Thirteen

 

Approaches for External Fixation

 

 

 

The Humerus

The Radius, Ulna, and Wrist The Pelvis

The Femur

The Tibia and Fibula The Ankle

Although a wide variety of external fixators can be constructed, they all consist of only two elements. Pins or wires are inserted into the bone to anchor the external fixator to the skeleton. These pins or wires 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). Wires always transfix the limb. Transfixion pins or wires can be connected at both their ends; therefore, external fixators that use these pins or wires provide the greatest stability. Because transfixion pins or wires 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 transfixion pins or wires are only used in situations that require the greatest stability as, for example, distraction osteogenesis.

Skeletal pins are usually 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.

There are three main uses for external fixators. Fixators may be used as a temporary way of immobilizing a fracture in cases where the soft tissues are so injured that immediate open surgery is impossible. These fixators frequently span joints and the pins used are ideally placed outside the zone of injury so that they do not interfere with subsequent definitive surgery. Fixators may also be used as a way of stabilizing long bone fractures especially those occurring in the femur in cases of polytrauma (damage control orthopedics). However, 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. Also be aware that pin placement should not interfere with the definitive soft tissue and bone treatment of open fractures. For these reasons, 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 that uses pins 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 and in the emergency management of polytrauma patients with associated severe lung damage.

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. Try to avoid inserting pins 5 to 7 cm distal to the acromion.

 

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. The radial nerve penetrates the lateral intermuscular septum about two-thirds of the way down the arm. This point is very close to the insertion point of lateral to medial locking bolts used in intramedullary nailing. Such bolts should be avoided and external fixator pins should not be inserted from the lateral side of the bone in this area.

 

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).

 

 

 

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.

 

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.

 

Middle Third

 

In the middle third of the radius, dorsally inserted half pins can be used

with a mini-open technique.

 

Distal Third

 

In the distal third of the radius, the lateral insertion of half pins is safe. The radial artery passes anterior to these pins. Because the branches of the superficial radial nerve are variable in position, it is important to make a small incision and dissect down to bone to avoid them, rather than inserting pins blindly (Fig. 13-2).

 

The Wrist

 

Percutaneous pinning is frequently used in the treatment of distal radial fractures. Pins are commonly inserted through the radial styloid or through the dorsal aspect of the distal radius. Insertion of pins through the fracture site (intrafocal pinning) is also a commonly used technique.1

External fixation of the wrist is used in complex fractures that are not amenable to internal fixation. External fixators commonly span the carpus from the distal radius to the second metacarpal, with half pins used in both bones. If the distal radial fragment is large enough, however, nonspanning external fixators can be used.2

 

The Radius

 

The superficial branch of the radial nerve always runs very close to the insertion point of the pins. Damage to the superficial branch of the radial nerve creates an area of anesthesia on the dorsum of the wrist and thumb, but the most common complication following damage to the nerve is the creation of a painful neuroma, which can be very troublesome to the patient. Place distal radial pins, using an open technique to avoid damage to the superficial branch of the radial nerve. Make 1-cm skin incisions and carefully deepen the incision down to the bone, using blunt dissection, taking care to identify and preserve any branches of the nerve. Pins should be inserted in the transverse plane at 45 degrees to the frontal plane and 45 degrees to the sagittal plane; two pins are usually used (Figs. 13-3 and 13-4).

Second Metacarpal Bone

The dorsal aspect of the second metacarpal bone is subcutaneous. Pins should be spread widely in the bone to increase the strength of the frame.

Because the bone is relatively small, fluoroscopy is useful to aid accurate pin placement. The inclination of the pin should be identical to those of the radial pins. As with the distal radius, a skin incision should be made, and the dissection should be carried out in a blunt fashion down to the bone, taking care to avoid damage to the extensor tendons and interosseous muscles.

 

 

 

Figure 13-2 The position of pins in the radius and ulna is dictated by the presence of the neurovascular bundles in the forearm. Pin placements vary depending on the level of bone to be used. The degree of supination and pronation of the forearm will need to be varied to allow different pin placements in the radius. A: Proximal third: Because the posterior interosseous nerve winds around the neck of the radius and has a variable relationship to the bone, it is not possible to insert pins safely in the proximal third of the radius except under direct vision. In the ulna, use half pins, taking care to avoid the ulnar nerve in the region of the elbow joint. B: Middle third: Place anterior half pins into the radius. Take care not to penetrate the bone too deeply to avoid damage to the superficial radial nerve and radial artery.

Place half pins through the subcutaneous surface of the ulna. C: Distal third: Place laterally inserted half pins into the distal radius. Take care to avoid the superficial branches of the radial nerve. Place half pins through the ulna, using its subcutaneous surface.

 

 

 

Figure 13-3 The superficial branch of the radial nerve is ever present along the distal third of the radius and branches over the anatomic snuff box and the second

metacarpal. Dissections here for pin placements should be done with the mini-open technique.

 

 

 

Figure 13-4 The incisions used are big enough to enable the surgeon to see that any nerve branches are protected, but the pins are placed in the middle of the bone in a bicortical fashion. Pins should be perpendicular to the bone and at 45 degrees to the frontal sagittal planes.

 

The Pelvis ‌

External fixation of the pelvis is used in life-threatening situations to achieve hemodynamic stability. The anterior pelvis is accessible in the supine trauma patient, and the anterior superior iliac spine is the most useful landmark for pin placement. Fluoroscopy should be used.

Two techniques are available for pin placement.

 

Iliac Crest

 

Place two external fixation pins immediately posterior to the anterior superior iliac spine through the subcutaneous surface of the iliac crest. Only drill the outer cortex and introduce the pins by hand, allowing them to slide between the two tables of the iliac wing. Be aware that the plane of the iliac wing is difficult to assess, particularly when the bone is displaced by trauma. Introducing long needles on either side of the iliac wing, keeping them in contact with the bone, provides a useful guide to correct pin placement (Fig. 13-5A–E).

The lateral femoral cutaneous nerve of the thigh is vulnerable at the level of the anterior superior iliac spine; therefore, pins should always be inserted under direct vision through small incisions rather than through blind stab incisions.

Anterior Inferior Iliac Spine External Fixation

This technique provides more stability than the iliac crest technique but is much more technically demanding.

Identify the anterior inferior iliac crest using a C-arm image intensifier and mark the skin. Make a 2-cm longitudinal incision centered over the anterior inferior iliac spine. Deepen the incision down to the deep fascia. Divide the fascia in line with the skin incision and identify the interval between the sartorius muscle and the tensor fasciae latae (Fig. 13-6B). Explore the intermuscular interval with a finger and palpate the bone. Insert a K-wire into the bone and check its position with the image intensifier. The wire should be at least 2 cm above the joint line to avoid inadvertent penetration of the hip joint capsule.

Alternatively make a stab incision over the anterior superior iliac crest and insert a pin down to bone through muscle under image intensifier control.3–5

 

 

 

 

 

 

Figure 13-5 A: Incision for iliac crest external fixation pins. B: Two wires placed on either side of the iliac crest mark the dissection for the unicortical drill hole. C:

The pin will find its way between the inner and outer table of the ilium using this open technique. D: The pins should be in the thick iliac tubercle. E: Two pins in place in the iliac wing.

 

Drill the outer cortex using fluoroscopy and introduce the pin by hand allowing it to slide into the supra-acetabular region (Fig. 13-6A–E).

The hip joint capsule is the structure that is most at risk during this procedure. The lateral cutaneous nerve of the thigh is always close but the main neurovascular bundle is always well medial to the approach.

 

 

 

 

 

 

Figure 13-6 A: A small stab 3 cm distal to the anterior superior iliac spine over the anterior inferior iliac spine. B: Small open incision using fluoroscopy. C: Blunt dissection finds the anterior inferior iliac spine. D: Unicortical drilling establishes a portal for the pin above the hip joint in the supra-acetabular region. E: Pin in place above the hip joint in the anterior inferior iliac spine.

 

The Femur ‌

The femoral artery enters the thigh in direct anterior relationship to the head of the femur (the femoral pulse). The artery courses down the limb, passing to the medial side of the bone in its middle third, and crosses the knee joint in direct posterior relationship to the distal femur. The sciatic nerve enters the thigh posterior to the femoral head and maintains this posterior relationship as it runs distally. At a variable point in the thigh, the nerve splits into its tibial and common peroneal components. The tibial nerve joins the femoral artery in the back of the knee joint. The common peroneal nerve runs with the tendon of the biceps muscle posterolateral to the bone.

Half pins can be inserted laterally throughout the entire length of the femur without damage to any of the neurovascular structures. These pins

do tether the fascia lata and vastus lateralis muscles, however, and it often is not possible to mobilize the knee successfully with them in position. In the distal third of the femur, laterally inserted half pins can be extended medially to transfix the limb. Be aware that these pins may penetrate the knee joint occasionally, resulting in leakage of synovial fluid and possible septic arthritis of the knee.

In the middle third of the bone, anteriorly inserted half pins also are safe. Care should be taken not to penetrate the posterior cortex too deeply, though, to avoid damage to the tibial nerve (Fig. 13-7).

 

 

 

Figure 13-7 The variable relationship of the femoral artery to the femur dictates different pin positions depending on the level of pin placement. A: Proximal third: Insert half pins from the lateral surface of the bone. Avoid penetrating the medial cortex too deeply to avoid damage to the profunda femoris artery and its tributaries. B: Middle third: Place laterally inserted half pins. Avoid penetrating the medial cortex too far to avoid damage to the femoral artery. Alternatively, place anteriorly inserted half pins. Avoid penetrating the posterior cortex too deeply to prevent damage to the sciatic nerve. C: Distal third: Place transfixion pins through the bone in a medial to lateral direction. Be aware that transcondylar pins will penetrate the knee joint synovium.

The Tibia and Fibula ‌

The anterior and posterior neurovascular bundles course down the leg on either side of the interosseous membrane lying between the tibia and fibula.

 

Fibula

 

The intimate relationship of the common peroneal nerve to the neck of the fibula makes pin insertion into the upper third of the fibula hazardous. Fortunately, this rarely is necessary.

 

Tibia

 

The tibia has a broad subcutaneous surface throughout its entire length. Because the bone is triangular in shape, the middle of this surface lies anterior to both neurovascular bundles. The subcutaneous surface of the bone can be used throughout its entire length for the placement of half pins. This route allows good bony anchoring without the risk of soft tissue tethering (Fig. 13-8).

 

 

 

Figure 13-8 Because the neurovascular bundles lie largely posterior to the tibia and it has a subcutaneous surface, pin placement is relatively straightforward. A:

Proximal third: Insert anterior half pins through the subcutaneous surface of the bone. If half pins are used, avoid penetrating the bone too far to protect the posterior neurovascular bundle—the posterior tibial artery and the tibial nerve. B: Middle third: Insert anterior half pins through the subcutaneous surface. C: Distal third: Insert anteriorly placed half pins.

 

The Ankle ‌

The bony prominences of the distal tibia, calcaneus, and first metatarsal provide access for pin placement. The neurovascular bundle running behind the medial malleolus should be avoided (Fig. 13-9). Half pins should be used, except in the calcaneus, where transfixion pins provide a very stable pin construct to be placed for trauma-related problems.

Half pins are placed in the distal tibia and in the first metatarsal using a mini-open technique. The calcaneal pin is a transfixion pin posterior and distal to the neurovascular bundle. The calcaneal pin should be placed from medial to lateral to avoid injury to the neurovascular bundle. The pin in the first metatarsal is placed perpendicular to the long axis of the first ray but distal to the tibialis anterior tendon.

 

 

Figure 13-9 Pin placement for external fixation around the ankle.

 

REFERENCES

1. MITTELMEIER W, BRAUN C, SCHäFER R: The Kapandji technique for fixation of distal radius fractures—a biomechanical comparison of primary stability. Arch Orthop Trauma Surg. 2001;121(3):135–138.

2. MCQUEEN MM: Non-spanning external fixation of the distal radius.

   Hand Clin. 2005;21(3):375–380.

3. KIM WY, HEARN TC, SELEEM O ET AL: Effect of pin location on stability of pelvic external fixation. Clin Orthop Relat Res. 1999; (361):237–244.

4. HAIDUKEWYCH GJ, KUMAR S, PRPA B: Placement of half-pins for supra-acetabular external fixation: an anatomic study. Clin Orthop Relat Res. 2003;(411):269–273.

5. LIDDER S, HEIDARI N, GäNSSLEN A ET AL: Radiological landmarks for the safe extra-capsular placement of supra-acetabular half pins for external fixation. Surg Radiol Anat. 2013;35(2):131–135.