1. Intramedullary Nailing Basic Principles
Presented by
Michael Sochacki, MD
Developed by
Rahul Banerjee, MD

2. A locked IM Nail has what advantage over an unlocked nail?
Higher union rate
Lower incidence of immediate post-op malrotation
Shorter OR time
Less radiation exposure
Higher rate of maintenance of reduction

3. Which of the following is the stiffest IM nail?
A. Ender pin
B. 10 mm diameter cannulated titanium nail
C. 9 mm diameter solid titanium nail
D. 8 mm diameter cannulated stainless steel nail
E. 8 mm diameter solid stainless steel nail

4. B. 10 MM DIAMETER CANNULATED TITANIUM NAIL
ANSWER
B. 10 MM DIAMETER CANNULATED TITANIUM NAIL

5. Intramedullary Nail Functional Requirements Design Features
Biology (Reaming)
Nail Biomechanics:
To better understand a commonly used long bone fracture fixation implant, intramedullary nail or rod, it will help to have a general understanding of nail biomechanics.
CLICK
To do this we will explore these 3 topics.

6. Functional Requirements
Maintain the alignment of a fractured or weakened bone until healed
Length
Rotation
Coronal alignment
Sagittal alignment
The functional requirement or job of an IM nail is:
1. Maintain length
2. Maintain rotational control
3. Maintain alignment
Like other metallic fracture fixation implants a nail is under a fatigue loading condition. Nails will eventually break if bone healing doesn’t occur.

7. Functional Requirements
How does the IM nail accomplish the functional requirements?
Internal Splint to Resist Bending
Load sharing device
(implant works with bone)
Nails act as an internal splint resisting the anatomic loading conditions on the long bones. Relative stability is achieved, meaning some amount of movement is expected at the fracture site.
Nails are a load sharing type of device. A good connection between the implant and bone is needed.
As with most fracture fixation devices the nail has the task of supporting those loads the broken bone can not.
Bending , Rotation , Compression Loads , Tensile Loads
Click

8. Functional Requirements
How does the IM nail accomplish the functional requirements?
Internal Splint to Resist Bending
Locking Screws Resist Fracture Rotation and Shortening
Locking elements are used for transmitting rotation and translation loads to the nail.
Click

9. Nail Design Features
Nail stiffness is ideal when loads are distributed throughout entire length.
Minimizes stress risers
At locking screws
Along nail bone interface
Nails allow us to place a metallic implant practically the entire length of the long bone. To take advantage of this good nail designs distribute the anatomical bending loads throughout the length of the implant.
Click
This reduces the stress risers at the nail locking holes and locking screw bone interface.
Stress is also better distributed along the nail bone interface.

10. Internal Splint to Resist Bending
Design Features
Internal Splint to Resist Bending
Compared to a plate, a nail location is closer to the anatomic axis of the bone.
The shorter moment arm results in reduced bending loads seen by the implant.
Moment arm length
Comparing the loading condition of a plate to a nail can help in understanding how it’s designed to perform the functions discussed previously.
Since nails are placed in the intramedullary canals of long bones, the nail is much closer to the biomechanical axis of the bone than a plate. This may also be referred to as the natural axis. The closer the implant is to this axis the shorter the moment arm. As we will see later in the presentation this equates to lower bending stresses on the implant and implant to bone interfaces.

11. Design Features Internal Splint to Resist Bending
Mechanics of a nail are similar in frontal and lateral planes
Nail cross section is round resisting loads equally in all directions.
Plate cross section is rectangular resisting greater loads in one plane versus the other.
The cross section of a nail is typically round in shape which is a distinct mechanical advantage.
Click
The intramedullary canal permits this shape to be used unlike the outside surfaces of the bone. For instance to achieve the bending strength required of a round tibia diaphyseal plate, the cross section would have to be 8mm in diameter. Obviously a soft tissue problem.
Since the nail can be round bending loads are resisted equally in a directions.

12. Internal Splint to Resist Bending
Design Features
Internal Splint to Resist Bending
Nail Geometry - Longitudinal Curvature - Cannulation - Interlocking

13. Design Features: Longitudinal Curvature
Early nail designs were straight Newer designs match shape of bone with slight mismatch

14. Design Features: Cannulation
Allows insertion over a guidewire
Reduction of intramedullary pressure while inserting the nail

15. Design Features: Interlocking
Original Gerhard Küntscher designs relied on a frictional fit between nail and bone
Read slide

16. Interlocking Nails
Original Gerhard Küntscher designs relied on a frictional fit between nail and bone.
Advancements in image intensifiers in the late 1960’s made it possible to interlock IM nails.
The frictional fit between the nail and bone of these early nails was the only means of controlling rotational and translation bone reduction. A design improvement was needed if the indications of nailing were to be expanded.
Click
Interlock nail design have since been created to improve nail rotational and translational fragment control.

17. Design Features: Interlocking
Thanks to Peter Trafton

18. Design Features: Interlocking
5 mos
Thanks to Peter Trafton

19. Biology Locking Elements: Screws ‘Shear Pin’
Core diameter (Ø) is very important
Thread Depth is not the critical feature
Minimal Screw Pullout loads
It’s the core diameter of a nail locking screw that is important. These screws are under shear loads and bending loads.

21. Biology Intramedullary Nailing is biologically friendly
Minimally invasive
No direct exposure of fracture
Load-sharing device

22. Reaming Why do we ream? Increases blood flow and healing response
Allows placement of larger nail….but!

23. Reaming Too much reaming can be dangerous! Leung, JBJS (Br) 1996
Osteocutaneous necrosis by aggressive reaming  ultimately resulting in osteomyelitis

25. Reaming
Old School = Ream to “chatter” and place largest nail possible

26. Biology
Reaming must balance placement of an appropriately sized nail with encouraging the blood flow and biologic response
New School = LIMITED REAMING

27. Biology What is LIMITED REAMING?
Think of the reamer as a “rotating intramedullary sound – to determine the normal canal diameter – not to change it very much.” – B. Brumbach

28. Reaming Limited Reaming Hupel et. al. 1996, 2001 Ziran et. al. 2004
Better viability and blood flow with limited reaming
Ziran et. al. 2004
Limited reaming acceptable in open fractures

29. Reaming General Principles Sharp reamer
Full speed rotation, but slow advancement
Limited reaming
Pass reamer limited (2-3 at most) number of times to determine canal size and encourage biology

30. Summary Functional requirements of IM Nail How? Biology
Resist bending, rotation, translation
How?
Internal Splint on biomechanical axis
Interlocking elements (ESSENTIAL)
Biology
Limited reaming
Read Slide

31. THANK YOU

32. SWOTA : 2010 Resident Course - Fundamentals of Fracture Care
Principles of Intramedullary Nailing
Helpfulness of Material
A) B) C) D) E)
Worst Bad OK Good Best
COMMENTS Please

33. Quality of Presentation
SWOTA : 2010 Resident Course - Fundamentals of Fracture Care
Principles of Intramedullary Nailing
Quality of Presentation
A) B) C) D) E)
Worst Bad OK Good Best
COMMENTS Please