FRCS Case Studies

Stress-Strain Curve

Candidate:

The stress-strain curve is a graphical representation of the deformation of a material under distinct intervals of load. It is unique to each material and differs depending on whether compression, tension, or shear forces are applied. In orthopaedics, the curve is significant as it helps us understand how materials behave under different loading conditions and how they respond to mechanical stresses.

Candidate:

Stress is the force per unit area in a structure and represents the intensity of internal force. It is point and direction specific within a structure and can be tensile, compressive, or shear. In orthopaedics, stress is measured in units of N/m2 or Pascal.

Candidate:

Strain is defined as the ratio or percentage value of the change in length over the original length when a force is applied to a material. In orthopaedics, strain is clinically relevant as it can help predict the risk of fracture. For example, fractures of cortical bone occur at 2% strain, while fractures of cancellous bone occur at 75% strain.

Candidate:

Stiffness is the ability of an object to resist deformation. In the stress-strain curve, stiffness is represented by the slope of the elastic zone. The steeper the slope, the stiffer the structure.

Candidate:

Yield stress is the transition zone between elastic and plastic (irreversible) deformation. It comprises of three points: the proportional limit, the elastic limit, and the yield point. In orthopaedics, yield stress is important as it helps us understand how materials behave under loading conditions that are close to the failure point. It is also a useful parameter for designing implants and prostheses.

Candidate:

Strain hardening is the process by which plastic deformation increases the dislocation density in a material and increases its strength. This prevents or makes it harder for any further movement, thus more stress is required to move them, causing the curve to go up until it reaches ultimate tensile strength. This phenomenon is also known as work hardening or cold working.

Candidate:

The toughness of a material represents the amount of energy it can absorb before failure and is represented by the area under the entire stress-strain curve. In orthopaedics, the toughness of a material is important as it helps us predict the ability of a material to withstand repeated loading over time. This is particularly relevant for implants and prostheses, which are subjected to cyclic loading in vivo.

Candidate:

Stress shielding occurs when there is a Young’s modulus mismatch between an implant and the bone. This is seen with stiffer implants and extensively porous coated stems. Proximal coating allows proximal bone loading and protects against stress shielding. In orthopaedics, stress shielding can lead to bone resorption and implant loosening, which can compromise the long-term success of the implant.

FRCS Case Studies

Curriculum

Stress-Strain Curve

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