A post is fixed at each of the internal corners of the frame by means of a 5 x 19 mm screw passing through a slot in the center of the post and secured with a specially-shaped locking piece. A post is fixed at each of the internal corners of the frame by means of a 5 x 19 mm screw passing through a slot in the center of the post and secured with a specially-shaped locking piece. This arrangement allows you to adjust the height of the frame in relation to the patient's head.