Introduction

In order to perform a successful anterior cruciate ligament (ACL) reconstruction, the surgeon must make a number of steps that require correct judgment and execution, but there is evidence that the most frequent cause of failure is malpositioning of the graft tunnel in the femur. This is not surprising, due to the anatomy of the interior of the knee joint and the difficulty of seeing the femoral attachment of the ACL through an arthroscope. The femoral attachment of the native ACL is located posteriorly on the lateral wall of the intercondylar notch. Its anterior-distal boundary, which may be described as being “shallow” in the notch, as seen through the arthroscope with the knee in 90 degrees of flexion, is marked by a bony ridge often called the “resident’s ridge.” The posterior-proximal border (or “deep”) of the femoral ACL attachment extends close to the margin of the articular cartilage at the posterior aspect of the femoral condyle. The lateral bifurcate ridge runs between these shallow and deep margins, and has been described as the division between the two dominant fiber bundles of the ACL, but has a somewhat variable presence between patients and can be difficult to find, even after shaving the ACL remnant fibers from the bone surface. When present, these bony ridges create two shallow depressions in the bone surface, which correspond to the anteromedial (AM) and posterolateral (PL) fiber bundles of the ACL ( Fig. 47.1 ). These landmarks are key references when intending to place the femoral tunnels in the native ACL attachment. If they are not clearly visualized during surgery, there is plenty of scope to err with the tunnel placement.

The femoral anterior cruciate ligament attachment with the areas of the anteromedial (AMB) and posterolateral (PLB) fiber bundles. These areas cover both the direct and indirect fiber insertion areas.

The aim of this chapter is to describe the evolution of knowledge regarding ACL graft placement on the femur, which relates closely to our understanding of the function of the ACL itself. The trend toward double-bundle “anatomical” reconstructions, with two grafts in parallel that attempt to reproduce the two functional fiber bundles of the ACL, has helped the understanding of anatomy, but has also influenced the paradigm of research. It is therefore important to note that although the organization and function of the ACL fibers are matters of debate, a majority of papers rely on the understanding that there are two main functional bundles—namely, the AM and PL bundles. The tibial attachment is not considered here because changes of the femoral attachment have a much larger effect on ACL graft tension and length changes. More recently, the term “anatomical” reconstruction has also been used for single-bundle ACL reconstructions, again implying that the single graft tunnel in each of the femur and tibia are positioned within the anatomical ACL attachment area.

In this chapter, two distinct sets of terminology will be used to describe femoral graft tunnel positions: (1) anatomical nomenclature for describing positions when the knee is in extension (anterior-posterior, proximal-distal) and (2) surgical nomenclature for describing what the surgeon views when the knee is flexed approximately 90 degrees (high-low, deep-shallow, respectively).

Functional Anatomy of the Anterior Cruciate Ligament Related to Graft Tunnels

The ACL has a complex fiber structure composed of many fascicles bound together within a synovial covering layer. Thus the ACL may be split into many small fiber bundles, and their tibial and femoral attachments mapped. The fibers are not arranged simply in parallel, and this gives rise to the cross-sectional area being less at midlength than at the bony attachments: the fibers must splay out toward the bones. The functional significance of this architecture is not understood. However, at a gross level, the fibers of the ACL are arranged as a flat band, and all are tensed when the knee is extended. This fiber band is oriented in a sagittal plane so that the ACL fits into and fills the narrow slot between the posterior cruciate ligament (PCL), which occupies most of the width of the intercondylar notch, and the lateral femoral condyle. The sagittal plane of the ACL orientation means that it attaches to the tibia over an area that is oriented anteroposterior. The ACL attaches to the femur over an area that is oriented from anteroproximal to posterodistal. This femoral attachment is close to and bounded posteriorly by the condylar articular cartilage and has an overall alignment approximately 35 degrees posterior-distal to the axial (see Fig. 47.1 ).

When the knee flexes, the axis of rotation moves within the distal femur, and the kinematics are affected by the loads imposed on the knee, but the overall effect in the intact knee is that the most anterior-proximal fibers of the ACL remain close to a constant length and thus are often described as being “isometric.” Meanwhile, the more posterior-distal the fibers, the more they slacken as the knee flexes—up to 90 degrees flexion. These length-change patterns have been measured in a number of studies, and an “isometry map” can be derived from such measurements. A surgical navigation system can produce such maps in response to the surgeon moving the knee during ACL reconstruction procedures, giving patient-specific feedback on the likely length changes associated with choices of graft tunnel positions around the intercondylar notch ( Fig. 47.2 ).

A map of fiber attachment length changes produced during anterior cruciate ligament reconstruction surgery by a navigation system. The “contour lines” represent areas with a given length change measured over a range of knee flexion. They converge toward a central zone of minimal length change.

With thanks for permission to Dr. Philippe Colombet, Merignac, France.

Functional Anatomy of the Anterior Cruciate Ligament Related to Graft Tunnels

The ACL has a complex fiber structure composed of many fascicles bound together within a synovial covering layer. Thus the ACL may be split into many small fiber bundles, and their tibial and femoral attachments mapped. The fibers are not arranged simply in parallel, and this gives rise to the cross-sectional area being less at midlength than at the bony attachments: the fibers must splay out toward the bones. The functional significance of this architecture is not understood. However, at a gross level, the fibers of the ACL are arranged as a flat band, and all are tensed when the knee is extended. This fiber band is oriented in a sagittal plane so that the ACL fits into and fills the narrow slot between the posterior cruciate ligament (PCL), which occupies most of the width of the intercondylar notch, and the lateral femoral condyle. The sagittal plane of the ACL orientation means that it attaches to the tibia over an area that is oriented anteroposterior. The ACL attaches to the femur over an area that is oriented from anteroproximal to posterodistal. This femoral attachment is close to and bounded posteriorly by the condylar articular cartilage and has an overall alignment approximately 35 degrees posterior-distal to the axial (see Fig. 47.1 ).

When the knee flexes, the axis of rotation moves within the distal femur, and the kinematics are affected by the loads imposed on the knee, but the overall effect in the intact knee is that the most anterior-proximal fibers of the ACL remain close to a constant length and thus are often described as being “isometric.” Meanwhile, the more posterior-distal the fibers, the more they slacken as the knee flexes—up to 90 degrees flexion. These length-change patterns have been measured in a number of studies, and an “isometry map” can be derived from such measurements. A surgical navigation system can produce such maps in response to the surgeon moving the knee during ACL reconstruction procedures, giving patient-specific feedback on the likely length changes associated with choices of graft tunnel positions around the intercondylar notch ( Fig. 47.2 ).

A map of fiber attachment length changes produced during anterior cruciate ligament reconstruction surgery by a navigation system. The “contour lines” represent areas with a given length change measured over a range of knee flexion. They converge toward a central zone of minimal length change.

With thanks for permission to Dr. Philippe Colombet, Merignac, France.

Anterior Cruciate Ligament Isometry and Reconstruction

The observation that the anterior fibers of the ACL remain tight across the range of knee flexion, whereas the more posterior parts slacken with knee flexion, led to the belief that the anterior fibers were the most important. The more anterior fibers are denser when the ACL is examined by histology. This was reinforced by the finding that the more anterior fibers had a greater material failure strength, which suggests that they have adapted to a more mechanically demanding role. A similar finding has been made for the PCL. These findings have been correlated with a higher collagen density in the anterior fiber bundles of both the ACL and PCL. A more practical reason to place a graft isometrically is that this implies that the graft will not be subjected to cyclical length changes when the knee is moving, thus helping to protect it from fatigue or loosening effects. For example, O’Meara et al. reported that isometric grafts survived cyclical motion in a continuous passive motion machine, whereas nonisometric grafts did not.

The problem with this line of reasoning is that isometry measurements depend on the ACL being intact; otherwise the kinematics may be abnormal. Even when the ACL is intact, the isometric area on the femoral condyle is influenced sensitively by the loads imposed on the knee while it is being moved. This was shown by Zavras et al., who published a map showing a range of different recommended isometric graft locations from the previous literature. Their reproduction of published works confirmed that isometric behavior could be found reliably for attachment points only at the extreme anteroproximal corner of the natural ACL attachment area. This means that “isometric” ACL reconstructions are nonanatomical, with the femoral graft tunnel centered higher and deeper in the notch (with the knee flexed) than the natural attachment area. Despite this, the mainstream opinion through the 1990s favored femoral graft tunnels placed isometrically. Although many clinical papers were published to report a high percentage of good and excellent results, there remained a high level of interest in ACL research and development, reflecting an underlying dissatisfaction with clinical outcome and a desire to find ways for improvement.

One of the underlying principles that emerged from the isometry research studies was that there is a transition line between attachments that cause graft tightening or slackening with knee flexion. The transition line passes through the isometric point at the anteroproximal edge of the ACL attachment, at the posterior end of Blumensaat line, and from there runs distal and slightly posterior. Attachments anterior to the transition line lead to graft tightening with knee flexion, whereas grafts posterior to the transition line slacken.

A recent study has unified the observations of ACL fiber isometry and of collagen density, using robotic testing to discover which area of the ACL fibers actually transmits most of the restraining load to the femur. This was prompted by the anatomical observations of Mochizuki et al., who described the femoral attachment as having an elongated area of dense direct collagen fiber insertion into the bone, and anterior and posterior “fan-like extension” areas, where the ACL fibers approach the bone tangentially and dissipate into the periosteum ( Fig. 47.3 ). This morphology has been supported by the linear attachment of the ribbon-like ACL described by Śmigielski et al. Histological sectioning showed that the dense direct fiber insertion area corresponded to the resident’s ridge, which may therefore be akin to a tuberosity for tensile force transmission. Kawaguchi et al. framed the ACL attachment with a grid, which was divided into 12 small areas, in 3 rows: anterior fan-like extension, central direct insertion area, and posterior fan-like extension area. These were then cut off at the femoral attachment, and the robot measured their force contributions by repeated draw and rotational tests. A clear picture emerged: the majority (approximately 70%) of the resistance to tibial anterior translation from the ACL was provided by the proximal half of the direct fiber insertion area ( Fig. 47.4 ). This matched both the isometric zone of Grood et al. and the histological demonstration of high collagen density. It also corresponded almost exactly to what is understood to be the AM fiber bundle attachment area. This is a logical place for an ACL graft tunnel, if the aim is to transmit the tension in the ACL along the physiological line of action; it is also a return to graft tunnel positions that were in regular use 20 years ago, prior to the current fashion for “anatomical” grafts, which have had larger failure rates associated with their use.

The femoral attachment of the anterior cruciate ligament (ACL) has an elongated central area of dense direct collagen fiber insertion into the bone (outlined by solid line), and anterior and posterior (outlined by interrupted line) “fan-like extension” areas, where the ACL fibers approach the bone tangentially and dissipate into the periosteum.

Reproduced from Mochizuki T, Fujishiro H, Nimura A, et al. Anatomic and histologic analysis of the mid-substance and fan-like extension fibres of the anterior cruciate ligament during knee motion, with special reference to the femoral attachment. Knee Surg Sports Traumatol Arthrosc . 2014;22(2):336–344, with kind permission of Springer Science and Business Media.

In this knee, the anterior cruciate ligament (ACL) femoral attachment is divided into panels: the posterior fan-like extension, the central dense fiber attachment area, and the anterior fan-like extension. The majority of the contribution of the ACL (approximately 70%) to resisting tibial anterior translation was carried by the proximal half of the direct fiber insertion area, which corresponds to the anteromedial bundle.

At present, the principal method for objective assessment of the restoration of normal mechanics to the knee after ACL reconstruction is the measurement of tibiofemoral anterior translation laxity—that is, how far anteriorly the tibia moves in response to a known displacing force at a given angle of knee flexion. Very little work has been done to examine how well different ACL graft positions can restore anterior laxity to normal across the range of knee flexion. Even an incorrect graft placement might restore anterior drawer to normal at one angle of knee flexion (by adjusting the tension appropriately), but then it might behave abnormally and either overconstrain or allow excessive laxity as the knee moves away from the posture where the graft had been tensed.

A study of alternative graft attachments investigated the effect of moving to different attachment points either at or around the isometric area on the femur. Five attachment points were investigated: isometric, then anteroproximal, anterodistal, posteroproximal, or posterodistal to the isometric point. It was found to be possible to restore tibiofemoral anterior laxity close to normal across the range of knee flexion investigated, with attachments that were either on the isometric transition line or just posterior to it. The tendency of anterior femoral attachments to cause the graft to tighten with knee flexion led to overconstraint of the flexed knee; this was accompanied by elevated graft tension as the knee flexed. Grafts placed distal and posterior to the isometric point, which meant that they were in the anatomical ACL attachment, restored anterior laxity to that of the intact knee across the range of knee flexion investigated.