Residency · Residency · Orthopedic Surgery

Principles of Fracture Reduction and Internal Fixation

Fracture Reduction Principles

Goals of Reduction

The fundamental goals of fracture reduction are to restore anatomic alignment (length, rotation, and angulation), re-establish articular congruity for intra-articular fractures, achieve acceptable alignment for extra-articular fractures (where acceptable parameters vary by anatomic location), and create the optimal mechanical and biologic conditions for fracture healing.

Direct (Anatomic) Reduction

Direct reduction involves open visualization and manual manipulation of fracture fragments back into their anatomic position. Instruments employed include pointed reduction clamps, ball-spike pushers, dental picks, and Kirschner wires used as joysticks. Direct reduction is mandatory for intra-articular fractures, where the articular surface must be restored to within 1-2 mm of anatomic alignment. The disadvantages of this approach are that it requires a larger surgical exposure, inevitably involves some degree of periosteal stripping, and may devascularize small bone fragments.

Indirect Reduction

Indirect reduction restores alignment without directly visualizing the fracture site. Techniques include ligamentotaxis (applying traction to allow intact ligaments and periosteum to reduce the fracture), using the implant itself as a reduction tool, and applying percutaneous clamps. This approach preserves fracture biology by minimizing periosteal and soft tissue stripping. It is ideal for extra-articular metaphyseal and diaphyseal fractures and is the foundation of bridge plating, intramedullary nailing, and minimally invasive plate osteosynthesis (MIPO).

Stability Concepts

Absolute Stability

Absolute stability means there is no motion at the fracture site under physiologic loading conditions. It is achieved through interfragmentary compression using lag screws or compression plating. This environment is required for primary (direct) bone healing without callus formation. Absolute stability is indicated for simple fracture patterns and intra-articular fractures that require anatomic reduction. A critical principle is that applying absolute stability constructs to comminuted fractures leads inevitably to implant failure, because the fixation cannot withstand the full load without cortical bone-to-bone contact.

Relative Stability

Relative stability permits controlled micromotion at the fracture site, which stimulates secondary (indirect) bone healing with callus formation. It is achieved with bridge plating, intramedullary nailing, and external fixation. This approach is appropriate for comminuted fractures and diaphyseal fractures where anatomic reduction of every fragment is neither possible nor necessary. The construct must be stiff enough to prevent excessive motion (which leads to nonunion) but flexible enough to allow the strain environment that promotes callus formation.

Strain Theory

Perren's strain theory provides the biologic rationale for these stability concepts. Strain is defined as the change in gap width divided by the original gap width. Bone tissue cannot form if local strain exceeds approximately 2% (the absolute stability zone). Callus formation occurs when strain is between 2% and 10% (the relative stability zone). When strain exceeds 10%, only fibrous tissue forms, leading to nonunion. Simple fractures with small gaps experience high strain from even small amounts of motion, necessitating absolute stability. Comminuted fractures distribute the same total motion across many fragment interfaces (larger total gap length), resulting in lower strain at each site and tolerating relative stability constructs well.

Lag Screw Technique

Principles

The lag screw converts the rotational force of screw insertion into axial compression across a fracture plane. For this to work, the screw must glide freely through the near cortex (via a glide hole) while its threads purchase only in the far cortex (via a thread hole). The screw should be oriented perpendicular to the fracture line for maximum compression. The lag effect can be achieved either with a partially threaded screw (where the smooth shaft spans the near cortex) or by overdrilling the near cortex to a diameter equal to the screw's outer thread diameter.

Technique

The step-by-step technique involves drilling the glide hole in the near cortex (matching the screw's thread diameter), then drilling the thread hole in the far cortex through a centering guide (matching the screw's core diameter). The length is measured, the far cortex is tapped (for non-self-tapping screws), and the screw is inserted. As the threads engage the far cortex, continued tightening draws the fragments together under compression. A washer may be placed under the screw head to increase the area of force distribution on the near cortex, reducing the risk of the head sinking into the bone.

Position Screw vs. Lag Screw

A position screw is a fully threaded screw that crosses a fracture, holding fragments in their reduced position without generating active compression. A lag screw actively compresses the fracture surfaces together. Both types may be used in the same construct depending on the fracture geometry and the surgeon's goals.

Plating Techniques

Neutralization (Protection) Plate

A neutralization plate is applied after lag screw fixation of a simple fracture pattern. Its role is to protect the lag screw construct from the bending, rotational, and shear forces that act on the bone during functional use. The plate itself does not generate compression; it works in concert with the lag screw, which provides the interfragmentary fixation.

Compression Plating

The Dynamic Compression Plate (DCP) generates axial compression through eccentric screw placement within its oval holes. As a screw is tightened in the inclined portion of the hole, it slides down the ramp, translating the bone fragment toward the fracture and producing compression. The Limited-Contact DCP (LC-DCP) has undercuts on its undersurface that reduce plate-bone contact, preserving periosteal blood supply while maintaining compression capability. Compression plating is appropriate for simple diaphyseal fractures where anatomic reduction has been achieved.

Bridge Plating

Bridge plating spans a comminuted fracture zone without any attempt at direct fragment reduction. It provides relative stability, allowing secondary bone healing with callus formation while preserving the biology of the comminuted zone by avoiding periosteal stripping. Key design parameters include plate length, screw density, and working length. A longer plate with fewer screws in the comminuted zone creates a more elastic construct that promotes callus. The recommended screw density ratio (number of screws divided by number of available plate holes) is less than 0.5 for bridge plating. Working length, defined as the distance between the closest screws on either side of the fracture, determines construct flexibility.

Buttress (Antiglide) Plating

A buttress plate resists an axial load that would otherwise displace a fracture fragment. It is applied on the side where the deforming force acts. The classic application is a buttress plate for tibial plateau split-depression fractures, where it prevents subsidence of the elevated articular surface. The antiglide plate, applied posteriorly on the distal fibula, uses the screw heads to prevent distal fragment displacement, essentially converting the plate into a buttress against the oblique fracture's tendency to shorten.

Locking vs. Non-Locking Plates

Non-Locking (Conventional) Plates

Conventional plates rely on friction between the plate undersurface and the bone to generate stability. This requires the plate to be precisely contoured to the bone surface. The compression of plate against bone can compromise periosteal blood supply. In osteoporotic bone, screws may toggle within their holes, leading to progressive loosening and loss of fixation.

Locking Plates

Locking plates represent a fundamentally different biomechanical concept. The screw heads thread directly into the plate holes, creating a fixed-angle construct that functions as an internal external fixator. Because stability does not depend on friction between plate and bone, locking plates can stand off from the bone surface, preserving the periosteum. They are superior in osteoporotic bone and periarticular locations because each screw acts as a fixed-angle device that cannot toggle. Disadvantages include the inability to use standard locking holes for compression, higher cost, and the requirement for symmetric loading across the construct.

Combination Holes

Modern plates often feature combination holes that accept both locking and conventional screws. This allows the surgeon to use conventional screws first for fracture reduction and compression, then add locking screws for definitive fixation, combining the advantages of both systems in a single construct.

Screw Types and Biomechanics

Cortical Screws

Cortical screws have finer pitch (more threads per unit length) and are designed for dense cortical bone. They are fully threaded, functioning as position screws unless the near cortex is overdrilled to create a lag effect. Standard sizes are 3.5 mm for upper extremity and 4.5 mm for lower extremity applications.

Cancellous Screws

Cancellous screws feature wider pitch, deeper threads, and a larger thread-to-core diameter ratio, all optimizing purchase in the lower-density metaphyseal and epiphyseal bone. They are available in partially threaded versions (providing an inherent lag function) or fully threaded. Standard sizes are 4.0 mm and 6.5 mm. A washer is often employed to distribute compression forces over a larger area of the cancellous bone surface.

Cannulated Screws

Cannulated screws have a hollow core that allows placement over a pre-positioned guide wire under fluoroscopic guidance. This enables percutaneous insertion with precise trajectory control. Common applications include femoral neck fractures, scaphoid fractures, and syndesmotic fixation. The trade-off is slightly reduced strength compared to solid screws of equivalent outer diameter.

Pullout Strength Factors

The resistance of a screw to pullout depends on several factors: screw diameter (the most important single factor), length of thread engagement in bone, bone mineral density, pilot hole size, and screw design characteristics including root diameter and thread pitch.

Reduction Aids and Techniques

Instruments

A variety of specialized instruments facilitate fracture reduction. Pointed reduction forceps (Weber clamps) compress fragments together percutaneously. Large pelvic reduction clamps (Jungbluth, Farabeuf) manipulate heavy pelvic and acetabular fragments. Ball-spike pushers allow indirect manipulation through small incisions. Collinear clamps enable reduction directly through a plate. Kirschner wires provide provisional fixation and serve as joysticks for fragment manipulation. The universal distractor and femoral distractor restore length via ligamentotaxis.

Intraoperative Assessment

Reduction quality is assessed through fluoroscopy (evaluating alignment, rotation, and joint congruity), direct visualization for articular reduction, clinical comparison of rotation with the contralateral limb, and assessment of overall limb mechanical axis alignment.

<image>A technical illustration demonstrating the lag screw technique in a simple oblique fracture of a long bone diaphysis. Show a cross-sectional view with the near cortex having a glide hole (larger diameter) and the far cortex with a thread hole (smaller diameter). Illustrate how tightening the screw generates interfragmentary compression with directional force arrows. Label the glide hole, thread hole, screw head, and compression vector perpendicular to the fracture line.</image>

<image>A comparison diagram of four plating techniques applied to a femoral shaft: (1) compression plate with eccentric screw placement generating axial compression across a simple transverse fracture, (2) neutralization plate protecting a lag screw in a simple oblique fracture, (3) bridge plate spanning a comminuted fracture zone with screws only in proximal and distal fragments, and (4) buttress plate supporting an articular fragment against axial loading. Label each technique and show force vectors.</image>

<image>A detailed cross-sectional comparison of a conventional (non-locking) screw-plate interface versus a locking screw-plate interface. On the left, show the conventional screw with a smooth head compressing the plate against the bone surface with friction arrows. On the right, show the locking screw with threaded head engaging threaded plate hole creating a fixed-angle construct with the plate standing off from the periosteal surface. Label all components and highlight the biomechanical differences.</image>

Clinical Pearls

The fixation strategy must always match the fracture personality: absolute stability for simple patterns and relative stability for comminuted patterns. Applying absolute stability constructs (lag screws and compression plating) to comminuted fractures leads to implant failure because the fixation cannot withstand bearing all the load without cortical bone contact. For bridge plating, use a longer plate with fewer screws and a longer working length to create a more elastic construct that promotes callus formation. In osteoporotic bone, locking plates outperform conventional plates because the fixed-angle construct prevents screw toggle. Always reduce before fixing; provisional fixation with K-wires and clamps should precede definitive screw and plate placement. For intra-articular fractures, the principle is "articular simple, metaphyseal complex": reduce and compress the articular block anatomically, then bridge to the diaphysis with a relative stability construct. Screw density ratio should be less than 0.5 for bridge plating to optimize construct flexibility. Lag screws must be oriented perpendicular to the fracture plane, not perpendicular to the bone shaft.

References

• Ruedi TP, Buckley RE, Moran CG. AO Principles of Fracture Management. 3rd ed. Thieme; 2018.
• Perren SM. Evolution of the internal fixation of long bone fractures. J Bone Joint Surg Br. 2002;84(8):1093-1110.
• Gautier E, Sommer C. Guidelines for the clinical application of the LCP. Injury. 2003;34 Suppl 2:B63-76.
• Tornetta P, Ricci WM, et al. Rockwood and Green's Fractures in Adults. 9th ed. Wolters Kluwer; 2020.
• Wagner M. General principles for the clinical use of the LCP. Injury. 2003;34 Suppl 2:B31-42.

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