Introduction: Temporary external fixation has been widely utilized in the stabilization of plateau fractures while waiting for an optimization of the soft tissue conditions before subsequent permanent internal fixation. Simultaneously, MRI is beneficial in the assessment of concomitant damage to ligaments and menisci so that these injuries could be promptly identified, and surgical planning executed at the time of definitive fixation of the bony injury. Increasing numbers of side-bars and pins have been previously suggested to increase frame rigidity, but at the same time, several studies have indicated the presence of MRI artifacts which may obscure key anatomical structures, even when MRI-compatible fixation devices are used. This study aims to identify, among six potential configurations, the construct that maximizes stability while most minimizing the number of MRI artifacts generated among different configurations commonly used.
Hypothesis: There is one or more configurations among the others that maximize stability while preserving a clinically acceptable level of MRI quality.
Material and methods: Six constructs were recreated on cadaveric specimens and identified by the disposition of the bars: H, Anterior, Flash, Hashtag, Rhomboid, and Diamond. Stage one evaluated the amount of artifact produced during MRI on instrumented cadaveric legs, as well as the signal-to-noise ratio (SNR) and the contrast-to-noise ratio (CNR) at five specific regions of interest. Stage two assessed the amount of compressional and torsional stiffness of the configurations on bone surrogate models.
Results: Image artifacts were not detected within the knee joint for all considered constructs. In terms of SNR The H, Anterior, Hashtag, and Diamond configurations were not significantly different from their control (p>0.366) while the others were significantly different (p<0.03). The values of CNR found for the H and Hashtag configurations were not significantly different from their controls (p>0.07) while the remaining configurations were significantly different (p<0.03). In compression, the H and Diamond configurations had similar stiffness (p=0.468) of 35.78N/mm and 31.44N/mm, respectively, and were stiffer than the other configurations. In torsion, the constructs have shown different stiffness (p<0.001) with a minimum value of 0.66 Nm/deg for the Rhomboid configuration, which was significantly less stiff than the Anterior configuration (1.20 Nm/deg [p<0.001]). There was no difference between the Diamond and H configurations (p=0.177) or between them and the Hashtag configuration (p=0.215).
Discussion: An external fixator construct directly bridging the femur and tibia without interconnections is the most stable and produces MRI scans without image artifacts that would interfere with diagnostic quality.
Level of evidence: V, basic science study, diagnostic imaging and mechanical testing.
Keywords: External fixation stability; Image artifact; Magnetic resonance imaging; Tibial plateau fracture.
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