Kinematic Chains & Arthrokinematics

• Kinematic chain comprises articulated, segmented links such as the pelvis, thigh, leg, and foot in the lower extremity.

Open vs Closed Kinematic Chain

• Open kinematic chain: the distal segment moves freely (e.g., raising your hands up).
• Closed kinematic chain: the proximal segment moves freely (e.g., standing from sitting).

Fundamental Arthrokinematics

• Roll: Multiple points along one rotating articular surface contact multiple points on another articular surface.
• Slide: A single point on one articular surface contacts multiple points on another articular surface.
• Spin: A single point on one articular surface rotates on a single point on another articular surface.

Analogy for Roll, Slide, and Spin

• Roll: Like a ball rolling on a smooth surface.
• Slide: Like a tire gliding.
• Spin: Like a spinning toy top on one spot.

Convex-Concave Rule

• For convex on concave surfaces, the convex member rolls and slides in opposite directions.
• For concave on convex surfaces, the concave member rolls and slides in similar directions.
• The convex concave rule on the humeral head affects sliding as the arm abducts.

Combination of Movements

• Flexion and extension of the knee are examples of combined roll, slide, and spin arthrokinematics.

Stance

• When standing from a squat, the femur rolls forward (anterior) and slides backward (posterior) on the tibia.
• Internal rotation, or medial spin, occurs as the knee reaches full extension on the stationary tibia.
• The obligatory spinning rotation depends on the shapes of the articular surfaces.
• Conjunct rotation helps securely lock the knee joint.

Closed Kinematics

• During a squat (knee flexion), the femur and tibia move according to roll, glide, and spin directions.

Roll and Slide in Joints

• Sliding increases with more congruent joint surfaces.
• Rolling increases with more incongruent joint surfaces.
• The direction of rolling is always the same as the moving bone, regardless of the surface shape (convex or concave).
• Congruent refers to surfaces that match or fit together well.

Joint Classifications

• Structural: Fibrous, cartilaginous, and synovial joints.
• Functional:
  • Synarthrosis (immovable, little movement), such as sutures of the cranial bones.
  • Amphiarthrosis (slight movement), such as the symphysis pubis.
  • Diarthrosis (freely movable), also known as synovial joints.
• Biomechanical: Simple, compound, and complex joints.

Synovial Joints

• Synovial joints can be classified into four categories based on the type of axis.
• The components that make up a synovial joint.

Uniaxial Joints

• Uniaxial joints facilitate motion around one axis within one plane.
• Hinge joints (e.g., elbow/humeroulnar, ankle/talocrural) permit one plane of movement, like a door hinge.
• Pivot joints (e.g., humeroradial and atlanto-axial in the craniocervical region) allow rotational movement around an axis, analogous to a doorknob.

Biaxial Joints

• Biaxial joints allow motion to occur about two axes, and motion occurs within two planes.
• Condyloid: One bone has a shallow concave cup shape, and the other bone is spherical convex; allows 2 planes of motion (flexion-extension and abduction-adduction/flexion-extension and axial rotation).
  • e.g. Metacarpophalangeal joint and knee joint (tibiofemoral).
• Ellipsoid: One bone is elongated concave, and the other is a (flattened) elongated convex ellipsoid; enables 2 planes of motion (flexion-extension and abduction-adduction).
  • e.g. Radiocarpal joint.
• Saddle: Similar in structure and function to sitting in a saddle; allows flexion-extension and abduction-adduction.
  • e.g. carpometacarpal of thumb, sternoclavicular.

Triaxial Joints

• Triaxial joints provide motion around three axes within three planes.
• The primary type of triaxial synovial joint is the ball-and-socket joint.
• One bone has a ball-like convex surface fitting into a concave socket.
• Examples include the hip and shoulder joints.

Nonaxial Joints

• Nonaxial joints allow motion within a plane, but not around an axis.
• A gliding movement occurs where one bone surface translates (glides) along another.
• The bones' surfaces are typically flat or slightly curved, and the joint is also known as a gliding joint.
• Examples include joints between carpal bones (intercarpal joints), facet joints of the spine, and carpometacarpal joints of digits 2-5.

Joint Positions: Closed- vs Loose-Packed

• Closed-packed position features maximum congruency, taut ligaments, and minimal accessory movements, providing natural stability.
• Loose-packed positions are all positions other than close-packed. The ligaments and capsules are relatively slackened, allowing increased accessory movements, and the joint has the least congruency near its midrange.
• The hip joint is an exception. Maximum stability occurs in full extension (closed-packed) due to taut ligaments. The articular surfaces aren't in optimal contact due to incongruency (femoral head larger than acetabulum).
• Full hip joint congruency (head lies completely in the acetabular cavity) occurs at 90º hip flexion, slight abduction, and slight lateral rotation (loose-packed), where ligaments are slack and there's less stability.

Understanding kinematic chains, including open and closed chains, is crucial in biomechanics. Arthrokinematics, involving roll, slide, and spin, dictates joint movement. The convex-concave rule further explains the relationship between joint surfaces during motion.