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Transtibial Versus Anteromedial Portal Drilling for Anterior Cruciate Ligament Reconstruction: A Cadaveric Study of Femoral Tunnel Length and Obliquity
Transtibial Versus Anteromedial Portal Drilling for Anterior Cruciate Ligament Reconstruction: A Cadaveric Study of Femoral Tunnel Length and Obliquity Asheesh Bedi, M.D., Brad Raphael, M.D., Alex Maderazo, M.D., Helene Pavlov, M.D., and Riley J. Williams III, M.D.
Purpose: To compare the obliquity and length of femoral tunnels prepared with transtibial versus anteromedial portal drilling for anterior cruciate ligament (ACL) reconstruction and identify potential risks associated with the anteromedial portal reaming technique. Methods: We used 18 human cadaveric knees (9 matched pairs) without ACL injury or pre-existing arthritis for the study. Femoral tunnels for ACL reconstruction were prepared by either a transtibial (n ⫽ 6) or anteromedial portal (n ⫽ 12) technique. For the anteromedial portal technique, a guidewire was advanced through the medial portal in varying degrees of knee flexion (100° [n ⫽ 4], 110° [n ⫽ 4], or 120° [n ⫽ 4]) as measured with a goniometer. By use of a 6-mm femoral offset guide, two 6-mm femoral tunnels were reamed with the guide placed (1) as far posterior and lateral in the notch as possible and (2) as far medial and vertical in the notch as possible to define the range of maximal and minimal achievable coronal obliquity for each technique. All knees were imaged with high-resolution, 3-dimensional fluoroscopy to define (1) coronal tunnel obliquity relative to the lateral tibial plateau, (2) sagittal tunnel obliquity relative to the long axis of the femur, (3) intraosseous tunnel length, and (4) the presence of posterior cortical wall blowout. Data analysis was performed with a paired t-test and repeated-measures analysis of variance, with P ⬍ .05 defined as significant. Results: Preparation of a vertical tunnel was possible with both transtibial and anteromedial portal drilling. The maximal achievable coronal obliquity, however, was significantly better with an anteromedial portal compared with transtibial drilling. However, 7 of 36 tunnels (19.4%) showed violation of the posterior tunnel wall, and all of these cases occurred with the anteromedial portal drilling technique. In addition, 1 of 6 oblique femoral tunnels (16.7%) drilled with the transtibial technique and 5 of 12 oblique femoral tunnels (41.7%) drilled with the anteromedial portal had an intraosseous length less than 25 mm. Increasing knee flexion with anteromedial portal drilling was associated with a significant reduction in tunnel length, increase in coronal obliquity, increase in sagittal obliquity, and increased risk of posterior wall blowout (P ⬍ .05). Conclusions: The anteromedial portal technique allows for slightly greater femoral tunnel obliquity compared with transtibial drilling. However, there is a substantially increased risk of critically short tunnels (⬍25 mm) and posterior tunnel wall blowout when a conventional offset guide is used. Increasing knee flexion with anteromedial portal drilling allows for greater coronal obliquity of the femoral tunnel but is accompanied by a greater risk of critically short tunnels and posterior wall compromise. Clinical Relevance: Our findings provide insight into the potential risks and advantages of a transtibial versus an anteromedial femoral tunnel drilling technique in ACL reconstruction.
From the Sports Medicine and Shoulder Service (A.B., B.R., R.J.W.) and Department of Radiology and Imaging (A.M., H.P.), Hospital for Special Surgery, New York, New York, U.S.A. Supported by the Institute for Sports Medicine Research, Hospital for Special Surgery. The authors report no conflict of interest. Received June 29, 2009; accepted December 7, 2009. Address correspondence and reprint requests to Asheesh Bedi, M.D., Sports Medicine and Shoulder Service, Hospital for Special Surgery, New York, NY 10021, U.S.A. E-mail: [email protected] © 2010 by the Arthroscopy Association of North America 0749-8063/10/2603-9394$36.00/0 doi:10.1016/j.arthro.2009.12.006
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Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 26, No 3 (March), 2010: pp 342-350
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hereas sagittal graft alignment has long been recognized as critical for restoration of anteroposterior stability, coronal graft obliquity has only recently been recognized as a critical parameter to restore with anterior cruciate ligament (ACL) reconstruction. There has been an increased recognition of ACL reconstruction failure attributable to vertically oriented grafts in recent years. Although anterior tibial translation can be well controlled with isometric femoral positioning and a vertical graft orientation, patients often have residual rotational instability and a persistent pivot shift postoperatively that preclude their ability to return to previous level of athletic activity.1-4 The restoration of normal knee kinematics and improvement in tibial rotational control with greater femoral tunnel obliquity have been recently established in a number of biomechanical studies.1-4 The surgical techniques by which to achieve sufficient graft coronal obliquity, however, remain controversial. A number of authors have described technical modifications to the transtibial technique to improve femoral tunnel obliquity and restore the native femoral ACL footprint. Howell et al.1 recommended creating a tibial tunnel at a coronal angle of 65° to 70° to reduce loss of flexion and anterior laxity. Chhabra et al.5 provided guidelines for appropriate external landmarks and reported that a tibial starting point at the midpoint between the tibial tubercle and posteromedial corner achieved a coronal angle of approximately 70°. Golish et al.6 recently reported increased femoral tunnel obliquity using a transtibial drilling technique with a tibial tunnel starting point that encroaches on the anterior fibers of the medial collateral ligament (MCL). Some surgeons, however, have advocated independent drilling of the femoral tunnel for ACL reconstruction. Giron et al.7 reported an inability to restore the anatomic femoral origin of the ACL with transtibial drilling techniques despite technical modifications of coronal angle and starting position. Cha et al.8 recommended independent drilling of the femoral tunnel through a medial arthroscopic portal with the knee placed in hyperflexion. Although laboratory studies have reported favorable femoral tunnel obliquity by use of this technique, the risks associated with anteromedial (AM) portal drilling remain undefined. The purpose of this study was to compare the obliquity, length, and wall compromise of femoral tunnels prepared with transtibial versus AM portal drilling for ACL reconstruction. Furthermore, our goal was to define the dimensions and location of femoral tunnels prepared with an AM portal technique in varying
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degrees of knee flexion to define the potential risks or benefits associated with this technique. We hypothesized that a medial portal drilling technique would allow for preparation of a femoral socket with greater coronal obliquity compared with the transtibial technique but is accompanied by a greater risk of critically short femoral tunnels and posterior wall blowout. METHODS After institutional review board approval was obtained, 18 human cadaveric knees (9 matched pairs) without ACL injury or pre-existing arthritis were obtained for the study. Each femur was secured in a custom jig allowing free flexion of the knee. A 30° 3.5-mm arthroscope (Stryker, Warsaw, IN) was introduced into the knee through a standard anterolateral portal. Through an AM portal placed approximately 4 mm medial to the border of the patellar tendon, the ACL was arthroscopically resected and a notchplasty with minimal bone resection was performed with a 4.0-mm full-radius resector (Stryker) to allow direct visualization of the over-the-top position. Care was taken to ensure that the medial portal allowed direct approximation of the center of the femoral ACL footprint without risk of medial or lateral femoral condyle injury. Femoral tunnels were subsequently reamed by either a transtibial (n ⫽ 6) or AM portal (n ⫽ 12) technique. Matched pairs were used to minimize anatomic differences between specimens. This distribution allowed for the evaluation of the 2 primary study questions regarding (1) transtibial versus AM portal drilling femoral socket position and (2) the effect of increasing knee flexion on tunnel length and position with AM drilling. We therefore required double the number of knees for the AM portal group to isolate the effect of a variable flexion angle. For the transtibial technique, a commercial tibial tunnel ACL guide (Acufex Director; Smith & Nephew Arthroscopy, Andover, MA) was set at 50° to prepare a 10-mm tunnel. The intra-articular exit point of the guide pin was directed 5 mm lateral to the medial tibial spine and 3 to 4 mm posterior to the anterior horn of the lateral meniscus as described by Rue et al.9 A 6-cm incision was then created over the medial face of the tibia, and the pes anserinus was retracted inferiorly to define the tibial insertion of the MCL. The external starting point was placed at the anterior border of the MCL insertional fibers in all cases to allow for oblique orientation of the guide of approximately 60° from the horizontal. When satisfactory intra- and extra-articular starting points were confirmed, the
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guide pin was placed and over-reamed with a 10-mm cannulated acorn reamer (Fig 1). A commercially available 6-mm offset aimer (Arthrex, Naples, FL) was passed retrograde through the tunnel and placed in the over-the-top position on the posterior cortex of the femur. The angle of knee flexion with the transtibial technique was dictated by the offset guide resting on the back wall; it is not a technical variable that can be independently manipulated as with the AM portal technique. Two 6-mm femoral tunnels were subsequently reamed with the guide rotated (1) as far posterior and lateral in the notch as possible and (2) as far medial and vertical in the notch as possible to define the range of maximal and minimal achievable coronal obliquity (Fig 2). For the AM portal technique, a guidewire was advanced through the AM portal in varying degrees of knee flexion (100° [n ⫽ 4], 110° [n ⫽ 4], or 120° [n ⫽ 4]) as measured with a goniometer and maintained by a surgical assistant. By use of a 6-mm offset aimer, two 6-mm femoral tunnels were reamed with the guide rotated (1) as vertical in the notch as possible and (2) as oblique in the notch as possible to define the range of minimal and maximal achievable coronal obliquity (Fig 2). Care was taken to optimize medial
portal position to avoid any iatrogenic injury to the medial femoral condyle with reaming. All knees were subsequently imaged in multiple planes on a Philips Multidiagnost-Eleva Fluoroscopy System (Royal Philips; Eindhoven, The Netherlands) to generate high-resolution, 3-dimensional images of the prepared tunnels. ViewForum 4.1 integrated view and processing software (Royal Philips) was used to define (1) coronal tunnel obliquity, (2) sagittal tunnel obliquity, (3) intraosseous tunnel length, and (4) the presence of posterior cortical wall compromise for each tunnel. Coronal obliquity was defined by the angle subtended between the tunnel and a horizontal axis defined by the lateral tibial plateau. Clock-face description of coronal obliquity was defined by Rue et al.,9 with each 30° increment from the vertical axis (12 o’clock) equivalent to one clock-face position. Sagittal obliquity was defined by the angle subtended between the tunnel and longitudinal axis of the femur (Fig 3). Tunnel length was measured on selected images that were coplanar with the tunnel allowing for direct measure from intra-articular to extraarticular aperture. Posterior wall compromise was defined by any breach in the posterior cortical wall of the tunnel evident on axial computed tomography (CT) images. All knees were disarticulated and the femurs stripped of soft tissue to confirm anatomic relations of the tunnels identified on imaging. Data analysis was performed with a paired t-test and repeatedmeasures analysis of variance, with P ⬍ .05 defined as significant. RESULTS
FIGURE 1. Coronal image obtained with high-resolution, 3dimensional fluoroscopy of 10-mm tibial tunnel prepared by technique as described by data from Rue et al.9,15
Vertical femoral tunnels could be prepared with transtibial and AM portal drilling techniques (79.55° ⫾ 9.32° and 77.88° ⫾ 7.90°, respectively). The maximal achievable coronal obliquity, however, was significantly greater with an AM portal compared with transtibial drilling (46.85° ⫾ 6.89° v 54.08° ⫾ 7.17°, P ⬍ .05). In contrast, sagittal tunnel orientation was not significantly different between groups (Table 1). Of 36 tunnels, 7 (19.4%) showed compromise of the posterior tunnel wall on axial CT images (Fig 4). These occurred in 1 maximally vertical tunnel and 6 maximally oblique tunnels, all of which were drilled with an AM portal drilling technique (Table 2). The incidence of posterior wall compromise increased with higher degrees of knee flexion during AM portal drilling (Table 2). Mean tunnel length of the maximally vertical tunnel prepared with a transtibial technique was 47.9 ⫾ 8.23
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FIGURE 3. Sagittal obliquity was defined by the angle subtended between the tunnel and longitudinal axis (red circle) of the femur. M1 is the exit point of this femoral socket through the anterior cortex of the distal femur.
mm (range, 44.0 to 61.6 mm) compared with 53.0 ⫾ 8.2 mm (range, 40.7 to 65.8 mm) for the AM portal technique (P ⫽ .22). Mean tunnel length of the most oblique tunnel prepared with a transtibial technique was 28.7 ⫾ 3.4 mm (range, 23.9 to 32.0 mm) compared with 26.0 ⫾ 5.7 mm (range, 15.0 to 34.4 mm) for the AM technique (P ⫽ .30). However, 1 of 6 oblique femoral tunnels (16.7%) drilled with a transtibial technique compared with 5 of 12 oblique femoral tunnels (41.7%) drilled with the AM portal had an intraosseous length less than 25 mm (P ⬍ .05) (Fig 5). The influence of knee flexion on tunnel obliquity, length, and posterior wall compromise with AM portal drilling was assessed. Increasing knee flexion with AM portal drilling was associated with a significant, progressive reduction in tunnel length (30.93 ⫾ 2.63 mm and 21.30 ⫾ 4.80 mm for 100° and 120° of
4 ™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™ FIGURE 2. (A) Diagram showing maximally vertical and oblique 6-mm femoral tunnels prepared for each specimen, defining a range of potential coronal obliquity (represented by trapezoid between vertical and oblique femoral sockets). (B) Corresponding arthroscopic photo of intercondylar notch showing intra-articular aperture locations of these tunnels. (C) Notch view of a dissected femoral specimen showing defined range of coronal obliquity between vertical and maximally oblique femoral sockets (as marked by lines).
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A. BEDI ET AL. TABLE 1. Characterization of Femoral Socket Prepared With Transtibial Versus AM Portal Drilling: Comparison of Length, Obliquity, and Incidence of Posterior Wall Blowout of Femoral Tunnels
Vertical tunnel Transtibial AM portal Oblique tunnel Transtibial AM portal
Coronal Angle (°)
Sagittal Angle (°)
Length (mm)
Posterior Wall Blowout
79.6 ⫾ 9.3 77.9 ⫾ 7.9
41.4 ⫾ 13.5 37.1 ⫾ 7.9
47.9 ⫾ 8.2 53.0 ⫾ 8.2
0% 8.0%
54.1 ⫾ 7.17 45.9 ⫾ 6.9*
44.4 ⫾ 9.1 50.0 ⫾ 7.5
28.7 ⫾ 3.4 26.0 ⫾ 5.7
0% 50.0%*
*P ⬍ .05.
flexion, respectively) (Fig 5), increase in coronal obliquity (52.63° ⫾ 4.78° and 41.73° ⫾ 1.11° for 100° and 120° of flexion, respectively), increase in sagittal obliquity (51.00° ⫾ 4.56° and 43.83° ⫾ 6.24° for 100° and 120° of flexion, respectively), and increased risk of posterior wall blowout (25% and 75% for 100° and 120° of flexion, respectively) (P ⬍ .05) (Table 2). Inspection of the femoral tunnels after stripping of all soft tissues confirmed the tunnel lengths and obliquity as measured on CT images (Figs 5B and 6). On gross dissection, the exit of oblique femoral tunnels prepared through an AM portal technique showed proximity to the posterolateral (PL) articular cartilage margin of the knee and was less than 4 mm in 4
FIGURE 4. Axial image of a case of posterior wall blowout that occurred with AM portal drilling of an oblique tunnel. The incidence of posterior wall compromise increased with higher degrees of knee flexion during AM portal drilling.
specimens (Fig 6). However, no correlation between increasing flexion angle and proximity to the articular cartilage was identified. Frank penetration of the articular cartilage was not observed in any of the specimens. DISCUSSION The purpose of this study was to compare the obliquity, length, and wall compromise of femoral tunnels prepared with transtibial versus AM portal drilling for ACL reconstruction. Using CT analysis of tunnel position, we found that the AM portal technique allows for slightly greater femoral tunnel obliquity compared with transtibial drilling, but this is accompanied by an increased risk of critically short tunnels (⬍25 mm) and posterior tunnel wall blowout when using a conventional offset guide. Increasing knee flexion with AM portal drilling allows for greater coronal obliquity of the femoral tunnel but is accompanied by a greater risk of critically short tunnels and posterior wall compromise. A vertical femoral tunnel has been cited as one of the most common causes of clinical failure after ACL reconstruction, with patients complaining of persistent rotational instability and an inability to return to their previous level of athletic activity. A number of recent studies analyzing ACL tunnel position have shown that traditional, single-incision ACL reconstruction techniques may result in vertical graft orientation because of inherent technical limitations in achieving an oblique femoral tunnel.1,2,10 The clinical result can be persistent rotational instability and a positive pivotshift examination postoperatively despite successful restoration of anteroposterior stability of the knee. As a result, current trends in ACL reconstruction include drilling a more oblique femoral tunnel to place the graft in a more anatomic position on the lateral femoral condyle and provide increased rotational stability.2,4,10-13
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TABLE 2. Effect of Increasing Knee Flexion on Femoral Socket Preparation With AM Portal Drilling: Comparison of Length, Obliquity, and Incidence of Posterior Wall Blowout of Femoral Tunnels AM Portal
Coronal Angle (°)
Sagittal Angle (°)
Length (mm)
Posterior Wall Blowout
100° of flexion 110° of flexion 120° of flexion
52.6 ⫾ 4.8 46.2 ⫾ 8.4 41.7 ⫾ 1.1
43.8 ⫾ 6.2 52.1 ⫾ 9.5 51 ⫾ 4.6
30.9 ⫾ 2.6 25.7 ⫾ 5.4 21.3 ⫾ 4.8
25.0% 50.0% 75.0%
The biomechanical superiority of increased coronal obliquity of the femoral tunnel has been well established in both laboratory and clinical studies.2,4,10-13 In a cadaveric study Arnold et al.14 showed that more oblique placement of the femoral tunnel better reproduced the normal ACL’s U-shaped tension curve during passive extension and flexion. Scopp et al.4 showed that rotational stability in cadaveric knees is restored to normal with oblique femoral tunnel placement, and Yamamoto et al.2 showed that lateral femoral tunnel placement restores rotatory and anterior translation knee stability in extension. Musahl et al.11 used a surgical robot system to compare 2 femoral tunnel locations on the femoral notch, either inside the anatomic footprint of the ACL or at the “over-the-top” position chosen for the best graft isometry. Neither position completely restored the normal kinematics of the native knee, but the oblique femoral tunnel inside of the anatomic footprint more closely approximated normal knee kinematics than did the tunnel chosen for graft isometry. Loh et al.12 compared anterior tibial translation between cadaveric knees in which the ACL was reconstructed with either a 10-o’clock or 11o’clock femoral position. Both constructs showed similar anteroposterior stability to controlled loads, but the more oblique construct showed smaller amounts of anterior tibial translation to combined rotatory valgus-internal tibial torque loads. Howell et al.1 and Simmons et al.13 showed a decrease in tension across the ACL graft, increased motion, and decreased posterior cruciate ligament impingement with a graft placed more horizontally in the coronal plane. In addition, some surgeons have advocated that a more lateral femoral tunnel placement favorably increases the moment arm of ACL reconstruction.10,11 Although the importance of coronal obliquity of the femoral tunnel is well established, the surgical techniques by which to achieve optimal graft position have undergone considerable evolution and remain controversial. Some authors have advocated that oblique femoral tunnel preparation is readily achievable with technical modifications to the conventional transtibial
technique. Howell et al.1 have recommended creating a tibial tunnel at a coronal angle of 65° to 70° to achieve sufficient femoral tunnel obliquity. Chhabra et al.5 provided guidelines to use external landmarks to achieve sufficient tibial and femoral tunnel obliquity and reported that a tibial starting point at the midpoint between the tibial tubercle and posteromedial corner achieved a coronal angle of approximately 70°. Golish et al.6 recently reported increased femoral tunnel obliquity at the 10:30 position using a transtibial drilling technique with a starting point that encroaches on the anterior fibers of the MCL. Rue et al.15 effectively showed in a cadaveric model that a 10-mm lateralized femoral tunnel prepared by a transtibial technique places the graft in a location that replaces approximately half of the AM and PL bundle footprints. Other surgeons have advocated independent drilling of the femoral tunnel for ACL reconstruction. Giron et al.7 reported an inability to restore anatomic femoral origin of the ACL with transtibial drilling techniques despite technical modifications of coronal angle and starting position. Cha et al.8 recommended independent drilling of the femoral tunnel through a medial arthroscopic portal with the knee placed in hyperflexion. Preliminary laboratory studies have reported favorable femoral tunnel obliquity using this technique, although concerns regarding tunnel length, grafttunnel mismatch, and fixation methods have been reported.6 This study shows that the drilling of a suboptimal vertical femoral tunnel at a clock-face position of approximately 11:30 is equally possible with the transtibial and AM portal techniques. Although the surgical goal is not to achieve maximal tunnel verticality, this finding is pertinent and shows that the AM portal technique is not protective against poor tunnel position. In this regard, meticulous attention to guide pin positioning on the wall of the lateral femoral condyle is critical, because satisfactory obliquity is not obligatory with independent preparation of the femoral tunnel.
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A. BEDI ET AL. scribed the anatomy of the AM and PL bundles of the ACL and localized the center of the femoral attachments corresponding to the 10:30 position on the intercondylar wall. Furthermore, Rue et al.9 recently showed that reaming of a 10-mm femoral tunnel at the 10:30 position places the graft in a location that replaces approximately half of the AM and PL bundle footprints. These findings suggest that both the transtibial and AM portal techniques of femoral tunnel preparation, when appropriately performed, can achieve an ACL graft position that accurately reproduces native anatomy and thereby confers improved anteroposterior and rotational stability to the knee. The maximal achievable coronal obliquity of the femoral tunnel was significantly greater with the AM portal technique in this study. However, the clinical significance of approximately 7° increased coronal obliquity (10:45 to 10:30 clock-face position) is unclear. In addition, the results suggest that this marginal gain in potential obliquity may be accompanied by an increased risk of complications, including critically short tunnel length and posterior tunnel wall compromise. Although the mean femoral tunnel lengths between the transtibial and AM portal techniques were not significantly different, 1 of 6 oblique femoral tunnels (16.7%) drilled with the transtibial technique compared with 5 of 12 oblique femoral tunnels (41.7%) drilled with the AM portal technique were less than 25 mm. These findings are in concordance with the results reported by Golish et al.,6 supporting concerns regarding graft-tunnel mismatch, insufficient
FIGURE 5. (A) Coronal tomographic image and (B) corresponding gross dissection of a critically short tunnel (19.7 mm) obtained with drilling of an oblique femoral tunnel through the AM technique with the knee flexed to 110°. Of 12 oblique femoral tunnels drilled through the AM portal, 5 (41.7%) had an intraosseous length less than 25 mm.
The results of this study also show that both the transtibial and AM portal techniques can allow for preparation of an oblique femoral tunnel at a clock-face position of approximately 10:30. Yasuda et al.16 de-
FIGURE 6. Gross dissection specimen showing position of oblique femoral tunnel obtained with an AM portal drilling technique with knee flexed to 120°. Note the proximity of the tunnel exit to the articular cartilage of the lateral femoral condyle.
PORTAL DRILLING FOR ACL RECONSTRUCTION graft-tunnel interface for healing, and limited options for fixation with such short tunnels. In addition, 100% of the cases of posterior tunnel wall blowout in our study occurred with the AM portal technique and most commonly occurred during the preparation of an oblique femoral tunnel. In this study we identified a predictable relation between increasing knee flexion during AM portal drilling and femoral tunnel length and position. Knee hyperflexion has been recommended with this technique to provide safe clearance of the medial femoral condyle with passage of the reamer and increased tunnel length.8 However, well-defined recommendations regarding the degree of flexion and its effect on tunnel position have not been defined to date. In our study increased femoral tunnel obliquity was achieved with progressive increases in knee flexion but was associated with a paradoxical reduction in tunnel length and an increased risk of posterior tunnel wall blowout. Seventy-five percent of tunnels drilled at 120° of flexion showed posterior wall compromise and had a mean length of 21.3 mm. Although these findings are in contrast to current recommendations of knee hyperflexion to increase tunnel length and protect against wall blowout with AM portal reaming, we believe that these results reflect the use of a conventional offset guide to prepare the femoral tunnels in this study. Referencing of the posterior wall and over-the-top position with an offset guide when drilling through the AM portal may paradoxically increase the posterior trajectory of the guidewire, thereby increasing the risk of short tunnels and wall blowout. This finding is of paramount importance and suggests that surgeons using the AM portal technique should avoid conventional transtibial offset guides that reference the posterior wall when placing the femoral guide pin. Rather, the guide pin should be placed freehand and reference the center of the anatomic femoral footprint of the native ACL. In this capacity, a trajectory that maximizes tunnel length and obliquity while restoring native anatomy is not restricted by offset guides that were designed for conventional transtibial drilling techniques. Careful medial portal placement that is not restricted by guides is also critical to avoid iatrogenic injury to both the medial and lateral femoral condyles. In a cadaveric study Zantop et al.17 showed that drilling of the femoral PL bundle tunnel through a high medial portal at low knee flexion angles greatly increases the risk of damage to the subchondral bone of the lateral compartment. Our study design is not without limitations. The transtibial technique used in this study was performed
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as described by Rue et al.,9 with oblique positioning of the guide at 60° in the coronal plane relative to the proximal tibia joint surface and intra-articular referencing of 5 mm lateral to the medial tibial spine and 3 to 4 mm posterior to the posterior edge of the anterior horn of the lateral meniscus. Despite the clinical success that has been achieved with these techniques, recent studies have suggested that these intra-articular references place the aperture of the tibial tunnel relatively posterior in the native ACL footprint and may result in biomechanically inferior constructs.18,19 It is possible that intra-articular referencing defined by the center of the native tibial footprint would have limited the achievable obliquity with the use of the transtibial technique. In addition, our study analyzed tunnel position using CT analysis and was not referenced relative to the native femoral footprint. In this regard, we recognize that the reported tunnel position addresses coronal obliquity but does not account for the relative proximal or distal location of the intra-articular aperture within the footprint. Although it is true that a vertical tunnel is often located outside of the normal femoral ACL footprint, theoretically, a tunnel centered in the femoral insertion might also have a vertical direction. On the other hand, a socket located very high and shallow in the notch might have an oblique direction. Despite this limitation, the proximal-distal location of the tunnel was standardized by use of the offset guide for both techniques, and work by Rue et al.15 has recently shown that a 10-mm tunnel placed at the 10:30 location captures 50% of both bundle footprints. In addition, the femoral footprint is often not available for referencing during femoral guide pin positioning after debridement and notchplasty have been performed. We also recognize that a 6-mm femoral tunnel may not be a clinically relevant size that is used with single-bundle reconstruction techniques. However, a small tunnel size was essential to our study design to create independent tunnels that defined a range of achievable coronal obliquity with both techniques. Characterization of tunnel position and length by use of CT is a significant strength of our study design. Numerous prior studies have attempted to define tunnel obliquity based on analysis of anteroposterior-, lateral-, and notch-view plain radiographs. The inherent limitations of this technique, however, have been recently recognized. Hoser et al.20 reported that ACL femoral tunnels were invisible on plain anteroposterior radiographs in 92.2% (46 of 50) cases and on lateral radiographs in 21.6% (8) cases. In 2 cases the wrong anatomic structure was identified as the femo-
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ral tunnel by radiologists. In contrast, accurate tunnel characterization was possible in multiple planes in 48 patients by use of CT technology.20 CONCLUSIONS The AM portal technique allows for slightly greater femoral tunnel obliquity compared with transtibial drilling. However, there is a substantially increased risk of critically short tunnels (⬍25 mm) and posterior tunnel wall blowout with AM portal compared with transtibial drilling when a conventional offset guide is used. Increasing knee flexion with AM portal drilling allows for greater coronal obliquity of the femoral tunnel at the expense of a greater risk of critically short tunnels and posterior wall compromise. REFERENCES 1. Howell SM, Gittins ME, Gottlieb JE, Traina SM, Zoellner TM. The relationship between the angle of the tibial tunnel in the coronal plane and loss of flexion and anterior laxity after anterior cruciate ligament reconstruction. Am J Sports Med 2001;29:567-574. 2. Yamamoto Y, Hsu WH, Woo SL, Van Scyoc AH, Takakura Y, Debski RE. Knee stability and graft function after anterior cruciate ligament reconstruction: A comparison of a lateral and an anatomical femoral tunnel placement. Am J Sports Med 2004;32:1825-1832. 3. Lee MC, Seong SC, Lee S, Chang CB, Park YK, Kim CH. Vertical femoral tunnel placement results in rotational knee laxity after anterior cruciate ligament reconstruction. Arthroscopy 2007;23:771-778. 4. Scopp JM, Jasper LE, Belkoff SM, Moorman CT III. The effect of oblique femoral tunnel placement on rotational constraint of the knee reconstructed using patellar tendon autografts. Arthroscopy 2004;20:294-299. 5. Chhabra A, Diduch DR, Blessey PB, Miller MD. Recreating an acceptable angle of the tibial tunnel in the coronal plane in anterior cruciate ligament reconstruction using external landmarks. Arthroscopy 2004;20:328-330. 6. Golish SR, Baumfeld JA, Schoderbek RJ, Miller MD. The effect of femoral tunnel starting position on tunnel length in anterior cruciate ligament reconstruction: A cadaveric study. Arthroscopy 2007;23:1187-1192. 7. Giron F, Cuomo P, Edwards A, Bull AM, Amis AA, Aglietti P. Double-bundle “anatomic” anterior cruciate ligament reconstruction: A cadaveric study of tunnel positioning with a transtibial technique. Arthroscopy 2007;23:7-13.
8. Cha PS, Brucker PU, West RV, et al. Arthroscopic doublebundle anterior cruciate ligament reconstruction: An anatomic approach. Arthroscopy 2005;21:1275.e1-1275.e8. Available online at www.arthroscopyjournal.org. 9. Rue JP, Ghodadra N, Lewis PB, Bach BR Jr. Femoral and tibial tunnel position using a transtibial drilled anterior cruciate ligament reconstruction technique. J Knee Surg 2008;21: 246-249. 10. Harner CD, Honkamp NJ, Ranawat AS. Anteromedial portal technique for creating the anterior cruciate ligament femoral tunnel. Arthroscopy 2008;24:113-115. 11. Musahl V, Plakseychuk A, VanScyoc A, et al. Varying femoral tunnels between the anatomical footprint and isometric positions: Effect on kinematics of the anterior cruciate ligament-reconstructed knee. Am J Sports Med 2005;33:712-718. 12. Loh JC, Fukuda Y, Tsuda E, Steadman RJ, Fu FH, Woo SL. Knee stability and graft function following anterior cruciate ligament reconstruction: Comparison between 11 o’clock and 10 o’clock femoral tunnel placement. 2002 Richard O’Connor Award paper. Arthroscopy 2003;19:297-304. 13. Simmons R, Howell SM, Hull ML. Effect of the angle of the femoral and tibial tunnels in the coronal plane and incremental excision of the posterior cruciate ligament on tension of an anterior cruciate ligament graft: An in vitro study. J Bone Joint Surg Am 2003;85:1018-1029. 14. Arnold MP, Verdonschot N, van Kampen A. ACL graft can replicate the normal ligament’s tension curve. Knee Surg Sports Traumatol Arthrosc 2005;13:625-631. 15. Rue JP, Ghodadra N, Bach BR Jr. Femoral tunnel placement in single-bundle anterior cruciate ligament reconstruction: A cadaveric study relating transtibial lateralized femoral tunnel position to the anteromedial and posterolateral bundle femoral origins of the anterior cruciate ligament. Am J Sports Med 2008;36:73-79. 16. Yasuda K, Kondo E, Ichiyama H, et al. Anatomic reconstruction of the anteromedial and posterolateral bundles of the anterior cruciate ligament using hamstring tendon grafts. Arthroscopy 2004;20:1015-1025. 17. Zantop T, Haase AK, Fu FH, Petersen W. Potential risk of cartilage damage in double bundle ACL reconstruction: impact of knee flexion angle and portal location on the femoral PL bundle tunnel. Arch Orthop Trauma Surg 2008;128:509-513. 18. Pearle AD, Shannon FJ, Granchi C, Wickiewicz TL, Warren RF. Comparison of 3-dimensional obliquity and anisometric characteristics of anterior cruciate ligament graft positions using surgical navigation. Am J Sports Med 2008;36:15341541. 19. Purnell ML, Larson AI, Clancy W. Anterior cruciate ligament insertions on the tibia and femur and their relationships to critical bony landmarks using high-resolution volume-rendering computed tomography. Am J Sports Med 2008;36:20832090. 20. Hoser C, Tecklenburg K, Kuenzel KH, Fink C. Postoperative evaluation of femoral tunnel position in ACL reconstruction: Plain radiography versus computed tomography. Knee Surg Sports Traumatol Arthrosc 2005;13:256-262.
Purpose: To compare the obliquity and length of femoral tunnels prepared with transtibial versus anteromedial portal drilling for anterior cruciate ligament (ACL) reconstruction and identify potential risks associated with the anteromedial portal reaming technique. Methods: We used 18 human cadaveric knees (9 matched pairs) without ACL injury or pre-existing arthritis for the study. Femoral tunnels for ACL reconstruction were prepared by either a transtibial (n ⫽ 6) or anteromedial portal (n ⫽ 12) technique. For the anteromedial portal technique, a guidewire was advanced through the medial portal in varying degrees of knee flexion (100° [n ⫽ 4], 110° [n ⫽ 4], or 120° [n ⫽ 4]) as measured with a goniometer. By use of a 6-mm femoral offset guide, two 6-mm femoral tunnels were reamed with the guide placed (1) as far posterior and lateral in the notch as possible and (2) as far medial and vertical in the notch as possible to define the range of maximal and minimal achievable coronal obliquity for each technique. All knees were imaged with high-resolution, 3-dimensional fluoroscopy to define (1) coronal tunnel obliquity relative to the lateral tibial plateau, (2) sagittal tunnel obliquity relative to the long axis of the femur, (3) intraosseous tunnel length, and (4) the presence of posterior cortical wall blowout. Data analysis was performed with a paired t-test and repeated-measures analysis of variance, with P ⬍ .05 defined as significant. Results: Preparation of a vertical tunnel was possible with both transtibial and anteromedial portal drilling. The maximal achievable coronal obliquity, however, was significantly better with an anteromedial portal compared with transtibial drilling. However, 7 of 36 tunnels (19.4%) showed violation of the posterior tunnel wall, and all of these cases occurred with the anteromedial portal drilling technique. In addition, 1 of 6 oblique femoral tunnels (16.7%) drilled with the transtibial technique and 5 of 12 oblique femoral tunnels (41.7%) drilled with the anteromedial portal had an intraosseous length less than 25 mm. Increasing knee flexion with anteromedial portal drilling was associated with a significant reduction in tunnel length, increase in coronal obliquity, increase in sagittal obliquity, and increased risk of posterior wall blowout (P ⬍ .05). Conclusions: The anteromedial portal technique allows for slightly greater femoral tunnel obliquity compared with transtibial drilling. However, there is a substantially increased risk of critically short tunnels (⬍25 mm) and posterior tunnel wall blowout when a conventional offset guide is used. Increasing knee flexion with anteromedial portal drilling allows for greater coronal obliquity of the femoral tunnel but is accompanied by a greater risk of critically short tunnels and posterior wall compromise. Clinical Relevance: Our findings provide insight into the potential risks and advantages of a transtibial versus an anteromedial femoral tunnel drilling technique in ACL reconstruction.
From the Sports Medicine and Shoulder Service (A.B., B.R., R.J.W.) and Department of Radiology and Imaging (A.M., H.P.), Hospital for Special Surgery, New York, New York, U.S.A. Supported by the Institute for Sports Medicine Research, Hospital for Special Surgery. The authors report no conflict of interest. Received June 29, 2009; accepted December 7, 2009. Address correspondence and reprint requests to Asheesh Bedi, M.D., Sports Medicine and Shoulder Service, Hospital for Special Surgery, New York, NY 10021, U.S.A. E-mail: [email protected] © 2010 by the Arthroscopy Association of North America 0749-8063/10/2603-9394$36.00/0 doi:10.1016/j.arthro.2009.12.006
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hereas sagittal graft alignment has long been recognized as critical for restoration of anteroposterior stability, coronal graft obliquity has only recently been recognized as a critical parameter to restore with anterior cruciate ligament (ACL) reconstruction. There has been an increased recognition of ACL reconstruction failure attributable to vertically oriented grafts in recent years. Although anterior tibial translation can be well controlled with isometric femoral positioning and a vertical graft orientation, patients often have residual rotational instability and a persistent pivot shift postoperatively that preclude their ability to return to previous level of athletic activity.1-4 The restoration of normal knee kinematics and improvement in tibial rotational control with greater femoral tunnel obliquity have been recently established in a number of biomechanical studies.1-4 The surgical techniques by which to achieve sufficient graft coronal obliquity, however, remain controversial. A number of authors have described technical modifications to the transtibial technique to improve femoral tunnel obliquity and restore the native femoral ACL footprint. Howell et al.1 recommended creating a tibial tunnel at a coronal angle of 65° to 70° to reduce loss of flexion and anterior laxity. Chhabra et al.5 provided guidelines for appropriate external landmarks and reported that a tibial starting point at the midpoint between the tibial tubercle and posteromedial corner achieved a coronal angle of approximately 70°. Golish et al.6 recently reported increased femoral tunnel obliquity using a transtibial drilling technique with a tibial tunnel starting point that encroaches on the anterior fibers of the medial collateral ligament (MCL). Some surgeons, however, have advocated independent drilling of the femoral tunnel for ACL reconstruction. Giron et al.7 reported an inability to restore the anatomic femoral origin of the ACL with transtibial drilling techniques despite technical modifications of coronal angle and starting position. Cha et al.8 recommended independent drilling of the femoral tunnel through a medial arthroscopic portal with the knee placed in hyperflexion. Although laboratory studies have reported favorable femoral tunnel obliquity by use of this technique, the risks associated with anteromedial (AM) portal drilling remain undefined. The purpose of this study was to compare the obliquity, length, and wall compromise of femoral tunnels prepared with transtibial versus AM portal drilling for ACL reconstruction. Furthermore, our goal was to define the dimensions and location of femoral tunnels prepared with an AM portal technique in varying
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degrees of knee flexion to define the potential risks or benefits associated with this technique. We hypothesized that a medial portal drilling technique would allow for preparation of a femoral socket with greater coronal obliquity compared with the transtibial technique but is accompanied by a greater risk of critically short femoral tunnels and posterior wall blowout. METHODS After institutional review board approval was obtained, 18 human cadaveric knees (9 matched pairs) without ACL injury or pre-existing arthritis were obtained for the study. Each femur was secured in a custom jig allowing free flexion of the knee. A 30° 3.5-mm arthroscope (Stryker, Warsaw, IN) was introduced into the knee through a standard anterolateral portal. Through an AM portal placed approximately 4 mm medial to the border of the patellar tendon, the ACL was arthroscopically resected and a notchplasty with minimal bone resection was performed with a 4.0-mm full-radius resector (Stryker) to allow direct visualization of the over-the-top position. Care was taken to ensure that the medial portal allowed direct approximation of the center of the femoral ACL footprint without risk of medial or lateral femoral condyle injury. Femoral tunnels were subsequently reamed by either a transtibial (n ⫽ 6) or AM portal (n ⫽ 12) technique. Matched pairs were used to minimize anatomic differences between specimens. This distribution allowed for the evaluation of the 2 primary study questions regarding (1) transtibial versus AM portal drilling femoral socket position and (2) the effect of increasing knee flexion on tunnel length and position with AM drilling. We therefore required double the number of knees for the AM portal group to isolate the effect of a variable flexion angle. For the transtibial technique, a commercial tibial tunnel ACL guide (Acufex Director; Smith & Nephew Arthroscopy, Andover, MA) was set at 50° to prepare a 10-mm tunnel. The intra-articular exit point of the guide pin was directed 5 mm lateral to the medial tibial spine and 3 to 4 mm posterior to the anterior horn of the lateral meniscus as described by Rue et al.9 A 6-cm incision was then created over the medial face of the tibia, and the pes anserinus was retracted inferiorly to define the tibial insertion of the MCL. The external starting point was placed at the anterior border of the MCL insertional fibers in all cases to allow for oblique orientation of the guide of approximately 60° from the horizontal. When satisfactory intra- and extra-articular starting points were confirmed, the
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guide pin was placed and over-reamed with a 10-mm cannulated acorn reamer (Fig 1). A commercially available 6-mm offset aimer (Arthrex, Naples, FL) was passed retrograde through the tunnel and placed in the over-the-top position on the posterior cortex of the femur. The angle of knee flexion with the transtibial technique was dictated by the offset guide resting on the back wall; it is not a technical variable that can be independently manipulated as with the AM portal technique. Two 6-mm femoral tunnels were subsequently reamed with the guide rotated (1) as far posterior and lateral in the notch as possible and (2) as far medial and vertical in the notch as possible to define the range of maximal and minimal achievable coronal obliquity (Fig 2). For the AM portal technique, a guidewire was advanced through the AM portal in varying degrees of knee flexion (100° [n ⫽ 4], 110° [n ⫽ 4], or 120° [n ⫽ 4]) as measured with a goniometer and maintained by a surgical assistant. By use of a 6-mm offset aimer, two 6-mm femoral tunnels were reamed with the guide rotated (1) as vertical in the notch as possible and (2) as oblique in the notch as possible to define the range of minimal and maximal achievable coronal obliquity (Fig 2). Care was taken to optimize medial
portal position to avoid any iatrogenic injury to the medial femoral condyle with reaming. All knees were subsequently imaged in multiple planes on a Philips Multidiagnost-Eleva Fluoroscopy System (Royal Philips; Eindhoven, The Netherlands) to generate high-resolution, 3-dimensional images of the prepared tunnels. ViewForum 4.1 integrated view and processing software (Royal Philips) was used to define (1) coronal tunnel obliquity, (2) sagittal tunnel obliquity, (3) intraosseous tunnel length, and (4) the presence of posterior cortical wall compromise for each tunnel. Coronal obliquity was defined by the angle subtended between the tunnel and a horizontal axis defined by the lateral tibial plateau. Clock-face description of coronal obliquity was defined by Rue et al.,9 with each 30° increment from the vertical axis (12 o’clock) equivalent to one clock-face position. Sagittal obliquity was defined by the angle subtended between the tunnel and longitudinal axis of the femur (Fig 3). Tunnel length was measured on selected images that were coplanar with the tunnel allowing for direct measure from intra-articular to extraarticular aperture. Posterior wall compromise was defined by any breach in the posterior cortical wall of the tunnel evident on axial computed tomography (CT) images. All knees were disarticulated and the femurs stripped of soft tissue to confirm anatomic relations of the tunnels identified on imaging. Data analysis was performed with a paired t-test and repeatedmeasures analysis of variance, with P ⬍ .05 defined as significant. RESULTS
FIGURE 1. Coronal image obtained with high-resolution, 3dimensional fluoroscopy of 10-mm tibial tunnel prepared by technique as described by data from Rue et al.9,15
Vertical femoral tunnels could be prepared with transtibial and AM portal drilling techniques (79.55° ⫾ 9.32° and 77.88° ⫾ 7.90°, respectively). The maximal achievable coronal obliquity, however, was significantly greater with an AM portal compared with transtibial drilling (46.85° ⫾ 6.89° v 54.08° ⫾ 7.17°, P ⬍ .05). In contrast, sagittal tunnel orientation was not significantly different between groups (Table 1). Of 36 tunnels, 7 (19.4%) showed compromise of the posterior tunnel wall on axial CT images (Fig 4). These occurred in 1 maximally vertical tunnel and 6 maximally oblique tunnels, all of which were drilled with an AM portal drilling technique (Table 2). The incidence of posterior wall compromise increased with higher degrees of knee flexion during AM portal drilling (Table 2). Mean tunnel length of the maximally vertical tunnel prepared with a transtibial technique was 47.9 ⫾ 8.23
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FIGURE 3. Sagittal obliquity was defined by the angle subtended between the tunnel and longitudinal axis (red circle) of the femur. M1 is the exit point of this femoral socket through the anterior cortex of the distal femur.
mm (range, 44.0 to 61.6 mm) compared with 53.0 ⫾ 8.2 mm (range, 40.7 to 65.8 mm) for the AM portal technique (P ⫽ .22). Mean tunnel length of the most oblique tunnel prepared with a transtibial technique was 28.7 ⫾ 3.4 mm (range, 23.9 to 32.0 mm) compared with 26.0 ⫾ 5.7 mm (range, 15.0 to 34.4 mm) for the AM technique (P ⫽ .30). However, 1 of 6 oblique femoral tunnels (16.7%) drilled with a transtibial technique compared with 5 of 12 oblique femoral tunnels (41.7%) drilled with the AM portal had an intraosseous length less than 25 mm (P ⬍ .05) (Fig 5). The influence of knee flexion on tunnel obliquity, length, and posterior wall compromise with AM portal drilling was assessed. Increasing knee flexion with AM portal drilling was associated with a significant, progressive reduction in tunnel length (30.93 ⫾ 2.63 mm and 21.30 ⫾ 4.80 mm for 100° and 120° of
4 ™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™™ FIGURE 2. (A) Diagram showing maximally vertical and oblique 6-mm femoral tunnels prepared for each specimen, defining a range of potential coronal obliquity (represented by trapezoid between vertical and oblique femoral sockets). (B) Corresponding arthroscopic photo of intercondylar notch showing intra-articular aperture locations of these tunnels. (C) Notch view of a dissected femoral specimen showing defined range of coronal obliquity between vertical and maximally oblique femoral sockets (as marked by lines).
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A. BEDI ET AL. TABLE 1. Characterization of Femoral Socket Prepared With Transtibial Versus AM Portal Drilling: Comparison of Length, Obliquity, and Incidence of Posterior Wall Blowout of Femoral Tunnels
Vertical tunnel Transtibial AM portal Oblique tunnel Transtibial AM portal
Coronal Angle (°)
Sagittal Angle (°)
Length (mm)
Posterior Wall Blowout
79.6 ⫾ 9.3 77.9 ⫾ 7.9
41.4 ⫾ 13.5 37.1 ⫾ 7.9
47.9 ⫾ 8.2 53.0 ⫾ 8.2
0% 8.0%
54.1 ⫾ 7.17 45.9 ⫾ 6.9*
44.4 ⫾ 9.1 50.0 ⫾ 7.5
28.7 ⫾ 3.4 26.0 ⫾ 5.7
0% 50.0%*
*P ⬍ .05.
flexion, respectively) (Fig 5), increase in coronal obliquity (52.63° ⫾ 4.78° and 41.73° ⫾ 1.11° for 100° and 120° of flexion, respectively), increase in sagittal obliquity (51.00° ⫾ 4.56° and 43.83° ⫾ 6.24° for 100° and 120° of flexion, respectively), and increased risk of posterior wall blowout (25% and 75% for 100° and 120° of flexion, respectively) (P ⬍ .05) (Table 2). Inspection of the femoral tunnels after stripping of all soft tissues confirmed the tunnel lengths and obliquity as measured on CT images (Figs 5B and 6). On gross dissection, the exit of oblique femoral tunnels prepared through an AM portal technique showed proximity to the posterolateral (PL) articular cartilage margin of the knee and was less than 4 mm in 4
FIGURE 4. Axial image of a case of posterior wall blowout that occurred with AM portal drilling of an oblique tunnel. The incidence of posterior wall compromise increased with higher degrees of knee flexion during AM portal drilling.
specimens (Fig 6). However, no correlation between increasing flexion angle and proximity to the articular cartilage was identified. Frank penetration of the articular cartilage was not observed in any of the specimens. DISCUSSION The purpose of this study was to compare the obliquity, length, and wall compromise of femoral tunnels prepared with transtibial versus AM portal drilling for ACL reconstruction. Using CT analysis of tunnel position, we found that the AM portal technique allows for slightly greater femoral tunnel obliquity compared with transtibial drilling, but this is accompanied by an increased risk of critically short tunnels (⬍25 mm) and posterior tunnel wall blowout when using a conventional offset guide. Increasing knee flexion with AM portal drilling allows for greater coronal obliquity of the femoral tunnel but is accompanied by a greater risk of critically short tunnels and posterior wall compromise. A vertical femoral tunnel has been cited as one of the most common causes of clinical failure after ACL reconstruction, with patients complaining of persistent rotational instability and an inability to return to their previous level of athletic activity. A number of recent studies analyzing ACL tunnel position have shown that traditional, single-incision ACL reconstruction techniques may result in vertical graft orientation because of inherent technical limitations in achieving an oblique femoral tunnel.1,2,10 The clinical result can be persistent rotational instability and a positive pivotshift examination postoperatively despite successful restoration of anteroposterior stability of the knee. As a result, current trends in ACL reconstruction include drilling a more oblique femoral tunnel to place the graft in a more anatomic position on the lateral femoral condyle and provide increased rotational stability.2,4,10-13
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TABLE 2. Effect of Increasing Knee Flexion on Femoral Socket Preparation With AM Portal Drilling: Comparison of Length, Obliquity, and Incidence of Posterior Wall Blowout of Femoral Tunnels AM Portal
Coronal Angle (°)
Sagittal Angle (°)
Length (mm)
Posterior Wall Blowout
100° of flexion 110° of flexion 120° of flexion
52.6 ⫾ 4.8 46.2 ⫾ 8.4 41.7 ⫾ 1.1
43.8 ⫾ 6.2 52.1 ⫾ 9.5 51 ⫾ 4.6
30.9 ⫾ 2.6 25.7 ⫾ 5.4 21.3 ⫾ 4.8
25.0% 50.0% 75.0%
The biomechanical superiority of increased coronal obliquity of the femoral tunnel has been well established in both laboratory and clinical studies.2,4,10-13 In a cadaveric study Arnold et al.14 showed that more oblique placement of the femoral tunnel better reproduced the normal ACL’s U-shaped tension curve during passive extension and flexion. Scopp et al.4 showed that rotational stability in cadaveric knees is restored to normal with oblique femoral tunnel placement, and Yamamoto et al.2 showed that lateral femoral tunnel placement restores rotatory and anterior translation knee stability in extension. Musahl et al.11 used a surgical robot system to compare 2 femoral tunnel locations on the femoral notch, either inside the anatomic footprint of the ACL or at the “over-the-top” position chosen for the best graft isometry. Neither position completely restored the normal kinematics of the native knee, but the oblique femoral tunnel inside of the anatomic footprint more closely approximated normal knee kinematics than did the tunnel chosen for graft isometry. Loh et al.12 compared anterior tibial translation between cadaveric knees in which the ACL was reconstructed with either a 10-o’clock or 11o’clock femoral position. Both constructs showed similar anteroposterior stability to controlled loads, but the more oblique construct showed smaller amounts of anterior tibial translation to combined rotatory valgus-internal tibial torque loads. Howell et al.1 and Simmons et al.13 showed a decrease in tension across the ACL graft, increased motion, and decreased posterior cruciate ligament impingement with a graft placed more horizontally in the coronal plane. In addition, some surgeons have advocated that a more lateral femoral tunnel placement favorably increases the moment arm of ACL reconstruction.10,11 Although the importance of coronal obliquity of the femoral tunnel is well established, the surgical techniques by which to achieve optimal graft position have undergone considerable evolution and remain controversial. Some authors have advocated that oblique femoral tunnel preparation is readily achievable with technical modifications to the conventional transtibial
technique. Howell et al.1 have recommended creating a tibial tunnel at a coronal angle of 65° to 70° to achieve sufficient femoral tunnel obliquity. Chhabra et al.5 provided guidelines to use external landmarks to achieve sufficient tibial and femoral tunnel obliquity and reported that a tibial starting point at the midpoint between the tibial tubercle and posteromedial corner achieved a coronal angle of approximately 70°. Golish et al.6 recently reported increased femoral tunnel obliquity at the 10:30 position using a transtibial drilling technique with a starting point that encroaches on the anterior fibers of the MCL. Rue et al.15 effectively showed in a cadaveric model that a 10-mm lateralized femoral tunnel prepared by a transtibial technique places the graft in a location that replaces approximately half of the AM and PL bundle footprints. Other surgeons have advocated independent drilling of the femoral tunnel for ACL reconstruction. Giron et al.7 reported an inability to restore anatomic femoral origin of the ACL with transtibial drilling techniques despite technical modifications of coronal angle and starting position. Cha et al.8 recommended independent drilling of the femoral tunnel through a medial arthroscopic portal with the knee placed in hyperflexion. Preliminary laboratory studies have reported favorable femoral tunnel obliquity using this technique, although concerns regarding tunnel length, grafttunnel mismatch, and fixation methods have been reported.6 This study shows that the drilling of a suboptimal vertical femoral tunnel at a clock-face position of approximately 11:30 is equally possible with the transtibial and AM portal techniques. Although the surgical goal is not to achieve maximal tunnel verticality, this finding is pertinent and shows that the AM portal technique is not protective against poor tunnel position. In this regard, meticulous attention to guide pin positioning on the wall of the lateral femoral condyle is critical, because satisfactory obliquity is not obligatory with independent preparation of the femoral tunnel.
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A. BEDI ET AL. scribed the anatomy of the AM and PL bundles of the ACL and localized the center of the femoral attachments corresponding to the 10:30 position on the intercondylar wall. Furthermore, Rue et al.9 recently showed that reaming of a 10-mm femoral tunnel at the 10:30 position places the graft in a location that replaces approximately half of the AM and PL bundle footprints. These findings suggest that both the transtibial and AM portal techniques of femoral tunnel preparation, when appropriately performed, can achieve an ACL graft position that accurately reproduces native anatomy and thereby confers improved anteroposterior and rotational stability to the knee. The maximal achievable coronal obliquity of the femoral tunnel was significantly greater with the AM portal technique in this study. However, the clinical significance of approximately 7° increased coronal obliquity (10:45 to 10:30 clock-face position) is unclear. In addition, the results suggest that this marginal gain in potential obliquity may be accompanied by an increased risk of complications, including critically short tunnel length and posterior tunnel wall compromise. Although the mean femoral tunnel lengths between the transtibial and AM portal techniques were not significantly different, 1 of 6 oblique femoral tunnels (16.7%) drilled with the transtibial technique compared with 5 of 12 oblique femoral tunnels (41.7%) drilled with the AM portal technique were less than 25 mm. These findings are in concordance with the results reported by Golish et al.,6 supporting concerns regarding graft-tunnel mismatch, insufficient
FIGURE 5. (A) Coronal tomographic image and (B) corresponding gross dissection of a critically short tunnel (19.7 mm) obtained with drilling of an oblique femoral tunnel through the AM technique with the knee flexed to 110°. Of 12 oblique femoral tunnels drilled through the AM portal, 5 (41.7%) had an intraosseous length less than 25 mm.
The results of this study also show that both the transtibial and AM portal techniques can allow for preparation of an oblique femoral tunnel at a clock-face position of approximately 10:30. Yasuda et al.16 de-
FIGURE 6. Gross dissection specimen showing position of oblique femoral tunnel obtained with an AM portal drilling technique with knee flexed to 120°. Note the proximity of the tunnel exit to the articular cartilage of the lateral femoral condyle.
PORTAL DRILLING FOR ACL RECONSTRUCTION graft-tunnel interface for healing, and limited options for fixation with such short tunnels. In addition, 100% of the cases of posterior tunnel wall blowout in our study occurred with the AM portal technique and most commonly occurred during the preparation of an oblique femoral tunnel. In this study we identified a predictable relation between increasing knee flexion during AM portal drilling and femoral tunnel length and position. Knee hyperflexion has been recommended with this technique to provide safe clearance of the medial femoral condyle with passage of the reamer and increased tunnel length.8 However, well-defined recommendations regarding the degree of flexion and its effect on tunnel position have not been defined to date. In our study increased femoral tunnel obliquity was achieved with progressive increases in knee flexion but was associated with a paradoxical reduction in tunnel length and an increased risk of posterior tunnel wall blowout. Seventy-five percent of tunnels drilled at 120° of flexion showed posterior wall compromise and had a mean length of 21.3 mm. Although these findings are in contrast to current recommendations of knee hyperflexion to increase tunnel length and protect against wall blowout with AM portal reaming, we believe that these results reflect the use of a conventional offset guide to prepare the femoral tunnels in this study. Referencing of the posterior wall and over-the-top position with an offset guide when drilling through the AM portal may paradoxically increase the posterior trajectory of the guidewire, thereby increasing the risk of short tunnels and wall blowout. This finding is of paramount importance and suggests that surgeons using the AM portal technique should avoid conventional transtibial offset guides that reference the posterior wall when placing the femoral guide pin. Rather, the guide pin should be placed freehand and reference the center of the anatomic femoral footprint of the native ACL. In this capacity, a trajectory that maximizes tunnel length and obliquity while restoring native anatomy is not restricted by offset guides that were designed for conventional transtibial drilling techniques. Careful medial portal placement that is not restricted by guides is also critical to avoid iatrogenic injury to both the medial and lateral femoral condyles. In a cadaveric study Zantop et al.17 showed that drilling of the femoral PL bundle tunnel through a high medial portal at low knee flexion angles greatly increases the risk of damage to the subchondral bone of the lateral compartment. Our study design is not without limitations. The transtibial technique used in this study was performed
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as described by Rue et al.,9 with oblique positioning of the guide at 60° in the coronal plane relative to the proximal tibia joint surface and intra-articular referencing of 5 mm lateral to the medial tibial spine and 3 to 4 mm posterior to the posterior edge of the anterior horn of the lateral meniscus. Despite the clinical success that has been achieved with these techniques, recent studies have suggested that these intra-articular references place the aperture of the tibial tunnel relatively posterior in the native ACL footprint and may result in biomechanically inferior constructs.18,19 It is possible that intra-articular referencing defined by the center of the native tibial footprint would have limited the achievable obliquity with the use of the transtibial technique. In addition, our study analyzed tunnel position using CT analysis and was not referenced relative to the native femoral footprint. In this regard, we recognize that the reported tunnel position addresses coronal obliquity but does not account for the relative proximal or distal location of the intra-articular aperture within the footprint. Although it is true that a vertical tunnel is often located outside of the normal femoral ACL footprint, theoretically, a tunnel centered in the femoral insertion might also have a vertical direction. On the other hand, a socket located very high and shallow in the notch might have an oblique direction. Despite this limitation, the proximal-distal location of the tunnel was standardized by use of the offset guide for both techniques, and work by Rue et al.15 has recently shown that a 10-mm tunnel placed at the 10:30 location captures 50% of both bundle footprints. In addition, the femoral footprint is often not available for referencing during femoral guide pin positioning after debridement and notchplasty have been performed. We also recognize that a 6-mm femoral tunnel may not be a clinically relevant size that is used with single-bundle reconstruction techniques. However, a small tunnel size was essential to our study design to create independent tunnels that defined a range of achievable coronal obliquity with both techniques. Characterization of tunnel position and length by use of CT is a significant strength of our study design. Numerous prior studies have attempted to define tunnel obliquity based on analysis of anteroposterior-, lateral-, and notch-view plain radiographs. The inherent limitations of this technique, however, have been recently recognized. Hoser et al.20 reported that ACL femoral tunnels were invisible on plain anteroposterior radiographs in 92.2% (46 of 50) cases and on lateral radiographs in 21.6% (8) cases. In 2 cases the wrong anatomic structure was identified as the femo-
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ral tunnel by radiologists. In contrast, accurate tunnel characterization was possible in multiple planes in 48 patients by use of CT technology.20 CONCLUSIONS The AM portal technique allows for slightly greater femoral tunnel obliquity compared with transtibial drilling. However, there is a substantially increased risk of critically short tunnels (⬍25 mm) and posterior tunnel wall blowout with AM portal compared with transtibial drilling when a conventional offset guide is used. Increasing knee flexion with AM portal drilling allows for greater coronal obliquity of the femoral tunnel at the expense of a greater risk of critically short tunnels and posterior wall compromise. REFERENCES 1. Howell SM, Gittins ME, Gottlieb JE, Traina SM, Zoellner TM. The relationship between the angle of the tibial tunnel in the coronal plane and loss of flexion and anterior laxity after anterior cruciate ligament reconstruction. Am J Sports Med 2001;29:567-574. 2. Yamamoto Y, Hsu WH, Woo SL, Van Scyoc AH, Takakura Y, Debski RE. Knee stability and graft function after anterior cruciate ligament reconstruction: A comparison of a lateral and an anatomical femoral tunnel placement. Am J Sports Med 2004;32:1825-1832. 3. Lee MC, Seong SC, Lee S, Chang CB, Park YK, Kim CH. Vertical femoral tunnel placement results in rotational knee laxity after anterior cruciate ligament reconstruction. Arthroscopy 2007;23:771-778. 4. Scopp JM, Jasper LE, Belkoff SM, Moorman CT III. The effect of oblique femoral tunnel placement on rotational constraint of the knee reconstructed using patellar tendon autografts. Arthroscopy 2004;20:294-299. 5. Chhabra A, Diduch DR, Blessey PB, Miller MD. Recreating an acceptable angle of the tibial tunnel in the coronal plane in anterior cruciate ligament reconstruction using external landmarks. Arthroscopy 2004;20:328-330. 6. Golish SR, Baumfeld JA, Schoderbek RJ, Miller MD. The effect of femoral tunnel starting position on tunnel length in anterior cruciate ligament reconstruction: A cadaveric study. Arthroscopy 2007;23:1187-1192. 7. Giron F, Cuomo P, Edwards A, Bull AM, Amis AA, Aglietti P. Double-bundle “anatomic” anterior cruciate ligament reconstruction: A cadaveric study of tunnel positioning with a transtibial technique. Arthroscopy 2007;23:7-13.
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