Deflection Of A Head Fixated In A Stereotactic Frame With Insulated Posts.

Thomas Arn1, Per Carlsson1

1Elekta Instrument AB, Stockholm,Sweden

Keywords: technique, gamma knife, magnetic resonance imaging, stereotactic frame, Imaging

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     The accuracy of a stereotactic treatment is highly dependent on the fixation being rigid to prevent movements of the target during treatment. Plastic thread inserts have been introduced lately to prevent side effects in MRI, such as skin burns at the fixation screw tips. Since the plastic thread inserts are attached into the post, they introduce an interface which is not applicable for posts made of solid aluminum.
     The aim of this investigation was to determine the movement of a head, fixated in the G-frame, as a result of applied load. This investigation shows the evaluation of how this affects the rigidity of the fixation.
     We have fixated two sizes of plastic head dummies (59 cm circumference, an adult male size and 50 cm circumference, the average size of a three year old boy respectively) with different fixation screw torques and post configurations. The head dummies were exposed to an anterior-posterior load of 50 N, 5 mm below the frame whilst inducing a torque to the head versus the frame. The movements as a result of the load have been monitored at four different measurement points on the head with dial indicators.
     The largest deflection is measured for the smallest head size (50 cm) where the screw length between post and head is largest. The deflection measured at a simulated target 82 mm below the screw plane is 0.44 mm with insulated and 0.31 mm for non-insulated posts. For the larger skull (59 cm) the influence of the screw lengths is much smaller resulting in the absolute deflection differences between insulated and non insulated posts being 0.02 to 0.06 mm
     We did not test all types of head shapes and volumes.
     Our recommendation is to use curved anterior posts for patients with small head sizes.
     This will minimize the screw length between the head and the posts and thereby reduce the possibility of head movement during treatment.


Project Roles:

T. Arn (), P. Carlsson ()