Orthopedic Implants: Screws, Plates, Nails, and Drill Bits

1. OVERVIEW

• Stainless steel - historically dominant; used in plates, sliding hip screws, some flexible nails
• Titanium alloy - preferred today for most IM nails, femoral stems; modulus closer to bone, higher fatigue strength, better MRI compatibility
• Cobalt-chromium - used in joint replacement bearings

2. SCREWS

Screw anatomy: The root (minor/core) diameter determines bending strength. Pitch = distance between threads = how far the screw advances per full turn.

Screw Anatomy (from Rockwood & Green 10th Ed, 2025)

Types of Screws by Bone Type

A. Cortical Screws

• Used in: Dense diaphyseal (shaft) cortical bone
• Design: Small pitch, shallow thread depth
• Sizes: Typically 3.5 mm, 4.5 mm (large fragment); 2.7 mm, 2.0 mm (small fragment)
• Drill bit used = minor (core) diameter of screw

B. Cancellous Screws

• Used in: Metaphyseal/epiphyseal cancellous (spongy) bone
• Design: Large pitch, deep thread depth to maximize purchase in porous bone
• Sizes: 4.0 mm (small), 6.5 mm / 7.3 mm (large, used in femoral neck)
• Can be fully threaded or partially threaded

C. Malleolar Screws

• Used specifically for ankle fracture fixation (medial malleolus)
• Features a larger head and partially threaded shaft

Types of Screws by Function

Lag Screw Technique (detailed)

1. Drill a glide hole in near fragment = same diameter as screw's major (thread) diameter
2. Use a "top-hat" drill guide to drill a pilot hole in far fragment = minor (core) diameter
3. As screw advances, threads bite only the far fragment; head compresses near fragment = interfragmentary compression

• Both holes must be co-linear and perpendicular to fracture line for maximal compression

• Thread-free shaft glides through near fragment automatically
• Used for cancellous lag screws (e.g., 6.5 mm cancellous across femoral neck)

Tapping

Bicortical vs. Unicortical Screws

• Bicortical: Penetrate both cortices; better purchase, greater pullout resistance - standard for diaphyseal plating
• Unicortical: Remain in proximal cortex only; used in locking plate systems; less stress shielding, lower refracture risk on removal

3. PLATES

Types of Plates by Function

A. Dynamic Compression Plate (DCP) / LC-DCP

• Oval "load-screw holes" with an inclined ramp
• As a screw is tightened eccentrically in the hole, the plate slides and generates compression at the fracture site
• Used for transverse/short oblique diaphyseal fractures requiring absolute stability
• Primary bone healing (Haversian remodeling) expected - no visible callus

B. Locking Compression Plate (LCP)

• Combination holes accept both standard screws (compression mode) and locking screws (angle-stable mode)
• Locking screws have threaded heads that lock into the plate, creating a fixed-angle construct
• The plate does NOT need to contact bone - acts as an internal fixator
• Does not rely on plate-to-bone friction = works in osteoporotic bone where conventional screws pull out
• Single most important innovation in fracture fixation in the last 30 years

C. Buttress Plate

• Undercontoured plate applied at vertically oriented partial articular fractures
• Prevents axial collapse; an "axillary screw" in the stable segment generates compression
• Used at: distal radius, proximal tibia, posterior malleolus (distal tibia)
• Produces absolute stability = primary bone healing

D. Antiglide Plate

• Prevents shortening (shearing) in oblique fractures
• Does not necessarily apply compression (distinguishes it from buttress)
• Classic use: posterior/posterolateral plating of distal fibula

E. Bridge Plate

• Used for highly comminuted fractures where screw purchase across all fragments is not possible
• Spans the comminuted zone; screws only in proximal and distal intact segments
• Provides relative stability = secondary bone healing with callus formation

F. Tension Band Plate

• Applied to the convex (tension) surface of an eccentrically loaded bone
• Converts tensile forces to compressive forces
• Used at: olecranon (proximal ulna), patella
• Only appropriate for transverse fracture patterns with an intact concave surface

G. Neutralization Plate

• Plate spans the fracture but doesn't generate compression itself
• Protects lag screws from torsion, bending, shear
• Common in oblique/spiral fractures of tibia, fibula, forearm

H. Reconstruction Plate

• Highly malleable; can be contoured in three planes
• Used for complex pelvic/acetabular fractures, mandible, clavicle

Small Fragment vs. Large Fragment Sets

• Small fragment: Screws 2.0-3.5 mm, for small bones (distal fibula, hand)
• Large fragment: Screws 4.5-6.5 mm, for large bones (femur, tibia, humerus)

4. INTRAMEDULLARY (IM) NAILS

Intraoperative fluoroscopy showing an IM nail with interlocking screws

Types of IM Nails

Key Design Features

Solid vs. Cannulated

• Solid nails: Stronger in torsion
• Cannulated (hollow) nails: More flexible, conform to canal shape, easier insertion; most modern nails are cannulated to allow insertion over a guide wire

Reamed vs. Unreamed

• Reamed: Progressively larger reamers prepare the canal; allows larger diameter nail = stronger construct
• Unreamed: Preserves endosteal blood supply; used in contaminated wounds or high-energy open fractures

Interlocking Screws

• Placed at proximal and distal ends to prevent rotation and shortening
• Static locking: Both ends locked (for comminuted, length-unstable fractures)
• Dynamic locking: One end unlocked to allow controlled axial compression with weight-bearing

Working Length

• The distance between the most proximal and distal fixation points
• Longer working length = more flexible construct, greater relative motion, secondary bone healing
• Shorter working length = stiffer construct

Biomechanics

• Bending stiffness increases with the square of nail radius (r²)
• Bending stiffness inversely proportional to the square of working length
• Modern nails are titanium alloy (modulus closer to bone than stainless steel; higher fatigue strength)
• Anatomically pre-curved to match the anterior bow of the femur or tibia

5. DRILL BITS IN ORTHOPEDIC SURGERY

Structure of an Orthopedic Drill Bit

• Flutes - helical cutting edges that remove bone debris; 2-4 flutes common
  • 4-flute bits = easier insertion + greater holding power
• Tip - pointed cutting tip; most are twist (spiral) design
• Shank - the non-cutting end that connects to the power drill
  • Jacobs chuck (round) shank - universal fit
  • SQC (Standard Quick Connect) shank - snap-in/out, used in most modern orthopedic power tools
• Cannulated - hollow core allowing placement over a K-wire (used with cannulated screws)
• Non-cannulated - solid; general purpose

Drill Bit Types

Key Drill Bit Sizes (AO/Synthes Standard)

Practical Drill Technique Rules

1. Use a drill sleeve / tissue protector to protect soft tissues and maintain axial alignment
2. Use slow speed (500-1000 rpm) in cortical bone to prevent thermal necrosis (bone dies above 47°C)
3. Use irrigation (saline) or intermittent drilling to reduce heat generation
4. Perpendicular to fracture line for maximum lag screw compression
5. Do NOT force the drill - let the bit do the work; excessive pressure blunts flutes and increases heat
6. 4-flute bits are preferred over 2-flute for better torque distribution and debris clearance
7. After drilling, measure depth with a depth gauge to select correct screw length
8. In dense bone, follow drilling with tapping before screw insertion

Thermal Necrosis Risk

• High RPM
• Blunt/dull drill bit
• No irrigation
• Heavy axial pressure
• Dense cortical bone

6. SUMMARY TABLE

• Rockwood & Green's Fractures in Adults, 10th Ed 2025, Chapter 12 (Implants and Biomechanics), pp. 471-491
• Campbell's Operative Orthopaedics, 15th Ed 2026 - multiple chapters on fracture fixation techniques
• Indian J Musculoskeletal Radiology - Orthopedic Hardware in Trauma
• AO Surgery Reference - Lag Screw Technique