Significance of research concerning in vivo stability of knee joints
Knee injury certainly brings kinematic and stability changes, leads to knee joint degeneration and instability, ultimately, results in osteoarthritis[12-13]. However, it is difficult to diagnose knee instability at the early times via physical examination and auxiliary examination. Most of these patients suffered from surgical trauma history or symptoms, but it is hard to diagnose or treat as knee instability. More and more patients suffered knee joint injury and degeneration with time prolong, thus, various methods are used to recover the stability and reconstruct the functions of knee joint, including anterior cruciate ligament reconstruction, ligament reconstruction under an arthroscope and total knee replacement. However, parts of patients still needed a second ligament reconstruction if he suffered the same symptoms after ligament reconstruction. Patients also presented with same functional differences after total knee replacement, such as joint instability or soft tissue unbalance. All of these situations need to evaluate functions of injury knees, which lack of precise evaluating methods.
Acquisition of kinematic information of bone structure of knee joint makes it possible to test knee stability precisely. Here, 3D reconstruction of normal knee joints were obtained by computer image reconstruction, namely, two vertically projected C-arm machines were used to collect 2D dynamic information of knee, which was transformed into 3D simulated motion models via 2D/3D image registration technique, thus, the dynamic motion of knee joint was reduced. Based on anatomical location of interarticular ligaments, the mean value of in vivo ligaments with each angle and length changes of ligaments with flexion variation were obtained, accordingly, the evaluation system for dynamic stability of knee joints were established. It helps us to measure the kinematic parameter of injury knee joint, and compared the differences from normal knee, to evaluate knee functions exactly, thus, confirm the damage threshold of knee and realize the early diagnose and treatment for knee joint injury.
Length changes of collateral ligaments under different flexion angles
Collateral ligaments also easily suffered injury during sports. Many in vitro test have shown the collateral ligaments injury[14], but the reports concerning kinematic measurement of collateral ligaments in vivo, in particularly, in stimulated physiological flexions, are few. Collateral ligaments play an important role in repairing knee joint injury or soft tissue balance after knee replacement.
In this paper, we found that, the length of MCL/LCL was shortened with flexion angle increasing, and the differences had significance. It suggests that, collateral ligament was in a high tension at the small angle to maintain the medial/lateral stability of knee. The ligament tension gradually reduced with flexion angle increasing. Knee instability resulted from any damage can lead to changes in MCL/LCL length and tendency, thus, collateral ligament injury can be diagnosed at early time by comparing the normal and injury knees. The end point of collateral ligament is fan-shaped, here, mid-point measurement was used during point selection, accordingly, there are different changes in the first half part and second half part of the ligaments during knee flexion, that is, the tension increased in the first half part but decreased in the second half part. In the further experiment, we will measure the anterior, middle and posterior collateral ligaments in order to obtain precise kinematic features of in vivo collateral ligaments.
Effects of ACL injury on length changes of collateral ligaments
Collateral ligaments play an important role during knee flexion[5]. After ACL injury, the length of injury MCL was increased with flexion angle increasing, and there were significances between normal and injury knees at 0°, 15° and 30° (P < 0.05). The MCL length of injury knees increased 1.9 mm at 0°, which was increased 1.48 and 1.26 mm at 15° and 30°. Though the length also increased at 60° and 90°, there was no significance compared with normal knees. Inversely, the length of injury LCL was reduced with flexion angle increasing, and there was significance at 0°, 15° and 30° (P < 0.05), but the difference had no significance at 60° and 90°.
Length changes of collateral ligaments is resulted from ACL plays crucial effects on restricting tibial antedisplacement and intorsion, which similar to other in vitro experiment results[12-22]. The LCL is located at the posterior of bottom dead center, thus, the LCL arranged from anterior-superior to posterior-inferior. The line feed of MCL was basically parallel to tibia. Accordingly, if ALC injury leads to tibial intorsion, the bottom dead center of LCL made forward rotation, which closed to LCL dead center, thus, the length was shortened. In contradictory, the length of ligament was increased when the MCL made backward rotation. However, the backward rotation tendency gradually disappeared when the angle greater than 30°, thus, the length of collateral ligaments presented with no obviously changes. DeFrate et al[23] found that ALC can not only prevent tibial antedisplacement and intorsion, but also plays crucial effects on restricting tibial ingression (mean 2 mm).
Our data suggested that, the length of ligament increased due to its effects on preventing tibial antedisplacement and intorsion. The ACL fracture certainly results in stress concentration on MCL, and obviously increased the risk of MCL tearing[24-29]. In addition, traumatic ACL fracture always combined with MCL injury. Animal experiments have demonstrated that the natural healing rate of MCL was only 10% when the ACL and MCL were cut down, because ALC fracture bring over weight on MCL, which block MCL healing[30].
From here, we found that ACL injury affects knee collateral ligaments obviously; similarly, collateral ligaments injury affects knee stability. Accordingly, collateral ligament should arouse fully consideration during reconstruction of knee stability. In further studies, we will elevate examination precision, enlarge sample numbers, and compare the results prior to and after surgery in order to obtain more detailed information addressing kinematic features of in vivo collateral ligaments after ACL injury, and to provide reference for clinical treatment.
