation, and energy data were normalized to each control. A Newman-Keuls repeated measures analysis of variance was used to infer significant differences.

RESULTS

Diagonal cross-link configurations provided the most rigid construct. Transverse cross-links did not significantly change torsional behavior compared with the unlinked control. Rotation and energy expended were not significantly greater torsional stiffness compared with other constructs tested (p<.01).

CONCLUSIONS

The diagonal cross-link configuration provided increased torsional stiffness as compared with unlinked or transverse configurations. This observation should be considered in future cross-link designs.