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Knee stability assessment on anterior cruciate ligament injury: Clinical and biomechanical approaches
© Lam et al; licensee BioMed Central Ltd. 2009
Received: 23 September 2008
Accepted: 27 August 2009
Published: 27 August 2009
Anterior cruciate ligament (ACL) injury is common in knee joint accounting for 40% of sports injury. ACL injury leads to knee instability, therefore, understanding knee stability assessments would be useful for diagnosis of ACL injury, comparison between operation treatments and establishing return-to-sport standard. This article firstly introduces a management model for ACL injury and the contribution of knee stability assessment to the corresponding stages of the model. Secondly, standard clinical examination, intra-operative stability measurement and motion analysis for functional assessment are reviewed. Orthopaedic surgeons and scientists with related background are encouraged to understand knee biomechanics and stability assessment for ACL injury patients.
Sports injury is common, ranking the second highest (21%) in terms of cause of injury  and leading to long-term disabilities and handicaps especially in patients with knee injuries . Among all sport-related knee injuries, one-fifth (20%) involves the anterior cruciate ligament (ACL) – the most commonly traumatized structure . ACL rupture results in knee instability , prohibits the athletes back to sports, and results in early retirement . Conservative treatments can somewhat enhance the sense of stability and rehabilitation, but not in objective outcome assessment  and rate of returning to sports . Therefore, operative treatments are often prescribed to reconstruct the ACL in order to restore the knee stability and return the athletes to sports and active lifestyle .
Numerous anatomy studies showed that the intact human ACL consists of an anteromedial (AM) bundle, and a posterolateral (PL) bundle , while some studies even reported an intermediate bundle in between . Biomechanics studies showed that AM and PL bundles mainly contribute to anterior-posterior and rotational stability of the knee respectively [11, 12]. Traditional surgical methods employ a single bundle bone-patellar-tendon-bone or hamstrings autograph, however, the methods provide good resistance to anterior tibial loads but not to rotational loads . Therefore, the unique anatomical and biomechanics characteristics of the two bundles provide a rationale to the recent emerge of anatomical double-bundle ACL reconstruction approach [14, 15] to better mimic and restore the anatomy and biomechanics of the intact ACL in the reconstructed knee . However, this advantage of rotational stability has not been widely proved on living human.
Returning to high level athletic activity is an ultimate goal for patient who undergoes ACL reconstruction. However, standardized and objective criteria to assess athletes' safe return-to-sports are limited. Functional knee stability is proposed to be one of the key factors influencing safe return-to-sports . Before recommending reconstructed patients to return to activity with pre-injury level, good knee stability should be attained when performing similar on-field movements such as stop-jumping and cutting in the laboratory setting. Therefore, functional knee stability evaluated by kinematics assessment definitely provides valuable information on standardization for safe return-to-sports. This article reviews the knee stability assessments for injury diagnosis, treatment evaluation and long term standard for safe return-to-sports for ACL deficient knee. It aims to provide the basic introduction in knee biomechanics and the importance of stability assessments for orthopaedic surgeons, physiotherapists and scientists with related background.
The knee and its movement
The lower extremity is composed of three major joints: the hip joint, the knee joint and the ankle joint. Located in between hip and ankle joint, knee provides balance and transformation of load of body even when we perform a rapid change of speed and direction. Study has shown that unanticipated cutting maneuvers would increase the risk of non-contact knee ligament injury due to the increased external varus/valgus and internal/external rotation moments applied to the knee . Even in straight running, the ground reaction force can be up to three times the body weight . Therefore, being with the function of supporting the entire body weight during stance phase, knee is one of the most vulnerable joints suffering acute injury  and long term development of osteoarthritis [20, 21].
Anterior cruciate ligament
Biomechanical presentation of knee motion
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 . 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  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 . 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 . Therefore, a well understanding of knee kinematics assessments is crucial for the said purpose of this study.
Contribution of knee stability assessment at different stages of ACL injury
Knee stability assessments contribute three main roles in the management model for ACL injury – (i) clinical assessment provides a quick and reliable way for the diagnosis of ACL injury, (ii) intra-operative assessment evaluates immediate effect of operative treatment and compares different reconstruction techniques, (iii) functional assessment acts as long-term guidelines during or after rehabilitation program, indicating if the athlete is fully recovered in terms of stability to pre-injury activity level. These three main roles are then elaborated in the following sections.
Diagnosis of anterior cruciate ligament injury – clinical assessment
Accurate diagnosis of ACL injury relies on injury history , clinical assessment  as well as advanced imaging technique . Being different from others, clinical assessment provides a stability evaluation of the injured knee to test if excessive motion exists. It varies considerably within the normal population and a greater motion would be found in hyper-laxity group . It is always recommended to compare the motion of the injured side to the normal side  if the patients have unilateral knee injury. The potential limitations should be kept in mind, including the uncontrolled force applied and the reflex resistance of the patient because of anxiety and pain. The first clinical examination after an acute knee trauma is suggested to have a low diagnostic value . Therefore, clinical assessments should be performed by skillful and experienced examiner. Several typical assessments for diagnosing ACL injury are demonstrated below.
Pivot shift test
Operation treatment evaluation – intra-operative assessment of ACL reconstruction
Computer assisted navigation system
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  and total joint surgery . 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 details and measurement
Image-free navigation  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.  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. , Pearle et al.  and Colombet et al.  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.
Long term evaluation during and after rehabilitation – functional assessment
Passive and active motion
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 . 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 . 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.
Optical motion analysis with reflective skin marker
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.
The dynamic movement
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  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.
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 . Biomechanics study has also shown that unplanned cutting is identified as a risk factor of noncontact ACL injury . 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.
Standard clinical tests, such as Anterior Drawer test and Lachman test, are commonly used to assess AP stability before and after reconstructing the graft. With the help of validated navigation system, knee kinematics stability test can be assessed during operation procedure, enabling the evaluation of immediate effect of ACL reconstruction. The clinical result in terms of laxity is more reliable using navigation system when compared to conventional procedure . To investigate if ACL reconstruction with anatomical double-bundle technique better improve rotational stability, Robinson et al  suggested that PL bundle was important than AM bundle in controlling rotational component during Pivot Shift test. In another intra-operative study  in which the surgeon applied manual maximum force to test anterior-posterior and rotational stability, however, found no significant different between single-bundle technique and double-bundle technique in restoring knee kinematics. It is still a controversial issue for double-bundle technique before it comes to a consensus from different research groups.
Patients with ACL deficiency report that they feel giving way rather than anterior-posterior instability during cutting movement in sports. Pivot Shift test is a dynamic test containing multiple directional motion to assess abnormal joint excursion . Using navigation system, stability in terms of rotational displacement and anterior translation can be objectively monitored during Pivot Shift test. However, the manual force applied by the surgeon remains one of major limitations in these intra-operative studies [53, 60, 61]. Robotic testing systems have been employed in cadaveric experiments to simulate Pivot Shift test to a combined valgus and internal rotatory loads [44, 62]. This kind of equipments with controlled manual force should be implemented to the operation theatre for future study which aims at a more scientific proof for having double-bundle technique on ACL injury patients.
In the study conducted by Ristanis et al , the range of internal rotation was reported to be significantly higher in deficient knees than that in intact knees. The authors, however, did not mention about the other knee kinematics data such as valgus angle during the landing phase, which might be an important implication of instability of ACL deficient patients. This kinematics study maneuver, which demonstrates a similar clinical result , not only further confirms the rotational laxity in ACL deficient patients, but also provides an adequate assessment for the long term evaluation of anatomical double-bundle ACL reconstruction.
The knee stability assessments in different stages of management model for ACL injury are important in sports medicine. Related researches on clinical examination, intra-operative navigation ACL reconstruction and functional evaluation with motion analysis system are highlighted for better understanding of how these assessments contribute to the diagnosis of ACL injury, the immediate evaluation of operation treatments and the establishment of safe return-to-sports criteria respectively. The clinical relevance is for orthopaedic surgeons, physiotherapists and scientists with related background to apply appropriate assessments for ACL injury patients.
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