EXTERNAL FIXATORSJAYANT SHARMAM.S.,D.N.B.,M.N.A.M.S.

HISTORY

In 1853, Malgaigne described a clawlike device used percutaneously to compress and immobilize the major fragments of fractured patellae.

In 1893, Keetley, noting the frequency of malunions in the femur, recommended that rigid pins be inserted percutaneously and held in a special external fixation device

HISTORY

In 1897, Parkhill described the use of two half-pins above and two half-pins below.

Freeman published a series of papers from 1909 to 1919 advocating the use of external pins

HISTORY

Lambotte in 1912 and Humphry in 1917 were the first to advocate the use of threaded pins, but they used only one above and one below the fracture site.

HISTORY

In 1948, Charnley popularized his compression device to facilitate arthrodesis of joints, and this technique rapidly grew in popularity.

In 1966 and 1974, Anderson et al. reported the use of transfixing pins incorporated into a plaster cast for the successful management of large series of tibial shaft fractures.

HISTORY

Sladek and Kopta. From 1968 to 1970,

Vidal and Vidal et al. --modified the original Hoffmann device from a single half-pin unit to a quadrilateral bicortical frame, greatly increasing its rigidity

EXTERNAL FIXATORS- LANDMARKS MALGAIGNE- 19TH CENTURY LAMBOTTE-1902- UNILATERAL FRAME RAUL HOFFMAN -1938- UNIVERSAL BALL

JOINTS ROGER ANDERSON-USA.- THRU AND THRU

PINS. OTTO SADER-1937- VETERINARY SURGEON-

DISTRACTION VIDAL-QUADILATERAL FRAME USING HOFFMAN A.O-1977, TUBULAR FIXATOR,MONOPLANAR,

MONOLATERAL

ADVANTAGES Provides rigid fixation of the bones Compression, neutralization, or fixed

distraction of the fracture fragments is possible

Allows direct surveillance of the limb and wound status, including wound healing, neurovascular status

Immediate motion of the proximal and distal joints is allowed

Insertion can be performed with the patient under local anesthesia

Disadvantages of external fixation    1.    Meticulous pin insertion technique and skin and

pin track care are required to prevent pin track infection.    2.    The pin and fixator frame can be mechanically

difficult to assemble by the uninitiated surgeon.   3.    The frame can be cumbersome, and the patient

may reject it for esthetic reasons. 4.    Fracture through pin tracks may occur. 5.    Refracture after frame removal may occur, unless

the limb is adequately protected until the underlying bone can become accustomed to stress again.

6.    The equipment is expensive. 7.    A noncompliant patient may disturb the appliance

adjustments

INDICATIONS Accepted indications    1.    Severe type II and III open fractures

   2.    Fractures associated with severe burns   3.    Fractures requiring subsequent cross leg flaps, free vascularized grafts, or other reconstructive procedures   4.    Certain fractures requiring distraction (e.g., fractures associated with significant bone loss or fractures in paired bones of an extremity in which maintenance of equal length of the paired bones is important)   5.    Limb lengthening   6.    Arthrodesis   7.    Infected fractures or nonunions   8.    Correction of malunions

Possible Indications

  1.    Certain pelvic fractures and dislocations   2.    Open, infected pelvic nonunions   3.    Reconstructive pelvic osteotomy (i.e., exstrophy of the bladder)   4.    Fixation after radical tumor excision with autograft or allograft replacement   5.    Femoral osteotomies in children (use of this method eliminates the necessity of subsequent removal of internal fixation appliances such as plates and screws)   6.    Fractures associated with vascular or nerve repairs or reconstructions   7.    Limb reimplantation

ComplicationsPin Track InfectionNeurovascular ImpalementMuscle or Tendon ImpalementDelayed UnionCompartment SyndromeRefractureLimitation of Future Alternatives

FRAME BIOMECHANICS

LARGE PIN FIXATORS USE OF HALF PINS OF DIAMETER

BETWEEN 2-6MM. PINS WITH THREAD IN BETWEEN— THE LARGE PINS HAVE AN ABILITY

TO NEUTRALIZE DEFORMING FORCES THEY PROVIDE RIGID FIXATION,

AVOID COLLAPSE OF FRACTURE.

PINS

LARGE THREAD DIAMTER- CANCELLOUS BONE

SMALL PITCH ANGLE AND NARROW THREAD DIAMETER FOR DENSE CORTICAL BONES

HYDROXY-APPETITE COATED- FOR BONE APPOSITION

Conic schanz screw (trocar tip)

Non conic schanz screw (trocar tip)

Non conic schanz screw cancellous thread (trocar tip)

Conic schanz screws (regular tip)

Non conic schanz screw (regular tip)

PINS

A PIN WITH SCREW HOLE >30% OF BONE DIAMETER HAS 45% REDUCTION IN TORSIONAL STRENGTH.

PIN DIAMETER MORE THAN 6MM INCREASE THE RISK OF STRESS FRACTURES.

RADIAL PRELOAD IM PLANT –BONE INTERFACE HAS AN EFFECT

ON PIN LOADING. IT IS THE CONCEPTOF PRESTRESSES THE

BONE –PIN INTERFACE IN CIRCUMFERENTIAL PATTERN.

FIXATORPINS ARE PLACED WITH SLIGHT MISMATCH IN THE GREATER THREAD DIAMETER VERSUS CORE DIAMETER OF THE PILOT HOLE.

THIS REDUCES THE CHANCES OF PIN LOOSENING.

LARGE MISMATCH RESULTS IN HIGH DEGREE OF RADIAL PRELOAD AND CHANCES OF STRESS

FRACTURES, AND MICROSCOPIC STRUCTURAL

DAMAGE TO THE BONE SURROUNDING THE PIN.

PIN INSERTION

TYPES OF PIN

A)PREDRILLED

B) TO BE DRILLED

The new AO Schanz.type screws for unilateral external fixation

Combined deep- and shallow-thread shanz tip screw. The thread near the tip has a 4.5-mm outer and 3.2-mm core diameter,

the tip is self-cutting. The shaft is 46 mm in diameter and connects with a conical part. It is furthermore provided with a shallow thread to engage and drive the screw forwards when the thread near the tip ha not yet engaged in the far cortex. This Schanz screw is designed for use in cortical bone providing automatic radial preload. It is used wherever the axial holding strength requires priority.

Shallow-thread Schanz,-type screw. This screw is provided with only a shallow thread and is especially designed

for use in cancellous bone where transverse surfaces arc present (good strength in relation to a force acting perpendicularly to the long axis). This screw is of adsantage wherever the resistance to transverse forces has priority over the axial holding strength (e.g.. in short metaphyseal fragments).

PIN PLACEMENT

PIN PLACED PERPENDICULAR TO LONG AXIS OF BONE ACT AS CANTILEVER.TION

THEY DONOT REDUCE THE SHEAR FORCE VECTOR.

HALF PINS PLACED PARALLEL TO FRACTURE ARE IN DIRECT OPPOSITION TO SHEAR FORCE– WHICH GETS CONVERTED TO COMPRESSION.

THIS WAY COMPRESSION IS DEPENDANT ON SHEAR LOAD.

PIN PLACEMENT

AN EXTERNAL FIXATOR THAT ALLOWS PIN OFFSET ANGLE OF 60 DEGREES CAN EQUALIZE THE FORCES IN SAGGITAL AND CORONAL PLANE.

THIS PROVIDES MECHANICAL STIMULATION CLOSER TO NORMAL.

IN OBLIQUE FRACTURES THERE IS INHERENT SHEAR PRESENT.

HERE STEERAGE PINS SHOULD BE PLACED.

IF OBLIQUITY IS MORE THAN 60 DEGREES THESE STEERAGE PINS ARE ALSO IN EFFECTIVE SO THE FRAME SHOULD BE A NEUTRALIZING DEVICE.

Improved Effectiveness of External Fixator Pins

In the past decade it has become clear that pin loosening and pin tract infections can be significantly reduced (Behrens 1982. 1989) by

(1) using larger pins and (2) generating radial preload with the inserted pins.

Based on early clinical experience (Behrens 1982) the AO introduced, in 1984. short- threaded Schanz screws with a core diameter of 3.5 mm and a shaft diameter of 4.5 mm. These screws provided excellent purchase in cortical bone while making the smooth shaft of 4.5 mm the effective pin diameter in the near cortex.

These Schanz screws were considerably stiffer than the 5-mm screws previously used, where the threaded portion (core diameter of 4.0 mm) engaged in both cortices.

Clinical experience in the early 1980s indicated that using pins with short threads inserted in pin holes smaller than the shaft diameter lowered the empirical rate of pin tract infections and loosening (Behrens 1982).

With these systems, misfits between drill hole and pin shaft (radial preload) of somewhere between 0.5 mm and 1.0 mm were created (Behrens 1986. 1989).

a )An exact fit between pin and bone results in pin loosening without prior mechanical damage to the interfacing bone. b )Oversize of 0.1 mm results in the best bone structure and avoidance of pin loosening due to adequate fit. c )Oversize of 0.3 mm results in mechanical damage. instability, and consequent micromotion-induced bone resorption. d )Oversize of 0.5 mm results in massive mechanical destruction of bone by overload.

------------------------------------------------------------------------ Riliouri s et. al. 1989

On closer examination of this issue, Gasser (1989) predicted on theoretical grounds that mismatches between core and shaft diameter of 0.1—0.2 mm would be highly effective, yet small enough to prevent mechanical disruption of the surrounding cortex.

Hyldahl et al. (1989) further established the effectiveness of radial preload using a pneumatically operated pin motion system in the sheep tibia .

While similar experiments carried out by Biliouris et al. (1989) confirmed Gasser’s theoretical prediction

Based on these insights, a new AO Schanz-type screw for unilateral external fixation has been developed .

It has a 3.4-mm core diameter and a 4.5-mm outer diameter of the thread near the trocar tip.

A very shallow thread with 4.7-mm core diameter and 5.0-mm outer diameter connects to the 5-mm drive shaft.

The portion of the screw with the shallow thread produces forward propulsion of the pin until the far cortex is engaged by the tip portion.

These special design features result in a 0.2-mm misfit for all parts of the new Schanz screw.

It is, therefore, not a conical screw but a conically preloaded cylindrical screw,

this avoids the disadvantage of the conical threads, that they require a predetermined position along the long axis of the screw, i.e., a predetermined position in relation to the depth within the bone.

a uniform design of the Schanz screw has been adopted.

which exerts radial preload on both cortices and requires only drilling of both cortices to 4.5 mm. This simplifies Schanz screw application significantly .

BIOMATERIALS

STAINLESS STEEL– INCREASESSTRESS AT BONE PIN INTERFACE

TITANIUM– LESS ELASTIC MODULUS HA PINS.– BETTER FOR CANCELLOUS

BONES, PAINFUL WHILE REMOVING.

FACTORS AFFECTING STABILITY PIN NUMBER PIN PROXIMITY PIN SEPARATION BONE BAR DISTANCE PIN TO CENTRE OF ROTATION INSUFFICIENT HOLDING OF CLAMP

TO BAR/PIN.

MONOLATERAL FRAMES

FLEXIBLITY BUILD UP AND DOWN CONCEPT COMPONENTS CAN BE REMOVED

FOR STRESS AT FRACTURE SITE. ANGLE VARIATION FRACTURE REDUCTION

DELTA FRAMES

TWO MONOPLANAR CONSTRUCTS AT 90 DEGREES

HELPS TRANSFER MORE LOAD UNILATERAL BIPLANAR DELTA FRAME

MORE STABLE THAN BILATERAL TRANSFIXING DEVICES.

MONOTUBE FIXATORS

FIXED PIN PLACEMENT TELESCOPIC TUBE– AXIAL COMPRESSION

AND DISTRACTION NO PIN SPREAD POSSIBLE PLACED NEAR THE BONE TO INCREASE

STABILITY RIGIDITY INCREASED BY LARGE

DIAMETER TUBE HIGH BENDING STIFFNESS & TORSIONAL

STIFFNESS, VARIABLE AXIAL STIFFNESS.

MONOTUBE FIXATORS HAVE BALL JOINTS– REQUIRE FREQUENT

TIGHTENING.(CHAO) INSUFFICIENT HOLDING STRENGTH OF PIN

DECREASES RIGIDITY. PINS PLACED AT 60 DEGREES AND10

DEGREES OF SEPARATION DECREASED TORSIONAL STRESS BY 97%

PINS PLACED DIVERGENT TO EACH OTHER– PIN PLACED OUT OF PLANE—INCREASED THE ANGULAR AND TORSIONAL RIGIDITY

The Development ofTube to-Tube Clamps

A recent contribution to external fixation consists in the “tube-to-tube” fixation as realized by Fernández Dell ‘Oca (1989).

Tube-to-tube fixation allows for very versatile assembly of the tubes which connect the pins.

According to the philosophy of the AO group. the fragments of the fracture are fixed subsequent to appropriate reduction.

Therefore extensive secondary Corrections are actually a less important

indication for tube-to-tube fixation.

More important is the possibility to apply pins in distinctly different planes.

A good example for such application is the external fixation of the humerus, where damage to the radial nerve can best be avoided by applying the pins in Iwo planes, at right angles to each other.

Tube-to-tube fixation Tube-to-tube fixation is a typical example of AO

ingenuity. Versatility is achieved without complicated

additions to the system of instrumentation and implants.

In spite of the extended possibilities offered, the

system remains simple and can be taught and learned with little effort.

Quality and simplicity are equally import ant guidelines for the development of implants and instruments.

DYNAMIZATION

CONVERTS STATIC FIXATOR WHICH NEUTRALIZES ALL FORCES,INCLUDING THE AXIAL MOTION, AND ALLOWS THE PASSAGE OF FORCES ACROSS THE FRACTURE SITE.

DYNAMIZATION

HELPS TO RESTORE CORTICAL CONTACT. PRODUCE STABLE FRACTURE PATTERN

WITH INHERENT MECHANICAL SUPPORT. ALSO KNOWN AS SECONDARY CONTACT

HEALING. DECREASES TRANSLATION SHEAR FORCES-- PRODUCES FIBROUS UNIONOCCURS WITH WEIGHT BEARING, WITH

PROGRESSIVE CLOSURE OF FRACTURE GAP.

THEORY OF DYNAMIZATION Once there is evidence of biologic activity (early fracture callus), there

should be a slow and progressive load transfer to the healing callus. As hypothesized by Pauwel and later explained in different terms by

Perren (with his interfragmentary strain theory), pure compression and hydrostatic pressure will stimulate the mesenchymal cells to differentiate toward chondrogenesis and subsequently endochondral ossification.

Strain will result in the formation of collagenous tissue and subsequent intramembranous ossification.

Combinations of these two temporally spaced events (compression then strain) can manifest themselves as callus healing or, as is the case with use of the Ilizarov principle, regenerate formation.

All of this, however, depends on adequate blood flow because, in its absence, there will be no bone-healing, regardless of the type of fracture fixation. Thus, as the initial construct with the stiff fixator begins to demonstrate some biologic activity, the fixator undergoes a "controlled destiffening" so that there is a slow but definitive transfer of load-bearing from the fixator to the bone. This load-sharing will gradually stimulate the developing callus until solid bone-healing has occurred.

DYNAMIZATION

MICROMOTION IS A VERY IMPORTANT MECHANICAL STIMULUS FORCE IN A CONSTRUCT,

THESE FORCES ARE IMPARTED TO THE PERIOSTEAL CALLUS

THE QUANTITY , MAGNITUDE AND TIMING OF MICROMOTION IS STILL UNDER STUDY.

DYNAMIZATION IN FIXATORS ADJUSTING PIN BAR CLAMPS RELEASING THE BODY IN MONOTUBE RELEASING TENSION OF WIRES IN

ILIZAROV.

DYNAMIZATION

THERE IS A RACE BETWEEN THE LOAD CARRYING CAPACITY OF HEALING BONE AND FAILURE OF PIN BONE INTERFACE.

DYNAMIZATION DECREASES THESE STRESSES, AND PROLONGS THE LIFE OF FIXATORS

IN UNSTABLE FRACTURES– LOCALIZED YIELDING FAILURE DUE TO PIN BONE INTERFACE.

Ligamentotaxis

The term ligamentotaxis, common in the European literature, suggests that certain intraarticular fractures can be treated by external fixation using traction by the fixator on the capsular and ligamentous structures around the joint

This concept works well in comminuted intraarticular fractures of the distal radius, for which pins and plaster have commonly been employed