Thoracoscopic discectomy and fusion in an animal model: safe and effective when segmental blood vessels are spared

Testing the impact of discoplasty on the biomechanics of the intervertebral disc with simulated degeneration: An in vitro study

Percutaneous Cement Discoplasty has recently been developed to relieve pain in highly degenerated intervertebral discs presenting a vacuum phenomenon in patients that cannot undergo major surgery. Little is currently known about the biomechanical effects of discoplasty. This study aimed at investigating the feasibility of modelling empty discs and subsequent discoplasty surgery and measuring their impact over the specimen geometry and mechanical behaviour. Ten porcine lumbar spine segments were tested in flexion, extension, and lateral bending under 5.4 Nm (with a 200 N compressive force and a 27 mm offset). Tests were performed in three conditions for each specimen: with intact disc, after nucleotomy and after discoplasty. A 3D Digital Image Correlation (DIC) system was used to measure the surface displacements and strains. The posterior disc height, range of motion (ROM), and stiffness were measured at the peak load. CT scans were performed to confirm that the cement distribution was acceptable. Discoplasty recovered the height loss caused by nucleotomy (p = 0.04) with respect to the intact condition, but it did not impact significantly either the ROM or the stiffness. The strains over the disc surface increased after nucleotomy, while discoplasty concentrated the strains on the endplates. In conclusion, this preliminary study has shown that discoplasty recovered the intervertebral posterior height, opening the neuroforamen as clinically observed, but it did not influence the spine mobility or stiffness. This study confirms that this in vitro approach can be used to investigate discoplasty.

Novel dual-rod screw for thoracoscopic anterior instrumentation: biomechanical evaluation compared with single-rod and double-screw/double-rod anterior constructs

STUDY DESIGN

A novel dual-rod screw was designed to provide a second-rod augmentation at the critical apical/middle segments of the single-rod thoracoscopic anterior construct. Biomechanical testing was performed on pig thoracic spines instrumented with 7-segment anterior scoliosis constructs.

OBJECTIVES

To analyze the biomechanical performance of the new implant, and compare it to a single-rod and double-rod anterior constructs.

SUMMARY OF BACKGROUND DATA

Using single-rod thoracoscopic anterior instrumentation for thoracic scoliosis, the complications of rod breakage at apex, high rate of nonunion, and resultant loss of coronal and sagittal correction has been reported. Inadequate construct stiffness because of a smaller diameter single rod has been implicated as the cause of these complications.

METHODS

Twelve pig thoracic spines were instrumented over 7 segments with: (1) single-rod construct, (2) short second-rod augmentation at the apex of the single-rod construct, (3) long second-rod augmentation at middle segments of the single-rod construct, and (4) double-screw/double-rod anterior construct. The spines were tested in flexion-extension, left-right lateral bending, and torsion, using pure bending moments. Strain gauges attached to the primary single rod at the cephalad, middle, and caudal portions were used and the maximum tensile stress was recorded.

RESULTS

In the single-rod construct, the middle portion stress was 39% to 51% greater than the stress in the cephalad and caudal portions in flexion-extension (P < 0.05), and the cephalad portion stress was 39% to 65% greater than the stress in the middle and caudal portions in right lateral bending and torsion (P < 0.05). When a second rod was added at the apical/middle portion, the middle portion stress decreased from 50% to 72% in flexion-extension and right lateral bending (P < 0.05). In addition, the second rod decreased the primary single-rod stress at the cephalad portion by 48% (left torsion) and the caudal portion by 50% (flexion). Double-screw/double-rod construct significantly increases the construct stiffness in comparison with the single-rod construct. However, it did not add any construct stiffness at the critical apical segments when compared to the constructs in which the second rod augmented the single-rod constructs.

CONCLUSION

A novel dual-rod screw was designed to combine the standard single-rod construct with the addition of a second rod at the critical apical/middle segments and increase construct stiffness and stability. This implant may therefore prevent pseudarthrosis and rod breakage by enhancing construct stiffness.

Segmental vessel sparing during convex growth arrest surgery--a modified technique

BACKGROUND CONTEXT

Further evidence of the importance of segmental vessel ligation in the development of neurological complications has been recently published. The more levels the ligation encompasses, the higher the risk of spinal cord damage. Therefore, caution should be taken when several segmental arteries are to be ligated in the clinical setting.

PURPOSE

To prevent ligation of segmental vessels during convex growth arrest surgery and thus decrease the risk of spinal cord ischemia and neurological injury.

STUDY DESIGN

A report of a modified technique of convex growth arrest surgery used in two consecutive patients in our unit.

METHODS

In two consecutive patients the segmental vessels were mobilized, elevated, and protected by using surgical slings. The rib graft was then slid beneath the elevated vessels into the prepared vertebral body channel and punched into place. The pleura then closed over the rib graft and spared vessels.

RESULTS

Three of the five segmental vessels in the first patient were spared. All five segmental vessels were spared in the second patient. No neurological complications occurred.

CONCLUSION

We report a straightforward technique, which obviates the need for segmental vessel ligation, and therefore decreases the risk of neurological injury in an already high-risk group.

Video-assisted thoracoscopic surgery in idiopathic scoliosis: evaluation of the learning curve

STUDY DESIGN

Retrospective review of patients with idiopathic scoliosis who underwent same-day or staged anterior and posterior spinal fusion and segmental spinal instrumentation.

OBJECTIVE

Evaluation of our learning curve with video-assisted thoracoscopic surgery (VATS) with respect to operative time, blood loss, and complications in patients with idiopathic scoliosis.

SUMMARY OF BACKGROUND DATA

VATS is a minimally invasive alternative to thoracotomy in the management of idiopathic scoliosis. An increased or steep learning curve has been described in the initial application of this technique.

METHODS

We began performing VATS in 1998. We compared our first 25 consecutive VATS patients (Group 2) and subsequent 28 consecutive VATS patients (Group 3) to our previous 16 consecutive patients (Group 1) with a thoracotomy (1991-1998) for idiopathic scoliosis. Training at a sponsored regional course was obtained before performing our first VATS procedure.

RESULTS

VATS allowed more disc to be excised in Group 2 (4.5 +/- 1, 5.7 +/- 1, and 4.4 +/- 1 discs in Group 1, Group 2, and Group 3, respectively) and significantly decreased the anterior operative time (215 +/- 33, 260 +/- 56, and 177 +/- 47 minutes) and time per individual disc excision (50 +/- 13, 47 +/- 12, and 41 +/- 12 minutes), while providing comparable correction of the thoracic deformity (67% +/- 12%, 66% +/- 10%, and 70% +/- 13% correction). There was no increase in estimated intraoperative anterior blood loss (228 +/- 213, 183 +/- 136, and 211 +/- 158 mL), estimated blood loss per disc excised (51 +/- 42, 34 +/- 29 and 48 +/- 37 mL), or complications in the VATS groups. Complications were primarily pulmonary and resolved with medical therapy. Postoperative chest tube drainage (855 +/- 397, 462 +/- 249, and 561 +/- 261 mL) and total perioperative anterior blood loss (1083 +/- 507, 647 +/- 309, and 773 +/- 308 mL) were significantly decreased in the VATS groups, but this was attributed to the use of Amicar.

CONCLUSIONS

VATS is an effective procedure for anterior spinal fusion in idiopathic scoliosis. The learning curve is short, provided appropriate training is obtained.

Histomorphometric assessment of thoracoscopically assisted anterior release in a porcine model: safety and completeness of disc discectomy with surgeon learning curve

STUDY DESIGN

A retrospective study using histomorphometric analysis to quantify the percentage of discectomy following thoracoscopic anterior release and fusion in a porcine model.

OBJECTIVE

To investigate the safety and completeness of disc and endplate removal with respect to the learning curve of the surgeon in a porcine thoracoscopic anterior fusion model.

SUMMARY OF BACKGROUND DATA

The thoracoscopic approach has been used to perform an anterior release and fusion before an open posterior instrumentation, however, there is concern that the technique may not provide sufficient visualization or exposure to perform safely and completely.

METHODS

A total of 32 pigs (160 discs) were assigned to 2 groups (early experience, n = 16; late experience, n = 16), and underwent 5 level thoracoscopic anterior release followed by anterior instrumentation and fusion from T5 to T10. At 4 months after surgery, the spines were harvested, and each discectomy disc was histomorphometrically analyzed to determine the percentage of disc excision and amount of endplate removal.

RESULTS

There were no significant differences in the percent disc excision between the early (67% +/- 11%) and late groups (69% +/- 10%). Greater than 50% of the disc was excised in 151 of 160 discectomies (94%). Both superior and inferior endplates were resected in 92 of 160 disc levels (57%). The amount of endplate removal had improved over time in both early and late groups (P < 0.0001). The histologic examination revealed no evidence for posterior longitudinal ligament disruption or spinal canal encroachment in any disc.

CONCLUSIONS

Video-assisted thoracoscopic discectomy is safe and allows for a significant amount of disc material excision. This study did not demonstrate a learning curve with respect to the amount of disc material excised, but a learning curve was seen for endplate excision.

Comparison of the lowest instrumented, stable, and lower end vertebrae in "single overhang" thoracic adolescent idiopathic scoliosis: anterior versus posterior spinal fusion

STUDY DESIGN

A retrospective multicenter study.

OBJECTIVE

To investigate the relationship between the lowest instrumented, stable, and lower end vertebrae in patients with "single overhang" thoracic (main thoracic) curves treated with anterior or posterior spinal fusion.

SUMMARY OF BACKGROUND DATA

Previous studies have shown "saving" fusion levels with anterior spinal fusion, as opposed to posterior spinal fusion; however, to our knowledge, none of these studies evaluated the relative position to the lower end vertebra to compare study groups accurately. For clarification, "single overhang" includes Lenke 1A and 1B curves. For these thoracic curves, the lumbar curve does not cross the midline.

MATERIALS AND METHODS

A retrospective multicenter study of adolescent idiopathic scoliosis was performed to identify specifically patients with "single overhang" thoracic (Lenke 1A and 1B) curves with more than a 2-year follow-up. To analyze relative fusion levels, the differences were computed as follows: (1) the difference between the vertebra position for the stable vertebra of the main thoracic (MT) curve and the lowest instrumented vertebra, as noted on postoperative radiographs, or [equation: see text] (2) the difference between the vertebra position for the lower end vertebra of the main thoracic (MT) curve and the lowest instrumented vertebra, as noted on postoperative radiographs, or [equation: see text].

RESULTS

A total of 298 "single overhang" thoracic curves (148 Lenke 1A, 150 Lenke 1B) were identified, of which 293 had either an anterior spinal fusion or posterior spinal fusion; 5 patients underwent a combined anterior-posterior spinal fusion. Anterior spinal fusion was performed in 70 patients (23.9%) and posterior spinal fusion in 223 (76.1%). While comparing the lowest instrumented vertebra to the stable vertebra with anterior spinal fusion, the lowest instrumented vertebra was identified either at the level of the stable vertebra or above in 97% of 1A/B curves (P < 0.001). Using posterior spinal fusion techniques, the lowest instrumented vertebra was identified either at the stable vertebra or above in 65% of the 1A/B curves (P < 0.05).

CONCLUSIONS

These data confirm that anterior spinal fusion techniques result in a mean shorter fusion of 1.5 vertebral segments/patient when compared to posterior spinal fusion techniques with respect to the position of the lowest instrumented and stable vertebrae for "single overhang" thoracic (Lenke 1A/B) curves. However, because this is a retrospective multicenter study over 10 years, it represents various posterior spinal fusion techniques that do not include all pedicle screw constructs.

Regional differences in anatomical landmarks for placing anterior instrumentation of the thoracic spine in both normal patients and patients with adolescent idiopathic scoliosis

STUDY DESIGN

A retrospective analysis of patients who had magnetic resonance imaging (MRI) of the thoracic spine, comparing those with a normal straight spine and those with a right thoracic adolescent idiopathic scoliosis (AIS).

OBJECTIVE

To analyze the position of the rib head with respect to the spinal canal and vertebral body in normal patients and those with right thoracic AIS using MRI.

SUMMARY OF BACKGROUND DATA

When placing anterior vertebral body screws in thoracic AIS, the most cephalad screws are most at risk for loosening because of smaller vertebral body size and the position of the rib heads, which may obscure more of the vertebral bodies. To our knowledge, there are no studies defining the relationship of the rib head to the vertebral anatomy in thoracic AIS.

METHODS

Transverse MRIs of the vertebral bodies from the 4th thoracic (T4) vertebra to the 12th thoracic (T12) vertebra in normal patients (n = 21) and patients with AIS (n = 21) group were analyzed regarding the following parameters: (1) percent vertebra obscured by rib head (i.e., the percent of the sagittal plane vertebral body length obscured by the overlapping rib head); (2) posterior safe angle, defined as the most posterior angle a screw can be placed, which avoids the spinal canal; and (3) anterior safe angle, defined as the most anteriorly directed screw trajectory that safely obtained good screw purchase.

RESULTS

In both the normal and AIS groups, the percent vertebra obscured by rib head significantly decreased from T4 (30% in normal group and 34.7% in AIS group) to T12 (-0.4% in normal group and 3.5% in AIS group) (P < 0.05). The rib head was positioned more anterior to the vertebral body in the cephalad-thoracic spine when compared to a more posterior position in the caudal thoracic spine. In each group, the posterior safe angle significantly decreased from T4 (23 degrees in normal group and 20.8 degrees in AIS group) to T12 (-0.9 degrees in normal group and 2.1 degrees in AIS group) (P < 0.05), while the anterior safe angle significantly increased from T4 (27.5 degrees in normal group and 26.6 degrees in AIS group) to T12 (38.3 degrees in normal group and 38.5 degrees in AIS group) (P < 0.05).

CONCLUSIONS

It is important to understand the relationship of the rib head to the vertebral body to provide-excellent screw purchase within the vertebral body without risking penetration into the spinal canal. In both normal and AIS groups, the relationship of the rib head to the vertebral body and spinal canal changes so that the rib head is positioned more anteriorly in the cephalad-thoracic spine and more posteriorly in the caudal thoracic spine. When placing anterior thoracic screws, at the cephalad- thoracic spine (T4, T5, T6, and T7), removal of rib heads is recommended to allow for good screw purchase. However, at the caudal thoracic spine (T10-T12), staying anterior to the rib head is important to avoid penetration into the spinal canal.

Reduction of vertebral blood flow by segmental vessel occlusion: an intraoperative study using laser Doppler flowmetry

STUDY DESIGN

During anterior spinal surgery, vertebral perfusion was assessed by laser Doppler flowmetry. Blood flow changes were assessed after unilateral ligation and contralateral compression of the segmental vessels.

OBJECTIVE

To assess the influence of unilateral and bilateral segmental vessel occlusion on vertebral blood flow.

SUMMARY OF BACKGROUND DATA

During anterior spinal surgery, segmental vessels are frequently being ligated. The reduced blood supply to the vertebrae may impair intervertebral fusion, and the decreased spinal cord perfusion may lead to ischemic myelopathy. To our knowledge, this is the first in vivo study to investigate vertebral blood flow. METHODS.: There were 10 patients who underwent anterior release for adult idiopathic scoliosis (n = 6), Scheuermann disease (n = 3), and posttraumatic kyphosis (n = 1). A high-power laser Doppler flowmeter was used to assess vertebral blood flow. Measurements were performed in 19 thoracic and 4 lumbar vertebrae (n = 23) after unilateral segmental vessel ligation and additional temporary digital compression of the contralateral vessels.

RESULTS

Initial mean blood flow was 49.1 +/- 27.6 arbitrary units, and all signals were pulsatile. The blood flow decreased by a mean of 8% after unilateral ligation of the segmental vessels. With additional compression of the contralateral vessels, the signal heights decreased significantly by 54% (mean 18.3 +/- 7.8 arbitrary units, P = 0.00003), and a loss of the pulsatile pattern was observed in 75% of the vertebrae. On release of digital compression, the signal height as well as the pulsatility promptly returned.

CONCLUSIONS

Unilateral ligation of segmental vessels led only to a slight decrease of the vertebral blood flow. Future studies may show whether sparing the segmental vessels during anterior fusion enhances bone graft incorporation, thus decreasing the rate of pseudarthrosis. According to clinical data, the risk of neurologic injury through unilateral ligation is negligible. Bilateral segmental vessel occlusion markedly reduced vertebral bloodflow. Therefore, when treating patients with a higher neurologic risk or in revision cases, the surgeon should always consider sparing the segmental vessels.

Animal models for spinal fusion

Animal models for spinal fusion are essential for preclinical testing of new fusion methods and adjuncts. They allow for control of individual variables and quantification of outcome measures. Model characteristics are considered. Preclinical experiments to evaluate proof of concept, feasibility, and efficacy are generally studied in an orderly progression from smaller to larger animal models with an evolving cascade of evidence which has become known as the "burden of proof". Methods of fusion analysis include manual palpation, radiographs, computed tomography, histology, biomechanical testing, and molecular analysis. Models which have been developed in specific species are reviewed. This sets the stage for the interpretation of studies evaluating bone graft materials such as allograft, demineralized bone matrices, bone morphogenetic proteins, ceramics, and others with consideration of the variables affecting their success. As evidence accumulates, clinical trials and applications are defined.

Video-assisted thoracoscopic surgery: the Cincinnati experience

Video-assisted thoracoscopic surgery is an alternative to open thoracotomy. We analyzed our experience during a consecutive series of 100 patients who had this procedure and who were available for study at 3-year followup. Video-assisted thoracoscopic surgery was done on patients with the following diagnoses: idiopathic scoliosis (n = 49), neuromuscular spinal deformity (n = 15), Scheuermann kyphosis (n = 15), congenital and infantile scoliosis (n = 5), neurofibromatosis (n = 5), Marfan (n = 1), postradiation scoliosis (n = 1), and repair of pseudoarthrosis (n = 1). Four patients had excision of the first rib to treat thoracic outlet syndrome. One patient had excision of an intrathoracic neurofibroma and one a benign rib tumor. One had anterior arthrodesis after fracture-dislocation of the thoracic spine and another had anterior fusion for vertebral osteomyelitis. The average operative time for the thoracoscopic anterior release with discectomy and arthrodesis was 253 minutes. The average number of discs excised was 8. Final postoperative scoliosis and kyphosis corrections were 68% and 90%, respectively. Complications related to thoracoscopy occurred in eight patients. Video-assisted thoracoscopic surgery provides a safe and effective alternative to open thoracotomy in the treatment of thoracic pediatric spinal deformities.

Anterior vertebral body screw position placed thoracoscopically: a function of anatomy and surgeon experience in a porcine model

STUDY DESIGN

Manual digital measurements, in a porcine model of thoracoscopic anterior instrumentation, divided into two studies: 1) comparing the actual screw position within each vertebra with the perceived ideal screw position and correlation with the learning curve of the surgeon; and 2) a quantitative analysis of the vertebral bodies from T3 to T12 to determine the ideal screw insertion point and trajectory.

OBJECTIVE

To investigate whether the position of thoracoscopically placed screws is related to the portion of the thoracic spine instrumented and to the experience of the surgeon and to define the ideal starting position and direction of anterior thoracic screws in a porcine model.

SUMMARY OF BACKGROUND DATA

Anterior screw insertion using a thoracoscopic approach is generally considered to be technically demanding and potentially dangerous. To our knowledge, there is no study analyzing the proper position of anterior vertebral screws using thoracoscopic technique and no study analyzing the ideal starting position of anterior vertebral body screws.

METHODS

In Study 1, 26 pigs were assigned to two groups (early experience, n = 16; late experience, n = 10) and underwent thoracoscopic anterior instrumentation and fusion from T5 to T10. The screw position was determined in the sagittal plane and in the frontal plane, respectively. In Study 2, 10 thoracic pig spines (T3-T12) were obtained. Each vertebra was digitally measured in the midtransverse plane. Using a point directly adjacent to the rib head as the assumed ideal screw starting point, the percent of vertebral body "obscured" by the rib head and the maximum safe anterior and posterior screw insertion angles were determined.

RESULTS

In Study 1, screw positions of Group 1 were significantly more anterior from T5 to T7 compared with the ideal screw position (P < 0.05). Most screws were in the ideal position in Group 2. In both groups, the screw position within T5 was too inferior (P < 0.05). The majority of posterior cortical disruptions occurred in the distal vertebrae. Most anterior disruptions occurred in the proximal vertebrae. In Study 2, the percent of vertebral body behind the rib head significantly decreased from T3 (61.5%) to T12 (25.7%) (P < 0.0001). The maximum anterior insertion angle significantly increased from T3 (11 degrees) to T12 (24 degrees) (P < 0.0001) while the maximum posterior insertion angle significantly decreased from T3 (28 degrees) to T12 (11 degrees) (P < 0.0001).

CONCLUSIONS

The position of thoracoscopically placed vertebral screws is dependent on the level of the spine instrumented and surgeon experience. Screws placed in the proximal thoracic spine tend to be too anterior and too inferior while posterior placement of screws occurs in the distal thoracic spine. When using the rib head as the starting point for anterior screw insertion, a slight anterior angle is required in the distal thoracic spine while a slight posterior insertion angle is required in the proximal thoracic spine. A learning curve does exist, however; the proximal screw positioning continues to be a challenge.