Plate Span Ratio in Orthopaedic Surgery
Direct Answer
The optimal plate span ratio should be high (maximizing the ratio of plate length to fracture zone length) to reduce mechanical load on the plate and individual screws, allowing for lower screw density while maintaining stable fixation. 1
Understanding Plate Span Ratio
The plate span ratio represents the relationship between the total length of the plate and the length of the fracture or comminuted zone. A high plate span ratio is biomechanically advantageous because:
- Reduces load concentration on the plate itself by distributing forces over a longer working length 1
- Decreases stress on individual screws, which reduces the risk of screw pullout or hardware failure 1
- Permits lower plate-screw density (fewer screws needed), which preserves bone biology and blood supply 1
Practical Application Guidelines
Minimum Plate Length Requirements
Place the plate with at least three holes on each main fragment for standard fracture patterns 2:
- Simple two-fragment transverse fractures: Use a 2+2 screw configuration (two screws per main fragment) with minimum three-hole plate length per side 2
- Fractures with interfragmentary zones or comminution: Use a 3+3 screw configuration (three screws per main fragment) 2
- Extensive comminution, bone defects, or segmental fractures: Use four plate holes per fragment, but not more 2
Working Length Concept
The working length is the unsupported span of plate between the most proximal and most distal screws:
- Longer working length reduces individual screw loading, making the construct more forgiving 1
- High working length with high plate span ratio creates optimal mechanical environment for callus formation in bridging fixation 1, 3
- Excessive rigidity from too many screws or too short working length can suppress secondary bone healing and lead to nonunion 2
Biomechanical Principles
Strain Theory and Fixation Stability
The mechanobiologic environment determines healing pathway 4:
- Granulation tissue tolerates 100% strain 4
- Cartilage tolerates 10-15% strain 4
- Bone tolerates only 2% strain 4
Flexible fixation with appropriate plate span ratio stimulates callus formation through controlled interfragmentary motion 3, while maintaining sufficient stability to prevent excessive strain that would inhibit healing.
Common Pitfalls to Avoid
Locking Plate Considerations
Locking compression plates (LCP) have NOT demonstrated substantial advantages over conventional 3.5mm DCP plates and may actually create problems 2:
- Excessive rigidity from locked constructs can suppress bone healing 2
- Fixed screw trajectories may result in suboptimal screw placement, particularly with volar plating 2
- Shorter locking plates increase fixation rigidity excessively and often lead to nonunion 2
Screw Placement Strategy
Direct drill trajectory toward the interosseous border to maximize screw length and purchase 2:
- Longest screw trajectory provides best rotational stability 2
- Perforation of anterior or posterior cortex shortens screw trajectory and reduces fixation stability 2
Plate-Screw Density
Keep plate-screw density low by using fewer screws distributed appropriately along a longer plate 1:
- Fewer screws preserve periosteal blood supply 1
- High plate span ratio compensates for lower screw density by reducing per-screw loading 1
Clinical Context
While specific guidelines vary by fracture location and pattern, the fundamental principle remains: maximize plate span ratio to optimize load distribution, minimize hardware stress, and create appropriate mechanical environment for bone healing 1, 3. This approach balances stability requirements with biological preservation necessary for successful fracture union.