Targeting inaccuracy caused by mechanical distortion of the Leksell stereotactic frame during fixation

BACKGROUND: The stereotactic frame represents the mainstay of accuracy for targeting in stereotactic procedures. Any distortion of the frame may induce a significant source of error for the stereotactic coordinates. OBJECTIVE: To analyze the sources of distortion of the Leksell frame G induced by fixation to the patient's head and to evaluate the clinical impact of frame distortion on the accuracy of targeting in stereotactic procedures. METHODS: We analyzed the torques exerted on the fixation screws after frame placement in a series of patients treated stereotactically by an experienced team. We studied the risk for frame bending in an experimental model of stereotactic frame fixation, with increasing torque of fixation screws in a homogeneous and heterogeneous distribution of torques between the four screws. We assessed the impact of expanding dimensions of bending of the Leksell frame both on surgeries utilizing the stereotactic frame, and on radiosurgical procedures with the Gamma Knife. RESULTS: Frames were fixed clinically at a range of torques of 0.147–0.522 Nm (mean = 0.348 Nm). The torques did not vary significantly with time. Heterogeneity between the two opposite pairs of screws is often limited, but can reach 96.3%. Distortion of the frame may occur even at minimal levels of torque. Heterogeneity between the two opposite pairs of screws will significantly raise the amount of frame distortion. We found a direct correlation between measures of the frame distortion and extend of the deviation from the stereotactic target in clinical models of stereotactic procedures. CONCLUSION: Stereotactic frames were subjected to distortion due to the torque used for frame fixation. The risk of distortion increased with the torque used and the heterogeneity between the torques of the fixation screws. Distortion of the frame was a significant source of inaccuracy of targeting for stereotactic procedures in clinical practice.