The tibiofemoral joint, as the largest joint in the body, is comprised of the distal fe-mur and proximal tibia articulations. The medial condyle of the fefe-mur is larger than the lateral femur, in both anteroposterior and proximodistal directions contributing
Figure 2.1: Frontal (left) and transverse (right) views of the knee joint.
to the valgus alignment of the knee. Also, the tibia has ~3 degrees lateral inclination relative to the joint line and ~9 degrees posterior slope [66]. As a result, the knee typically has ~10-12 degrees of valgus [66]. The menisci, situated between the femur and the tibia, effectually provide the tibiofemoral joint with a continued contact area and stress distribution across the joint, shock absorption, and joint lubrication [67].
The intrinsic material properties of the meniscus such as low permeability and low compressive stiffness (compared to its stiffness in the knee transverse plane) account for the above-mentioned functions of the tissue [67, 68].
It is thought that the muscles acting across the knee stabilize and control the knee flexion/extension DoF. Apart from the flexion/extension DoF, ligaments within the tibiofemoral joint (i.e., anterior and posterior cruciate ligaments and lateral and me-dial collateral ligaments) primarily restrain the relative translation and rotation of the femur and tibia in sagittal, frontal, and transverse planes.
The anterior cruciate ligament (ACL) is attached to the medial surface of the lateral femoral condyle in one end. On its other end, it is attached to the front and lateral edge of the medial tibial plateau (Figure 2.1). The ACL primarily restrains posterior translation of the femur relative to tibia whilst it does not resist anterior translation of the femur relative to the tibia [69–71]. The ACL secondarily restrains adduction rotation, external rotation, and hyperextension of the femur [69–71].
The femoral attachment site of the posterior cruciate ligament (PCL) is the lateral surface of the medial femoral condyle. The PCL tibial attachment is located in a hallow between the medial and lateral tibia plateaus and extends to the posterior surface of the tibia (Figure 2.1). The PCL primarily restrains the anterior translation of the femur relative to the tibia at all angles of knee flexion, with no resistance in the backward translation of the femur relative to the tibia [70–72]. The PCL, secondarily, restrains the abduction rotation and internal rotation of the femur [70–72].
The medial collateral ligament (MCL) can be divided into superficial and deep bundles, both originating from the medial epicondyle of the femur (immediately below the adductor tubercle). The superficial MCL is subdivided into anterior and posterior portions. The anterior superficial MCL stretches downward to the medial tibia, whereas the posterior superficial MCL passes obliquely backward and inserts into the medial meniscus [73, 74]. The deep MCL bundle is subdivided into menis-cotibial and meniscofemoral ligaments. The menismenis-cotibial portion originates from the lateral and posterior medial tibial plateau (inferior to the tibial cartilage) and inserts at the outer surface of the medial meniscus. The meniscofemoral ligament originates from the medial epicondyle of the femur and inserts at the posterior horn of the lateral menisci. The MCL, and mostly its superficial bundle, is an important restraint to abduction rotation, external rotation, and mediolateral translation of the femur relative to the tibia [66, 73, 75].
The lateral collateral ligament (LCL) stretches from the lateral epicondyle of the femur to the head of the fibula. In contrast to the MCL, the LCL is not connected with other tissues such as menisci. LCL primarily resists knee adduction rotation and secondarily restrains internal rotation and anterior translation of the femur rel-ative to tibia [66, 75, 76].
2.1.2 The patellofemoral joint
The patellofemoral articulation is a saddle joint (cellar joint) between the patella and the femoral trochlea. The movement and alignment of the patella are con-trolled by muscle forces (i.e., quadriceps and hamstrings), contact forces (between
the patella and trochlear groove), and restraints of ligaments [66]. The ligaments which are attached to the patella consist of the patellar ligament, medial and lateral patellofemoral ligaments (MPFL and LPFL), medial patellotibial ligament, medial patellomeniscal ligament, medial retinaculum, and lateral retinaculum.
The patella provides the quadriceps muscles with a greater moment arm around the center of rotation of the knee joint, decreasing up to 30% of the quadriceps force required to extend the knee [66]. Interestingly, the moment arm of the patellar ligament is slightly longer than the moment arm of the quadriceps tendon [77, 78].
As a result, the ratio of the quadriceps force to the patellar ligament force varies at different knee flexion angles [79]. This feature of the patellofemoral joint is of interest, especially in the rehabilitation of the quadriceps muscle-tendon complex [79]. The patella also acts as a bony shield to protect the trochlea and distal femur, e.g., when the knee is flexed.
2.1.3 Functions of the muscles
Approximately 660 skeletal muscles, ~40% of body mass, provide the human body with coordination, movement, and stability under the control of the central ner-vous system (CNS) [80]. At each joint of the body, e.g., the knee joint, a certain magnitude of moments (on each joint’s DoF) is required to perform either static or dynamic tasks. These moments are provided actively via muscles or passively via, e.g., ligaments. In other words, the force generated by a muscle crossing a joint may have a twofold effect on the joint kinetics. First, the muscle force applies a moment around the center of rotation of the joint, which is equal to the muscle force multi-plied by its moment arm. The moment arm of each muscle varies (dominantly) as a function of the body kinematics. Consequently, the required moment at the joint is controlled by the CNS through adjusting the magnitude of the generated muscle forces. The second effect of the muscle force on the joint kinetic is a force equal to the force generated by the muscle. As a result, the total joint contact force (JCF) may exceed multiples of the bodyweight due to the interaction of several muscles simultaneously acting on the joint.
Muscles acting on a joint can be classified (typically) into four categories during every specific task. The prime mover (agonist) muscles are those producing most of the moment required to perform the task. For instance, hamstring muscles are the agonists during knee flexion. A muscle that aids the agonist is termed as a synergistic muscle. For example, medial and lateral gastrocnemius are considered as synergistic muscles during knee flexion. The third group of muscles, which are termed antagonist, are the muscles that oppose the agonist muscles. Antagonist muscles provide more control over the joint and prevent excessive movement, in-appropriate action, or joint injury. When flexing the knee, the quadriceps act as antagonists. The fourth group of the muscles are called fixators since they improve the stability of the joint such as relative translations of bones when other muscles drive the joint. The recruitment coordination of these four muscle functional roles when performing an action is termed muscle synergy or the muscle activation pat-tern [81]. Muscle synergy may be altered in individuals with different MS disorders despite the comparable body kinematics [39–45, 82].
2.2 MICROSCALE COMPOSITION AND STRUCTURE OF THE KNEE