CHAPTER 8 External Fixation of Distal Radius Fractures
External fixation has been used for the treatment of distal radius fractures for more than 50 years. Although the fixator configurations have undergone considerable modification over time, the type of fixator itself is not as important as the underlying principles that provide the foundation for external fixation. Although volar plate fixation is currently popular, the indications for external fixation remain largely unchanged. Newer fixator designs also have expanded the traditional usage to include nonbridging applications, which allow early wrist motion. This chapter focuses on the myriad uses for external fixation and the shortcomings and potential pitfalls.
Anatomy
There are some important anatomical points one must bear in mind when considering external fixation of the distal radius. The articular surface of the radius is triangular with the apex of the triangle at the radial styloid. It slopes in a volar and ulnar direction with a radial inclination of 23 degrees (range 13 to 30 degrees), a radial length of 12 mm (range 8 to 18 mm), and an average volar tilt of 12 degrees (range 1 to 21 degrees).1 The dorsal surface of the distal radius is convex and irregular, and it is covered by the six dorsal extensor compartments. The dorsal cortex is thin, which often results in comminution that may lead to an abnormal dorsal tilt. Lister’s tubercle acts as a fulcrum for the extensor pollicis longus (EPL) tendon, which lies in a groove on the ulnar side of the tubercle. The volar side of the distal radius, which is covered by the pronator quadratus, is flat and makes a smooth curve that is concave from proximal to distal. When inserting the dorsal pins, it is important to engage the volar ulnar lip of the distal radius where the bone density is highest, especially in osteopenic bone.2
The dorsum of the radius is cloaked by the arborizations of the superficial radial nerve (SRN) and the dorsal cutaneous branch of the ulnar nerve. The SRN exits from under the brachioradialis approximately 5 cm proximal to the radial styloid and bifurcates into a major volar and a major dorsal branch at a mean distance of 4.2 cm proximal to the radial styloid (Fig. 8-1). Either partial or complete overlap of the lateral antebrachial cutaneous nerve with the SRN occurs up to 75% of the time.3 The dorsal cutaneous branch of the ulnar nerve arises from the ulnar nerve 6 cm proximal to the ulnar head and becomes subcutaneous 5 cm proximal to the pisiform. It crosses the ulnar snuffbox and gives off three to nine branches that supply the dorsoulnar aspect of the carpus, small finger, and ulnar ring finger. Open pin insertion allows identification and protection of these branches.
The proximal pins are placed at the junction of the proximal and middle thirds of the radius. At this level, the radius is covered by the tendons of the extensor carpi radialis longus, extensor carpi radialis brevis, and extensor digitorum communis. The proximal pins can be inserted in the standard mid lateral position by retracting the brachioradialis tendon and the SRN, in the dorsoradial position between the extensor carpi radialis longus and extensor carpi radialis brevis, or dorsally between the extensor carpi radialis brevis and extensor digitorum communis, which carries less risk of injury to the SRN.4
Ligamentotaxis
Limitations of Ligamentotaxis
Ligamentotaxis has many shortcomings when applied to the treatment of displaced intra-articular fractures of the distal radius. First, because ligaments exhibit viscoelastic behavior,5 there is a gradual loss of the initial distraction force applied to the fracture site through stress relaxation.6 The immediate improvements in radial height, inclination, and volar tilt are significantly decreased by the time of fixator removal (Fig. 8-2).7
FIGURE 8-2 Limitations of Ligamentotaxis. A, Initial overdistraction restores radial length. B, Note the persistent dorsal tilt of the articular surface even with maximum distraction.
(© South Bay Hand Surgery, LLC; 2007. Used by permission.)
Traction does not correct the dorsal tilt of the distal fracture fragment because the stout volar radiocarpal ligaments are shorter, and they pull out to length before the thinner dorsal radiocarpal ligaments exert any traction.8 Excessive traction may increase the dorsal tilt (Fig. 8-3).9 A dorsally directed vector is still necessary to restore the normal volar angulation. This vector is usually accomplished by applying manual thumb pressure over the dorsum of the distal fragment. With intra-articular fractures, ligamentotaxis reduces the radial styloid fragment, but for the aforementioned reasons, it does not reduce a depressed lunate fragment.10 When there is a sagittal split of the medial fragment, traction causes the volar medial fragment to rotate, which often necessitates an open reduction. External fixation cannot control radial translation and cannot be used with an unstable distal radioulnar joint (Fig. 8-4).
Biomechanical Considerations for External Fixation
Fracture Site Loads
External fixation is considered flexible fixation.11 The biomechanical requirements of external fixation for fractures of the distal radius have not been ascertained because until more recently, the magnitude and direction of the physiological loads on the distal radius were dynamic and unknown. Work by Rikli and colleagues12 has shed new light on this point, however. Using a new capacitive pressure-sensory device, these investigators measured the in vivo dynamic intra-articular pressures under local anesthesia in the radioulnocarpal joint of a healthy volunteer. With the forearm in neutral rotation, the forces ranged from 107N with wrist flexion to 197N with wrist extension. The highest forces of 245N were seen with the wrist in radial deviation and the forearm in supination. Presumably, any implant or external fixator would need to be strong enough to neutralize these loads to permit early active wrist motion. Rikli and colleagues12 also identified two centers of force transmission. The first center was opposite the scaphoid pole, which would represent the radial column. The second center, which would represent the intermediate column of the wrist, took a considerable amount of the load and was opposite the lunate, extending ulnarly over the triangular fibrocartilaginous complex.
Construct Rigidity
Increasing the rigidity of the fixator does not appreciably increase the rigidity of fixation of the individual fracture fragments.13 The stability of the construct can be augmented in many ways, however. After restoration of radial length and alignment by the external fixator, percutaneous pin fixation can lock in the radial styloid buttress and support the lunate fossa fragment.14 A fifth radial styloid pin attached to the frame of a spanning AO external fixator (Synthes, Paoli, PA) prevents a loss of radial length through settling and leads to improved wrist range of motion compared with a four-pin external fixator.15 The addition of a dorsal pin attached to a sidebar easily corrects the dorsal tilt found in many distal radius fractures.16,17
Kirschner wire (K-wire) fixation enhances the stability of external fixation. The combination of an external fixator augmented with 0.62-inch K-wires approaches the strength of a 3.5-mm dorsal AO plate (Synthes, Paoli, PA).18 Supplemental K-wire fixation is more crucial to the fracture fixation than the mechanical rigidity of the external fixator itself.13 Stabilizing a fracture fragment with a nontransfixing K-wire that is attached to an outrigger is just as effective as a K-wire that transfixes the fracture fragments.19
Bridging External Fixation
Temporary External Fixation: Indications
Compared with conventional plate fixation, bridging external fixation may be used in a temporary manner, or it may be used for definitive management of the distal radius fracture. Bindra20 listed the following indications for this technique:
FIGURE 8-5 A, Open distal radius fracture. B, Temporary stabilization with a bridging external fixator.
(© South Bay Hand Surgery, LLC; 2007. Used by permission.)
Rikli and colleagues21 use temporary bridging external fixation for complex fractures to aid in the provisional fracture reduction and to allow a better computed tomography (CT) evaluation of the fracture characteristics before double-plate fixation.
Definitive External Fixation: Indications
Indications for definitive external fixation include the following:
Complications
Fixator loosening with loss of fracture position can be avoided by periodically checking and tightening the fixator connections. Fixator failure by itself is uncommon, but many commercially available fixators are approved for single use only because of the risk of unrecognized material fatigue or failure of any locking ball joints. Pin site complications include infection, loosening, and interference with extensor tendon gliding. The risk of injury to branches of the SRN mandate open pin site insertion. Bad outcomes associated with external fixation are often related to overdistraction. One biomechanical study documented the effect of distraction of the wrist on metacarpophalangeal joint motion. More than 5 mm of wrist distraction increases the load required for the flexor digitorum superficialis to generate metacarpophalangeal joint flexion for the middle, ring, and small fingers. For the index finger, however, 2 mm of wrist distraction significantly increases the load required for flexion at the metacarpophalangeal joint.22 Many cases of intrinsic tightness and finger stiffness that are attributed to reflex sympathetic dystrophy are a consequence of prolonged and excessive traction, which can be prevented by limiting the duration and amount of traction and instituting early dynamic metacarpophalangeal flexion splinting even while in the fixator.
The degree and duration of distraction correlate with the amount of subsequent wrist stiffness.23 Distraction, flexion, and locked ulnar deviation of the external fixator encourage pronation contractures (Fig. 8-6). Distraction also increases the carpal canal pressure,24 which may predispose to acute carpal tunnel syndrome. Metaphyseal defects should be grafted to diminish bending loads and to allow fixator removal after 6 to 7 weeks, which minimizes the fixator-related complications.
Results
Margaliot and associates25 did a meta-analysis of 46 articles with 28 (917 patients) external fixation studies and 18 (603 patients) internal fixation studies. They did not detect a clinically or statistically significant difference in pooled grip strength, wrist range of motion, radiographic alignment, pain, or physician-rated outcomes between the two treatment arms. There were higher rates of infection, hardware failure, and neuritis with external fixation and higher rates of tendon complications and early hardware removal with internal fixation. Considerable heterogeneity was present in all of the studies, which adversely affected the precision of the meta-analysis.
Augmented External Fixation
The use of supplemental K-wire fixation can expand the indications for external fixation. As noted earlier, K-wire fixation not only enhances the reduction of the fracture fragments, but also increases the rigidity of the entire construct. Many authors have stressed the importance of using the external fixator as a neutralization device rather than a traction device. Ligamentotaxis is used to obtain a reduction of the fracture fragments, which is then captured with percutaneous K-wire fixation. The traction on the fixator can be reduced, which allows positioning of the wrist in neutral or slight extension (Fig. 8-7).9 This positioning serves to reduce extensor tendon tightness and facilitates finger motion. In a study of intrafocal pinning, Weil and Trumble26 noted that in patients older than 55 years and younger patients with comminution involving two or more surfaces of the radial metaphysis (or >50% of the metaphyseal diameter), bridging fixation was necessary in addition to percutaneous pin fixation to prevent late fracture collapse. In four-part fractures in which there is a sagittal split of the medial fragment, longitudinal traction accentuates the palmar translation and rotation of the volar medial fragment (Fig. 8-8). Dorsal to volar K-wire placement carries the risk of injury to the volar neurovascular bundles, especially with K-wire migration. For these reasons, any sagittal split of the articular surface typically requires open treatment27 (Fig. 8-9).
FIGURE 8-9 Limited Internal Fixation. A, Anteroposterior x-ray of a displaced four-part fracture. B, Lateral view shows a sagittal split of the medial fragment. C, Internal fixation of medial fragment through a limited volar ulnar approach. D, Subchondral K-wire inserted to support the joint surface. E, Second K-wire replaced by a percutaneous screw, which stops short of the distal radioulnar joint. F, Lateral view shows restoration of normal tilt and reduction of the sagittal split.
(© South Bay Hand Surgery, LLC; 2007. Used by permission.)
Results
Kreder and associates28 compared the results of open reduction and internal fixation (ORIF) versus external fixation and pinning. The study randomly assigned 179 adult patients with displaced intra-articular fractures of the distal radius to receive indirect percutaneous reduction and external fixation (n = 88) or ORIF (n = 91). There was no statistically significant difference in the radiological restoration of anatomical features or the range of movement between the groups at 2 years. The patients who underwent indirect reduction and percutaneous fixation had a more rapid return of function and a better functional outcome, however, than the patients who underwent ORIF, provided that the intra-articular step and gap deformity were minimized.
