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See also Meniscal Injuries: Causes, Consequences and Treatments and Long-term Consequences of ACL Injuries, as well as Knee Alignment, Component Interdependency, and Biomechanics (including Gait Dynamics)
and Proprioception and Neuromuscular Considerations and also
Osteotomies and Complex Bone-Realignment Surgeries Brief Overview of Knee Anatomy and Physiology, Michael Frind, May 18, 2004, with subsequent revisions. Comments: This synopsis of knee anatomy and physiology serves as a good primer of how the knee works. Includes diagrams. The article is not overly technical, yet it covers the salient topics with a depth and breadth which transcends that normally provided in general-level literature. The Role of the Anteromedial and Posterolateral Bundles of the Anterior Cruciate Ligament in Anterior Tibial Translation and Internal Rotation, Thore Zantop et al.; The American Journal of Sports Medicine, Baltimore; February 2007, Vol 35, p. 223-227. Comments: This brilliantly perceptive, eye-opening article discusses the intricate double-bundle (anteromedial and posterolateral) structure of the natural ACL. The authors clearly show that the only hope of reproducing the complex functioning of the natural ACL is with a double-bundle graft.
While it would be biomechanically wonderful if all knee-involved orthopedists were to perform double-bundle ACL grafts, the problem is that double-bundle grafting is technically far more complicated than single-bundle ACL grafting (which in itself is already technically very demanding). This means that double-bundle grafting, while kinematically far superior to single-bundle ACL grafting, would introduce a greater risk of surgeon error. Hopefully, improvements and standardization in ACL-grafting techniques, combined perhaps with technological enhancements (tools and perhaps electronic tunnel-drilling guidance systems), will make double-bundle ACL grafting feasible for mass implementation. 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, Vasileios Chouliaras et al.; The American Journal of Sports Medicine, Baltimore; February 2007, Vol 35, p. 189-196.
Comments: These authors confirm and amplify a long-standing (see, for example, Furia-AJSM-May97.shtml) biomechanical worry: that the standard single-bundle ACL-graft construct, as has been used for years in hundreds of thousands of patients, unsatisfactorily reproduces the intricate biomechanics and motion of the natural ACL. The biomechanical need for a double-bundle graft construct is abundantly clear. Other good articles dealing with ACL-graft biomechanics, and highlighting the benefits of a double-bundle ACL graft construct, are Petersen-AJSM-Feb07.shtml and Zantop-AJSM-Feb07.shtml. However, there is one practical problem with double-bundle ACL-grafting: such grafting is technically more complicated than single-bundle ACL grafting, and thus makes far greater demands on the surgeon's skill. This means there is greater risk of surgeon error with a double-bundle graft. In the future, widespread implementation of double-bundle ACL grafting might be feasible through the use of technological advancements such as computerized bone-tunnel-drilling guidance systems. Such systems would have to somehow scan the knee bones (remember that each person's knee is unique), accurately map the individual's knee and its motion in three dimensions, and indicate where the bone tunnels should be drilled and what tension should be applied to the grafts upon installation. 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 a thought-provoking biomechanical comparison of single- versus double-bundle ACL graft, please see the superb February 2007 article Biomechanical Evaluation of Two Techniques for Double-Bundle Anterior Cruciate Ligament Reconstruction --
One Tibial Tunnel Versus Two Tibial Tunnels, by Wolf Petersen et al., in the Choosing a Knee-Ligament Graft Subsection. For profound insight into the fundamental material differences (e.g. lower strength, easier breakability, less elasticity, more frangibility) between the female and male ACLs, please see the superb December 2006 article Sex-based differences in the tensile properties of the human anterior cruciate ligament, by Naveen Chandrashekar et al., in the Female-Athlete Knee-Injury Incidence and Prevention Subsection. The 6 Degrees of Freedom Kinematics of the Knee After Anterior Cruciate Ligament Deficiency -- An In Vivo Imaging Analysis, Louis E. DeFrate et al.; The American Journal of Sports Medicine, Baltimore; August 2006, Vol 34, p. 1240-1246. Comments: This study, done on single-ACL-torn people using fluoroscopy (i.e. X-ray video; note that this technique results in a high X-ray dose because it uses the same X-ray source as is used for static imaging, but the shutter remains open, thus exposing the imaged area to the equivalent of 50 still X-ray shots every second!) and MRI (which is completely harmless), shows how the biomechanics of the ACL-deficient knee varies from the natural knee.
The six degrees of freedom (i.e. 3 translations and 3 rotations) are universal to any field involving motion. They are anterior-posterior (front-to-back translation), medial-lateral (side-to-side translation), superior-inferior (up-down translations, also known as proximal-distal), internal-external rotation (i.e. rotation in a transverse plane, i.e. any plane parallel to the person's belt line), varus-valgus rotation (i.e. what you get if you force the knee inwards or outwards in such a way as to tear the MCL or LCL, respectively; this is a rotational motion in a coronal/frontal plane, which is any plane parallel to the bed if you are lying on your back), and saggital-plane rotation (i.e. the normal flexion and extension of the knee). (In aviation, these 6 degrees of freedom are known as x, y, z, and roll, pitch, and yaw.) The authors conclude that the ACL plays a role not only in anterior-posterior stability, but in medial-lateral stability as well (and in addition to contributing to extension limitation and constraining inwards tibial rotation). For insight into PCL biomechanics and kinematics, including ACL-PCL interdependency (i.e. the two cruciate ligaments work closely together to govern the sagittal-plane roll-and-glide motion of the knee) 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 anatomical and physiological differences between male and female ACLs, please see the October 2005 article Sex-Based Differences in the Anthropometric Characteristics of the Anterior Cruciate Ligament and Its Relation to Intercondylar Notch Geometry -- A Cadaveric Study, by Naveen Chandrashekar et al., in the Female-Athlete Knee-Injury Incidence and Prevention Subsection. For insight into the topic of combined PCL (posterior cruciate ligament) and PLC (posterolateral corner, also known as posterolateral structures) reconstruction, and keeping in mind that PLC injuries sometimes accompany ACL injuries, please see the March 2005 article Biomechanical Analysis of a Combined Double-Bundle Posterior Cruciate Ligament and Posterolateral Corner Reconstruction, by Jon Sekiya et al., in the PCL Injuries and Reconstructive Surgeries Subsection. An alternative method of anthropometry of anterior cruciate ligament through 3-D digital image reconstruction, J. Hashemi et al.; Journal of Biomechanics; March 2005, Vol 38, p. 551-555. Comments: This intriguing article describes a new method for determing ACL dimensions and volume via an inexpensive camera-based optical scanning arrangement. Although the system is impractical for use in vivo (since the femoral condyles obstruct the view of the ACL from certain angles), it is useful for research into intricate multi-bundle (multifascicular) structure of the ACL. This type of research is useful for determining better ACL-graft constructs (i.e. dual-bundle graft, as is already being done for PCL reconstructions) and installation-alignment procedures. Effects of Applied Quadriceps and Hamstrings Muscle Loads on Forces in the Anterior and Posterior Cruciate Ligaments, Keith L. Markolf; The American Journal of Sports Medicine, Baltimore; July 2004, Vol 32, p. 1144-1149. Comments: This study shows how even normal muscle contraction forces can generate very high tensile forces in knee-ligament grafts. The findings show how both quadriceps and hamstring musculature affect both the ACL and the PCL. For good insight into the biomechanical consequences of chronic ACL deficiency, in particular with regards to the resulting greatly increased stresses on the knee's lateral structures and accelerated medial-compartment degeneration (and how devices such as special shoe inserts can be helpful here), see the November 2003 article The Effect of Wedged Insoles on the Lateral Thrust of Anterior Cruciate Ligament-Insufficient Knees, by Yoshimura et al., in the The Degenerate Knee: Arthritis Subsection. For insight into forwards tibial sliding of the ACL-deficient knee during the transition-to-weightbearing portion of the gait cycle, and also for an evaluation of the application of functional knee bracing in this context, see The Effect of Anterior Cruciate Ligament Deficiency and Functional Bracing on Translation of the Tibia Relative to the Femur During Nonweightbearing and Weightbearing, Bruce D. Beynnon et al., in the Functional Knee Bracing Subsection. For insight into the forces the ACL is subjected to during alpine skiing, see Injury to the Anterior Cruciate Ligament During Alpine Skiing -- A Biomechanical Analysis of Tibial Torque and Knee Flexion Angle, by Sharon L. Hame et al., in the General Knee-Injury Epidemiology and Prevention Subsection. 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.
For penetrating insight into the functional anatomy and biomechanics/kinematics of natural versus reconstructed ACLs, be sure to read the pair of current-concepts articles by F.H. Fu: Current Trends in Anterior Cruciate Ligament Reconstruction; Part 1: Biology and Biomechanics of Reconstruction, and Current Trends in Anterior Cruciate Ligament Reconstruction; Part 2: Operative Procedures and Clinical Correlations , in the ACL Reconstruction via Patellar-Tendon Autografting Subsection. For excellent insight into the reasons why the ACL is not amenable to repair via stitching of the ends (i.e. in the synovial-fluid-filled intra-articular environment), and also for insight into the process of ACL-graft tendon-to-bone healing (histologically, therefore on cellular level), please see "Studies of Tendon-to-Bone Healing: Exploring Ways to Improve Graft Fixation Following Anterior Cruciate Ligament Reconstruction", by Scott A. Rodeo, MD, and also "Effect of the Intra-Articular Environment on Healing of the Ruptured Anterior Cruciate Ligament", by Martha Meaney Murray, MD. Both of these intriguing commentaries are supplementary documents to the extremely insightful and superbly written August 2001 Noyes-and-Barber-Westin article Revision Anterior Cruciate Surgery with Use of Bone-Patellar Tendon-Bone Autogenous Grafts, and they appear at the end of said article. The commentaries include full-colour photographs, taken through a microscope, of stained-and-mounted intact and ruptured ACL cells.
The History of ACL Surgery, P. Colombet et al., Bordeaux-Mérignac Centre of Orthopaedic and Sports Surgery, Mérignac, France. Comments: This easy-to-read article covers the history of ACL reconstruction, and shows the evolution of current procedures. Note: If you are looking for a concise overview of current ACL-graft options, please go here for the article "ACL Graft Choices", by F.L. Avery. For insight into the structure of the ACL (i.e. different collagen types and roles) and the influence of hormones (particularly in the female athlete), see the October 2003 article Association of Menstrual-Cycle Hormone Changes with Anterior Cruciate Ligament Laxity Measurements, by VanLunen et al., in the Female-Athlete Knee-Injury Incidence and Prevention Subsection. For insight into the differing biomechanics/kinematics of the female knee versus the male knee, please see the August 2003 article Gender Differences in Surface Rolling and Gliding Kinematics of the Knee, by Hollman et al., in the Female-Athlete Knee-Injury Incidence and Prevention Subsection. For insight into the biomechanical ramifications of the single-bundle structure of the typical ACL graft, see also the October 2003 article Fixed tibial subluxation after successful anterior cruciate ligament reconstruction, by Almekinders et al., in the Evaluation of the Reconstructed Knee Subsection. For insight into the dynamic stabilization of the knee, and therefore the synergistic interaction between the ACL, menisci, and leg musculature, see the July 2001 article Knee Stability Controlled by Hamstrings and Functional Knee Brace, by Torry et al., in the Functional Knee Bracing Subsection. The Biomechanical Interdependence between the ACL graft and the Medial Meniscus, Christos D Papageorgiou; The American Journal of Sports Medicine, Baltimore; Mar/Apr 2001, Vol 29/2, p. 226. Comments: This article underscores the close interdependency of the knee's cruciate ligaments and the medial meniscus. The article makes it clear not only that ACL deficiency is devastating to meniscal tissue, but that meniscal loss increases demands on the nascent ACL. Direct evidence of ACL-hamstring reflex arc in humans, Eiichi Tsuda; The American Journal of Sports Medicine, Baltimore; Jan/Feb 2001, Vol 29/1, p. 83. Comments: This article discusses the ACL-protective hamstring reflex, thus highlighting the importance of strong hamstring musculature in protecting said ligament. Electromyographic and kinematic analysis of cutting maneuvers: Implications for anterior cruciate ligament injury, Scott Colby et al.; American Journal of Sports Medicine, Baltimore; Mar/Apr 2000, Vol 28/2, pages 234-240 Comments: This fascinating study looks at the muscle-firing patterns underlying various cutting-type movements. Given that roughly 70% of ACL injuries occur in the context of sports, an improved understanding of the biomechanical/kinematic precursors to ACL injuries offers the potential to engender improvements in athletic training. One method that greatly reduces torque at the knee during cutting-type manoevres entails learning to pivot on the front portion of the foot (rather than planting the entire shoe sole). Such a technique is especially worthwhile for athletes with knee-injury histories. Colby et al. also provide a good description of the stride and stance phases of walking and running. Biomechanics of Knee Ligaments, Savio L-Y Woo; The American Journal of Sports Medicine, Baltimore; Jul/Aug 1999, Vol 27/4, p. 533. Comments: This article gives a good quantitative overview of the biomechanical-engineering fundamentals germane to knee ligaments. For insight into the demands placed on the ACL during various types of free-weight closed-kinetic-chain exercises, see the November 1996 article Comparison of lntersegmental Tibiofemoral Joint Forces and Muscle Activity During Various Closed Kinetic Chain Exercises, by Stuart et al., in the Physiotherapy, Rehabilitation, and Post-Operative Aspects Pertaining to Ligament Surgeries Subsection. For insight into the process of ACL-graft incorporation/ligamentization and the related aspects of biomechanics, see the August 1992 article The Effect of a Ligament-Augmentation Device on Allograft Reconstructions for Chronic Ruptures of the Anterior Cruciate Ligament, by Frank R. Noyes and Sue D. Barber-Westin, in the Other Topics Pertaining to the Knee (RSD, Synthetic Grafts, etc.) section. Note: A deep archive of articles and abstracts from Cincinnati Sports Medicine (CSM) Centre, including many landmark studies in the realm of knee biomechanics and trend-setting articles in the fields of ACL reconstruction and meniscal repair, can be found in directly on the CSM Publications Website. The article-publication dates range from 1980 to present. Please note that most of these articles are already present directly in this Knee Library. On the CSM site, full-text articles are given as scanned-to-PDF files. Microsoft Windows users should, due to file-size considerations, download these by right-clicking and choosing "Save Target As". (The files must then be viewed using Adobe/Xerox Acrobat Reader, which is freely available for all computing platforms here.) Some articles on the CSM site are available only as abstracts; however, full-text printed copies may be obtained by contacting Sue Barber-Westin at sbwestin(at)csmref.org.
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