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Commentary and Perspective   |    
Non-Contact ACL Injury: Can Anatomic Factors Be Used in Screening At-Risk Athletes?Commentary on an article by Christopher J. Wahl, MD, et al.: “An Association of Lateral Knee Sagittal Anatomic Factors with Non-Contact ACL Injury: Sex or Geometry?”
Elizabeth A. Arendt, MD1; Gregory A. Brown, MD1
1 University of Minnesota, Minneapolis, and TRIA Orthopaedic Center, Bloomington, Minnesota
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Copyright © 2012 by The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2012 Feb 01;94(3):e20 1-2. doi: 10.2106/JBJS.K.01488
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The authors are to be congratulated on their extensive case-control study of the sagittal geometry of the lateral knee compartment and its association with non-contact anterior cruciate ligament (ACL) injury. The underlying motivation for identifying risk factors is to recognize individuals at high risk for ACL injury and to intervene (when possible) to reduce the risk in those individuals. Unfortunately, if the results and conclusions of the study are reviewed from the perspective of a clinician hoping to identify individuals at high risk for non-contact ACL injury, there is no “tool” or guideline for how the risk factors can be used to assist the clinician.
Female athletes have a risk of non-contact ACL injury that is two to five times that of male athletes. One might therefore expect female athletes to demonstrate the greatest effect size in the risk factors associated with non-contact ACL injury. However, the results of this study show no difference between injured and uninjured female subjects with respect to any of the six geometric parameters pertaining to the lateral knee compartment—the maximal anteroposterior femoral articular width (FAP), distal femoral radius of curvature (Fr), maximal anteroposterior tibial articular width (TPAP), tibial plateau radius of curvature (TPr), ratio of femoral width to tibial width (FAP:TPAP), or ratio of femoral to tibial radius of curvature (Fr:TPr). Indeed, the corresponding p values (0.65, 0.83, 0.77, 0.68, 0.75, and 0.80) do not even approach significance. What value for any of these measured parameters would a clinician use as a cutoff to identify a high-risk female athlete?
The authors report thirty-six statistical comparisons in six tables. If a corresponding Bonferroni correction is applied to the significance level, the p value corresponding to a statistically significant difference would change from 0.05 to 0.05/36 = 0.0014. Fifteen of the thirty-six comparisons are significant at the corrected significance level, and five of these significant differences are between the uninjured female and uninjured male subgroups. The authors need to specify a priori comparisons to specifically address their hypothesis and not simply perform all possible comparisons between the male and female, injured and uninjured, and combined and uncombined subgroups. How do significant differences between uninjured female and uninjured male athletes support their hypothesis?
Ideal screening methods for risk factors need to be relatively low-cost and simple, with minimal time requirements. Use of the method in this paper would require prospective magnetic resonance (MR) imaging of all athletes as well as relatively complex digital analysis to measure the parameters. Furthermore, these parameters would be dynamic during skeletal maturation. At what age would MR imaging be (ideally) performed?
The authors report “almost perfect” interobserver agreement with their method of MR image measurement. However, they reduced the intraobserver and interobserver variability with two steps. First, “each series of T2-weighted proton-density MRIs was prescreened by the senior author to ensure that the axial, coronal, and sagittal reconstructed images were orthogonal to the posterior femoral condyles.” Second, “this sagittal reference plane was agreed on by the three observers and was used for all measurements in this study.” Ideally, each observer would have been given three-dimensional MR images and would have independently determined the planes orthogonal to the posterior femoral condyles, independently determined the femoral longitudinal axis, independently determined the sagittal reference plane, and then independently measured the sagittal-plane femoral and tibial parameters. Each observer would then have repeated these measurements for the same subject multiple times to calculate intraobserver as well as interobserver correlation coefficients. This four-step process would likely have yielded much lower values for these correlation coefficients.
The authors have articulated a set of possible sagittal-plane anatomic risk factors for a non-contact ACL injury. However, they have not demonstrated any correlation involving these risk factors in female subjects, arguably the highest-risk population. No cutoff values for identifying high-risk athletes are articulated. The reproducibility of their method also requires further quantification to judge its value.
Furthermore, use of these measurements as a screening tool for high-risk athletes would require obtaining MR images in this at-risk population; this is a formidable task. Perhaps this method could find clinical utility as a screening tool to help counsel patients who are contemplating a revision ACL reconstruction regarding their risk of reinjury. For the method to be applicable, it would need to be applied to standard MR imaging sequences, with good reproducibility and better-defined parameters for the predictive ability of this set of anatomic variables.
Even without clinical utility, the authors have further refined a particular set of anatomic variables that may play an important role in the risk equation for non-contact ACL injury.

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Elizabeth A. Arendt, MD, Gregory A. Brown, MD
Posted on March 06, 2012
Response to Christopher J. Wahl and Amy M. Cizik
Department of Orthopaedic Surgery, University of Minnesota; TRIA Orthopaedics, Bloomington, MN

We would like to thank the authors for their thoughts on our “Commentary and Perspective” on “An Association of Lateral Knee Sagittal Anatomic Factors with Non-Contact ACL Injury: Sex or Geometry?” Our comments were based on our perspective as two active clinicians. The perspective differences between ourselves and the authors may be characterized as “pragmatic” science versus “pure” science. As clinicians and readers of The Journal of Bone and Joint Surgery, we review each Journal article with the intent of increasing our evidence-based knowledge base to improve the clinical care and outcomes of our patients. Scientific research begins with a testable hypothesis. “We hypothesized that … the articular surfaces of the lateral aspect of the tibial plateau and/or the distal aspect of the femur to be more highly convex in patients with an ACL injury compared with patients of similar age and activity level without an ACL injury.” Table IV of the authors’ results show no difference of any of the six measured parameters for females with and without ACL injuries. The null hypothesis cannot be rejected and the conclusion should be that lateral knee articular geometry is not associated with ACL injury in females. Anatomic risk factors for ACL injuries are typically ratios, angles or other normalized parameters, such as posterior tibial slope (PTS), patellar ligament inclination angle (PLIA), femoral notch width ratio, body mass index (BMI), and footwear/surface coefficient of friction (μ). Absolute geometric measurements are highly correlated with height and therefore correlated with sex and are not independent risk factors for ACL injury. “We further hypothesized that female athletes would demonstrate greater absolute and relative articular convexity in the lateral compartment of the knee joint, which could help explain the female predisposition toward ACL injury.” Because absolute anatomic measurements are highly correlated with height and sex, the absolute lateral knee compartment convexity is not an appropriate independent risk factor. Therefore, the only appropriate parameter to test the second hypothesis is the ratio of the femoral radius of curvature and the tibial plateau radius of curvature, Fr:TPr. Table I compares the lateral knee geometry between females and males. The p value for the female and male comparison of Fr:TPr is 0.56. Again, the null hypothesis cannot be rejected and the conclusion should be that there is no relative articular convexity difference between females and males. The authors seem to be subject to the cognitive error of accepting confirming evidence and rejecting disconfirming evidence. They accept the data that confirms their hypotheses and reject the relevant data that does not confirm their hypotheses. The engineering term for two convex surfaces contacting with compressive forces is a “metastable” state. The compressive forces must be collinear with the contact point or the articulating surfaces become unstable. Therefore, any hypothesis or model that attempts to demonstrate that lateral knee compartment articular geometry is a non-contact ACL injury risk factor should explain how the female metastable state becomes unstable more often than the male metastable state. As “thoughtful clinicians and scientists,” we are not rejecting the possibility that lateral knee compartment articular geometry is a risk factor for non-contact ACL injuries. However, we do not believe that the evidence provided in this article supports the authors’ hypotheses. We also believe that reporting negative findings is important. Perhaps a more appropriate title for the article would be “Lateral Knee Sagittal Anatomic Factors Are Not Associated with Non-Contact ACL Injuries in Females.”

Christopher J. Wahl MD, Amy M. Cizik MPH
Posted on February 07, 2012
Authors Response to Drs. Arendt & Brown
University of Washington, Department of Orthopaedics and Sports Medicine

Response to: Non-Contact ACL Injury: Can Anatomic Factors Be Used in Screening At-Risk Athletes? Commentary on an article by Christopher J. Wahl, MD, et al.: “An Association of Lateral Knee Sagittal Anatomic Factors with Non-Contact ACL Injury: Sex or Geometry?” Author(s) Christopher J. Wahl MD, Amy M. Cizik MPH Affiliation(s) University of Washington, Department of Orthopaedics and Sports Medicine Authors Response to Drs. Arendt & Brown We read with interest the thoughtful Commentary & Perspective submitted by Drs. Elizabeth A. Arendt and Gregory A. Brown on our manuscript “An Association of Lateral Knee Sagittal Anatomic Factors with Non-Contact ACL Injury: Sex or Geometry?” (J Bone Joint Surg Am. 2012,94:e20(1-2). While they certainly present some valid criticisms, we believe Drs. Arendt and Brown may have missed the crux intention and conclusions of our study. Among their comments, Drs. Arendt and Brown stated, “Unfortunately, if the results and conclusions of the study are reviewed from the perspective of a clinician hoping to identify individuals at high risk for non-contact ACL injury, there is no “tool” or guideline for how the risk factors can be used to assist the clinician.” Note that the hypothesis of our manuscript never set out to specifically identify a clinical tool to assess the likelihood of ACL injury. Rather, our hypothesis explicitly stated that we expected to find differences in the articular length and curvatures of the lateral compartment that could correlate with the ACL injured state and could help to explain the female predisposition to this injury. We initially set upon this study to confirm or refute an observation made by the lead author (CJW) that persons with ACL injury and those with normal ACLs in the setting of knee laxity appeared to have an increased curvature in the mid-sagittal plane visible on MRI. In this respect, this study was meant to evaluate this observation and see if further investigation of the phenomenon was warranted. Indeed, at our institution we have continued with a biomechanical robotic study to specifically test the hypothesis that articular curvature may lead to higher in-situ stress/strain on the anterior cruciate ligament. Furthermore, the commentary was critical of the manuscript because we did not find geometric differences between ACL-injured and uninjured patients in our population of females in whom “one might expect to find the greatest effect size” based on the female predisposition. This criticism in many ways indicates that Drs. Arendt and Brown missed the crux of discussion in our paper. As authors, we expected to find differences in these anatomic variables between injured and uninjured persons, regardless of sex. We were similarly surprised that the data indicated such differences existed only for males. The criticism in the commentary hits upon the scientific difference between inductive and deductive study, however. As thoughtful clinicians and scientists, we should not discount the findings of an investigation because the results do not fit within our current paradigm of thinking (e.g. that only certain females will be at risk for ACL injury). Rather, we must interpret what we observed within the context of what is known. In this respect, it is not a failure of the study that we didn’t identify these anatomic differences between ACL-injured and uninjured female athletes—conversely, that fact informed our conclusion: that ACL injury is perhaps more common in females because all females share this at risk geometry while only some males do. Said another way; if short length and steep curvature are among risk factors for an ACL injury, and most females but only some males have this risk factor, then one might expect that more females will suffer ACL injuries—the phenomenon we observe clinically. This addresses another question that Drs. Arendt and Brown brought up (“How do significant differences between uninjured female and uninjured male athletes support their hypothesis.”) One would expect sex differences in the measured anatomy based on size alone. This, and the fact that differences still existed when controlling for size differences using the radius and length ratios between uninjured males and females (Fr:TPr and FAP:TPAP, respectively) supports our stated hypothesis that “females would demonstrate greater absolute and relative articular convexity in the lateral compartment of the knee joint, which could help to explain the female predisposition toward ACL injury.” We are appreciative and respect Drs. Arendt and Brown’s recommendation to apply the Bonferroni correction in the interpretation of our statistical results. I take responsibility as senior author to have not recognized the need for this correction a priori as our pre-study power analysis set out to enroll patients to identify a minimum difference of p<0.05, not p<0.0014. This stated, even with the correction applied, statistical significance of p<0.0014 was achieved with respect to geometric differences in males versus females (Table III) and injured versus uninjured males (Table V), while no differences existed between females regardless of injury status (Table IV), or between ACL-injured males and females (Table VI). The only variable that did not achieve a statistical significance with the Bonferroni correction applied was lateral knee geometry between injured and uninjured when the sexes combined (Table II). While this may imply that no difference exists between ACL-injured versus uninjured, combining the sexes actually skews the data away from a difference because of the scant variability in the female geometry regardless of injury. In short, application of the Bonferroni correction does not substantively change the outcome and conclusions of the investigation. The extant literature regarding anatomic factors and ACL injury identifies population differences that may be implicated or portend ACL injury among populations of patients, but these differences are generally too small to be useful prognosticators at the level of the individual. I would refer you to the cited work of Hashemi[1,2], Musahl[3], Meister[4], and others[5-10]. These studies similarly identify measureable anatomic variables that may play a role in the development of injury, but cannot be used clinically to stratify risk. This is obviously in part because the development of non-contact ACL injuries is multifactorial—no yet identified single predisposing pathologic trait (anatomic, neuromuscular, or otherwise) serves as a reliable clinical marker for risk. In this respect, ACL injury is similar to almost all other orthopaedic and medical conditions. It was the aim and hope that this study may further open the door for thoughtful clinicians and scientists to further consider the three-dimensional articular anatomy of the knee and assess what role it may have on the development of ACL injuries. If this can be more carefully tested and/or quantified, in the future there may be a role for its use in risk stratification in the general population or to recommend more durable (e.g. extra-articular derotational augmentation with ACL reconstruction) in injured individuals with at risk geometry. REFERENCES: [1] Hashemi J, Chandrashekar N, Gill B, et al.: The geometry of the tibial plateau and its influence on the biomechanics of the tibiofemoral joint. J Bone Joint Surg Am 2008:90:2724-2734. [2] Hashemi J, Chandrashekar N, Mansouri H, et al.: Shallow medial tibial plateau and steep medial and lateral tibial slopes: new risk factors for anterior cruciate ligament injuries. Am J Sports Med 2010:38:54-62. [3] Musahl V, Ayeni OR, Citak M, Irrgang JJ, Pearle AD, Wickiewicz TL: The influence of bony morphology on the magnitude of the pivot shift. Knee Surg Sports Traumatol Arthrosc 2010:18:1232-1238. [4] Meister K, Talley MC, Horodyski MB, Indelicato PA, Hartzel JS, Batts J: Caudal slope of the tibia and its relationship to noncontact injuries to the ACL. Am J Knee Surg 1998:11:217-219. [5] Brandon ML, Haynes PT, Bonamo JR, Flynn MI, Barrett GR, Sherman MF: The association between posterior-inferior tibial slope and anterior cruciate ligament insufficiency. Arthroscopy 2006:22:894-899. [6] Chaudhari AM, Zelman EA, Flanigan DC, Kaeding CC, Nagaraja HN: Anterior cruciate ligament-injured subjects have smaller anterior cruciate ligaments than matched controls: a magnetic resonance imaging study. Am J Sports Med 2009:37:1282-1287. [7] Hewett TE, Myer GD, Ford KR, et al.: Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med 2005:33:492-501. [8] Houseworth SW, Mauro VJ, Mellon BA, Kieffer DA: The intercondylar notch in acute tears of the anterior cruciate ligament: a computer graphics study. Am J Sports Med 1987:15:221-224. [9] Hudek R, Schmutz S, Regenfelder F, Fuchs B, Koch PP: Novel measurement technique of the tibial slope on conventional MRI. Clin Orthop Relat Res 2009:467:2066-2072. [10] Todd MS, Lalliss S, Garcia E, DeBerardino TM, Cameron KL: The relationship between posterior tibial slope and anterior cruciate ligament injuries. Am J Sports Med 2010:38:63-67.

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