The Musculoskeletal System
The musculoskeletal system gives the body structure, enables movement, protects vital organs, and stores minerals. It is made up of bones, joints, muscles, tendons, ligaments, and cartilage working together. As a medical assistant, you will assist with fracture care, orthopedic procedures, and range-of-motion assessments. You also need to protect yourself and your patients through proper body mechanics every day.
The Skeletal System: 206 Bones
Adults have 206 bones. Infants have about 270 to 300 because many bones fuse during development. Bone serves multiple functions:
- Support and structure: The framework that gives the body its shape.
- Protection: Skull protects the brain; rib cage protects heart and lungs; vertebrae protect the spinal cord.
- Movement: Acts as levers for muscles to pull on.
- Mineral storage: Bone is the main reservoir for calcium and phosphorus.
- Blood cell production: Red bone marrow in flat bones and ends of long bones produces red blood cells, white blood cells, and platelets (hematopoiesis).
Bone Classifications
- Long bones: Humerus, femur, tibia, fibula. Longer than they are wide; shaft (diaphysis) with ends (epiphyses).
- Short bones: Carpals, tarsals. Roughly cube-shaped.
- Flat bones: Skull, sternum, scapulae, ribs. Protect organs; contain red marrow.
- Irregular bones: Vertebrae, hip bones. Complex shapes.
- Sesamoid bones: Embedded in tendons; the patella (kneecap) is the largest.
Joints
A joint is where two or more bones meet. Joints are classified by structure and by the movement they allow.
- Fibrous joints (synarthroses): No movement. Bones connected by fibrous tissue. Example: skull sutures.
- Cartilaginous joints (amphiarthroses): Slight movement. Example: pubic symphysis, intervertebral discs.
- Synovial joints (diarthroses): Freely movable. Most joints in the body. Have a joint cavity filled with synovial fluid for lubrication. Types include hinge (elbow, knee), ball-and-socket (shoulder, hip), pivot (atlas-axis for head rotation), gliding (wrist carpals), saddle (thumb), and condyloid (wrist).
Muscle Types
The body has three types of muscle tissue:
- Skeletal muscle: Attached to bones via tendons. Voluntary control. Striated (striped) appearance under microscope. Responsible for movement, posture, and heat production.
- Smooth muscle: Found in walls of hollow organs (GI tract, blood vessels, bladder, uterus). Involuntary control. Not striated.
- Cardiac muscle: Found only in the heart. Involuntary. Striated. Self-excitatory (can generate its own electrical impulse).
Tendons connect muscle to bone. Ligaments connect bone to bone. Both are made of collagen and are slow to heal after injury due to limited blood supply.
Fracture Types
A fracture is any break in a bone. Types appear frequently on MA certification exams.
- Simple (closed) fracture: Bone breaks but does not pierce the skin. The most common type.
- Compound (open) fracture: Broken bone penetrates the skin. High risk of infection. More serious than a simple fracture.
- Comminuted fracture: Bone shatters into three or more pieces. Common in high-impact trauma and in patients with osteoporosis.
- Greenstick fracture: Incomplete break; bone bends and cracks on one side but does not break all the way through. Occurs mainly in children because their bones are more flexible.
- Spiral fracture: Twisting force causes a helical break along the shaft of a long bone. Can be a red flag for abuse, especially in young children who are not yet walking.
- Transverse fracture: Break perpendicular to the long axis of the bone. Usually from direct impact.
- Stress fracture: Small cracks from repetitive force. Common in athletes, especially runners (metatarsals, tibia).
- Pathological fracture: Break caused by disease weakening the bone (osteoporosis, cancer metastases) rather than by significant trauma.
Common Musculoskeletal Conditions
Osteoporosis
Decreased bone density and deterioration of bone microstructure, increasing fracture risk. Most common in postmenopausal women (estrogen protects bone density) and older adults. Diagnosed by DEXA scan (bone density test). Often asymptomatic until a fracture occurs. Common fracture sites: hip, vertebrae (compression fracture), wrist. Prevention: adequate calcium and vitamin D, weight-bearing exercise, smoking cessation. Treatment: bisphosphonates (e.g., alendronate), calcium, vitamin D.
Arthritis
Two main types for the exam:
- Osteoarthritis (OA): Degenerative wear-and-tear of articular cartilage. Age-related. Affects weight-bearing joints (knees, hips) and hands. Pain worsens with activity and improves with rest. Bony enlargements on finger joints: Heberden nodes (DIP joints) and Bouchard nodes (PIP joints).
- Rheumatoid arthritis (RA): Autoimmune inflammatory disease attacking synovial membranes. Symmetrical joint involvement. Morning stiffness lasting over an hour. Can affect multiple organ systems. Treated with DMARDs (disease-modifying antirheumatic drugs) like methotrexate.
Sprains vs. Strains
These terms are often confused:
- Sprain: Injury to a ligament (bone to bone). Common at the ankle. Graded I (mild stretch) to III (complete tear).
- Strain: Injury to a muscle or tendon (muscle to bone). Common in the lower back and hamstring.
Treatment for both: RICE (Rest, Ice, Compression, Elevation) in the acute phase.
Body Mechanics for MAs
Back injuries are one of the most common occupational injuries for healthcare workers. Proper body mechanics protect you and your patient.
- Wide base of support: Stand with feet shoulder-width apart for stability.
- Bend at the knees and hips, not the waist: Keep your back straight when lifting.
- Hold the load close to your body: Reduces leverage and strain on the spine.
- Do not twist while lifting: Pivot your whole body using your feet. Twisting is the most common cause of back injury.
- Push rather than pull: When moving equipment or patients, pushing uses larger muscle groups more efficiently.
- Ask for help: If a patient needs assistance getting up or transferring, use a gait belt and a second person when needed. Never attempt a transfer alone if the patient is at high fall risk.
Know the six fracture types (simple, compound, comminuted, greenstick, spiral, stress), the difference between sprains and strains, the key features of osteoporosis versus osteoarthritis, and the body mechanics principles. The number of bones in the adult body (206) and the three muscle types are also commonly tested. Be ready to identify what connects muscle to bone (tendon) versus bone to bone (ligament).
Practice Questions
Question 1: A 6-year-old child presents with an arm injury after falling from monkey bars. X-ray shows the bone bent and partially cracked on one side but not broken all the way through. What type of fracture is this?
Answer: Greenstick fracture. This is an incomplete fracture where one side of the bone bends and cracks but does not break completely through. It occurs primarily in children because their bones contain more collagen and are more flexible than adult bone, causing them to bend rather than snap cleanly.
Question 2: What is the difference between a sprain and a strain? Give an example of each.
Answer: A sprain is an injury to a ligament (which connects bone to bone). Example: twisting the ankle tears the anterior talofibular ligament. A strain is an injury to a muscle or tendon (which connects muscle to bone). Example: lifting a heavy object improperly strains the lumbar muscles. Both are treated initially with RICE: Rest, Ice, Compression, Elevation.
Question 3: When lifting a box from the floor, which body mechanics principle is most important for preventing back injury?
Answer: Bend at the knees and hips while keeping the back straight (do not bend at the waist). Also keep the load close to your body and avoid twisting while lifting. Using leg muscles (the largest and strongest muscles) to lift rather than back muscles reduces spinal compression and the risk of disc injury.