Among other tests, the authors of this paper studied the effect of working length on plate strain between the two innermost plate holes (area of interest) of a single LCP implant.1 In Table 3, the authors report no significant difference in the strain between SWL and MWL, but the strain in LWL was significantly higher than that of both SWL and MWL. Nevertheless, in the abstract, the authors state that “single plate construct strain was significantly, incrementally, higher as working length was extended.” This statement gives the wrong impression as it creates a pattern of a direct relationship between plate strain and working length, which does not reflect the findings.
In the last paragraph of the Discussion, the authors state that they used four-point bending “to produce a constant bending moment across the entire construct to allow comparison between constructs of varied stiffness.” This statement correctly suggests that the bending moment in the area of interest was the same at SWL, MWL, and LWL. [Fig. 1] of this letter illustrates a model and its graphical representation of the four-point bending used in the paper. Considering that the area of interest is composed of a homogenous material and isotropic, with a constant cross-sectional area and a longitudinal plane of symmetry within which the bending moment lies, the stress at various points of this area can be calculated using the flexure formula ([Fig. 1]). From the formula, it is apparent that the stress at any point in that area depends on parameters that were constant among SWL, MWL, and LWL. This suggests that the flexural stresses should also be constant at any point within the area. The flexural stresses would be the only normal stresses applied in the area of interest as the area was under pure bending. The same stress will produce the same strain within the elastic deformity boundaries where the experiment took place. Contrary to that, the authors found a significant difference between strains in SWL and MWL and LWL. It would be acceptable for the authors not to reject the null hypothesis for this test, as, despite the small test value, the result does not apply in real life since it is against the mathematical theory of pure bending.[2] The results could be attributed to inaccuracies in measurement or because the strain at 300 N was estimated by interpolation from a linear model they created using a small number of measurements, or other reasons.
