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Enhancement of Fixator Stability



- Methods:
    - increasing pin diameter:
         - most important factor in fixator stability;
         - adult tibia: usually requires fixation w/ 4.5 to 6.0 mm pins;
         - pins must be < 1/3 bone diameter to prevent pin hole fractures;
         - reference:
               - Biomechanical analysis of pin placement and pin size for external fixation of distal radius fractures.
    - widely separated pins within single fragment;
    - placement of pins near the fracture site;        
    - number of pins:
         - three pins usually provide axial stability even w/ segmental comminution;
         - little is gained w/ a 4th pin in single segment;
         - short fragments fixed w/ 2 pins in same plane will provide stability in plane of pins but will be relatively unstable in plane at a right angle to the pins;
    - number of support bars:
    - additional planes of fixation:
         - a short fragment may not allow 3 pins in single plane but may allow additional pins in a different plane;
         - unilateral external fixators must stabilize the frx from an eccentric off axis position, and are most able to control frx site bending and shear when there is frx site opposition;
         - multiplane fixation or circular wire fixators help limit frx site bending and shear and allow load sharing at the frx site;
    - proximity to the extremity: (decreasing bone to support bar distance)
    - fibular fixation:
         - see frx of the tibia w/ intact fibula;
         - reference: Plating of the fibula. Its potential value as an adjunct to external fixation of the tibia.



Fracture Site Motion with Ilizarov and "Hybrid" External Fixation.
    - biomechanical study which compared stability of Ilizarov fixator to a Hybrid fixator in a frx model;
    - findings:
          - in a completely unstable frx model, a four ring Ilizarov fixator has substantially more stability than a unilateral Hybrid fixator;
    - weakness of study:
          - most fractures have at least some bone apposition which confers load sharing;
          - the frx model used in their study used longer half pins than would be used for most tibial frx;
                 - as would be expected, increasing the distance between the half pins and bone will decreased frx stability;
          - in most cases, Ilizarov transfixation wires cannot be placed at right angles to each other, and therefore, the construct will not be as stable as the one in the biomechanical study;



- Case Example:

    - 30-year-old prisoner who sustained a patellar fracture and a distal tibia fracture;
    - he was treated w/ an external fixator but several treatment principles were ignored;
    - the fracture was left distracted, the pins were not spread out and were placed well away from the fracture site, and the fixator bar was not near the skin;
    - as might be expected, this went on to a non union

           



Plating of the fibula. Its potential value as an adjunct to external fixation of the tibia.

The Role of Supplemental Lag-Screw Fixation for Open Fractures of the Tibial Shaft Treated With External Fixation.

Medial external fixation with lateral plate internal fixation in metaphyseal tibia fractures. A report of eight cases associated with severe soft-tissue injury.

Fracture Site Motion with Ilizarov and "Hybrid" External Fixation

Mechanical considerations in using tensioned wires in a transosseous external fixation system.

The effect of rigidity on fracture healing in external fixation.