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

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

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

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

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.

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.

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

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

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.

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.

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.

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

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

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

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

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.

Design Features: Interlocking Thanks to Peter Trafton

Design Features: Interlocking 5 mos Thanks to Peter Trafton

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.

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

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

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

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

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

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

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

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

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

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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

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