Andrew D. Pearle, MD, David McAllister, MD, and Stephen M. Howell, MD
Authors’ Disclosure Statement: Dr. Pearle reports that he consults for and receives royalties from Biomet. Dr. McAllister reports that he consults for and receives royalties from Biomet. Dr. Howell reports that he consults for Biomet Sports Medicine.
Multiple cadaveric studies have investigated the relationship between femoral tunnel positioning and time-zero stability. These studies often demonstrated superior time-zero control of knee stability, particularly in pivot type maneuvers, with a femoral tunnel placed more centrally in the femoral footprint than with a tunnel placed outside the footprint.34-37 However, an emerging body of literature is finding no significant difference in time-zero stability between an anteriorly placed femoral tunnel within the anatomical footprint (eccentrically located in the footprint) and a centrally placed graft.38,39 Returning to the more isometric tunnel position, still within the femoral footprint, would be expected to confer the benefits of an anatomically based graft position with the advantageous profile of improved isometry, as compared with a centrally placed or PL graft. Biomechanical studies40 have documented that ACL graft fibers placed posteriorly (low) in the footprint cause high graft forces in extension and, in some cases, graft rupture (Figure 4). Accordingly, the importance of reconstructing the posterior region of the footprint to better control time-zero stability is questioned.41
In addition to time-zero control of the stability examination, restoring the low tension-flexion pattern in the ACL graft to replicate the tension-flexion behavior of the native ACL is a fundamental biomechanical principle of ACLR.15,33,42,43 These studies have demonstrated that a femoral tunnel localized anterior (high) and proximal (deep) within the footprint better replicates the tension-flexion behavior of the native ACL, as compared with strategies that attempt to anatomically “fill the footprint.”40 Together, these studies have demonstrated that an eccentric position in the footprint, in the anterior (high) and proximal (deep) region, not only maximizes isometry and restores the direct fibers, but provides favorable time-zero stability and a low tension-flexion pattern biomechanically, particularly as compared with a tunnel in the more central or posterior region of the footprint.
5. Clinical Data
Clinical studies of the traditional transtibial ACLR have shown good results.44,45 However, when the tibial tunnel in the coronal plane was drilled vertical with respect to the medial joint line of the tibia, the transtibially placed femoral tunnel migrated anterior to the anatomical femoral footprint, often on the roof of the notch.10,14 This nonanatomical, vertical placement of the femoral tunnel led to failed normalization of knee kinematics.46-50 Indeed, a higher tension-flexion pattern was found in this nonanatomical “roof” position for the femoral tunnel as compared with the native ACL—a pattern that can result in either loss of flexion or recurrent instability.13,15,51
Clinical results of techniques used to create an anatomical ACLR centrally within the footprint have been mixed. Registry data showed that the revision rate at 4 years was higher with the AM portal technique (5.16%) than with transtibial drilling (3.20%).52 This higher rate may be associated with the more central placement of the femoral tunnel with the AM portal technique than with the transtibial technique, as shown in vivo with high-resolution magnetic resonance imaging.12 Recent reports have documented a higher rate of failure with DB or central ACLR approaches than with traditional transtibial techniques.53 As mentioned, in contrast to a more isometric position, a central femoral tunnel position would be expected to demonstrate 5 to 7 mm of length change, whereas moving the graft more posterior in the footprint (closer to the articular cartilage) would result in more than 1 cm of length change through the range of motion.31,32 As such, these more central grafts, or grafts placed even lower (more posterior) in the footprint, would be expected to see high tension in extension (if fixed in flexion), or to lose tension completely during flexion (if the graft is fixed at full extension).32 This may be a mechanistic cause of the high failure rate in the more posterior bundles of the DB approach.54
Together, these clinical data suggest that the femoral tunnel should be placed within the anatomical footprint of the ACL. However, within the footprint, a more eccentric femoral tunnel position capturing the isometric and direct region of the insertion may be preferable to a more central or posterior (low region) position.
Summary
Anatomical, histologic, isometric, biomechanical, and clinical data from more than 4 decades collectively point to an optimal position for the femoral tunnel within the femoral footprint. This position can be summarized by the acronym I.D.E.A.L., which refers to placing a femoral tunnel in a position that reproduces the Isometry of the native ACL, that covers the fibers of the Direct insertion histologically, that is Eccentrically located in the anterior (high) and proximal (deep) region of the footprint, that is Anatomical (within the footprint), and that replicates the Low tension-flexion pattern of the native ACL throughout the range of flexion and extension (Figure 5).
