Spinal implants are medical devices used to stabilize parts of the spine after injury, deformity, or degeneration. In the lumbar spine, implants can reduce painful motion, restore alignment, and protect nerves by holding vertebrae in a safer position. Common parts include pedicle screws, rods, interbody cages, and bone graft material.
Together, they act like an internal brace while the body heals.
Key Facts
- Pedicle screws anchor into the strong bony pedicles of vertebrae to hold rods in place.
- Rods connect screws across multiple vertebrae and help maintain spinal alignment.
- An interbody cage sits in the disc space and helps restore height between vertebrae.
- Bone graft provides a scaffold for new bone growth during fusion.
- Stress = force / area, so spreading load across an implant and bone can reduce local stress.
- Fusion aims to turn two or more moving vertebrae into one solid bone segment.
Vocabulary
- Pedicle screw
- A threaded implant placed through a vertebral pedicle to anchor spinal rods and stabilize the spine.
- Spinal rod
- A metal bar connected to screws that helps hold vertebrae in a corrected and stable position.
- Interbody cage
- A spacer placed between vertebral bodies to support disc height and create space for bone fusion.
- Bone graft
- Bone or bone-like material used to encourage new bone growth between vertebrae.
- Spinal fusion
- A surgical process in which two or more vertebrae grow together into one solid section of bone.
Common Mistakes to Avoid
- Thinking the rods permanently do all the work, which is wrong because the goal is for new bone to fuse the vertebrae and take over much of the support.
- Confusing a cage with an artificial disc, which is wrong because a fusion cage is designed to reduce motion and support bone growth, not preserve normal disc motion.
- Assuming screws go into the spinal cord, which is wrong because pedicle screws are placed through bone pathways beside the spinal canal.
- Ignoring load sharing between bone and implant, which is wrong because implants can fail if the spine does not fuse or if forces remain concentrated in the hardware.
Practice Questions
- 1 A spinal construct uses 6 pedicle screws. If each screw can safely support 250 N in a simplified model, what is the total safe load supported by the screws?
- 2 An interbody cage restores a disc space from 6 mm to 11 mm. By how many millimeters did the disc height increase, and what is the percent increase relative to the original height?
- 3 Explain why a surgeon might use both rods with screws and an interbody cage instead of using only one type of implant.