INJURED WORKER WAS INSTALLING SECTIONS OF BACKING PLATES FOR THE FINISH MILL CENTER DIAPHRAGM. THE PIECE OF PLATE HE WAS HANDLING WAS APPROX. 14"" X 18"" X 1 1/2"" THICK. AS HE PLACED THE PLATE NEXT TO OTHER PLATES ALREADY IN PLACE AND SECURE AT KNEE HEIGHT, THE PLATE SHIFTED & SMASHED HIS RIGHT THUMB AGAINST AN ADJACENT PLATE. FRACTURED PROXIMAL BONE - SPLINT - WORK RESTRICTIONS.