The effect of myelography dye on bone density measurements utilizing Hounsfield units on CT.

OBJECTIVE: Hounsfield unit (HU) measurements of bone density on CT are increasingly used for preoperative planning in spine surgery. Postmyelogram CT is another common preoperative diagnostic study. However, there is no current literature evaluating whether HU measurements on CT are affected by the presence of myelography dye. The purpose of the current study was to determine if the presence of myelography dye affects HU measurements of bone density in CT studies. METHODS: Twenty-nine preoperative spine surgery patients who underwent both standard and postmyelography CT performed within 6 months of each other were identified. HU measurements were obtained from an elliptical region of interest using the available software on a standard PACS. Measurements were obtained on the axial cut at the midvertebral body on all lumbar vertebrae on three separate occasions and an average value was calculated for comparative analysis. A 6-week gap was used between measurements of the CT scans and the CT myelograms to diminish bias. RESULTS: The mean age of the cohort was 69 years and the average BMI was 32 kg/m2. Five patients were male and 24 were female. Six of the patients had instrumentation placed prior to the initial CT scan. The average HU measurements for CT levels L1-5 were 165, 171, 145, 154, and 225, respectively, whereas HU measurements for CT myelography of levels L1-5 were 168, 177, 148, 170, and 239, respectively. Strong correlations were noted between the HU measured on CT and CT myelography for L1 (r2 = 0.951), L2 (r2 = 0.966), L4 (r2 = 0.820), and L5 (r2 = 0.900), and moderate for L3 (r2 = 0.668). CONCLUSIONS: The presence of myelography dye had no clear effect on CT HU measurements of bone density. The results of this study support the use of CT myelograms for bone density assessment in the absence of standard CT images.

Dual pitch screw design provides equivalent fixation to upsized screw diameter in revision pedicle screw instrumentation: a cadaveric biomechanical study.

BACKGROUND CONTEXT: There are situations that require the replacement of pedicle screws. They are often exchanged when loose or broken or to accommodate a different sized rod or pedicle screw system. Traditionally, pedicle screws are replaced by up-sizing the core diameter until an interference fit is obtained. However, this method carries a risk of pedicle screw breach. PURPOSE: To determine if dual pitch screws, with cancellous pitch in the vertebral body and cortical pitch throughout the pedicle, allows for in-line screw revision without upsizing screw diameter. STUDY DESIGN: Cadaveric biomechanical Study PATIENT SAMPLE: Not applicable OUTCOME MEASURES: Not applicable METHODS: Pedicle screws were tested in the lumbar vertebrae from eleven cadavers. Standard pitch 5.5 mm screws were inserted and loaded using a "break-in" protocol. Screws were removed and replaced with one of four screw types: 5.5 mm Standard Pitch, 5.5 mm Dual Pitch, 6.0 mm Standard Pitch, or 6.0 mm Dual Pitch. Failure testing was done using a stepwise increasing cyclic loading protocol for 100 cycles at each increasing load level. The loading consisted of a combined axial and bending load simulating the load seen by the most inferior screw. RESULTS: Failure was consistent, with the tip of the screw displacing inferiorly into the vertebral body while simultaneously pulling out. Failure strength was lowest in the 5.5mm Standard (135.8±29.4N) followed by 6.0mm Standard (141.8±38.6N), 5.5mm Dual (158.1±53.8N), and 6.0mm Dual (173.6±52.1N, p=.023). There was no difference in the failure strength between the 5.5mm Dual and 6.0mm Standard. Lumbar level (p=.701) and donor spine (p=.062) were not associated with failure strength. CONCLUSIONS: After pedicle screw removal, screws with a larger core diameter or with a dual pitch have similar failure strengths. Dual pitch screws may allow for in-line revision of screws without upsizing screw diameter, minimizing the risk of pedicle breach or fracture. CLINICAL SIGNIFICANCE: Dual pitch screws, with cancellous pitch in the vertebral body and cortical pitch through the pedicle, allows for in-line revision of pedicle screws without upsizing screw diameter; reducing the risk of pedicle breach or fracture when exchanging screws.

Minimally invasive lateral approaches for the treatment of spinal tumors: single-position surgery without the "flip".

Although primary tumors of the spine and neural elements are rare, metastatic disease to the spine is quite common. Traditionally, surgical treatment for spinal tumor patients involves open decompression with or without stabilization. The single-position minimally invasive (MIS) lateral approach, which has been recently described over the recent decade, allows simultaneous access to the anterior and posterior columns with the patient positioned in the lateral decubitus position. Herein, we review the application of single-position MIS lateral surgery for the treatment of spinal neoplasm. The aim was to review the evolution, operative technique, outcomes, and complications associated with MIS lateral approaches for spinal tumors. The history of spinal tumor diagnosis and management are reviewed and discussed as well as the author's experience and literature regarding spinal tumor treatment outcome and surgical complications, with particular attention to single-position, MIS lateral approaches. In addition, the author's surgical technique is outlined in detail for thoracic, thoracolumbar and lumbar tumors. Furthermore, there are specific indications and complications associated with the surgical treatment of spinal tumors, and the MIS, single-position lateral approach, when applied appropriately, allows for concurrent access to the anterior and posterior column while mitigating the complications associated with traditional, open posterior-based approaches. In the treatment of spinal neoplasms, the goals of surgery are dictated by a number of tumor-specific and patient-specific factors. Therefore, operative treatment of tumors in the future may be a consolidation of historical surgical techniques and MIS, single-position lateral approaches. Regardless, multidisciplinary management is imperative for the individualized treatment of the patient and optimization of outcome.