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Leaders discuss concepts on femoral tunnel drilling in ACL reconstruction

Issue: April 2013

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Recent attention on the anatomy of the femoral footprint of the ACL has resulted in an increase in double-bundle reconstructions, as well as a desire to place single-bundle grafts in a more “anatomic” position. This anatomic placement is lower on the wall of the notch, and many surgeons have altered their drilling technique in order to make this location. This Orthopedics Today Round Table discussion focuses on our current understanding of the ideal femoral tunnel for ACL reconstruction and the optimal means of getting there technically.

Brett D. Owens, MD, LTC USA MC
Moderator

Brett D. Owens, MD, LTC, USA: Dr. Wolf, your recent study in the American Journal of Sports Medicine studied surgeon agreement on tunnel placement. Can we even agree on what an ideal tunnel looks like?

Brian R. Wolf, MD, MS: The research I conducted with the Multicenter Orthopaedic Outcomes Network (MOON) group would suggest no … and yes. Our recent paper demonstrated some difference of opinion between surgeons on the “ideal” tunnel placement on the femur and the tibia. In this cadaver study, the surgeons who created the ACL tunnels judged the tunnels to be ideal on the femur 92% of the time and on the tibia 95% of the time. In contrast, three independent surgeons arthroscopically assessed the tunnels as ideal 69% and 57% of the time, respectively. Interestingly, tunnels made using a two-incision technique were found ideal on the femur 86% of the time, medial portal 66% of the time and transtibial 51% of the time. On the femur, tunnels were most frequently criticized for being too posterior and/or too vertical.

However, the disagreement is not so bad. When judging if the ACL tunnels were “acceptable” as opposed to “ideal,” the surgeon agreement was better. The operating surgeons found 98% of their femoral tunnels and 100% of their own tibial tunnels acceptable compared to 82% and 90% for the independent surgeons. In this same cadaver study, we also used 3-D CT to analyze the tunnels relative to the published measures of the ACL footprints. Despite some disagreement on arthroscopic analysis of the tunnels, 3-D CT demonstrated relatively good consistency among 12 surgeons drilling ACL tunnels with 88% of femoral and 78% of tibial tunnels falling within ACL footprint measurement criteria. This suggests that tunnel position can vary between surgeons a bit, but still be “anatomic” in most cases.

Owens: Is it possible to get to the ideal position on the femoral side using a transtibial drilling technique? How does this affect the tibial tunnel?

Wolf: I believe it is possible for the ideal femoral tunnel position to be achieved using a transtibial technique. We, in the MOON group, have used 3-D CT to analyze tunnel placement in both a cadaver study and in a clinical study (in press). Our data demonstrates that you can achieve tunnels within the ACL femoral footprint using a transtibial technique. However, like other research groups, we have found that the most common error, regardless of preferred femoral drilling technique, is to have a tibial tunnel either in the posterior aspect of the tibial footprint or posterior to the footprint. In addition, the transtibial technique was most frequently associated with posterior tibial tunnels in our studies.

The other potential compromise of the tibial tunnel relates to tunnel angulation needed to achieve an anatomic femoral tunnel. In the coronal plane, to achieve a low and posterior femoral tunnel, the tibial tunnel often is cheated medial, close to or into the superficial medial collateral ligament (MCL). In the sagittal plane, the tunnel is often cheated proximal which can result in a short tibial tunnel. Both of these angulation issues can result in difficulty with graft to tunnel length matching and fixation issues. All these issues can absolutely be managed and appropriate tunnels can be achieved. However, in my opinion, there are more potential pitfalls with transtibial than tibial tunnel independent techniques.

Mark E. Steiner, MD: Theoretically it is possible to use transtibial drilling and place central tibial and femoral tunnels, but this would be impractical because it would require a tibial tunnel that is too close to the joint line and too short. What is controversial is how much both tunnels can be compromised from their ideal locations using transtibial drilling and still have a successful ACL reconstruction.

At one end of the spectrum is the “traditional” transtibial method of starting the tibial tunnel near the pes and MCL then directing the tunnel 7 mm anterior to the posterior cruciate ligament (PCL). This method often resulted in non-anatomic femoral tunnels with vertical grafts that failed to restore normal laxity. Whether transtibial drilling can be revised to place a femoral tunnel in the proximal end of the femoral footprint by starting the tibial tunnel medially and directing the tibial tunnel posterolateral, and whether this is good enough, is debatable. This placement may be acceptable with a patellar tendon graft where the bone plug can rotated within the tunnel to adjust its location, but soft tissue grafts with this placement may result in knees with too much abnormal motion.

Mark D. Miller, MD: It may be possible to get to the anatomic center of the ACL footprint with transtibial drilling, but not consistently. In our cadaveric study, we found that transtibial drilling results in adequate femoral tunnel placement 60% of the time. Accessory medial portal (AMP) drilling was accurate almost 100% of the time. The problem is recognizing when transtibial placement “misses the mark,” and being comfortable with other options. In another study, we found that transtibial drilling actually increases the tibial tunnel aperture by almost 30%. This may lead to late graft laxity and tunnel osteolysis.

Owens: The anteromedial portal technique was described in 1998, but seems to have taken off during the late 2000s. Dr. Miller, you have done work comparing AMP drilling to transtibial drilling. Can you tell us about your findings?

Miller: Dr. Freddie Fu deserves much of the credit for teaching us the importance of achieving more anatomic ACL reconstructions. Many of us, including me, were slow in recognizing that transtibial drilling often resulted in vertical femoral tunnel placement. Part of the reason that I was slow to change techniques was that I believed a more medial tibial starting point would allow me to achieve a more horizontal femoral tunnel. While this is true, the resulting tunnel is still often non-anatomic and, as we later learned, the “clock face method” of describing femoral tunnel placement is inaccurate.

We were also concerned that independent femoral drilling would result in short femoral tunnels. A subsequent clinical study showed that drilling with the knee in hyper flexion will result in adequate tunnel length. Our cadaveric study previously described convinced me that independent femoral tunnel drilling through an AMP is consistently accurate and reliable.

Owens: Dr. Steiner, you have done some recent work on flexible instruments for drilling the femoral socket through both transtibial and anteromedial portals. What were your findings?

Steiner: We have found that flexible instruments primarily make it easier to create an anatomic femoral tunnel through an anteromedial portal. Flexible pins and reamers obviate the need for hyper flexion and also eliminate the need to rigidly maintain knee flexion during pin placement and reaming. Transtibial drilling is an option with these instruments, but this technique is more demanding than a direct anteromedial approach.

We documented that flexible instruments used through an anteromedial portal created straight femoral tunnels with more than adequate length that did not violate the posterior cortex of the femur. It is the direction the pin is directed toward that is important, and the curved aimers provide the opportunity to optimally place the pin. Our results, along with those of other studies, indicate that anteromedial drilling with traditional rigid instruments requires hyper flexion of at least 125° to place a safe tunnel with adequate length. This can be challenging, therefore, for the role of flexible instruments.

Owens: The outside-in drilling has traditionally been performed using a rear-entry guide brought into the joint through a second incision (two-incision technique). However, this can now also be performed using a guide brought in through an anterior portal. New reverse drilling instruments also allow a socket to be drilled. Are any of you using this technique, and are there any specific indications for this?

Wolf: I have used the retro-drilling instruments (Arthrex Inc.), and it seems to mirror traditional two-incision technique without the second incision. It is a good option for primary and revision ACL reconstruction as it facilitates a lower and less vertical femoral tunnel compared to transtibial technique. Portal placement is less crucial than medial portal techniques, and the tunnel can be drilled without knee hyper-flexion which allows for easy visualization. However, the retro-drilling does make using bone-tendon-bone (BTB) grafts, Achilles grafts and quads tendon grafts more challenging. There are suspensory fixation devices available, but using traditional interference screws is difficult due to the tunnel angle, unless you make the traditional second incision. Also BTB graft passage can sometimes be a challenge as the bone block needs to make a nearly 90° turn in the notch. A key is to make your bone block for the femoral side a bit shorter (~20 mm).

Miller: I am a firm believer in the concept that it does not matter how you get there (anatomic footprint of the ACL), just that you get there. I do not routinely use a second lateral incision, but I often do so in the revision setting, and always for physeal-sparing ACL reconstructions in children.

Steiner: I have not used reverse drilling instruments, but their presence underscores the concern many surgeons have with transtibial drilling. A traditional two-incision technique with an aimer placed through a portal and then reaming conducted outside-in is still a successful reconstruction method used by some outstanding surgeons. I have used a distal lateral thigh incision when there have been problems with graft fixation, and the incision does not add any significant morbidity to the operation.

Owens: Can you share your current femoral drilling technique and any pearls about transition from a transtibial technique?

Miller: My preferred technique for femoral tunnel placement in ACL reconstruction is via an AMP. I debride the torn ACL and attempt to leave a “crew-cut” showing the ACL footprint. With the knee in 90° of flexion and viewing through the anteromedial portal, I create an accessory medial portal after localizing it with a spinal needle (it is typically 1 cm to 2 cm medial and slightly inferior to the anteromedial portal). I mark the center of the femoral footprint (sometimes cheating 1 mm or 2 mm posteriorly, because I think these fibers are more important) with an electrocautery device. I then use a microfracture awl to further define this location, and then hyper flex the knee. It is critical to focus on the target and ignore the distorted anatomy with the knee in hyper flexion.

I insert the beath needle from the AMP into the target area and drill it out the lateral thigh. I then place a protective sheath between the beath needle and the medial femoral condyle and drill the femoral tunnel with a partially threaded drill bit (smooth on one side) providing further protection to the medial femoral condyle. After debriding the tunnel, I pass a #5 looped suture into the femoral tunnel with the beath needle to facilitate later graft passage.

Steiner: The femoral tunnel is created first to avoid extravasation of fluid out the tibial tunnel. While surgeons use various methods to find the site of an anatomic femoral tunnel, a reliable method has been to base the tunnel’s location on its height up the lateral wall of the notch. With the knee in 90° flexion a traditional 7-mm offset aimer placed through the anteromedial portal and positioned approximately 7 mm to 8 mm from the low point of the lateral wall will direct a guide pin slightly deep to the center of the ACL (Figure 1). This approach focuses on how low the tunnel is, in addition to how deep in the notch the tunnel lies (Figure 2). This point is marked manually, and then an awl is used to enlarge the target point.

Figure 1. A 7-mm offset aimer is introduced through the anteromedial portal and positioned approximately 7 mm to 8 mm up the lateral wall. A pin placed through this aimer will locate a point slightly deep to the ACL center.

Images: Steiner ME

A 42° curved aimer is introduced through the anteromedial portal with its tip directed toward the anterolateral femur. The knee is slightly flexed more than 90°. The guide pin is drilled out the thigh then the appropriate sized flexible femoral reamer is used through the anteromedial portal to create a tunnel of at least 35 mm in length. This tunnel will be slightly deep to lowest point on the lateral wall and be approximately 8 mm up the lateral wall (Figure 3).

Figure 2. The target point for the femoral tunnel visualized from the anteromedial portal.

Wolf: My preferred femoral drilling technique is through the anteromedial portal aiming for the center of the anatomic footprint. I use a low profile straight reamer with the knee flexed to around 110° to 120°.

The pearls that helped me make the transition from a transtibial technique just involved portal placement. I use a trans-patellar tendon viewing portal placed through the lateral half of the patellar tendon just below the level of the inferior pole. This has been advocated by Dr. Stephen Howell and made a huge difference for me. This portal, without any notchplasty, allows for excellent visualization of the posterior notch and footprint area so much better than the traditional anterolateral portal, and yet, it keeps the camera sufficiently away from my drilling portal. My patients have had no problems with this portal postoperatively.

Next, I make the medial drilling portal using a spinal needle to precisely locate its position. I prefer to initially place the spinal needle just above the medial meniscus with the knee in relative extension and valgus. I shoot for the junction of the anterior fat pad and the smooth medial capsule, which usually is the ideal location. Then, I hyper flex the knee and aim the needle to the femoral ACL footprint to be check that I will not hit the medial condyle.