The elusive predictors of anterior cruciate ligament failure in the young: Null findings in registry data

The recent study by Cristiani et al. [4], which identified no significant predictors of revision surgery following primary anterior cruciate ligament (ACL) reconstruction in a young population, presents a critical challenge to our current understanding of ACL failure. While the study leverages the considerable strength of a national registry, its provocative null finding demands a substantive, critical examination that probes beyond its presented conclusions [4]. The most substantial methodological concern is the pronounced risk of Type II error. With a revision event rate of 4.2% (n = 100) distributed across a cohort of 2374 patients, the statistical power to detect anything but the most robust risk factors is severely limited. For instance, to reliably detect a hazard ratio of 2.0 for a binary predictor with a 20% prevalence (e.g., a specific meniscal pathology), a sample would require approximately 120–150 event occurrences. The present analysis falls short of this benchmark. Consequently, the stated lack of association may not be conclusive but rather a reflection of an underpowered analysis. True, clinically relevant associations with moderate effect sizes are almost certainly obscured [1, 4]. Furthermore, the aggregation of disparate surgical techniques into a single analytical model represents a significant oversimplification. Transphyseal, all-epiphyseal, and hybrid techniques have fundamentally different risk profiles concerning graft fixation, physeal violation, and resultant graft maturity. Pooling these techniques likely introduces profound confounding. A risk factor critical in one technique (e.g., graft diameter in transphyseal reconstructions) could be diluted to non-significance when averaged across all procedures. The absence of detailed data on graft fixation methods and tunnel positioning—factors with a well-established link to graft failure—further limits the depth of the intraoperative analysis [1, 2]. The definition of the endpoint itself, revision surgery, is a pragmatic but incomplete capture of failure. This endpoint fails to account for patients with symptomatic graft insufficiency who decline further surgery or are managed nonoperatively. A composite endpoint incorporating patient-reported outcomes alongside revision would provide a more clinically nuanced definition of failure. The sole reliance on revision likely misclassifies a proportion of poor outcomes as successes, thereby biasing the reported associations toward the null [2, 3]. Most critically, the study's framework omits the modern paradigm of dynamic risk, which is defined by return-to-sport (RTS) readiness. A wealth of recent evidence demonstrates that the strongest predictors of a second ACL injury are not found in the operating room, but in the quality of rehabilitation and the objective metrics at the time of RTS clearance [3]. Deficits in quadriceps strength symmetry (<90%), persistent asymmetries in hop tests, and, crucially, lack of psychological readiness are powerfully associated with re-injury. The absence of this functional data creates a critical gap in the analytical model, leaving the most probable drivers of failure unmeasured and unaccounted for. It is plausible that the variance attributed to “unmeasured factors” in this study would be substantially explained by incorporating these dynamic functional phenotypes [3, 5]. In conclusion, while the work by Cristiani et al. usefully highlights the complexity of predicting ACL failure in the young, its conclusions should not be interpreted as evidence that such predictors do not exist. Instead, they underscore the limitations of a registry-based analysis that lacks granular surgical data, functional outcomes, and is potentially underpowered for its primary aim. The path forward requires prospective, hypothesis-driven studies powered for revision endpoints, with mandatory stratification by surgical technique and, most importantly, the systematic integration of objective, time-dependent functional assessments throughout the rehabilitation and RTS continuum. 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.