Orthopedic Screw Selection Calculator

Selecting an orthopedic screw is a sizing problem, not just a stocking question: the screw has to match the bone it's going into and the job it's doing. This calculator follows the diameter and drill-bit sizing conventions the AO/ASIF group (Arbeitsgemeinschaft für Osteosynthesefragen / Association for the Study of Internal Fixation) codified in its 1969 Manual of Internal Fixation -- still the reference standard behind cortical, cancellous, locking, and cannulated screw families across trauma implant systems -- starting from the screw application -- cortical (lag or positional), cancellous, locking, or cannulated -- because that choice, not bone diameter, is what determines the standard diameter family a recommendation is drawn from: cancellous screws jump straight to 4.0/6.5 mm regardless of the exact bone width, while cortical screws step through 2.7/3.5/4.5 mm as bone diameter grows. Bone diameter is, however, the dominant driver of screw LENGTH: wider bone diameter shifts the length recommendation up through the standard 2 mm increments, and for a bicortical screw, length is essentially plate thickness plus bone diameter plus a couple of millimeters of far-cortex purchase, rounded to the nearest standard 2 mm increment. A unicortical screw's length instead comes from cortex thickness rather than the full bone diameter -- plate thickness plus the near cortex thickness itself plus about 2 mm of engagement past it -- since it's only meant to purchase the near cortex, not travel through the medullary canal to the far side; bone diameter has no effect on screw length at all in unicortical mode, unlike in bicortical mode where it's the dominant driver, so switching the Cortices toggle changes which input actually controls this output.

Drill bit (thread-hole) diameter is read from a standard AO/ASIF sizing table keyed to the recommended screw's outer diameter, rather than computed as a fixed percentage of it, since the real published relationship between outer and thread-hole diameter isn't a constant ratio across the different screw families. Cortical bone generally needs pre-tapping before screw insertion; cancellous and cannulated screws are typically self-tapping, which this calculator reflects in the pre-tapping flag. This is a planning aid based on general sizing principles, not a substitute for direct intraoperative measurement -- actual screw selection also depends on fracture pattern, bone quality, and implant system-specific instrumentation that this calculator does not model.