Interlocking Nail

IntroductionThese implants are introduced into the bone remote to the fracture site and share compressive, bending, and torsional loads with the surrounding osseous structures

IM nails act as internal splints with load-sharing characteristics.

HISTORYStimson in 1883 described the insertion of an

ivory peg in medullary canal.

Rush brothers described their IM pinning system in 1927.

Gerhard KuntschnerFemoral Nail-1939

Stable Osteosynthesis

Principle of fixation was based on compression between bone and implant

Flexible NailsRush pins

Ender nails

Morote Nails

Nancy Nails

MechanicsAct by stabilizing fractures with three or

four point compression

Equilibrium between the tensioned pin and the bone with its soft attached tissues will hold the alignment.

Bending movements are neutralized but telescopining and rotational torsion are not prevented.

ApplicationsChildren- Mainly for forearm fractures femur fracture.

Adults-Clavicle fracture Proximal humerus fracture Humeral shaft fracture Forearm fracture

DisadvantagesAdditional immobilization is often required.

Secondary loss of reduction

Shortening with loading

Intramedullary NailingUnlocked Nail

Interlocking Nail

Unreamed Nail

Reamed Nail

Unlocked Nail

Mechanics Elastic Deformation is principle of nail stability Nail insertion causes radially oriented force Force is proportional to the contact area

between the bone and nail

Produced friction stops the nail from pulling out

“Elastic Locking”

Elastic LockingBending of the nail (curvature)

Cross-sectional shape (particularly the geometry of the surface of the implant), and its diameter

The corresponding properties of the canal (eg, size, shape, bone quality)

Interlocking NailingThese nails have proximal and distal locking screws.

The resistance to axial and torsional forces is mainly dependant on screw bone interphase.

Interlocking screws placed proximal and

distal to the fracture site restrict translation

and rotation at the fracture site; however,

minor movements occur between the nail

and screws, allowing toggling of the bone.

Nail Bio-MechanicsIntrinsic

Extrinsic

Intrinsic FactorsMaterial properties

Cross-sectional shape Anterior bow

Diameter

Extrinsic FactorsReaming of the medullary canal

Fracture stability (comminution)

The use and location of locking bolts

Stability Nail size Number of locking screws or bolts, and

Distance of the locking screw or bolt from the fracture site.

Nail Diameter Bending rigidity is proportional to the nail

diameter to the third power, The torsional rigidity is proportional to the

fourth power

Working LengthIt is the distance between proximal and

distal locking screws

The working length influences nail stiffness in bending and torsion.

Physiological loading of the Nail

Screw BreakageWith cortical bone contact weight is transmitted

through bone also.

However in its absence four point bending can occur

Implant FailuresUnlocked nails typically fail either at the

fracture site or through a screw hole or slot.

Locked nails fail by screw breakage or fracturing of the nail at locking hole sites, most commonly at the proximal hole of the distal interlocks

PATHOPHYSIOLOGY OF NAILINGLocal effects

Systemic effects.

These effects are described with reamed nailing.

Local effectsDamage to endosteal blood supplyHeat necrosisWith intact soft tissue envelop reaming

increases the circulation in the surrounding muscles

Rate of non union is less with reamed nail as compared to unreamed nail.

Systemic Reaming causes transient raise of the pulmonary arterial pressure

IM instrumentation causes liberation of bone marrow contents to blood stream

They undergo an increase in size due to platelet adhesions

Leads to a transient decrease in perfusion

Subsequent cascade reaction follows.

Unreamed nailsIt is said that unreamed nailing is

advantageous in treatment of Gustilo IIIB open fractures.

It has got less amount of superficial infection and malunion as compared to external fixation.

DisadvantagesNonunion

Distal Screw breakage

Special Designs Proximal Femoral Nails

Retrograde Nails

Distraction Nails

Knee Arthrodesis

Proximal Femoral NailSub trochanteric fracture

Fracture NOF

Intertrochanteric Fractures

Retrograde NailsDistal femur

fractures

Humeral fractures

Periprosthetic fractures

IM SKELETAL KINETIC DISTRACTOR

PrinciplesA motor with sub-cutaneous receiver for

gradual lengthening

A mechanical function with one way cluches

Advantages over external fixatorsLimb lengthening by external fixators is

associated with problems such as Pain at the pin tracts Pin tract infections Reduced joint motion and Prolonged fixation time.

Coated ImplantsHydroxyapatite

Growth Factors

Antibiotics

Hydroxyapatite coated implants The extraction torque of HAP coated implants found to be higher.

Coating of the dynamic screw reduced significantly rate of cutout

Growth FactorsLocal application of the growth factors

significantly accelerates the fracture healing in early phase

The sustained release of growth factors doesn’t induce HO.

Antibiotics Gentamycin coated implants have shown

reduced rate of infection in animal studies.