Key takeaway
Orthopedic screws are not a single interchangeable device category. In fracture fixation, they may compress bone fragments, anchor a plate, hold an intramedullary nail, support a small joint fusion, or work with a washer to distribute load at the screw head. The right design depends on the bone, fracture pattern, desired stability, implant system, material, and evidence behind the device. For readers comparing fixation technologies, the practical point is straightforward: screw selection is a biomechanical decision, not just a size choice. Cortical, cancellous, cannulated, headless compression, locking, nonlocking, fully threaded, and partially threaded screws each address different fixation needs and carry different limitations.
What orthopedic screws do in fracture fixation
Orthopedic screws hold bone fragments in a reduced position while biological healing takes place. They may be used alone, with plates, with rods or nails, or as part of more complex internal fixation constructs. Patient-facing educational material from the American Academy of Orthopaedic Surgeons notes that screws are used in internal fixation more often than any other implant type, and that different screw designs are chosen according to the fracture and intended use.
The basic clinical objective is stable fixation, but “stable” does not mean the same thing in every case. Some constructs aim for absolute stability, where compression limits motion at the fracture line. Others use relative stability, where controlled motion and load sharing support callus formation. A screw may therefore be selected to generate interfragmentary compression, secure a plate to bone, create a fixed-angle construct, or help prevent shortening and rotation around a nail.
Because screw performance depends on the full construct, evaluating a screw in isolation can be misleading. Bone density, cortical thickness, screw trajectory, plate design, working length, pilot hole preparation, insertion torque, and post-operative loading all influence whether fixation remains stable until healing progresses.
Main orthopedic screw types and where they fit
Orthopedic screw terminology often overlaps. A screw may be described by thread form, head design, shaft design, material, intended anatomical region, or the way it interacts with a plate. The categories below are useful for understanding the clinical and engineering intent behind common designs.
Cortical screws
Cortical screws are designed for dense cortical bone. They typically have finer threads than cancellous screws and are often used in diaphyseal bone or to attach a nonlocking plate to the cortex. In plate fixation, a conventional cortical screw can pull the plate toward the bone as it is tightened. This can support compression in a well-contoured construct, but it can also disturb reduction if the plate is not properly adapted.
Cancellous screws
Cancellous screws are intended for less dense metaphyseal or epiphyseal bone. They usually have deeper, coarser threads to improve purchase in trabecular bone. Partially threaded cancellous screws may be used as lag screws when the goal is to draw one fragment toward another and create interfragmentary compression.
Cannulated screws
Cannulated screws have a central channel that allows placement over a guidewire. This can support more controlled positioning in anatomically constrained regions or in minimally invasive approaches. Their use still depends on accurate guidewire placement, appropriate screw length, and avoidance of joint penetration or unintended cortical breach.
Headless compression screws
Headless compression screws are designed to sit below the bone or cartilage surface while generating compression across a fracture or fusion site. They are often discussed in small bone, articular, and fusion applications. Their main advantage is low-profile placement, but the design requires precise sizing and trajectory because removal and revision can be more difficult than with standard headed screws.
Locking screws
Locking screws engage with threaded holes in a compatible locking plate. AO Surgery Reference describes the principle as a screw head locking into the threaded plate hole, creating stability without necessarily pulling the bone to the plate. This changes construct mechanics: the plate and screws can behave more like an internal fixator, with stability depending less on plate-to-bone compression.
Design choices that influence fixation stability
Several screw features directly affect fixation behavior. These details matter because a screw must tolerate insertion, resist loosening or pullout, and transfer load without damaging bone or the implant system.
- Major diameter: A larger diameter generally increases the bone-screw interface and can improve mechanical strength, but anatomy limits how large a screw can be used safely.
- Core diameter: The root or minor diameter contributes to torsional strength. A screw with a larger core may better resist twisting failure, although thread geometry and material also matter.
- Thread pitch and depth: Coarser threads may improve purchase in cancellous bone, while finer threads are more typical for cortical fixation.
- Threaded length: Fully threaded screws can hold along their length. Partially threaded screws may be used to create compression when the near fragment is overdrilled or otherwise allowed to glide.
- Tip design: Self-tapping and self-drilling features affect insertion technique, heat generation, insertion torque, and the need for predrilling or tapping.
- Head design: Standard, low-profile, headless, and locking heads each change how force is transferred to bone or to a plate.
- Washer compatibility: A washer may spread load at the screw head-bone interface, especially where the screw head could sink into softer bone.
The FDA’s November 22, 2024 final guidance for orthopedic non-spinal metallic bone screws and washers treats mechanical performance as more than a single strength value. It highlights torsional strength, driving torque, axial pullout strength, sterilization or reprocessing validation, and biocompatibility evaluation. That regulatory framing is useful for industry readers because it separates insertion risk, pullout risk, and biological safety instead of reducing them to a broad claim of “strong fixation.”
Locking versus nonlocking screws in plate constructs
One of the most important distinctions in modern plate fixation is whether the screw locks to the plate. A nonlocking screw compresses the plate toward the bone as it is tightened. A locking screw, by contrast, threads into the plate hole and creates a fixed-angle relationship between the screw and plate.
| Feature | Nonlocking screw with plate | Locking screw with plate |
|---|---|---|
| Main mechanical action | Pulls plate toward bone and may help create compression | Locks screw head into plate to form a fixed-angle construct |
| Plate-bone contact | Usually more dependent on accurate plate contouring | Less dependent on exact plate-to-bone contact |
| Common advantage | Useful for compression and traditional plating techniques | Useful where angular stability or reduced plate compression on bone is desired |
| Common limitation | Poor contouring can pull fragments out of reduction | Incorrect drilling or trajectory can still compromise fixation, even if the screw locks |
Locking technology does not make a construct automatically superior. It solves certain problems and introduces others. A locking construct may be valuable in osteoporotic bone, metaphyseal regions, periarticular fractures, or bridge plating strategies, but excessive construct stiffness may not be desirable in every fracture pattern. Nonlocking screws remain important when the goal is compression across a fracture or plate-to-bone adaptation.
The decision is therefore contextual. Surgeons evaluate fracture morphology, soft tissue status, bone quality, planned reduction method, implant availability, and the desired balance between rigidity and load sharing. From a device evaluation perspective, locking and nonlocking screws should also be assessed as part of their intended plate system rather than as isolated fasteners.
Materials, standards, and regulatory evidence
Most orthopedic screws used for internal fixation are metallic, commonly stainless steel or titanium alloy, with some devices using cobalt-chromium alloys depending on indication and design. The FDA guidance for non-spinal metallic bone screws identifies several recognized material standards, including titanium, titanium alloy, stainless steel, and cobalt-chromium alloy specifications, when those materials fall within the guidance scope. See also: Implants.
In the United States, non-spinal metallic bone fixation screws are generally associated with Class II orthopedic device regulation under 21 CFR 888.3040. FDA product classification listings include “screw, fixation, bone” and “screw, fixation, bone, non-spinal, metallic” under that regulation. This classification should not be read as a broad statement that every screw-like implant follows the same pathway. Spinal, craniofacial, mandibular, absorbable, additively manufactured, complex, or system-specific devices may fall outside a particular guidance scope and may require different evidence.
ASTM F543 is especially important because it is the recognized consensus standard for metallic medical bone screws. FDA recognized ASTM F543-23 in December 2024, and the 2024 FDA guidance points to the FDA-recognized version for mechanical bench testing. For industry readers, the useful takeaway is that a credible screw discussion should address defined test methods and acceptance criteria, not only material grade or marketing descriptors.
| Evidence area | What it evaluates | Example from FDA’s 2024 guidance |
|---|---|---|
| Torsional strength | Resistance to twisting failure during insertion or loading | Worst-case screw designs are evaluated by nominal diameter; a 3.5 mm screw has a listed torsional yield criterion of 2.1 Nm |
| Driving torque | Insertion and removal torque relative to screw strength | Maximum insertion and removal torque in at least 20 pcf bone foam should be 50% or less of torsional yield strength |
| Axial pullout | Resistance to being pulled out along the screw axis | Theoretical pullout criteria are based on dimensions such as major diameter, minor diameter, pitch, and axial thread length |
| Sterility and reprocessing | Cleanliness, sterile barrier integrity, and reprocessing validation where applicable | Validation should demonstrate sterility or the ability to clean and sterilize to a sterility assurance level of 10^-6 |
| Biocompatibility | Biological response to tissue and bone contact | The guidance treats covered implants as tissue/bone-contacting devices with prolonged or permanent contact duration greater than 30 days |
These criteria do not replace clinical judgment. Bench testing can show whether a device meets defined mechanical and biological expectations, but it does not determine the best surgical plan for an individual patient. Clinical success still depends on reduction quality, implant placement, patient biology, rehabilitation, infection control, and load management.
Implications for clinicians, manufacturers, and content readers
For clinicians, screw terminology should be linked to a specific fixation objective. Asking whether a screw is cortical or cancellous is not enough; the more important question is what the screw is expected to do in that construct. Is it creating compression, fixing a plate, locking into a plate, resisting rotation, supporting a fusion, or distributing load through a washer?
For manufacturers and regulatory teams, the 2024 FDA guidance reinforces the need to define scope. A conventional non-spinal metallic screw made from recognized materials and familiar geometry may be evaluated differently from a resorbable, additively manufactured, complex, fenestrated, spinal, craniofacial, or system-specific implant. Claims about compatibility, strength, sterilization, or biological response should be traceable to the intended use and test evidence.
For readers comparing fixation topics, the strongest content usually explains trade-offs. A locking screw may improve angular stability, but it is not a universal replacement for compression plating. A larger screw may increase interface area, but it may not be appropriate for a small fragment. A headless screw may reduce prominence, but it can be less forgiving if placement is wrong. A washer may improve load distribution, but it adds another implant component and may not be needed in every case.
The practical conclusion is that orthopedic screws sit at the intersection of anatomy, mechanics, surgical technique, and regulatory evidence. Useful analysis should not reduce them to a product list; it should explain why a given design fits a specific fixation problem.
Frequently asked questions
Are orthopedic screws always removed after a fracture heals?
No. Screws may be left in place or removed depending on the fracture, symptoms, implant location, healing status, and surgeon judgment. Removal is not automatically required and can carry its own risks.
What is the difference between a locking screw and a regular screw?
A regular nonlocking screw tightens the plate toward bone. A locking screw has a threaded head that engages a compatible threaded plate hole, creating a fixed-angle relationship between screw and plate.
Why are some bone screws partially threaded?
Partially threaded screws can help create compression when the threads engage the far fragment while the near fragment can glide. This lag effect is useful only when the fracture pattern and technique are appropriate.
Do stronger screws always mean better fixation?
Not necessarily. Strength is important, but fixation also depends on bone quality, screw placement, construct stiffness, fracture biology, soft tissue condition, and rehabilitation. Excessive stiffness or poorly planned placement can be problematic.
Are all orthopedic screws covered by the same FDA guidance?
No. The FDA’s 2024 performance criteria guidance focuses on non-spinal metallic bone screws and associated washers within a defined scope. Spinal, craniofacial, resorbable, additively manufactured, and complex devices may require different evidence or pathways.