AOA Critical Issues Symposium: Mind the Gap: Addressing Confidence, Imposter Syndrome, and Perfectionism in Surgical Training.

ABSTRACT: Orthopaedic surgeons in training and in their careers can experience a lack of confidence and imposter syndrome. Confidence is built early through continuous improvement, accomplishments, support, and reinforcement. Although it is normal to lack confidence at times, the goal is to recognize this issue, work on visualizing success, and know when to seek help. Mentors can help mentees to build confidence and to normalize thoughts of insecurity and imposter syndrome. It is critical to develop and to maintain resilience, grit, emotional intelligence, courage, and vulnerability during training and throughout one's entire orthopaedic career. Leaders in the field must be aware of these phenomena, be able to talk about such issues, have methods to combat the harmful effects of imposter syndrome, and create a safe, supportive environment conducive to learning and working. Leading well builds not only confidence in oneself but also self-confidence in others. Leaders who are able to build the confidence of individuals will enhance team dynamics, wellness, and overall productivity as well as individual and organizational success.

Does Resection of the Posterior Longitudinal Ligament Affect the Stability of Cervical Disc Arthroplasty?

BACKGROUND: The need for posterior longitudinal ligament (PLL) resection during cervical total disc arthroplasty (TDA) has been debated. The purpose of this laboratory study was to investigate the effect of PLL resection on cervical kinematics after TDA. METHODS: Eight cadaveric cervical spine specimens were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) to moments of ±1.5 Nm. After testing the intact condition, anterior C5-C6 cervical discectomy was performed followed by PLL resection and implantation of a compressible, 6-degrees-of-freedom disc prosthesis (M6-C, Spinal Kinetics Inc, Sunnyvale, California). Next, a second prosthesis was implanted at C6-C7 with PLL intact. Finally, the C6-C7 PLL was resected while the disc prosthesis remained in place. Segmental range of motion (ROM) and stiffness in the high flexibility zone around the neutral posture were analyzed using repeated measures ANOVA. RESULTS: At C5-C6, following TDA and PLL resection, FE, LB, and AR ROMs decreased significantly. Anterior and posterior disc height, segmental lordosis, and flexion stiffness increased significantly. At C6-C7, TDA with the PLL intact resulted in a significant increase in anterior disc height and segmental lordosis with no change in posterior disc height. FE, LB, and AR ROMs all decreased significantly, while flexion stiffness increased significantly compared to intact. PLL resection at C6-C7 did not result in a notable change compared to TDA with PLL intact. At the same level, flexion stiffness decreased following PLL resection compared to TDA with a value closer to intact. Two-level TDA (C5-C7) with PLL resection did not result in a loss of segmental stability. CONCLUSION: PLL resection did not significantly affect motion segment kinematics following cervical TDA using a prosthesis with inherent stiffness. Motion segment stiffness loss after PLL resection can be compensated for by a TDA design that can provide resistance to angular motion.

Cervical sagittal balance: a biomechanical perspective can help clinical practice.

PURPOSE: In this article, we summarize our work on understanding the influence of cervical sagittal malalignment on the mechanics of the cervical spine. METHODS: Biomechanical studies were performed using an ex vivo laboratory model to study the kinematic and kinetic response of human cervical spine specimens in the setting of cervical sagittal imbalance. The model allowed controlled variations of C2-C7 Sagittal Vertical Alignment (C2-C7 SVA) and T1-Slope so that clinically relevant sagittally malaligned profiles could be prescribed, while maintaining horizontal gaze, and their biomechanical consequences studied. RESULTS: Our results demonstrated that increasing C2-C7 SVA caused flexion of lower cervical (C2-C7) segments and hyperextension of suboccipital (C0-C1-C2) segments to maintain horizontal gaze. An increase in C2-C7 SVA increased the lower cervical neural foraminal areas. Conversely, increasing T1-slope predominantly influenced subaxial cervical lordosis and, as a result, decreased cervical neural foraminal areas. Therefore, we believe patients with increased upper thoracic kyphosis and radicular symptoms may respond with increased forward head posture (FHP) as a compensatory mechanism to increase their lower cervical neural foraminal area and alleviate nerve root compression as well as reduce the burden on posterior muscles and soft and bony structures of the cervical spine. Increasing FHP (i.e., increased C2-C7 SVA) was associated with shortening of the cervical flexors and occipital extensors and lengthening of the cervical extensors and occipital flexors, which corresponds to C2-C7 flexion and C0-C2 extension. The greatest shortening occurred in the suboccipital muscles, suggesting considerable load bearing of these muscles during chronic FHP. Regardless, there was no evidence of nerve compression within the suboccipital triangle. Finally, cervical sagittal imbalance may play a role in exacerbating adjacent segment pathomechanics after multilevel cervical fusion and should be considered during surgical planning. CONCLUSIONS: The results of our biomechanical studies have improved our understanding of the impact of cervical sagittal malalignment on pathomechanics of the cervical spine. We believe this improved understanding will assist in clinical decision-making.

Biomechanical Stability Analysis of a Stand-alone Cage, Static and Rotational-dynamic Plate in a Two-level Cervical Fusion Construct.

OBJECTIVE: To test the following hypotheses: (i) anterior cervical discetomy and fusion (ACDF) using stand-alone interbody spacers will significantly reduce the range of motion from intact spine; and (ii) the use of a static or a rotational-dynamic plate will significantly augment the stability of stand-alone interbody spacers, with similar beneficial effect when compared to each other. METHODS: Eleven human cadaveric subaxial cervical spines (age: 48.2 ± 5.4 years) were tested under the following sequence: (i) intact spine; (ii) ACDF at C4 -C5 using a stand-alone interbody spacer; (iii) ACDF at C5 -C6 and insertion of an interbody spacer (two-level construct); and (iv) randomized placement of either a two-level locking static plate or a rotational-dynamic plate. RESULTS: Insertion of stand-alone cage at C4 -C5 and C5 -C6 caused a significant decrease in the range of motion compared to intact spine (P < 0.05). Placement of both the locking and the rotational dynamic plate further reduced the range of motion at C4 -C5 and C5 -C6 compared to stand-alone cage (P < 0.01). No significant differences in range of motion restriction at either C4 -C5 or C5 -C6 were found when the two plating systems were compared (P > 0.05). CONCLUSIONS: Cervical stand-alone interbody spacers caused significant restriction in the range of motion. Both plates significantly augmented the stability of stand-alone interbody spacers, with similar stabilizing effect.

Cervical Spine Muscle-Tendon Unit Length Differences Between Neutral and Forward Head Postures: Biomechanical Study Using Human Cadaveric Specimens.

BACKGROUND: Forward head posture (FHP) may be associated with neck pain and poor health-related quality of life. Literature describes only qualitative muscle length changes associated with FHP. OBJECTIVE: The purpose of this study was to quantify how muscle-tendon unit lengths are altered when human cadaveric specimens are placed in alignments representing different severities of FHP. DESIGN: This biomechanical study used 13 fresh-frozen cadaveric cervical spine specimens (Occiput-T1, 54±15 y). METHODS: Specimens' postural changes simulating increasing FHP severity while maintaining horizontal gaze were assessed. Specimen-specific anatomic models derived from computed tomography-based anatomic data were combined with postural data and specimen-specific anatomy of muscle attachment points to estimate the muscle length changes associated with FHP. RESULTS: Forward head posture was associated with flexion of the mid-lower cervical spine and extension of the upper cervical (sub-occipital) spine. Muscles that insert on the cervical spine and function as flexors (termed "cervical flexors") as well as muscles that insert on the cranium and function as extensors ("occipital extensors") shortened in FHP when compared to neutral posture. In contrast, muscles that insert on the cervical spine and function as extensors ("cervical extensors") as well as muscles that insert on the cranium and function as flexors ("occipital flexors") lengthened. The greatest shortening was seen in the major and minor rectus capitis posterior muscles. These muscles cross the Occiput-C2 segments, which exhibited extension to maintain horizontal gaze. The greatest lengthening was seen in posterior muscles crossing the C4-C6 segments, which exhibited the most flexion. LIMITATIONS: This cadaver study did not incorporate the biomechanical influence of active musculature. CONCLUSIONS: This study offers a novel way to quantify postural alignment and muscle length changes associated with FHP. Model predictions are consistent with qualitative descriptions in the literature.

Is Cervical Sagittal Imbalance a Risk Factor for Adjacent Segment Pathomechanics After Multilevel Fusion?

STUDY DESIGN: A biomechanical study using human spine specimens. OBJECTIVE: The aim of this study was to assess whether the presence of cervical sagittal imbalance is an independent risk factor for increasing the mechanical burden on discs adjacent to cervical multilevel fusions. SUMMARY OF BACKGROUND DATA: The horizontal offset distance between the C2 plumbline and C7 vertebral body (C2-C7 Sagittal Vertical Axis (SVA)) or the angle made with vertical by a line connecting the C2 and C7 vertebral bodies (C2-C7 tilt angle) are used as radiographic measures to assess cervical sagittal balance. There is level III clinical evidence that sagittal imbalance caused by kyphotic fusions or global spinal sagittal malalignment may increase the risk of adjacent segment pathology. METHODS: Thirteen human cadaveric cervical spines (Occiput-T1; age: 50.6 years; range: 21-67) were tested first in the native intact state and then after instrumentation across C4-C6 to simulate in situ two-level fusion. Specimens were tested using a previously validated experimental model that allowed measurement of spinal response to prescribed imbalance. The effects of fusion on segmental angular alignments and intradiscal pressures in the C3-C4 and C6-C7 discs, above and below the fusion, were evaluated at different magnitudes of C2-C7 tilt angle (or C2-C7 SVA). RESULTS: When compared with the pre-fusion state, in situ fusion across C4-C6 segments required increased flexion angulation and resulted in increased intradiscal pressure at the C6-C7 disc below the fusion in order to accommodate the same increase in C2-C7 tilt angle or C2-C7 SVA (P < 0.05). The adjacent segment mechanical burden due to fusion became greater with increasing C2-C7 tilt angle or SVA. CONCLUSION: Cervical sagittal imbalance arising from regional and/or global spinal sagittal malalignment may play a role in exacerbating adjacent segment pathomechanics after multilevel fusion and should be considered during surgical planning. LEVEL OF EVIDENCE: N/A.

Postural Consequences of Cervical Sagittal Imbalance: A Novel Laboratory Model.

STUDY DESIGN: A biomechanical study using human spine specimens. OBJECTIVE: To study postural compensations in lordosis angles that are necessary to maintain horizontal gaze in the presence of forward head posture and increasing T1 sagittal tilt. SUMMARY OF BACKGROUND DATA: Forward head posture relative to the shoulders, assessed radiographically using the horizontal offset distance between the C2 and C7 vertebral bodies (C2-C7 [sagittal vertical alignment] SVA), is a measure of global cervical imbalance. This may result from kyphotic alignment of cervical segments, muscle imbalance, as well as malalignment of thoracolumbar spine. METHODS: Ten cadaveric cervical spines (occiput-T1) were tested. The T1 vertebra was anchored to a tilting and translating base. The occiput was free to move vertically but its angular orientation was constrained to ensure horizontal gaze regardless of sagittal imbalance. A 5-kg mass was attached to the occiput to mimic head weight. Forward head posture magnitude and T1 tilt were varied and motions of individual vertebrae were measured to calculate C2-C7 SVA and lordosis across C0-C2 and C2-C7. RESULTS: Increasing C2-C7 SVA caused flexion of lower cervical (C2-C7) segments and hyperextension of suboccipital (C0-C1-C2) segments to maintain horizontal gaze. Increasing kyphotic T1 tilt primarily increased lordosis across the C2-C7 segments. Regression models were developed to predict the compensatory C0-C2 and C2-C7 angulation needed to maintain horizontal gaze given values of C2-C7 SVA and T1 tilt. CONCLUSION: This study established predictive relationships between radiographical measures of forward head posture, T1 tilt, and postural compensations in the cervical lordosis angles needed to maintain horizontal gaze. The laboratory model predicted that normalization of C2-C7 SVA will reduce suboccipital (C0-C2) hyperextension, whereas T1 tilt reduction will reduce the hyperextension in the C2-C7 segments. The predictive relationships may help in planning corrective strategy in patients experiencing neck pain, which may be attributed to sagittal malalignment. LEVEL OF EVIDENCE: N/A.

Effect of physiological loads on cortical and traditional pedicle screw fixation.

STUDY DESIGN: Human cadaveric biomechanical study. OBJECTIVE: To determine the fixation strength of laterally directed, cortical pedicle screws under physiological loads. SUMMARY OF BACKGROUND DATA: Lateral trajectory cortical pedicle screws have been described as a means of obtaining improved fixation while minimizing soft-tissue dissection during lumbar instrumentation. Biomechanical data have demonstrated equivalent strength in a quasi-static model; however, no biomechanical information is available comparing the fixation of cortical with traditional pedicle screws under cyclic physiological loads. METHODS: Seventeen vertebral levels (T11-L5) underwent quantitative computed tomography. On 1 side, a laterally directed, cortical pedicle screw was inserted with a traditional, medially directed pedicle screw placed on the contralateral side. With the specimen constrained in a testing apparatus, each screw underwent cyclic craniocaudal toggling under incrementally increasing physiological loads until 2 mm of head displacement occurred. Next, uniaxial pullout of each toggled screw was performed. The number of craniocaudal toggle cycles and load (N) required to achieve pedicle screw movement as well as axial pullout resistance (N) were compared between the 2 techniques. RESULTS: The mean trabecular bone mineral density of the specimens was 202 K2HPO4 mg/cm. Cortical pedicle screws demonstrated significantly improved resistance to toggle testing, requiring 184 cycles to reach 2 mm of displacement compared with 102 cycles for the traditional pedicle screws (P=0.002). The force necessary to displace the screws was also significantly greater for the cortical versus the traditional screws (398 N vs. 300 N, P=0.004). There was no statistical difference in axial pullout strength between the previously toggled cortical and traditional pedicle screws (1722 N vs. 1741 N, P=0.837). CONCLUSION: Laterally directed cortical pedicle screws have superior resistance to craniocaudal toggling compared with traditional pedicle screws. LEVEL OF EVIDENCE: N/A.

The effect of posterior decompressive procedures on segmental range of motion after cervical total disc arthroplasty.

STUDY DESIGN: We quantified the segmental biomechanics of a cervical total disc replacement (TDR) before and after progressive posterior decompression. We hypothesized that posterior decompressive procedures would not significantly increase range of motion (ROM) at the index TDR level. OBJECTIVE: To quantify the kinematics of a cervical total disc replacement (TDR) before and after posterior cervical decompression. SUMMARY OF BACKGROUND DATA: A reported yet unaddressed issue is the potential for the development of same-segment disease after implantation of a cervical TDR and the implications of same-segment posterior decompression on TDR mechanics. METHODS: Eight human cadaveric cervical spines C3-C7 were tested in flexion-extension, lateral bending, and axial rotation while intact, after C5-C6 TDR, C5-C6 unilateral foraminotomy, C5-C6 bilateral foraminotomies, and after C5 laminectomy in combination with the bilateral foraminotomies. Moment versus angular motion curves were obtained for each testing step, and the load-displacement data were analyzed to determine the range of angular motion for each step. RESULTS: Unilateral foraminotomy did not result in a statistically significant increase in flexion-extension ROM, and did not increase the ROM to a degree greater than normal. Although bilateral foraminotomies did increase flexion-extension ROM, motion remained within a physiological range. A full laminectomy added to the bilateral foraminotomies significantly increased ROM and was also associated with distortion of the load-displacement curves. CONCLUSION: With respect to segmental biomechanics as demonstrated, we think that for same-segment disease, a unilateral foraminotomy can be performed safely. However, the impact of in vivo conditions was not accounted for in this model, and it is possible that cyclical loading and other physiological stresses on such a construct may affect the behavior and lifespan of the implant in a way that cannot be predicted by a biomechanical study. Bilateral foraminotomies would require close observation and additional clinical follow-up, whereas complete laminectomy combined with bilateral foraminotomies should be avoided after TDR given the significant changes in kinematics. In addition, future disc replacement designs may need to account for changes after posterior decompression for same-segment disease. LEVEL OF EVIDENCE: N/A.

Biomechanical evaluation of a low-profile, anchored cervical interbody spacer device at the index level or adjacent to plated fusion.

STUDY DESIGN: In vitro biomechanical study. OBJECTIVE: To test the hypotheses: (1) an anchored spacer device would decrease motion similarly to a plate-spacer construct, and (2) the anchored spacer would achieve a similar reduction in motion when placed adjacent to a previously fused segment. SUMMARY OF BACKGROUND DATA: An anchored spacer device has been shown to perform similar to the plate-spacer construct in previous biomechanical evaluation. The prevalence of adjacent segment disease after fusion is well established in the literature.There is currently no evidence supporting the use of an anchored interbody spacer device adjacent to a previous fusion. METHODS: Eight human cervical spines (age: 45.1 ± 13.1 yr) were tested in moment control (±1.5 Nm) in flexion-extension, lateral bending, and axial rotation without preload. Flexion-extension was then retested under 150-N preload. Spines were tested intact and after anterior cervical discectomy and fusion (ACDF) at C4-C5 and C6-C7 with either a plate-spacer or anchored spacer construct (randomized). The specimens were tested finally with an ACDF at the floating C5-C6 segment using the anchored spacer device adjacent to the previous fusions. RESULTS: Both the plate-spacer and anchored spacer significantly reduced motion from the intact spine in flexion-extension, lateral bending, and axial rotation (P < 0.005). There was no statistically significant difference between the 2 fusion constructs in their abilities to reduce motions (P = 1.0). ACDF using the anchored spacer at the floating C5-C6 level (in between the plate-spacer and anchored spacer constructs) resulted in significant motion reductions in all modes of testing (P < 0.05). These motion reductions did not significantly differ from those of a single-level anchored-spacer construct or a single-level plated ACDF. CONCLUSION: The anchored spacer provided significant motion reductions, similar to a plated ACDF, when used as a single-level fusion construct or placed adjacent to a previously plated segment. LEVEL OF EVIDENCE: N/A.

Compressive preload reduces segmental flexion instability after progressive destabilization of the lumbar spine.

STUDY DESIGN: Biomechanical human cadaveric study. OBJECTIVE: We hypothesized that increasing compressive preload will reduce the segmental instability after nucleotomy, posterior ligament resection, and decompressive surgery. SUMMARY OF BACKGROUND DATA: The human spine experiences significant compressive preloads in vivo due to spinal musculature and gravity. Although the effect of destabilization procedures on spinal motion has been studied, the effect of compressive preload on the motion response of destabilized, multisegment lumbar spines has not been reported. METHODS: Eight human cadaveric spines (L1-sacrum, 51.4 ± 14.1 yr) were tested intact, after L4-L5 nucleotomy, after interspinous and supraspinous ligaments transection, and after midline decompression (bilateral laminotomy, partial medial facetectomy, and foraminotomy). Specimens were loaded in flexion (8 Nm) and extension (6 Nm) under 0-N, 200-N, and 400-N compressive follower preload. L4-L5 range of motion (ROM) and flexion stiffness in the high-flexibility zone were analyzed using repeated-measures analysis of variance and multiple comparisons with the Bonferroni correction. RESULTS: With a fixed set of loading conditions, a progressive increase in segmental ROM along with expansion of the high-flexibility zone (decrease of flexion stiffness) was noted with serial destabilizations. Application of increasing compressive preload did not substantially change segmental ROM, but did significantly increase the segmental stiffness in the high-flexibility zone. In the most destabilized condition, 400-N preload did not return the segmental stiffness to intact levels. CONCLUSION: Anatomical alterations representing degenerative and iatrogenic instabilities are associated with significant increases in segmental ROM and decreased segmental stiffness. Although application of compressive preload, mimicking the effect of increased axial muscular activity, significantly increased the segmental stiffness, it was not restored to intact levels; thereby suggesting that core strengthening alone may not compensate for the loss of structural stability associated with midline surgical decompression. This suggests that there may be a role for surgical implants or interventions that specifically increase flexion stiffness and limit flexion ROM to counteract the iatrogenic instability resulting from surgical decompression. LEVEL OF EVIDENCE: N/A.

Biomechanical evaluation of a low profile, anchored cervical interbody spacer device in the setting of progressive flexion-distraction injury of the cervical spine.

INTRODUCTION: Anterior cervical decompression and fusion is a well-established procedure for treatment of degenerative disc disease and cervical trauma including flexion-distraction injuries. Low-profile interbody devices incorporating fixation have been introduced to avoid potential issues associated with dissection and traditional instrumentation. While these devices have been assessed in traditional models, they have not been evaluated in the setting of traumatic spine injury. This study investigated the ability of these devices to stabilize the subaxial cervical spine in the presence of flexion-distraction injuries of increasing severity. METHODS: Thirteen human cadaveric subaxial cervical spines (C3-C7) were tested at C5-C6 in flexion-extension, lateral bending and axial rotation in the load-control mode under ±1.5 Nm moments. Six spines were tested with locked screw configuration and seven with variable angle screw configuration. After testing the range of motion (ROM) with implanted device, progressive posterior destabilization was performed in 3 stages at C5-C6. RESULTS: The anchored spacer device with locked screw configuration significantly reduced C5-C6 flexion-extension (FE) motion from 14.8 ± 4.2 to 3.9 ± 1.8°, lateral bending (LB) from 10.3 ± 2.0 to 1.6 ± 0.8, and axial rotation (AR) from 11.0 ± 2.4 to 2.5 ± 0.8 compared with intact under (p < 0.01). The anchored spacer device with variable angle screw configuration also significantly reduced C5-C6 FE motion from 10.7 ± 1.7 to 5.5 ± 2.5°, LB from 8.3 ± 1.4 to 2.7 ± 1.0, and AR from 8.8 ± 2.7 to 4.6 ± 1.3 compared with intact (p < 0.01). The ROM of the C5-C6 segment with locked screw configuration and grade-3 F-D injury was significantly reduced from intact, with residual motions of 5.1 ± 2.1 in FE, 2.0 ± 1.1 in LB, and 3.3 ± 1.4 in AR. Conversely, the ROM of the C5-C6 segment with variable-angle screw configuration and grade-3 F-D injury was not significantly reduced from intact, with residual motions of 8.7 ± 4.5 in FE, 5.0 ± 1.6 in LB, and 9.5 ± 4.6 in AR. CONCLUSIONS: The locked screw spacer showed significantly reduced motion compared with the intact spine even in the setting of progressive flexion-distraction injury. The variable angle screw spacer did not sufficiently stabilize flexion-distraction injuries. The resulting motion for both constructs was higher than that reported in previous studies using traditional plating. Locked screw spacers may be utilized with additional external immobilization while variable angle screw spacers should not be used in patients with flexion-distraction injuries.

Would an anatomically shaped lumbar interbody cage provide better stability? An in vitro cadaveric biomechanical evaluation.

STUDY DESIGN: A biomechanical cadaveric study of lumbar spine segments. OBJECTIVE: To compare the immediate stability provided by parallel-shaped and anatomically shaped carbon fiber interbody fusion I/F cages in posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF) constructs with posterior pedicle screw instrumentation. SUMMARY OF BACKGROUND DATA: Few biomechanical data are available on the anatomically shaped cages in PLIF and TLIF constructs. METHODS: Twenty human lumbar segments were tested in flexion-extension (FE) (8 N m flexion, 6 N m extension), lateral bending (LB) (± 6 N m), and torsional loading (± 5 N m). Each segment was tested in the intact state and after insertion of interbody cages in one of 3 constructs: PLIF with 2 parallel-shaped or anatomically shaped cages and TLIF with 1 anatomically shaped cage. All cages received supplementary pedicle screw fixation. The range-of-motion (ROM) values after cage insertion and posterior fixation were compared with the intact specimen values using analysis of variance and multiple comparisons with Bonferroni correction. RESULTS: All constructs significantly reduced segmental motion relative to intact (P < 0.001). The motion reductions in FE, LB, and axial rotation were 85 ± 15%, 83 ± 18%, and 67 ± 6.8% for the PLIF construct using parallel cages, 79 ± 5.5%, 87 ± 10%, and 66 ± 20% for PLIF using anatomically shaped cages, and 90 ± 6.8%, 87 ± 12%, and 77 ± 22% for TLIF with an anatomically shaped cage. In FE and LB, the reductions in the ROM caused between the 3 constructs were equivalent (P > 0.05). In axial rotation, the TLIF cage provided significantly greater limitation in the ROM compared with the parallel-shaped PLIF cage (P = 0.01). CONCLUSIONS: The parallel-shaped and anatomically shaped I/F cages provided similar stability in a PLIF construct. The greater stability of the TLIF construct was likely due to a more anterior placement of the TLIF cage and preservation of the contralateral facet joint.

Relaxation response of lumbar segments undergoing disc-space distraction: implications to the stability of anterior lumbar interbody implants.

STUDY DESIGN: A biomechanical study of human cadaveric lumbar spine segments undergoing disc-space distraction for insertion of anterior lumbar interbody implants. OBJECTIVE: To measure the distraction force and its relaxation during a period of up to 3 hours after disc-space distraction as a function of the distraction magnitude and disc level. SUMMARY OF BACKGROUND DATA: Interbody implants depend on compressive preload produced by disc-space distraction (annular pretension) for initial stabilization of the implant-bone interface. However, the amount of preload produced by disc-space distraction due to insertion of the implant and its subsequent relaxation have not been quantified. METHODS: Twenty-two fresh human lumbar motion segments (age: 51 ± 14.8 years) were used. An anterior lumbar discectomy was performed. The distraction test battery consisted of a tension stiffness test performed before and after each relaxation test, 2 distraction magnitudes of 2 and 4 mm, and a recovery period before each distraction input. The distraction forces and lordosis angles were measured. RESULTS.: Peak distraction force was significantly larger for the 4-mm distraction (431.8 ± 116.4 N) than for the 2-mm distraction (204.9 ± 55.5 N) (P < 0.01). The distraction force significantly decreased over time (P < 0.01), approximating steady-state values of 146.1 ± 47.3 N at 2-mm distraction and 289.8 ± 92.8 N at 4-mm distraction, respectively. The distraction force reduced in magnitude by more than 20% of peak value in the first 15 minutes and reduced by approximately 30% of the peak value at the end of the testing period. The spine segment relaxed by the same amount of force, regardless of the disc level (P > 0.05). CONCLUSION: The "tightness of fit" that the surgeon notes immediately after interbody device insertion in the disc space degrades in the very early postoperative period, which could compromise the stability of the bone-implant interface.

Kinematics of cervical total disc replacement adjacent to a two-level, straight versus lordotic fusion.

STUDY DESIGN: In vitro biomechanical study. OBJECTIVE: To characterize cervical total disc replacement (TDR) kinematics above two-level fusion, and to determine the effect of fusion alignment on TDR response. SUMMARY OF BACKGROUND DATA: Cervical TDR may be a promising alternative for a symptomatic adjacent level after prior multilevel cervical fusion. However, little is known about the TDR kinematics in this setting. METHODS: Eight human cadaveric cervical spines (C2-T1, age: 59 ± 8.6 years) were tested intact, after simulated two-level fusion (C4-C6) in lordotic alignment and then in straight alignment, and after C3-C4 TDR above the C4-C6 fusion in lordotic and straight alignments. Fusion was simulated using an external fixator apparatus, allowing easy adjustment of C4-C6 fusion alignment, and restoration to intact state upon disassembly. Specimens were tested in flexion-extension using hybrid testing protocols. RESULTS: The external fixator device significantly reduced range of motion (ROM) at C4-C6 to 2.0 ± 0.6°, a reduction of 89 ± 3.0% (P < 0.05). Removal of the fusion construct restored the motion response of the spinal segments to their intact state. The C3-C4 TDR resulted in less motion as compared to the intact segment when the disc prosthesis was implanted either as a stand-alone procedure or above a two-level fusion. The decrease in motion of C3-C4 TDR was significant for both lordotic and straight fusions across C4-C6 (P < 0.05). Flexion and extension moments needed to bring the cervical spine to similar C2 motion endpoints significantly increased for the TDR above a two-level fusion compared to TDR alone (P < 0.05). Lordotic fusion required significantly greater flexion moment, whereas straight fusion required significantly greater extension moment (P < 0.05). CONCLUSION: TDR placed adjacent to a two-level fusion is subjected to a more challenging biomechanical environment as compared to a stand-alone TDR. An artificial disc used in such a clinical scenario should be able to accommodate the increased moment loads without causing impingement of its endplates or undue wear during the expected life of the prosthesis.

Effect of increasing implant height on lumbar spine kinematics and foraminal size using the ProDisc-L prosthesis.

STUDY DESIGN: A biomechanical study using human lumbar spines. OBJECTIVE: To test the hypotheses that with increasing implant height (1) the range of motion (ROM) of the implanted segment will decrease, (2) the segmental lordosis will increase, and (3) the size of the neural foramens will increase. SUMMARY OF BACKGROUND DATA: Little is known about the effects of the implant height on the segmental motion and foraminal size at the implanted level. METHODS: Seven human lumbar spines (age, 54.4+/-11.4 years; L1-sacrum) were tested intact, and after discectomy at L4-L5 and sequential insertion of ProDisc-L implants (Synthes Spine, Paoli, PA) of increasing heights (10, 12, and 14 mm). The specimens were tested in flexion (8 Nm) and extension (-6 Nm) with a 400 N follower preload as well as in lateral bending (+/-6 Nm) and axial rotation (+/-5 Nm) without preload. Three-dimensional motions were measured at L4-L5. Foraminal sizes at L4-L5 were measured in the specimen's neutral posture under a 400 N preload for the intact spine and after each implantation using finely graded cylindrical probes. Segmental lordosis was measured in the specimen's neutral posture under a 400 N preload by analyzing digital fluoroscopic images. Effects of implant height on the kinematics, foraminal size, and segmental lordosis were assessed using paired comparisons with Bonferroni correction. RESULTS: Increasing implant height from 10 mm to 14 mm caused a significant decrease (P<0.05) in segmental ROM by up to 37%+/-21% in flexion/extension, 33%+/-18% in lateral bending, and 29%+/-28% in axial rotation. Increasing implant height also produced a significant increase in segmental lordosis (P<0.05): from 9.7 degrees+/-2.9 degrees at 10 mm, to 16.1 degrees+/-5.1 degrees at 14 mm. The increase in foraminal size, while significant, was only 4.6%+/-3.2% when comparing 10 mm to 14 mm implants. CONCLUSION: These results suggest that a smaller implant height should be selected to optimize the ROM of the implanted segment and maintain sagittal balance.

Cauda equina syndrome in an eleven-month-old infant following sacrococcygeal teratoma tumor resection and coccyx excision: case report.

STUDY DESIGN: A case report of cauda equina syndrome (CES) in an 11-month-old infant, following sacrococcygeal teratoma tumor resection and coccyx excision leading to a spinal epidural hematoma (SEH). OBJECTIVE: To illustrate a rare case of CES and SEH in an infant, and discuss the need for sealing access to the spinal canal after sacrococcygeal surgical resection and reconstruction. SUMMARY OF BACKGROUND DATA: To the authors' knowledge, this is the youngest patient reported to develop a SEH and CES, and the only patient reported in the literature to develop a SEH after coccyx excision. METHODS: Seventeen days after undergoing sacrococcygeal tumor resection and coccyx excision, the patient presented to the emergency room with a large distended bladder, loss of rectal tone, and significant weakness in the lower extremities. Magnetic resonance imaging of the thoracic and lumbar spine showed a large lesion in the dorsal epidural space extending from T12 to the tip of the communicating with the prior operative site by means of the previous coccyx resection. The infant was emergently brought to the operating room for decompression. RESULTS: The patient was discharged 6 days later with diminished neurologic function, but demonstrated significant improvement over the next 18 months and currently remains disease free and neurologically normal at age 7. CONCLUSION: This case demonstrates the need for future examination of sacrococcygeal surgical resection and subsequent reconstruction of excised structures to decrease the risk of communication with the epidural space.

Biomechanical evaluation of segmental occipitoatlantoaxial stabilization techniques.

STUDY DESIGN: Biomechanical study using human cadaveric cervical spines. OBJECTIVE: To evaluate the construct stability of 3 different segmental occipitoatlantoaxial (C0-C1-C2) stabilization techniques. SUMMARY OF BACKGROUND DATA: Different C0-C1-C2 stabilization techniques are used for unstable conditions in the upper cervical spine, all with different degrees of risk to the vertebral artery. Techniques with similar stability but less risk to the vertebral artery may be advantageous. METHODS: Six human cadaveric cervical spines (C0-C5) (age: 74 +/- 5.0 years) were used. After testing the intact spines, instability was created by transecting the transverse and alar ligaments. The spines were instrumented from the occiput to C2 using 3 different techniques which varied in their attachment to C2. All spines had 6 screws placed into the occiput along with lateral mass screws at C1. The 3 variations used in attachment to C2 were (1) C2 crossing laminar screws, (2) C2 pedicle screws, and (3) C1-C2 transarticular screws. The C1 lateral mass screws were removed before placement of the C1-C2 transarticular screws. Range of motion across C0-C2 was measured for each construct. The data were analyzed using repeated measures ANOVA. The following post hoc comparisons were made: (1) intact spine versus each of the 3 techniques, (2) laminar screw technique versus the pedicle screw technique, and (3) laminar screw technique versus the transarticular screw technique. The level of significance was alpha = 0.01 (after Bonferroni correction for 5 comparisons). RESULTS: All 3 stabilization techniques significantly decreased range of motion across C0-C2 compared to the intact spine (P < 0.01). There was no statistical difference among the 3 stabilization methods in flexion/extension and axial rotation. In lateral bending, the technique using C2 crossing laminar screws demonstrated a trend toward increased range of motion compared to the other 2 techniques. CT scans in both axial and sagittal views demonstrated greater proximity to the vertebral artery in the pedicle and transarticular screw techniques compared to the crossing laminar screw technique. CONCLUSION: Occipitoatlantoaxial stabilization techniques using C2 crossing laminar screws, C2 pedicles screws, and C1-C2 transarticular screws offer similar biomechanical stability. Using the C2 crossing laminar screw technique may offer an advantage over the other techniques due to the reduction of the risk to the vertebral artery during C2 screw placement.