Orthopedic Implants: NEET PG Essentials
1. Common Materials
- Stainless Steel (316L): Cheap, strong, but prone to corrosion and high “stress shielding.” Often used for temporary fixation (plates/screws).
- Titanium Alloys: Excellent biocompatibility, lower modulus of elasticity (closer to bone, reducing stress shielding). Used for permanent implants (joint replacements/stems).
- Cobalt-Chrome: High wear resistance; often used in the femoral head/ball of hip replacements.
- Ultra-High Molecular Weight Polyethylene (UHMWPE): Standard material for the acetabular liner (the “cup”) in total hip arthroplasty.
2. Fixation Systems
| Device | Clinical Application |
|---|---|
| Intramedullary (IM) Nail | Gold standard for femoral and tibial shaft fractures; load-sharing device. |
| DHS (Dynamic Hip Screw) | Used for intertrochanteric hip fractures; allows for “controlled collapse” to promote healing. |
| K-Wires (Kirschner) | Thin, stainless steel wires are used for temporary fixation of small, comminuted fractures. |
3. NEET PG Examination Pearls
- Stress Shielding: Occurs when an implant is significantly stiffer than the surrounding bone ($E_{implant} >> E_{bone}$), leading to bone resorption. Titanium is preferred over steel to mitigate this.
- Load-Sharing vs. Load-Bearing: IM nails are load-sharing (bone and implant share the load), whereas plates are often load-bearing (the implant carries most of the load).
- Ceramics: Used in arthroplasty to minimize wear debris (osteolysis), which is a common cause of implant loosening over time.
- Corrosion: Occurs more frequently at the interface of dissimilar metals (e.g., steel screw in a titanium plate); known as galvanic corrosion.