Although a wide variety of external fixators can be constructed, they all consist of only two elements. Pins are inserted into the bone to anchor the external fixator to the skeleton. These pins are then connected to provide stability. Pins may be inserted by transfixing the limb (transfixion pins), or most commonly they may stop just beyond the far cortex of the bone into which they are inserted (half pins). Transfixion pins can be connected at both their ends; therefore, external fixators that use these pins provide the greatest stability. Because transfixion pins transfix the soft tissues on both sides of the bone, they tether soft tissues more than half pins do. This makes it difficult to mobilize joints above and below the external fixator. Fixators using transfixation pins are only used in situations that require the greatest stability.
Skeletal pins are inserted through small open incisions in the skin in a “blind” fashion, with little dissection of the soft tissues. Exceptions occur where nerves are close to pin placement, as in the distal third of the radius. Studies of cross-sectional anatomy in cadaveric material reveal a large number of possible pin placements for any given bone in any given position. The most common indication for the use of an external fixator is in open fractures. These injuries usually are associated with fracture displacement, and the normal anatomy frequently is distorted. When skeletal pins are used, nerves and vessels may be damaged as they course down the limb. Distortion of the normal anatomy or normal anatomic variation may make apparently safe routes hazardous. For this reason, only a few of the safest pin placements are discussed for each bone in this chapter. More specific information should be obtained from other sources if an external fixator is being used for a specialized procedure such as leg lengthening/distraction osteogenesis.
The rigidity of an external fixator system can be modified in many ways. As mentioned above, transfixion pins provide more stability than do half pins. Spreading the pins widely and increasing their number also adds to the rigidity of the system. Stability is increased by utilizing larger pins as well as pins that are slightly tapered (radial preload).
The number of bars used also increases stability; the closer the bars are to the skin, the more stable the construct will be. Placement of the pins is influenced not only by the underlying anatomy, but also by the biomechanical requirements of the fixation system. Finally, soft-tissue damage also may dictate pin position.
Skin incisions for pin insertion should be generous, because tight skin around a pin inevitably leads to low-grade sepsis, which in turn can cause pin loosening.
The Humerus
External fixation of the humerus has few indications but is invaluable when severe soft-tissue injuries are associated with fractures. External fixation is also used occasionally after infection, especially in the initial treatment of infected nonunion.
Because of the intimate relationship of the neurovascular bundles to the bone, the humerus is one of the most difficult bones in which to apply external fixators safely.
The median nerve runs with the brachial artery. In the upper two thirds of the arm, it is almost exactly medial to the humerus, but in the distal third of the humerus, it crosses over laterally to lie anterior to the bone at the level of the elbow joint.
The ulnar nerve runs with the median nerve in the upper two thirds of the arm and then courses posteriorly to run in direct relationship to the posteromedial aspect of the humerus at the level of the elbow joint.
The radial nerve crosses the posterior aspect of the humerus in a medial to lateral direction roughly in the middle third of the bone. At the level of the elbow joint, it is anterolateral to the humerus.
Proximal Third
In the proximal third of the bone, half pins may be inserted via a lateral route. These pins should not protrude very far beyond the medial cortex to avoid damage to the neurovascular bundle. Anterior insertion of half pins also is possible, although the biceps tendon may be damaged. Both anterior and laterally inserted half pins may damage the axillary nerve as it courses around the bone on the deep surface of the deltoid muscle.
P.697
Middle Third
Anterior half pins may be inserted in the middle third of the humeral shaft. The radial nerve runs across the back of the humerus in the middle third of the bone, however, and its course is variable. Care should be taken that these pins do not penetrate the far cortex too deeply.
 |
Figure 13-1 The placement of skeletal pins in the humerus varies with anatomic site. The variable relationship of the neurovascular bundles to the bone dictates different pin placement for the proximal, middle, and distal thirds. (A) Proximal third: Insert a half pin from the lateral side of the bone. Take care not to penetrate the medial cortex too far to avoid damage to the neurovascular bundle (brachial artery and median nerve). (B) Middle third: Place a half pin anteriorly. Take care not to penetrate the far cortex too deeply to avoid damage to the radial nerve, which courses in a medial to lateral direction on the posterior aspect of the middle third of the bone. (C) Distal third: Insert transfixion pins from the medial to the lateral point. Take care to avoid the ulnar nerve, as it runs in the groove on the back of the medial humeral epicondyle where the nerve is easily palpable. |
Distal Third-Elbow Joint
At the level of the elbow joint, half pins may be inserted in a lateral to medial direction, avoiding the neurovascular bundles that lie anterior and posterior to the epicondyles of the humerus (Fig. 13-1).
P.698
The Radius, Ulna, and Wrist
The relationships of the radius and ulna to the neurovascular structures are fundamentally different, and the pin placement required in each bone is distinct.
Ulna
The ulna has an easily palpable subcutaneous surface throughout its entire length. The ulnar nerve enters the forearm on the anteromedial aspect of the ulna, but passes rapidly into the anterior compartment of the forearm to run down on the anterior aspect of the bone together with the ulnar artery.
Half pins can be inserted throughout the entire length of the ulna from either side of the subcutaneous surface of that bone. In the proximal end of the ulna, the ulnar nerve is at risk, but it can be palpated easily as it crosses the back of the medial epicondyle of the humerus to allow safe pin placement in the subcutaneous surface.
Radius
The radial artery and sensory branch of the radial nerve run down the forearm roughly on the anterolateral aspect of the radius.
Proximal Third
The posterior interosseous nerve winds around the proximal third of the radius in an anterolateral to posteromedial direction and is very close to the bone. Because radial fractures nearly always involve a rotational deformity of the bone, the exact position of the posterior interosseous nerve in the proximal third of the radius cannot be predicted safely. For this reason, pin placement in the upper third of this bone is not recommended unless it is performed as an open procedure.
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
