Improved understanding and management of the risk factors associated with osteoarthritis of the knee are clearly desirable, especially considering the shifts in the age distribution of the US population toward older ages. Among the various genetic, constitutional, and biomechanical insults that are believed to cause metabolic and structural changes in the synovial joint, two of the recognized local adverse biomechanical factors are rupture of the anterior cruciate ligament and malalignment of the lower limb.
Over the past three decades, both in vitro and in vivo biomechanical studies have documented the complex anatomy of the anterior cruciate ligament and its function in stabilizing the knee in multiple degrees of freedom. An in vitro robotic testing system was used to assess the various biomechanical variables of knee flexion and to measure the in situ forces in the normal anterior cruciate ligament as well as its replacement grafts (see "Evaluation of Knee Stability with Use of a Robotic System," by Woo and Fisher). To relate the in vitro findings to those obtained during in vivo weight-bearing knee flexion, a combined dual fluoroscopic-magnetic resonance imaging system of analysis was developed; this system revealed that there is an abnormal mediolateral translation in addition to the anterior and rotational instability that is seen in knees with anterior cruciate ligament deficiency (see "Evaluation of Kinematics of Anterior Cruciate Ligament-Deficient Knees with Use of Advanced Imaging Techniques, Three-Dimensional Modeling Techniques, and Robotics," by Van de Velde et al.). When investigating more complex maneuvers such as the pivot shift, computer navigation provides a precise tool to quantify the pathologic kinematics in vivo (see "The Pivot Shift Phenomenon During Computer-Assisted Anterior Cruciate Ligament Reconstruction," by Pearle et al.), whereas gait analysis is most suitable for the kinematic analysis of repetitive cyclic loading in normal, anterior cruciate ligament-deficient, and malaligned knee joints (see "Gait Mechanics Influence Healthy Cartilage Morphology and Osteoarthritis of the Knee," by Andriacchi et al.).
Recent results suggest that malalignment is not a risk factor for osteoarthritis but rather a marker of disease severity and/or progression (see "Alignment and Osteoarthritis of the Knee," by Hunter et al.), emphasizing the need for accurate restoration of neutral hip-knee-ankle alignment. Computer navigation is a promising technology, aiding the surgeon in the determination of the mechanical axis during procedures such as high tibial osteotomy (see "Reliability of Image-free Navigation to Monitor Lower-Limb Alignment," by Pearle et al.) and fracture reduction (see "Navigated Long-Bone Fracture Reduction," by Kahler). 