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Feasibility Of Percutaneous Unilateral Transpedicular Balloon Kyphoplasty For The Mid & Upper Thoracic Spine – A Laboratory Study 2013

Category Interventional Georgios Vastardis, MD
Anna Marjan, BS
Brian Dial, BS
Michael Stojanovic, BS
Tejaswy Potluri, MS
Gerard Carandang, MS
Alexander Hadjipavlou, MD, PhD, FACS
Leonard Voronov, MD, PhD
Michael Zindrick, MD
Avinash Patwardhan, PhD
Purpose It is difficult to perform kyphoplasty through the pedicle in the mid and upper thoracic spine due to the size and angulation of the pedicles in the sagittal and axial planes.  Surgeons are reluctant to attempt a transpedicular kyphoplasty at these upper thoracic levels, especially with large-gauge trocars (e.g., an 8-gauge has an outer diameter of 4.2mm).  The favored modality for transpedicular kyphoplasty is to use a bilateral approach to ensure augmentation of the entire vertebral body.  Augmenting only a single half of the vertebral body could lead to medial/lateral angular deformation of the vertebral body.  With the advent of balloons that fit down an 11-guage trocar (outer diameter of 3.0mm) and the curved tipped cement applicator, AVAflex (Carefusion, McGaw Park, IL), one can augment both the ipsilateral and contralateral regions of the mid to upper thoracic vertebral bodies using a unilateral approach.  Using this approach with a small gauge needle decreases both the surgical risk and surgical time of the kyphoplasty procedure. The purpose of the study was to (1) define the general morphology of the T1-T12 thoracic pedicles, (2) perform unilateral transpedicular kyphoplasty, and (3) assess the placement of cement in the various compartments of the vertebral bodies. Materials & Methods Four fresh frozen human osteoporotic thoracic specimens (74±1.8 years) with intact rib cages and skin were screened to exclude pre-existing fractures and then scanned using Computed Tomography (CT) at a 0.6mm slice thickness. The CT images were uploaded into the Mimics software package (Materialize Inc, Ann Arbor, MI) where the following parameters were measured: pedicle height, width, and angulation in sagittal and axial planes. Measurements were made for all specimens by 4 observers in an inter- and intra-observation scheme separated by 1 week's time. The kyphoplasty procedure was performed at T1-T12 levels on the same specimens using the 11-gauge trocar (3mm diameter) for pedicle insertion, followed by the use of the 11-gauge AVAflex curved applicator and 11-gauge AVAmax balloon for vertebral body cavitation, and 11-gauge AVAflex curved applicator for cement injection.  For all specimens the transpedicular approach was conducted through the right pedicle.  Surgery was performed under C-arm fluoroscopy to facilitate proper alignment of the trocar with the pedicle axis.  Qualitative observations of cortical wall encroachment and fracture of the pedicle were made by observing post-op CT scans. Postoperative CT scans and 3D reconstructions of T1-T12 vertebral bodies were obtained for each specimen using the Mimics Software package.  The 3D reconstructions were imported into MatLab for volumetric analysis.  Each vertebral body was divided into 12 zones to assess the distribution of the injected cement. The percent augmentation in each of the twelve zones was determined and a percentage was calculated comparing the volume of cement and the total volume of the vertebral body in each zone. Results The anatomically-limiting parameter for height (average of 4 specimens) was in the upper levels (T1-T4) at 5.38±1.08mm, highlighting a linear trend from T1 to T12.  The width was most limiting in the middle levels (T5-T8) at 2.46±0.32mm, which emphasizes a parabolic trend from T1-T12.  The absolute minimum for height was at T1 with left and right pedicle measurements of 3.80 and 3.87mm. Similarly, this minimum for width was at the left pedicle of T4 (2.10mm) and right pedicle of T5 (2.00mm). Kyphoplasty was successfully performed in T1 in one specimen, T2 in three specimens, and T3-T12 in all four specimens. The curved needle applicator allowed cement augmentation in all 12 zones of the vertebral body.  On average, 43.22% of the vertebral body volume was augmented with cement at T2-T12 levels. Conclusion The inner diameter of the pedicle at its narrowest point, the isthmus, shows the highest correlation between the observers, making it the defining criteria for any anatomically-limiting parameters. The 11-gauge trocar has an outer diameter of 3.0 mm.  The trocar’s 3.00mm diameter exceeds the isthmus in the upper and middle levels whose average widths were 2.68±0.50 and 2.46±0.32mm respectively.  Post operative CT scans indicated no fracturing of the pedicles in the upper and middle thoracic levels or any other vertebral levels.  This unexpected finding indicates the potential for elastic properties inherent to cortical bone and/or angular leeway allowing for trocar penetration without fracture.  We could augment both the ipsilateral and contralateral regions of the mid to upper thoracic vertebral bodies with a unilateral approach using the curved tipped cement applicator. It is critical to target cement placement at the desired location.  The 11-gauge curved needle cement applicator allowed for cement augmentation in all twelve zones of the vertebral body. The instrumentation was shown to be effective in percutaneous transpedicular balloon kyphoplasty of the thoracic spine using a unipedicular approach. References None