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Dr. Burkhart's statements that the findings of our recent article¹ are counter to Newtonian mechanics are not correct. In fact, basic mechanical analysis resolves the tendon force vector into two force components—one acting parallel to the anchor (potential pullout) and the other perpendicular to the anchor end (potential tilting or “windshield wipering”). When the anchor position becomes more vertical (e.g., from 45° to 90°), the perpendicular force on the anchor end thus decreases, as does the cause for tilting movement of the anchor. Of course, clinically, the propensity for this manner of anchor instability is affected by the direction of the tendon force (arm position) and its magnitude (activity). Dr. Burkhart is correct that anchor depth (which can affect the direction of the tendon force) and anchor design (such as the ability to achieve cortical engagement and increased fixation at the anchor end) are also important factors for anchor stability and should have received greater stress in our article's discussion.

Because current anchors generally have high pullout strengths, attention should be focused on improving the tendon-suture interface. We have shown that its strength is affected by the direction of the applied tendon force.²

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References 

1. Strauss E, Frank D, Kubiak E, Kummer F, Rokito A. The effect of the angle of suture insertion on fixation failure at the tendon-suture interface after rotator cuff repair: Deadman's angle revisited. Arthroscopy. 2009;25:597–602
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2. Kummer FJ, Iesaka K. The role of graft materials in suture augmentation for tendon repairs and reattachment. J Biomed Mater Res B Appl Biomat. 2005;74:789–791
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