The flat posterior cruciate ligament graft: Unresolved questions on biomechanical equivalence and clinical translation

We read with great interest the recent article by Deichsel et al. [1] entitled ‘A flat posterior cruciate ligament reconstruction restores native knee kinematics, comparable to a double-bundle reconstruction—A biomechanical robotic investigation’. The authors present a compelling biomechanical case for a novel, flat-graft, single-bundle (SB) posterior cruciate ligament (PCL) reconstruction technique. Their robotic study, demonstrating equivalent restoration of posterior and rotational stability to a double-bundle (DB) reconstruction, is a significant contribution. However, while the data are robust within their controlled experimental scope, a prospective and critical examination reveals several pivotal methodological and translational considerations that must be addressed before this technique can be endorsed for clinical practice [1, 2]. The foremost concern lies in the long-term structural fate of the flat graft. The proposed biomechanical advantage hinges entirely on the persistence of the graft's noncylindrical geometry [4]. However, the process of ligamentization involves profound biological and architectural remodelling. There is a substantial risk that the carefully engineered flat configuration may succumb to circumferential fibrosis or ‘rounding’ at the tunnel apertures due to the combined effects of the ‘bungee’ and ‘windshield wiper’ phenomena under cyclical loading [4, 5]. If such remodelling occurs, the initial kinematic superiority would be negated, potentially reverting the construct to the performance profile of a traditional SB technique. The study's single-cycle loading protocol, while standard for proof-of-concept, fails to interrogate this critical vulnerability. Stress-shielding of the graft's central fibres by the peripheral, more tensioned fibres in a flat construct could further lead to heterogeneous graft healing and weakening, a phenomenon not observed in conventional grafts [5]. Second, the clinical validity of the model is limited by its isolation to the PCL. In vivo, high-grade PCL injuries are frequently part of a multi-ligamentous injury pattern, most commonly involving the posterolateral corner (PLC). The native PCL and PLC structures function synergistically to resist coupled posterior translation and external rotation [2, 5]. The critical test for any new PCL reconstruction is its performance in this combined instability scenario. It is plausible that the flat SB graft, while equivalent to DB in a PCL-deficient model, may demonstrate significant shortcomings under the complex, multiplanar loads of a PCL/PLC-deficient knee. The DB reconstruction, with its two distinct bundles and often broader footprint, may provide a more robust restraint to these combined forces. The absence of data from a combined injury model represents a significant gap in the claim of clinical comparability [2]. Furthermore, the focus on gross kinematics overlooks the critical aspect of in situ forces within the graft. A key rationale for DB reconstruction is the reciprocal tensioning of its anterolateral and posteromedial bundles throughout the range of motion, which more closely mimics the function of the native PCL. While the flat graft may achieve a similar net kinematic result, the distribution of forces within its substance likely differs [2]. It is possible that the flat graft experiences higher peak stresses at its edges or undergoes abnormal internal shear strains during dynamic activities, which could predispose to premature failure. Robotic technology allows for the measurement of these in situ forces; the absence of such data limits our understanding of the graft's true functional performance and long-term durability [2, 3]. Finally, the biological integration of this novel graft design remains entirely speculative. The broader contact area with the femoral wall could theoretically enhance healing. Conversely, the altered collagen fibre alignment and potential for central avascularity within a thick, flat graft could impair cellular repopulation and nutrient diffusion, leading to problematic graft incorporation or central necrosis [5]. The long-term consequences of a flat graft on tunnel osteointegration and the risk of tunnel enlargement, a known issue in PCL surgery, are also unknown. In conclusion, the work by Deichsel et al. [1] is commendable for its innovation. However, proclaiming the flat-graft SB technique as a biomechanically equivalent substitute for DB reconstruction is premature. Its purported advantages remain provisional, pending investigations into its remodelling stability under cyclical loading, its efficacy in combined injury models, its internal force distribution, and its biological healing potential. Until these substantive questions are resolved, the search for the optimal PCL reconstruction technique continues. We received fund from The Affiliated Traditional Chinese Medicine Hospital of Southwest Medical University, Biomechanical research on cartilage transplantation based on the theory of ‘equal emphasis on muscle and bone’, 2023ZYQJ02. The authors declare no conflicts of interest. Ethical approval was not required. No data were generated or analysed for or in support of this paper.