Lecture 22

Question 1

Why study biomechanics?

Answer 1

Help determine surgical interventions eg. Where to detach and reattach.
Design rehabilitation procedures eg. Which muscles to target.
Develop computer models.

Question 2

What is a movement arm?

Answer 2

The perpendicular distance between the joint’s centre of rotation and the line of action of the muscle or line of action of force.
Every joint has a centre of rotation however it may change position during movement.
The torque capacity of a muscles (eg. In elevation, rotation etc) is determined by the movement arm.
Increase the leverage = more mechanical advantage.
The further away the fulcrum = the more leverage.

Question 3

What is the line of action?

Answer 3

Direction of the force created when a muscle shortens.
Vector in 3D space.
The stabilising or destabilising potential of a muscle is determined by it’s line of action.
Vector sum of each line = resultant force.
Rotator cuff muscles are horizontally orientated & have a compressive effect to stop the shoulder from dislocating.
If the resultant force vector of the shoulder becomes more vertically orientated shoulder dislocation or subacromial impingement can occur.
Generally coincides with the orientation of the muscle fibres.

Question 4

What were the specific aims of the shoulder movement arm measurement study?

Answer 4

To measure the movement arms of 18 muscle sub regions spanning the shoulder joint throughout abduction and flexion.
To measure the lines of action of 18 muscle sub regions spanning the shoulder joint throughout abduction and flexion.
With this you can determine if a muscle has a lever arm for elevation, depression or rotation etc. And whether it’s a stabilising muscle or a destabilising muscle.

Question 5

What technique was used in the shoulder movement arm measurement study?

Answer 5

The shoulder joint was passively abducted and flexed and the muscle line of action were visualised and digitised. 
The movement arms of the shoulder musculature were measured using the tendon excursion technique with a dynamic shoulder cadaver testing apparatus (DSCTA). 
Movement arm (lever arm) = gradient of curve (tendon excursion/joint angle).

Question 6

What age range of people are rotator cuff tears seen in and why?
What does it cause?
How is it fixed?

Answer 6

Rotator cuff tear: age related degeneration.
Rotator cuff tears are usually seen in elderly people.
If you don’t use it, you lose it.
Painful – person will keep the joint immobile.
Epidemiology: 50% of people in their seventh decade have some variety of a full-thickness rotator cuff tear.
People with a posterior superior rotator cuff tear have no way of elevating their arm without dislocating it.
Causes osteoarthritis (cartilage has eroded away) = debilitating.

Joint replacement surgery? But no cuff muscles = joint will still dislocate.
Reverse total shoulder arthroplasty:
More stable than anatomical shoulder ball and socket.
Semi constrained joint.
Well connected, tends not to dislocate.

Question 7

What were the aims of the shoulder replacement study?

Answer 7

To compare moment arms, muscle forces, joint forces (bone-to-bone contact forces) and joint translations between:

1. Normal shoulder
2. Rotator cuff deficient shoulder
3. Reconstructed shoulder

Question 8

What were the key findings of the shoulder replacement study?

Answer 8

Surgically significantly increased the moment arm of the deltoid
- Increased leverage and decreased force.

Glenohumeral joint translation was significantly reduced and the joint stabilised post surgery

• Reduced joint dislocation
• Improved joint stability and function.

Surgery significantly increased superior joint shear

• Increased component failure die to large superior forces.
• Superior shear force – deltoid is more superiorly inclinced than usual.
• Potential for superior dislocation.
• Must check patients suitability before surgery.