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

Implant Distribution Versus Implant Density in Lenke Type 1 Adolescent Idiopathic Scoliosis: Does the Position of the Screw Matter?

STUDY DESIGN

Retrospective study.

OBJECTIVE

Previous studies have demonstrated that increased implant density (ID) results in improved coronal deformity correction. However, low-density constructs with strategically placed fixation points may achieve similar coronal correction. The purpose of this study was to identify key zones along the spinal fusion where high ID statistically correlated to improved coronal deformity correction. Our hypothesis was that high ID within the periapical zone would not be associated with increased percent Cobb correction.

METHODS

We identified patients with Lenke type 1 curves with a minimum 2-year follow up. The instrumented vertebral levels were divided into 4 zones: (1) cephalad zone, (2) caudal zone, (3) apical zone, and (4) periapical zone. High and low percent Cobb correction groups were compared, high percent Cobb group was defined as percent correction >67%. Total ID, total concave ID, total convex ID, and ID within each zone of the curve were compared between the groups. A multivariable analysis was performed to identify independent predictors for coronal correction. Subsequently increased and decreased thoracic kyphosis (TK) groups were compared, increased TK was defined as post-operative TK being larger than preoperative TK and decreased TK was defined as post-operative TK being less than preoperative TK.

RESULTS

The cohort included 68 patients. The high percent Cobb group compared with the low percent Cobb group had significantly greater ID for the entire construct, the total concave side, the total convex side, the apical convex zone, the periapical zone, and the cephalad concave zone. The high percent Cobb group had greater pedicle screw density for the total construct, total convex side, and total concave side. In the multivariate model ID and pedicle screw density remained significant for percent Cobb correction. Ability to achieve coronal balance was not statistically correlated to ID (P = .78).

CONCLUSIONS

Increased ID for the entire construct, the entire convex side, the entire concave side, and within each spinal zone was associated with improved percent Cobb correction. The ability to achieve coronal balance was not statistically influence by ID. The results of this study support that increasing ID along the entire length of the construct improves percent Cobb correction.

Increasing loads and diminishing returns: a biomechanical study of direct vertebral rotation

STUDY DESIGN

Biomechanical simulation of DVR and pure-moment testing on thoracic spines.

OBJECTIVES

Characterize load-deformation response of thoracic spines under DVR maneuvers until failure, and compare to pure-moment testing of same spines. Despite reports of surgical complications, few studies exist on increase in ROM under DVR torque. Biomechanical models predicting increases from surgical releases have consistently used "pure-moments", a standard established for non-destructive measurement of ROM. Yet, DVR torque is not accurately modeled using pure moments and, moreover, magnitudes of torque applied during DVR maneuvers may be substantially higher than pure-moment testing.

METHODS

Cadaveric thoracic spines (N = 11) were imaged, then prepared. Polyaxial pedicle screws were implanted at T7-T10 after surgical releases. Bilateral facetectomies and Ponte osteotomies were completed at T10-T11. A custom apparatus, mounted into an 8-dof MTS load frame, was used to attach to pedicle screws, allowing simulation of surgical DVR maneuvers. Motions of vertebrae were measured using optical motion tracking. Torque was increased until rupture of the T10-T11 disc or fracture at the pedicle screw sites at any level. The torque-rotation behavior was compared to its behavior under pure-moment testing performed prior to the DVR maneuver.

RESULTS

Under DVR maneuvers, failure of the T10-T11 discs accompanied in most cases by pedicle screw loosening, occurred at 13.7-54.7 Nm torque, increasing axial rotation by 1.4°-8.9°. In contrast, pure-moment testing (4 Nm) increased axial rotation by only 0.0°-0.9°.

CONCLUSIONS

DVR resulted in substantially greater correction potential increases compared to pure-moment testing even at the same torque. These results suggest increased flexibility obtained by osteotomies and facetectomies is underestimated using pure-moment testing, misrepresenting clinical expectations. The present study is an important and necessary step toward the establishment of a more accurate and ultimately surgically applied model.

LEVEL OF EVIDENCE

III.

Biomechanical cadaver study of proximal fixation in a minimally invasive bipolar construct

STUDY DESIGN

Biomechanical human cadaver study.

OBJECTIVE

To determine the three-dimensional intervertebral ranges of motion (ROMs) of intact and hook-instrumented thoracic spine specimens subjected to physiological loads, using an in vitro experimental protocol with EOS biplane radiography. Pedicle screws are commonly used in thoracic instrumentation constructs, and their biomechanical properties have been widely studied. Promising clinical results have been reported using a T1-T5 thoracic hook-claw construct for proximal rod anchoring. Instrumentation stability is a crucial factor in minimizing mechanical complications rates but had not been assessed for this construct in a biomechanical study.

METHODS

Six fresh-frozen human cadaver C6-T7 thoracic spines were studied. The first thoracic vertebrae were instrumented using two claws of supra-laminar and pedicle hooks, each fixed on two adjacent vertebrae, on either side of a single free vertebra. Quasi-static pure-moment loads up to 5 Nm were applied to each specimen before and after instrumentation, in flexion-extension, right and left bending, and axial rotation. Five steel beads impacted in each vertebra allowed 3D tracking of vertebral movements on EOS biplanar radiographs acquired after each loading step. The relative ranges of motion (ROMs) of each pair of vertebras were computed.

RESULTS

Mean ROMs with the intact specimens were 17° in flexion-extension, 27.9° in lateral bending, and 29.5° in axial rotation. Corresponding values with the instrumented specimens were 0.9°, 2.6°, and 7.3°, respectively. Instrumentation significantly (P < 0.05) decreased flexion-extension (by 92-98%), lateral bending (by 87-96%), and axial rotation (by 68-84%).

CONCLUSION

This study establishes the biomechanical stability of a double claw-hook construct in the upper thoracic spine, which may well explain the low mechanical complication rate in previous clinical studies.

LEVEL OF EVIDENCE

Not applicable, experimental cadaver study.

Cadaveric biomechanical testing of torque - to - failure magnitude of Bilateral Apical Vertebral Derotation maneuver in the thoracic spine

It remains unclear what is the real safe limit of torque magnitude during Bilateral Apical Vertebral Derotation (BAVD) in thoracic curve correction. Up to author's knowledge there is no study except this one, to reproduce in-vivo real measurements and intraoperative conditions during BAVD maneuver. The objective of this study was to evaluate the torsional strength of the instrumented thoracic spine under axial rotation moment as well as to define safety limits under BAVD corrective maneuver in scoliosis surgery. 10 fresh, full-length, young and intact human cadavers were tested. After proper assembly of the apparatus, the torque was applied through its apical part, simulating thoracic curve derotation. During each experiment the torque magnitude and angular range of derotation were evaluated. For more accurate analysis after every experiment the examined section of the spine was resected from the cadaver and evaluated morphologically and with a CT scan. The average torque to failure during BAVD simulation was 73,3 ± 5,49Nm. The average angle of BAVD to failure was 44,5 ± 8,16°. The majority of failures were in apical area. There was no significant difference between the fracture occurrence of left or right side of lateral wall of the pedicle. There was no spinal canal breach and/or medial wall failure in any specimen. The safety limits of thoracic spine and efficacy of BAVD for axial plane correction in the treatment of Adolescent Idiopathic Scoliosis (AIS) were established. It provided qualitative and quantitative information essential for the spinal derotation under safe loading limits.

Determination of optimal screw number based on correction angle for main thoracic curve in adolescent idiopathic scoliosis

BACKGROUND

Reducing the number of screw insertions while maintaining good clinical outcomes can improve the efficiency and cost-effectiveness of scoliosis surgery. However, the optimal minimum number of pedicle screws remains unclear. This study searched for factors to estimate the fewest number of pedicle screws required between end vertebrae in relation to preoperative main thoracic curve flexibility.

METHODS

Sixty-nine subjects (4 male and 65 female, mean age: 14.8 ± 2.5 years) who underwent skip pedicle screw fixation for Lenke type 1-4 or 6 curves and were followed for at least 1 year were enrolled. Intervention technique was selected according to the size and flexibility of the preoperative main thoracic curve. Surgery-related variables included pedicle screw number, rod material and diameter, and extent of Ponte osteotomy. The effect on postoperative correction angle (i.e., the difference between the preoperative supine position maximum bending and postoperative standing Cobb angles of the main thoracic curve) according to surgical intervention technique was estimated using multiple linear mixed regression models with the preoperative supine position maximum bending correction angle (i.e., the difference between the standing preoperative and supine position maximum bending Cobb angles) as a random effect.

RESULTS

The preoperative maximum bending correction angle was 8-42° and had a moderate negative correlation with postoperative correction angle (r = -0.65, P < 0.01). Multivariate analysis revealed a 1.7° (95% CI 0.7-2.6; P < 0.01) correction gain per single-screw insertion and a 1.8° (95% CI 0.5-3.1; P < 0.01) gain per intervertebral level in Ponte osteotomy.

CONCLUSIONS

The number of pedicle screws necessary to correct main thoracic adolescent idiopathic scoliosis curves can be estimated by calculating correction gains of 1.7° per pedicle screw and 1.8° per Ponte osteotomy intervertebral level. Based on these results, it may be possible to reduce invasiveness and cost for patients requiring a smaller degree of correction.

Biomechanical Comparison of the Load-Sharing Capacity of High and Low Implant Density Constructs With Three Types of Pedicle Screws for the Instrumentation of Adolescent Idiopathic Scoliosis

STUDY DESIGN

Biomechanical numerical simulation analysis of implant design and density in adolescent idiopathic scoliosis posterior instrumentation.

OBJECTIVES

To evaluate the combined effect of pedicle screw design and density on deformity correction and construct load-sharing capacity.

SUMMARY OF BACKGROUND DATA

Screw density is an area of popular study because of the impact of cost and potential patient morbidity of higher-density constructs. Using fewer screws raises concern about reduced correction and greater forces on each screw.

METHODS

Personalized spinal numerical models were created for five patients. The correction techniques from five spine surgeons using both a high- and a low-density implant pattern (2 vs. 1.4 ± 0.22 screws/level) with uniaxial, multiaxial, and favored angle screws were simulated. The predicted correction and forces sustained by the implants were compared. The postoperative load-sharing capacity of a high- and a low-density construct, with or without crosslinks, was compared by simulating daily activities motions.

RESULTS

The major coronal curve correction was similar with high- and low-density constructs (73% ± 10% vs. 72% ± 10%; p > .05) but was higher when using uniaxial (77% ± 8%) compared to multiaxial (69% ± 11%) and favored angle screws (71% ± 10%; p = .009). High- and low-density constructs sustained similar intraoperative peak forces (305 ± 61 N vs. 301 ± 73 N; p = .23) regardless of screw design (all p > .05). Multiaxial and favored angle screws reduced the peak axial force by 23% and 38% compared to uniaxial screws (p = .007). The high-density construct reduced the postoperative loads sustained by each implant by 31% (p = .006). Crosslinks had no effect on load sharing (p = .23).

CONCLUSION

High- and low-density implant patterns achieved similar coronal correction with equivalent capacity to share corrective forces regardless of the screw design. Increased degrees of freedom of the screw head reduces the capacity to correct coronal deformity but generates lower bone-screw forces. The reduced number of screws increased the postoperative forces sustained by each screw, but its effect on potential complications requires further investigations.

LEVEL OF EVIDENCE

Level 4.

Significant variability in surgeons' preferred correction maneuvers and instrumentation strategies when planning adolescent idiopathic scoliosis surgery

BACKGROUND

Increased implant number is thought to provide better control on the scoliotic spine, but there is limited scientific evidence of improved deformity correction and surgical outcomes with high-density constructs. The objective is to assess key anchor points used by experienced spinal deformity surgeons and to evaluate the effect of implant density pattern on correction techniques.

METHODS

Seventeen experienced spine surgeons reviewed five Lenke 1 adolescent idiopathic scoliosis cases and provided their preferred posterior correction technique (implant pattern, correction maneuvers, and implants used for their execution) and an alternative technique with the minimal implant density they felt would be acceptable (170 surgical plans total). Additionally, for each case, they selected acceptable screw patterns for surgery from seven published implant configurations. Variability in the surgeons' plans was assessed, including instrumentation and correction strategies.

RESULTS

The preferred correction plan involved an average of 1.65 implants/vertebra, with 88% of the available anchor points at the apex ± 1 vertebra used for the execution of correction maneuvers and only 43% of possible anchor points used proximal and distal to the apical area. The minimal density that surgeons found acceptable was 1.24 implants/vertebra. The minimal density plan involved more in situ rod contouring (53 vs. 41%), fewer vertebral derotation maneuvers (82 vs. 96%), and fewer implants used for compression/distraction maneuvers (1.18 and 1.42 respectively) (p < 0.05). Implant placement at alternate levels or dropout of convex implants above and below the apical area was most frequently considered acceptable (> 70% agreement).

CONCLUSIONS

Implant position and number affect surgeons correction maneuvers selection. For low implant density constructs, dropout in the convexity and particularly in the periapical region is accepted by surgeons, with minor influence on planned correction maneuvers. Thus, preoperative implant planning must take into account which anchor points are needed for desired correction maneuvers.

Challenging the Conventional Standard for Thoracic Spine Range of Motion: A Systematic Review

BACKGROUND

Segmental motion is a fundamental characteristic of the thoracic spine; however, studies of segmental ranges of motion have not been summarized or analyzed. The purpose of the present study was to present a summary of the literature on intact cadaveric thoracic spine segmental range of motion in each anatomical plane.

METHODS

A systematic MEDLINE search was performed with use of the terms "thoracic spine," "motion," and "cadaver." Reports that included data on the range of motion of intact thoracic human cadaveric spines were included. Independent variables included experimental details (e.g., specimen age), type of loading (e.g., pure moments), and applied moment. Dependent variables included the ranges of motion in flexion-extension, lateral bending, and axial rotation.

RESULTS

Thirty-three unique articles were identified and included. Twenty-three applied pure moments to thoracic spine specimens, with applied moments ranging from 1.5 to 8 Nm. Estimated segmental range of motion pooled means ranged from 1.9° to 3.8° in flexion-extension, from 2.1° to 4.4° in lateral bending, and from 2.4° to 5.2° in axial rotation. The sums of the range of motion pooled means (T1 to T12) were 28° in flexion-extension, 36° in lateral bending, and 45° in axial rotation.

CONCLUSIONS

The pooled ranges of motion were similar to reported in vivo motions but were considerably smaller in magnitude than the frequently referenced values reported prior to the widespread use of biomechanical testing standards. Improved reporting of biomechanical testing methods, as well as specimen health, may be beneficial for improving on these estimations of segmental cadaveric thoracic spine range of motion.

An in Vitro Biomechanical Model of Differing Pedicle Screw Configurations for Long Construct Segmental Thoracic Fixation

BACKGROUND

The optimum pattern of pedicle screw (PS) fixation during long-segment thoracic fixation has not been determined.

OBJECTIVE

To evaluate rod stress and construct stability with minimal, alternating, skipped, and bilateral PS constructs in the iatrogenically destabilized thoracic spine.

METHODS

Eight cadaveric thoracic specimens (T3-T12) were initially tested intact to ±5 Nm using a custom 6 degree-of-freedom spine testing apparatus in flexion-extension (FE), lateral bending (LB), and axial rotation. Specimens were instrumented with T4-T10 bilateral PS, with Ponte osteotomies to introduce instability. Rods were bent to fit the PS and then spines were tested with the minimal, alternating, skipped, and bilateral fixation patterns. Range of motion (ROM) was calculated from T4-T10 and segmentally. In addition, strain gauges fixed to the spinal rods measured rod stress under FE and LB. Results were compared using ANOVA and post hoc Holm Sidak tests.

RESULTS

All fixation patterns provided significant reductions in ROM with respect to the intact spine. In all motion planes, minimal provided the least amount of rigidity, while bilateral provide the greatest; however, no statistically significant differences were detected in FE. In LB and axial rotation, skipped, alternating, and bilateral were all significantly more rigid than minimal (P < .01). Rod strains were greatest under LB and correlated with overall construct ROM, where bilateral had significantly lower strain than the other patterns (P < .05).

CONCLUSION

All constructs effectively decreased thoracic ROM. There was significant improvement in stabilization and decreased rod stress when more fixation points beyond the minimal construct were included.

Are There 3D Changes in Spine and Rod Shape in the 2 Years After Adolescent Idiopathic Scoliosis Instrumentation?

STUDY DESIGN

Retrospective analysis of spine and rods 3D shape in the 2 years after adolescent idiopathic scoliosis (AIS) posterior surgical instrumentation.

OBJECTIVE

To evaluate spine and rod shape 3D changes in the 2 years after AIS instrumentation with different rod materials.

SUMMARY OF BACKGROUND DATA

Postoperative loss of correction has been reported with modern segmental instrumentation. The postoperative 3D rod shape changes of different rod materials and their implication in postoperative loss of correction have never been studied.

METHODS

A total of 42 Lenke 1 AIS cases who underwent surgery all with pedicle screws and 5.5 mm rods of three different materials were retrospectively reviewed (14 cases for each of the three rod material subgroups: titanium, cobalt-chromium, stainless steel). 3D reconstructions of the spine were performed using biplanar radiographs preoperatively, 1-week postoperatively (1WPO), and 2 years follow-up (2YFU). Rods 3D shape also was reconstructed at 1WPO and 2YFU. The spine and rods shapes were analyzed and compared between time points.

RESULTS

An average of 1.93 implant per vertebra was used. The main thoracic curve (61° ± 9°) was corrected on average by 75% (15° ± 6°, P < 0.01) with no change at 2YFU (17° ± 7°, P = 0.14). The apical vertebral rotation (23° ± 7°) was corrected by 44% (13° ± 9°, P < 0.01) with no change at 2YFU (14° ± 9°, P = 0.64). The thoracic kyphosis (24° ± 12°) remained unchanged (P = 0.78). The orientation of the planes of maximal curvature with respect to the sagittal plane of the main thoracic curve (39° ± 15°) and of the rods (concave: 28° ± 23°, convex: 12° ± 10°) at 1WPO was unchanged at 2YFU (all P > 0.05). Rod curvature and deflection also remained unchanged (all P > 0.05). 3D curve correction was maintained in the 2YFU for all rod materials subgroups (all P > 0.05). The lumbar lordosis changed from 1WPO (47° ± 8°) to 2YFU (56° ± 9°, P < 0.01). At preoperatively, 57% of the cases had a balanced posture (sacral vertical axis/sacral femoral distance ≤0.5 and sacral vertical axis ≤0), 33% at 1WPO and 79% at 2YFU.

CONCLUSION

There was no significant 3D shape change of the instrumented thoracic spine or of the rods postoperatively for any of the segmental pedicle screw constructs with titanium, stainless steel, and cobalt-chromium rods. Patients overall unbalanced sagittal posture documented immediately after surgery came back to balanced 2 years after surgery.

LEVEL OF EVIDENCE

3.

Thoracic spine morphology of a pseudo-biped animal model (kangaroo) and comparisons with human and quadruped animals

PURPOSE

Based on the structural anatomy, loading condition and range of motion (ROM), no quadruped animal has been shown to accurately mimic the structure and biomechanical function of the human spine. The objective of this study is to quantify the thoracic vertebrae geometry of the kangaroo, and compare with adult human, pig, sheep, and deer.

METHODS

The thoracic vertebrae (T1-T12) from whole body CT scans of ten juvenile kangaroos (ages 11-14 months) were digitally reconstructed and geometric dimensions of the vertebral bodies, endplates, pedicles, spinal canal, processes, facets and intervertebral discs were recorded. Similar data available in the literature on the adult human, pig, sheep, and deer were compared to the kangaroo. A non-parametric trend analysis was performed.

RESULTS

Thoracic vertebral dimensions of the juvenile kangaroo were found to be generally smaller than those of the adult human and quadruped animals. The most significant (p < 0.001) correlations (Rho) found between the human and kangaroo were in vertebrae and endplate dimensions (0.951 ≤ Rho ≤ 0.963), pedicles (0.851 ≤ Rho ≤ 0.951), and inter-facet heights (0.891 ≤ Rho ≤ 0.967). The deer displayed the least similar trends across vertebral levels.

CONCLUSIONS

Similarities in thoracic spine vertebral geometry, particularly of the vertebrae, pedicles and facets may render the kangaroo a more clinically relevant human surrogate for testing spinal implants. The pseudo-biped kangaroo may also be a more suitable model for the human thoracic spine for simulating spine deformities, based on previously published similarities in biomechanical loading, posture and ROM.

A pedicle screw system and a lamina hook system provide similar primary and long-term stability: a biomechanical in vitro study with quasi-static and dynamic loading conditions

PURPOSE

For the stabilization of the thoracolumbar spine area, various stabilization techniques have been developed in recent decades. The aim of these techniques is to immobilize the treated segment to repositioning or correct the spine and guaranty long-term stability to achieve a reliable fusion. The aim of this study was to simulate in an in vitro experiment the postoperative long-term situation in elderly osteoporotic patients to compare two different stabilization principles; a pedicle screw system and a lamina hook system.

METHODS

Two comparable groups with respect to age and bone mineral density with each n = 6 fresh-frozen human, bi-segmental thoracolumbar spine specimens (T11-L1) were used. Antero-posterior and lateral radiographs were taken before the test, to assess the spinal status. Then the intact specimens were biomechanically characterized with pure moments in the three anatomical planes in different states in terms of range of motion and neutral zone. After implantation of either, a pedicle screw system or a lamina hook system, the primary stability was determined under the same conditions. Subsequently the specimens were cyclically loaded under complex loading, using a custom-made set-up in a dynamic materials testing machine with increasing moments from 3 to 66 Nm until 100,000 cycles or until one of the three defined "failure" criteria was reached. (1) A failure of a bony structure. (2) Exceeding of the threefold ROM of the primary stability after implantation in flexion plus extension. (3) Reaching of the ROM based on the intact state before implantation both in flexion plus extension.

RESULTS

The results showed that the ROM was strongly reduced after instrumentation similar for both implant systems in all motion planes. The highest stabilization was found in flexion/extension. During cyclic loading with increasing moments, the ROM increased continuously for both systems. The number of load cycles until one of the failure criteria was reached varied only slightly between the two groups. In the pedicle screw group 30,000 (median) loading cycles (range 5000-80,000) with a corresponding moment of 24 Nm (range 9-54) could be reached. In the lamina hook group 32,500 load cycles (range 20,000-45,000) could be achieved with a corresponding moment of 25.5 Nm (range 18-33). There was a slight trend that the pedicle screw system is influenced more by bone mineral density.

CONCLUSION

Both implant systems provide similar primary stability and similar long-term stability. In the pedicle screw group, there was a stronger correlation between bone mineral density and the reached number of load cycles.

Return to sports after surgery to correct adolescent idiopathic scoliosis: a survey of the Spinal Deformity Study Group

BACKGROUND CONTEXT

There are no guidelines for when surgeons should allow patients to return to sports and athletic activities after spinal fusion for adolescent idiopathic scoliosis (AIS). Current recommendations are based on anecdotal reports and a survey performed more than a decade ago in the era of first/second-generation posterior implants.

PURPOSE

To identify current recommendations for return to sports and athletic activities after surgery for AIS.

STUDY DESIGN/SETTING

Questionnaire-based survey.

PATIENT SAMPLE

Adolescent idiopathic scoliosis after corrective surgery.

OUTCOME MEASURES

Type and time to return to sports.

METHODS

A survey was administered to members of the Spinal Deformity Study Group. The survey consisted of surgeon demographic information, six clinical case scenarios, three different construct types (hooks, pedicle screws, hybrid), and questions regarding the influence of lowest instrumented vertebra (LIV) and postoperative physical therapy.

RESULTS

Twenty-three surgeons completed the survey, and respondents were all experienced expert deformity surgeons. Pedicle screw instrumentation allows earlier return to noncontact and contact sports, with most patients allowed to return to running by 3 months, both noncontact and contact sports by 6 months, and collision sports by 12 months postoperatively. For all construct types, approximately 20% never allow return to collision sports, whereas all surgeons allow eventual return to contact and noncontact sports regardless of construct type. In addition to construct type, we found progressively distal LIV resulted in more surgeons never allowing return to collision sports, with 12% for selective thoracic fusion to T12/L1 versus 33% for posterior spinal fusion to L4. Most respondents also did not recommend formal postoperative physical therapy (78%). Of all surgeons surveyed, there was only one reported instrumentation failure/pullout without neurologic deficit after a patient went snowboarding 2 weeks postoperatively.

CONCLUSIONS

Modern posterior instrumentation allows surgeons to recommend earlier return to sports after fusion for AIS, with the majority allowing running by 3 months, noncontact and contact sports by 6 months, and collision sports by 12 months.

Implant distribution in surgically instrumented Lenke 1 adolescent idiopathic scoliosis: does it affect curve correction?

STUDY DESIGN

Retrospective review of prospective multicenter database of patients with adolescent idiopathic scoliosis who underwent posterior spinal fusion.

OBJECTIVE

To analyze implant distribution in surgically instrumented Lenke 1 patients and evaluate how it impacts curve correction.

SUMMARY OF BACKGROUND DATA

Although pedicle screw constructs have demonstrated successful surgical results, the optimal pedicle screw density and configuration remain unclear.

METHODS

A total of 279 patients with adolescent idiopathic scoliosis treated with pedicle screws were reviewed. Implant density was computed for each side of the instrumented segment, which was divided into 5 regions: distal and proximal ends (upper/lower instrumented vertebra +1 adjacent vertebra), apical region (apex ± 1 vertebra), and the 2 regions in between (upper/lower periapical). Centralized measurement of Cobb angle and thoracic kyphosis was performed on preoperative and at 1-year postoperative radiographs as well as percent curve flexibility.

RESULTS

The mean implant density was 1.66 implants per level fused (1.08 to 2) with greater available pedicles filled on the concavity (92%, 53%-100%) compared with the convex side (73%, 23%-100%, P < 0.01). The concave distal end region had the highest density with 99% of pedicles filled (P < 0.01), followed by the other concave regions and the convex distal end region (88%-94%) (P > 0.05). Other convex regions of the construct had less instrumentation, with only 54% to 78% of pedicles instrumented (P < 0.01). Implant density in the concave apical region (69%, 23%-100%) had a positive effect on curve correction (P = 0.002, R = 0.19).

CONCLUSION

Significant variability exists in implant distribution with the greatest variation on the convex side and lowest implant density used in the periapical convex regions. Only instrumentation at the concave side, particularly at the apical region, was associated with curve correction. This suggests that for a low implant density construct, the best regions for planned screw dropout may be in the periapical convexity.

LEVEL OF EVIDENCE

3.

Flexibility of thoracic spines under simultaneous multi-planar loading

PURPOSE

The corrective potential of two posterior-only destabilization procedures for scoliosis deformity was quantified under single and multi-planar loading using cadaveric spines.

METHODS

Ten full-length human cadaveric thoracic spines were mounted in an 8-df servohydraulic load frame. Cyclic, pure moments were applied in: (1) flexion-extension, (2) lateral bending, (3) axial rotation, (4) flexion-extension with axial rotation, and (5) lateral bending with axial rotation at 0.5°/s, to ±4 Nm. Each specimen was tested intact, and again after nine en bloc bilateral total facetectomies, and one, two, three, and four levels of Ponte osteotomies. Motion was measured throughout loading using optical motion tracking.

RESULTS

Under single-plane loading, facetectomies and Ponte osteotomies increased thoracic spine flexibility in all three planes. Compared to total facetectomies, higher per-level increases were seen following Ponte osteotomies, with increases in total range of motion (total ROM) of up to 2.7° in flexion-extension, 1.4° in lateral bending, and 3.1° in axial rotation following each osteotomy. Compared to the facetectomies, four supplemental osteotomies increased total ROM by 23 % in flexion (p < 0.01) and 8 % in axial rotation (p < 0.01). Increases in lateral bending were smaller. Under multi-planar loading, each Ponte osteotomy provided simultaneous increases of up to 1.4°, 1.6°, and 2.2° in flexion-extension, lateral bending, and axial rotation.

CONCLUSIONS

Ponte osteotomies provided higher per-level increases in ROM under single-plane loading than total facetectomies alone. Further, Ponte osteotomies provided simultaneous increase in all three planes under multi-planar loading. These results indicated that, to predict the correction potential of a surgical release, multi-planar testing may be necessary.

Would CoCr rods provide better correctional forces than stainless steel or titanium for rigid scoliosis curves?

STUDY DESIGN

Comparative in vitro, biomechanical study.

OBJECTIVE

Compare the effect of rod curvature and material properties on rod flattening and correctional forces.

SUMMARY OF BACKGROUND DATA

Traditional methods of correction for large progressive deformities involve 3-dimensional correction, performed with an attempt to reach a balanced correction in all planes, spinal instrumentation, and fusion. Increasing attention to the transverse plane correction has developed after the introduction of segmental pedicle screws into the treatment of idiopathic scoliosis. Approximation of the spine (pedicle screws or hooks) to the rods remains the heart of many deformity procedures. Therefore, it is crucial that the instrumentation used provide and maintain the initial correction of the spinal deformity while minimizing potential intraoperative failures.

METHODS

Two experiments were performed using 80 rods made from 4 different materials namely: stainless steel (SS), titanium (Ti), cobalt chromium (CoCr), and ultrahigh strength stainless steel (UHSS). Half of the rods were contoured to 20 degrees, whereas the reaming contoured to 30 degrees. Half of the rods were approximated to a synthetic spine models to measure the flattening of the rods when approximated to highly rigid spine. The other half was used to measure the correctional forces produced by each rod type and curvature.

RESULTS

For the 20-degree pre-bend rods, Ti was the best in maintaining its original shape followed by UHSS, SS, and CoCr of 90%, 77%, 62.5%, and 54.4%, respectively. The 30-degree pre-bend showed exactly a similar trend with 80.7% for Ti, 71% for UHSS, 54.6% for SS, and 48.1% for the CoCr rods. For 30-degree pre-bend CoCr and UHSS rods, the intraoperative reduction forces were almost 42% and 10% higher than the Ti and SS rods, respectively. The correctional force produced by the Ti 30-degree pre-bend rod was approximately 67% that of a CoCr and UHSS rods.

CONCLUSIONS

CoCr and UHSS rods have the ability to produce the highest correction forces, however, both can plastically deform in a very rigid curves. Therefore, it is critical to have sense of the quality of the bone fixation as well as the curve flexibility when selecting for appropriate rod size material and contouring the rod to the desired shape.

Pediatric pedicle screws: comparative effectiveness and safety: a systematic literature review from the Scoliosis Research Society and the Pediatric Orthopaedic Society of North America task force

BACKGROUND

Pedicle screws are widely used in spinal surgery. There is extensive published literature concerning the use of pedicle screw instrumentation for spinal surgery in adults. Now there is a trend to use pedicle screws in pediatric patients, including the very young. A systematic review of the current English-language literature on the use of pedicle screw instrumentation in the pediatric age group was performed to specifically determine (1) the pedicle screw placement accuracy in patients with spine deformity and (2) the effect size of all-pedicle screw constructs compared with other methods of spinal instrumentation in terms of the percentage of scoliosis correction.

METHODS

English-language studies of pedicle screw use in pediatric patients (defined as those younger than eighteen years of age) were included. Descriptive statistics synthesized the accuracy of pedicle screw placement. Accuracy rates were compared between pediatric and adult patients. The effect of pedicle screw instrumentation on scoliosis correction was calculated with use of Cobb angle measurements.

RESULTS

On the basis of the literature search, 1181 articles were screened, 320 abstracts were examined, and ninety full-text articles representing 5761 patients were reviewed in detail. Seventeen studies met the inclusion criteria for the analysis of pedicle screw placement accuracy. A total of 13,536 pedicle screws were placed in 1353 pediatric patients. The overall placement accuracy rate in pediatric patients was 94.9%, which was higher than the rate of 91.5% reported for adults. The weighted, geometric, and 5% trimmed mean accuracy rates of pedicle screw placement were 91.9%, 88.5%, and 89.1%, respectively (standard deviation = 10%; interquartile range = 10%). Sixteen comparative studies met the inclusion criteria for the analysis of the effect of pedicle screw instrumentation on scoliosis correction. Pedicle screw constructs had a significantly larger percentage of Cobb angle correction compared with hooks (Cohen's d = 1.14) and hybrid constructs (Cohen's d = 0.49).

CONCLUSIONS

The accuracy of pedicle screw placement in the pediatric spine exceeds the accuracy rate reported in adults. Pedicle screw instrumentation constructs are significantly more effective for scoliosis correction, as determined on the basis of Cobb angle measurements, than are hook constructs and hybrid constructs.

Biomechanical evaluation of fixation strength of conventional and expansive pedicle screws with or without calcium based cement augmentation

BACKGROUND

The expansive pedicle screw was originally developed to be installed in the bone of compromised quality, but there are some concerns whether it can provide enough fixation strength in the spine with osteoporosis or severe osteoporosis.

METHODS

Twelve fresh human cadaver spines were stratified into four levels: normal, osteopenia, osteoporosis and severe osteoporosis. The vertebra was bilaterally instrumented with pedicle screws according to four protocols, including conventional pedicle screw without augmentation, expansive pedicle screw without augmentation, conventional screw with augmentation and expansive screw with augmentation. Screw pullout tests were conducted.

FINDINGS

Given the same specimen, the fixation strength of expansive screw was significantly higher than that of the conventional screw. When the same type of screw was used, the fixation strength of the calcium based cement augmented group was stronger than that of the non-augmented group. The pullout strength and stiffness of the expansive screw, augmented conventional screw and augmented expansive screw groups at the osteoporotic level were comparable to those of the conventional pedicle screw group at the osteopenic level. However, under the severely osteoporotic bone environment, the pullout strength of pedicle screw with whatever placement protocol was significantly lower than that of the conventional screw group at the osteopenic level.

INTERPRETATION

Our results demonstrate that (i) the expansive pedicle screw appears feasible and safe in either osteopenic or osteoporotic spine; (ii) calcium based cement augmentation can offer improved initial fixation strength of pedicle screws.; and (iii) no screw placement protocol we examined is efficacious in the bone at the severely osteoporotic level.

A computed tomography-based morphometric study of thoracic pedicle anatomy in a random United States trauma population

OBJECT

The objective of this study was to establish normative data for thoracic pedicle anatomy in the US adult population. To this end, CT scans chosen at random from an adult database were evaluated to determine the ideal pedicle screw (PS) length, diameter, trajectory, and starting point in the thoracic spine. The role of patient sex and side of screw placement were also assessed. The authors postulated that this information would be of value in guiding safe implant size and placement for surgeons in training.

METHODS

One hundred patients (50 males and 50 females) were selected via retrospective review of a hospital trauma registry database over a 6-month period. Patients included in the study were older than 18 years of age, had axial bone-window CT images of the thoracic spine, and had no evidence of spinal trauma. For each pedicle, the pedicle width, pedicle-rib width, estimated screw length, trajectory, and ideal entry point were measured using eFilm Lite software. Statistical analysis was performed using the Student t-test.

RESULTS

The shortest mean estimated PS length was at T-1 (33.9 ± 3.3 mm), and the longest was at T-9 (44.9 ± 4.4 mm). Pedicle screw length was significantly affected by patient sex; men could accommodate a PS from T1-12 a mean of 4.0 ± 1.0 mm longer than in women (p < 0.001). Pedicle width showed marked variation by spinal level, with T-4 (4.4 ± 1.1 mm) having the narrowest width and T-12 (8.3 ± 1.7 mm) having the widest. Pedicle width had an obvious affect on potential screw diameter; 65% of patients had a least 1 pedicle at T-4 that was < 5 mm in diameter and therefore would not accept a 4.0-mm screw with 1.0 mm of clearance, as compared with only 2% of patients with a similar status at T-12. Sex variation was also apparent, as thoracic pedicles from T-1 to T-12 were a mean of 1.4 ± 0.2 mm wider in men than in women (p < 0.001). The PS trajectory in the axial plane was measured, showing a marked decrease from T-1 to T-4, stabilization from T-5 to T-10, followed by a decrease at T11-12. When screw trajectory was stratified by side of placement, a mean of 1.7° ± 0.5° of increased medialization was required for ideal pedicle cannulation from T-3 to T-12 on the left as compared with the right side, presumably because of developmental changes in the vertebral body caused by the aorta (p < 0.05 for T3-12, except for T-5, where p = 0.051). The junction of the superior articular process, lamina, and the superior ridge of the transverse process was shown to be a conserved surface landmark for PS placement.

CONCLUSIONS

Preoperative CT evaluation is important in choosing PS length, diameter, trajectory, and entry point due to variation based on spinal level, patient sex, and side of placement. These data are valuable for resident and fellow training to guide the safe use of thoracic PSs.

Evaluation of pullout strength and failure mechanism of posterior instrumentation in normal and osteopenic thoracic vertebrae

Object

There is limited data on the pullout strength of spinal fixation devices in the thoracic spine among individuals with different bone quality. An in vitro biomechanical study on the thoracic spine was performed to compare the pullout strength and the mechanism of failure of 4 posterior fixation thoracic constructs in relation to bone mineral density (BMD).

Methods

A total of 80 vertebrae from 11 fresh-frozen thoracic spines (T2–12) were used. Based on the results from peripheral quantitative CT, specimens were divided into 2 groups (normal and osteopenic) according to their BMD. They were then randomly assigned to 1 of 4 different instrumentation systems (sublaminar wires, pedicle screws, lamina claw hooks, or pedicle screws with wires). The construct was completed with 2 titanium rods and 2 transverse connectors, creating a stable frame. The pullout force to failure perpendicular to the rods as well as the pattern of fixation failure was recorded.

Results

Mean pullout force in the osteopenic Group A (36 vertebrae) was 473.2 ± 179.2 N and in the normal BMD Group B (44 vertebrae) was 1414.5 ± 554.8 N. In Group A, no significant difference in pullout strength was encountered among the different implants (p = 0.96). In Group B, the hook system failed because of dislocation with significantly less force than the other 3 constructs (931.9 ± 345.1 N vs an average of 1538.6 ± 532.7 N; p = 0.02). In the osteopenic group, larger screws demonstrated greater resistance to pullout (p = 0.011). The most common failure mechanism in both groups was through pedicle base fracture.

Conclusions

Bone quality is an important factor that influences stability of posterior thoracic implants. Fixation strength in the osteopenic group was one-fourth of the value measured in vertebrae with good bone quality, irrespective of the instrumentation used. However, in normal bone quality vertebrae, the lamina hook claw system dislocated with significantly less force when compared with other spinal implants. Further studies are needed to investigate the impact of different transpedicular screw designs on the pullout strength in normal and osteopenic thoracic spines.

Surgical management of two- versus three-column injuries of the cervicothoracic junction: biomechanical comparison of translaminar screw and pedicle screw fixation using a cadaveric model

STUDY DESIGN

An in vitro cadaveric biomechanical study.

OBJECTIVE

To determine the stability of translaminar screws compared to pedicle screws at T1-T2 for constructs bridging the cervicothoracic junction.

SUMMARY OF BACKGROUND DATA

Instrumented fixation of the cervicothoracic junction is challenging both biomechanically, due to the transition from the mobile cervical to the rigid thoracic spine, and technically, due to the anatomic constraints of the T1-T2 pedicles. For these reasons, an alternate fixation technique at T1-T2 that combines ease of screw insertion and a favorable safety profile with biomechanical stability would be clinically beneficial.

METHODS

A 6-degree of freedom spine simulator was used to test multidirectional flexibility in 8 human cadaveric specimens. Flexion, extension, lateral bending, and axial rotation were tested in the intact condition, followed by destabilization via a simulated 2-column injury at C7-T1. Specimens were reconstructed using C5-C6 lateral mass screws and either translaminar or pedicle screws placed at T1, followed by caudal extension to T2. A 3-column injury at C7-T1 was then performed and specimens were tested using a posterior only approach with either translaminar or pedicle screws placed at T1 and T1-T2. Finally, anterior fixation at C7-T1 was added and multidirectional flexibility testing performed as previously described.

RESULTS

Following a 2-column injury at C7-T1, there were no significant differences in segmental flexibility at C7-T1 between translaminar and pedicle screw fixation when placed at T1-T2 (P>0.05). For a 3-column injury treated posteriorly, translaminar screws at T1-T2 provided increased flexibility compared to pedicle screws in flexion/extension (P<0.05). There were no differences in segmental flexibility at C7-T1 between the 2 techniques following the addition of anterior fixation (P>0.05).

CONCLUSION

Translaminar screws in the upper thoracic spine offer similar stability to pedicle screw fixation for constructs bridging the cervicothoracic junction. Small differences in range of motion must be weighed clinically against the potential benefits of translaminar screw insertion at T1-T2.

Long adult spinal deformity fusion to the sacrum using rhBMP-2 versus autogenous iliac crest bone graft

STUDY DESIGN

Comparative study.

OBJECTIVE

To compare the radiographic outcome of patients undergoing long spinal deformity surgery to the sacrum/ilium, using either rhBMP-2 without iliac crest bone graft (ICBG) or ICBG without rhBMP-2.

SUMMARY OF BACKGROUND DATA

rhBMP-2 has been shown to be more effective in promoting successful bone union in patients undergoing single level lumbar spinal fusion than ICBG. However, to the best of our knowledge, there are no studies that compare the efficacy of rhBMP-2 versus ICBG in long spinal deformity surgery.

METHODS

To obtain uniform background, we selected patients with adult spinal deformity who underwent primary spinal fusion from the thoracic spine to the sacrum/ilium and had a minimum 2-year follow-up. Fifty-five consecutive patients, consisting of 32 patients who underwent a fusion using ICBG without rhBMP-2 (ICBG group) and 23 patients who underwent a fusion using rhBMP-2 without ICBG (BMP group) were analyzed.

RESULTS

The 2 groups were similar with respect to age, gender, smoking history, comorbidity, and body mass index. The average number of vertebrae fused (11.3 in both groups) and the degree of preoperative deformity (major Cobb angle 58.3 degrees in ICBG group vs. 54.2 degrees in BMP group) were also similar in both groups. All but 2 patients had both anterior and posterior surgery. Both groups were similar in terms of final deformity correction. The average total amount of rhBMP-2 used in the BMP group was 119.2 mg (anterior 11.6 mg/level; posterior 10.0 mg/level). Of the 32 patients in the ICBG group, 9 patients (28.1%) developed a pseudarthrosis, while only 1 of 23 patients (4.3%) in the BMP group developed a pseudarthrosis with the caveat that the follow-up period was shorter in the BMP group (average follow-up of 4.9 vs. 2.7 years).

CONCLUSION

The pseudarthrosis rate in the BMP group compares favorably to pseudarthrosis rate in ICBG group, suggesting that the use of rhBMP-2 without iliac harvesting leads to a competitive fusion rate in long adult spinal deformity surgery, while avoiding ICBG harvest site morbidity.

Use of all-pedicle-screw constructs in the treatment of adolescent idiopathic scoliosis

All-pedicle-screw constructs are safe and biomechanically advantageous in the management of adolescent idiopathic scoliosis. Safe and reproducible placement of thoracic pedicle screws is dependent on a thorough understanding of normal and abnormal anatomy, meticulous technique, and the use of neuromonitoring and fluoroscopy. Improvement in the biomechanical properties secondary to the use of pedicle screw fixation has led to shorter fusions with improved deformity correction. Coronal, sagittal, and rotational correction is superior to that obtained with hook instrumentation. Improved derotation may decrease the need for thoracoplasty, thus eliminating the risk of associated morbidity. Superior control of the deformity with all-pedicle-screw fixation, as well as the use of adjunctive posterior releases, often obviates the need for an anterior approach, even in severe curves measuring 70 degrees to 100 degrees . Improved correction, shorter fusion, and the lower morbidity associated with posterior-only approaches may compensate for higher implant costs.

Pedicle screw instrumentation for adult idiopathic scoliosis: an improvement over hook/hybrid fixation

STUDY DESIGN

A matched cohort comparison of adult idiopathic scoliosis (AdIS) patients treated with all pedicle screw constructs compared to hook/hybrid constructs.

OBJECTIVE

To compare clinical and radiographic results of AdIS treatment using all pedicle screw constructs versus hook/hybrid constructs.

SUMMARY OF BACKGROUND DATA

Pedicle screw instrumentation has demonstrated excellent clinical efficacy in the treatment of pediatric spinal deformity. No prior reports have compared the outcomes of pedicle screw only constructs to hook/hybrid constructs in the treatment of AdIS.

METHODS

We analyzed 34 consecutive patients undergoing posterior-only correction for AdIS, using pedicle screw instrumentation at minimum 2-year follow-up. Thirty-four matching patients (11 with anterior releases) were selected from a cohort of 58 patients treated with hook/hybrid constructs based on similar age, curve type, magnitude, and fusion levels.

RESULTS

Significantly greater curve correction was seen in the pedicle screw compared to the hook/hybrid group (56 vs. 40%, P < 0.01). Coronal and sagittal imbalance were equivalent between the groups (P = 0.91 and 0.23, respectively). Thoracic kyphosis (T5-T12) was maintained in the pedicle screw patients but significantly increased in the hybrid/hook patients over time (P < 0.05). Scoliosis Research Society outcome scores significantly improved in both groups. Blood loss was equivalent but operative time was longer in the hook/hybrid patients.No pedicle screw patients were revised for instrumentation complications with 1 lumbosacral nonunion revised at 5 years postoperative (3% revision rate). Eight of 58 patients among the hook/hybrid cohort underwent 9 revisions for instrumentation failure (n = 3) or nonunion (n = 6) (14% revision rate; P = 0.04).

CONCLUSION

Pedicle screw correction of AdIS is safe and effective. Compared to hook/hybrid constructs, these patients displayed significantly improved correction of the major curve (even in the absence of anterior releases), maintenance of thoracic kyphosis, and a lower revision rate. Similar SRS scores and blood loss were noted. The hook/hybrid patients had significantly longer operative times. The use of BMP-2 in 7 patients and TLIF/ALIF also in 7 of the pedicle screw patients may have decreased the need for revision surgery up to the latest follow-up.

Treatment of thoracic scoliosis: are monoaxial thoracic pedicle screws the best form of fixation for correction?

STUDY DESIGN

Multicenter retrospective cohort study of 100 consecutive patients.

OBJECTIVE

Compare the coronal and sagittal plane radiographic outcomes and clinical rib hump deformity correction in patients who underwent posterior spinal fusion for adolescent idiopathic scoliosis using hybrid instrumentation, polyaxial (POLY) segmental pedicle screw fixation, or monoaxial (MONO) segmental pedicle screw fixation.

SUMMARY OF BACKGROUND DATA

Instrumentation for the treatment of scoliosis has evolved. Current techniques include use of a combination of hooks, sublaminar wires, and pedicle screws (HYBRID), as well as segmental pedicle screw fixation with either monoaxial or polyaxial screw anchors.

METHODS

Data were obtained from a multicenter prospective database registry. Radiographic assessment was performed from radiographs taken before surgery and 2 years after surgery. Intraoperative parameters assessed included operative time, estimated blood loss, and whether or not a thoracoplasty was performed. Clinical evaluation of the angle of trunk rotation was done with an inclinometer.

RESULTS

Preoperative thoracic curvature and curve flexibility, age, Lenke curve type, and number of levels fused was similar for each group. Significantly more anchors per level were used in the MONO group (1.69) and in the HYBRID group (1.24) compared to the POLY group (1.06). There were no statistical differences among the groups with respect to major thoracic curve correction (MONO: 69%; POLY: 68%; HYBRID: 62%, P = 0.22). POLY constructs maintained thoracic kyphosis, whereas MONO and HYBRID had a tendency toward loss of thoracic kyphosis and there was a trend toward a greater percent thoracic angle of trunk rotation correction in the MONO group (55%) compared with the POLY group (32%, P = 0.10), but no differences compared with Hybrid (55%, P > 0.05).

CONCLUSION

Similar coronal and sagittal plane correction was achieved in thoracic adolescent idiopathic scoliosis with 3 different constructs. There was a trend toward improved correction of clinical rib hump deformity with MONO screw constructs compared with POLY screw constructs.

The successful short-term treatment of flexion-distraction injuries of the thoracic spine using posterior-only pedicle screw instrumentation

STUDY DESIGN

In this retrospective study, the results of treating unstable flexion-distraction injuries (FDI) of the thoracic spine with posterior-only thoracic pedicle screw (P/TPS) instrumentation were investigated.

OBJECTIVE

The objective was to determine the ability of P/TPS to correct and maintain the focal kyphosis of the injured spine. Clinical outcome and complications of the surgical procedure were also a focus of the study.

SUMMARY OF BACKGROUND DATA

The treatment of FDI of the thoracic spine remains controversial. There continues to be difficulty in maintaining the corrected kyphosis angle regardless of the surgical approach used.

METHODS

Eighteen patients with FDI of thoracic spine who underwent P/TPS were identified. The initial and corrected focal kyphosis was evaluated radiographically. Failure of treatment was defined as a >5-degree progression of corrected kyphosis from initial to latest follow-up. Clinical evaluation included complication rate, Injury Severity Score, and Frankel grade.

RESULTS

Of the 18 eligible patients, 15 (83%) had adequate follow-up for evaluation. The mean length of follow-up care was 16.1 months. The average Injury Severity Score was 23. The average number of instrumented levels was 6.8. The focal kyphosis of the injury was reduced from a mean of 19.60 to 5.73 degrees (P<0.001), with an average progression of only 2.27 degrees (P=0.128) at final follow-up. Complications were limited to wound infections and occurred in 2 study participants. Neurologic status returned to normal in all patients with incomplete cord injuries.

CONCLUSIONS

FDI of the thoracic spine may be amendable to a uniform surgical approach with P/TPS. This procedure carried a relatively low complication rate and allowed for reduction and restoration of the posterior tension band with a biomechanically rigid construct.

A biomechanical assessment of thoracic spine stapling

STUDY DESIGN

An in vitro immature bovine study of thoracic spine fixation using shape memory alloy for minimally invasive adolescent scoliosis treatment.

OBJECTIVE

The purpose of this study was to detect differences in thoracic spine range of motion due to the placement of spinal staples manufactured from a common shape memory alloy.

SUMMARY OF BACKGROUND DATA

Scoliosis surgery attempts to both correct the existing deformity and prevent further curve progression. The gold standard in surgical treatment of scoliosis is posterior instrumentation and fusion using pedicle screws or hooks. Fusionless techniques generally use less invasive procedures than fusion methods. One such technique, anterior stapling of the vertebrae, theoretically enables preservation of growth, motion, and spinal function. However, the degree of stability afforded by this method has not been reported.

METHODS

Eight immature bovine specimens (T4-T9) were used in this study. Nitinol staples were cooled to below the transition temperature and placed on the spine such that they spanned the disc space. The specimen was then heated above the critical temperature and the staple tines engaged the vertebral endplates. A common flexibility protocol was used to determine the ranges of motion (ROM) in flexion-extension, lateral bending, and axial rotation. The intact spine and 4 construct variants (combinations of staple type and placement) were evaluated using this testing protocol. ANOVA statistics with post hoc testing was used to discern statistical differences.

RESULTS

Not all staple variants were able to achieve significant reductions with respect to the intact condition. ROM was significantly restricted in axial rotation and lateral bending with the introduction of staple instrumentation. Further, there seemed to be a mechanical equivalence between a single double-prong staple and 2 single-prong staples. Our data indicate that staple fixation does not result in consistently elevated adjacent segment motion.

CONCLUSIONS

Our results imply that staples are able to significantly restrict motion while not achieving motion reductions that one would achieve with fusion-promoting instrumentation. The choice between double- and single-prong staples remains a matter of preference. Neither staple variant provided a mechanical advantage. The single-prong staple did allow more control in the placement of the staple over the disc space. The addition of an anterior staple significantly reduced the overall flexion-extension ROM.

Surgical Strategies and Choosing Levels for Spinal Deformity: How High, How Low, Front and Back

The purpose of this article is to describe general strategies in the surgical treatment of adolescent and adult scoliosis, including radiographic evaluation, curve selection, principles guiding the selection of the upper and lower instrumented vertebrae, and indications for anterior surgery. Sagittal plane deformity, including Scheuermann's kyphosis, is discussed. Avoidance and treatment of postoperative flatback deformity is also briefly mentioned. There are multiple and sometimes conflicting considerations that must be reviewed when planning surgical stabilization of spinal deformity. Although there may be significant variation in surgeon decision making, careful adherence to primary principles, such as achieving coronal and sagittal balance in all patients and minimizing fusion levels, particularly in young patients, should be of paramount importance.