BioMed Central Page 1 of 9 (page number not for citation purposes) Journal of Orthopaedic Surgery and Research Open Access Review Biomechanics and anterior cruciate ligament reconstruction Savio L-Y Woo*, Changfu Wu, Ozgur Dede, Fabio Vercillo and Sabrina Noorani Address: Musculoskeletal Research Center, Department of Bioengineering, University of Pittsburgh, Pennsylvania, USA Email: Savio L-Y Woo* - ddecenzo@pitt.edu Changfu Wu - chw54@pitt.edu Ozgur Dede - ozd2@pitt.edu Fabio Vercillo - fabio.vercillo@poste.it Sabrina Noorani - syn1@pitt.edu * Corresponding author Abstract For years, bioengineers and orthopaedic surgeons have applied the principles of mechanics to gain valuable information about the complex function of the anterior cruciate ligament (ACL). The results of these investigations have provided scientific data for surgeons to improve methods of ACL reconstruction and postoperative rehabilitation. This review paper will present specific examples of how the field of biomechanics has impacted the evolution of ACL research. The anatomy and biomechanics of the ACL as well as the discovery of new tools in ACL-related biomechanical study are first introduced. Some important factors affecting the surgical outcome of ACL reconstruction, including graft selection, tunnel placement, initial graft tension, graft fixation, graft tunnel motion and healing, are then discussed. The scientific basis for the new surgical procedure, i.e., anatomic double bundle ACL reconstruction, designed to regain rotatory stability of the knee, is presented. To conclude, the future role of biomechanics in gaining valuable in-vivo data that can further advance the understanding of the ACL and ACL graft function in order to improve the patient outcome following ACL reconstruction is suggested. Background An anterior cruciate ligament (ACL) rupture is one of the most common knee injuries in sports. It is estimated that the annual incidence is about 1 in 3,000 within the gen- eral population in the United States, which translates into more than 150,000 new ACL tears every year [1,2]. Unlike many tendons and ligaments, a mid-substance ACL tear cannot heal and the manifestation is moderate to severe disability with "giving way" episodes in activities of daily living, especially during sporting activities with demand- ing cutting and pivoting maneuvers. Further, it can cause injuries to other soft tissues in and around the knee, par- ticularly the menisci, and lead to early onset osteoarthritis of the knee. Therefore, surgical treatment using tissue autografts or allografts is frequently performed by sur- geons on patients with a ruptured ACL. It is estimated that approximately 100,000 primary ACL reconstruction sur- geries are performed annually in the United States [1,3]. The direct cost for these operations is estimated to be over $2 billion [4]. The goal of an ACL reconstruction is to reproduce the functions of the native ACL. Over the past three decades, clinically relevant biomechanical studies have provided us with important data on the ACL, particularly on its complex anatomy and functions in stabilizing the knee joint in multiple degrees of freedom (DOF). As such, sur- gical reconstruction of the ACL has not been able to repro- duce its complex function. Both short and long term clinical outcome studies reveal an 11���32% less than satis- Published: 25 September 2006 Journal of Orthopaedic Surgery and Research 2006, 1:2 doi:10.1186/1749-799X-1-2 Received: 13 June 2006 Accepted: 25 September 2006 This article is available from: http://www.josr-online.com/content/1/1/2 �� 2006 Woo et al licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Journal of Orthopaedic Surgery and Research 2006, 1:2 http://www.josr-online.com/content/1/1/2 Page 2 of 9 (page number not for citation purposes) factory outcome for patients [5-8], among whom up to 10% may require revision ACL reconstruction [9]. Indeed, ACL reconstruction remains a significant clinical problem to date as there have been over 3,000 papers published in the last 10 years, with over half focusing on techniques, a large number on complications and related issues, and only a small percentage on clinical outcome. This review paper will provide a perspective on how bio- mechanics has helped in understanding the complex function of the normal ACL as well as in advancing ACL reconstruction. Firstly, the anatomy and function of the ACL as well as available tools in ACL-related biomechan- ical study are briefly introduced. Secondly, the contribu- tions of biomechanics in determining some key factors that affect the surgical outcomes of ACL reconstruction are discussed. Thirdly, the role of biomechanics in developing a new ACL reconstruction procedure, i.e., anatomic dou- ble bundle ACL reconstruction, is presented. Finally, the future role of biomechanics in gaining the needed in-vivo data to further improve the results of ACL reconstruction for better patient outcome is suggested. Anatomy and biomechanics of the ACL The ACL extends from the lateral femoral condyle within the intercondylar notch, to its insertion at the anterior part of the central tibial plateau. The cross-sectional areas of the ACL at the two insertion sites are larger than those at the mid substance. The cross-sectional shape of the ACL is also irregular[10]. Functionally, the ACL consists of the anteromedial (AM) bundle and the posterolateral (PL) bundle [11]. It has been shown that the AM bundle lengthens and tightens in flexion, while the PL bundle does the same in extension [12]. These complex anato- mies make the ACL particularly well suited for limiting excessive anterior tibial translation as well as axial tibial and valgus knee rotations. Laboratory studies have determined load-elongation curve of a bone-ligament-bone complex by a uniaxial ten- sile test. The stiffness and ultimate load are obtained to represent its structural properties. In the same test, a stress-strain relationship can also be obtained, from which the modulus, tensile strength, ultimate strain, and strain energy density can be measured to represent the mechanical properties [13]. In addition, forces in the ACL can be measured by studying the knee kinematics in 6 DOF in response to externally applied loads. For instance, when a knee is subjected to an anterior tibial load, it undergoes anterior tibial translation, as well as internal tibial rotation. Thus, biomechanics is useful to determine the inter-relationships between the ACL and knee kine- matics as the data serve as the basis for the goal of a replacement graft. Discovery of tools for biomechanical studies of the ACL and ACL grafts There have been many tools, including buckle transduc- ers, load cells, strain gauges, and so on, designed to meas- ure the forces within the ACL when a load is applied to the knee [14-19]. All have contributed significantly to the knowledge of the function of the ACL. However, they all make contact with the ACL. Other investigators prefer to measure the force in the ACL without contact. These include the use of radiographic or kinematic linkage systems attached to the bones and determine the forces in the ACL by combining kinematic data from the intact knee and the load-deformation curves of the ACL [12,20]. More recently, computer modeling and simulations have also been used to estimate the forces in the ACL during gait [21]. In our research center, we have pioneered the use of a robotic manipulator together with a 6-DOF universal force-moment sensor (UFS), as illustrated in Figure 1[22]. (a) The robotic/universal force-moment sensor (UFS) testing system designed to measure knee kinematics and in situ forces in DOF Figure 16 (a) The robotic/universal force-moment sensor (UFS) testing system designed to measure knee kinematics and in situ forces in 6 DOF. (b) A human cadaveric knee specimen mounted on the robotic/UFS testing system.