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Lag Screw Theory

- See: Screw Menu

- Discussion:
    - involves placement of one or more screws across a frx or osteotomy site to achieve interfragmentary compression;
    - note that the direction along which the compression acts (inclination of lag screw) must coincide well w/ perpendicular
           axis of the fracture surface;
           - as Johner, et al (1983) have shown, frx sliding will occur if the compression is applied at an angle greater than 20 deg in
                    relation to axis perpendicular to the fracture surface;
    - it has been written that if more than one lag screw can be positioned across a frx site, then plate fixation of the frx is not necessary;
          - fortunately, only one or two disaters w/ this statedgy are required for the orthopaedist to realize that this is not true;
    - self tapping screws:
         - if self tapping screws are inadvertently angled, they will cut a new path and destroy already cut thread, which is a disadvantage;
         - self tapping screws should therefore not be used as lag screws;

- Technique:
    - lag screw is best positioned at right angles to the fractures plane;
    - reduction:
         - in order to find best location and inclination, forceps compressing fracture temporarily substitutes for function of the lag screw;
         - lag screw replaces the forceps;
    - gliding hole:
         - the gliding hole (near cortex) is overdrilled w/ the appropriately sized bit;
         - the appropriate sized dril bit corresponds to the outer diameter of the screw;
         - take care not to drill past the far cortex;
    - thread hole:
         - insert the "golf tee" or a drill sleeve into the overdrilled cortex;
         - for gliding hole to function properly, the screw must be positioned in axis of the drill hole;
         - appropriate sized drill bit is inserted to drill the far cortex;
                  - 1.5 mm sized screw requires 1.1 mm drill bit
                  - 2.0 mm sized screw requires 1.5 mm drill bit
                  - 2.7 mm sized screw requires 2.0 mm drill bit
                  - 3.5 mm sized screw requires 2.5 mm drill bit
                  - 4.5 mm sized screw requires 3.2 mm drill bit
    - measure screw length:
          - screw length must be long enough to allow screw tip to protrude 2 mm beyond far cortex;
    - bone tap
          - insert appropriately sized bone tap to cut across far cortex;
    - counter-sink:
          - apply the countersink to make the screw head to improve seating on the anterior fibula and to make the head less prominent;
          - the countersink will cause the screw to sink deeper, causing it to be more proud

- References:
       - Comparison of three methods for maintaining inter-fragmentary compression after fracture reduction and fixation.
       - Rigidity of pure lag-screw fixation as a function of screw inclination in an in vitro spiral osteotomy.