The management of pelvic ring injuries can be complex, requiring a multidisciplinary approach for treatment.1 Anterior pelvic ring injuries can manifest as symphyseal dislocations, ramus fractures, or combinations of both.2 Functional recovery, treatment options, and residual deformity varies widely between fractures classified as stable and unstable.3–5 Anterior pelvic ring instability results from significant disruption of the pubic symphysis (often defined as greater than 2.5 cm of symphyseal diastasis, although the significance of this absolute number has recently been called into question6) and/or ramus fractures combined with posterior ligament equivalent disruptions, including complete sacral fractures.7 Various options for anterior fixation have been proposed, including anterior external fixators with use of supra-acetabular and/or iliac wing pins and internal fixation in the form of plates and screws.
INDICATIONS FOR EXTERNAL FIXATION
The routine use of external fixation for stabilization of the anterior pelvic ring has declined in recent years. As recently as 2 decades ago, external fixators were used often in the treatment of anterior ring injuries. External fixation has maintained a role at trauma centers, especially in the resuscitative phase of treatment, owing to its ease and efficiency in the application of fixators for symphyseal disruptions and ramus fractures.8 However, the risk of pin site infections, interference with abdominal access, the cumbersome nature of the frame—especially with sitting upright—and inability to accurately control posterior ring instability in addition to the widespread use of pelvic sheeting or binders for provisional pelvic stabilization during acute resuscitation has limited their use in modern pelvic fixation.9
Currently, the primary role of anterior pelvic external fixation is stabilization of the anterior pelvic ring when open reduction and internal fixation is precluded. This may occur in the presence of bladder rupture where contamination of plate and screw constructs is increased, open/contaminated wounds, or wounds potentially contaminated due to anterior laparotomy incisions. Patients presenting with refractory hemodynamic instability indicated for pelvic packing may undergo external fixation in conjunction with this emergent procedure, as external fixation provides pelvic volume control with packing to allow for tamponade of pelvic bleeding. Additional patient and injury-specific considerations may also drive consideration of external fixation, including certain complex and comminuted anterior ring injury patterns that may not be amenable to open reduction and internal fixation and morbidly obese patients with a large anterior pannus, in whom the risk of wound complications associated with open instrumentations is deemed unacceptably high.
Special consideration can be made when treating females of childbearing age. As internal fixation can prevent relaxation of the pelvic ring at the time of delivery, this may necessitate cesarean section. External fixation provides temporary stabilization of the pelvic ring with no residual fixation to inhibit relaxation of the pelvic ring at the time of delivery.10 In the study by Vallier et al, there was a trend toward increased caesarean delivery in women with retained internal fixation.11
BIOMECHANICAL CONSIDERATIONS FOR EXTERNAL FIXATION
Prudent decisions regarding stabilization of pelvic ring injuries require knowledge of mechanical stability differences with respect to internal and external fixation. McBroom and Tile found that all existing external frames would stabilize the pelvic ring sufficiently, provided the posterior osseous ligamentous hinge remained intact. However, no external frame could provide adequate stability for mobilization of a patient with the posterior hinge disrupted. They additionally found that internal fixation provided greater stability to the pelvic ring than external fixation.12
Hearn and Tile demonstrated that no significant difference exists in controlling external rotation of the hemipelvis when comparing anterior internal and external fixation.13 However, vertical displacement was resisted to a greater extent with use of anterior internal fixation as opposed to external fixation.14 Furthermore, pure symphyseal disruptions treated with anterior external fixation have been reported to have an increased incidence of mechanical failure in comparison to internal fixation, with obesity additionally increasing the risk of mechanical failure.15
External Fixation Treatment Methods
Two common methods exist for the placement of anterior pelvic external fixation: iliac crest pins and supra-acetabular pins. The classic iliac crest frame traditionally involves the use of two or three 5-mm partially threaded Schanz pins placed in to each iliac wing starting at the iliac crest and bound together by an anterior frame.16 It is important to place the most anterior pin at least 2 cm dorsal to anterior superior iliac spine (ASIS) to avoid injuring the lateral femoral cutaneous nerve. In the study by Rupp et al, a thick zone of bone was identified 2–3 cm posterior to the ASIS and extending 6–8 cm posteriorly along the crest. This region of bone was found to be hourglass-shaped and followed the superior gluteal ridge to the acetabular region with a maximal thickness of 4 cm, which would minimize risk of cortical perforation along the tables of the ilium.17 Historically, the technique involved dissection into the internal iliac fossa to help guide trajectory of the pins and minimize perforation. The most significant advantage of this technique is that it can be performed without fluoroscopic guidance and can be performed expeditiously. However, intraoperative fluoroscopy can be used to ensure proper placement of the pins, with an outlet view demonstrating whether the pin is out of the crest, and an obturator oblique view showing whether the pins remain between the inner and outer tables. The iliac crest pin frame, however, uses pins placed in inferior-quality bone, and recent biomechanical studies have shown inferior ability to resist internal and external rotation. Furthermore, the less ideal vector for closure of the unstable ring occasionally causes an abduction moment not seen in the supra-acetabular frame.18–23
The supra-acetabular frame uses 2 pins placed from the anterior inferior iliac spine to the posterior ilium in the robust bone of the sciatic buttress.24,25 The placement of these pins requires intraoperative fluoroscopy, beginning first with an obturator oblique outlet view to visualize the “tear drop” (represents the inner and outer tables of the ilium and the top of the greater sciatic notch inferiorly) for the start site followed by an iliac oblique to demonstrate that the pin is proximal to the hip joint and directed toward the sciatic buttress superior to the greater sciatic notch. The final necessary view is the obturator oblique inlet, which demonstrates placement of the pin within the inner and outer tables of the pelvis (Fig. 1).26 As these pins extend back to the posterior inferior iliac spine, they offer some, albeit minimal, control of the posterior ring. Supra-acetabular pin placement can be associated with injury to the lateral femoral cutaneous nerve and hip capsule, with Haidukewych et al showing the mean distance from pin insertion site to the lateral femoral cutaneous nerve being 10 mm, and as close as 2 mm, and the mean superior extent of the hip capsule being 16 mm, ranging from 11 to 20 mm.27 The starting point and trajectory for these pins is traditionally described as being identical to those employed by the insertion of “Lateral Column-2 (LC-2) screws,” inserted for fixation of a posterior iliac “crescent” fracture. However, the trajectory may be modified and directed more toward the greater sciatic notch to provide more clearance for hip flexion and facilitate a seated posture for those patients being treated definitively in an anterior frame. Additionally, iliac crest pins can be added to a supra-acetabular frame to increase mechanical stability.19
A third option, the subcristal pelvic external fixation frame is less common. The starting point of the subcristal pin lies medial to the center of the palpable ASIS and just lateral to the inner cortex of the ilium. The pin is placed toward the iliac tuberosity between the 2 iliac cortices. Intraoperative fluoroscopy can be used to confirm intraosseous placement of the pin in the iliac crest, although it is not needed. Solomon et al reported low complication rates while allowing for easier pin placement and less interference with hip flexion.28
Postoperative infection after external fixation of the unstable anterior ring usually manifests as pin tract infections. Pin tract infection rates have varied in the literature, with recent reports suggesting an 18% rate while some historical literature suggesting up to a 50% incidence of pin tract infections.29 These can typically be managed with appropriate release of the skin around the pins and dressing changes as necessary. Antibiotics may be warranted with persistent drainage or the development of cellulitis. Persistent infections can cause pin loosening, at which point the pin must be removed and the pin tract debrided.30
Loss of reduction can occur with use of external fixation, especially when used for definitive fixation (Fig. 2). In the study by Tosounidis et al, type B pelvic ring injuries displaced to a lesser extent at final follow-up than type C pelvic ring injuries. However, increased displacement at final follow-up did not correlate statistically with functional outcomes.31
The application of external fixation for the unstable anterior pelvic ring can also exacerbate the deformity of the ring injury. Dickson et al reported on the presence of the “external fixator deformity,” defined as worsening of flexion, internal rotation, or both with application of an anterior fixator in the setting of complete posterior ring disruption. They recommended caution when applying the external fixator and suggested adjusting the reduction vector to circumvent the worsening deformity. Additionally, the authors recommended sheeting or binder placement around the greater trochanters before external fixation to help reduce the pelvis while limiting the occurrence of pelvic deformity.32
Pelvic ring injuries can vary in severity and deformity. Although external fixation of the unstable anterior pelvic ring has largely given way to internal fixation, there remain indications for its use. In the setting of acute hemodynamic instability requiring pelvic packing, external fixation maintains a crucial role in stabilization of the patient. The 2 most common methods used have been the classic iliac crest frame and the fluoroscopy-dependent supra-acetabular frame. Like all treatment options, complications remain. Pin site infections remain the most commonly reported complication, although this is often managed with minimal effort. In summary, external fixation remains a viable form of treatment for the unstable anterior pelvic ring, and as such should remain in the armamentarium of the surgeon.
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