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. Author manuscript; available in PMC: 2021 Jul 1.
Published in final edited form as: Am J Sports Med. 2020 Jun 9;48(8):1893–1899. doi: 10.1177/0363546520924168

Reconsidering reciprocal length patterns of the anteromedial and posterolateral bundles of the anterior cruciate ligament during in vivo gait

Zoë A Englander 1,2, Jocelyn Wittstein 1, Adam P Goode 1,4, William E Garrett 1, Louis E DeFrate 1,2,3
PMCID: PMC7693121  NIHMSID: NIHMS1647751  PMID: 32515986

Abstract

Background:

Some cadaveric studies have indicated that the anterior cruciate ligament (ACL) consists of anteromedial (AM) and posterolateral (PL) bundles that display reciprocal function with regard to knee flexion. However, several in vivo imaging studies have suggested that these bundles elongate in parallel with regard to flexion. Furthermore, the most appropriate description of the functional anatomy of the ACL is still debated, with the ACL being described as consisting of two or three bundles, or as a continuum of fibers.

Hypothesis:

We hypothesized that as long as their origination and termination locations are defined within the ACL attachment site footprints, ACL bundles elongate in parallel with knee extension during gait.

Study Design:

Descriptive laboratory study

Methods:

High-speed biplanar radiographs of the right knee joint were obtained during gait in 6 healthy male subjects (BMI: 25.5±1.2 kg/m2, Age: 29.2±3.8 years) with no history of lower extremity injury or surgery. 3D models of the right femur, tibia, and ACL attachment sites were created from magnetic resonance (MR) images. The bone models were registered to the biplanar radiographs, thereby reproducing the in vivo positions of the knee joint. For each knee position, the distances between the centroids of the ACL attachment sites were used to represent ACL length. The lengths of 1000 virtual bundles were measured for each subject by randomly sampling locations on the attachment site surfaces, and measuring the distances between each pair of locations. Spearman-rho rank correlations were performed between the virtual bundle lengths and ACL length.

Results:

The virtual bundle lengths were highly correlated with the length of the ACL defined as the distance between the centroids of the attachment sites (rho = 0.91±0.1, mean±standard deviation across subjects, P<5×10−5). The lengths of the bundles that originated and terminated in the anterior and medial aspects of the ACL were positively correlated (rho=0.81±0.1, P<5×10−5) with the lengths of the bundles that originated and terminated in the posterior and lateral aspects of the ACL.

Conclusion:

As long as their origination and termination points are specified within the footprint of the attachment sites, ACL bundles elongate in parallel as the knee is extended.

Clinical Relevance:

This data elucidates ACL functional anatomy and may help guide ACL reconstruction techniques.

Keywords: kinematics, anatomy, strain, flexion, gait, reconstruction

Summary:

This study examined the attachment site to attachment site length changes of numerous subdivisions of the ACL using MRI and high-speed biplanar radiography. We found that as long as their origination and termination points are specified within the ACL attachment site footprints, ACL subdivisions elongate in parallel as the knee is extended during in vivo gait.

Introduction

Anterior cruciate ligament (ACL) reconstructive surgery is one of the most commonly performed sports medicine procedures.20 The goal of ACL reconstruction is to restore the stability of the knee joint after ACL rupture. However, while reconstructive surgery has been shown to improve functional outcomes following ACL injury, some studies have suggested that these procedures do not fully restore normal ACL function9, 14, 27, 30 or prevent long term degenerative changes in the knee joint.23 Thus, in an effort to improve the efficacy of ACL reconstruction, there has been a focus on creating reconstruction techniques that more closely restore the anatomic function of the ACL.1, 2, 7, 9, 18, 28

It has been suggested that a double-bundle graft may more accurately reflect the structure of the native ACL and improve deficits in knee kinematics that persist with a single-bundle reconstruction.7, 19, 35 These assertions are related to findings from cadaveric studies that have suggested that there may be two functional bundles of the ACL, defined as the anteromedial (AM) and posterolateral (PL) bundles, that have reciprocal length patterns throughout the flexion path.3, 8, 15 Specifically, these studies have suggested that the PL bundle becomes relatively lax in flexion and taut in extension while the AM bundle becomes more taut in flexion and lax in extension.3, 8, 15 Furthermore, it has been suggested that the AM bundle functions mainly to resist anterior translation, while the PL bundle plays a greater role in restraining rotational motion.13, 34, 37

However, there remains significant debate in the literature regarding ACL subdivision organization and length patterns.29 Specifically, while some studies have identified two or more distinct bundles3, 16, 24, 25, other studies have described the ACL as a continuum of numerous bundles.6 Furthermore, the findings of several in vivo imaging studies have been in contrast to the findings of previous work with regard to the length patterns of the AM and PL bundles during flexion. Specifically, in vivo imaging studies that have measured the length patterns of AM and PL bundles have suggested that both bundles are elongated with knee extension and are less elongated in flexion during weight-bearing17, 22, 32 and non-weight bearing knee flexion36, single legged hopping10, and gait.11, 12, 33

Thus, in order to better elucidate the functional anatomy of the ACL, we aimed to comprehensively investigate the length patterns of numerous subdivisions of the ACL during gait using high-speed biplanar radiography integrated with models of the knee joint derived from magnetic resonance (MR) images. We aimed to answer the question of how virtual bundles, defined by various origination and termination points, elongate during gait. Furthermore, we aimed to investigate whether there are subsets of virtual bundles that display reciprocal length patterns, as has been suggested by ex vivo studies. The methodology used here allows for precise measurement of in vivo length changes of the native ACL during common dynamic activities.4, 10, 12 We hypothesized that the virtual bundles would elongate in parallel as the knee extends during gait. Furthermore, we hypothesized that the lengths of the bundles that originated and terminated in the anterior and medial aspects of the ACL would be positively correlated with the lengths of the bundles that originated and terminated in the posterior and lateral aspects of the ACL.

Materials and Methods:

Six healthy male subjects (body mass index (BMI): 25.5±1.2 kg/m2, Age: 29.2±3.8 years mean ± standard deviation) with no previous history of lower extremity injury or surgery prior to testing were evaluated using an IRB approved protocol. All subjects provided written informed consent. One knee of each subject was imaged using a 3T magnetic resonance imaging (MRI) scanner (Trio Tim, Siemens Medical Solutions USA, Malvern, PA). Sagittal, coronal, and axial images were acquired from the subjects while lying supine, using a double-echo steady-state sequence (DESS) and an eight-channel knee coil (resolution: 0.3×0.3×1 mm; flip angle: 25°, repetition time: 17 ms, echo time: 6 ms). Outlines of the femur and tibia, as well as the footprints of the ACL attachment sites on the femur and tibia were segmented manually using solid-modeling software in all three planes (Rhinoceros 4.0, Robert McNeel and Associates, Seattle, WA) (Figure 1A,B). These segmentations were compiled into 3D models of the joint26 (Figure 1C). Additionally, the ACL attachment site footprints on the femur and tibia were divided into anteromedial (AM) and posterolateral (PL) bundles (Figure 1D). Specifically, the AM and PL bundle subdivisions were estimated by dividing the femoral and tibial ACL attachment sites such that their surface areas were evenly distributed between the anterior and medial aspects and the posterior and lateral aspects of the footprints. The positions and shapes of the ligament attachment sites were confirmed in the three orthogonal imaging planes. Prior validation studies have demonstrated that this approach can locate the anatomic center of the ACL footprint to within 0.3±0.2 mm.1, 31

Figure 1 (Adapted from11):

Figure 1 (Adapted from11):

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The outer contours of the femur and tibia were outlined in double echo steady state (DESS) magnetic resonance (MR) images in the (A) sagittal plane (B) coronal plane and axial plane (not shown). (B) ACL attachment sites (shown in red) were outlined in the coronal plane. (C) These outlines were compiled into 3D surface models of the femur and tibia and associated ACL attachment sites (shown in red). (D) Anteromedial (AM) and posterolateral (PL) subdivisions of the ACL attachment sites were estimated (shown in red and green, respectively).

Following MR image acquisition, the subjects were imaged using a high-speed biplanar radiography system consisting of two x-ray generators (EMD technologies), two x-ray tubes (G296, Varian), and two image intensifiers (41 cm diameter, TH 9447 QX, Thales) which are coupled to two high-speed cameras (Phantom v9.1, Vision Research). Each radiograph had a resolution of 1152×1152 pixels2. First, the positions of the sources and intensifiers were adjusted in order to ensure that the full gait cycle could be captured within the field of view without interference from the contralateral leg. Next, calibration images were acquired in order to map the geometry of the imaging set up and correct image distortion, as described previously.12 Subjects then ambulated at a speed of 1 m/s on a dual belt treadmill (Bertec) while high-speed biplanar radiographs were obtained at a frame rate of 120 Hz.11 Each in vivo experiment used a radiographic protocol not exceeding 110 kVp/200 mA.10, 11 To assess radiation risk to participants, the radiation effective dose was calculated by our institution’s radiation safety department from the total skin entrance exposure and energy absorption by the tissues and was found to be less than 0.14 mSv per participant.

Data analyses were performed using Matlab (version R2016B, Mathworks, Natick MA). Following data collection, the 3D bone models, the calibration images, and biplanar radiographs were imported into custom registration software.12 Subsequently, the software was employed to move each bone separately within six degrees of freedom until its projections onto the two imaging planes from the perspective of the radiographic sources matched the outlines of bones as seen in the radiographs (Figure 2).

Figure 2:

Figure 2:

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Each bone was moved separately in six degrees of freedom until its projections onto the two imaging planes from the perspective of the radiographic sources matched the bones as seen in the radiographs. Previous validation of this technique has shown to have a precision of approximately 70 μm in measuring the relative distances between two matched bones.

After reproducing the in vivo positions of the bones during the full gait cycle, the knee flexion angle and the length of the ACL, defined as the distances between attachment site centroids, were determined for each knee position.10, 32 We then measured the lengths of 1000 virtual ACL bundles for each subject by randomly sampling 1000 locations each on the femoral and tibial attachment site surfaces, and measuring the distances between pairs of these locations. Spearman-rho rank coefficients were used to determine the relationships between each of the 1000 virtual bundle lengths and ACL length throughout gait for each subject. Virtual bundles were then classified into virtual AM bundles, which originated and terminated in the anterior and medial aspects of the ACL attachment site footprints, and virtual PL bundles, which originated and terminated in the posterior and lateral aspects of the ACL attachment site footprints. The bundles that originated in the anterior medial aspect of the femur and terminated in the posterior lateral aspect of the tibia (or originated in the anterior medial aspect of the tibia and terminated in the posterior lateral aspect of the femur) could not be classified as either virtual AM or virtual PL bundles.

Statistical Analysis:

Spearman rho rank correlations were performed between each of the 1000 virtual bundle lengths and ACL length throughout gait. Additionally, Spearman rho rank correlations were performed between the lengths of virtual AM bundles and the lengths of the virtual PL bundles. A significance level threshold of P<5×10−5 was used, and average correlation coefficients across subjects were calculated using only correlations that reached statistical significance. The Bonferroni corrected significance level threshold was set at P<5×10−5 to account for the number of comparisons being made. The number of virtual bundles selected for each subject was informed by our prior work where we varied the locations of the ACL attachment site centroids 20 times within a 5 mm radius of the original attachment site centroids, and measured the lengths of these subdivisions.11 Based upon this prior work we chose a higher number of virtual bundles (i.e., n=1000) because this number of bundles ensured the bundle attachment site origination and termination points would be distributed across the entire surface of the femoral and tibial ACL attachment site footprints.

Results

During gait, the minimum flexion angle was −6.8 ± 10.0° (mean ± standard deviation across subjects), and the maximum flexion angle was 55.5 ± 5.2°. ACL length at the minimum flexion angle was 29.4 ± 2.1 mm as compared to 25.3 ± 1.4 mm at the maximum flexion angle. The mean virtual AM bundle length at the minimum flexion angle was 33.3 ± 2.5 mm as compared to 31.6 ± 2.9 mm at the maximum flexion angle. The mean virtual PL bundle length at the minimum flexion angle was 27.3 ± 3.7 mm as compared to 22.1 ± 1.7 mm at the maximum flexion angle.

95±3% (mean ± standard deviation across subjects) of correlations between virtual bundle lengths and ACL length were significant using P<5×10−5 as a threshold. Furthermore, virtual bundle lengths were consistently positively correlated with ACL length (rho = 0.91±0.1, mean ± standard deviation), using P<5×10−5 as a significance threshold. Figure 3 shows an example in one subject of the ACL length, defined as the as the centroid to centroid distance between attachment site footprints (left) and a randomly selected 100 of the 1000 virtual ACL bundle lengths (right). Virtual bundle lengths that were significantly positively correlated with ACL length are shown in red, and virtual bundle lengths that were negatively correlated with ACL length are shown in blue. Figure 4 shows an example in one subject of ACL length (red) and the 1000 virtual bundle lengths (gray) during gait.

Figure 3:

Figure 3:

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An example in one subject of the ACL length, measured as the centroid to centroid distance between ACL attachment sites, (left) and a randomly selected 100 of the 1000 virtual ACL bundle lengths (right). Virtual bundle lengths that were positively correlated with ACL length are shown in red, and virtual bundle lengths that were negatively correlated with ACL length are shown in blue.

Figure 4:

Figure 4:

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An example in one subject of the ACL length (red) and the lengths of the 1000 virtual bundles (gray) during gait. Virtual bundle lengths were highly positively correlated with ACL length (rho = 0.91±0.1, mean ± standard deviation of rho values across subjects, using P<5×10−5 as a significance threshold).

Of the 1000 virtual bundles for each subject, 261±43 (mean±standard deviation across subjects) of them were classified as virtual AM bundles. These bundles were highly positively correlated with ACL length (rho = 0.86±0.1, P<5×10−5). Furthermore, 251±46 bundles were classified as virtual PL bundles. The virtual PL bundles were also highly positively correlated with ACL length (rho = 0.89±0.1, P<5×10−5). The bundles that originated in the anterior medial aspect of the femur and ended in the posterior lateral aspect of the tibia (or originated in the anterior medial aspect of the tibia and terminated in the posterior lateral aspect of the femur) were not included in this analysis, as they could not be classified as either virtual AM or virtual PL bundles. Importantly, the virtual AM and PL bundle lengths were highly positively correlated with each other over the entire gait cycle (rho = 0.81±0.1, P<5×10−5).

Discussion

The present study used the technique of integrating 3D models of the knee joint created from MR images with high-speed biplanar radiographs12 to measure in vivo ACL lengths, defined as the distances between attachment sites, during the full gait cycle. ACL lengths were highly positively correlated with the lengths of 1000 virtual bundles throughout gait (rho = 0.91±0.1, P<5×10−5). Furthermore, virtual AM bundle lengths demonstrated a consistent and strong positive correlation with virtual PL bundle lengths (rho = 0.81±0.1, P<5×10−5). Therefore, the findings of this study suggest that AM and PL bundles unlikely to lengthen in a reciprocal fashion during gait, no matter how the AM and PL bundle attachment sites are defined within the footprint of the ACL attachment site.

The findings of this study are in line with other in vivo studies that have specifically measured AM and PL bundle lengths using similar imaging and data analysis methods, suggesting that the AM and PL bundles elongate with knee extension during quasi-static17, 22, 32 and dynamic tasks10–12, 33. Specifically, Jordan et al.17 measured AM and PL bundle lengths during quasi-static lunges of varying flexion angle using static biplanar radiography and MRI. This study found that both subdivisions decreased in length as knee flexion increased. Additionally, a study by Utturkar et al.32 found that both the AM and PL bundles of the ACL decreased in length when the knee was flexed to 30° as compared when positioned in full extension using a similar technique. With regard to dynamic activity, a study by Wu et al.33 found that both the AM and PL bundles were maximally elongated when the knee was extended during the stance phase of gait, specifically during mid-stance and around the time of heel strike. In addition, prior studies from our lab found that both the AM and PL bundles of the ACL follow similar elongation patterns with regard to knee flexion during gait11, 12 and single legged hopping.10 Specifically, these studies found that AM and PL bundle lengths were highly correlated with each other throughout motion. Together with the data from the present study, these studies suggest that the AM and PL bundles likely elongate in parallel during in vivo loading, with both bundles being taut in extension and lax in flexion. Importantly, the present study expands on this previous literature by examining the length patterns of numerous subdivisions of the ACL during dynamic activity, and suggests that as long as their origination and termination points are specified within the footprint of the attachment sites, ACL subdivisions elongate as the knee is extended during walking.

Importantly, it has been suggested that selection of the attachment sites that define the ACL may affect the measurement of ligament length during the flexion path.22 To address this concern, a study by Li et al.22 performed a sensitivity analysis to assess the dependence of in vivo ligament length measurements to the locations of the ligament attachment sites. This study determined that a 5 mm variation in the location of the attachment site footprints had minimal effect on the observed relationship between ACL bundle lengths and flexion angle. The present study adds to these findings of this prior study by demonstrating that as long as the attachment sites are specified within the footprint of the ACL, bundle lengths are highly positively correlated with overall ACL length throughout gait. Thus, it is unlikely that the AM and PL bundles elongate reciprocally with respect to flexion in vivo during gait. Furthermore, these data suggest that the centroid to centroid estimation of ACL length is a robust approximation to identify knee motions where the ACL is likely to be maximally loaded.

The findings of the present study and other in vivo imaging studies are in contrast to cadaveric studies that have indicated that the AM and PL bundles behave with reciprocity3, 15, 28, with the PL bundle being more taut in extension and lax in flexion and the AM bundle being more lax in extension and taut in flexion. A potential reason for this discrepancy is the difficulty in fully reproducing the mechanical environment of the knee joint during dynamic in vivo activities in a cadaveric model. This is particularly important, given that the length of the ACL depends on specific loading conditions21, and the complex loading conditions of dynamic movement are likely different than the simulated loading conditions of in vitro experiments. Furthermore, recent studies have suggested that the microstructural and material properties of the ACL may vary across the ligament29, rather than differ distinctly between bundles. This suggests that the anatomy of the ACL may be better described as a continuum of fascicles6 rather than as two distinct bundles with separate functions.

In this study, the length of the ACL is defined as the straight line distance between the centroids of the attachment sites. Additionally, the lengths of the virtual bundles are also defined as the straight line distances between their origination and termination points. However, the ACL is a complex structure comprised of many fibers, and thus these straight line distance measurements represent approximations of ACL length. Another important consideration for this study is that these measurements are geometric in nature and do not directly measure tissue load, as could be achieved in an ex vivo experiment. However, the techniques used in this study are advantageous in that they allow for a non-invasive in vivo estimation of the length of the ACL and its various subdivisions under physiologic loading conditions. Furthermore, these measurements do not take into account a case where the ACL or the virtual bundles would be slack. Nonetheless, this technique can identify knee positions in which the ACL and its bundles are maximally strained, which has important implications for ACL graft design. Future work toward understanding dynamic in vivo ACL function should include measurements of ACL length during other dynamic activities across a larger range of motion (up to 90° of flexion), as well assessment of potential differences in dynamic loading of additional subdivisions of the ligament. Additionally, future work should include assessment of length changes associated with more demanding motions in order to better elucidate mechanisms of ACL injury. Furthermore, inclusion of female subjects may clarify differences in ACL length patterns between males and females which may contribute to the increased incidence of ACL rupture in females.5

Importantly, while it may be challenging to fully reproduce its native function, understanding the mechanical function of the ACL and its subdivisions is critical for improving grafts such that they more accurately mimic native ACL function. The data presented here indicate that as long as their origination and termination points are specified within the footprints of the ACL, bundle lengths are highly positively correlated with overall ACL length throughout gait. Specifically, these data demonstrate that the ACL and its subdivisions are elongated with knee extension during gait. As such, these data suggest that if a double bundle reconstruction is performed, both bundles should be more elongated with the knee extended relative to flexed.

What is known about this subject:

Cadaveric studies have indicated that the ACL consists of AM and PL bundles that function reciprocally during knee flexion. However, several in vivo imaging studies have suggested that ACL bundles elongate in parallel as the knee is extended.

What this study adds to existing knowledge:

This study found that virtual bundle lengths derived from randomly generated locations on the attachment site surfaces were highly correlated with ACL length throughout the full gait cycle, regardless of the virtual bundle origination and termination location. Furthermore, the lengths of the bundles that originated and terminated in the anterior and medial aspects of the ACL were positively correlated with the lengths of the bundles that originated and terminated in the posterior and lateral aspects of the ACL. Therefore, the results of this study indicate that ACL subdivisions elongate in parallel with as the knee is extended. Thus it is unlikely that there are AM and PL bundles of the ACL that function reciprocally with respect to flexion.

Footnotes

Conflict of Interest Statement: The authors of this manuscript have no conflicts of interest pertaining to this work to disclose.

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