Screw implants for bone fusion

A fixation screw assembly comprising:

• a bone screw including a head and a shank, the head defining an equator plane, the shank including a first window and a second window, wherein the shank includes a dual inner diameter including a first portion with shallow threads and a second portion with deep threads;
• and
• a washer disposed around the head of the bone screw, wherein the washer comprises a plurality of vertical slits that provide flexibility to the washer and is configured for polyaxial movement relative to the bone screw, and at least a portion of the washer is positioned below the equator plane at any polyaxial angle of the washer,
• wherein the bone screw is monolithically formed, the plurality of vertical slits accommodate expansion of the washer around the bone screw head, and the head of the bone screw includes a groove at a proximal-most end of the head and the washer includes a lip at a proximal-most end of the washer such that, when the washer is sufficiently angled relative to the longitudinal axis of the bone screw, the lip of the washer bottoms out on the groove of the head.