Where Are We Now?
Only after decades of studies systematically unraveling the structure and function of the native ACL  did orthopaedic surgeons accept anatomic ACL reconstruction as a suitable surgical procedure. Independent drilling of the femoral tunnel, most commonly through an anteromedial portal, consistently positions the tunnel within the anatomic footprint, which better restores the knee kinematics compared to a non-anatomic graft placed through transtibial drilling [8, 9]. While exhaustive research efforts have been made to understand the effects of intercondylar tunnel position on knee kinematics and clinical outcomes following ACL reconstruction, far less is known about the effect of drilling techniques on the position of the lateral structures of the femur relative to the tunnel outlet. Through cadaveric dissection, Laprade and colleagues  described the relative positions and sizes of several posterolateral knee structures. More recent studies, utilizing both three-dimensional (3-D) models and cadaveric specimens, investigated tunnel outlet position relative to posterolateral knee structures when drilling was performed through an anteromedial portal [4, 12]. Using angles such as 90°, 110°, 130°, but never exceeding 135° of knee flexion, these studies found that drilling under high knee-flexion angles reduced the risk of violating lateral structures [4, 12]. In another cadaveric in vitro study, Basdekis and colleagues  recommended against maximal knee flexion when drilling due to high variability in the maximum flexion angle achieved across specimens, as well as higher tunnel acuity.
In the current study, Chung and colleagues  retrospectively studied CT 3-D reconstructed knee models of patients who underwent anatomic single-bundle ACL reconstruction with a transportal technique using quadruple hamstring autografts. Although the knees were maximally flexed at the time of femoral tunnel drilling, the knee flexion angle measured during surgery varied across three groups of patients: < 120°, 120° to 129°, and ≥ 130°. Chung and colleagues  found a safe window of 121° to 131° in which the risk of the tunnel outlet violating any posterolateral structure was minimized. These findings provide clinical evidence for the aforementioned cadaveric studies in which knee flexion angles ≤ 90° or ≥ 130° were associated with increased risk of either violating lateral structures or variable tunnel positioning.
Where Do We Need To Go?
With the continued adoption of anatomic ACL reconstruction technique by independent tunnel drilling (transportal versus outside-in), the results from Chung and colleagues  should give us pause. Notably, the variability in anatomy of the posterolateral structures across individuals remains unclear, as compared to the now extensively documented variation in ACL dimensions and morphology . Even so, it is difficult to characterize ACL morphology, as the insertion sites vary in geometric shape across individuals  and the midsubstance adopts different shapes depending on knee flexion angle and loading conditions [6, 13]. The relative position of the femoral attachments of the posterolateral structures from the current study were derived from a previous study  in which the centroid of each posterolateral structure was averaged across 10 cadaveric specimens and expressed relative to the lateral femoral epicondyle. While the present study can tell us that, on average, drilling the femoral tunnel with 121° to 131° of knee flexion reduces the risk of iatrogenic injury of lateral structures, we still lack predictive tools to fully prevent this complication for a given patient. Furthermore, few studies have reported on such complications following anatomic ACL reconstruction, suggesting they are underreported, that they are underdiagnosed, or that they don’t affect important clinical outcomes to any great degree. Studies investigating tunnel outlet position relative to lateral structures, as assessed on post-operative MRI, could provide an incidence rate of iatrogenic damage. The correlation of iatrogenic lateral structure injury with objective and subjective outcomes could substantiate or refute the clinical consequence of such a complication.
How Do We Get There?
In order to individualize anatomic ACL reconstruction, both the qualitative and quantitative nature of a patient’s anatomy must be better understood. For example, potential autograft sizes can be determined pre-operatively by MRI and ultrasound so as to better match the dimensions of the patient’s native ACL . Pre-operative planning with 3-D modeling is not commonly performed for primary ACL reconstruction due to its high frequency and the high radiation exposure of CT. However, emerging technologies such as 3-D MRI may make pre-operative planning a reality , as it could potentially facilitate a greater understanding of the spatial relationships of soft tissues, which, at present, can only be seen in bone-specific CT.
Finally, 3-D imaging modalities may serve to improve the accuracy and reliability of computer-assisted navigation in ACL reconstruction. Three-dimensional imaging models would help surgeons determine where the tunnel will exit the lateral femur relative to the surrounding soft-tissue structures before entering the femoral footprint from the intercondylar notch. Perhaps soon, the ideal knee flexion angle at which the femoral tunnel is drilled will be individualized to each patient.
To achieve this, navigation systems utilizing models reconstructed from 3-D MRI, must be developed. Initial optimization and validation in cadaveric in vitro studies should report the effect of knee flexion angle not only on conventional tunnel characteristics such as length, angle, and positioning in anatomic footprint, but also the location of the tunnel outlet relative to lateral soft tissue structures. The accuracy and reliability of these systems will then need to be validated in clinical studies. In the interim, clinical studies investigating the incidence and consequence of iatrogenic lateral structure injury are needed. Indeed, we should know what is on the other end of the tunnel before entering.
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