For insight into how a chronic ACL deficiency spells eventual doom for the MCL and LCL, please see the February 2007 article The Effect of Anterior Cruciate Ligament Deficiency on the In Vivo Elongation of the Medial and Lateral Collateral Ligaments, by Samuel K. Van de Velde et al., in the Long-term Consequences of ACL Injuries Subsection.

For insight into the biomechanical importance of thoroughly understanding the double-bundle (anteromedial and posterolateral) structure of the ACL, please see the February 2007 article The Role of the Anteromedial and Posterolateral Bundles of the Anterior Cruciate Ligament in Anterior Tibial Translation and Internal Rotation, by Thore Zantop et al., in the Knee Biomechanics, Functional Anatomy of ACL and Other Ligaments Subsection.

For insight into the "firmware settings" inherent in landing a jump (e.g. knee-flexion angle, hip flexion angle, hamstring activation for eccentric contraction), and for insight into the female-versus-male differences in these "settings", please see the February 2007 article Kinematics and Electromyography of Landing Preparation in Vertical Stop-Jump -- Risks for Noncontact Anterior Cruciate Ligament Injury, by Jonathan D. Chappell et al., in the Female-Athlete Knee-Injury Incidence and Prevention Subsection.

For a good discussion on the biomechanical shortfalls of the traditional single-bundle ACL graft, and for a clear warning that dual-bundle ACL grafting is the only long-term-appropriate way to go, please see the superb February 2007 article Effectiveness of Reconstruction of the Anterior Cruciate Ligament With Quadrupled Hamstrings and Bone-Patellar Tendon-Bone Autografts -- An In Vivo Study Comparing Tibial Internal-External Rotation, by Vasileios Chouliaras et al., in the Knee Biomechanics, Functional Anatomy of ACL and Other Ligaments Subsection.

Interaction of Arch Type and Footwear on Running Mechanics, Robert J. Butler et al.; The American Journal of Sports Medicine, Baltimore; December 2006, Vol 34, pages 1998-2005. Comments: The human arch is intricately linked with the shock-absorption characteristics of the foot. If you place a rigid filler underneath the arch of your foot, you will notice the dramatic increase in impact felt. While cushioning underneath the heel is always helpful, it cannot replace the shock-absorbing function of the arch. Ideally, the entire shoe is designed with shock absorption in mind. Of course, some compromise must be made between cushioning to absorb shock and firmness to prevent excessive motion of the foot, which could lead to ankle inversion or eversion injuries. (And, ankle injuries can lead to knee injuries, since ankle injuries are typically followed by the use of taping or stiffening via motion-restricting braces. The ankle is one of those joints which naturally moves in many ways, and any loss of this motion could translate into increased knee-injury risk.) With regards to a person's foot arches, severely flat-footed (i.e. no arch to speak of) or low-arched person would be expected to have less shock-absorption capability than someone who has high arches. How these attributes interact with shoe design is an interesting area of research. This study looked at a variety of footwear-related factors pertinent to the running gait, and in particular as influencing various types of injuries. A full biomechanical analysis, using a well-equipped gait-analysis laboratory (complete with a six-infrared-camera 3D motion-analysis system and floor-mounted force plates), enabled accurate quantification of the movements (displacements), velocities, accelerations, forces, and moments (torques) involved. Footwear is intricately linked not only to forces at the ankle (rearfoot), but also with the forces at the knee (and in fact, the influence affects the entire leg, as well as the spine). Clearly, shoe biomechanics can have a major effect on the knee. The authors chose their study variables with an eye towards injury causes. So, their foot-related variables reflected excessive pronation and ankle eversion. Their knee-related variables reflected compression (including impact loadings), shearing, and twisting at the knee. (Although this study was supported in part by a footwear manufacturer, the general findings are pertinent to the shoes from other firms too.) The authors found that running mechanics (and not structural attributes such as arch height) are the most useful parameter for shoe selection; ideally, every runner would undergo a complete gait analysis in a well-equipped biomechanics laboratory. The authors note that shoes designed for controlling foot motion necessarily are more rigid and provide less shock absorption than cushion trainer types.

For a penetratingly insightful discussion into the importance of having serviceable intactness in all major tensile structures in the knee, and for an insightful evaluation of failures of posterolateral reconstructions, please see the September 2006 article An Analysis of the Causes of Failure in 57 Consecutive Posterolateral Operative Procedures, by Noyes, Barber-Westin and Albright, in the Injuries and Surgeries pertaining to Posterolateral Structures (includes LCL) Subsection.

For insight into six degrees of freedom inherent in any motion study, and for insight into the multiple functions of the ACL, please see the August 2006 article The 6 Degrees of Freedom Kinematics of the Knee After Anterior Cruciate Ligament Deficiency -- An In Vivo Imaging Analysis, by Louis E. DeFrate et al., in the Knee Biomechanics, Functional Anatomy of ACL and Other Ligaments Subsection.

For insight into PCL biomechanics and kinematics, including ACL-PCL interdependency and the importance of the PCL to normal knee motion, please see the following three articles: Anatomical Posterior Cruciate Ligament Transplantation -- A Biomechanical Analysis, by Davis et al., In Vivo Function of the Posterior Cruciate Ligament During Weightbearing Knee Flexion, by DeFrate et al., and The Effect of Posterior Cruciate Ligament Deficiency on Knee Kinematics , by Logan et al., all in the PCL Injuries and Reconstructive Surgeries Subsection.

For insight into the proprioceptive and neuromuscular consequences of ACL deficiency, see the October 2005 article Balance in Single-Limb Stance in Patients With Anterior Cruciate Ligament Injury -- Relation to Knee Laxity, Proprioception, Muscle Strength, and Subjective Function, by Ageberg et al., in the Proprioception and Neuromuscular Considerations Subsection.

For insight into the complex interrelationship between ACL serviceability, meniscal intactness, and articular-cartilage health/longevity, see the September 2004 article Histology of the Torn Meniscus -- A Comparison of Histologic Differences in Meniscal Tissue Between Tears in Anterior Cruciate Ligament–Intact and Anterior Cruciate Ligament–Deficient Knees, by Meister et al., in the Meniscal Injuries: Causes, Consequences and Treatments section

For insight into the painful and worrisome long-term consequences of partial or complete meniscus removal, please see the July 2004 article Meniscal Transplantation in Symptomatic Patients Less Than Fifty Years Old, by Noyes and Barber-Westin, in the Meniscal Injuries: Causes, Consequences and Treatments Subsection.

Abnormal Rotational Knee Motion During Running After Anterior Cruciate Ligament Reconstruction Scott Tashman et al.; American Journal of Sports Medicine, Baltimore; June 2004, Volume 32, pages 975-983. Comments: This article makes clear the need for ongoing refinements and enhancements to ACL-reconstruction techniques. Although the current method (entailing implantation of a single sliver of pilfered tendon) certainly brings good results in terms of restoring knee stability, the motion (kinematics) of the reconstructed knee still leaves something to be desired. The problem is that the natural ACL has a multifascicular structure (two or three bundles, depending on the reference source consulted), an attribute which explains the complex three-dimensional movement of the natural knee. (The knee flexes and extends in the anterior-posterior direction, hence in the sagittal plane; this movement itself is a combination of roll and glide. And, to facilitate locking during standing, the knee also twists slightly at full extension; this is movement in the transverse plane. And, the knee effectively becomes slightly knock-kneed at full extension; this valgus-type movement occurs in the frontal plane [also known as the coronal plane].) In an ACL-reconstructed knee, the movement is detectably different from that of a never-injured knee, particularly when dynamic, high-kinetic-energy movements are involved. (Note that kinetic energy increases with speed squared, and so running at thrice the speed of walking translates into nine times as much kinetic energy which the knee is forced to deal with when changing direction or accelerating/decelerating.) The authors note that further research is needed in order to ascertain whether the lingering abnormalities in the movement of the reconstructed knee are a harbinger of premature osteoarthritis. (Keep in mind that about 85% of ACL injuries are accompanied by bone-bruising [articular-cartilage damage], and that such damage itself leaves permanent aftereffects. Meniscal-cartilage damage is also a common accompaniment to ACL tearing. Because all types of joint-cartilage damage leads to early-onset arthritic degeneration, it is not easy to determine whether joint deterioration is a consequence of cartilage damage from the initial injury [or from subsequent giving-way incidents that may have occurred prior to reconstruction] or whether it arises as a result of abnormal knee motion. Also note that the motion of a completely-ACL-deficient-but-non-reconstructed knee is far more abnormal than that of an ACL-reconstructed knee.) The authors note that double-bundle ACL grafts might be a future possibility.

See also the April 2004 article Prospective Evaluation of 1485 Meniscal Tear Patterns in Patients With Stable Knees, Michael H. Metcalf et al., in the Meniscal Injuries: Causes, Consequences and Treatments section

Anatomy, David A. Schulz; Knee Ligament Rehabilitation, edited by Todd S. Ellenbecker. Philadelphia, Pennsylvania: Churchill Livingstone (Harcourt), 2000. Pages 1-15. Comments: David Schulz, a seasoned physiotherapist, delves into the structural anatomy of the knee. The knee is able to move in 6 degrees of freedom, as defined by bearing-surface shape, but note that from the viewpoint of overall movement, flexion-extension rotation is the most important. The complex movement of the knee, most notably the combination roll-and-glide in the sagittal plane, is governed by the cruciate ligaments. This article provides important insight into how the knee ligaments work, and by implication into the types of injuries that can be expected given certain modes of forcing. (For definitions of anatomical-orientation terms such as frontal/coronal, sagittal, transverse, distal, proximal, medial and lateral, please see this document.)

Biomechanics, Richard R. Boeckmann and Todd S. Ellenbecker; Knee Ligament Rehabilitation, edited by Todd S. Ellenbecker. Philadelphia, Pennsylvania: Churchill Livingstone (Harcourt), 2000. Pages 16-23. Comments: This chapter discusses how the knee works internally, and focuses on the forces (biomechanics) borne by the ligaments during the range of motion. The authors note the importance of both cruciate ligaments in governing the roll-and-glide motion of the knee, and they note that all four primary ligaments (ACL, PCL, MCL, and LCL) play a role in limiting how far the knee extends. (No wonder severe hyperextension-type injuries result in massive, widespread ligament damage.) The authors also note that residual MCL laxity (from an insufficiently scarred-over torn MCL) means more stress on the ACL. The authors additionally describe the demands made on the secondary restraints in the event of chronic ligamentous damage. For example, a torn ACL results in the menisci being pressed into service (i.e. to prevent the tibia from sliding too far forwards). In general, damage to any knee structure means more stress on the remaining intact structures. Since the remaining intact structures are not intended to handle loadings they were not designed for, they eventually deteriorate. Ultimately, a knee left with an unaddressed chronic deficiency in any major structure can be counted on to self-destruct.

Foot Mechanics and Knee Pathology, Robert Donatelli and Bruce Greenfield; Knee Ligament Rehabilitation, edited by Todd S. Ellenbecker. Philadelphia, Pennsylvania: Churchill Livingstone (Harcourt), 2000. Pages 307-320. Comments: This article provides penetrating insight into the importance of considering foot mechanics when treating knee injuries. Many knee problems can be precipitated or aggravated by foot-related problems such as collapsed or overly high arches. Also very problematic can be outwards-tilted feet (e.g. ankle sticks outwards, hence varus angulation of the ankle) or inwards-tilted feet (i.e. ankle protrudes inwards, hence valgus angulation). Custom-made foot orthotics can work wonders here, but they must be designed and fitted by an experienced orthotist. Keep in mind that rigid orthotics can cause problems with the foot's natural shock-absorption capabilities, which depend partly on flexion of the arches. Rigid orthotics can also impede the shock-absorption capabilities of athletic footwear, and so flexible orthotics are generally preferable. Foot problems can have major implications in the context of knee-ligament injuries, in particular those involving the ACL. In some cases, foot orthotics can be used to help treat the secondary problems that arise as a consequence of chronic ACL deficiency. Or, orthotics can be used to address some preexisting underlying biomechanical issues which, together with newfound ACLlessness, translate into daily-living functional impairments. For people with ACL tears who are contemplating not having surgery, it should be kept in mind that even the best foot mechanics and the strongest leg muscles, albeit helpful, cannot entirely compensate for the unique functions of the knee ligaments. For example, the ACL is responsible for controlling inwards tibial twisting, and helps the MCL control inwards forcing of the knee; also, the ACL limits forwards tibial sliding (also known as anterior drawer). And, the ACL works in concert with the PCL to limit extension of the knee. (Like the ACL, the PCL fulfills multiple roles too; e.g. control of outwards twisting and rearwards tibial sliding.) A knee that is unstable due to ACL deficiency or PCL deficiency can often be helped through the use of orthotics (along with other measures such as muscle strengthening and application of functional bracing). However, especially for patients who wish to return to knee-demanding activities, there are practical limits to how much can be accomplished without pursuing ligament-reconstructive surgery.

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