Skeletal Diseases Study Guide


As weird as it is to think about, your skeleton is actually alive. And just like you can get an ear infection or catch a cold, your bones can get sick too. It’s usually a little bit more complicated than something you can treat with ibuprofen and time, but that doesn’t mean it can’t be treated (or at least managed).

There are a TON of different bone diseases out there. Here, we’ll cover six of the most common ones: osteoporosis, bone fractures, osteoarthritis, rheumatoid arthritis, Paget’s disease, and rickets. Let’s get started.

Lesson Objectives

  • Learn about 6 common skeletal system diseases
  • Understand their causes, symptoms, and risk factors


Your bones are constantly being broken down and replaced, but certain factors can sometimes make it hard for your body to keep up. When more bone is broken down than can be restored, your bone density decreases, causing them to become brittle and frail with a higher risk of fracture. This loss of bone mass is a disease aptly named Osteoporosis, which literally translates to “porous bone”. In the 3D model below, you can see the difference between a healthy bone and one that’s developed osteoporosis.

One of the worst parts of osteoporosis is that you usually can’t tell it’s happening until it’s too late. You might have back pain, a stooped posture, and a gradual loss of height over time, but most people don’t know they have it until they break a bone from something as mild as bending over or coughing (can you imagine breaking a bone just because you accidentally swallowed water the wrong way? Yikes).

The likelihood that you’ll develop osteoporosis depends a lot on how much bone you make when you’re growing up. Before you turn twenty, you’re building new bone faster than it gets broken down, which makes your bone mass increase and your skeleton get bigger. This growth peaks around around thirty–after that, your bone mass decreases faster than it’s created.

Your peak bone mass is the major determinant of whether or not you’ll develop osteoporosis. The higher the peak, the more you have to spare as bone loss occurs in the later stages of your life. Unfortunately, this is largely determined by factors that are out of your control, including sex, age, race, family history, and body frame size.

  • Sex: Females are more likely to develop osteoporosis than males, especially after menopause.

  • Age: The older you get, the more bone gets broken down, increasing your risk.

  • Race: People of white and Asian descent are more likely to develop osteoporosis than other ethnicities.

  • Family History: Osteoporosis is partly genetic, so you’re at greater risk if someone in your family has it.

  • Body Frame: Men and women who have smaller body frames tend to develop osteoporosis more than their larger counterparts because they already have less bone to begin with.

Fortunately there are some things you can do to help prevent it. If you stay active, curb your alcohol and tobacco use, eat enough food, and make sure to get enough calcium, your risk decreases.

Bone Fracture

Before we get to our next bone disease, let’s quickly touch on one of the things that osteoporosis can cause: Bone Fractures. When you break a bone, it’s called a bone fracture. It can be caused by a multitude of things, including excessive stress on the bone or an impact trauma (like a car crash or a sports injury). Osteoporosis alone is responsible for over 2,000,000 fractures every year (30% of the 6,300,000 breaks that happen annually in the US!).

Symptoms of a broken bone are pretty much what you’d imagine them to be. Other than the searing pain, symptoms can include difficulty using the limb, an unusual bump, bend or twist, and swelling.

There are five different types of fractures: closed or open fractures, complete fractures, partial fractures, displaced fractures, and stress fractures. What kind do you see in the 3D model below?

  • Closed or Open Fracture: A closed fracture is one that doesn’t break the skin. If it does, it’s called an open fracture.

  • Complete Fracture: A fracture that goes completely through the bone.

  • Partial Fracture: A fracture that does not go completely through the bone.

  • Displaced Fracture: A fracture with a gap between the break in the bone.

  • Stress Fracture: A crack in the bone, which is tough to detect without an x-ray.


Your joints are responsible for your ability to move. The rest of your musculoskeletal system helps, but without joints, you’d be stuck in the same position for the entirety of your life. Joints are defined as an area where two or more bones meet. To keep those bones from knocking together, most of them have a layer of connective tissue called cartilage between them that help the joint move smoothly without friction. When that cartilage starts to wear down, it causes a disease that we know as Arthritis. The most common forms of this disease are osteoarthritis and rheumatoid arthritis, both of which we’ll go over below.


Osteoarthritis is first on the leaderboard for the most common types of arthritis. It’s caused by the general wear and tear that your body goes through in your life, eroding the cartilage between your bones. It can affect any joint in your body, but the most common ones are your hands, knees, hips, and spine.

If you look at the 3D model below, you can see the difference between a healthy joint and one that’s developed osteoarthritis. The osteoarthritis bone has noticeable damage to its cartilage, exposing the bone to the cartilage below. Unfortunately, this damage can’t be reversed. Once you have it, you’re stuck with it.

Symptoms can be managed, but they tend to develop slowly and get worse over time. They might include pain, stiffness, tenderness, a loss of flexibility, bone spurs, swelling, or a grating sensation, all of which can make daily tasks both painful and difficult (we’re having a lot more empathy for our grandmas right now).

Some people are more at risk for osteoarthritis than others. Factors include age, sex, obesity, injuries, repeated stress, genetics, bone deformities, and certain metabolic diseases. Let’s talk about how these influence your risk below.

  • Age: Your risk of developing osteoarthritis increases with age.

  • Sex: Women are more likely to develop osteoarthritis than men (though we’re not totally sure why).

  • Obesity: Increased weight adds extra stress to your weight-bearing joints, like your hips and knees. Fat tissue also produces proteins that can cause inflammation around your joints.

  • Joint Injuries: Any injuries that you’ve gotten over your lifetime can contribute to osteoarthritis, even if they happened years and years ago and seemed to heal just fine.

  • Repeated Stress: If your job or usual activities involve putting repetitive stress on your joints, they might develop osteoarthritis (like ours probably will from doing all this typing, but we’ll take one for the team).

  • Genetics: Some people are just more likely to get osteoarthritis than others based solely on luck of the draw.

  • Bone Deformities: If you’re born with a bone abnormality like a malformed joint or defective cartilage, osteoarthritis is more likely to develop.

  • Metabolic Diseases: Certain diseases like diabetes or a condition called hemochromatosis (when your body has too much iron in it) increase your risk of developing osteoarthritis.

Rheumatoid arthritis

Rheumatoid Arthritis, like osteoarthritis, causes stiffness and swelling in your joints. Unlike osteoarthritis however, it’s not caused by general wear and tear. Rheumatoid arthritis is an autoimmune inflammatory disease, which means it’s caused by your immune system attacking your body by mistake (it may be a little confused, but it’s got the spirit!).

Instead of your cartilage wearing down and causing pain, this form of arthritis causes painful swelling in the lining of your joints that can lead to bone erosion and joint deformity. You can see this inflammation depicted in the 3D model below. It can affect other parts of your body too, like your skin, eyes, lungs, heart, and blood vessels. Advances in medications have improved treatment options for rheumatoid arthritis, but it can still cause physical disabilities if it’s severe enough.

We don’t know exactly what causes your immune system to attack your body like this yet, but it seems like there’s definitely a genetic component involved. Other risk factors that can play into it include sex, age, family history, smoking, and excess weight. Let’s elaborate on those a little bit below.

  • Sex: Females are mor likely to develop rheumatoid arthritis than males.

  • Age: While it can occur at any age, rheumatoid arthritis tends to set in somewhere around middle age.

  • Family History: If someone in your family has rheumatoid arthritis, you’re more likely to get it too.

  • Smoking: Cigarettes increase your risk of rheumatoid arthritis, especially if you already have the gene that predisposes you to it. It can also increase its severity once you have it.

  • Excess Weight: We don’t know exactly why, but people who are overweight seem to be at a higher risk for developing rheumatoid arthritis (possibly because fat tissue produces proteins that can cause inflammation in your body).

Paget’s Disease of Bone

Remember when we were talking about how your bones are constantly being broken down and replaced? In Paget’s Disease, this process happens way faster than it normally does. Because there’s so much more breakdown and regrowth, your bones become bigger and softer than they’d usually be, causing them to become misshapen or easily fractured.

We’re not sure what causes Paget’s disease, but the current theory is that it has something to do with genetics and environmental factors (kind of like rheumatoid arthritis). A few genes have been linked to it, but that’s pretty much all we know. Weirdly, the disease has become less common and less severe over the past few years, and no one’s sure why. If you’re interested in learning more about potential theories, the National Organization for Rare Diseases (NORD) is a good place to start.

Usually, people with Paget’s don’t experience symptoms, but if they do, they generally depend on the area the disease is affecting. It could be one or two areas, or widespread over the entire body. A few common places where symptoms occur are the pelvis, skull, spine, and leg. Let’s briefly talk about what effects you’d experience with each before moving on to our last disease.

  • Pelvis: In the pelvis, Paget’s can cause hip pain.

  • Skull: If the bones in your skull are affected, it can cause headaches or hearing loss.

  • Spine: Paget’s can cause the nerve roots in your spine to become compressed, which causes pain, tingling, and numbness in your arms or legs.

  • Leg: Bones weakened by Paget’s may start to bend, making you bowlegged. Enlarged bones can put extra stress on your joints, which can cause osteoarthritis in your knees or hips (because apparently one bone disease isn’t enough).


There are a lot of places where we can get vitamin D, mostly from the sun, but also from foods like fatty fish, mushrooms, egg yolks, and other supplements. When you don’t get enough vitamin D as a child, it can cause a disease called Rickets. Rickets occurs specifically from an extreme, prolonged vitamin D deficiency that affects bone formation (inherited problems can cause it too, but those are super super rare).

Children affected by rickets can develop skeletal deformities such as thickened wrists and ankles, breastbone projection, and most commonly, bowed legs (which you can see in the model below). Signs and symptoms can include delayed growth and motor skills, muscle weakness, and pain in the spine, pelvis, and legs.

Why is vitamin D so important for your bones though? Isn’t that supposed to be calcium? Well, it’s both. Vitamin D is the nutrient that helps your body absorb calcium and phosphorus from your food. No matter how much of either you put into your body, it doesn’t matter if you can’t take it in. There are some medical conditions that can make it hard to absorb vitamin D too, like celiac disease, inflammatory bowel disease (IBS), cystic fibrosis, or kidney problems.

There are some risk factors involved in developing rickets, including dark skin, a vitamin D deficiency during pregnancy, location, premature birth, certain medications, and exclusive breastfeeding. Let’s elaborate on those a little below before moving on to our last bone condition.

  • Dark Skin: People with darker skin have more melanin, which makes it harder for their skin to make vitamin D from sunlight.

  • Vitamin D Deficiency During Pregnancy: If a mother has a severe vitamin D deficiency while pregnant, her child can be born with symptoms of rickets or develop them within a few months after birth.

  • Location: Children that live in northern latitudes receive less sunlight than those in lower latitudes, and are thus at higher risk of developing rickets.

  • Premature Birth: Babies that are born early tend to have lower levels of vitamin D because they have less time to absorb it from their mothers in the womb.

  • Medications: Certain medications like anti-seizure or antiretroviral ones can have the side effect of interfering with the body’s ability to use vitamin D

  • Exclusive Breastfeeding: Babies who are exclusively breastfed are more likely to develop a vitamin D deficiency if not provided with supplements.

✅ Conclusion

Now that you’ve made it through this study guide, we hope that you know a lot more about skeletal diseases than you did before. They have different causes, ranging from genetic deformities, to wear and tear, to autoimmune diseases, to nutrient deficiencies, to plain old aging . There are plenty of risk factors, most of which you can’t really control. But what you CAN do is maintain a healthy lifestyle, eat plenty of nutritious foods, avoid smoking , curb alcohol intake, stay active, and take care of your body in general. And that’s all we’ve got for you today! Take a look at out the FAQs to check your understanding about skeletal diseases below 🥳🦴💀☀️🍣🎓.

  • Skeletal diseases in the human body have different causal factors.

  • While genetic abnormalities cause some, others are caused by nutrition deficiency and old age among other factors.

  • Treatment depends on the types of bone diseases, with the approach varying based on the severity of the disease.


1. What is the major determinant of whether or not you’ll develop osteoporosis?

Your peak bone mass, fully developed around the age of 30.

2. What is the most common form of arthritis?


3. What is an autoimmune disease, and what skeletal disease is associated with it?

An autoimmune disease occurs when your immune system attacks your body by mistake. Rheumatoid arthritis is an example of a skeletal autoimmune disease.

4. Is your bone growing too quickly or too slowly when you have Paget’s disease?

Your bones are being replaced too quickly with Paget’s disease, causing them to become bigger and softer than they normally would.

5. Why is vitamin D important for bone health?

Vitamin D is the nutrient that allows you to absorb calcium. Children with a prolonged and severe vitamin D deficiency can develop Rickets.

6. What are the signs and symptoms of a bone fracture?

Severe pain, difficulty using the affected limb, an unusual bump, bend, or twist, and swelling.

7. What is the difference between osteoarthritis and rheumatoid arthritis?

Osteoarthritis is caused by general wear and tear as you body gets older, while rheumatoid arthritis is an autoimmune disease.

8. What causes Paget’s disease?

We aren’t entirely sure, but theories include genetics and environmental factors.

9. What skeletal disease is responsible for 30% of bone fractures?


10. What kind of fracture was depicted in the 3D model?

A complete fracture.

We hope you enjoyed studying this lesson and learned something cool about Skeletal Diseases! Join our Discord community to get any questions you may have answered and to engage with other students just like you! Don’t forget to download our App to experience our fun, VR classrooms – we promise, it makes studying much more fun! 😎


  1. Skeletal Diseases. Accessed 30 Nov, 2021.

  2. Skeletal System Problems and Diseases. Accessed 30 Nov, 2021.

  3. FIVE COMMON SKELETAL DISEASES. Accessed 30 Nov, 2021.

  4. Leukemia. Accessed 30 Nov, 2021.


  6. Skeletal limb abnormalities. Accessed 30 Nov, 2021.

  7. Muscle Disorders. Accessed 30 Nov, 2021.


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