Plate Stress Does Not Decrease When Working Length Is Increased
I would like to comment on a statement that was made in the discussion section of the article describing a biomechanical comparison of two conical coupling plate constructs. The testing model simulated a bridging plate and the authors included a discussion of working length. They made the following statement—‘When using locking plates in bridging mode, leaving empty screw holes over the fracture gap decreases the internal stress of the plate by virtue of establishing a longer working length’—and I feel that this is incorrect. Plate stress does not decrease when the working length is increased by leaving more empty holes over the fracture gap—it stays the same, or may even increase.
They attribute this statement to two articles.  The second article does not make any statements about bridging plates, and measures strain in the bone, not in the implant. The first article is a very elegant study of plate deformation and fatigue for many different construct configurations. Plate stress for these different constructs was examined using finite element model simulations. There is one VERY specific scenario where they found that increasing the working length resulted in a decrease in plate stress. That scenario was a simulated fracture gap of 1 mm. With this very small gap, when the working length was increased, the construct was less stiff and, after initial loading, the bone fragments made contact. When they did that, the plate was prevented from deforming further, as much of the load was transferred through the bone contact, and, therefore, the plate stress was lower than for the short working length model, where bone contact did not happen. However, in this same article, when the model simulated a 6 mm gap, plate stress in empty screw holes was the same for short and long working lengths. This finding has been very elegantly reinforced in a study of plate strain in a gap model. Using surface strain mapping, plate strain was greater in long working length models compared with short working length models.
In clinical situations, bridging plates span large gaps and there is no bone contact, so intentionally increasing working length by leaving more screw holes empty is not beneficial, and should not be done. In the article used to support this statement, the more important and clinically relevant finding is that the yield load and cycles to failure were reduced for constructs with longer working lengths. Surgeons should aim to minimize working length to reduce the possibility of acute or long-term implant failure.
Received: 14 August 2020
Accepted: 24 August 2020
28 September 2020 (online)
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