Ortho Implant

1. Bone Structure and Healing

Bone is a complex biological tissue with a hierarchical structure designed for optimal strength and minimum mass.
There are two main types of bone: cortical (compact) bone (dense, low porosity) and cancellous (trabecular or spongy) bone (highly porous, network of trabeculae).
Bone healing is a complex process that occurs in three stages: inflammation, bone production (soft and hard callus formation), and bone remodeling.
A key factor guiding healing is interfragmentary movement, which influences tissue strain and cellular activity.

2. Orthopedic Implants

Definition: Medical devices used to replace or provide fixation of bone, or to replace joint surfaces.
Primary Materials: Stainless steel and titanium alloys for strength, often combined with plastic liners for joint articulation.

3. Orthopedic Screws

Function: The most common device for fracture fixation, used both standalone and to secure other implants like plates.
Key Anatomy: A screw has four parts:
- Head: For the screwdriver and to provide a counterforce.
- Shaft (Shank): The smooth section between the head and threads.
- Thread: Defined by its root (core) diameter, thread (outside) diameter, and pitch (distance between threads).
- Tip: Can be round (requires pre-tapping) or self-tapping.
Critical Biomechanical Considerations:
- Pull-out Strength: The resistance of a screw to being pulled from the bone. It is predominantly reliant on bone density and is critically affected by the screw's outer diameter and length of engagement.
- Interfragmentary Compression: A screw can be used to compress fracture fragments together to promote healing.
- Stress Shielding: The significantly stiffer metallic screw carries most of the mechanical load, shielding the adjacent bone. According to Wolff's Law, this reduced mechanical stimulus can lead to bone resorption (osteopenia) and eventual screw loosening.

4. Orthopedic Plates

Function: Used to neutralize deforming forces (bending, torsion, shear) that cannot be counteracted by screws alone.
Tension Band Principle: A fundamental engineering concept. The plate must be applied to the tension (convex) side of the bone to convert tensile forces into compressive forces at the fracture site. Application to the compression side leads to failure.
Advantages: Allow for anatomic reduction and stability for early joint function.
Disadvantages: Risk of stress shielding and osteoporosis beneath the plate, re-fracture after removal, and soft tissue irritation.
Plate Types & Evolution:
- Dynamic Compression Plate (DCP): Relies on friction between the plate and bone generated by screw torque.
- Locking Compression Plate (LCP): Uses screws that lock into the plate, creating a fixed-angle construct. It does not rely on friction and minimizes plate-bone contact, preserving blood supply.
- Other designs include neutralization, compression, buttress, and bridge plates, each with a specific functional goal.