An improvement of a polyaxial screw head disclosed in the U.S. Pat. No. 8,277,490. A saddle resides within a tulip. The saddle is configured to mate with a rod. An outer set screw engages an inner surface of the tulip. An inner set screw is disposed within a lumen of an outer set screw. A force distributor has a concave surface configured to mate with the rod. The force distributor has a bore configured to accept a conical end of the inner set screw. The force distributor is configured to distribute the force applied onto the bore by the inner set screw along the contact area with rod. The force distributor allows the rod to pivot and have sagittal motion when the inner set screw is untightened. The force distributor immobilizes the rod within the saddle when the inner set screw is tightened.
Johnson, Wesley M. and Freeman, Thomas B., "Adaptation sphere saddle" (2018). USF Patents. 1040.
University of South Florida