Screw Biomechanics

A screw converts rotational torque into axial advancement and fixation force, and it is the fundamental fixation device in orthopaedic surgery. Its biomechanics govern implant selection, insertion technique and construct stability.

Why it matters. Screw pullout or loosening is a common mode of fixation failure, and an understanding of thread engagement, the effect of bone density and screw selection is what prevents it. Lag technique for interfragmentary compression, the choice between locking and non-locking screws, and bicortical versus unicortical fixation all rest on the same principles, and improper technique compromises stability.

Three mechanical modes. Screws function in three mechanical modes, and understanding them is fundamental to screw selection:

• Position screw - holds fragments in the reduced position without compression, its threads engaging both fragments equally
• Lag screw - generates interfragmentary compression, its threads engaging the far fragment only
• Locking screw - its threaded head locks to the plate, creating a fixed-angle construct

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The head. The head transfers torque from the driver to the screw, through a hex, cruciate or star drive, and it is the compression surface, either countersunk or buttress. It may be threaded, as in a locking screw, or smooth, as in a non-locking screw. How it meets a plate defines the screw:

• Conventional (compression) head - smooth, slides in the plate hole and generates compression between plate and bone
• Locking head - threaded to engage the plate, giving a fixed-angle construct
• Variable-angle locking head - allows off-axis insertion and direction adjustment with locking capability

The shank and the threaded length. The shank is the unthreaded portion of the screw. A fully threaded screw has threads along its entire length and acts as a position screw unless a gliding hole is drilled. A partially threaded screw has threads at the tip only, and its smooth shaft glides, which makes it a lag screw by design. A cannulated screw has a hollow core for guidewire placement.

Core diameter sets strength. The core (root) diameter is the diameter at the base of the threads, the smallest diameter of the screw. It determines bending and torsional strength, which depend on the core diameter cubed (moment of inertia). It is also the stress-concentration point during loading, and so the weak point where screws break.

Outer diameter sets pullout. The outer (major) diameter, at the tips of the threads, determines thread contact area and therefore pullout strength and holding power in bone. The trade-off is that a larger outer diameter increases pullout but needs a larger hole, which weakens the bone.

Thread depth. Thread depth is the radial distance from core to outer diameter, (outer - core) / 2. Greater depth increases thread surface area, which is why cancellous screws have deeper threads for trabecular engagement. Threads that are too deep reduce core strength.

Thread angle. The angle between the thread flank and the perpendicular to the screw axis is typically 40-60 degrees, the optimal range. A smaller angle increases stripping resistance; a larger angle makes insertion easier but lowers holding power.

Pitch and lead. Examiners frequently test this distinction. Pitch is the distance between adjacent threads, measured parallel to the axis: a smaller pitch puts more threads in a given length and gives greater holding power. Lead is the axial distance the screw advances in one complete rotation:

• Single-lead (most common) - lead equals pitch
• Dual-lead - two helical thread paths, so lead is twice the pitch, giving faster insertion with less rotation
• Triple-lead - lead is three times the pitch

Thread forms. Three thread profiles are named:

• V-thread - standard, with good general properties
• Buttress thread - asymmetric, with better axial pullout resistance
• Hi-Low thread - variable thread depth, combining cortical and cancellous properties

The tip. Self-tapping screws cut their own threads; non-self-tapping screws need the hole tapped first. Tips may be self-drilling, trocar (sharp) or blunt.

Materials. Screws are made from three groups of material:

• Stainless steel - high strength, corrosion resistant
• Titanium - biocompatible, with a lower modulus and reduced stress shielding
• Bioabsorbable (PLLA/PGA) - used in low-stress applications

Named designs. Some screws are known by name or by site:

• Herbert screw - headless, variable pitch, buried fixation
• Syndesmotic screw - crosses the tibiofibular syndesmosis
• Pedicle screw - a large cancellous screw for spine fixation

Pullout strength is the force required to extract a screw from bone, and its loss is the primary mode of fixation failure in many constructs. It depends on the screw and on the bone, and understanding both is what allows fixation strength to be optimised.

Screw factors. Outer diameter is the most important screw factor. Pullout force is proportional to it, doubling the diameter approximately doubles pullout strength, and thread surface area increases with it. Thread engagement length has a linear relationship with pullout strength, and a minimum of 3-4 threads is required for adequate purchase. Thread geometry adds the rest: depth increases surface area, angle affects stripping resistance, and pitch sets the number of engaged threads per unit length.

Bone factors. Bone density is the most important bone factor, and pullout strength is proportional to BMD. Cortical bone is 3-4 times stronger than cancellous, and osteoporotic bone reduces pullout strength by 50-70%, with the loss already under way in osteopenic bone:

Where the threads engage. In bicortical fixation the outer cortex provides 70% of pullout strength and the far cortex adds 30%, so near-cortex engagement is critical.

Pullout failure. A screw pulls out of bone in one of three ways:

1. Interface failure, the most common. The threads shear through bone when the bone is weaker than the screw-bone interface, typically in osteoporotic or cancellous bone. A larger diameter, longer engagement or cement augmentation prevents it.
2. Bone fracture. The cylinder of bone around the screw fractures, with oversized screws or poor technique, and the permanent damage reduces the strength of any re-fixation. Proper hole size and avoiding over-tightening prevent it.
3. Screw deformation. The threads deform under excessive load. This is rare in modern screws with adequate material strength but may occur with cyclic loading and fatigue, and an adequate screw size for the expected loads prevents it.

Enhancing pullout. There are two routes, modifying the screw and augmenting the bone, and in challenging bone they can be combined.

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Screw design optimised for the bone type improves fixation strength and reduces complications.

The cortical screw. Dense cortical bone gives high pullout strength per thread, so the cortical screw uses a fine pitch to maximise the number of engaged threads, and its high thread count spreads force over more of the bone-screw interface. Shallow threads keep the core diameter large, so the high core-to-outer ratio buys a stronger core. The 3.5mm and 4.5mm cortical screws used for plate fixation are the examples.

The cancellous screw. Trabecular bone needs a larger thread area for equivalent strength. The coarse pitch allows deeper penetration into trabecular bone, the wider thread spacing engages more trabeculae per thread, and deep threads increase thread surface area in porous bone. The price is the core: a cancellous screw has a larger outer diameter for pullout but a smaller core, and so less bending strength. The 6.5mm and 7.3mm cancellous screws for metaphyseal fixation are the examples.

Lag screws generate interfragmentary compression, and they are the gold standard for fracture fixation when anatomic reduction and absolute stability are desired.

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The principle. A lag screw achieves compression by engaging its threads in the far cortex or fragment only, while the shaft glides freely through the near cortex or fragment. As it is tightened, the head contacts the near fragment and the threads pull the far fragment towards it. Continued tightening increases the compression force, friction at the fracture site provides stability, and rotational torque has been converted into compressive force across the fracture.

Two holes make it work. Each has its own diameter:

• Gliding hole in the near cortex or fragment, of a diameter equal to or slightly larger than the screw's outer diameter, so the shaft passes with no thread engagement and no purchase
• Thread hole in the far cortex or fragment, of a diameter equal to the core diameter, so the threads cut into bone during insertion and provide the purchase for compression

Lag by design or by technique. Both achieve interfragmentary compression through the same principle.

A lag screw by design is partially threaded, smooth proximally and threaded distally. It needs a single drill hole of thread-hole size, but its smooth shaft must be longer than the near fragment is thick, and its threads long enough to engage the far fragment. The 6.5mm and 7.3mm cannulated screws are common examples.

A lag screw by technique is a fully threaded screw with a deliberate gliding hole. It is more versatile, since any screw can be used as a lag screw, and it allows precise control of where the compression goes.

Screw selection depends on fracture pattern, bone quality, fixation goals and anatomic location.

Compression or bridging. There are two fundamental strategies. Absolute stability with compression (lag screws, compression plates) suits simple fractures with good bone contact. Relative stability with bridging (locked plates, external fixators) suits comminuted fractures or osteoporotic bone. Screw choice follows the strategy: lag screws, partially threaded or by technique, for compression; locking or position screws for bridging; and locking plate-screw constructs where angular stability is needed.

Matching the bone. Use cortical screws in cortical bone, with bicortical purchase when possible, and cancellous screws in cancellous bone, maximising thread engagement. In osteoporotic bone, use locking screws and consider augmentation.

Screw length. Measure depth accurately with the depth gauge. Go bicortical when strength is needed, and avoid excessive protrusion, which irritates soft tissue.

Osteoporotic bone. Conventional screws rely on friction between plate and bone, which fails in weak bone, whereas locking screws reduce the reliance on bone quality through angular stability. The strategy:

• Prefer locking screws for angular stability
• Consider cement augmentation for critical screws
• Maximise screw length and bicortical purchase
• Avoid compression, which risks fragmentation
• Use plates with greater working length to distribute stress

High-energy trauma. In high-energy injuries the strategy is:

• Bridge plating to preserve soft tissue and biology
• Locking screws for angular stability across the comminution
• Minimise periosteal stripping, using locked plates as internal fixators
• Consider lag screws for the main fragments only
• Leave the comminuted zone unfixed to allow callus formation

Working length is the main stiffness dial. Omitting one screw hole either side of the fracture makes the construct roughly twice as flexible in both compression and torsion. More than three screws per fragment adds little axial stiffness, and keeping the plate close to the bone matters, because increasing the plate-to-bone distance reduces construct stability.

The rule differs by fracture pattern, and this catches people out. In a simple fracture with a small gap and cortical contact, deliberately omit one or two holes each side to allow the micromotion that produces callus. In a comminuted fracture with a large gap and no bone contact, place the innermost screws as close as practicable to the fracture: a long working length without cortical contact failed earlier under dynamic loading.

Screw spacing. Avoid clustering screws, which concentrates stress, and maintain adequate bone bridges between them.

A headless compression screw is buried entirely beneath the articular or cortical surface, so it can cross a joint surface or fix a small bone with no prominent head to impinge. How a screw with no head generates compression is a recurring basic-science viva question.

Differential pitch. The leading (far) threads are coarser than the trailing (near) threads, so they advance further per turn. As the screw is driven, the far fragment is pulled towards the near fragment and the gap closes, producing compression along the screw without any head pressing on the cortex.

A cannulated screw has a hollow axial core so that it can be railed over a pre-placed guidewire. That gives accurate, reproducible and percutaneous placement where the trajectory is critical: the femoral neck, scaphoid, SCFE and sacroiliac joint.

The strength cost is small. The hollow core does reduce bending, torsional and fatigue strength relative to a solid screw of the same outer diameter, but the penalty is modest. Bending stiffness rises steeply with radius, with the fourth power, and the central core is the lowest-stressed material, so removing it costs relatively little. Pullout, a thread-bone property, is essentially preserved.

In practice. Manufacturers often make cannulated designs a touch larger in diameter to offset the difference. The practical cautions are guidewire bending, inadvertent advancement, and breakage during drilling.

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Locking screw technology revolutionised fracture fixation by creating fixed-angle constructs with angular stability.

How locking works. The screw head has threads that engage matching threads in the plate hole, fixing the angle between screw and plate, typically perpendicular. The screw cannot toggle or back out, and load passes through the screw-plate interface rather than through friction.

The internal fixator. A locked plate spans the fracture without compressing the periosteum and needs no contact with bone. It works as an internal fixator, and its biomechanics are fundamentally different from those of a conventional construct.

When to lock. Locking screws have the advantage in:

• Osteoporotic bone - angular stability reduces reliance on pullout strength
• Comminuted fractures - bridge plating without compressing the fragments
• Periarticular fractures - fixed-angle support for subchondral bone
• Periprosthetic fractures - avoid stress shielding while providing stability
• Minimally invasive plating - preserves soft tissue and periosteal blood supply

When not to. Conventional screws keep advantages of their own:

• Compression - interfragmentary compression with lag technique
• Correction - the screw angle can be adjusted to correct alignment during insertion
• Bone contact - plate-bone friction provides additional stability
• Simple fractures - adequate fixation at lower cost
• Revision - easier to remove if needed

Hybrid constructs. Combining the two brings the best of both technologies to complex fracture patterns: lag screws through the plate compress the main fragments, and locking screws in the metaphyseal regions give angular stability.

Locking-specific complications. Locking brings failure modes of its own:

• Cross-threading - the screw threads misalign with the plate threads when insertion is not perpendicular, losing the locking mechanism and angular stability and weakening the construct. Prevented with the drill guide, a perpendicular approach and visual confirmation.
• Cold welding - titanium screw threads gall titanium plate threads, with excessive insertion torque or cyclic loading, making removal difficult or impossible. Avoid over-tightening.
• Stress concentration - the fixed-angle construct concentrates stress at the screw-bone interface, more than toggling non-locking screws do, and can cause screw cutout in osteoporotic bone. Use adequate screw length and consider cement augmentation.

Standard insertion. Four steps:

1. Drill the pilot hole with the drill bit appropriate to the screw's core diameter
2. Measure depth with the depth gauge
3. Tap the hole if the screw is not self-tapping
4. Insert the screw to the appropriate torque

Locking screws. Ensure the plate is properly seated before drilling, and use the threaded drill guide to hold the trajectory. Drill bicortically when possible, although unicortical purchase is adequate with locking. Do not over-torque, because cross-threading damages the mechanism.

Cannulated screws. Placement follows the guidewire:

1. Place the guidewire under fluoroscopy
2. Confirm its position in two planes
3. Drill over the wire, protecting it from spinning
4. Insert the screw, and remove the wire once the screw is seated

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Cement augmentation. The cement goes in before the screw:

1. Create a slightly undersized pilot hole
2. Inject low-viscosity cement into the hole, through a fenestrated screw or by syringe
3. Insert the screw into the cement before polymerisation
4. Hold until the cement sets, 2-3 minutes

Thread stripping. Stripping occurs when insertion torque exceeds the bone's capacity for purchase, and it is more common in osteoporotic bone. Stop at the appropriate torque, when increased resistance is felt at final seating, and avoid overtightening. The salvage is a larger-diameter screw, cement augmentation or an alternative fixation point.

Screw pullout. Purchase is lost when cyclic loading exceeds pullout strength. Osteoporosis, unicortical fixation and short screws are the risk factors; maximising thread engagement, bicortical fixation, locking screws and cement prevent it.

Screw breakage. Screws fail by fatigue under cyclic loading at stress-concentration points, the thread-shaft junction. Delayed union, a high activity level and inadequate construct stability are the risk factors, and adequate construct stability, bridging long segments, prevents it.

Hardware prominence. A screw that is too long or at the wrong angle causes soft-tissue irritation and tendon damage. Accurate measurement and fluoroscopic confirmation prevent it.

Stress shielding. When a rigid construct carries the load, the bone does not remodel normally. It is more common with stiff locking constructs, and plate removal after healing can be considered.

Thermal necrosis. Drilling without irrigation generates heat greater than 47°C, causing bone necrosis and screw loosening. Sharp drill bits, irrigation and intermittent drilling prevent it.

Infection at screw sites. Biofilm forms on the implant surface, and stainless steel is more resistant to adhesion than titanium. Management is debridement, with implant exchange if the implant is loose.

Reading the failing construct. A radiograph showing a problematic screw is a classic viva and CIM scenario, and distinguishing the mode of failure dictates the salvage strategy.

Weight-bearing. Progression is based on construct stability, fracture pattern and bone quality. Locking constructs may allow earlier weight-bearing in osteoporotic bone, whereas lag screw fixation requires protected weight-bearing until healing.

Follow-up. Serial radiographs assess fracture healing and implant position, watching for screw loosening (lucency around the threads), migration or breakage. Clinically, look for screw prominence causing soft-tissue irritation, signs of loosening or migration, and symptoms suggesting hardware failure. Screw back-out suggests inadequate initial purchase or excessive motion at the fracture site, and broken hardware indicates fatigue failure, from nonunion or construct insufficiency, and should prompt investigation for nonunion.

Activity. Restrict high-impact activities during the healing phase, and progress gradually on radiographic and clinical progress, taking account of age, bone quality and compliance.

Hardware removal. Removal is not routine in adults unless the hardware is symptomatic. Consider it in young patients with forearm plates, because of the refracture risk. It is typically done 12-24 months after complete healing.

What decides success. Beyond bone quality, fixation success depends on surgical technique (proper drilling, tapping and insertion torque), construct design (appropriate screw selection for the application) and patient factors (compliance, comorbidities and smoking status).

Union rates. By fixation type:

• Lag screw fixation - 90-95% union for simple fractures
• Conventional plate fixation - 85-95%, depending on fracture pattern
• Locking plate fixation - similar union rates, advantageous in osteoporotic bone

Locking versus conventional. Meta-analyses show equivalent union rates in normal bone, and locking constructs are superior in osteoporotic metaphyseal fractures. Cost-effectiveness favours conventional screws when bone quality is adequate.

Hardware removal rates. Overall, symptomatic hardware runs at 5-15%, higher in subcutaneous locations such as the ankle, wrist and clavicle. Locking screws may be harder to remove.

Timing of failure. Early failure, in less than 6 weeks, usually reflects a technical error or an inadequate construct. Late failure, after 6 weeks, suggests nonunion or fatigue from excessive motion. Screw breakage indicates stress concentration and cyclic loading beyond the fatigue limit.

References

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2. Lee SJ, Lee JH, Lee HJ, Oh JW, Park IH. Pullout strength of pedicle screws using cadaveric vertebrae with or without artificial demineralization. Spine J. 2021;21(9):1580-1586. doi:10.1016/j.spinee.2021.04.010
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