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Can flexible instruments create adequate femoral tunnel lengths at 90[degrees] of knee flexion in anterior cruciate ligament reconstruction?

Anatomical reconstruction of the Anterior Cruciate Ligament (ACL) involves several principles, one of which is the accurate placement of femoral tunnel at the femoral footprint. An anteromedial portal may be used to independently drill the femoral tunnel as close as possible to the native ACL footprint. (1) There is growing evidence that the medial portal or an accessory medial portal is more reliable than the transtibial technique. (2-4) However, there is a concern regarding inadequate femoral length when the femoral tunnel is drilled via an anteromedial portal. (1) The femoral tunnels are shorter via anteromedial portal drilling than the transtibial method. (5) The degree of knee flexion is another important determinant of the length of the femoral tunnel and its distance from the critical structures on the lateral aspect of the knee. (6) Higher degrees of knee flexion aid in creating a longer femoral tunnel, and a minimum of 110[degrees] of knee flexion is recommended. (1,7) Hyperflexion of the knee can create longer tunnels and prevent lateral or posterior wall disruption but can obscure visualization and requires an assistant to support the leg or the use of a leg holding device. (8) Many surgeons find it convenient to keep the knee at 90[degrees] of flexion for better orientation and appreciation of the ACL femoral footprint anatomy.

An alternative method of drilling the femoral tunnel is to use flexible instruments. These can potentially mitigate posterior wall tunnel blowouts as the curved guide directs the guide pin at an angle away from the posterior femoral cortex. (1) Flexible reamers can also create longer femoral tunnels than rigid ones. (9) Surgeons familiar with the transtibial approach may also use flexible instruments to get low on the femoral condyle via a transtibial approach. (10)

A recent study demonstrated that "hyperflexion" is not necessary for flexible instruments to create a well-placed and adequately long femoral tunnel, and the guide pins exit at a safe distance from the lateral sided anatomical structures of the knee, specifically the peroneal nerve and the lateral collateral ligament (LCL). (10) The flexible pins were drilled with the knee in 110[degrees] of flexion. Another study emphasized that flexing the knee to 120[degrees] is safer than 90[degrees] and advocated as much flexion as possible even with flexible instruments. (11) The pins were drilled with an open surgical approach. But, it is unclear whether flexible instrumentation creates adequately long tunnels at lower knee flexion angles, such as 90[degrees], when the tunnels are drilled arthroscopically and whether this technique places the peroneal nerve or the lateral collateral ligament at risk. This study aims to study femoral tunnel lengths and the distance to important lateral structures obtained by flexing the knee at various angles and by drilling the guide pins arthroscopically to resemble clinical practice.

The purpose of this cadaveric study was twofold: 1. to determine whether femoral tunnel lengths of greater than 20 mm can be created with a flexible reamer system at 90[degrees] of knee flexion and 2. to determine whether the lateral structures of the knee are safe with this technique. Our hypothesis was that sufficient femoral tunnel length can be obtained at 90[degrees] of knee flexion, defined as a minimum of 20 mm for graft placement and fixation. (12,13) We also hypothesized that the peroneal nerve and the lateral collateral ligament (LCL) were at a critical distance from the exit point of the guide pin at all angles of knee flexion. We defined the critical distance as 35 mm for the peroneal nerve and 20 mm to the LCL femoral origin. These values were the ones used in the only other study that looked at these variables using flexible instrumentation. (10)

Materials and Methods

Ten fresh cadaveric knees were utilized in this study (age range: 58 to 86). Specimens were examined manually, and no gross pathology was apparent. All specimens were deep frozen at -20[degrees]C and thawed at room temperature 24 to 48 hours before experiments. Standard anteromedial and anterolateral portals were created at the inferior patellar border (Fig. 1). The ACL was excised and debrided arthroscopically with retention of the ACL footprint. Flexible pins were used as a surrogate measure to determine the tunnel length and to measure the distance from the lateral structures. Femoral tunnels were not actually created over the pin since the tunnel length can be considered to be just equal to or slightly less than the length of bone measured between the entry and the exit point of the flexible pin, referred to as the intra-osseous length. This intra-osseous length can be measured with a specially designed flexible guide pin. The flexible pin has a laser mark up to which it is drilled. Then, as per the manufacturer's guidelines (Stryker Endoscopy, New Jersey, USA), the direct measuring device is inserted through a stab incision on the lateral aspect of the knee. The intra-osseous length is measured directly off the measuring device. Flexible pins were inserted with the knee at 70[degrees], 90[degrees], and 120[degrees] of flexion. The knee was held at flexion angles 70[degrees], 90[degrees], and 120[degrees] by an assistant and measured with a goniometer, corresponding to each trial. We decided to measure at 70[degrees] as well to confirm the trend that the more flexion, the safer it is. Also, it is possible that the difference may not be significant between 90[degrees] and 120[degrees], but the differences between 70[degrees] and 120[degrees] will be. The pin was inserted at the center of the ACL footprint with the help of a curved femoral guide. Although these femoral guides have an offset to reference off the posterior femoral cortex, the center of the footprint was used as the anatomical landmark. This was approximately at the 10 o'clock position. The pin was then drilled all the way out from the lateral femoral cortex and withdrawn from the outside until the laser mark was seen at the cortex in the intercondylar notch. The intra-osseous length was measured with the measuring device. The pins were then drilled out through the exit point until the proximal tip receded just under the intercondylar notch. Using the same entry point, the knee was flexed to 90[degrees], and the second guide pin was inserted. The steps were repeated with the knee in 120[degrees] of flexion. To confirm that the pins do not track the same route at progressive angles of flexion, x-rays were obtained in a few specimens (Figs. 2 and 3). Each specimen was dissected around the lateral aspect of the knee to identify the critical structures, the common peroneal nerve, and the LCL (Fig 4). The distance from the guide pins to the common peroneal nerve and femoral attachment of the LCL were measured with a standard flexible paper ruler to the nearest millimeter. We defined hyperflexion as 110[degrees] or above based on the number commonly referred to in the literature. (1)

Statistical Analysis

Ten knee specimens were used. A pilot study of four samples was utilized to calculate the specimens needed to achieve intra-osseous tunnel lengths greater than 20 mm in the 90[degrees] flexion group, as there were no parallel tunnel length data using flexible reamers. It was determined through power analysis that eight specimens were needed to achieve statistical significance. Power analysis was also performed for the secondary outcome measure of distance to common peroneal nerve based on a parallel study using rigid pins at 70[degrees], 90[degrees], and 120[degrees] of flexion. (6) The assumption of 20% greater distance to common peroneal nerve was used, and it was determined that a power of 0.90 would be achieved with 10 specimens; therefore, we proceeded with 10 knee specimens. One-Way ANOVA tests at the 0.05 level were carried out to detect differences among each variable (tunnel length, distance to common peroneal nerve, and distance to LCL) stratified by flexion angle. Paired t-tests were subsequently carried out to detect individual differences within each variable. Descriptive statistics, as well as significance analysis, were performed using Statistical Analysis Software (SAS Institute Inc, Cary, NC).


Intra-Osseous Length

The minimum and maximum distances, mean values, and standard deviations of the intra-osseous length for the 10 specimens are depicted in Table 1. There is a trend for progressively increasing mean intra-osseous length associated with increased flexion of the knee. The mean intra-osseous length for 70[degrees] flexion was 25.2 mm (20 mm to 32 mm) compared to mean intra-osseous lengths of 32.1 mm (22 mm to 45 mm) and 38.0 mm (34 mm to 45 mm) in the 90[degrees] and 120[degrees] flexion groups, respectively. Paired t-tests revealed significant differences in mean intra-osseous lengths between 70[degrees] and 90[degrees] as well as 70[degrees] and 120[degrees]. Statistical significant difference was also noted for 90[degrees] versus 120 [degrees] of knee flexion.

Distance to Lateral Collateral Ligament (LCL)

Minimum and maximum distances, mean values, and standard deviations are depicted in Table 1. There were no significant differences among the groups.

Distance to Common Peroneal Nerve

Minimum and maximum distances, mean values, and standard deviations are depicted in Table 1. There is a trend towards longer distances to the common peroneal nerve with increased flexion. There was no statistically significant difference in the distance to common peroneal nerve when comparing 120[degrees] versus 90[degrees] or 90[degrees] versus 70[degrees]. There was, however, a statistically significant difference when comparing 120[degrees] versus 70[degrees].


ACL reconstruction is one of the most common procedures performed in orthopaedics with excellent patient outcomes. (14-18) Nonetheless, many modifications in techniques have been proposed to decrease failure rates. The concept of anatomic single bundle in lieu of double bundle has been proposed for technical ease and equivocal results. (17,19) So far, the biomechanical results have shown to effectively restore the knee kinematics in a single bundle anatomic ACL reconstruction. (20-23) To achieve the anatomic position for the femoral tunnel, some surgeons recommend drilling via a medial or accessory medial portal. There are several types of reamers available for this purpose, including half-reamers, retrograde reamers, and flexible reamers. The aim is to get to the anatomical and position on the intercondylar notch. Flexible reamers aim to recreate the approach familiar to most surgeons. However,

drilling through an anteromedial tunnel produces shorter tunnel lengths than drilling a transtibial tunnel, (5,24) and rigid reamers produce shorter tunnels than flexible ones. (9) Thus, there is concern about shorter femoral tunnels and inadequate graft length in the tunnel. The minimum femoral tunnel length needed for adequate tendon-bone healing is not clear in the literature. (13,25) Two studies indicate that the ultimate load to failure is not significantly different for shorter tunnels, (12,13) whereas one study does show that maximizing tunnel length is beneficial. (25) These studies were done in animals and hence cannot be directly applied to humans. One of them reported that a tunnel as short as 15 mm may be sufficient for graft to tunnel healing. (13) Our hypothesis was that flexible reamers would produce adequate tunnel lengths at 90[degrees] of flexion, a commonly used position for ACL reconstruction. We chose a minimum of 20 mm length as adequate for graft fixation based on these studies.

Our study has its limitations, the most important being that we did not measure the distance from the exit point of pin on the lateral femoral condyle to the posterior cortex. This has significant clinical implications as to the risk of posterior wall blowout of the femoral tunnel. However, this distance to the posterior wall at 90[degrees] of knee flexion has been measured in a previous study and was found to be minimum of 9.8 mm, well within the safe limits. (11) All of our pins exited out of the lateral cortex and not the posterior cortex. Also, most of these studies that are concerning for posterior wall blowout have been done using rigid instrumentation and not flexible ones. Flexible instrumentations have a curve in the femoral guide that helps direct the pin away from the posterior cortex. This, in fact, brings the pin into the femoral condyle rather than direct it up the femoral shaft leading to a more horizontal trajectory. This potentially increases the length of the tunnel without compromising the posterior cortex. Another limitation of our study was that the pins in every specimen had the same entry point. This was done to minimize variability and limit the number of specimens needed. This could have potentially led the pins to follow the same track as the previous one. We did take x-rays to confirm that the pins were divergent. This technique was similar to a study previously described. (7) Other studies have looked at anteromedial portal drilling and tunnel lengths at 90[degrees] knee flexion. In the study by Baskedis, (7) rigid instrumentation was used, and the minimum tunnel length obtained was 18 mm with a mean of 27 mm. They argue that 25 mm or less may be unacceptable for interference or suspensory type of fixation. Our study looked at flexible pins, and our tunnel lengths were only slightly longer. However, the "minimum" acceptable tunnel is debatable. Lubowitz and Konicek reported a minimum tunnel length of 18.4 mm (mean: 30.5 mm) at 120[degrees] knee flexion. (26) Again, rigid pins were used. Only one other study looked at flexible instrumentation at 90[degrees] of knee flexion, and the minimum tunnel length obtained was 33.7 mm (mean: 38.3 mm). (11) However, they used an open arthrotomy approach as opposed to arthroscopic methods in our study. Our results show that the flexible instruments create femoral tunnel lengths that are significantly shorter at 70[degrees] and 90[degrees] (mean intra-osseous length of 25.2 mm and 32.1 mm) when compared to 120[degrees] of flexion (mean intra-osseous length of 38.0 mm). However, even at 90[degrees] of knee flexion, we had a mean length of 32.1 mm, and the minimum length obtained in one of the specimens was 22 mm. Our hypothesis that a flexible reamer would create adequate femoral tunnel length at 90[degrees] of knee flexion was supported by this study. However, care must be taken while using conventional suspensory types of fixation to accommodate the length of the suspensory device that is often 10 mm or above. Other types of suspensory fixations that involve looping the graft directly around the button maybe used for shorter tunnels.

The peroneal nerve is a potential structure at risk with lesser degrees of knee flexion. (6) Our study found that the distance from the exit point of the flexible pin to the peroneal nerve was safe. The minimum distance was 18 mm at 70[degrees] of flexion. For the LCL, we found a minimum distance of 15 mm at 120[degrees] of flexion. This distance was not significantly different at 70[degrees] and 90[degrees] of flexion. Only one other study looked at this distance to the critical structures using flexible instrumentation and found the minimum distance to be 35 mm for the peroneal nerve and 20 mm to the LCL femoral origin. (10)


This study that shows that adequate femoral tunnel lengths can be safely created without knee hyperflexion using flexible instruments via an anteromedial portal.

Kunal P. Kalra, M.D., Department of Orthopedics, Detroit Medical Centre, Detroit, Michigan. Edward Y. Tang, M.D., Department of Orthopedics, Contra Costa Regional Medical Center, Martinez, California. Omar N. Khatib, M.D., Department of Orthopaedics, Medical College of Wisconsin, Milwaukee, Wisconsin. Abiola A. Atanda, M.D., Steven Shamah, B.A., Robert J. Meislin, M.D., and Laith M. Jazrawi, M.D., NYU Hospital for Joint Diseases, New York, New York.

Correspondence: Laith M. Jazrawi, M.D., NYU Hospital for Joint Diseases, 301 East 17th Street, Suite 1402, New York, NY 10003;

Disclosure Statement

None of the authors have a financial or proprietary interest in the subject matter or materials discussed, including, but not limited to, employment, consultancies, stock ownership, honoraria, and paid expert testimony.


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Caption: Figure 1: Left knee cadaveric specimen mounted with anteromedial and anterolateral arthroscopic portals demonstrated. To view this figure in color, see

Caption: Figure 2: Anteroposterior x-ray of knee specimen demonstrating wire placement: from bottom to top, pins inserted at 70[degrees], 90[degrees], and 120[degrees] of flexion.

Caption: Figure 3: Lateral x-ay of knee specimen demonstrating wire placement: from left to right, pins inserted at 70[degrees], 90[degrees], and 120[degrees] of flexion.

Caption: Figure 4: Left knee dissection after pin insertion at varying angle of knee flexion with the origin of the lateral collateral ligament, the insertion of biceps femoris, and the common peroneal nerve shown. To view this figure in color, see

Table 1 Mean, Standard Deviations, and Ranges of Tunnel
Length Achieved Along with How Much Distance There was
Between the Critical Lateral Structures of the Common
Peroneal Nerve and Lateral Collateral Ligament

N = 10      Tunnel Length (mm)

            70[degrees]   90[degrees]   120[degrees]

Minimum     20            22            34
Maximum     32            45            45
Mean        25.2          32.1 *        38.0 ([dagger])
Std. Dev.    4.3           7.5           3.5

N = 10      Dist. To Peroneal Nerve (mm)

            70[degrees]   90[degrees]   120[degrees]

Minimum     18            35            48
Maximum     74            81            101
Mean        43.9          55.8           65.7
Std. Dev.   14.8          13.6           15.9

N = 10      Dist. To LCL (mm)

            70[degrees]   90[degrees]   120[degrees]

Minimum     16            13            15
Maximum     32            32            41
Mean        23.5          20.7          23.2
Std. Dev.    6.1           5.5           8.9

* Statistical Significance relative to 70[degrees]
(p < 0.05); ([dagger]) Statistical Significance
relative to 90[degrees] (p < 0.05).


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Article Details
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Author:Kalra, Kunal P.; Tang, Edward Y.; Atanda, Abiola A.; Khatib, Omar N.; Shamah, Steven; Meislin, Rober
Publication:Bulletin of the NYU Hospital for Joint Diseases
Article Type:Report
Date:Apr 1, 2016
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