Regardless of the subspecialty you’re rotating on as a medical student, it’s pretty much inevitable that you’ll see fractures fixed with plates and/or screws. While the nuances of orthopedic implants can get confusing, a little knowledge of the basics goes a long way, especially on your trauma rotation!

In this first article, we’ll focus on screws, used in fixation and compression of bone. In order to understand different types of screws and how they work, it’s helpful to start with the “anatomy.” A screw is composed of a head, a shaft, threads that surround the shaft, and a tip.

The head of the screw allows us to advance the screw with a screwdriver, but also prevents it from sinking completely into bone—a helpful property when our goal is to compress fragments together.

The shaft can be measured by the core diameter (the diameter of the narrowest threaded portion, or where the threads emerge from the shaft) or by the thread diameter (the diameter at the widest threaded portion, or the outer edge of the threads). The threads on the shaft are critical in determining the “pullout strength” of the screw. The wider the thread diameter, the more contact they will have with the bone, and thus the more resistant the screw will be to dislodging. The pitch of the threads describes how spread apart the threads are, which determines the distance the screw will travel with each rotation.

The tip of the screw can have different properties as well. For a non self-tapping, non-self-drilling screw, the order of operations would be to create a drill hole, create channels for your threads with a device called a tap, and then drive in your screw. A self-tapping screw, on the other hand, could be inserted right after the drill hole was created—it has a sharp, fluted tip that allows it to cut its own thread channels. Some screws are even self-drilling, so you can just drive them straight into bone without a preliminary drill hole.

Now that we know some of the basic screw components, we can understand a few different types of screws:

1. Cortical screws: these screws are 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.
2. Cancellous screws: in contrast to cortical screws, 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.
3. Cannulated screws: remember the Seldinger technique from your general surgery rotation? Same idea here. A cannulated screw is 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. 

So what can screws do for us? 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.

But wait…cortical screws can act as lag screws too, despite the fact that they have threads all down their shaft. How is this accomplished? By over-drilling the near side of the fracture, or drilling a hole equal to the thread diameter, so that the threads do not engage on the near side. The far side, by contrast, is drilled to fit the core diameter of the screw, allowing the threads to engage and compress the two fragments together.

While compression by lag screw technique is an important function of screws, they can help us in other ways as well. Screws don’t have to compress—for example, a syndesmotic screw placed during an ankle ORIF simply holds the syndesmosis together to allow healing to occur. Screws can, of course, be used to fix plates to bone as well (more to come in part 2 of this article), and can either help pull the plate closer to the bone, or in the case of locking screws, simply lock to the plate to stabilize the fixation construct. These locking screws have threaded heads to make sure they are secured directly to the plate. Thought screws were only used in fracture fixation? Think again—they can also be used to fix soft tissue to bone; for instance, in ACL reconstruction.

So now that we’ve “drilled in” some basic knowledge, you’re unlikely to “screw up” the next time you’re asked about lag techniques in the OR! If this article “drives” you to learn more about screws and other implants (ok, enough terrible puns for now), check out the resources listed below. And of course, keep an eye out for Part 2, where we will go over the basics of orthopedic plates!

Written by: Jessa Fogel, MS4

Sources and further reading:

1. “ Basics of Internal Fixation.” Cambridge Orthopedics, http://www.cambridgeorthopaedics.com/easytrauma/classification/commonfiles/basics_of_internal_fixation.htm.
2. Cole, Brian J., et al. “Fixation of Soft Tissue to Bone.” Journal of the American Academy of Orthopaedic Surgeons, vol. 24, no. 2, 2016, pp. 83–95., https://doi.org/10.5435/jaaos-d-14-00081.
3. Jajodia, Neha, et al. “Orthopaedic Bone Tap: A ‘Jugaad’ for Zygomatic Bone Reduction.” Journal of Maxillofacial and Oral Surgery, vol. 17, no. 4, 2017, pp. 630–631., https://doi.org/10.1007/s12663-017-1076-x.
4. “Locking Plate Principles.” AO Surgery Reference, https://surgeryreference.aofoundation.org/cmf/basic-technique/locking-plate-principles#locking-versus-nonlocking-plates-advantages-to-a-locking-plate-screw-system.
5. “Orthopedic Hardware.” UW Radiology, 18 Apr. 2017, https://rad.washington.edu/about-us/academic-sections/musculoskeletal-radiology/teaching-materials/online-musculoskeletal-radiology-book/orthopedic-hardware/.
6. Purushothaman, Rajesh. “Bone Screws in Orthopaedic Surgery.” OrthopaedicPrinciples.com, https://orthopaedicprinciples.com/2013/06/bone-screws-in-orthopaedic-surgery/.
7. “Screws.” Wheeless’ Textbook of Orthopaedics, 22 July 2020, https://www.wheelessonline.com/bones/hand/screws/.
8. Singh, Dr Arun Pal. “Bone Screws Used in Orthopedics.” Bone and Spine, 27 Nov. 2018, https://boneandspine.com/bone-screws-used-in-orthopedics/.
9. “Technique of Syndesmotic Fixation.” Wheeless’ Textbook of Orthopaedics, 22 July 2020, https://www.wheelessonline.com/bones/technique-of-syndesmotic-fixation/.
10. Weinstein, Robert B. “ORTHOPEDIC SCREW MECHANICS.” Podiatry Update, 2011. http://www.podiatryinstitute.com/pdfs/Update_2011/2011_36.pdf