The anterior cruciate ligament is a band of dense connective tissue which courses from the femur to the tibia 22. It is a major knee ligament to stabilize the joint movement against anterior tibial translation 23 and rotational loads 24. While Norwood and Cross 12 in 1979 suggested ACL to have three separate bundles, most anatomical studies 2526 agreed that AM bundle and PL bundle are the only two components of ACL (Figure 1). The AM and PL bundles behave differently in length 27 and in situ force 11 during passive flexion. Due to the different bony orientation attachment 27 of the two bundles, AM and PL bundles are responsible for resisting anterior tibial load and rotational load respectively. Biomechanical study 28 revealed the ultimate load of ACL to failure can be as high as three times of the body weight. Video analysis 29 reported that ACL rupture occurs within 100 ms, indicating a huge explosive force acting to the knee joint during ACL injury.

The knee joint motion is the relative movement between the femur and the tibia. Theoretically, it is capable to a six degree-of-freedom movement: both translation and rotation in three body planes. Clinically, excessive motion in specific direction (anterior-posterior direction) during physical examination may be an indication of knee ligament injury 30. The result of these assessments is often determined by the subjective feeling and experience of the examiners. Biomechanical presentation of the knee motion, instead, provides precise information for comparison between intact and deficient knee when assessing knee stability. To describe the geometric representation, Grood and Suntay 31 proposed a joint coordinate system for measuring three dimensional translation and rotation motions of the knee joint. This is essential for studying ligament injury as knee ligaments govern the motion of the knee. For example, ACL rupture would lead to excessive motion in AP translation and tibial rotation 22. In cadaveric study, it is also suggested that an isolated excision of ACL would increase anterior drawer and tibial rotation in both flexion and extension 26. Therefore, a well understanding of knee kinematics assessments is crucial for the said purpose of this study.

Lachman test has a high accuracy for diagnosing ACL injury 42. Before the test, the examiner should ensure that the tibia is not subluxated posteriorly to avoid false-positive result in a posterior cruciate ligament deficient knee. The patient is asked to lie supine and the knee flexes around 30°. The examiner stabilizes the femur and applies an anterior force on tibia without restraining axial rotation (Figure 3). A positive result of an ACL deficient knee will be presented with proprioceptive or visible anterior translation of the tibia 43. The anterior translation of 1 mm to 5 mm is defined as grade I laxity, 6 mm to 10 mm as grade II, and greater than 10 mm or without a displacement limit (end point) as grade III 40. End point is further graded as firm, marginal or soft.

Pivot shift test is relatively complex since it is a combination of internal rotation torque and valgus torque 44. This test is highly dependent to the technique and experience of the examiner as the result is graded subjectively according to the knee laxity. However, a positive result for the pivot shift test is the best for ruling in an ACL rupture 37. To perform the test, the basic principle is to apply valgus torque and internal rotation to the leg. The test starts with the knee in full extension and then gently flexed to about 40° (Figure 4). A positive pivot shift test is defined as a forward subluxation of tibia during sudden change in direction. It is a reproduction of event that occurs when the knee gives way because of the loss of ACL.

This is an instrument that has been developed for an objective measurement for knee anterior-posterior laxity on sagittal plane. The patient lies in a supine position with knee flexion of about 20° to 30°, supporting with a firm platform placed proximal to the popliteal space. The patient is told to relax in this position. The KT-1000 arthrometer is then placed above the tibia and attached firmly by two bands. After the zero adjustment, the arthrometer is pulled anteriorly to the tibia in order to provide an anterior force (Figure 5). An audible indication will be noticed at 15, 20 and 30 pounds of force. The anterior displacement is measured in millimeter while the laxity is often presented in side-to-side difference.

Computer-assisted surgery has gone through lots of evolutions in recent 15 years. One of the technologies for orthopaedic surgery is the navigation system, which has been applied in spine surgery 45 and total joint surgery 46. It has two basic components for ACL reconstruction:

• A set of optical camera to locate the surgical joint and limb, and to create a picture or image of the operation site.

• Computer programs which integrate these images with surgical information and assist the surgeon during the operation.

Navigation systems improve the accuracy of surgical procedures 47. The computer provides information of the real time relative positions between the instruments and the bone to assist surgeon during surgical procedures. Moreover, by locating joint centre between two relative bodies, it accurately measures the kinematics data in sagittal, coronal or transverse plane (Figure 6). This technique has been utilized in the total knee replacement surgery to guide the balancing of ligaments 46. In the same way, computer assisted navigation system is employed to collect intra-operative details on the laxity of knee in different planes both before and after the ACL reconstruction 48. Therefore, it would be a good way to assess immediate effect of ACL reconstruction, especially to compare single-bundle technique and anatomical double-bundle technique in terms of anterior translation and tibial rotation.

Fluoroscopic navigation system 4749 is based on the pre imaging data (both AP and lateral views), such as computer tomography or C-arm shots, for the model formation to be displaced in the computer software. A pointer containing integrated reflective markers discs is also attached to the C-arm image. By holding the pointer to the known anatomical landmarks, the surgeon reviews the accuracy of the images when acquiring the AP and lateral images. To accurately locate the navigated tools in relationship to the selected anatomic landmarks, surgical instrument with passive marker spheres must be fixed securely to the patient's femur and tibia (Figure 7). Optoelectronic camera system with infrared light-emitting diodes tracks all passive markers throughout the surgical procedure. The line of sight must be guaranteed once after the navigated procedure starts.

Image-free navigation 50 has been widely established recently. Based on the intra-operative knee alignment measurement such as knee axis and joint lines, the system provides virtual illustration of the anatomical structures. With the information after digitizing the cartilage surface of the femur and the tibia, this method combines the existing model and patient's knee as defined by surface matching.

For both fluoroscopic and image-free navigations, since one reference base is fixed each to the femur and the tibia the relative motion in space can be measured precisely. A few studies have reported the validity and reliability of the navigation system. Tsuda et al. 50 validated the navigation system for femoral tunnel placement in double-bundle ACL reconstruction with motion analysis system (digital camera). The average differences between the two measurement systems were less than 3% for both AM and PL tunnels. Another studies conducted by Martelli et al. 51, Pearle et al. 52 and Colombet et al. 53 reported that navigation system is reliable to quantify knee kinematics during stability examinations, particularly in the setting of complex rotatory patterns such as pivot shift test. This suggests an accurate and precise evaluation of different techniques of ACL reconstruction.

By applying a certain force on specific direction to the relaxed knee, ligament injury would be identified if laxity is found when comparing to the other side. This is a usual clinical examination for suspected knee injury without any patients' active movement. However, it may not be the best assessment when it comes to the rehabilitation stage after operative treatment as clinical examinations do not produce sufficient force to stimulate physical activity 40. The ultimate goal for clinical treatment in sports medicine is to allow patients' safe return-to-sports. It was also suggested that functional knee stability should be one of the criteria that determine a safe return-to-sports 16. Being different from static knee stability test such as KT-1000, dynamic functional test, which mimics real game situation during sports, involves patients' muscle strength and neuromuscular perception, demand of specific movement and confidence for performing. To monitor the knee stability during this specific dynamic movement, motion analysis is a good way to achieve.

Patients with ACL injury can be assessed using motion analysis before and after ACL reconstruction. The functional assessment is conducted in a gait laboratory (Figure 8), which is equipped of more than three high-speed cameras and data processing software, providing 10 × 5 m2 captured volume. The three dimensional coordinates of nine-millimeter reflective markers can be recognized in the captured volume by means of infra-red light emitting cameras. In the centre of the captured volume, force plates are placed on floor level in order to collect ground reaction force during the movement.

Marker model is essential for motion analysis. It consists of several reflective skin markers that depend on outcome parameters. Ristanis et al 54 adopt the method described by Vaughan 55 for measuring knee joint kinematics. Fifteen markers are stuck on anatomical landmarks of lower extremities including anterior superior iliac spine (ASIS), greater trochanter, lateral femoral epicondyle, tibial tubercle, lateral malleolus, heel, metatarsal head V on both sides and sacrum (Figure 9). Before capturing the dynamic movement, anthropometric data which include weight, ASIS breadth and thigh length, medthigh circumference, calf length, calf circumference, knee diameter, foot length, malleolus height, malleolus diameter, foot breadth on both sides, are collected.

After data collection, the evaluation period should be well defined and trimmed. In clinical practice, stance phase is chosen for evaluation due to the landing risk factor of noncontact ACL injury. A standing trial with anatomical position is needed to define the offset degree for all segmental movements in all planes. Kinematics of knee joint such as flexion angle, tibial rotation and valgus angle are calculated using programming software. Anthropometric measurements combined with three dimensional coordinates from standing trial provide joint centre position and axes of joint rotations. Joint kinematics is then calculated from the position of reflective markers during the movements.

Dynamic movement should be clinically based and specific to the research objective. ACL injury would lead translational and rotational instability. The movement that performed by the ACL deficient and reconstructed patients should be high demanding, giving a rotational and valgus stress to the knee. Ristanis et al 54 employ a combined movement in assessing ACL deficient and reconstructed patients. The movement involves jumping, landing, pivoting and running. The patient is required to jump forward from a 40 cm high platform, land with both feet, pivot to the right or left at 90° and run away with their maximum speed. It is treated as a high demand of activity in which the movement has to resist a high rotational stress to the knee during pivoting.

The injury mechanism can be an implication of how we assess ACL injury patients. It is reported that over 70% of ACL injury occur in non-contact situation (Figure 10), which involves landing, decelerating and changing direction 33. If the patient has good stability during these 'high risk' movements, it would be an important indication for safe return-to-sports. Therefore, movements such as 3-way cutting 56 and 4-way jumping 57 maneuvers are useful to assess the knee stability. In the cutting task, patients are required to run and cut with single leg in three directions: 90° cut, 45° cut and crossover cut. The 4-way jumping task consists of straight run and a two-footed landing followed by a two-footed takeoff maximum jumping in four directions: vertical, anterior, right and left. After data collection, stance phase is trimmed for further evaluation. Kinematics and kinetics data will be measured from the motion analysis system and the force-plate. Both injury (deficient or reconstructed) and intact knee should be assessed since comparison is important when investigating knee laxity.

In most situations during sports, movements such as landing and sudden change of direction are often unexpected. Planned laboratory experiments and actual athletic competition would result different biomechanics performances 17. Biomechanics study has also shown that unplanned cutting is identified as a risk factor of noncontact ACL injury 58. In order to investigate this unanticipated effect, a device containing photo cell receiver with light source is instrumented across the runway. When the patient passes through the device, a randomized signal will be generated from the computer connecting to the device. It will then create a visual cue for the cutting and jumping direction through a monitor placing in front of the patient This laboratory setting would only allow subjects' short time decision so that a game-like situation is reproduced in the laboratory.