A countersink hole is drilled at the outer end of this hole to accommodate the screw head. The distal portion of the hole is drilled with a diameter corresponding to the core diameter of the screw. As the screw is tightened its distal end engages the threaded hole of the distal fragment. The undersurface of the screw head contacts the countersink hole and pulls the distal fragment against the proximal fragment causing compression. A lag effect can only occur if the screw can pass freely through the gliding hole.
Lag screws are most appropriate for oblique fractures.
2-3 lag screws are needed to ensure stability. The ASIF recommends drilling holes so that the axis of the holes bisects the angle formed by the fracture and the cortical surface to prevent shearing. The last form of fixation to be discussed is the reconstruction plate. These plates are a form of internal surgical splint. They serve to buttress fragments against displacent and to absorb the functional load while healing occurs.