Types of bone screws

The bone screw is used in orthopedics as an implant to hold two or more objects together and achieve the LAG effect, that is, to compress two surfaces. In the AO system, there are the two most common types of bone screws, depending on the type of fracture and the consistency of the bone tissue: the cortical screw and the cancellous bone screw.

• Cortex screws: This type of orthopedic bone screw is specifically used for diaphyseal bones. These screws have smaller threads and a smaller pitch. A cortex screw can be self-tapping and not self-tapping.
• Cancellous screws: A cancellous screw is used as a positioning screw to secure a bone plate in the metaphyseal and epiphyseal region. These screws have a deeper thread, a larger pitch, and a larger outside diameter than the cortex screw.
• Schanz screw: These screws are used in open and closed fracture fixation. Schanz’s latest screw design has a shallower thread for less sharp edges. These screws have a larger diameter core. It offers better resistance to forces acting perpendicular to the major axis of the screw, and when used with precision, the self-tapping thread is a good buy.

Locked screws

These screws were recently developed for the diaphyseal application of internal fixators (PC-Fix and LISS). They can be self-drilling, self-tapping, and monocortical, and the screw head locks into the hole in the plate. They are “locked screws” because their inclination relative to the “body of the plate” is fixed. These screws act as a fixed angle device, which is useful for metaphyseal fractures when using a minimally invasive technique.

Herbert bone screw

The Herbert Bone Screw is a variable pitch cannulated screw because there is a thread on each end of the screw. This is a headless compression screw. This screw is used for interfragmentary compression using its variable pitch. This screw was primarily used for fractures of the scaphoid, carpal bones, and other small joint fusions of the wrist and hand.

Axle bolts

The operation of a lag screw is best understood when looking at a short thread screw and shank, core diameter or thread. The smooth shaft mainly has a diameter that corresponds to the inner core of the thread. When applied to a tubular diaphyseal bone, the suture is anchored in the near and opposite cortex. The shaft does not generate any axial force, but the head provides support against the axial tension.

Type of application of a fully threaded lag screw

If a thread close to the screw head is prevented from engaging the cortex, a fully threaded screw can be used as a lag screw. This can be done by drilling a clearance or slip hole with a diameter slightly larger than the outside diameter of the screw thread in the adjacent cortex. When an angled screw thread creates axial force, it tends to move the screw head along the bone surface in the direction of the fracture. Under these conditions, it may be recommended to use a lag screw with a shaft that matches the outside diameter of the thread, i.e. shaft bolts. Otherwise, the screw thread may get stuck in the sliding hole and there may be a loss of efficiency. In diaphyseal bones, rod screws or fully threaded screws are used primarily because partially threaded cancellous bone screws are not easily removed after healing.

Screwdriver with and without torque limit

Since the screws generate a large axial force, the screws should not be tightened to their maximum limit. Thus, a surgeon must tighten the screws enough to be optimal and compatible with the screw and / or bone strength. It can then on average 86% of the tearing torque of the wire.

If the clamping force of a screw is fully utilized by the static preload, a limited clamping force remains unlike an additional functional load. The new PC-Fix and LISS self-locking screws snap into place when tightened into the hole in the plate, securely holding the screw and bone. With these new techniques, a torque limiting screwdriver is useful in preventing screw heads from getting stuck in tapered holes whether they are threaded or not. In cases where bone density and thickness vary from region to region, torque limiting screwdrivers are less useful.