Musculoskeletal System Lecture 30

Question 1

What is zonation?

Answer 1

Zonation refers to the different layers or zones within the articular cartilage.

Question 2

Zonation in articular cartilage

Answer 2

As you move from the surface zone down to the deep zone, there is a gradual increase in proteoglycan (PG) content.
Proteoglycans and glycosaminoglycans (GAGs) are crucial for creating swelling forces that help maintain the cartilage’s structure.
The deep zone, rich in proteoglycans, experiences the greatest swelling force.
Collagen helps resist the swelling force by tethering different zones together, preventing excessive expansion of the cartilage.

Question 3

Collagen Organization and Shear Forces

Answer 3

Surface Zone:
The surface of the cartilage contains fine, densely packed collagen fibers, oriented to resist shear forces. This orientation makes the surface smooth and strong.
These fibers are parallel to the surface, which helps protect against the wear and tear of joint movement.
Middle Zone:
In the middle zone, collagen fibers are thicker and less densely packed, arranged at 45-degree angles.
These fibers attach to the surface zone and extend down to the deeper layers of cartilage.
Deep Zone:
In the deep zone, collagen fibers are perpendicular to the surface and extend down to the subchondral bone.
This fiber arrangement tethers the surface to the subchondral bone, preventing the cartilage from separating or pulling away when it swells with water.

Question 4

Collagen and Cartilage Strength

Answer 4

Collagen makes up 75% of the dry weight of articular cartilage, making it a dominant structural component.
The collagen network helps resist the swelling forces from the high water content in the cartilage, holding it firmly in place.
As cartilage absorbs water, the surface wants to move away from the subchondral bone, but the collagen fibers anchor it securely to the underlying bone.

Question 5

Age-Related Changes in Cartilage

Answer 5

As we age, the functional zones of cartilage thin significantly:
In children, these zones might be 5 mm thick, but by the age of 60-65, they can reduce to 2 mm.
This thinning occurs because the ability of chondrocytes (cartilage cells) to repair and maintain the tissue diminishes with age.
Chondrocytes in younger cartilage are more active in repairing tissue, but this function declines over time, leading to thinner, less resilient cartilage.

Question 6

Unique Challenges of Cartilage

Answer 6

vascularity: Cartilage contains no blood vessels. This is a significant challenge because most tissues rely on a blood supply for nutrients.
Cartilage is white in appearance due to its poor blood supply, as seen in specimens like the cow’s knee.
Tissues under long-term load, like cartilage, cannot afford to have blood vessels, as the compression would block circulation, depriving the tissue of nutrients.
No Nerve Supply: Cartilage is aneural, meaning it does not contain nerves.
This is beneficial because you wouldn’t want to feel pain every time your joints move.
No Lymphatics: Cartilage lacks lymphatic vessels, which is less critical but still part of its structure.
Diffusion: Chondrocytes rely solely on diffusion to receive nutrients and oxygen.
Most tissues are close to a blood source and can rely on diffusion over short distances, but the cartilage is far from blood sources, making diffusion less efficient.
The nearest blood source is located in the articular capsule, making nutrient diffusion to the chondrocytes challenging.

Question 7

Glycosaminoglycans (GAGs)

Answer 7

Often abbreviated as GAGs.
Made from monosaccharides, typically six-carbon rings like glucose or fructose.
A monosaccharide is a basic sugar (e.g., glucose or fructose).
Disaccharides are formed by linking two monosaccharides, which serve as the basic structural units of GAGs.
The disaccharides in GAGs often contain a carboxyl or sulphate group, which can give up protons in solution, making them negatively charged.
These negative charges are crucial for the function of cartilage, as they repel each other, creating resistance during compression.
Repeating disaccharide units form long-chain GAGs.
Examples of GAGs include chondroitin sulphate and keratin sulphate, both commonly found in cartilage.
Chondroitin sulphate can have up to 50 repeating disaccharide units, forming very long chains.

Question 8

Proteoglycans (PGs)

Answer 8

Formed when GAGs are attached to a protein core.
The formula is: GAGs + protein core = PG.
A common proteoglycan found in cartilage is aggrecan.
Aggrecan is made by attaching about 125 chondroitin sulphate chains and 50 keratin sulphate chains to a protein core.
PGs have a “bottle brush” structure, where each “bristle” is a GAG chain with negative charges along its length.
The negative charges on the GAG chains repel each other, giving proteoglycans a spring-like function.
This allows proteoglycans to resist compression, a critical property for tissues like cartilage that experience constant mechanical stress. When compressed, the negative charges push back against each other, and when released, the PGs spring back to their original shape.

Question 9

Hyaluronic Acid (HA)

Answer 9

Also known as hyaluronin, the salt form commonly found in the body.
Hyaluronic acid is a special type of GAG, much longer than others, with over 25,000 repeating disaccharide units.
PGs like aggrecan can attach to the hyaluronic acid backbone. Up to 200 PGs can attach to a single hyaluronic acid chain, forming a large proteoglycan complex.
This complex structure, with multiple PGs attached to a single HA chain, creates a massive molecule.
The GAGs in this complex are negatively charged, giving it a high capacity for attracting water.

Question 10

The loading cycle of articular cartilage: Cartilage Structure

Answer 10

Cartilage has three main zones: surface zone, middle zone, and deep zone (not showing calcified zone or subchondral bone).
Collagen fibers:
- Surface zone contains a felt-like layer of collagen.
- Middle and deep zones have arcading bundles of collagen that anchor the surface zone to the underlying bone.
Proteoglycan (PG) complexes:
- Scattered in the cartilage matrix, especially concentrated in the middle and deep zones.
- The combination of collagen fibers and PG complexes forms the fixed solid component of cartilage.

Question 11

The loading cycle of articular cartilage: Unloaded Cartilage

Answer 11

• When cartilage is unloaded (no compressive force), the negative charges on the GAGs in the middle and deep zones attract positive ions (e.g., calcium, potassium, sodium) from the synovial fluid.
• The ion concentration in the matrix increases, setting up an osmotic gradient.
• This gradient draws water into the cartilage, causing it to swell.

Question 12

The loading cycle of articular cartilage:
Swelling and Tension

Answer 12

• As water enters, the cartilage swells, putting tension on the collagen fibers.
• Eventually, the tensional forces from the collagen fibers equal the swelling forces, and the cartilage reaches a state called unloaded equilibrium.
• If the collagen in the deeper zones were cut, the cartilage would keep swelling. In its intact state, the cartilage is in a pre-stressed state, which helps it resist compressive forces.

Question 13

The loading cycle of articular cartilage: Introducing Load

Answer 13

• When a load (compressive force) is applied, fluid is initially prevented from escaping by the felt-like surface layer.
• Fluid compression acts like a hydraulic blanket, protecting the solid component from immediate compression.
• If the load persists, fluid is gradually squeezed out of the cartilage and back into the joint space or non-compressed areas of the cartilage.

Question 14

The loading cycle of articular cartilage: Creep and Loaded Equilibrium

Answer 14

• As fluid is forced out, the cartilage volume decreases, a process known as creep.
• The solid components (collagen and PG complexes) move closer together, pushing the negative charges on the GAGs closer. These charges repel each other, creating resistance to further compression.
  -Eventually, the swelling force = tensional forces, reaching loaded equilibrium.

Question 15

The loading cycle of articular cartilage: Nutrient and Waste Exchange

Answer 15

• During unloading, as water enters, it brings in dissolved oxygen and nutrients for the chondrocytes (cartilage cells) since cartilage lacks nearby blood vessels.
• During loading, when fluid is forced out, CO₂ and waste products are expelled, helping to flush the cartilage.

Question 16

The loading cycle of articular cartilage: Exercise and Cartilage Health

Answer 16

Gentle weight-bearing activities like walking, swimming, and cycling are important because they promote fluid exchange in the cartilage.
Regular loading and unloading help maintain cartilage thickness by ensuring chondrocytes have the necessary nutrients for repair and maintenance.

Question 17

Overview of the loading cycle

Answer 17

Question 18

Arthritic Finger vs. Normal Finger

Answer 18

Arthritic finger shows significant degradation compared to a child’s finger, which is healthy and unaffected by arthritis.

Question 19

What is arthritis?

Answer 19

Inflammation of the joints, leading to pain, swelling, stiffness, and reduced range of motion.

Question 20

Arthritis and Bone Degradation

Answer 20

Osteoporosis:
The spongy bone in an arthritic finger shows thin trabeculae, making the bone look more porous. This is a classic sign of osteoporosis, where bone density decreases, leading to a weaker bone structure.
In comparison, a child’s finger has thicker, healthier trabeculae.

Cartilage Degradation:
Normal cartilage can still be seen in some areas, protecting the bone.
Degraded cartilage: In arthritic joints, some cartilage attempts to regrow but is often less dense and lighter in color, suggesting improper or incomplete regrowth.
Complete cartilage erosion: In some areas, cartilage is entirely worn away, exposing the subchondral bone. This leads to direct bone-on-bone contact, which causes severe pain due to the rich nerve supply in bones.

Inflammatory Response:
Bone-on-bone grinding triggers an inflammatory response, resulting in pain, swelling, and edema (excess fluid in the tissue), worsening the symptoms of arthritis.

Osteophytes:
Arthritis often leads to the development of osteophytes, which are bony growths or spurs. These form as the body tries to repair and compensate for the damaged bone, but they appear in places where they should not grow, further contributing to joint dysfunction.
Unwanted Bone Growth:
In advanced cases, bone growth can even occur within the articular capsule, the protective tissue surrounding the joint. These unwanted bones, sometimes referred to as sesamoid bones, develop due to pressure and joint damage caused by arthritis.

Question 21

Hip Joint and Hip Replacement: Normal Hip Joint

Answer 21

Normal Hip Joint:
The ball of the femur forms half of the hip joint. The tissue shown is from a donor cadaver and has been embalmed, which gives it a yellowish tinge.
Articular cartilage is smooth and intact, protecting the bone and preventing exposure of the subchondral bone.
The cartilage may not be fully hydrated due to embalming, but the key observation is that it still provides protection to the bone.

Question 22

Hip Joint and Hip Replacement: Osteoarthritis in the Hip Joint

Answer 22

In a hip joint affected by osteoarthritis, the cartilage has worn away, exposing the subchondral bone.
The exposed bone surface is rough, like sandpaper, and can grind against the cartilage on the opposite side, further accelerating joint damage.
Osteoarthritis causes pain because bone-on-bone contact triggers an inflammatory response due to the nerve supply in bones.

Question 23

Hip Joint and Hip Replacement: Challenges of Cartilage Repair

Answer 23

Cartilage is difficult to repair due to its unique structure, which consists of collagen fibers and proteoglycans.
Research is ongoing, but currently, replacing the joint is the most effective option, especially if arthritis significantly impacts the patient’s quality of life.

Question 24

Hip Joint and Hip Replacement: Joint Replacement (Arthroplasty)

Answer 24

The most common joints replaced are the hip and knee due to their high load-bearing roles in the body, especially in the elderly.
In a total hip replacement (hip arthroplasty), both the acetabulum (hip socket) and the head of the femur are replaced with prosthetic components.

Question 25

Hip Joint and Hip Replacement: Hip Replacement Procedure

Answer 25

The surgeon uses a cheese grater-like tool to remove the damaged cartilage and expose the bloody bone, which is necessary for attaching the prosthetic components.
The head of the femur is removed, and the medullary cavity of the femur is bored out to insert a large spike that will help distribute the load across the bone. This is crucial to prevent bone fractures, especially in elderly patients with weak bones.
A prosthetic acetabular component (hip socket) is placed in the pelvis. These components are typically made from cobalt chromium, surgical steel, or titanium.
The back of the acetabular component is porous, allowing the bone’s trabeculae to grow into it, securing the prosthesis. Holes may also be present for screws to temporarily hold it in place.
A polyethylene liner (a smooth, slippery plastic) is placed inside the cup to act as a lubricating surface for the artificial joint.
The femoral component includes a spike inserted into the medullary cavity to distribute the load, and the head of the femur can be adjusted in size based on the patient’s anatomy.

Question 26

Hip Joint and Hip Replacement:

Answer 26

Acetabular Component:
The acetabular component of a hip replacement is made from cobalt chromium or surgical steel. Nowadays, most replacements are made from titanium, which is stronger and lighter.
The backside of the acetabular component is often porous or sintered, which allows the bone’s trabeculae to grow into it and secure the prosthesis, ensuring it stays in place.
Some acetabular components have holes for screws, which can be used to temporarily hold the prosthesis in place during healing.
In cases where the backside is smooth, the surgeon may use artificial bone cement to glue the prosthesis to the bone.

Polyethylene Liner:
The cup of the acetabular component is lined with polyethylene, a slippery, greasy plastic that serves as the lubricating surface of the artificial joint.
The liner allows the prosthetic joint to function smoothly by reducing friction between the ball and socket.

Femoral Component:
The femoral component includes a giant spike that is inserted into the medullary space of the femur to distribute the load across the bone. This is essential to prevent fractures, especially in the elderly with weaker bones.
The size of the femoral head can be adjusted to fit the patient’s anatomy—some may need a larger head, while others might require a smaller one.
Once assembled, the hip joint functions as a healthy, artificial joint.

Longevity of Joint Replacements:
A hip replacement typically lasts around 15 years before needing a replacement due to wear and tear.
Joint replacements are now common and provide an effective solution to arthritis, particularly in patients whose quality of life is significantly affected by the disease.

Purpose of Joint Replacements:
Hip replacements, along with knee replacements, are among the most common joint surgeries. They allow patients to regain mobility and reduce pain caused by severe arthritis.
This type of surgery is the primary approach to treating advanced arthritis when it has a profound impact on daily activities and pain management.

Question 27

What are synovial joints?

Answer 27

Synovial joints are a type of joint in the human body where two bones are connected by a fluid-filled cavity, allowing for smooth and free movement. These joints are the most common and most movable type of joint in the body. The key features of synovial joints include:

Articular Cartilage: This smooth, slippery cartilage covers the ends of the bones in the joint, reducing friction and allowing for easy movement.

Joint Cavity: A small space between the bones filled with synovial fluid, which lubricates the joint, nourishes the cartilage, and reduces friction.

Articular (Joint) Capsule: This capsule surrounds the joint, providing stability and enclosing the joint cavity. The capsule has two layers:

Fibrous Layer: The outer, tough layer made of dense connective tissue that helps stabilize the joint.

Synovial Membrane: The inner layer that produces synovial fluid, keeping the joint lubricated.

Synovial Fluid: A viscous, egg-white-like fluid that fills the joint cavity, reducing friction and providing nutrients to the articular cartilage.

Ligaments: Strong bands of dense connective tissue that connect bones and reinforce the joint, helping to control its range of motion.

Proprioceptors and Nerves: These monitor joint position and help control movements.

Question 28

Answer 28

Question 29

Layers of the Capsule

Answer 29

Fibrous layer (outer layer):
Dense connective tissue, bundles of collagen resist tension.
Can be irregular (collagen fibers oriented in multiple planes) or regular (fibers in one plane).
Thicker areas are called capsular ligaments.
Blends with the periosteum of bones, sometimes inserting as Sharpey’s fibers.
Contains fibroblasts to maintain collagen and nerves (for pain detection and proprioreception).
Some blood vessels present, but tissue is avascular, making healing slow.

Synovial membrane (inner layer):
Loose connective tissue lining non-articular surfaces.
Can vary in thickness, often with villi to increase surface area.
Two layers: Intima (close to synovial cavity) and Sub-intima (outside intima).
Contains fibroblasts, macrophages, blood vessels (leak fluid to form synovial fluid), and fat cells (act as cushions).

Synovial Fluid:
An ultrafiltrate of blood plasma with secretions from synoviocytes (secrete hyaluronic acid and lubricating proteins).
Helps lubricate joints and nourish chondrocytes.
Balances fluid levels to avoid separating cartilage too much or drying the joint.

Question 30

Articular Capsule:

Answer 30

Encloses synovial joints, forming a sleeve connecting bones.
Synovial joints are surrounded by this capsule.
Connects two bones with fibrous tissue, also called ligaments (dense regular connective tissue connecting bone to bone).
Tendons connect muscle to bone.
Capsule does not limit movement except at extreme ranges.
Can be loose in more mobile joints.

Question 31

Answer 31

Question 32

Joint cavity

Answer 32

The term joint cavity is misleading as it implies a large space within the joint. In reality the joint cavity is the small area between the articulating surfaces, while the peripheral margins of the
joint cavity are filled by the collapsing and in-folding of the synovial membrane (villi). This potential space contains a small amount of fluid called synovial fluid. The amount of synovial fluid inside a healthy joint cavity rarely exceeds 2ml, even in large human joints such as the knee.

Question 33

Synovial fluid

Answer 33

Synovial fluid is a clear or slightly yellowish fluid that is an ultrafiltrate of blood plasma that leaks out of the blood vessels in the synovial membrane (subintima) into the joint space.
Other components, not found in the blood filtrate, are secreted by the synoviocytes. One such component is Hyaluronic acid. Other lubricating proteins are also secreted. Free cells are also found in low concentrations within synovial fluid. These cells tend to be
monocytes, lymphocytes, macrophages and synoviocytes.
The function of synovial fluid includes joint lubrication, shock absorption, chondrocyte metabolism and overall joint maintenance.

Question 34

Functions of Muscle (Focus on Skeletal Muscle)

Answer 34

Energy Conversion: Muscles convert chemical energy (ATP) into mechanical energy. This is the fundamental function of muscle tissue.
Movement: Skeletal muscle pulls on bones to generate movement at joints. Other muscles, like smooth muscle, facilitate other forms of movement (e.g., moving gut contents or pushing blood through the heart).
Stability: Muscle stabilizes joints by maintaining posture and preventing unnecessary movement. Muscle use for stability requires energy (ATP), unlike passive stabilizers like ligaments and bone structure.
Communication: Muscle plays a role in subconscious communication (e.g., facial expressions, body language) and conscious actions (e.g., writing, speaking). Intercostal muscles and the diaphragm help in speech production.
Sphincter Control: Rings of muscle called sphincters control how things enter and exit the body. Skeletal muscle sphincters (e.g., around the mouth, bladder) are consciously controlled, while smooth muscle sphincters (e.g., pyloric sphincter) are controlled by the autonomic nervous system.
Heat Production: Muscle contributes significantly to heat production, making humans warm-blooded. This heat is largely a by-product of ATP consumption during muscle activity. Shivering is an involuntary process where skeletal muscle contractions increase body heat production by up to four times in cold conditions.

Question 35

Skeletal Muscle and Heat Regulation

Answer 35

Skeletal muscles play a major role in regulating body temperature. The body increases muscle activity (e.g., shivering) when it gets cold to generate heat.