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Kinematic Analysis of the Indirect Femoral Insertion of the Anterior Cruciate Ligament: Implications for Anatomic Femoral Tunnel Placement
Original Article With Video Illustration
Kinematic Analysis of the Indirect Femoral Insertion of the Anterior Cruciate Ligament: Implications for Anatomic Femoral Tunnel Placement Neil P. Pathare, M.D., Stephen J. Nicholas, M.D., Robb Colbrunn, Ph.D., and Malachy P. McHugh, Ph.D.
Purpose: To determine the effect of debriding the indirect insertion component of the femoral anterior cruciate ligament (ACL) attachment on tibiofemoral kinematics when compared with the intact knee. Methods: Knee kinematics were measured in 9 cadaveric knees with the ACL intact, after indirect insertion debridement, and after ACL transection. Three loading conditions were used: (1) a 134-N anterior tibial load, (2) a combined 10-Nm valgus and 5-Nm internal tibial torque, and (3) a simulated robotic pivot shift. Anterior tibial translation (ATT) was recorded in response to anterior and combined loads at 0 , 15 , 30 , 45 , 60 , and 90 of flexion. Posterior tibial translation and external tibial rotation were recorded during the simulated pivot shift. Results: With an anterior load, indirect insertion debridement increased ATT by 0.37 0.24 mm at 0 (P ¼ .002) and by 0.16 0.19 mm at 15 (P ¼ .033; increases <1 mm in all specimens). ACL transection increased ATT in response to an anterior load (P ¼ .0001) with maximum effect at 15 compared with the intact and debrided states (11.26 1.15 mm and 11.04 1.08 mm, respectively). With a combined load, indirect insertion debridement increased ATT by 0.17 0.11 mm at 0 (P ¼ .001; increases <0.3 mm in all specimens) with no effect at other angles. ACL transection increased ATT in response to a combined load (P ¼ .001) with maximum effect at 15 (4.45 0.85 mm v ACL intact and 4.44 0.84 mm v debrided indirect insertion). In the ACL intact condition, the pivot shift produced 1.29 1.34 mm of posterior tibial translation and 1.54 1.61 of external tibial rotation, as compared with 1.28 1.34 mm and 1.54 1.47 , respectively, after debridement (P ¼ .68 and P ¼ .99, respectively) and 12.79 3.22 mm and 17.60 4.30 , respectively, after ACL transection (P ¼ .0001). Conclusions: The indirect femoral ACL insertion contributes minimally to restraint of tibial translation and rotation. Clinical Relevance: Femoral tunnel positioning for anatomic ACL reconstruction should aim to recreate the biomechanically significant direct insertion.
T
he femoral anterior cruciate ligament (ACL) footprint has been described extensively in the literature.1-10 Despite this, controversy surrounding ideal femoral tunnel placement for ACL reconstruction continues. A recent study involving surgeons associated with the MOON (Multicenter Orthopaedic Outcomes Network) group showed that they do not currently
From Nicholas Institute of Sports Medicine and Athletic Trauma, Lenox Hill Hospital (N.P.P., S.J.N., M.P.M.), New York, New York; and the Department of Biomedical Engineering, Lerner Research Institute, and Orthopaedic Research Center, Cleveland Clinic (R.C.), Cleveland, Ohio, U.S.A. The authors report that they have no conflicts of interest in the authorship and publication of this article. Received February 13, 2014; accepted July 17, 2014. Address correspondence to Neil P. Pathare, M.D., Nicholas Institute of Sports Medicine and Athletic Trauma, Lenox Hill Hospital, 100 E 77th St, Second Floor, New York, NY 10075, U.S.A. E-mail: [email protected] Ó 2014 by the Arthroscopy Association of North America 0749-8063/14122/$36.00 http://dx.doi.org/10.1016/j.arthro.2014.07.017
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uniformly agree on ideal single-bundle tunnel placement.11 Trends in ACL reconstruction have been toward anatomic reconstruction, which aims to restore the location of the native anteromedial and posterolateral bundle insertion sites. The trend has thus been to drill femoral tunnels lower on the wall of the intercondylar notch, such that the inferior-most aspect of the tunnel sometimes even abuts the inferior cartilage margin.12-14 Macroscopic studies have reported that the femoral ACL attachment site is quite broad with variable size and shape, ranging from 93 mm2 to 200 mm2 (Fig 1A).1-10 More detailed histologic and immunohistologic analyses of the insertion site indicate that the broad ACL attachment is composed of both direct and indirect collagenous insertions (Fig 1B).15,16 The direct insertion has been shown to be a narrow, linear band extending up to the shallow (distal) articular cartilage margin. The central, inferior aspect of the direct insertion is consistently located approximately 5 to 6 mm
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Fig 1. Macroscopic view of the femoral ACL insertion in a left knee at 90 of flexion. (A) Arthroscopic nomenclature is followed by anatomic nomenclature in parentheses. The dashed line depicts the broad ACL attachment site. (B) The purple (anteromedial bundle) and blue (posterolateral bundle) rectangles depict the direct insertion. The red semicircle depicts the indirect insertion. (C) The double-headed arrow depicts the distance from the central, inferior aspect of the direct insertion to the inferior cartilage margin.
superior to the inferior cartilage margin (Fig 1C).15-17 The direct insertion represents a firm and fixed attachment and is considered the most biomechanically relevant portion of a ligament attachment because it allows for a gradual force distribution from ligamentous tissue to noncalcified fibrocartilage, calcified fibrocartilage, and finally, bone.15,16,18-20 The indirect insertion has been shown to consist of coarse fibrous tissue, type I collagen, and synovium and directly attaches to bone without a clear transition zone.15,16,21,22 The indirect insertion has been shown to be a crescentshaped area located between the inferior cartilage margin and the direct insertion (Fig 1B).15-17 The indirect insertion plays a lesser biomechanical role, acting as a dynamic anchorage of soft tissue to bone allowing certain shear movements.16,19,23 Recently, it has been suggested that ACL reconstruction should be tailored to each individual in an effort to provide maximum coverage of the native femoral ACL attachment site.24-26 As described by Siebold,27 the concept of maximum footprint coverage is based on the hypothesis that the restored biomechanical envelope of the knee is a function of the amount of reconstructed ACL attachment site area. The concept of maximum footprint coverage aims at filling both the direct and indirect insertion sites with graft material, with the assumption that both play a role in the kinematic profile of the ACL. If this concept holds true, it must be
assumed that both the indirect and direct insertion components of the femoral ACL attachment site play a role in the biomechanical profile of the ACL. To our knowledge, no biomechanical studies have delineated the kinematic contributions of the indirect and direct femoral ACL insertions. The purpose of this biomechanical study was to determine the effect of debriding the indirect insertion component of the femoral ACL attachment in response to an anterior tibial load, a combined rotatory load, and a dynamic pivot-shift load when compared with the intact knee. We hypothesized that debridement of the indirect insertion would not affect tibial translation or rotation during kinematic testing.
Methods Specimens In this study 11 fresh-frozen human cadaveric knees (6 female and 5 male knees) were used. The mean donor age was 55 4.3 years (range, 50 to 60 years). Before testing, the knees were stored at 20 C and thawed for 24 hours at room temperature. Once thawed, the knees were clinically examined. Two specimens were excluded because they were found to have a positive posterolateral drawer test with greater than 2 cm of posterolateral translation of the tibia. The remaining specimens underwent diagnostic arthroscopy with no evidence of
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damage to the cartilage surfaces, menisci, or ligamentous structures. Arthroscopic nomenclature is referenced with the knee in 90 of flexion throughout the study (Fig 1A). The femur and the tibia were cut approximately 20 cm from the joint line. The proximal and distal 7 cm of the femur and tibia were secured within thick-walled aluminum cylinders with Wood’s metal. The tibia was fixed to a 6 degrees of freedom (DOF) force/torque sensor (SI-1500-240; ATI Industrial Automation, Apex, NC), which was mounted rigidly to a crossbeam. The femur was similarly placed in a custom mount on a robotic manipulator (Rotopod R2000; Parallel Robotics Systems, Hampton, NH). To prevent exsiccation of the soft tissues, the skin of the specimens was left intact, and the specimens were moistened with normal saline solution throughout the testing procedure. Robotic/Universal Force-Moment Sensor Testing System A knee joint coordinate system was established using a MicroScribe coordinate measuring machine (Immersion, San Jose, CA). The memory-lock function of the testing system allowed the specimen to be removed from the robot for surgery and precisely returned (repeatability of <10 mm) for further testing. This robotic musculoskeletal simulation testing system has been previously validated.28 After a neutral position (position of the knee with a 20-N compressive load and zero translational or rotational forces) at full extension was established, the knee was flexed from 0 to 90 with 100 N of compression and the system found the positions of the knee that minimized all external forces and moments applied to the joint throughout the range of flexion. By use of the kinematic output during this passive flexion testing, the digitized coordinate system was then optimized to minimize changes in anterior and superior translations throughout the range of flexion. The resulting positions were accepted as starting positions for the application of the quasi-static external loading conditions. Testing Protocol Kinematic testing consisted of a series of quasi-static and dynamic loading conditions, which were controlled by a real-time force feedback controller. The 2 quasi-static loading conditions were an external anterior tibial load and a combined rotatory load. To simulate a clinical examination using the KT-1000 arthrometer (MEDmetric, San Diego, CA), anterior tibial translation was measured in response to a 134-N anterior tibial load with 20 N of compression (with all remaining off-axis loads minimized) at 0 , 15 , 30 , 45 , 60 , and 90 of flexion. An anterior load of 134 N has been used previously when evaluating injuries to the ACL using a robotic/universal force-moment sensor testing system.29-37 Anterior tibial translation was also measured in
response to a combined rotatory load of 10-Nm valgus torque and 5-Nm internal tibial torque with 20 N of compression (all remaining off-axis loads minimized) at 0 , 15 , 30 , 45 , 60 , and 90 of flexion. This load was chosen to simulate a pivot-shift test, and a similar load has been used in previous studies.29,31-38 These 2 quasistatic loading conditions were grouped and tested in 3 different, randomized flexion-angle orders. Kinematic testing also consisted of a dynamic loading condition, a novel, robotically controlled pivot-shift maneuver developed to more accurately reflect the clinical examination.39 This loading condition was developed by instrumenting an ACL-deficient cadaveric knee with optical tracking markers and a load cell. An experienced clinician performed a pivot-shift test, and the measured kinetics and kinematics were transformed to the anatomic reference frames of Grood and Suntay.40 The resulting dynamic trajectories of posterior tibial translation (PTT) versus flexion angle provided a distinctive profile of the ACL-transected condition in which the flexion angles represented the inflection points at the beginning and end of tibial reduction. In our study, the differences in PTT and external tibial rotation due to the dynamic pivot shift were calculated based on the kinematic data corresponding to the start and end flexion angles of tibial reduction. Because there was no significant tibial reduction (pivot shift) in the ACL-intact or indirect insertionedebridement conditions, the start and end angles from the ACLtransected condition were applied to the intact and debridement conditions for each respective specimen to calculate PTT and external tibial rotation differences among all 3 conditions. Debridement of Indirect Femoral Insertion After kinematic testing in the ACL-intact condition, superior traction was applied to the midsubstance of the ACL with the knee flexed to 90 . This prevented differential tensioning of the anteromedial and posterolateral bundles while allowing for easy differentiation between the indirect and direct fibers both visually and by palpation with an arthroscopic probe (Fig 2, Video 1, available at www.arthroscopyjournal.org). Debridement of the fibrous, indirect insertion was carried out with a 4.5-mm full-radius shaver under direct visualization until firm resistance from the fixed attachment of the direct insertion was reached. Care was taken to debride the entire indirect insertion, including its most shallow and deep extents. In our specimens this consistently yielded 5 to 6 mm of bare bone from the central, inferior aspect of the direct insertion to the inferior cartilage margin with a crescent shape analogous to previous studies detailing the architecture of the indirect insertion (Fig 3).15-17 An arthroscopic probe with a 5-mm tip was used to measure distances.
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Fig 2. (A) Arthroscopic view of the femoral ACL attachment in a left knee through the lateral portal. Tension was applied to the undersurface of the ACL, and the direct insertion fibers are easily visualized superior to the indirect insertion. (B) A close-up view of the femoral ACL attachment shows a clear distinction between the direct and indirect insertions (arrows).
In the final step the ACL was completely transected and kinematic testing was repeated. Statistics The effect of ACL debridement on anterior tibial translation was assessed by use of condition (ACL intact, indirect insertion debridement, and ACL transection) by knee flexion angle (0 , 15 , 30 , 45 , 60 , and 90 ). A repeated-measures analysis of variance (ANOVA) was performed on the average of 3 separate, randomized trials. Separate ANOVAs were performed for trials with an anteriorly directed force on the tibia and trials with a combined valgus and internal rotation load on the tibia. Because we hypothesized that debridement of the indirect insertion would not affect anterior tibial displacement (negative finding), pairwise comparisons were not adjusted for multiple comparisons. This increased the risk of a type I error but minimized the risk of a type II error. Thus, the level of significance was set at P < .05 (with mean standard deviation reported in the text and displayed in the
figures). The 95% confidence intervals (95% CIs) are reported for changes in anterior tibial displacement with debridement and transection. Tibiofemoral position in 6 of motion was recorded during the pivot-shift maneuver. The knee flexion angles at the start and end of tibial reduction were identified in the ACL-transected condition. These tibiofemoral positions were then identified at the same knee flexion angles for the intact and debridement conditions. Differences in tibiofemoral translations and rotations among the 3 conditions (ACL intact, indirect insertion debridement, and ACL transected) were assessed using repeated-measures ANOVA. The data for the ACL-deficient condition were excluded for 2 specimens that showed progressively higher anterior tibial translation with each trial. It was determined that secondary restraints were disrupted by the application of the 134-N load during the first of the 3 trials in the ACL-transected condition. Thus, full factorial analyses were performed on a sample of 7 specimens, and ACL-intact versus indirect insertionedebridement analyses were performed on all 9 specimens. On the basis of pilot data, it was estimated that 9 specimens would be required to detect a 1.4-mm difference in anterior tibial translation between intact and debridement conditions with 80% power at an a level of .05.
Results
Fig 3. Arthroscopic view of a right knee after debridement of the indirect insertion. A 5-mm probe is positioned at the central, inferior aspect of the direct insertion.
Knee Kinematics in Response to 134-N Anterior Tibial Force Anterior tibial translation in response to a 134-N anteriorly directed force was dependent on knee flexion angle and ACL condition (condition by angle, P ¼ .0001) (Fig 4). Debridement of the indirect insertion significantly increased tibial translation by 0.37 0.24 mm at 0 (95% CI, 0.18 to 0.55 mm; P ¼ .002) and by 0.16 0.19 mm at 15 (95% CI, 0.02 to 0.31 mm; P ¼ .033). At 0 , this increase was less than 1 mm for all 9 specimens, and at 15 , it was less than 0.5 mm for all
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Fig 4. Anterior tibial translation with an anteriorly directed 134-N force on the tibia. P ¼ .0001 for interaction of Condition (ACL intact, indirect insertion debrided, or ACL deficient) Knee flexion angle (6 angles). The “Results” section contains additional details. Data are shown as mean standard deviation.
9 specimens. There was no significant effect of debridement at any of the other test angles. ACL transection increased anterior tibial translation in response to a 134-N anteriorly directed force at all knee flexion angles (P ¼ .0001) compared with the ACL-intact condition (average increase, 9.20 1.30 mm; 95% CI, 8.00 to 10.40 mm; maximum increase, 11.26 1.15 mm at 15 ; 95% CI, 10.19 to 12.32 mm) and compared with the debridement condition (average increase, 9.03 1.22 mm; 95% CI, 7.90 to 10.15 mm; maximum increase, 11.04 1.08 mm at 15 ; 95% CI, 10.04 to 12.04 mm). The relative contribution of the indirect insertion to restraining anterior tibial translation in response to an anteriorly directed force on the tibia was computed by dividing the intact versus debridement difference by the intact versus transected difference. For example, if the difference in anterior tibial translation between the ACL-intact condition and the debridement condition was 1 mm and the difference between the ACL-intact and ACL-transected conditions was 10 mm, then the indirect fibers would be contributing 10% to restraint of the tibia (1/10 ¼ 0.1). By use of this computation, the indirect fibers contributed 5.5% to restraint of the tibia at 0 and 1.9% at 15 . Knee Kinematics in Response to Combined Rotatory Load Anterior tibial translation in response to a combined valgus and internal tibial rotation torque was similarly dependent on knee flexion angle and ACL condition (condition by angle, P ¼ .0001) (Fig 5). Debridement of the indirect insertion significantly increased tibial translation by 0.17 0.11 mm at 0 (95% CI, 0.09 to
0.25 mm; P ¼ .001) with no effect at other test angles. At 0 , this increase was less than 0.3 mm for all 9 specimens. ACL transection increased anterior tibial translation in response to a combined internal rotation and valgus force at 0 (P ¼ .0001), 15 (P ¼ .0001), 30 (P ¼ .001), and 45 (P ¼ .015) of knee flexion compared with the ACL-intact condition (average increase, 2.49 0.74 mm; 95% CI, 1.81 to 3.18 mm; maximum increase, 4.48 0.85 mm at 15 ; 95% CI, 3.68 to 5.26 mm). The combined loading condition showed similar differences when comparing transection with the debridement condition (average increase, 2.46 0.76 mm; 95% CI, 1.76 to 3.16 mm; maximum increase, 4.44 0.84 mm at 15 ; 95% CI, 3.66 to 5.22 mm; P ¼ .0001 at 0 , P ¼ .0001 at 15 , P ¼ .001 at 30 , and P ¼ .018 at 45 ). The relative contribution of the indirect fibers to restraining anterior tibial translation in response to a combined internal rotation and valgus force was 4.8% at 0 , 0.8% at 15 , 0% at 30 , and 2.7% at 45 . Knee Kinematics in Response to Simulated Pivot Shift During the pivot-shift maneuver, the start of tibial reduction was detected at 22 8 and was complete by 33 5 of knee flexion. During this maneuver in the ACL-transected condition, the tibia translated 12.79 3.22 mm posteriorly and rotated 17.60 4.30 externally (Fig 6). These values were significantly higher than for the ACL-intact and debridement conditions (P ¼ .0001) (Fig 6). For the ACL-intact and debridement conditions, posterior translation was 1.29 1.34 mm and 1.28 1.34 mm, respectively,
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Fig 5. Anterior tibial translation with combined internal rotation (IR)evalgus force on the tibia. P ¼ .0001 for the interaction of Condition (ACL intact, indirect insertion debrided, or ACL deficient) Knee flexion angle (6 angles). Data are shown as mean standard deviation.
whereas external tibial rotation was 1.54 1.61 and 1.54 1.47 , respectively. Neither posterior translation nor external tibial rotation was different between the intact and indirect insertionedebridement conditions (P ¼ .68, P ¼ .99), with no difference exceeding 0.15 mm or 0.5 . Medial-lateral translation, inferior-superior translation, and varus-valgus rotation were also significantly increased in the ACL-deficient condition versus the ACL-intact and debridement conditions (all P < .001). However, there were no differences in any rotations or translations between the ACL-intact and debridement conditions (all P > .15).
Fig 6. Posterior tibial translation and external tibial rotation during the reduction phase of a simulated pivot shift. P ¼ .0001 for effect of ACL status. The ACL-deficient condition showed greater translation and rotation than the intact and indirect insertionedebrided conditions (P ¼ .0001). There was no difference between the ACL-intact and indirect insertionedebrided conditions for posterior translation (P ¼ .68) and for external rotation (P ¼ .99). Data are shown as mean standard deviation.
Measurement Variability Over the 1,241 distinct quasi-static loading conditions, the root-mean-square errors between the prescribed and actual loads were 0.4 N, 0.7 N, 2.2 N, 0.05 Nm, and 0.02 Nm in the lateral, anterior, superior, valgus, and internal rotation degrees of freedom.
Discussion The results of this study support our hypothesis that the indirect insertion plays a minimal biomechanical role in knee kinematics when compared with the ACLintact condition. We found that the indirect insertion contributed less than 1 mm to restraint of tibial
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Fig 7. (A) A slightly superiorly positioned tunnel, in which the central, inferior aspect is 5 to 6 mm above the inferior cartilage margin (doubleheaded arrow), may be more optimal because it will position the soft-tissue aspect of the graft (a BPTB graft seen here) over the direct insertion and superior to the indirect insertion (green semicircle), as shown in (B). Extreme lowering of the femoral tunnel may place graft material within the biomechanically insignificant, indirect insertion. (AM, anteromedial; PL, posterolateral.)
translation and less than 0.5 to restraint of tibial rotation. We believe this is important because previous biomechanical studies, in contrast, have focused solely on analyzing the kinematic contributions of the anteromedial and posterolateral bundles of the ACL. Recent trends in ACL reconstruction have focused on restoring the anatomy of the native ACL attachment sites. To this end, femoral tunnel placement has evolved from the non-anatomic position, located high in the notch, to a lower position located within the ACL footprint, with the inferior aspect of the femoral tunnel at times abutting the inferior cartilage margin.12-14 Markolf et al.,41 however, have shown that a low posterolateral bundle produced ACL graft tensions that were much higher than the native ACL and could potentially lead to graft stretching. In addition, the Danish ACL registry reported that the revision rate for failed reconstruction with the anteromedial portal technique, which presumably allows for an inferiorly placed femoral tunnel, is 2 times greater than that for the transtibial technique.42 Although ACL reconstruction should restore the anatomy of the native ACL femoral footprint, Howell et al.43 advise that extreme lowering of the femoral tunnel should be met with caution until there is some agreement on intended targets, tunnel placement, and aperture morphology for boneepatellar tendonebone (BPTB) and hamstring grafts. We question the concept of maximum footprint coverage as a proposed goal of ACL reconstruction because our results suggest that the indirect insertion of the ACL plays a much less significant role when compared with the direct insertion.26,27 Furthermore, when a BPTB graft is used, the ability to provide maximum coverage of the entire native ACL footprint, including the direct and indirect insertions, is limited because of the smaller cross-sectional area of the softtissue portion of the graft when compared with a
doubled or quadrupled hamstring graft.44 The results of our study also question the recent trend of extreme lowering of the femoral tunnel. With use of a hamstring graft, an inferiorly placed tunnel may place the majority of graft material within the area of the biomechanically insignificant, indirect insertion. This concern is also present when using a BPTB graft because an inferiorly positioned tunnel will place the eccentrically positioned, soft-tissue aspect of the graft within the area of the indirect insertion (Fig 7). Limitations The generalizability of the current results may be limited because only 9 specimens were used for comparison between the ACL-intact and indirect insertionedebridement conditions. However, the results were remarkably consistent between specimens as indicated by the fact that mean differences of less than 0.5 mm between test conditions reached statistical significance. Thus, the ability to detect statistical significance was better than the pilot data indicated and far in excess of what might be deemed clinically significant. The robotic musculoskeletal simulations used here have previously been validated.28,39 Another limitation of our study is the ability to differentiate between the direct and indirect insertion fibers during the debridement procedure. Although this differentiation was performed on a macroscopic level, we believe the direct and indirect fibers can be readily delineated visually and tactilely after applying superior traction to the midsubstance of the ACL (Fig 2, Video 1, available at www.arthroscopyjournal.org). In addition, debridement of the indirect insertion was carried out until the firm, taut fibers of the direct insertion were directly visualized and palpated. This consistently yielded 5 to 6 mm of bare bone from the central, inferior aspect of the direct insertion to the inferior articular
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margin, with extension to the shallowest and deepest margins of the indirect insertion (Fig 3, Video 1, available at www.arthroscopyjournal.org). This area correlates to previous studies which have detailed the crosssectional area of the indirect insertion.15-17 An additional limitation was that the cross-sectional area of indirect insertion debridement was not measured.
Conclusions The indirect femoral ACL insertion contributes minimally to restraint of tibial translation and rotation.
Acknowledgment The authors thank Tara Bonner, M.S., for her support with the specimen preparation and robotic testing.
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Importance of femoral tunnel location on knee kinematics. Am J Sports Med 2008;36:678-685. Zantop T, Herbort M, Raschke MJ, Fu FH, Petersen W. The role of the anteromedial and posterolateral bundles of the anterior cruciate ligament in anterior tibial translation and internal rotation. Am J Sports Med 2007;35:223-227. Zantop T, Lenschow S, Lemburg T, Weimann A, Petersen W. Soft-tissue graft fixation in posterior cruciate ligament reconstruction: Evaluation of the effect of tibial insertion site on joint kinematics and in situ forces using a robotic/UFS testing system. Arch Orthop Trauma Surg 2004;124:614-620. Kanamori A, Woo SL, Ma CB, et al. The forces in the anterior cruciate ligament and knee kinematics during a simulated pivot shift test: A human cadaveric study using robotic technology. Arthroscopy 2000;16:633-639. Dumpe J, Saluan P, Bonner TF, Colbrunn RW, Barsoum WK. A novel robotic pivot shift that more reflects the clinical exam. Presented at the 54th Annual Meeting of the Society of Military Orthopaedic Surgeons, Naples, FL, December 2012. Grood ES, Suntay WJ. A joint coordinate system for the clinical description of three-dimensional motions: Application to the knee. J Biomech Eng 1983;105:136-144. Markolf KL, Park S, Jackson SR, McAllister DR. Anteriorposterior and rotatory stability of single and doublebundle anterior cruciate ligament reconstructions. J Bone Joint Surg Am 2009;91:107-118. Wagner LR, Thillemann TM, Mehnert F, Pederson A, Lind M. Increased risk of ACL revision after anteromedial compared with transtibial technique for femoral drillhole placement during ACL reconstruction. Result from the Danish registry of knee ligament reconstruction. Knee Surg Sports Traumatol Arthrosc 2012;20:1-3 (suppl 1). Howell SM, Hull M, McAllister D. Be sensible and cautious about criticizing tunnel placement in ACL reconstruction. J Bone Joint Surg Am 2012;94:e133. Iriuchishima T, Shirakura K, Yorifuji H, Aizawa S, Fu FH. Size comparison of ACL footprint and reconstructed auto graft. Knee Surg Sports Traumatol Arthrosc 2013;21:797-803.
Kinematic Analysis of the Indirect Femoral Insertion of the Anterior Cruciate Ligament: Implications for Anatomic Femoral Tunnel Placement Neil P. Pathare, M.D., Stephen J. Nicholas, M.D., Robb Colbrunn, Ph.D., and Malachy P. McHugh, Ph.D.
Purpose: To determine the effect of debriding the indirect insertion component of the femoral anterior cruciate ligament (ACL) attachment on tibiofemoral kinematics when compared with the intact knee. Methods: Knee kinematics were measured in 9 cadaveric knees with the ACL intact, after indirect insertion debridement, and after ACL transection. Three loading conditions were used: (1) a 134-N anterior tibial load, (2) a combined 10-Nm valgus and 5-Nm internal tibial torque, and (3) a simulated robotic pivot shift. Anterior tibial translation (ATT) was recorded in response to anterior and combined loads at 0 , 15 , 30 , 45 , 60 , and 90 of flexion. Posterior tibial translation and external tibial rotation were recorded during the simulated pivot shift. Results: With an anterior load, indirect insertion debridement increased ATT by 0.37 0.24 mm at 0 (P ¼ .002) and by 0.16 0.19 mm at 15 (P ¼ .033; increases <1 mm in all specimens). ACL transection increased ATT in response to an anterior load (P ¼ .0001) with maximum effect at 15 compared with the intact and debrided states (11.26 1.15 mm and 11.04 1.08 mm, respectively). With a combined load, indirect insertion debridement increased ATT by 0.17 0.11 mm at 0 (P ¼ .001; increases <0.3 mm in all specimens) with no effect at other angles. ACL transection increased ATT in response to a combined load (P ¼ .001) with maximum effect at 15 (4.45 0.85 mm v ACL intact and 4.44 0.84 mm v debrided indirect insertion). In the ACL intact condition, the pivot shift produced 1.29 1.34 mm of posterior tibial translation and 1.54 1.61 of external tibial rotation, as compared with 1.28 1.34 mm and 1.54 1.47 , respectively, after debridement (P ¼ .68 and P ¼ .99, respectively) and 12.79 3.22 mm and 17.60 4.30 , respectively, after ACL transection (P ¼ .0001). Conclusions: The indirect femoral ACL insertion contributes minimally to restraint of tibial translation and rotation. Clinical Relevance: Femoral tunnel positioning for anatomic ACL reconstruction should aim to recreate the biomechanically significant direct insertion.
T
he femoral anterior cruciate ligament (ACL) footprint has been described extensively in the literature.1-10 Despite this, controversy surrounding ideal femoral tunnel placement for ACL reconstruction continues. A recent study involving surgeons associated with the MOON (Multicenter Orthopaedic Outcomes Network) group showed that they do not currently
From Nicholas Institute of Sports Medicine and Athletic Trauma, Lenox Hill Hospital (N.P.P., S.J.N., M.P.M.), New York, New York; and the Department of Biomedical Engineering, Lerner Research Institute, and Orthopaedic Research Center, Cleveland Clinic (R.C.), Cleveland, Ohio, U.S.A. The authors report that they have no conflicts of interest in the authorship and publication of this article. Received February 13, 2014; accepted July 17, 2014. Address correspondence to Neil P. Pathare, M.D., Nicholas Institute of Sports Medicine and Athletic Trauma, Lenox Hill Hospital, 100 E 77th St, Second Floor, New York, NY 10075, U.S.A. E-mail: [email protected] Ó 2014 by the Arthroscopy Association of North America 0749-8063/14122/$36.00 http://dx.doi.org/10.1016/j.arthro.2014.07.017
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uniformly agree on ideal single-bundle tunnel placement.11 Trends in ACL reconstruction have been toward anatomic reconstruction, which aims to restore the location of the native anteromedial and posterolateral bundle insertion sites. The trend has thus been to drill femoral tunnels lower on the wall of the intercondylar notch, such that the inferior-most aspect of the tunnel sometimes even abuts the inferior cartilage margin.12-14 Macroscopic studies have reported that the femoral ACL attachment site is quite broad with variable size and shape, ranging from 93 mm2 to 200 mm2 (Fig 1A).1-10 More detailed histologic and immunohistologic analyses of the insertion site indicate that the broad ACL attachment is composed of both direct and indirect collagenous insertions (Fig 1B).15,16 The direct insertion has been shown to be a narrow, linear band extending up to the shallow (distal) articular cartilage margin. The central, inferior aspect of the direct insertion is consistently located approximately 5 to 6 mm
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Fig 1. Macroscopic view of the femoral ACL insertion in a left knee at 90 of flexion. (A) Arthroscopic nomenclature is followed by anatomic nomenclature in parentheses. The dashed line depicts the broad ACL attachment site. (B) The purple (anteromedial bundle) and blue (posterolateral bundle) rectangles depict the direct insertion. The red semicircle depicts the indirect insertion. (C) The double-headed arrow depicts the distance from the central, inferior aspect of the direct insertion to the inferior cartilage margin.
superior to the inferior cartilage margin (Fig 1C).15-17 The direct insertion represents a firm and fixed attachment and is considered the most biomechanically relevant portion of a ligament attachment because it allows for a gradual force distribution from ligamentous tissue to noncalcified fibrocartilage, calcified fibrocartilage, and finally, bone.15,16,18-20 The indirect insertion has been shown to consist of coarse fibrous tissue, type I collagen, and synovium and directly attaches to bone without a clear transition zone.15,16,21,22 The indirect insertion has been shown to be a crescentshaped area located between the inferior cartilage margin and the direct insertion (Fig 1B).15-17 The indirect insertion plays a lesser biomechanical role, acting as a dynamic anchorage of soft tissue to bone allowing certain shear movements.16,19,23 Recently, it has been suggested that ACL reconstruction should be tailored to each individual in an effort to provide maximum coverage of the native femoral ACL attachment site.24-26 As described by Siebold,27 the concept of maximum footprint coverage is based on the hypothesis that the restored biomechanical envelope of the knee is a function of the amount of reconstructed ACL attachment site area. The concept of maximum footprint coverage aims at filling both the direct and indirect insertion sites with graft material, with the assumption that both play a role in the kinematic profile of the ACL. If this concept holds true, it must be
assumed that both the indirect and direct insertion components of the femoral ACL attachment site play a role in the biomechanical profile of the ACL. To our knowledge, no biomechanical studies have delineated the kinematic contributions of the indirect and direct femoral ACL insertions. The purpose of this biomechanical study was to determine the effect of debriding the indirect insertion component of the femoral ACL attachment in response to an anterior tibial load, a combined rotatory load, and a dynamic pivot-shift load when compared with the intact knee. We hypothesized that debridement of the indirect insertion would not affect tibial translation or rotation during kinematic testing.
Methods Specimens In this study 11 fresh-frozen human cadaveric knees (6 female and 5 male knees) were used. The mean donor age was 55 4.3 years (range, 50 to 60 years). Before testing, the knees were stored at 20 C and thawed for 24 hours at room temperature. Once thawed, the knees were clinically examined. Two specimens were excluded because they were found to have a positive posterolateral drawer test with greater than 2 cm of posterolateral translation of the tibia. The remaining specimens underwent diagnostic arthroscopy with no evidence of
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damage to the cartilage surfaces, menisci, or ligamentous structures. Arthroscopic nomenclature is referenced with the knee in 90 of flexion throughout the study (Fig 1A). The femur and the tibia were cut approximately 20 cm from the joint line. The proximal and distal 7 cm of the femur and tibia were secured within thick-walled aluminum cylinders with Wood’s metal. The tibia was fixed to a 6 degrees of freedom (DOF) force/torque sensor (SI-1500-240; ATI Industrial Automation, Apex, NC), which was mounted rigidly to a crossbeam. The femur was similarly placed in a custom mount on a robotic manipulator (Rotopod R2000; Parallel Robotics Systems, Hampton, NH). To prevent exsiccation of the soft tissues, the skin of the specimens was left intact, and the specimens were moistened with normal saline solution throughout the testing procedure. Robotic/Universal Force-Moment Sensor Testing System A knee joint coordinate system was established using a MicroScribe coordinate measuring machine (Immersion, San Jose, CA). The memory-lock function of the testing system allowed the specimen to be removed from the robot for surgery and precisely returned (repeatability of <10 mm) for further testing. This robotic musculoskeletal simulation testing system has been previously validated.28 After a neutral position (position of the knee with a 20-N compressive load and zero translational or rotational forces) at full extension was established, the knee was flexed from 0 to 90 with 100 N of compression and the system found the positions of the knee that minimized all external forces and moments applied to the joint throughout the range of flexion. By use of the kinematic output during this passive flexion testing, the digitized coordinate system was then optimized to minimize changes in anterior and superior translations throughout the range of flexion. The resulting positions were accepted as starting positions for the application of the quasi-static external loading conditions. Testing Protocol Kinematic testing consisted of a series of quasi-static and dynamic loading conditions, which were controlled by a real-time force feedback controller. The 2 quasi-static loading conditions were an external anterior tibial load and a combined rotatory load. To simulate a clinical examination using the KT-1000 arthrometer (MEDmetric, San Diego, CA), anterior tibial translation was measured in response to a 134-N anterior tibial load with 20 N of compression (with all remaining off-axis loads minimized) at 0 , 15 , 30 , 45 , 60 , and 90 of flexion. An anterior load of 134 N has been used previously when evaluating injuries to the ACL using a robotic/universal force-moment sensor testing system.29-37 Anterior tibial translation was also measured in
response to a combined rotatory load of 10-Nm valgus torque and 5-Nm internal tibial torque with 20 N of compression (all remaining off-axis loads minimized) at 0 , 15 , 30 , 45 , 60 , and 90 of flexion. This load was chosen to simulate a pivot-shift test, and a similar load has been used in previous studies.29,31-38 These 2 quasistatic loading conditions were grouped and tested in 3 different, randomized flexion-angle orders. Kinematic testing also consisted of a dynamic loading condition, a novel, robotically controlled pivot-shift maneuver developed to more accurately reflect the clinical examination.39 This loading condition was developed by instrumenting an ACL-deficient cadaveric knee with optical tracking markers and a load cell. An experienced clinician performed a pivot-shift test, and the measured kinetics and kinematics were transformed to the anatomic reference frames of Grood and Suntay.40 The resulting dynamic trajectories of posterior tibial translation (PTT) versus flexion angle provided a distinctive profile of the ACL-transected condition in which the flexion angles represented the inflection points at the beginning and end of tibial reduction. In our study, the differences in PTT and external tibial rotation due to the dynamic pivot shift were calculated based on the kinematic data corresponding to the start and end flexion angles of tibial reduction. Because there was no significant tibial reduction (pivot shift) in the ACL-intact or indirect insertionedebridement conditions, the start and end angles from the ACLtransected condition were applied to the intact and debridement conditions for each respective specimen to calculate PTT and external tibial rotation differences among all 3 conditions. Debridement of Indirect Femoral Insertion After kinematic testing in the ACL-intact condition, superior traction was applied to the midsubstance of the ACL with the knee flexed to 90 . This prevented differential tensioning of the anteromedial and posterolateral bundles while allowing for easy differentiation between the indirect and direct fibers both visually and by palpation with an arthroscopic probe (Fig 2, Video 1, available at www.arthroscopyjournal.org). Debridement of the fibrous, indirect insertion was carried out with a 4.5-mm full-radius shaver under direct visualization until firm resistance from the fixed attachment of the direct insertion was reached. Care was taken to debride the entire indirect insertion, including its most shallow and deep extents. In our specimens this consistently yielded 5 to 6 mm of bare bone from the central, inferior aspect of the direct insertion to the inferior cartilage margin with a crescent shape analogous to previous studies detailing the architecture of the indirect insertion (Fig 3).15-17 An arthroscopic probe with a 5-mm tip was used to measure distances.
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Fig 2. (A) Arthroscopic view of the femoral ACL attachment in a left knee through the lateral portal. Tension was applied to the undersurface of the ACL, and the direct insertion fibers are easily visualized superior to the indirect insertion. (B) A close-up view of the femoral ACL attachment shows a clear distinction between the direct and indirect insertions (arrows).
In the final step the ACL was completely transected and kinematic testing was repeated. Statistics The effect of ACL debridement on anterior tibial translation was assessed by use of condition (ACL intact, indirect insertion debridement, and ACL transection) by knee flexion angle (0 , 15 , 30 , 45 , 60 , and 90 ). A repeated-measures analysis of variance (ANOVA) was performed on the average of 3 separate, randomized trials. Separate ANOVAs were performed for trials with an anteriorly directed force on the tibia and trials with a combined valgus and internal rotation load on the tibia. Because we hypothesized that debridement of the indirect insertion would not affect anterior tibial displacement (negative finding), pairwise comparisons were not adjusted for multiple comparisons. This increased the risk of a type I error but minimized the risk of a type II error. Thus, the level of significance was set at P < .05 (with mean standard deviation reported in the text and displayed in the
figures). The 95% confidence intervals (95% CIs) are reported for changes in anterior tibial displacement with debridement and transection. Tibiofemoral position in 6 of motion was recorded during the pivot-shift maneuver. The knee flexion angles at the start and end of tibial reduction were identified in the ACL-transected condition. These tibiofemoral positions were then identified at the same knee flexion angles for the intact and debridement conditions. Differences in tibiofemoral translations and rotations among the 3 conditions (ACL intact, indirect insertion debridement, and ACL transected) were assessed using repeated-measures ANOVA. The data for the ACL-deficient condition were excluded for 2 specimens that showed progressively higher anterior tibial translation with each trial. It was determined that secondary restraints were disrupted by the application of the 134-N load during the first of the 3 trials in the ACL-transected condition. Thus, full factorial analyses were performed on a sample of 7 specimens, and ACL-intact versus indirect insertionedebridement analyses were performed on all 9 specimens. On the basis of pilot data, it was estimated that 9 specimens would be required to detect a 1.4-mm difference in anterior tibial translation between intact and debridement conditions with 80% power at an a level of .05.
Results
Fig 3. Arthroscopic view of a right knee after debridement of the indirect insertion. A 5-mm probe is positioned at the central, inferior aspect of the direct insertion.
Knee Kinematics in Response to 134-N Anterior Tibial Force Anterior tibial translation in response to a 134-N anteriorly directed force was dependent on knee flexion angle and ACL condition (condition by angle, P ¼ .0001) (Fig 4). Debridement of the indirect insertion significantly increased tibial translation by 0.37 0.24 mm at 0 (95% CI, 0.18 to 0.55 mm; P ¼ .002) and by 0.16 0.19 mm at 15 (95% CI, 0.02 to 0.31 mm; P ¼ .033). At 0 , this increase was less than 1 mm for all 9 specimens, and at 15 , it was less than 0.5 mm for all
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Fig 4. Anterior tibial translation with an anteriorly directed 134-N force on the tibia. P ¼ .0001 for interaction of Condition (ACL intact, indirect insertion debrided, or ACL deficient) Knee flexion angle (6 angles). The “Results” section contains additional details. Data are shown as mean standard deviation.
9 specimens. There was no significant effect of debridement at any of the other test angles. ACL transection increased anterior tibial translation in response to a 134-N anteriorly directed force at all knee flexion angles (P ¼ .0001) compared with the ACL-intact condition (average increase, 9.20 1.30 mm; 95% CI, 8.00 to 10.40 mm; maximum increase, 11.26 1.15 mm at 15 ; 95% CI, 10.19 to 12.32 mm) and compared with the debridement condition (average increase, 9.03 1.22 mm; 95% CI, 7.90 to 10.15 mm; maximum increase, 11.04 1.08 mm at 15 ; 95% CI, 10.04 to 12.04 mm). The relative contribution of the indirect insertion to restraining anterior tibial translation in response to an anteriorly directed force on the tibia was computed by dividing the intact versus debridement difference by the intact versus transected difference. For example, if the difference in anterior tibial translation between the ACL-intact condition and the debridement condition was 1 mm and the difference between the ACL-intact and ACL-transected conditions was 10 mm, then the indirect fibers would be contributing 10% to restraint of the tibia (1/10 ¼ 0.1). By use of this computation, the indirect fibers contributed 5.5% to restraint of the tibia at 0 and 1.9% at 15 . Knee Kinematics in Response to Combined Rotatory Load Anterior tibial translation in response to a combined valgus and internal tibial rotation torque was similarly dependent on knee flexion angle and ACL condition (condition by angle, P ¼ .0001) (Fig 5). Debridement of the indirect insertion significantly increased tibial translation by 0.17 0.11 mm at 0 (95% CI, 0.09 to
0.25 mm; P ¼ .001) with no effect at other test angles. At 0 , this increase was less than 0.3 mm for all 9 specimens. ACL transection increased anterior tibial translation in response to a combined internal rotation and valgus force at 0 (P ¼ .0001), 15 (P ¼ .0001), 30 (P ¼ .001), and 45 (P ¼ .015) of knee flexion compared with the ACL-intact condition (average increase, 2.49 0.74 mm; 95% CI, 1.81 to 3.18 mm; maximum increase, 4.48 0.85 mm at 15 ; 95% CI, 3.68 to 5.26 mm). The combined loading condition showed similar differences when comparing transection with the debridement condition (average increase, 2.46 0.76 mm; 95% CI, 1.76 to 3.16 mm; maximum increase, 4.44 0.84 mm at 15 ; 95% CI, 3.66 to 5.22 mm; P ¼ .0001 at 0 , P ¼ .0001 at 15 , P ¼ .001 at 30 , and P ¼ .018 at 45 ). The relative contribution of the indirect fibers to restraining anterior tibial translation in response to a combined internal rotation and valgus force was 4.8% at 0 , 0.8% at 15 , 0% at 30 , and 2.7% at 45 . Knee Kinematics in Response to Simulated Pivot Shift During the pivot-shift maneuver, the start of tibial reduction was detected at 22 8 and was complete by 33 5 of knee flexion. During this maneuver in the ACL-transected condition, the tibia translated 12.79 3.22 mm posteriorly and rotated 17.60 4.30 externally (Fig 6). These values were significantly higher than for the ACL-intact and debridement conditions (P ¼ .0001) (Fig 6). For the ACL-intact and debridement conditions, posterior translation was 1.29 1.34 mm and 1.28 1.34 mm, respectively,
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Fig 5. Anterior tibial translation with combined internal rotation (IR)evalgus force on the tibia. P ¼ .0001 for the interaction of Condition (ACL intact, indirect insertion debrided, or ACL deficient) Knee flexion angle (6 angles). Data are shown as mean standard deviation.
whereas external tibial rotation was 1.54 1.61 and 1.54 1.47 , respectively. Neither posterior translation nor external tibial rotation was different between the intact and indirect insertionedebridement conditions (P ¼ .68, P ¼ .99), with no difference exceeding 0.15 mm or 0.5 . Medial-lateral translation, inferior-superior translation, and varus-valgus rotation were also significantly increased in the ACL-deficient condition versus the ACL-intact and debridement conditions (all P < .001). However, there were no differences in any rotations or translations between the ACL-intact and debridement conditions (all P > .15).
Fig 6. Posterior tibial translation and external tibial rotation during the reduction phase of a simulated pivot shift. P ¼ .0001 for effect of ACL status. The ACL-deficient condition showed greater translation and rotation than the intact and indirect insertionedebrided conditions (P ¼ .0001). There was no difference between the ACL-intact and indirect insertionedebrided conditions for posterior translation (P ¼ .68) and for external rotation (P ¼ .99). Data are shown as mean standard deviation.
Measurement Variability Over the 1,241 distinct quasi-static loading conditions, the root-mean-square errors between the prescribed and actual loads were 0.4 N, 0.7 N, 2.2 N, 0.05 Nm, and 0.02 Nm in the lateral, anterior, superior, valgus, and internal rotation degrees of freedom.
Discussion The results of this study support our hypothesis that the indirect insertion plays a minimal biomechanical role in knee kinematics when compared with the ACLintact condition. We found that the indirect insertion contributed less than 1 mm to restraint of tibial
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Fig 7. (A) A slightly superiorly positioned tunnel, in which the central, inferior aspect is 5 to 6 mm above the inferior cartilage margin (doubleheaded arrow), may be more optimal because it will position the soft-tissue aspect of the graft (a BPTB graft seen here) over the direct insertion and superior to the indirect insertion (green semicircle), as shown in (B). Extreme lowering of the femoral tunnel may place graft material within the biomechanically insignificant, indirect insertion. (AM, anteromedial; PL, posterolateral.)
translation and less than 0.5 to restraint of tibial rotation. We believe this is important because previous biomechanical studies, in contrast, have focused solely on analyzing the kinematic contributions of the anteromedial and posterolateral bundles of the ACL. Recent trends in ACL reconstruction have focused on restoring the anatomy of the native ACL attachment sites. To this end, femoral tunnel placement has evolved from the non-anatomic position, located high in the notch, to a lower position located within the ACL footprint, with the inferior aspect of the femoral tunnel at times abutting the inferior cartilage margin.12-14 Markolf et al.,41 however, have shown that a low posterolateral bundle produced ACL graft tensions that were much higher than the native ACL and could potentially lead to graft stretching. In addition, the Danish ACL registry reported that the revision rate for failed reconstruction with the anteromedial portal technique, which presumably allows for an inferiorly placed femoral tunnel, is 2 times greater than that for the transtibial technique.42 Although ACL reconstruction should restore the anatomy of the native ACL femoral footprint, Howell et al.43 advise that extreme lowering of the femoral tunnel should be met with caution until there is some agreement on intended targets, tunnel placement, and aperture morphology for boneepatellar tendonebone (BPTB) and hamstring grafts. We question the concept of maximum footprint coverage as a proposed goal of ACL reconstruction because our results suggest that the indirect insertion of the ACL plays a much less significant role when compared with the direct insertion.26,27 Furthermore, when a BPTB graft is used, the ability to provide maximum coverage of the entire native ACL footprint, including the direct and indirect insertions, is limited because of the smaller cross-sectional area of the softtissue portion of the graft when compared with a
doubled or quadrupled hamstring graft.44 The results of our study also question the recent trend of extreme lowering of the femoral tunnel. With use of a hamstring graft, an inferiorly placed tunnel may place the majority of graft material within the area of the biomechanically insignificant, indirect insertion. This concern is also present when using a BPTB graft because an inferiorly positioned tunnel will place the eccentrically positioned, soft-tissue aspect of the graft within the area of the indirect insertion (Fig 7). Limitations The generalizability of the current results may be limited because only 9 specimens were used for comparison between the ACL-intact and indirect insertionedebridement conditions. However, the results were remarkably consistent between specimens as indicated by the fact that mean differences of less than 0.5 mm between test conditions reached statistical significance. Thus, the ability to detect statistical significance was better than the pilot data indicated and far in excess of what might be deemed clinically significant. The robotic musculoskeletal simulations used here have previously been validated.28,39 Another limitation of our study is the ability to differentiate between the direct and indirect insertion fibers during the debridement procedure. Although this differentiation was performed on a macroscopic level, we believe the direct and indirect fibers can be readily delineated visually and tactilely after applying superior traction to the midsubstance of the ACL (Fig 2, Video 1, available at www.arthroscopyjournal.org). In addition, debridement of the indirect insertion was carried out until the firm, taut fibers of the direct insertion were directly visualized and palpated. This consistently yielded 5 to 6 mm of bare bone from the central, inferior aspect of the direct insertion to the inferior articular
KINEMATICS OF THE INDIRECT FEMORAL INSERTION
margin, with extension to the shallowest and deepest margins of the indirect insertion (Fig 3, Video 1, available at www.arthroscopyjournal.org). This area correlates to previous studies which have detailed the crosssectional area of the indirect insertion.15-17 An additional limitation was that the cross-sectional area of indirect insertion debridement was not measured.
Conclusions The indirect femoral ACL insertion contributes minimally to restraint of tibial translation and rotation.
Acknowledgment The authors thank Tara Bonner, M.S., for her support with the specimen preparation and robotic testing.
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