The bone screw is used as an implant in Orthopaedics to hold two or more objects together and to have the ‘LAG’ effect i.e. compressing two surfaces. In the AO system, there are the two most common types of bone screws on the basis of the type of fracture and the consistency of tissues of bones: the cortex and the cancellous bone screw.
- Cortex screw: These type of orthopedic bone screws is specifically used for diaphyseal bone. These screws have smaller threads and a lower pitch. A cortex screw can be self-tapping and non-self-tapping.
- Cancellous screws: A cancellous screw is used as a placement screw to fix a bone plate in metaphyseal and epiphyseal region. These screws have a deeper thread, a larger pitch and a larger outer diameter as compared to cortex screw.
- Schanz screws: These screws are used in both open and closed fracture fixation.The latest design of Schanz screw has a less deep thread in order to provide less sharp edges. These screws have a larger core diameter. It provides better strength against the forces that act perpendicular to the long axis of the screw and if used accurately, Its self-cutting thread provides a good purchase.
Locked screws
These screws are designed recently for diaphyseal application of internal fixators (PC-Fix and LISS) may be self-drilling, self-cutting, and unicortical and the screw head locks in the plate hole. They are “locked screws” because their inclination in relation to the “plate” body is fixed. These screws as a fixed angle device, which is helpful in metaphyseal fractures if a minimally invasive technique is used.Herbert bone screws
Herbert bone screw is a cannulated screw with changeable pitch as there is thread at each end of the screw. This is a headless compression screw. This screw is used for interfragmentary compression with the help of its variable pitch. This screw has been mainly used in scaphoid fractures, Carpal bones and other minor joints fusion in wrist and hand.Shaft screws
The working of a lag screw is better understood by examining a screw which has a short thread and a shaft, the diameter of the core or of the thread. The smooth shaft mainly has a diameter corresponding to the inner core of the thread. When it is applied to a tubular diaphyseal bone, the thread anchors within the far cortex and in the near cortex. The shaft does not produce any axial force but the head provides a buttress against axial tension.Mode of application of a fully threaded lag screw
If a thread is kept from engaging within the cortex near to the screw head, a fully threaded screw can be applied as lag screw. This can be done by drilling a clearance or gliding hole of a diameter slightly larger than the outer diameter of the screw thread within the near cortex. When an inclined screw thread produces axial force, it tends to shift the screw head along the bone surface in the direction the fracture. In this condition, the use of a lag screw with a shaft corresponding to the outer diameter of the thread i.e. shaft screws can be recommended. Or else, the screw thread can engage within the gliding hole and there may be loss of efficiency. Mainly, Shaft screws or fully threaded screws are used in diaphyseal bone as partly threaded cancellous bone screws are not easy to remove after healing.Screw-tightening and torque-limiting screwdrivers
Considering the fact that the screws produce a high amount of axial force, so the screws must not be tightened to their highest limit. In this way a surgeon should tighten the screws to the degree optimal and compatible with screw and/or bone strength then one can achieve 86% of the thread-stripping torque on average.Moreover, when the holding force of a screw is completely used by static preload, there is limited holding force left contrary to additional functional load. The new self-locking screws of the PC-Fix and the LISS lock upon tightening in the plate hole, and thus keep the screw and the bone safe. For these new techniques a torque-limiting screwdriver proves helpful to prevent the screw heads from getting jammed in the conical holes, whether it is threaded or not. But in the case where the bone density and thickness varies from one region to another, torque limiting screwdrivers proves less helpful.
