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Does Knee Flexion Influence the Relationship between the Femoral Tunnel and the Lateral Anatomic Structures During ACL Reconstruction?

Chung, Kwangho MD; Kim, Sung-Jae MD, PhD; Choi, Chong Hyuk MD, PhD; Kim, Sung-Hwan MD, PhD; Choi, Yunjong MD; Jung, Min MD, PhD

Clinical Orthopaedics and Related Research®: October 2019 - Volume 477 - Issue 10 - p 2228–2239
doi: 10.1097/CORR.0000000000000845
CLINICAL RESEARCH
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Background In ACL reconstruction, the relationship of the femoral tunnel outlet to the anatomy of the lateral knee is clinically important, but whether that relationship is influenced by knee flexion using a transportal technique is unclear.

Questions/purposes The purposes of this study were to (1) to describe the relationship between the outlet of the femoral tunnel and the lateral anatomic structures of the femur, including the lateral epicondyle, lateral collateral ligament, popliteus tendon, and lateral head of the gastrocnemius, as a function of knee flexion angle when the tunnel was created; and (2) to determine the knee flexion range of angles that best limits the risk of damage to these structures as the femoral tunnel is created during anatomic single-bundle ACL reconstruction using the transportal technique.

Methods Between January 2017 and June 2018, 113 patients underwent ACL reconstruction, of which 62 (55%) who had a primary single-bundle ACL reconstruction with transportal technique using autogenous quadruple hamstring graft were included. Patients who were treated with grafts other than autogenous quadruple hamstring tendon, and had concomitant ligamentous injury, revisional ACL reconstruction, previous operative history of the affected knee, osseous deformity and osteoarthritis were excluded. Included patients were divided into three groups according to their knee flexion angles as the femoral tunnel was created. The femoral tunnel was created with rigid straight reamer with the knee flexed as much as possible in figure-of-four position and the flexion angle was measured with a sterile 12-inch goniometer intraoperatively for all patients. Fourteen patients (23%) had the femoral tunnel created with the knee in < 120° of flexion, 23 (37%) had the tunnel created in 120° to 129° of flexion, and 25 (40%) had the tunnel created in ≥ 130° of flexion. The femoral tunnel’s outlet and the lateral anatomic structures of the femur, including the femoral origins of the lateral epicondyle, lateral collateral ligament, popliteus tendon, and lateral head of the gastrocnemius, were identified on a three-dimensional model that was reconstructed using CT images taken on postoperative day 1. The shortest distances from the femoral tunnel’s outlet to these lateral anatomic structures were measured by two observers and interobserver reliability was high (intraclass correlation coefficient > 0.75). The distances were compared among the groups, and a correlation analysis of the measured distances regarding the knee flexion angle during creation of the femoral tunnel was performed. The safe distance was set as 12 mm between the centers of the femoral tunnel’s outlet and the lateral structures considering the footprint of the lateral structures, diameter of the femoral tunnel, and femoral tunnel widening. Any anatomic structures that were closer to the femoral tunnel than the safe distance were noted, and the cutoff point of knee flexion for injury to the lateral anatomic structures was determined with a receiver operating characteristic curve.

Results As knee flexion angle increased, the distance from the femoral tunnel to the lateral head of the gastrocnemius increased (r = 0.657, p < 0.001), and the distance to the lateral epicondyle decreased (r = -0.627, p < 0.001), as did the distance of the tunnel to the lateral collateral ligament (r = -0.443, p < 0.001) and the popliteus tendon (r = -0.653, p < 0.001). The cutoff point of the knee flexion angle associated with structural injury was 131° (sensitivity, 70%; specificity, 73%) for the lateral collateral ligament and 121° (sensitivity, 86%; specificity, 67%) for the lateral head of the gastrocnemius.

Conclusions As knee flexion increased, the femoral tunnel’s outlet tended to move more anteriorly and distally. Consequently, the safe distance to the lateral head of the gastrocnemius increased and the distances to the lateral epicondyle, lateral collateral ligament, and popliteus tendon decreased with increased knee flexion. To avoid possible damage to the lateral anatomic structures and obtain stable fixation in ACL reconstruction using the transportal technique, we recommend creating a femoral tunnel within 121° and 131° of knee flexion.

Level of Evidence Level III, therapeutic study.

K. Chung, S.-J. Kim, C. H. Choi, S.-H. Kim, M. Jung, The Arthroscopy and Joint Research Institute, the Department of Orthopaedic Surgery, Yonsei University College of Medicine, Seoul, Republic of Korea

Y. Choi, The Department of Orthopaedic Surgery, Yonsei University College of Medicine, Seoul, Republic of Korea

M. Jung, Arthroscopy and Joint Research Institute, Department of Orthopaedic Surgery, Yonsei University College of Medicine, C.P.O. Box 8044, 134, Shinchon-dong, Seodaemun-gu, Seoul 120-752, Republic of Korea, Email: jmin1103@yuhs.ac

One of the authors (MJ) has received, during the study period, funding from Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science and ICT (no. 2017R1C1B5017548).

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.

Each author certifies that his or her institution approved the human protocol for this investigation and that all investigations were conducted in conformity with ethical principles of research.

Clinical Orthopaedics and Related Research® neither advocates nor endorses the use of any treatment, drug, or device. Readers are encouraged to always seek additional information, including FDA approval status, of any drug or device before clinical use.

This work was performed at the Arthroscopy and Joint Research Institute, the Department of Orthopaedic Surgery, Yonsei University College of Medicine, Seoul, Republic of Korea.

Received January 14, 2019

Accepted May 09, 2019

Online date: May 30, 2019

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Introduction

In ACL reconstruction, placement of the femoral tunnel for the graft is important. Femoral drilling techniques have evolved to improve stability and reduce adverse events [6, 24], with placement of anatomic femoral tunnels demonstrating improved rotational stability and knee kinematics compared with a nonanatomic position [16, 25]. Accordingly, independent femoral drilling techniques to create a femoral tunnel in the anatomic footprint, such as the transportal technique and outside-in technique, have become widely used in place of the conventional transtibial technique [3, 14, 17, 22].

The advantage of the transportal technique is that it does not require lateral femoral dissection using an additional incision, which is used in the outside-in technique [24]. However, a femoral tunnel created with the transportal technique through an accessory anteromedial portal has potential shortcomings such as short tunnel length [4, 7, 12], posterior wall blowout [4], and damage to the medial femoral cartilage [20, 27]. In addition to these concerns, previous cadaver studies showed there was a risk of iatrogenic injury to the lateral anatomic structures of the femur [10, 19-21]. When a suspensory device is used for fixation with the transportal technique, the outlet of the femoral tunnel has been noted to move distally compared with the transtibial technique [22], increasing the possibility of damaging the lateral anatomic structures of the femur. The location of the outlet of the femoral tunnel has important implications for the following reasons: damage to the lateral soft-tissue structures, stable fixation of the graft by securing the suspensory fixation device on the bone, and intertunnel relationship in multiligament reconstruction.

To the best of our knowledge, no study has examined the relationship between the outlet of the femoral tunnel and the lateral anatomic structures of the femur based on intraoperative knee flexion in anatomic single-bundle ACL reconstruction using the transportal technique.

The purposes of this study, therefore. were to (1) to describe the relationship between the outlet of the femoral tunnel and the lateral anatomic structures of the femur, including the lateral epicondyle, lateral collateral ligament, popliteus tendon, and lateral head of the gastrocnemius as a function of knee flexion angle when the tunnel was created; and (2) to determine the knee flexion range of angles that best limits the risk of damage to these structures during creation of the femoral tunnel during anatomic single-bundle ACL reconstruction using the transportal technique.

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Patients and Methods

Our institutional review board approved this study. We retrospectively reviewed data on patients diagnosed with an ACL rupture that was treated with ACL reconstruction between January 2017 and June 2018. A total of 113 patients underwent ACL reconstruction during the study period; all had single-bundle ACL reconstruction. We included patients who met the following criteria: (1) those who underwent primary single-bundle ACL reconstruction with the transportal technique using an autogenous quadruple hamstring graft; (2) those with an isolated ACL injury without concomitant ligamentous injury; (3) those with a femoral tunnel created in the ACL footprint based on a postoperative three-dimensional (3-D) CT analysis [26]; and (4) patients with femoral fixation of the graft using suspensory fixation via EndoButton CL Ultra (Smith & Nephew, Memphis, TN, USA). We excluded patients with (1) revision ACL reconstruction; (2) an operative history of the affected knee; (3) malalignment of the lower extremity (normal mechanical axis line passes 8 ± 7 mm medial to the center of knee line on standing hip-knee-ankle radiographs [23]; (4) osseous deformity including femoral or tibial fracture; and (5) osteoarthritis classified as greater than Kellgren-Lawrence Grade I. After applying the above-mentioned criteria, we included 62 patients in the present study (sequential exclusion of 51 patients: 26 = used grafts other than autogenous quadruple hamstring tendon; 11 = concomitant ligamentous injury; two = revision ACL reconstruction; three = operative history on affected knee; five = malalignment of the lower extremity; one = osseous deformity; three = osteoarthritis classified as greater than Kellgren- Lawrence grade I) (Fig. 1).

Fig. 1

Fig. 1

We divided the patients into three groups according to the knee flexion angle that was used while creating the femoral tunnel. During femoral tunnel creation, the knee was flexed as much as possible in the figure-of-four position and the flexion angle was measured with a sterile 12-inch goniometer intraoperatively for all patients. Group 1 (n = 14) consisted of patients in whom the femoral tunnel was created at knee flexion angles of less than 120°. Group 2 (n = 23) consisted of patients in whom the femoral tunnel was created at knee flexion angles between 120° and 130°. Group 3 (n = 25) consisted of patients in whom the femoral tunnel was created at knee flexion angles of 130° or more (Fig. 1).

The demographic data of these patients, including sex, age, height, weight, BMI, injured side, and elapsed time from injury to surgery, were not different among the groups (Table 1).

Table 1

Table 1

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Operative Procedure

A single surgeon (MJ) performed all surgical procedures. For quadruple hamstring tendon grafts, the semitendinosus and gracilis tendons were divided proximally at the musculotendinous junction with an open-loop tendon stripper. Both ends of each tendon were whipstitched with a Number 1 Ethibond suture (Ethicon Inc, Somerville, NJ, USA). The average diameter of graft was 7 mm (range, 6–9 mm). The femoral and tibial tunnels were created on the footprint of the ACL. The centers of the femoral and tibial footprints of the ACL were identified with reference to ACL remnants and anatomic landmarks [13, 26]. A tibial tunnel was made on the tibial ACL footprint using a tibial ACL guide. To create the femoral tunnel, we made an accessory anteromedial portal immediately above the medial meniscus and as far away as possible from the medial border of the patellar tendon, with the knee flexed at 90° in the figure-of-four position. We confirmed that the femoral ACL footprint could be reached through the accessory anteromedial portal. The trajectory through this accessory anteromedial portal was close to the medial femoral condyle without damaging the cartilage of the medial femoral condyle. After carefully flexing the knee as much as possible, the surgeon inserted a Beath pin through the accessory anteromedial portal and advanced it to the centrum of the femoral ACL footprint through the lateral femoral condyle and out the far cortex of the lateral femur. With the knee in hyperflexion, we made the femoral tunnel with a rigid straight reamer and the tunnel for the suspensory fixation device to pass through. The knee flexion angle during creation of the femoral tunnel was measured and recorded. We used a sterile goniometer with two 12-inch arms for the measurement of the knee flexion angle. To improve the consistency of the measurement, we used the same landmarks (greater trochanter, lateral epicondyle, fibular head and lateral malleolus) to measure the knee flexion angle based on a previous study [9]. The center of the goniometer body was placed on the lateral epicondyle of femur. A stationary arm was aligned with the femoral shaft, using the greater trochanter for reference. The moving arm was placed parallel to the line representing the tibial shaft, which connected the fibular head and the lateral malleolus, and the knee flexion angle was recorded. A single operating surgeon (MJ) performed the measurement. After positioning the graft, we secured the femoral side with the suspensory fixation device, and the tibial side was secured with a bioabsorbable interference screw and screw-and-washer construct.

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Postoperative 3-D CT Analysis and Measurement

A postoperative CT scan was taken on postoperative day 1. The CT scan was performed with the CT scanner Sensation 64 (Siemens Healthcare, Erlangen, Germany). The tube parameters were 120 kVp and 135–253mAs. The acquisition matrix was 512 × 512 pixels. The scan field of view was 134–271 mm, and the slice thickness was 0.6 to 1 mm. Digital Imaging and Communications in Medicine (DICOM) data of the postoperative CT scan were extracted from our picture archiving and communication system (Centricity PACS, GE Medical System Information Technologies, Milwaukee, WI, USA). Extracted images were imported into Mimics software (version 17; Materialise, Leuven, Belgium), and a 3-D model of the femur was constructed. The suspensory fixation device located on the lateral femoral condyle was digitally removed to clearly identify the femoral tunnel outlet.

As one of the above-mentioned inclusion criteria of the present study, all patients were confirmed to have a femoral tunnel within the ACL footprint (Fig. 2). The standard area of the center of the ACL’s femoral footprint was determined according to the method of a previous study [26]. Similar to the quadrant method using standard lateral radiographs that was described previously [5], a 4 × 4 grid was placed on the medial wall of the lateral femoral condyle from a true medial view of the femur that was established at 90° of knee flexion with a 3-D model of the femur [8]. As the reference for the grid alignment, the most anterior edge of the intercondylar notch replaced Blumensaat’s line on a standard lateral radiograph. The coordinates were set by the proportions of the grids along Blumensaat’s line and perpendicular to it. The coordinates of the standard area of the center of the ACL’s femoral footprint were as follows: the distance of the center of the ACL’s footprint parallel to Blumensaat’s line was 27.5% ± 4.6% along the line measured from the posterior border, and the distance of the center of the ACL’s footprint perpendicular to Blumensaat’s line was 35.9% ± 9.2% along the line measured from Blumensaat’s line [26]. Only patients whose femoral tunnel center’s coordinates were within this mean and SD were included in the study. There was no difference in the coordinate values of the intraarticular center of the femoral tunnel on the 4 x 4 grid at the medial wall of the lateral femoral condyle between the groups (Table 2).

Fig. 2

Fig. 2

Table 2

Table 2

To determine the positional relationship of the femoral tunnel’s outlet, we used bony anatomic landmarks to identify the lateral anatomic structures of the femur based on the protocol of a previous anatomic study [15] (Fig. 3). These authors described the quantitative locational relationship between the origin of the lateral anatomic structures of the femur with reference to bony landmarks including the lateral epicondyle, supracondylar process, and popliteal sulcus. The lateral epicondyle is the most prominent point of the lateral condyle of the femur, and the popliteal sulcus is a groove on the lateral femoral condyle near the border of the articular cartilage. The femoral origin of the lateral collateral ligament is a small bony depression that is 1.4 mm proximal and 3.1 mm posterior to the lateral epicondyle, and the femoral origin of the lateral head of the gastrocnemius is consistently located near or at the supracondylar process of the distal femur, which is 17.2 mm and 13.8 mm from the lateral epicondyle and the attachment of the lateral collateral ligament, respectively. The femoral origin of the popliteus tendon is at the most anterior one-fifth of the popliteal sulcus. The distance between the centers of the popliteus tendon and the lateral collateral ligament is 18.5 mm.

Fig. 3

Fig. 3

The lateral anatomic structures of the femur, including the lateral epicondyle and the femoral origins of the lateral collateral ligament, popliteus tendon, and lateral head of the gastrocnemius were determined based on that previous anatomic study [15]. The shortest distance from the center of the lateral anatomic structures of the femur to the center of the outlet of the femoral tunnel for ACL reconstruction was measured with the 3-D reconstructed model. The distance to the lateral epicondyle was defined as the distance between the center of the femoral tunnel’s outlet and the lateral epicondyle, the distance to the lateral collateral ligament was defined as the distance between the center of the femoral tunnel’s outlet and the femoral origin of the lateral collateral ligament, the distance to the popliteus tendon was defined as the distance between the center of the femoral tunnel’s outlet and the femoral origin of the popliteus tendon, and the distance to the lateral head of the gastrocnemius was defined as the distance between the center of the femoral tunnel’s outlet and the femoral origin of the lateral head of the gastrocnemius (Fig. 4). The length of the femoral tunnel was also measured. Two different orthopaedic surgeons (JJ, CW) who were blinded to the patients’ information measured these values to increase the reliability. The mean of two numerical values was used. The intraclass correlation coefficients for interobserver reliability of the distance from the outlet of the femoral tunnel to the lateral anatomic structures were 0.878 for the lateral epicondyle (95% CI, 0.798–0.927), 0.835 for the lateral collateral ligament (95% CI, 0.726–0.901), 0.847 for the popliteus tendon (95% CI, 0.746–0.908), and 0.879 for the lateral head of the gastrocnemius (95% CI, 0.799–0.927). The statistical power was 84.6% for the distance to the lateral collateral ligament and 99.5% for the distance to the lateral head of the gastrocnemius.

Fig. 4

Fig. 4

To determine the safe range that would not result in damage to the lateral anatomic structures, we calculated the radius of the footprint based on the surface area. According to an anatomic study [15], the average cross-sectional areas of the attachment sites of the lateral collateral ligament and popliteus tendon were 0.48 cm2 (range, 0.43–0.52 cm2) and 0.59 cm2 (range, 0.53–0.62 cm2), respectively. Accordingly, the approximate radii of the attachment site of the lateral collateral ligament and popliteus tendon were calculated as 3.90 mm and 4.33 mm, respectively. Because no quantitative data exist regarding the area size of the lateral head of the gastrocnemius’s femoral origin, the radius of the attachment site was presumed to be 5 mm by reference to the footprint size of the lateral collateral ligament and popliteus tendon. The diameter of the femoral tunnel’s outlet at the far cortex of the lateral femoral condyle was determined as the diameter of the femoral tunnel. According to the method of an earlier study [18], the diameter of the postoperative tunnel could be extended to 11.15 ± 1.46 mm because of tunnel widening in ACL reconstruction using a hamstring tendon and a suspensory fixation device. The approximate maximum radius of the femoral tunnel’s outlet was calculated as 6.30 mm. Accordingly, the sufficient minimum safe distance between the center of the femoral tunnel’s outlet and the lateral anatomic structures was determined as 12 mm, considering the sum of the above elements and individual deviation.

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Statistical Analysis

The Shapiro-Wilk test was used to test for normality. We used ANOVA to compare continuous variables among the groups. The chi-square or Fisher’s exact test was used for categorical variables. We used Pearson’s correlation analysis to assess the correlation between the knee flexion angle as the femoral tunnel was created and the distance from the lateral anatomic structures of the femur to the outlet of the femoral tunnel. We used a receiver operating characteristic curve to determine a cutoff point for the knee flexion angle for injury to the lateral collateral ligament and lateral head of the gastrocnemius. Twelve millimeters, which we determined as the minimum safe distance between the outlet of the femoral tunnel and the lateral anatomic structures by considering the sum of the radii of the footprint and femoral tunnel’s outlet, was established as the reference for the receiver operating characteristic curve. The cutoff point calculated by the receiver operating characteristic curve maximizes sensitivity and specificity. The area under the curve is widely recognized as discriminatory. The interobserver reliability of the measurement of variables was analyzed with an intraclass correlation coefficient set at a 95% CI. A p value < 0.05 was set as statistically significant. The IBM SPSS Statistics for Windows software program (version 23.0; IBM, Armonk, NY, USA) was used for the statistical analyses. The statistical power was calculated using G*Power version 3.1 (Düsseldorf, Nordrhein-Westfalen, Germany) [11].

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Results

As knee flexion angle during the femoral tunnel creation increased, the distance from the femoral tunnel to the lateral head of the gastrocnemius increased and the distance to the lateral epicondyle decreased, as did the distance of the tunnel to the lateral collateral ligament and the popliteus tendon. The knee flexion angle during femoral tunneling was negatively correlated with the distances to the lateral epicondyle (r = -0.627, p < 0.001) (Fig. 5A), lateral collateral ligament (r = -0.443, p < 0.001) (Fig. 5B), and popliteus tendon (r = -0.653, p < 0.001) (Fig. 5C) and positively correlated with the distance to the lateral head of the gastrocnemius (r = 0.657, p < 0.001) (Fig. 5D). The overall distributions of the femoral tunnel outlets according to the flexion angle as the femoral tunnel was created showed that as the flexion angle of the knee increased, the outlet of the femoral tunnel tended to move anteriorly and distally (Group 1: knee flexion angle < 120° [Fig. 6A]; Group 2: knee flexion angle 120°-129° [Fig. 6B]; Group 3: knee flexion angle ≥ 130° [Fig. 6C]; and overall distribution [Fig. 6D]). The distances from the outlet of the femoral tunnel to the lateral epicondyle, lateral collateral ligament, popliteus tendon, and lateral head of the gastrocnemius and femoral tunnel length showed differences among the groups (Table 2).

Fig. 5 A-D

Fig. 5 A-D

Fig. 6 A-D

Fig. 6 A-D

A knee flexion angle of 121° to 131° provided the lowest likelihood of damage to the lateral-sided structures. The cutoff point of the knee flexion angle associated with structural injury was 131° (sensitivity, 70%; specificity, 73%; area under curve 0.773) for the lateral collateral ligament (Fig. 7A) and 121° (sensitivity, 86%; specificity, 67%; area under curve 0.830) for the lateral head of the gastrocnemius (Fig. 7B). Knee flexion angle more than 131° increased the risk of damage for the lateral collateral ligament and knee flexion angle less than 121° increased the risk of damage for the lateral head of gastrocnemius. As the knee flexion angle increased, the number of patients with the distance between the center of the femoral tunnel outlet and the lateral anatomic structures measuring less than the safety distance of 12 mm increased for the lateral collateral ligament and decreased for the lateral head of the gastrocnemius (Table 3). There were no surgical complications, including tunnel breakage.

Fig. 7 A-B

Fig. 7 A-B

Table 3

Table 3

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Discussion

Because improper placement of the femoral tunnel is one of the main causes of unsuccessful ACL reconstruction, numerous studies have examined factors related to the tunnel, such as tunnel length, breakage of the posterior wall, and bending angle of the graft [1, 2, 8, 28]. However, most of these studies have overlooked iatrogenic damage to structures other than the tunnel itself. Femoral tunnel outlets located on the far cortex of the lateral femoral condyle tend to move more distally when a transportal technique is used than when a transtibial technique is used [22]. Accordingly, the risk of damage to the lateral anatomic structures of the femur is increased when the transportal technique is used, and this risk should be considered when creating the femoral tunnel. This risk of damage to the lateral anatomic structures has not been considered from the clinical perspective. The present study showed that as knee flexion increased, the distance from the femoral tunnel outlet to the lateral head of the gastrocnemius increased while the distance to the lateral epicondyle, the lateral collateral ligament and the popliteus tendon decreased. Further, we found that a knee flexion angle of 121° to 131° provided the lowest likelihood of damage to the lateral-sided structures.

In interpreting the results of this study, readers must consider numerous limitations. First, the patients were allocated to three groups based on the maximum knee flexion angle at the time of the femoral tunnel creation. Because the angle of knee flexion could have been affected by factors such as the thickness of the patient’s thigh, surgical drapes, a tourniquet worn on the thigh, or different undetermined factors in different ways for each patient, it was considered that different flexion angles would be measured for each patient. Surgeon preference did not play a role in predetermining the flexion angle for each patient. However, perfect randomization could not be achieved as a result. Second, the knee flexion angles during the femoral tunnel creation were measured by a sterile 12-inch goniometer intraoperatively. To improve the consistency of the measurement, we used the same landmarks (greater trochanter, lateral epicondyle, fibular head and lateral malleolus) to measure the knee flexion angle based on a previous study [9]. Even if the measurement was consistently performed in the same manner by the same operating surgeon, we acknowledge the practical limit to the accuracy of the measurement. Third, the number of patients in Group 1 (knee flexion angle < 120°) was relatively small. The reliability of the results can be affected by a small sample size. However, the statistical power was calculated to be high (greater than 80%) for the comparison of the distance to the lateral collateral ligament (84.6%) and the distance to the lateral head of the gastrocnemius (99.5%) among the groups, which were the main results of this study. Fourth, bony landmarks and quantitative relationships were used to identify the lateral anatomic structures of the femur. This was based on a prior anatomic study [15] that found quantitative relationships between each of the lateral anatomic structures examined in our study. Although numerical data and surface anatomy were based on comprehensive anatomic evidence, we acknowledge that some individual variation would still be expected among patients. Thus, it might not reflect the actual attachment site in each patient. Fifth, the measurement was performed with a 3-D reconstructed CT model and data that were obtained by assessing virtual measurements. Even though the measurement with a 3-D reconstructed model should be close to the actual value, the measured value and the actual value might differ slightly. Sixth, we selected 12 mm as the minimum safe distance between the center of the femoral tunnel’s outlet and the lateral anatomic structures, considering widening of the tunnel’s diameter and the surface area of the lateral anatomic structures. However, this tunnel diameter could vary among individuals, depending on the graft size and degree of tunnel widening. Therefore, the safe distance without injury may vary for each patient. Lastly, the location of the femoral origin of the lateral head of the gastrocnemius has been described in a previous study [15], but quantitative data for the surface area size of the femoral origin of the lateral head of the gastrocnemius has not been reported. In this study, the radius of the attachment site of the lateral head of the gastrocnemius was presumed to be 5 mm compared with the footprint size of the lateral collateral ligament and popliteus tendon.

As knee flexion at the time of femoral tunnel creation increased, the tunnel drew closer to the lateral epicondyle, the lateral collateral ligament and the popliteus tendon and further from the lateral head of the gastrocnemius. Previous experimental studies [10, 19, 21] have demonstrated the influence of change in the knee flexion angle while the femoral tunnel is being created on the location of the femoral tunnel’s outlet during ACL reconstruction with a transportal technique. A virtual simulation study with a 3-D knee model reconstructed from MRI [21] showed that an increased flexion angle moves the outlet of the femoral tunnel anteriorly and distally in double-bundle ACL reconstruction using a transportal technique. This study compared only 120° and 135° of knee flexion to examine the influence of knee flexion angle and showed that 120°of knee flexion created the posterolateral tunnel within or closer to the lateral head of gastrocnemius than 135° of knee flexion. Nakamae et al. [19] conducted a cadaveric study of double-bundle ACL reconstruction to compare the locations of femoral tunnel outlets created at three flexion angles: 90°, 110° and 130°. They likewise showed that the femoral tunnel outlet moves anteriorly and distally, approaches the femoral origin of the lateral collateral ligament, and moves away from the lateral head of gastrocnemius as the knee flexion angle increases from 90° to 130° at an interval of 20°. Our results are consistent with the results of these previous experimental studies. However, our study has more clinical significance because it involved actual patients who underwent ACL reconstruction with a flexion angle, which was the condition of the tunnel creation presented as a continuous value.

In terms of injury to the lateral anatomic structures, it appears desirable to create the femoral tunnel at a knee flexion angle between 121° and 131°. A few studies have described the risk of the femoral tunnel exit penetrating lateral structures and the recommended knee flexion angle during femoral tunnel creation. In the study of Osaki et al. [21], femoral tunnels for the posterolateral bundle created in 120° of knee flexion were more likely to be located under the lateral head of the gastrocnemius than tunnels created in 135° of knee flexion. Nakamae et al. [19] noted that the risk of damage to the lateral collateral ligament increased as the knee flexion angle increased to 130°, but penetration of the gastrocnemius occurred at 110° of knee flexion in their cadaveric study of posterolateral bundle reconstruction. They concluded that a knee flexion angle between 110° and 120° was recommended to avoid the risk of damage to these femoral structures. Another cadaveric study [10] also demonstrated that the tunnel outlet for the posterolateral bundle should be made in at least 110° of knee flexion to protect the cardinal lateral structures from iatrogenic injury in comparisons between 90°, 110°, and 130° of knee flexion. Since we studied anatomical single-bundle ACL reconstruction, which is more frequently performed than double-bundle reconstruction, the safe range of the knee flexion angle of our study differed slightly from those of studies on double-bundle ACL reconstruction. Our study more clearly delineates the safe range of knee flexion angle in femoral tunnel creation because the degree of knee flexion was studied over a continuum of knee flexion. This finding also would have clinical importance for multiligament reconstruction that includes ACL reconstruction. In multiligament reconstruction, such as combined ACL and posterolateral corner reconstruction or combined ACL and anterolateral ligament reconstruction, the relationship between tunnels without convergence must be established. Understanding the location of the femoral tunnel outlet when reconstructing the ACL can help reduce complications of tunnel convergence in multiligament reconstruction.

In conclusion, as the knee flexion angle increased, the outlet of the femoral tunnel tended to move more anteriorly and distally. Consequently, the distance to the lateral head of the gastrocnemius increased, and the distance to the lateral epicondyle, lateral collateral ligament, and popliteus tendon decreased with an increased knee flexion angle. To avoid possible damage to the lateral anatomic structures while still obtaining stable fixation, we recommend creating a femoral tunnel within 121° and 131° of knee flexion during ACL reconstruction when using a transportal technique. We addressed the flexion angle of the knee as a condition for creating the femoral tunnel and the effect that flexion had on the location of the femoral tunnel outlet. However, in addition to the flexion angle of the knee, there may be various other factors–such as transverse drill angles set according to the locations of the accessory anteromedial portal–that affect femoral tunnel creation, stability of the reconstruction, and iatrogenic damage to surrounding structures. Studies involving a larger number of patients that integrate such factors will be needed to reach more definitive conclusions.

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Acknowledgments

We wish to thank two orthopaedic surgeons, Jinyoung Jang MD and Chanmyoung Woo MD, for their assistance in the measurement of parameters.

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