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Application of External Fixators for Distal Radius Frx


- See:
        - Orthofix
        - Over Distraction of Wrist Joint

- Positioning:
     - supine, hand table, flouro;
     - prep for contralateral iliac crest (ipsilateral w/ less personel);

- Frame Configuration:
     - Lateral Frame:
     - 45 deg Oblique Frame:
            - good stabilization in AP plane;
            - will engage only a total of 4 metacarpal cortices hence need to engage cortical bone (not cancellous) at metacarpal metaphyseal diaphyseal flare;

- Reduction and Final Wrist Position:
      - neutral pronation/supination is essential for correct incisions;

- Operative Strategy:
      - apply distal metacarpal pin and proximal radial pin first;
      - then apply frame, and reduce fracture;
      - then apply proximal metacarpal pin and distal radial pin;

- Metacarpal Incision:
     - always performed first, to ensure proper length of fixator;
     - make longitudinal incision from base of 2nd metacarpal to its midshaft;
     - dissect down to bone, and insert metacarpal retractors (Homan's)
     - site of pin insertion at base of index metacarpal, terminal branches of radial sensory nerve can be identified & carefully retracted;
     - to preserve saggital band and first dorsal interosseous aponeurosis, second metacarpophalangeal joint should be flexed to 90 deg;
           - flexion causes the lateral band and interosseus tendon to move distally, thus minimizing the chance of trans-fixation;
           - metacarpal phalangeal joint is held in flexion, placing intrinsic under tension, & first dorsal interosseous muscle is pushed in anterior direction w/ fingers to move it out of direction of external fixation pins;
           - first dorsal interosseous muscle can be sharply elevated off base of index metacarpal, providing direct access and visualization of metaphyseal flare and proximal shaft of index metacarpal;

- Metacarpal Pin Insertion:
    - proximal metacarpal half pin is inserted first;
            - it is positioned close to base of bone on flare of the tubercle;
            - position of fixator screw is at metaphyseal-diaphyseal junction;
    - if proximal metacarpal pin is going to be placed at 45 deg angle, (piercing only 2 cortices, rather than the 4 cortices possible w/straight lateral placement), then it should be placed through cortical bone at the flare of the metacarpal base;
    - consider angling pins away from each of at 60 deg for radial preloading
    - in older individuals predilling may not be necessary;
    - EBI System:
           - uses a drilling template for the metacarpals, thru which is inserted a drill sleeve and trochar;
                  - the trochar helps identify the center of the bone;
           - once completed, the drill sleeve is replaced w/ a threaded screw guide;
           - insert the half pin w/ at least 2 mm of protrussion of the distal tip through the far cortex;
           - EBI half pins will not tolerated being backed out (due to conical shape);
    - controlled axial preload (1.5 mm predrilling, insertion of 2.5 mm half pin) is experimentally and clinincally satisfacrory;
           - in hard bone of young adults, predrilling w/ 2.0 mm drill bit is helpful when using 2.5 mm half pins;
           - in soft bone, no predrilling is necessary for metacarpal pins;

- Radial Skin Incision:

    - reduce fracture prior to incision to decrease skin tenting;
    - choose interneural interval for skin incision;
           - Brachioradialis - ECRL Interval;
           - Brachioradialis - FCR Interval;
           - typically the radial pins are placed dorsal to brachioradialis;
    - first incision is centered approx 10 cm proximal to radial styloid overlying radial aspect of forearm and one overlying dorsoradial
    - make two 1.5 cm incisions, parallel to distal half pins;
           - longitudinal skin incisions approx 1.5 cm long are planned linearly on dorsoradial aspect of the forearm and hand;
    - avoid placement of the pins more proximally in the mid forearm as this presents ptoential of greater soft tissue motion over pin, leading to pin track sepsis;

- Radial Nerve Sensory Branch:
      - sensory branch of  radial nerve can be avoided by placing pins at least 5 cm proximal to wrist joint & approaching radius from a dorsomedial direction;
             - this avoids sensory branches medially & extensor tendons dorsally;
      - through proximal incision, branches of lateral antebrachial cut nerve can be identified in superficial subcutaneous tissue while deeper and more prominent radial sensory nerve can be identified as it emerges from under Brachioradialis tendon at interval between the brachioradialis & ECRL at their myocutaneous junction;

- Half Pin Placement:
     - both the forearm & metacarpal pins are placed in line w/ ea other;
     - through drill guides, radius is predrilled w/ sharp fresh drill bits on power source;
     - drill bits are then removed & replaced with self tapping threaded half pins of appropriate size;
     - after precise location of the inserted half pins has been confirmed by flouro, soft tissue may be allowed to fall back into place and skin is sutured;
     - Pin Size:
          - complications can limited by limited open surgical approach for predrilling of fixator pins & using 4 mm half pins;
          - use a 2.0 mm drill for predrilling a 4.0 mm half pin;
          - compared to 3 mm pins, 4 mm self tapping half pins show sig higher pullout strength, & there is only small decr in bone torsional strength