Objectives:

After listening to this episode, we hope that you can:

1. Differentiate primary from secondary bone healing.
2. Describe the difference between cortical and cancellous screws.
3. Contrast the use of a lag screw by design versus by technique.
4. Identify different plate types and their specific indications.

Resources

https://naileditortho.com/heres-what-med-students-should-know-about-orthopedic-plates-and-screws-part-1/

Summary:

- Screw = composed of a head, a shaft, threads that surround the shaft, and a tip.

- Types of screws:cortical, cancellous, cannulated

- Cortical: “fully threaded along the shaft, with a smaller pitch than their cancellous counterparts. The threads themselves are small, thus lowering their threaded to core diameter ratio, which helps them to anchor inside dense cortical bone.”

- Cancellous screws: “ have a large thread to core diameter ratio, so that their coarse threads can anchor in soft cancellous bone. They are often partially threaded, meaning the first part of their shaft has no thread, allowing them to act as a lag screw (more on this in a moment!) Notably, cancellous screws are also self-tapping, allowing the threads to achieve a better grip as the screw is driven into the bone”

- Cannulated screws: “ hollow on the inside, allowing it to be inserted over a K-wire that has been placed in the correct position. While these screws can be placed with more precision than non-cannulated screws, this comes with the tradeoff of slightly diminished pullout strength.”

- Lag screw technique

- One very important principle in orthopedic fixation is the lag screw technique. A lag screw compresses two fragments of bone together, which can be done in a couple different ways. We hinted at this before when discussing the cancellous screw: because it’s partially threaded, when the screw is driven into the bone, the threaded portion will engage bone only on the far side of the fracture, pulling it closer towards the near side, where the unthreaded portion is not engaged. Think of it like a fish hook attached to a line dangling from a fishing rod: every turn of the lag screw to drive it into the bone is like reeling in the line to bring the hook (and your fish) closer to the rod, or the proximal and distal fragments of bone closer together.

https://orthopaedia.com/two-main-types-of-bone-healing/

- Primary healing: “Primary bone healing involves a direct attempt by the cortex to re-establish itself after interruption, without the formation of a fracture callus. Usually only seen after surgical plating”

- Secondary bone healing: occurs when the ends of the fractured bones are near enough to heal** but not perfectly opposed, or when there is some motion at the fracture site. This motion is commonly seen with cast immobilization or with the placement of an intramedullary nail or rod.

https://www.youtube.com/watch?v=AydMRUqzJOo&ab_channel=Dr.AhmedYoussif

Mastering the principles of Orthopaedic Surgery, one break at a time.

Hosted by Dr. Kaid van Kampen
Produced by the UBC Orthopaedic Continuing Professional Development Program

In Collaboration with COSSNET and the Canadian Orthopaedic Association