Bone, Cartilage and the Mechanics of a Joint
Bone is living connective tissue whose cells remove and replace matrix. Reduced loading can lead to bone loss. Joint surfaces and supporting tissues guide movement. To calculate muscle force in a still-bone model, balance opposing moments using perpendicular moment arms.
What a learner can do afterwards
- Explain bone remodelling using the matrix-removing action of osteoclasts and the matrix-forming action of osteoblasts
- Relate the structure of a named synovial joint to its permitted range of movement
- Work out the force in a muscle from the geometry of the lever it acts on
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Bone and cartilage are connective tissues, with cells in extracellular matrix. Osteoclasts resorb (remove) bone matrix. Osteoblasts deposit new matrix, which later mineralises. Repeated loading can help maintain bone strength. Reduced loading can let removal exceed formation, causing bone loss. Hormones and nutrition also affect this balance. Living repair cells can bridge a fracture; remodelling later reshapes the repair.
A synovial joint has a capsule around a cavity containing synovial fluid. Articular cartilage covers contacting bone surfaces, spreads load and helps reduce friction with the fluid. Ligaments link bone to bone and restrain excess motion. Tendons link muscle to bone and transmit force. Contracting muscle can produce tension without shortening. Opposite-acting muscles can contract together to steady a joint.
The shoulder and hip are ball-and-socket joints: both bend in several directions and rotate. The shoulder's shallow socket and loose capsule allow more movement; rotator cuff muscles help hold it in place. The hip's deeper socket and strong ligaments favour stability for weight bearing. The elbow's humeroulnar hinge mainly bends and straightens in one plane. Plane joints allow small glides. Shape and supporting tissues together limit a joint's range.
Model a bone held still, with the joint as pivot. Moment (turning effect) = force × moment arm. The arm is the perpendicular distance from the pivot to the force's line of action, not necessarily the distance to its attachment. Opposing moments balance: muscle force × muscle arm = load force × load arm, if other moments are neglected. Use matching length units. A joint reaction acting at the pivot has zero moment but contributes to force balance.
Take a model with a 50 N load at a 24 cm moment arm and one muscle at a 6 cm arm. The moments oppose; other moments are neglected. Muscle force = (50 × 24)/6 = 200 N. Muscle and load may act on the same side of the joint yet turn it in opposite directions. Their forces need not be equal because the joint reaction also helps balance them. Ignore the bone's weight only when the model says to.
Osteoclasts remove bone matrix; osteoblasts form it. Joint structure guides movement. For a still bone, balance opposing moments using perpendicular moment arms.
Lesson, questions and flashcards written for this dataset. Anatomy and Physiology and Khan Academy reference records informed the anatomical explanations.
Readings and flashcards written for this dataset. Video references: Khan Academy, Ligaments, tendons, and joints and Cartilage; TED-Ed, What happens when you break a bone? - Gurpreet Baht and Natalie Pang. Video licensing terms are unverified.
More to read
Bone renewal and repair
Bone tissue remains active throughout life. At a remodelling site, osteoclasts remove existing matrix. Osteoblasts then deposit replacement matrix, which mineralises. Different sites can be at different stages at the same time.
To understand a change in bone mass, compare formation with removal over time. More removal than formation produces a net loss. Reduced weight-bearing can contribute to this imbalance. Repeated loading can help maintain strength, but hormones and nutrition also affect bone turnover.
A fracture requires repair as well as remodelling. Living cells produce tissue that bridges the break. Bone formation rebuilds the damaged region, and later remodelling reshapes it. This ability depends on living tissue, not on mineral repairing itself.
Why the shoulder and hip move differently
Knowing that a joint is ball-and-socket does not tell you its whole range. The shoulder and hip both have this arrangement, yet their supporting structures differ.
The shoulder's shallow socket and loose capsule allow considerable movement. Rotator cuff muscles help keep the rounded head in position. The hip has a deeper socket and strong supporting ligaments, suited to stability during weight bearing. Both joints allow bending in several directions and rotation, but the shoulder has the wider range.
Their contacting surfaces are covered by articular cartilage. A capsule encloses a cavity containing synovial fluid. These features support low-friction movement. Explaining movement therefore requires both the joint's surface arrangement and the tissues that support it.
Naming a joint's movement
Watch what happens to the angle at an elbow. Bending reduces the angle between the upper arm and forearm; this movement is flexion. Straightening increases the angle and is extension. The humeroulnar joint permits these hinge movements mainly in one plane.
Rotation describes turning about an axis. Ball-and-socket joints at the shoulder and hip allow rotation as well as bending in several directions. Plane joints contribute small gliding movements.
These names describe motion. They do not give the force needed to produce or hold it. A muscle can develop tension while the bone stays still. To calculate that tension in a joint model, identify the pivot, the external load and the perpendicular moment arm of each force.
A muscle-force calculation
Start with a bone held still and choose the joint as pivot. A force's moment arm is the shortest distance from that pivot to the force's line of action. The moment equals the force multiplied by this perpendicular distance.
Consider one muscle with a 4 cm moment arm and an 80 N load with a 24 cm moment arm. Their moments oppose, and all other moments are neglected. The load produces 80 × 24 = 1920 N cm. The muscle must produce the same moment in the opposite direction, so its force is 1920/4 = 480 N.
Muscle and load may act on the same side of the joint and still produce opposite moments. The joint reaction also contributes to force balance. If it acts at the pivot, its moment about that pivot is zero.
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