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Cancellous Bone Screws: 4.5 and 6.5



- 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