Overview — The Role of Screws in Orthopaedic Fixation
Screws are the most versatile and widely used implants in orthopaedic surgery. They serve as standalone fixation devices (lag screws for interfragmentary compression), as components of plate constructs (plate screws providing plate-bone fixation), as part of intramedullary nail systems (interlocking screws), and as anchoring devices for soft tissue repair (suture anchors, interference screws). Understanding screw anatomy, the biomechanical principles that determine how screws generate compression or stabilise fragments, and the indications and limitations of each screw type is fundamental to orthopaedic surgery. The AO/ASIF group (Arbeitsgemeinschaft für Osteosynthesefragen — Association for the Study of Internal Fixation) systematised the principles of screw fixation in the 1960s, and the terminology and concepts they established remain the foundation of modern implant design and surgical technique.
- AO principles of fracture fixation — how screws fit in: the four AO principles are: (1) Anatomical reduction; (2) Stable fixation; (3) Preservation of blood supply; (4) Early active mobilisation; screws contribute to `stable fixation` either through ABSOLUTE STABILITY (interfragmentary compression — lag screw; eliminates all relative motion at the fracture, promoting primary bone healing without callus) or RELATIVE STABILITY (bridging constructs — positional screws in plates; allows controlled micromotion at the fracture, promoting secondary bone healing with callus); the choice between absolute and relative stability — and therefore the type and application of screws — depends on the fracture morphology, the surgeon`s goals, and the mechanical environment
Screw Anatomy — Terminology & Geometry
Every screw is characterised by a precise set of geometric parameters that determine its mechanical behaviour in bone. Understanding these parameters explains why different screw types behave differently in clinical use.
| Parameter | Definition | Clinical Significance |
|---|---|---|
| Outer diameter (major diameter) | The overall diameter of the screw measured from the outermost tip of one thread to the outermost tip of the opposite thread; the `size` of the screw as it appears from the outside | Determines the size of hole the screw occupies in bone; larger outer diameter = greater pull-out strength (more bone engaged); but also greater stress concentration and bone removal; the drill bit used for the NEAR cortex (gliding hole) matches the outer diameter |
| Core diameter (minor diameter / root diameter) | The diameter of the screw shaft measured at the ROOT of the thread — the smallest diameter within the thread profile | Determines the torsional strength and bending stiffness of the screw shaft; the drill bit for the FAR cortex (thread hole) matches the core diameter so the threads can engage the bone; a larger core-to-outer diameter ratio means the screw is stiffer and stronger but has shallower threads (less bone engagement per thread) |
| Thread pitch | The distance between adjacent thread crests (measured parallel to the screw axis); equivalently, the distance the screw advances per complete turn | A LARGER pitch (coarser threads — further apart) means more advancement per turn and greater pull-out strength in cancellous bone (the threads engage a greater volume of bone per thread); SMALLER pitch (finer threads — closer together) provides more threads per unit length and better purchase in dense cortical bone; cancellous screws have large pitch; cortical screws have small pitch |
| Thread depth (thread height) | The radial distance from the root (core) to the crest of the thread; = (outer diameter − core diameter) / 2 | Deeper threads (greater thread depth) engage more bone and provide greater pull-out strength; cancellous screws have deeper threads relative to their core to maximise bone purchase in soft trabecular bone; cortical screws have shallower threads (the cortical bone is dense enough for shallow threads to provide adequate purchase) |
| Thread profile (thread form) | The cross-sectional shape of the thread — V-shaped, buttress, square, or reverse-angle profiles | V-shaped (standard) — simplest, suitable for general fixation; buttress thread (asymmetric profile — the leading face is perpendicular to the axis, the trailing face is angled) — resists pull-out forces in one direction; used in cancellous screws to resist pull-out; square thread — maximum pull-out resistance; used in some high-performance implants |
| Head design | The shape of the proximal end of the screw and the recess for the screwdriver; hex, cruciate (Phillips-type), Torx (star-shaped), slotted | The head design determines how torque is transmitted to the screw during insertion and removal; Torx (star-drive) provides the best torque transmission and least risk of stripping the screw head; hex is standard for most cortical and cancellous screws; a countersunk (low-profile) head sits flush with or below the bone surface after insertion; a spherical head allows `toggling` within a plate hole (providing multi-axial positioning) |
| Shank (shaft) | The portion of the screw between the head and the threaded region; the shank is smooth (unthreaded) | A fully threaded screw has threads all the way to the head — the threads engage both cortices simultaneously (useful for positional fixation); a partially threaded screw has a smooth shank proximally — the smooth shank slides freely through the near cortex (the gliding hole), allowing the near cortex to be pulled toward the far cortex as the threads engage — this is the BASIS of the LAG SCREW TECHNIQUE |
| Self-tapping vs tapping required | Self-tapping screws have a cutting flute at the tip that cuts threads in the bone during insertion, eliminating the need for a separate tapping step; non-self-tapping screws require a tap to be run through the drill hole before screw insertion to create threads in the bone | Self-tapping screws simplify the procedure and reduce steps; most modern orthopaedic screws (cortical and cancellous) are self-tapping; non-self-tapping screws may provide marginally greater pull-out strength in dense cortical bone (the separately cut threads are more precise) but the difference is not clinically significant in most circumstances; always use a tap in very hard cortical bone (dense diaphysis) to avoid screw head stripping during insertion |
Cortical Screws
- Definition and design: a cortical screw is designed for purchase in DENSE CORTICAL BONE; characterised by: small thread pitch (fine threads — close together, to maximise thread count per unit length in hard bone); shallow thread depth (cortical bone is dense enough for shallow threads to provide adequate grip); small outer-to-core diameter ratio (stiffer core, shallower threads); fully or partially threaded; standard AO cortical screws: 3.5 mm (most commonly used for plates in long bone fractures — the standard AO 3.5 mm cortical screw has a 1.25 mm pitch, 2.4 mm core diameter); 4.5 mm (for larger plates — femoral shaft and tibia plating); 2.7 mm (for small bones — hand, wrist, midfoot); 2.4 mm and 2.0 mm (for very small bones — finger, toe)
- Drilling technique for a cortical screw: (1) drill through BOTH cortices with the CORE DIAMETER drill bit (e.g. 2.5 mm for a 3.5 mm cortical screw — the `tap drill`); (2) measure screw length with a depth gauge; (3) TAP the hole (or use a self-tapping screw); (4) insert the screw; the threads engage BOTH cortices simultaneously; the screw provides POSITIONAL stability (maintains the fragments in position against shear forces) but does NOT generate interfragmentary compression by itself unless used in lag mode
- Fully threaded cortical screw as a positional screw: when a fully threaded cortical screw is inserted through both cortices, the threads engage BOTH cortices and the screw `positions` the fragments but does NOT compress them (compression requires a gliding hole in the near cortex — see lag technique); positional screws are used when compression is undesirable (e.g. fixing a distracted fracture to maintain distraction — the `neutralisation` mode)
Cancellous Screws
- Definition and design: a cancellous screw is designed for purchase in SOFT TRABECULAR (cancellous) BONE, which has much lower density and pull-out strength than cortical bone; to compensate for the weaker bone, cancellous screws have: LARGE thread pitch (coarse threads — widely spaced, to engage large volumes of cancellous bone per thread); DEEP threads (large thread depth relative to core — to maximise the bone engaged per thread); wider outer diameter relative to core; partially threaded (the standard AO cancellous screw has a smooth shank — the smooth proximal portion acts as a gliding hole through the near cortex, converting the cancellous screw into a LAG SCREW when used across a fracture); standard AO cancellous screws: 6.5 mm partially threaded (standard for metaphyseal and epiphyseal fixation — femoral neck, tibial plateau, distal radius, calcaneus); 4.0 mm partially threaded (smaller cancellous bone — medial malleolus, small periarticular fractures); 7.0 mm and 7.3 mm (large cancellous — femoral neck in younger patients, sacrum, pelvis)
- Thread length options: cancellous screws come in various thread lengths — 16 mm, 32 mm (for the 6.5 mm screw); the thread should span ONLY the far fragment (the fragment that needs to be pulled toward the near cortex); if the threads engage BOTH sides of the fracture, the screw acts as a POSITION screw (no compression); if the threads are entirely within the FAR fragment, the smooth shank glides through the near cortex/fragment, generating compression as the head engages the near cortex and pulls the far fragment toward it — this is the CANCELLOUS LAG SCREW principle
- Washer use with cancellous screws: in osteoporotic bone, the screw head may sink into the soft cancellous bone surface under compression — `head sinkage`; a WASHER distributes the head load over a wider area, reducing the contact stress at the bone-implant interface and preventing head sinkage; washers are routinely used with cancellous screws in osteoporotic patients (tibial plateau fixation in the elderly, calcaneal fractures); the washer must be of appropriate size — too small provides no benefit; too large may impinge on adjacent structures
The Lag Screw Technique — Interfragmentary Compression
The lag screw principle is arguably the single most important concept in orthopaedic screw fixation. It describes the method by which a screw generates compressive force across a fracture line, achieving absolute stability and enabling primary bone healing. The principle can be applied using any screw type — a specifically designed lag screw (partially threaded), or any fully threaded screw used in `lag fashion` by overdrilling the near cortex.
- The lag screw principle — mechanism: the lag screw generates interfragmentary COMPRESSION by a differential thread engagement mechanism; the key is a GLIDING HOLE in the NEAR cortex (the cortex closest to the screw head) that is the same diameter as the OUTER DIAMETER of the screw — the threads cannot engage the near cortex (they pass through freely); in the FAR cortex (the far fragment), a THREAD HOLE is drilled at the CORE DIAMETER — the threads DO engage the far cortex; as the screw is tightened: the head engages the near cortex and pushes against it; the threads engage the far fragment and pull it toward the head; the result is that the far fragment is pulled TOWARD the near fragment → COMPRESSION at the fracture interface; this compression is the mechanical basis of absolute stability and primary bone healing
- Two methods to achieve the lag effect: (1) PARTIALLY THREADED screw (the `true lag screw`) — the smooth shank is the same diameter as the outer diameter of the thread; the smooth shank automatically provides a gliding hole in the near cortex if the near cortex is drilled with the outer diameter bit; standard cancellous screws (6.5 mm, 4.0 mm) are designed this way; (2) FULLY THREADED screw used in `lag fashion` — a fully threaded cortical screw can be used as a lag screw by OVERDRILLING the NEAR CORTEX with the outer diameter bit (creating a `gliding hole` manually) and drilling the far cortex with the core diameter bit; this converts a standard cortical screw into a lag screw; this `technique-dependent lag` is commonly used with cortical screws in long bone fracture fixation
- Optimal lag screw orientation: to maximise interfragmentary compression, the lag screw should be directed PERPENDICULAR TO THE FRACTURE PLANE — this generates the maximum compressive force directly across the fracture surface; however, a screw perpendicular to a spiral fracture may not be orthogonal to the bone`s long axis; the screw should also be as perpendicular to the BONE`S LONG AXIS as practical to resist the bending moments applied to it; in practice, the ideal lag screw orientation is at an angle that bisects the angle between `perpendicular to fracture` and `perpendicular to bone axis` — called the `bisecting angle` technique; for long spiral fractures (e.g. humeral shaft), multiple lag screws are placed at intervals along the fracture to prevent rotation
- Lag screw + neutralisation plate: a lag screw alone can generate excellent compression but may fail under functional loading (bending and torsional forces) because a single screw is a point fixation; a NEUTRALISATION PLATE is applied alongside the lag screw(s) to `neutralise` (protect) the lag screw from these forces while the fracture heals; the plate carries the bending and torsional loads; the lag screw carries the compressive load; this combination is the standard technique for long bone spiral and oblique fractures (distal radius ORIF, fibula ORIF, humeral shaft ORIF)
| Step | Cortical Screw in Lag Mode (e.g. 3.5 mm cortical) | Cancellous/True Lag Screw (e.g. 6.5 mm partially threaded) |
|---|---|---|
| 1. Drill near cortex (gliding hole) | 3.5 mm drill bit (= outer diameter of the 3.5 mm cortical screw) through the NEAR CORTEX ONLY (protect near fragment from drilling through); this creates a hole that the threads CANNOT engage — the screw slides freely through the near cortex | The smooth shank of the cancellous screw (= outer diameter) automatically provides the gliding effect — the smooth shank passes freely through the near cortex/fragment without thread engagement; the smooth shank diameter IS the gliding hole |
| 2. Drill far cortex (thread hole) | 2.5 mm drill bit (= core diameter of the 3.5 mm cortical screw) through the FAR CORTEX/FRAGMENT ONLY; this creates a hole that the threads WILL engage | Drill the far fragment with the appropriate cancellous core diameter bit if the bone is very dense; in soft cancellous bone, the cancellous screw can be self-tapping (no pre-drilling required) — the threads cut through the cancellous trabeculae as the screw is inserted |
| 3. Tap (if not self-tapping) | Tap the far cortex only (using the 3.5 mm tap); do NOT tap the near cortex (the threads must not engage the near cortex) | Most cancellous screws are self-tapping; tapping required only in dense subchondral bone |
| 4. Insert screw | The 3.5 mm cortical screw is inserted; as it is tightened, the head engages the near cortex, the smooth shank glides freely through the near cortex, and the threads engage the far cortex; tightening pulls the far fragment toward the near fragment → COMPRESSION | The smooth shank passes through the near fragment; the threads engage the far fragment; tightening generates interfragmentary compression |
Locking Screws & Locking Plate Constructs
- Conventional vs locking screws — a fundamental difference: a CONVENTIONAL screw achieves fixation by friction between the screw head and the plate hole (the screw is tightened against the plate, drawing the plate to the bone — the plate-bone interface generates friction); the screw head is spherical and fits in a rounded hole in the plate, allowing slight angulation during tightening; the plate is pressed against the bone by the friction of the screw head; in osteoporotic bone, conventional screws can pull out of the bone because the only thing holding the screw in the bone is the thread-bone interface, and in weak bone this may be insufficient; a LOCKING SCREW has a threaded head that LOCKS into a threaded hole in the plate; the screw head does not produce friction with the plate — instead it `locks` mechanically to the plate at a fixed angle; the locking screw therefore acts as a `bolt` that is rigidly connected to the plate; the pull-out force on a locking screw must overcome the strength of the bone-thread interface plus the angle-stability of the plate-screw connection simultaneously — this is significantly stronger than a conventional screw in osteoporotic bone
- Internal fixator concept: because locking screws are angle-stable (fixed to the plate at a fixed angle), the locking plate-screw construct behaves as an `internal fixator` — the screws do not need to press the plate against the bone; the plate can be placed at a distance from the bone (bridging) without losing fixation; this is the basis of MIPO (Minimally Invasive Plate Osteosynthesis) — the plate is slid through a small incision and tunnelled along the bone, with locking screws inserted percutaneously through stab incisions; the periosteal blood supply is preserved because the plate does not strip the periosteum
- Locking screw pull-out strength: in NORMAL bone, conventional screws and locking screws have similar pull-out strength; the advantage of locking screws is most pronounced in OSTEOPOROTIC bone; in osteoporotic bone, locking screws are significantly stronger in pull-out because: (1) the angle-stable construct distributes the pull-out force across all locking screws in the plate simultaneously (they must all fail together — the `fixed-angle construct`); (2) a conventional screw in osteoporotic bone can `toggle` progressively (the head sinks into the soft bone and the screw slowly loses purchase); a locking screw cannot toggle because it is angle-stable; key principle: locking screws MUST be placed perpendicular (axially aligned) to the plate hole — a locking screw inserted at an angle will cross-thread and fail to lock properly; this is a common technical error
- Combi-hole plates (combination plates): many modern plates have `combination holes` that accept BOTH conventional and locking screws in the same hole; the oval/compression portion of the hole accepts a conventional screw (allowing axial compression along the oval); the circular threaded portion accepts a locking screw; this allows the surgeon to use conventional screws for anatomical reduction and plate-bone apposition, and locking screws for angle-stable fixation in the same construct
- Failure modes of locking constructs: unlike conventional plate constructs that typically fail by screw pull-out, locking constructs fail differently — because the screws are rigidly connected to the plate, the construct `works as a unit`; failure modes include: (1) screw BREAKAGE (fatigue failure — the rigidity of the locking construct concentrates stress at the screw-plate interface); (2) plate BREAKAGE (at the level of an empty hole or at the fracture zone); (3) bone failure (the screws pull out a block of bone rather than individual screws pulling out); locking construct stiffness can be EXCESSIVE — in complex fractures with bone gaps, the rigid construct provides no stimulus for secondary healing (callus formation); this is the paradox of locking plates in comminuted fractures — too rigid to allow callus, not stable enough to achieve primary healing; the solution is `relative stability` — leave a small gap and allow some micromotion to stimulate callus
Cannulated Screws
- Design: cannulated screws have a hollow central channel (the `cannula`) running along the entire length of the screw shaft; the cannula allows the screw to be advanced over a pre-placed GUIDEWIRE; a guidewire (typically 1.14–2.0 mm) is inserted first under fluoroscopic guidance, positioning it precisely in the intended screw trajectory; the screw is then advanced over the guidewire, maintaining the exact trajectory confirmed fluoroscopically; the guidewire is then removed; available in cortical, cancellous, and headless compression configurations; the cannulation weakens the screw slightly (reduced core diameter) but this is not clinically significant in most applications
- Key advantages: (1) ACCURACY — the guidewire is visualised under fluoroscopy before committing to the final screw; if the guidewire position is suboptimal, it can be repositioned before drilling — impossible with a solid screw; (2) MINIMALLY INVASIVE — the guidewire can be placed percutaneously through a tiny stab incision; the screw is then placed over it; standard applications: cannulated lag screws for femoral neck fractures (3 parallel cannulated screws — standard for undisplaced femoral neck fractures); cannulated screws for scaphoid fixation (the Herbert screw system uses a cannulated technique to place the screw along the central scaphoid axis); hip screw systems (DHS guidewire placement); cannulated hip screws for paediatric hip fractures (Delbet Type III/IV)
- Disadvantages: more expensive than solid screws; the cannula reduces torsional strength slightly; the guidewire can break within the bone if over-advanced; in femoral neck fractures, the three-screw `inverted triangle` configuration (two inferior screws along the calcar + one superior screw) provides optimal resistance to varus collapse — the inferior calcar screw (placed along the medial femoral neck) is the most important of the three for preventing varus; all three screws should be within 5 mm of the subchondral bone of the femoral head to maximise purchase in the dense subchondral bone
Headless Compression Screws (Herbert-Type)
- Design principle: a headless compression screw (HCS) is a screw with NO HEAD — it has threads at BOTH the leading (distal) and trailing (proximal) ends, with DIFFERENT THREAD PITCHES at each end; the leading thread has a COARSER pitch (advances more per turn) than the trailing thread (FINER pitch — advances less per turn); as the screw is tightened, the leading threads advance faster into the far fragment than the trailing threads retreat from the near fragment — the net effect is that the two fragments are PULLED TOGETHER (compressed) as the screw advances; this is the `differential pitch` compression mechanism; because there is no head, the screw can be completely countersunk below the articular surface or the bone cortex, making it ideal for intra-articular or subcutaneous bone fixation where a protruding screw head would be symptomatic or damaging to adjacent articular cartilage
- Classic examples: Herbert screw (Tim Herbert, 1984 — designed for scaphoid fixation): the original headless compression screw; the leading end has a coarser pitch than the trailing end; the screw is inserted from the distal scaphoid pole along the central axis; after insertion it is completely buried within the scaphoid bone; the countersunk position prevents cartilage damage at the STT or scaphocapitate joint; Acutrak screw (fully cannulated; variable pitch — continuously increasing pitch from proximal to distal, providing progressive compression along the entire screw length; the most widely used scaphoid screw); headless compression screws are also used for: metacarpal and phalangeal fractures; DIP joint arthrodesis; metatarsal fractures; osteochondral fragment fixation; tibial plateau fracture fixation
- Technical points: the screw MUST be completely buried below the bone/cartilage surface — failure to countersink the trailing end causes articular cartilage damage; the guidewire must be placed precisely along the CENTRAL AXIS of the bone before screw insertion (verified on two perpendicular fluoroscopic views); the `rule of central thirds` for scaphoid screws — on both AP and lateral fluoroscopic views, the screw should be in the central third of the scaphoid; off-axis placement predicts non-union because the screw does not generate axial compression along the scaphoid`s load axis
Malleolar Screws
- Design: the malleolar screw (standard: 4.0 mm) is a partially threaded cancellous screw with a fully threaded tip and a smooth shank — specifically designed for fixation of the medial malleolus; it combines the lag screw properties of the 4.0 mm cancellous screw with a broader, more dome-shaped (round) head that provides a larger footprint on the smooth cortex of the medial malleolus, distributing the compressive load and reducing the risk of head sinkage; the screw is inserted as a lag screw for medial malleolus fractures — the smooth shank glides through the medial cortex of the distal tibia and the threads engage the fracture fragment; two malleolar screws (or one malleolar screw + one anti-rotation K-wire) are used for medial malleolus fixation to prevent rotation of the malleolar fragment under applied load; the screw is directed slightly proximally to engage the denser metaphyseal bone of the distal tibia rather than the soft subchondral cancellous bone near the joint
- Alternative: the medial malleolus can also be fixed with a partially threaded 4.5 mm cancellous screw or with tension band wiring (for very small fragments that cannot accept a screw); the choice depends on fragment size and bone quality
Interference Screws
- Design and mechanism: an interference screw does not fix a fracture — it fixes a SOFT TISSUE GRAFT within a bone tunnel; specifically used for graft fixation in ACL reconstruction (and other ligament reconstructions); the screw is inserted alongside the graft within the bone tunnel; as the screw advances, it `interferes` (impinges) against the graft and the tunnel wall simultaneously, generating a friction and compression force that holds the graft within the tunnel; the screw`s outer surface presses the graft against the tunnel wall; the graft is immobilised by the friction forces; as the graft incorporates into the bone tunnel (ligamentisation — 12–24 months), the mechanical hold transfers from screw-to-graft friction to biological bone-to-graft integration; interference screws are commonly made from: (1) metal (titanium — strong, produces MRI artefact, may require removal if the tunnel needs to be re-drilled in revision ACL surgery); (2) bioabsorbable (PLLA, PGA, TCP-composite — resorbs over 1–3 years, no removal required, less MRI artefact); (3) PEEK (polyetheretherketone — strong, biologically inert, no artefact, no resorption)
- Sizing: the interference screw should match or slightly exceed the tunnel diameter — for hamstring graft ACL reconstruction, the screw diameter is typically equal to the tunnel diameter (to provide maximum interference/friction); for BPTB graft, the screw diameter matches the bone plug diameter; the screw should be fully seated within the tunnel (the trailing end flush with or just below the tunnel aperture) to avoid cortical protrusion
Bioabsorbable Screws
- Materials: bioabsorbable (bioresorbable) screws are made from polymers that are gradually degraded and resorbed by the body over months to years; common materials: PLLA (poly-L-lactic acid) — degrades in 2–5 years; PGA (polyglycolic acid) — degrades faster (3–6 months); PLGA (copolymers of PLLA and PGA) — intermediate degradation; TCP (tricalcium phosphate) composites — ceramic-polymer hybrids with better biocompatibility; PEEK (polyetheretherketone) — not truly bioabsorbable but biologically inert and stable
- Advantages: no hardware removal required; no MRI artefact (or minimal); no stress shielding from a permanent metallic implant; bone eventually fills the resorption space; ideal for paediatric patients (avoids implant removal operation); disadvantages: lower initial strength than metal; risk of aseptic foreign body reaction (inflammatory reaction to resorption products — can mimic infection); risk of screw breakage during insertion (brittle material — requires pre-drilling and gentle technique); delayed resorption leaving a fluid-filled cyst; not suitable for high-load fixation (not appropriate for femoral neck screws or high-stress environments)
- Clinical applications: small bone fixation (hand and wrist — phalangeal and metacarpal fractures, especially in children); osteochondral fragment fixation; meniscal repair devices; ACL graft fixation; paediatric fracture fixation (avoiding the need for implant removal operations)
Positional Screws & Syndesmotic Screws
- Positional screw: a screw used NOT to generate compression but to MAINTAIN POSITION of two structures relative to each other without compression; classically — the syndesmotic screw (also called the position screw); the syndesmosis is the fibrous joint between the distal tibia and fibula, stabilised by the anterior and posterior inferior tibiofibular ligaments and the interosseous membrane; a syndesmotic disruption (from high-energy ankle fractures — Lauge-Hansen Pronation-External Rotation Stage IV, Weber C fibula fractures with deltoid disruption, Maisonneuve fractures) allows the fibula to drift laterally, widening the mortice and destabilising the ankle; a syndesmotic screw HOLDS the fibula in its reduced position relative to the tibia while the syndesmotic ligaments heal — it does NOT compress the syndesmosis (compression would narrow the mortice, causing pain and restricted dorsiflexion)
- Technique: the ankle is held in maximum dorsiflexion (foot at 90°) during syndesmotic screw insertion (or a bone spreader is used to maintain the correct fibular reduction) — this ensures the mortice is at its widest, preventing over-tightening which would restrict dorsiflexion; the screw passes through both fibular cortices and through the lateral cortex of the tibia; 3-cortex (bicortical fibula + unicortical tibia) vs 4-cortex (bicortical fibula + bicortical tibia) fixation is debated: 4-cortex provides stronger fixation but higher screw breakage rate; 3-cortex is more elastic and has lower breakage rate; number of screws: 1 or 2 syndesmotic screws (2 screws for higher-energy injuries with greater syndesmotic disruption)
- Syndesmotic screw removal vs retention: the question of whether to routinely remove syndesmotic screws is debated: PRO removal — the syndesmotic screw restricts normal fibular rotation during dorsiflexion (the fibula rotates externally and translates proximally during dorsiflexion — a screw fixing the fibula to the tibia theoretically restricts this motion); screws may break with weight-bearing if left permanently; AGAINST removal — the operation has its own risks; many patients do well without removal; broken screw fragments may not be symptomatic; current practice: many surgeons remove syndesmotic screws at 8–12 weeks before full weight-bearing, while others leave them unless symptomatic; newer suture-button devices (TightRope) do not require removal and allow physiological fibular motion — increasingly replacing screws for syndesmotic fixation
Screw Failure Mechanisms
| Failure Mode | Mechanism | Risk Factors | Prevention |
|---|---|---|---|
| Pull-out (axial failure) | The screw is pulled out of the bone in the axial direction (along the screw axis); the threads strip through the bone trabeculae; the most common failure mode in cancellous bone | Osteoporotic bone (low bone density reduces trabecular engagement per thread); undersized screw (too small outer diameter); inadequate thread engagement (screw too short — threads not fully within bone); re-use of previously tapped holes; holes too close together | Use locking screws in osteoporotic bone; correct screw length (threads fully within bone); bicortical purchase where possible; washers to distribute load; augmentation with bone cement (PMMA) in severely osteoporotic metaphyseal bone |
| Toggling (rocking) | The screw rocks back and forth within the bone hole under cyclic loading; this gradually enlarges the hole and destroys the bone-thread interface; progressive loosening without frank pull-out; results in implant failure and loss of reduction | Soft cancellous bone; conventional (non-locking) screws in a plate construct in osteoporotic bone; overloading before bone healing; inadequate plate length (stress concentration) | Locking screws (angle-stable — cannot toggle); adequate plate length; avoid early weight-bearing before sufficient healing; augment with bone cement if severe osteoporosis |
| Fatigue breakage | Cyclic loading causes crack initiation at a stress concentration point (the thread root — the `root radius` where stress is highest; or the screw-plate interface in locking constructs) and progressive crack propagation until the screw fractures; the screw breaks without exceeding its static yield strength | Delayed union or non-union (the fracture fails to heal, leaving the screw bearing full cyclic load indefinitely); overly rigid constructs (insufficient load sharing with bone); screw placed in empty plate holes over the fracture (bending moment concentrated at empty hole); syndesmotic screws left too long | Ensure fracture healing conditions are optimal (avoid `stress shielding` from excessively rigid constructs); remove syndesmotic screws before full weight-bearing; bone grafting for non-union before the implant fails; do not place screws in empty plate holes directly over the fracture (working length) |
| Stripping (torsional failure during insertion) | The screw strips during insertion — either the bone threads strip (the screw spins freely without advancing — `stripped hole`) or the screw head strips (the screwdriver no longer grips the head recess — `stripped head`); stripped bone threads leave the screw with no hold in the bone; stripped head makes the screw unremovable | Bone stripping: over-tightening; soft cancellous bone; incorrect drill hole size (too large for the screw); soft bone + under-drilling (too much resistance, causing stripping under torque); Head stripping: use of wrong screwdriver (mismatched head design); worn or poorly fitting screwdriver bits; excessive torque | Bone stripping: use a torque-limiting screwdriver; use correct drill size; locking screws in osteoporotic bone (angle-stable — less reliant on over-tightening); for a stripped hole — redirect a new screw 5 mm away, use a larger diameter screw, or augment with bone cement; Head stripping: use the correctly fitting screwdriver bit; inspect driver bits for wear; PEEK-tipped drivers reduce slippage |
| Cut-out | The screw cuts through the articular surface or epiphyseal bone into the joint (most common with hip screws — the lag screw or DHS screw cuts superiorly through the femoral head into the hip joint); not a mechanical failure of the screw itself — a failure of bone to support the implant | Varus malreduction (the screw is in a superior position within the femoral head — prone to superior cut-out under the compressive hip joint forces); excessive TAD (Tip-Apex Distance) >25 mm (the screw tip is too far from the subchondral bone of the femoral head — the region with the highest bone density); osteoporotic femoral head | Anatomical (or valgus) reduction for intertrochanteric fractures before screw insertion; TAD <25 mm (the most important modifiable predictor of cut-out for DHS/cephalomedullary nail screws); central screw placement in the femoral head on AP view; inferior screw placement on the lateral view (`inferior-central` position is optimal) |
Exam Pearls
- Cortical screw: small pitch, shallow threads, both cortices engaged, stiff core; drill BOTH cortices with CORE diameter; standard sizes: 3.5 mm (most common), 4.5 mm; lag mode = overdrill near cortex with OUTER diameter bit to create gliding hole
- Cancellous screw: large pitch, deep threads, designed for soft cancellous bone; partially threaded (smooth shank = automatic gliding hole through near cortex = built-in lag effect); 6.5 mm (metaphyseal/epiphyseal standard), 4.0 mm (small periarticular), 7.3 mm (femoral neck, sacrum)
- Lag screw principle: gliding hole (outer diameter) in NEAR cortex + thread hole (core diameter) in FAR cortex = screw head engages near cortex, threads pull far fragment toward near = INTERFRAGMENTARY COMPRESSION = absolute stability = primary bone healing; optimal orientation = perpendicular to fracture plane (maximum compression)
- Locking screws: threaded head locks to plate at fixed angle (angle-stable); behaves as `internal fixator`; greatest advantage in OSTEOPOROTIC bone (pull-out resistance much greater than conventional screws); MUST be inserted perpendicular to plate hole; failure mode = screw BREAKAGE (not pull-out) — rigidity concentrates stress at screw-plate interface
- Headless compression screw (Herbert/Acutrak): differential pitch = leading threads coarser than trailing threads = compression generated as screw advances; countersunk below articular surface = no head prominence; scaphoid fixation (central axis placement — `rule of central thirds`); confirm on two fluoroscopic views before advancing
- TAD (Tip-Apex Distance): sum of distances from lag screw tip to apex of femoral head on AP + lateral views; TAD <25 mm = low cut-out risk; TAD >25 mm = significantly increased cut-out risk; the most important intraoperative measurement for DHS and cephalomedullary nail positioning in hip fractures
- Syndesmotic screw = POSITIONAL screw (NOT a compression screw): maintains fibula position, does NOT compress syndesmosis; insert with ankle in maximum dorsiflexion; 3-cortex vs 4-cortex fixation debated; remove at 8–12 weeks OR use suture-button (TightRope) to allow physiological fibular motion; syndesmotic screws break under load if left permanently
- Interference screw in ACL reconstruction: friction fixation of graft within tunnel; metal (strong, MRI artefact) vs bioabsorbable (resorbs 1–3 years, less artefact, risk of foreign body reaction) vs PEEK (inert, no artefact, no resorption); screw diameter = tunnel diameter for maximum interference