Stress distribution of different lumbar posterior pedicle screw insertion techniques: a combination study of finite element analysis and biomechanical test.

At present, the pedicle screw is the most commonly used internal fixation device. However, there are many kinds of common posterior pedicle screw insertion techniques performed to reconstruct the lumbar stability. Therefore, spinal surgeons often face a difficult choice. The stress distribution of internal fixation system is an important index for evaluating safety. Unfortunately, little had been known about the difference of stress distribution of screw-rod systems that established by Roy-Camille, Magerl and Krag insertion techniques. Here, combination of finite element analysis and model measurement research was adopted to evaluate the difference of stress. Following different pedicle screw insertion techniques, three lumbar posterior surgery models were established after modeling and validation of the L1-S1 vertebrae finite element model. By analyzing the data, we found that stress concentration phenomenon was in all the postoperative models. Roy-Camille and Magerl insertion techniques led to the great stress on screw-rod systems. Then, fresh frozen calf spines were selected as a model for subsequent measurements. Fitted with a specially designed test pedicle screw, L5-L6 vertebrae were selected to repeat and verify the results of the finite element analysis. With the aid of universal testing machine and digital torque wrench, models simulated flexion, extension, lateral bending and rotation. Finally, the strain value was captured by the strain gauge and was then calculated as the stress value. Krag and Magerl were found to be the safer choice for pedicle screw insertion. Overall, our combination method obtained the reliable result that Krag insertion technique was the safer approach for pedicle screw implantation due to its relatively dispersive stress. Therefore, without the consideration of screw size, pedicle fill, bone density, and bone structures, we recommend the Krag insertion technique as the first choice to reconstruction of lumbar stability. Additionally, the combination method of finite element analysis and strain gauge measurement can provide a feasible way to study the stress distribution of spinal internal fixation.

Malignant pheochromocytoma in the anterior mediastinum with sternal invasion: a case report.

Pheochromocytomas are catecholamine-producing neuroendocrine tumors that usually occur in the adrenal medulla or sympathetic paraganglia. Anterior mediastinum involvement with pheochromocytoma is rare and may not present with typical symptoms. Its clinical manifestation may be unclear and a high index of suspicion is required for accurate diagnosis. We report a rare case of pheochromocytoma of the anterior mediastinum in a 51-year-old female. A painful hard mass on the sternum was the only clinical manifestation. Imageological examination indicated that there might be a malignant mass on the anterior mediastinum and thoracic wall. The patient accepted surgical curettage and thoracic wall reconstruction. Based on pathological results and WHO definition, the final diagnosis was malignant pheochromocytoma. After six months follow-up, the patient had no recurrence or any symptom. Malignant pheochromocytoma in the anterior mediastinum invading the sternum is rare. A local painful mass may be the only clinical manifestation without special laboratory results. Surgery remains as the first choice for these patients. For this rare case, 3D reconstruction by special software may be a good method to realize individualized treatment. The final decision of the diagnosis should be based on pathological results, past medical history and WHO definition. Long-term follow-up is necessary, while other suspicious lesions should also be given sufficient attention.