Morphological alterations of lumbar intervertebral discs in patients with adolescent idiopathic scoliosis.

BACKGROUND CONTEXT: Etiology of adolescent idiopathic scoliosis (AIS) is still unknown. Prior in vitro research suggests intervertebral disc pathomorphology as a cause for the initiation and progression of the spinal deformity, however, this has not been well characterized in vivo. PURPOSE: To quantify and compare lumbar disc health and morphology in AIS to controls. STUDY DESIGN/SETTING: Cross-sectional study. METHODS: All lumbar discs were imaged using a 3T MRI scanner. T2-weighted and quantitative T2* maps were acquired. Axial slices of each disc were reconstructed, and customized scripts were used to extract outcome measurements: Nucleus pulposus (NP) signal intensity and location, disc signal volume, transition zone slope, and asymmetry index. Pearson's correlation analysis was performed between the NP location and disc wedge angle for AIS patients. ANOVAs were utilized to elucidate differences in disc health and morphology metrics between AIS patients and healthy controls. α=0.05. RESULTS: There were no significant differences in disc health metrics between controls and scoliotic discs. There was a significant shift in the NP location towards the convex side of the disc in AIS patients compared to healthy controls, with an associated increase of the transition zone slope on the convex side. Additionally, with increasing disc wedge angle, the NP center migrated towards the convex side of the disc. CONCLUSIONS: The present study elucidates morphological distinctions of intervertebral discs between healthy adolescents and those diagnosed with AIS. Discs in patients diagnosed with AIS are asymmetric, with the NP shifted towards the convex side, which was exacerbated by an increased disc wedge angle. CLINICAL SIGNIFICANCE: Investigation of the MRI signal distribution (T2w and T2* maps) within the disc suggests an asymmetric pressure gradient shifting the NP laterally towards the convexity. Quantifying the progression of these morphological alterations during maturation and in response to treatment will provide further insight into the mechanisms of curve progression and correction, respectively.

Quantifying the effect of posterior spinal instrumentation on the MRI signal of adjacent intervertebral discs.

STUDY DESIGN: Ex vivo porcine imaging study. OBJECTIVES: Quantitatively evaluate change in MRI signal at the discs caudal to spinal fusion instrumentation. Individuals who receive posterior spinal instrumentation are at risk of developing accelerated disc degeneration at adjacent levels. Degeneration is associated with a loss of biochemical composition and mechanical integrity of the disc, which can be noninvasively assessed through quantitative T2* (qT2*) MRI techniques. However, qT2* is sensitive to magnetic susceptibility introduced by metal. METHODS: Nine ex vivo porcine lumbar specimens were imaged with 3 T MRI. Fast spin-echo T2-weighted (T2w) images and gradient-echo qT2* maps were acquired, both without and with posterior spinal fusion instrumentation. Average T2* relaxation times of the nuclei pulposi (NP) were measured at the adjacent and sub-adjacent discs and measurements were compared using t tests before and after instrumentation. The size of the signal void and metal artifact were determined (modified ASTM F2119-07) within the vertebral body and spinal cord for both MRI sequences. The relationship between T2* signal loss and distance from the instrumentation was evaluated using Pearson's correlation. RESULTS: There was no significant difference between adjacent and sub-adjacent NP T2* relaxation time prior to instrumentation (p = 0.86). Following instrumentation, there was a significant decrease in the T2* relaxation time at the adjacent NP (average = 20%, p = 0.02), and no significant difference at the sub-adjacent NP (average = - 3%, p = 0.30). Furthermore, there was a significant negative correlation between signal loss and distance to disc (r = - 0.61, p < 0.01). CONCLUSIONS: Spinal fusion instrumentation interferes with T2* relaxation time measurements at the adjacent disc but not at the sub-adjacent discs. However, there is sufficient signal at the adjacent disc to quantify changes in the T2* relaxation time following spinal fusion. Hence, baseline MRI scan following spinal fusion surgery are required to interpret and track changes in disc health at the caudal discs. LEVEL OF EVIDENCE: N/A.

Validation of an automated shape-matching algorithm for biplane radiographic spine osteokinematics and radiostereometric analysis error quantification.

Biplane radiography and associated shape-matching provides non-invasive, dynamic, 3D osteo- and arthrokinematic analysis. Due to the complexity of data acquisition, each system should be validated for the anatomy of interest. The purpose of this study was to assess our system's acquisition methods and validate a custom, automated 2D/3D shape-matching algorithm relative to radiostereometric analysis (RSA) for the cervical and lumbar spine. Additionally, two sources of RSA error were examined via a Monte Carlo simulation: 1) static bead centroid identification and 2) dynamic bead tracking error. Tantalum beads were implanted into a cadaver for RSA and cervical and lumbar spine flexion and lateral bending were passively simulated. A bead centroid identification reliability analysis was performed and a vertebral validation block was used to determine bead tracking accuracy. Our system's overall root mean square error (RMSE) for the cervical spine ranged between 0.21-0.49mm and 0.42-1.80° and the lumbar spine ranged between 0.35-1.17mm and 0.49-1.06°. The RMSE associated with RSA ranged between 0.14-0.69mm and 0.96-2.33° for bead centroid identification and 0.25-1.19mm and 1.69-4.06° for dynamic bead tracking. The results of this study demonstrate our system's ability to accurately quantify segmental spine motion. Additionally, RSA errors should be considered when interpreting biplane validation results.

Mechanical Performance of Posterior Spinal Instrumentation and Growing Rod Implants: Experimental and Computational Study.

STUDY DESIGN: Experimental and computational study of posterior spinal instrumentation and growing rod constructs per ASTM F1717-15 vertebrectomy methodology for static compressive bending. OBJECTIVE: Assess mechanical performance of standard fusion instrumentation and growing rod constructs. SUMMARY OF BACKGROUND DATA: Growing rod instrumentation utilizes fewer anchors and spans longer distances, increasing shared implant loads relative to fusion. There is a need to evaluate growing rod's mechanical performance. ASTM F1717-15 standard assesses performance of spinal instrumentation; however, effects of growing rods with side-by-side connectors have not been evaluated. METHODS: Standard and growing rod constructs were tested per ASTM F1717-15 methodology; setup was modified for growing rod constructs to allow for connector offset. Three experimental groups (standard with active length 76 mm, and growing rods with active lengths 76 and 376 mm; n = 5/group) were tested; stiffness, yield load, and load at maximum displacement were calculated. Computational models were developed and used to locate stress concentrations. RESULTS: For both constructs at 76 mm active length, growing rod stiffness (49 ±â€Š0.8 N/mm) was significantly greater than standard (43 ±â€Š0.4 N/mm); both were greater than growing rods at 376 mm (10 ±â€Š0.3 N/mm). No significant difference in yield load was observed between growing rods (522 ±â€Š12 N) and standard (457 ±â€Š19 N) constructs of 76 mm. Growing rod constructs significantly decreased from 76 mm (522 ±â€Š12 N) to 376 mm active length (200 ±â€Š2 N). Maximum load of growing rods at 76 mm (1084 ±â€Š11 N) was significantly greater than standard at 76 mm (1007 ±â€Š7 N) and growing rods at 376 mm active length (392 ±â€Š5 N). Simulations with active length of 76 mm were within 10% of experimental mechanical characteristics; stress concentrations were at the apex and cranial to connector-rod interaction for standard and growing rod models, respectively. CONCLUSION: Growing rod constructs are stronger and stiffer than spinal instrumentation constructs; with an increased length accompanied a decrease in strength. Growing rod construct stress concentration locations observed during computational simulation are consistent with clinically observed failure locations. LEVEL OF EVIDENCE: 5.

Noninvasive Assessment of Biochemical and Mechanical Properties of Lumbar Discs Through Quantitative Magnetic Resonance Imaging in Asymptomatic Volunteers.

Intervertebral disc degeneration is a prevalent phenomenon associated with back pain. It is of critical clinical interest to discriminate disc health and identify early stages of degeneration. Traditional clinical T2-weighted magnetic resonance imaging (MRI), assessed using the Pfirrmann classification system, is subjective and fails to adequately capture initial degenerative changes. Emerging quantitative MRI techniques offer a solution. Specifically, T2* mapping images water mobility in the macromolecular network, and our preliminary ex vivo work shows high predictability of the disc's glycosaminoglycan content (s-GAG) and residual mechanics. The present study expands upon this work to predict the biochemical and biomechanical properties in vivo and assess their relationship with both age and Pfirrmann grade. Eleven asymptomatic subjects (range: 18-62 yrs) were enrolled and imaged using a 3T MRI scanner. T2-weighted images (Pfirrmann grade) and quantitative T2* maps (predict s-GAG and residual stress) were acquired. Surface maps based on the distribution of these properties were generated and integrated to quantify the surface volume. Correlational analyses were conducted to establish the relationship between each metric of disc health derived from the quantitative T2* maps with both age and Pfirrmann grade, where an inverse trend was observed. Furthermore, the nucleus pulposus (NP) signal in conjunction with volumetric surface maps provided the ability to discern differences during initial stages of disc degeneration. This study highlights the ability of T2* mapping to noninvasively assess the s-GAG content, residual stress, and distributions throughout the entire disc, which may provide a powerful diagnostic tool for disc health assessment.