Bone Screws

Basic Mechanics

• Fn - normal (compressive) force against the screw head
• Ft - tangential (frictional) force along the thread
• Fr - resultant of the two forces
• Alpha (α) - angle of the screw thread; finer thread = smaller α = lower frictional force

Types of Bone Screws

1. Cortical Screws

• Designed for dense cortical bone
• Small pitch with shallow thread depth - optimizes insertion and holding power in hard bone
• Threaded their entire length
• Can function as positional screws (holds fragments without compression) or as lag screws (if the near cortex hole is over-drilled)
• Available in multiple diameters (e.g., standard 4.5 mm)

2. Cancellous Screws

• Designed for softer metaphyseal/cancellous bone
• Large pitch with deeper thread depth - maximizes purchase in porous bone
• More frequently used in metaphyseal regions (e.g., femoral neck, distal femur, tibial plateau, medial malleolus)
• The malleolar screw (4.5 mm) is a special variant with a self-tapping trephine tip
• Washers (plastic or metal) are often used with cancellous screws to increase surface area for compression and for reattaching ligamentous avulsions

Examples of cancellous and cortical screws for fracture fixation (Campbell's Operative Orthopaedics, 15th Ed)

3. Lag Screws

• Lag by technique: Drill an oversized glide hole in the near fragment (matching or larger than screw's major diameter), then drill a smaller hole in the far fragment approximating the minor diameter. As the screw tightens, the far fragment is drawn toward the near fragment.
• Lag by design (partially threaded screws): The unthreaded shaft glides through the near fragment automatically. The threaded portion must not cross the fracture line, or compression is lost.

Lag screw generating interfragmentary compression across a fracture (Rockwood & Green's, 10th Ed)

4. Cannulated Screws

• Hollow core allowing placement over a guidewire
• Useful for percutaneous applications: femoral neck fixation, sacrum/SI joint screw fixation
• Tradeoff: increased core diameter reduces thread depth and may compromise pullout strength; higher cost; potentially increased infection risk due to greater implant surface area

5. Self-Tapping / Self-Drilling Screws

• Have a small cutting flute at the tip to remove bone debris - no separate tapping step needed
• Available in the same sizes as cortical screws
• Slightly less pullout strength than pre-tapped screws due to their design
• Frequently used in external fixation pins

6. Locking Screws

• Have threads on the screw head that mate with matching threads in the plate hole
• Create a fixed-angle construct between screw and plate - do not rely on plate-to-bone friction
• Generally have larger core diameters and shallower thread depths (emphasizing bending strength over stripping torque)
• Advantages:
  • Particularly useful in poor bone quality (osteoporosis) or short fixation segments
  • Common locations: proximal humerus, distal radius, distal femur
  • Prevent disruption of fracture reduction (locked screws don't draw bone to plate)
• Disadvantages: higher profile (soft tissue irritation), significantly more expensive, may create too stiff a construct for callus formation

Screw Fixation Technique (AO/ASIF Principles)

• Threads are more horizontal than machine screws
• Originally non-self-tapping (drill hole must be pre-tapped)
• Modern screws are self-tapping, in many sizes/lengths, using hexagonal or torx-type screwdrivers
• Cortical, cancellous, and malleolar designs are all available

1. Reduce fracture; secure with forceps or provisional K-wires
2. Plan screw to enter the middle of the fragment, equidistant from fracture edges, directed perpendicular to the fracture line (maximizes compression)
3. Drill the glide hole in the near cortex
4. Use a "top-hat" drill guide to drill the thread hole in the far fragment, co-linear with the glide hole
5. Countersink the near cortex to increase contact area and reduce cracking risk
6. Insert the screw

Summary Table

• Rockwood and Green's Fractures in Adults, 10th Ed (2025) - Campbell's Operative Orthopaedics, 15th Ed (2026)*, Chapters 12 and 58

Parts of a Bone Screw

Structural Diagram

Design parameters of an orthopaedic bone screw (Campbell's Operative Orthopaedics, 15th Ed)

Screw nomenclature showing major diameter, crest, pitch, thread depth, and root (Rockwood & Green's, 10th Ed)

1. Head

• Engages the screwdriver - the drive recess determines how torque is applied. Common designs:
  • Hexagonal (most ASIF/AO screws)
  • Torx (star-shaped, used by many modern manufacturers)
  • Phillips / cruciate
  • Slotted
• Acts as a counterforce (buttress) - as the screw advances, the head contacts the near cortex or plate and prevents the screw from passing through entirely. This is how compression is generated: the head pushes a plate forward or drives one bone fragment toward another.

In locking screws, the head has peripheral threads that engage matching threads in the plate hole, creating a fixed-angle construct rather than relying on friction.

Countersinking - a shallow conical recess can be cut into the near cortex to receive the screw head flush with the bone surface, increasing contact area, reducing crack risk, and lowering implant prominence.

2. Shaft (Shank)

• In a fully threaded screw, the shaft is minimal or absent - threads run to the head.
• In a partially threaded screw (e.g., cancellous lag screws), the shaft is intentionally left smooth so it can glide freely through the near fragment without purchasing it - this is the basis of the lag by design mechanism.

• Minor diameter (core/root diameter) - the inner diameter of the screw, measured at the base of the threads. This determines bending strength. A wider root diameter = greater resistance to fatigue fracture.
• Major diameter (thread/outer diameter) - the outer diameter measured at the thread crests. This is what is called the "screw size" clinically (e.g., 4.5 mm cortical screw).

3. Thread

Thread Parameters

Thread Cross-Sectional Design

• Buttress thread (used in ASIF/AO screws) - asymmetric, with one near-vertical face and one sloped face. Resists pullout forces very effectively.
• V-thread (used in machine screws) - symmetric V-shape. Used less commonly in orthopaedic implants.

Thread Coverage (Partial vs. Full)

• Fully threaded screws - threads run the full length; used as positional screws or lag screws by technique (over-drilling the near cortex).
• Partially threaded screws - smooth shaft proximally, threads only distally; inherently function as lag screws by design.

Cortical vs. Cancellous Thread Design

4. Tip

• Blunt/round tip - requires the drill hole to be pre-tapped before insertion. A separate tap instrument (matching the screw's diameter and thread profile) is used to pre-cut the thread path in bone, reducing friction and insertion torque.
• Self-tapping tip - has sharp cutting flutes (typically 2-4 flutes) or a trocar point that cut the thread path as the screw is inserted, eliminating the need for a separate tapping step. Screws with four cutting flutes are easiest to insert and have the greatest holding power.
  • Tradeoff: slightly less pullout strength than pre-tapped screws in dense bone, because the cutting flutes remove some bone material.

In cancellous bone, pre-tapping may actually decrease pullout strength (it removes the very trabeculae the threads need to grip), so self-tapping tips are preferred. Exception: very dense cancellous bone (e.g., young patients with femoral neck fractures) may need tapping to prevent excessive friction rotating the bone fragment during insertion.

Summary: All Parts at a Glance

HEAD
 ├── Drive recess (hex, torx, Phillips, slotted)
 └── Bearing surface / buttress (or threaded in locking screws)

SHAFT (SHANK)
 ├── Major diameter = outer/thread diameter (= nominal screw size)
 ├── Minor diameter = core/root diameter (→ bending strength)
 └── Smooth portion = unthreaded in partially threaded screws

THREAD
 ├── Crest (outermost peak)
 ├── Root (innermost valley)
 ├── Pitch (crest-to-crest distance)
 ├── Lead (advance per full turn)
 ├── Thread depth = (major − minor diameter) / 2  (→ pullout & stripping torque)
 └── Thread profile: buttress (AO) or V-thread

TIP
 ├── Round/blunt → requires pre-tapping
 └── Self-tapping → fluted cutting tip (2–4 flutes)