Definitive conclusions on the relative efficacy of different ACL reconstruction techniques can only be drawn from in vivo, human studies. Cadaveric studies have contributed a wealth of information on the basic biomechanics and passive structural properties of the knee and can be beneficial for development and initial evaluation of surgical techniques. However, cadaver studies cannot reproduce the complex combination of gravitational, inertial, and active muscular forces that influence knee mechanics during functional activities. Cadaver studies also represent only “time zero” conditions and cannot account for biologic responses (such as healing, remodeling, tunnel enlargement, etc) that can have a significant influence on knee and ligament function. Conversely, animal models are well suited for studying biologic tissue response, but they differ too extensively in joint morphology and function for direct transfer of kinematic findings to humans (with the possible exception of large primates, which are rarely used for orthopedic studies because of cost and ethical considerations).

In vivo knee function can be evaluated under a wide range of conditions. Which measures are most relevant to outcomes after ACL reconstruction? With the progression from laxity testing to static weight-bearing to dynamic, functional activities, assessment becomes more technically demanding but also potentially more relevant to joint health. Central to most theories relating joint mechanics to OA development is the idea that altered joint contact patterns and joint loads, encountered during routine activities, can be detrimental to long-term cartilage health.^6,8 These theories would suggest that the most relevant measures of joint function are those that reflect the behavior of the knee during common, functional activities. It is especially important to distinguish between evaluations of laxity versus assessments of dynamic knee function and functional stability. In a recent review, Musahl and colleagues⁹ stated, “In biomechanical terms, laxity is the passive response of a joint to an externally applied force or torque. Stability, on the other hand, is a functional measure; that is, a knee, regardless of laxity, is only unstable if it ‘gives way’ during functional activities.” Laxity tests are typically performed without the compressive joint forces required to properly engage the conforming condylar surfaces, which play an important role in joint stabilization. Thus, although laxity tests may be effective for identifying structural deficits, the results cannot predict joint behavior during dynamic, functional activities. In fact, many studies relating static laxity and clinical/functional outcomes have reported correlations that are at best weak.^10–14

In vivo studies incorporating body-weight loading and active muscular control provide a much more comprehensive and realistic picture of the natural function of the knee joint as a complex neuromusculoskeletal system. However, the knee has a wide envelope of possible motions, and joint function is highly activity dependent. Patterns of joint motion and articular contact vary considerably with loading and activity, even during similar ranges of knee flexion.^15,16 Knee tissues are highly viscoelastic and respond nonlinearly to load magnitude and loading rate,^17,18 so the behavior of the knee under low-demand conditions cannot be simply “scaled up” to predict behavior during functional activities. Thus, studies incorporating body-weight loading during quasi-static activities (eg, sequential fixed knee angles¹⁹) or low-effort movements (eg, half-speed gait²⁰) may not predict knee behavior during more complex, demanding tasks. This may be especially important for ACL-injured athletes, who will routinely expose their joints to high-magnitude, rapidly changing loads after returning to sports.

Meaningful characterization of dynamic joint function during common activities poses unique challenges, especially for measurements directly relevant to soft tissue behavior. Peak ACL strains during activities of daily living are on the order of 4% or less²¹; for a typical ACL size, this represents a length change of only 1.2 mm. Assessing articular contact kinematics (arthrokinematics) requires a measurement error substantially smaller than the thickness of the cartilage layer (typically 2–4 mm thick for the tibiofemoral joint). The most widely used technology for studying knee function after ACL reconstruction is video-motion analysis, which tracks motion of multiple skin-mounted markers placed on the thigh and shank to determine limb movement. This technology is noninvasive, widely available, and reliable and has been effective for identifying differences in knee kinematics between ACL-intact, ACL-deficient, and ACL-reconstructed joints (as described later). But, conventional motion analysis cannot achieve the submillimeter accuracy required for tissue-relevant measurements, because of the displacement of skin-mounted markers relative to underlying bone.^22–24 Magnetic resonance (MR) imaging can achieve submillimeter accuracy and enables direct visualization of soft tissue, but sample rates are too slow and the imaging environment is too restrictive for most functional movement tasks.

Dynamic radiographic imaging enables direct visualization and 3-dimensional tracking of bone motion and has been gaining in popularity during the past decade. Although some measurements have been performed using a single imaging plane, dual or biplane imaging systems are generally required to obtain submillimeter resolution in all 3 movement planes. Many systems are now in use across the United States, with capabilities that vary based on the specific equipment and analysis techniques used. Conventional “C-arm” fluoroscopy systems are limited by low frame rates (≤30 Hz) and long exposure times (≥8 ms) but are adequate for quasi-static and low-speed activities.^25,26 Custom-built systems can achieve much higher sample rates and have validated submillimeter accuracy for more physically demanding tasks, such as running.^27–29