Review Question - QID 219573

The image depicts a double-stacked frame, which provides increased rigidity and stability to the overall construct.

External fixator placement and configuration can vary based on the anatomic location of a fracture, soft tissue needs, and individual preferences. However, there are several accepted methods of increasing the rigidity of an external frame. These include increasing the diameter of the pins, mounting the bar as close as possible to the bone, maximizing the distance between pins on each side of the fracture, placing pins and/or rods in different planes, and stacking rods (as in this case).

Bible et al. reviewed the principles and applications of external fixators. The authors note that modern external fixation's modularity and ease of application have expanded its potential use in managing fractures and other musculoskeletal conditions. They conclude, however, that the risk-to-benefit ratio of added stability needs to be assessed with each fixator and that soft-tissue management is critical during pin insertion to lessen the risk of loosening and infection.

Strebe et al. analyzed strategies to increase external fixator stiffness to evaluate whether or not double stacking is worth the cost. The authors evaluated double stacking, cross-linking, and using an oblique pin in simulated knee-spanning external fixator models using ultra-high-molecular-weight polyethylene bone models and commercially available external fixator components. The models were tested in anterior-posterior bending, medial-lateral bending, axial compression, and torsion, and they found that double stacking significantly increased construct stiffness under anterior-posterior bending (109%), medial-lateral bending (22%), axial compression (150%), and torsion (41%), while use of an oblique pin and cross-linking only significantly increased stiffness under torsion by 25% and 29%, respectively. All 3 strategies increased stiffness under torsion to varying degrees, but only double stacking increased stiffness in all 4 testing modalities (p < 0.05) at an increased cost of 84%, leading the authors to conclude that clinicians can use these data to help guide the most cost-effective strategy to increase construct stiffness based on
the plane in which stiffness is needed.

Figure A is a clinical photograph of a knee-spanning external fixator that incorporates a double-stacked rod configuration (arrow).

Incorrect Answers:
Answer 1: Double-stacking would increase the rigidity and, thus, load-to-failure of the construct.
Answer 2: Double-stacking does not involve placing additional pins, so pin-site infection risk should not increase.
Answer 4: Though adding a double-stacked rod is certainly more expensive, the added benefit is increased construct stability, which may be warranted despite the cost.
Answer 5: The placement of the bar closer to the skin determines the relative soft tissue clearance; double-stacking should not significantly reduce this, and the soft tissue clearance depicted in this image is appropriate.