Intramedullary Nail Mechanics

1. Intramedullary Nail Mechanics Comprehensive Fracture Course

2. Objectives • Indications/Advantages of IM Nailing • Goals of Treatment • Cases • Fracture reduction • Fracture stabilization

3. Indications Diaphyseal Segmental Comminution Metaphyseal (selected)

4. Advantages of IM Nailing • Stable fixation of diaphyseal fractures • Load-sharing device allowing for early weight-bearing Moment arm length

5. Advantages of IM Nailing • Stable fixation of diaphyseal fractures • Load-sharing device allowing for early weight-bearing • Entry point is distant from fracture site • Fracture site remains closed in most cases • Soft tissue integrity maximally preserved • Ultimate minimally invasive technique

6. 35 F, MVA Closed Goals? • Reduce and stabilize fracture

7. Goals of Treatment • Reduce fracture • Understand deforming forces • Plan and execute method of reduction • Stabilize Fracture • Understand fracture and implant factors contributing to stability

8. 36 M, MVA Fracture reduction • Deformity? • Reduction plan?

9. 36 M, MVA Deformity Shortened Varus Rotation Reduction?

10. Reduction Techniques • Traction • Manual • Distractor

11. 36 M, MVA Fracture reduction • Deformity  • Reduction plan Stabilize fracture?

12. Factors Contributing to Stability • Fracture Characteristics • Pattern • Location • Working Distance • Implant Characteristics • Material • Geometry • Interlocking Screws

13. Transverse Oblique Comminuted Axial Stability High Intermediate Low Rotational Stability Low Intermediate Low

14. Fracture Location Diaphysis • Narrow canal • Endosteal contact with nail Metaphysis • Capacious canal • Deforming forces

15. Working Length Distance between proximal and distal fixation • Comminution • Nail-endosteal contact • Interlocking screws Amount of motion at the fracture is directly proportional to the working length

16. Working Length Torsional rigidity is inversely proportional to the working length Bending rigidity is inversely proportional to square of working length 1/WL (1/WL)2

17. Working Length Short Working Length • Transverse fractures • Inherently stable with nail Long Working Length • Comminution • Inherently less stable • Consider stronger implant - Maximize stability - Reduce risk of implant fatigue failure short long

18. Factors Contributing to Stability • Fracture Characteristics • Pattern? • Location? • Working Length?

19. Factors Contributing to Stability • Fracture Characteristics • Pattern  • Location  • Working Distance  • Implant Characteristics • Material • Geometry • Interlocking Screws

20. Implant Factors • Materials • Most nails are made of titanium • Flexible • Modulus of elasticity is close to cortical bone • High fatigue strength • High ultimate strength

21. Implant Factors • Geometry • Strength of the nail is proportional to the radius • Strength increases by the fourth power of r • Small increases in r overcome differences in material strength • Cannulation of the nail has minimal impact on strength r4

22. Geometry • Radius of Curvature

23. Implant Factors • Reaming • Allows for larger nail • Increases surface area for endosteal contact ( • Biologic stimulus?

24. Implant Factors • Interlocking Screws • When should you use them? ALWAYS • How many? • Based on fracture pattern and location of fracture • Transverse midshaft pattern = 1 screw • Short segment, comminution = 2+ screws Brumback et. al. J Bone Joint Surg, 1999

25. Factors Contributing to Stability • Fracture Characteristics • Pattern  • Location  • Working Length  • Implant Characteristics • Material  • Geometry  • Interlocking Screws 

27. 24M fall down stairs 315#

28. Subtrochanteric Fracture Fracture reduction • Deforming forces? • Plan for reduction?

29. Subtrochanteric Fracture Deforming Forces Flexion Abduction External Rotation Reduction?

30. Subtrochanteric Fracture Reduction Traction • Fracture table • Distractor Ball Spike Bone hook

31. 24M fall down stairs 315# Fracture reduction • Deforming forces  • Plan for reduction  Stabilize fracture?

32. Factors Contributing to Stability • Fracture Characteristics • Pattern • Location • Implant Characteristics • Material • Geometry • Interlocking Screws

33. 24M fall down stairs 315# Fracture reduction • Deforming forces  • Plan for reduction  Fracture stability

34. 42 M playing soccer Fracture reduction • Deforming forces? • Plan for reduction? Fracture stability • Fracture? • Implant?

35. Reduction Techniques • Percutaneous Clamping

36. Reduction Techniques • Blocking Screws

37. Factors Contributing to Stability • Fracture Characteristics • Pattern  • Location  • Working Distance  • Implant Characteristics • Material • Geometry • Interlocking Screws

39. 25 M fall from height Closed Injury Fracture reduction • Deforming forces? • Plan for reduction? Fracture stability • Fracture? • Implant?

42. Reduction Techniques • Blocking Screws

43. Reduction Techniques • Unicortical Plate

44. Reduction Techniques • Nailing in Extension

45. 25 M fall from height Closed Injury Fracture reduction • Deforming forces? • Plan for reduction? Fracture stability • Fracture? • Implant?

46. Summary • Intramedullary Nails • Minimally invasive • Load-sharing device • Goals of treatment • Fracture reduction • Understanding of deforming forces • Plan the method of reduction • Fracture stability • Fracture Pattern, Location, Working Length • Implant material, geometry, interlocking screws

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