Anatomy & Physiology of Joints

1. Synovial Joint

Synovial joint or diarthrosis is the most common and movable joint in the body of a mammal. Such joints help to achieve rotational movements at the point of contact of articulating bones. The various structures that make up the synovial joints are:

Synovial cavity: represents the space between the two articulating bones and is filled with a fluid called the synovial fluid. The fluid acts as a lubricant and reduces the friction between two articulating bones.
Joint Capsule: It is a fibrous covering which is continuous with the periosteum of the articulating bones and surrounds the synovial joint uniting the two articulating bones. Te capsule comprises of outer layer and the inner synovial membrane. The outer layer contains ligaments while the inner synovial membrane secretes the synovial fluid. The joint capsule is innervated by nerves, but lacks blood supply. The ligaments provide stability of the knee or other parts during rotational movements.
Articular cartilage: The bones of the synovial joint are covered by hyaline cartilage which also absorbs shock on the bones during jumping or rotational movements.

Articular Discs: These are fibro-cartilage cushions situated between opposite surfaces in a joint. Sometimes these pads are made of adipose tissue, which buffers the articular cartilage.
Tendons: Tissues that joins muscle to bones and are made of collagen.
Accessory ligaments: made of fibrous tissue which resists strains in extreme movements (Rogers, 2010).

The parts of a typical synovial joint are represented in Figure 1.

Figure 1: Labelled parts of a typical synovial joint
Q2. Images of Ligaments

Fig 2: Radiographic view of Anterior cruciate ligament

Fig 3: Schematic view of Anterior Cruciate, Medial Collateral and posterior cruciate ligaments (Rogers, 2010).

Fig 4 : Shows the orientation of ACL and PCL in relation to each other and other ligaments

Fig 5 : represents the medial collateral ligament

Fig 6: represents fibular collateral ligament

3. The ligaments of the knee are the collateral ligaments and cruciate ligaments that are found around the joints of the knee and inside the joint respectively. These ligaments are fibrous bands that provide stability to the knees by holding the articulating bones together. The collateral ligaments like the medial collateral ligaments (MCL) and lateral collateral ligaments (LCL) are responsible for providing sideward stability of the knee by holding the femur and tibia together. The MCL provides stability to the knee by resisting forces from outside. On the other hand LCL resists forces from inner side of the knees and provides its stability. The cruciate ligaments as depicted above are the anterior and posterior cruciate ligaments. They are present in the middle of the joint in a criss-cross fashion. These ligaments are associated with forward and backward motion of the knees and are also responsible for proprioception (sense of touch). The ACL attaches to the front of tibia and back of femur. It stabilizes tibia from sliding too far forward in relation to femur in case of stretching. The PCL attaches to the back of tibia and front of femur. It stabilizes tibia from sliding too far backward in relation to femur in case of sudden forces (Rogers, 2010).

4. Differences between Knee Joint and Ankle Joint

Features Knee Joint Ankle Joint
Type of joint Hinge joint Hinge joint
Range of Movements Forward and backward Up and down movements which are termed dorsiflexion and plantarflexion.
Degree and extent of movements Less than 100 degrees More than 100 degrees
Type of ligaments presents Anterior and Posterior cruciate ligaments, Medial and Lateral collateral ligaments Anterior and Posterior talofibular ligaments, Calcaneofibular ligaments, Deltoid ligaments (Rogers, 2010)

5. The most common ankle injuries are sprains and fractures. Sprain represents an injury to ligaments of the ankle while a fracture represents a broken bone in the ankle. Such injuries occur during twist and turning of the ankles with sudden and awkward forces (Rogers, 2010).

References
Rogers, K. (2010). Bone and Muscle: Structure, Force, and Motion