The Biomechanics of Lameness Flashcards
What does stress take into account?
Force and cross-sectional area
How is a safety factor calculated?
Maximum stress a structure withstands until breakage/stress most likely to undergo during life time
What are the factors of failure?
Magnitude, frequency, speed of loading and duration of loading
What are the influences on the factors of failure?
Body mass (higher body mass higher force)
Speed (higher force)
Gait (length of time leg is on the ground not the number of limbs)
What forces do bones experience?
Tension, compression and bending
What forces are tendons exposed to?
Tension
What forces are joints exposed to?
Compression and shear forces
Why is failure of a structure in a live animal more complex than a man made structure?
Due to internal repair mechanisms and compensatory mechanisms
How has the horse adapted to be more efficient at locomotion?
Reduced muscle mass, reduces swing time, move at a greater speed with less force as foot spends more time in contact with ground
Leg is straight so decreased joint forces/moments
What strains do the tendons in the horses leg experience?
SDFT and SL are both high strain tendons and experience a similar force but the SDFT experiences highest stress
DDFT and DAL are both low strain tendons
What is strain?
% length change
What are the characteristics of muscles involved with flexor tendons?
Highly pennate
Stretch about 1cm under pressure so have a limited capacity for length change
How much energy is released as heat in tendons and what are the consequences of this?
7% of energy is released as heat
Results in temperatures of up to 45C at gallop with has been proved as non-lethal to tendon cells but could cause a predilection to core tendon injuries
What bone characteristics make locomotion more efficient?
Reduced by fusion of the radius and ulna
Lengthening of the limb to reduce mass but maintain stride length
One large bone has higher bending strength
What is the function of joints?
For movement and shock absorption
How has the fact that a horses leg only moves in the sagittal plane helped with efficiency?
The interlocking configuration only requires collateral ligaments and no muscles decreasing weight and metabolic cost
What are the functions of different parts of the hoof?
Horn capsule = protection (limits expansion)
Distal phalanx, frog and lamellae = shock absorption
Hoof wall and sole = grip and support
What are the protective mechanisms of the hoof?
Shape of the solar surface allowing heel movement, suspension of the distal phalanx transfers forces to distal border of hoof wall, digital cushion provides shock absorption, hoof sliding reduces impact and ground reaction force, rotation and translation of DIP joint
What are the different phases of locomotion?
Stance phase (impact, loading and propulsion) Swing phase
What is the duty factor?
Ratio of stance and stride time
How does speed affect forces in the limb?
Speed decreases stance time so force is generated in less time increasing energetic cost and peak limb fore is also increased which in turn increases the risk of injury
What occurs in first impact? What injuries are most likely during first impact?
Large accelerations and low forces so bruising to soft tissues and vibrations in bones
What occurs in second impact?
Low accelerations and high forces which is important for natural breaking action
What occurs in loading?
Large vertical force with excessive dorsiflexion of the fetlock causing tendon stress with force acting parallel to long bones with very tight safety margins
What stress occurs in the propulsion phase?
Tensile stress
How does high speed exercise influence strain?
Increases tendon strain but reduces fatigue life and is a potent stimulus for changes in structural properties
What is the PZM?
Point of zero moment
Force point of the foot where it hits the ground and is in the middle of the foot
What influences the PZM?
Toe length or foot angle
What happens if the PZM is in the wrong place?
Uneven loading or loading of structures with a low safety factor increasing the chance of injury
What do heel wedges do the PZM? What is the purpose of heel wedges? What are the disadvantages of heel wedges?
Move the PZM towards the heels unloading the DDFT and NB giving longer support through stance
Increases DIP joint pressure and shifts articular contact dorsally
