Lecture 7: Engineering the Musculoskeletal System

Question 1

What are the largest shoulder muscles?

Answer 1

the three most superficial shoulder muscles which are the deltoid, latissimus dorsi and pecs (prime movers)

Question 2

What is the role of the rotator cuff muscles?

Answer 2

prevent shoulder joint dislocation

Question 3

What is the role of force couples in the shoulder?

Answer 3

keeps the humerus pressed up against the shoulder joint at all times

Question 4

What are examples of force couples in the shoulder joint?

Answer 4

transverse plane: the subscapularis and infraspinatus force couple
scapular plane: the deltoid and the infraspinatus / subscapularis / teres minor force couple

Question 5

What is anatomic shoulder arthroplasty used to treat?

Answer 5

end-stage osteoarthritis, severe pain, reduced range of motion, and loss of muscle strength

Question 6

How many people will experience a rotator cuff tear?

Answer 6

50% of people in their seventh decade, and over 80% of people over the age of 80 will suffer a full-thickness rotator cuff tear

Question 7

What are the symptoms of a rotator cuff tear?

Answer 7

pain, shoulder instability, shoulder dislocation, loss of shoulder movement and inability to perform activities of daily living

Question 8

What is reverse total shoulder arthroplasty used in the treatment of?

Answer 8

rotator cuff tear arthropathy (when the glenohumeral joint is unstable), but also trauma and tumour resection

Question 9

How is muscle and joint function quantified after reverse total shoulder arthroplasty?

Answer 9

muscle moment arms, muscle lines of action, muscle forces and joint forces

Question 10

What is the muscle moment arm?

Answer 10

the perpendicular distance between a muscle’s line of action and the joint centre of rotation in which it spans -> muscle leverage

Question 11

Which muscle has the largest muscle moment arm in early elevation?

Answer 11

the supraspinatus

Question 12

Which muscle has the largest muscle moment arm in mid-late elevation?

Answer 12

the deltoid

Question 13

Which muscle has a large negative muscle moment arm? What makes it negative?

Answer 13

the latissimus dorsi
negative because it is involved in adduction as opposed to the other muscles which have positive muscle moment arms and are involved in abduction

Question 14

How does reverse total shoulder arthroplasty affect muscle moment arms?

Answer 14

significant increase in the mean abduction moment arm of anterior deltoid and middle deltoid

Question 15

What is the muscle line of action? What does the muscle line of action confer?

Answer 15

the unit vector direction of the force produced by a muscle

it confers muscle stabilising or destabilising potential

Question 16

How does reverse total shoulder arthroplasty affect muscle line of action?

Answer 16

the rotator cuff muscles are less inferiorly inclined and the deltoid muscles are more superiorly inclined

Question 17

How can muscle forces be estimated?

Answer 17

cannot be measured (non-invasively)

patient-specific musculoskeletal models can estimate muscle forces using physiological cost functions and constraints

Question 18

How are muscle force models derived and validated?

Answer 18

using experimental data e.g. cadaveric testing, instrumented implants etc.

Question 19

How does reverse total shoulder arthroplasty affect muscle forces in the deltoids and rotator cuff muscles and why? Where do muscle force peaks occur?

Answer 19

significantly smaller muscle forces (due to larger lever arms)
muscle force peaks occur at different joint positions

Question 20

What is the overall effect of RTSA?

Answer 20

⬆ in muscle moment arms reduces muscle forces (greater range of motion, improved joint function)
this ultimately ⬇ joint compression, which may lead to dislocation
more superior joint shear compression occurs

Question 21

What are the issues associated with TMJ prosthetics?

Answer 21

only two implant suppliers
three sizes fit all?
risk of nerve damage
poor fit in cases of complex anatomy

Question 22

What is the strategy for developing a TMJ prosthetic?

Answer 22

problem definition -> musculoskeletal model development -> design of prosthetic TMJ -> evaluate / iterate design using musculoskeletal model -> cadaveric testing -> 1st prosthesis recipient

Question 23

What is an important factor that must be determined before a prosthetic can be used in patients?

Answer 23

implant loading

Question 24

How is implant loading determined?

Answer 24

need to solve a mathematical problem: input is bite force, outputs are muscle forces and TMJ force
then, use calculated forces to drive finite element model simulation of implant loading

Question 25

How is bite force determined?

Answer 25

subject bites a rubber measuring device -> development of a subject-specific musculoskeletal model -> decomposing bite force into individual muscle forces -> musculotendon parameters scaled from a previous dataset

Question 26

Why is reverse shoulder arthroplasty performed?

Answer 26

to restore shoulder function in cases of rotator cuff tears and end-stage OA (i.e. rotator cuff tear arthropathy), improves joint stability but can present risk of superior subluxation

Question 27

In this lecture what does computational modelling and 3D printing facilitate?

Answer 27

personalised implant development for the jaw, with cost and practical benefits