Joint Classification & Synovial Joints

Joint Classifications

• Fibrous joints allow for the least movement and are held together by ligaments, for example, cranial sutures.
• Cartilaginous joints allow for some movement and are held together by fibrocartilage, for example, the pubic symphysis.
• Synovial joints allow for the most movement.

Synovial Joints

• These joints are free moving and located in most limbs.
• The amount and direction of movement is decided by the structure of the joint.
• Key features are a complex association of tissues and structures.
• They help in the facilitation of free movement, as well as controlling movement.
• Bone ends determine the range of motion at a joint.
• Bony Congruence which can be seen when comparing the hip vs knee.

Synovial Joint Strucutre

• Key parts include the bone ends and articular cartilage.
• Contains a joint capusle and cavity.
• The joint cavity contains potential space.
• The synovial membrane secretes fluid filling the cavity.
• Ligaments are bands of DFCT inside/outside of the joint
• Hyaline (articular) cartilage covers bone ends articulate.
• Subchondral bone.
• Hyaline (articular) cartilage facilitates smooth, frictionless movement.
• Joint Capsule features an outerlayer of DFCT
• The inner layer of the synovial membrane secretes synovial fluid, enabling lubrication of the joint and providing nutrients to avascular structures.
• Joint capsules hold bones together.
• They are tight and thick where more support is required, and loose where movement is allowed.

Capsular and Intracapsular Ligaments

• In some joints, the capsule has thickenings where added support is needed
• Collateral ligaments of the knee are an example
• Medial collateral ligaments (MCL) connect the femur to thetibia.
• They restrict abduction.
• Lateral collateral ligaments (LCL) connect the femur to the fibula.
• They restrict adduction.
• In some joints, there are additional bands of DFCT located inside the capusle that hold bons together.
• Cruciate ligaments of the knee.
• Anterior cruciate ligaments (ACL) go from the front of the tibia to the back of the femur
• Limits posterior displacement of femur.
• Posterior cruciate ligaments (PCL)
• Goes from the back of the tibia to the front of the femure
• Restricts anerterior dispalcement of femur.

Fibrocartilaginous Pads

• Some joints also contain small structures made of fibrocartilage.
• These structures fill space, provide cushioning for shock absorption and/or deepen articulations.
• Menisci (pl.)/meniscus (sing.) in the knee are half-moon structures in the joint between the femur and tibia.

Stability vs Mobility

• There is a tradeoff between stability and mobility.
• Quiz:
• Hyaline cartilage, synovial fluid, and smooth bone ends ensures smooth, frictionless movement.
• Ligaments and joint capsules hold bones together. -The shape of bone ends (bony congruence) and ligaments influence movements at a synovial joint.

Joint Movement

• Range of movement/motion (ROM) determines the amount and type of movement.
• Key determining factors:
• Bone end shape.
• Ligament (location and length..
• Body surface contact.
• Muscles.
• Movements happen within a plane.
• These have an axis like uniaxial (one axis), biaxial (two axes) and multiaxial (many axes).

Types of Synovial Joints

• Plane, hinge, pivot, condylar*, ellipsoid*, saddle, and ball & socket
• Condylar and Ellipsoid different in lectures compared to textbook
• Plane joints are multiaxial, allow for ‘sliding’ and ‘gliding’ movements, and have flat articular surfaces (e.g., intercarpal and intertarsal joints).
• Hinge joints are uniaxial and allow for flexion and extension (e.g., ankle, elbow, interphalangeal joints).
• Pivot joints are uniaxial and allow for rotation (e.g., radioulnar joints, C1-C2 vertebrae).
• Condylar joints have biaxial, allow for Flexion & extension, Rotation (when flexed), e.g. knee, temporomandibular joint (TMJ)
• Ellipsoid joints are biaxial, allows for Flexion & extension, Abduction & adduction and circumduction with no rotation, e.g. wrist joint (radiocarpal joint)
• Saddle joints are biaxial (+), and allow for Flexion & extension, Abduction & adduction, circumduction and rotation, which results in opposition (e.g., carpometacarpal joint at the base of the thumb).
• Ball & Socket Joints are multiaxial, and allow for Flexion & extension, Abduction & adduction, circumduction and rotation (e.g., shoulder & hip).

Tips for remembering joints:

• Move around the joints in your upper and lower limbs,
• Think what type of joint is each one, what movements can be performable at the joint and determine the terminology
• Know which planes the movements occur in, based on which axis?

Lecture summary

• The amount of movement defines the three classes of joint.
• Some synovial joints have common structures and added features.
• Synovial joints move a lot, there are 7 types
• The amount and direction of movement is determined by joint structure based on bone end shape, capsule & ligaments, and body surface contact.