Response Letter to: “The flat posterior cruciate ligament graft: Unresolved questions on biomechanical equivalence and clinical translation”

We thank the authors for their thoughtful letter and for the interest they have shown in our recently published work [2]. We appreciate the opportunity to respond to the points they have raised and to clarify several aspects of our study design, methodology, and interpretation of the results. Their correspondence highlights topics that are important for the ongoing discussion about posterior cruciate ligament (PCL) reconstruction techniques. The authors raise concerns that a flat graft may lose its geometry in the long term, yet their argument relies entirely on conjecture. None of the references they cite provide data that would support deformation, rounding, or structural collapse of a flat graft over time. In our technique, the graft is placed into a flat femoral tunnel and anchored along a flat bone surface, which provides a defined osseous bed. The rationale was to reconstruct the area of direct fiber insertions that was shown to provide the highest contribution to the function of the PCL, in a previous biomechanical study from our working group [1]. There is currently no evidence that such a construct would spontaneously become round, and the literature presented does not address this question in any way. In addition, a study from our working group demonstrated that flat-prepared hamstring tendons display comparable biomechanical properties to round tendons [3]. The authors correctly note that PCL injuries frequently occur together with damage to the posterolateral or posteromedial corner. In our biomechanical study, however, the posterolateral corner was intentionally left intact. This was done to limit confounding variables and to isolate the effect of the PCL reconstruction itself. This approach corresponds to standard methodology in biomechanical research, where associated structures that are often injured in vivo are kept intact in order to simulate their ‘perfect reconstruction’ and to allow a focused analysis of the structure under investigation, as was done in multiple previous biomechanical studies [4, 5]. Regarding the biological aspects, we agree that long-term graft behavior and tendon to bone healing are important, but the authors' statements about flat grafts are not supported by the literature they cite. In contrast, there is experimental and simulation based evidence that a more flattened tunnel and graft configuration can have positive biological and mechanical effects. Zhao et al. demonstrated in a rabbit ACL model that a flattened bone tunnel, with comparable cross sectional area to a round tunnel, led to faster tendon to bone healing in the early postoperative period, compared with a conventional round tunnel [7]. Xiao et al. used a finite element model of ACL reconstruction and found that using a flatter tunnel reduced peak graft stresses and produced a more favorable strain environment in the cancellous bone around the tunnel [6]. These data suggest that flat graft constructs may offer biological and mechanical advantages at the graft-tunnel interface. We agree that any new surgical concept must ultimately demonstrate its value in clinical practice. Our biomechanical study was designed as a proof of concept and should be interpreted as such. It was never our intention to suggest that the flat PCL reconstruction is superior to established double-bundle techniques. Rather, our goal was to show that a flat single bundle construct may represent a feasible alternative that warrants further investigation. The biomechanical data we report support this idea. In our model, a single bundle PCL reconstruction did not result in unacceptable laxity, and only small residual posterior tibial translation remained after reconstruction. This was particularly evident when an interference screw was used on the femoral side. These findings indicate that a flat single bundle approach can restore stability to a degree that is comparable to what has traditionally been reported for single bundle PCL techniques. Therefore, we conclude that the flat graft concept is a promising option that may broaden the reconstructive strategies available for PCL injuries. Future clinical studies will be needed to confirm whether the encouraging biomechanical behavior we observed translates into comparable patient outcomes. We appreciate the authors' interest in our work and hope that our clarification contributes to a constructive discussion on this topic. Elmar Herbst is Deputy Editor-in-Chief for the Knee Surgery, Sports Traumatology and Arthroscopy (KSSTA). Adrian Deichsel is Web Editor for the Knee Surgery, Sports Traumatology and Arthroscopy (KSSTA). The remaining authors declare no conflicts of interest.