- 6.5 mm cancellous bone screw:
- has an unthreaded shaft of diameter 4.5 mm, a thread diameter of 6.5 mm, and a core diameter is 3 mm,
and either a 16 mm or 32 mm thread length;
-
pre-drilling:
- correct drill bits are 3.2 mm for cancellous bone and 4.5 mm for cortical bone;
- however, once inserted into cortical bone, the cancellous screw cannot cut its way back
out and will have to be left in;
- 4.5 mm cancellous screw:
- shaft of
malleolar screws has the same diameter as the core, so that predrilling of 3.2 mm is sufficient;
- Tapping:
- cancellous bone screws are designed as non self tapping screws;
- their thread must be tapped only in the near cortex;
- these screws should not be tapped all way, only the first few turns, because they gain much
better when they cut their own way into cancellous bone;
- the screws holding power in increased if its thread is not cut because it tends to compress trabeculae
together when it is driven inwards;
- Optimizing Compression:
- engaging far cortex increases the holding power of the screw, but is only necessary in porotic bone;
- tip is pointed, however, and care must be taken not to damage any structures on the opposite side;
- to achieve interfragmentary compression it is esstential that thread be inserted beyond the plane
of the fracture so that only smooth shank crosses it;
