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 

- 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

Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Thursday, May 24, 2012 1:46 pm