BIOMECHANICAL PRINCIPLES OF FRACTURE FIXATION

INTRODUCTION

BONE HEALING

Events in fracture healing are responsible for debridement,
stabilization, and ultimately remodeling of the fracture site.
Healing can take place either primarily, in the presence of rigid
fixation, or secondarily in the absence of a rigid fixation.

PRIMARY BONE HEALING
· Occurs with direct and intimate contact between the fracture
fragements.
· The new bone grows directly across the compressed bone
ends to unite the fracture.
· Primary cortical bone healing is very slow and cannot bridge
fracture gaps.
· There is no radiographic evidence of a bridging callus with
this mode of healing.
· It usually occurs approximately 2 weeks from the time of
surgery.
· This is the only method of healing with rigid compression
fixation of the fracture.
· Rigid fixation requires direct cortical contact and an intact
intramedullary vasculature.

SECONDARY HEALING
It denotes mineralization and bony replacement of a
cartilage matrix with a characteristic radiographic
appearance of callus formation.
· The greater the motion at the fracture site, the greater will
be the quality of the callus.
· This external bridging callus adds stability to the fracture
site by increasing the bone width.
· This occurs with casting and external fixation as well as
intramedullary nailing of the fracture.


Fracture healing
There are three main phases of fracture healing


A. INFLAMMATORY PHASE
·In the inflammatory stage, a hematoma develops within
the fracture site during the first few hours and days.
· Inflammatory cells (macrophages, monocytes,
ly mphocytes, and polymorphonuclear cells) and
fibroblasts infiltrate the bone under prostaglandin
mediation.
· This results in the formation of granulation tissue,
ingrowth of vascular tissue, and migration of
mesenchymal cells.
· The primary nutrient and oxygen supply of this early
process is provided by the exposed cancellous bone
and muscle.


B. REPARATIVE PHASE

This phase lasts several months.
· The fracture hematoma is then invaded by
chondroblasts and fibroblasts, which lay down the
matrix for the callus.
· Initially a soft callus is formed, composed mainly of
fibrous tissue and cartilage with small amounts of bone.
· Osteoblasts are then responsible for mineralization of
this soft callus, converting in to a hard callus of woven
bone and increasing the stability of the fracture
· This type of bone is immature and weak in torque and

therefore cannot be stressed.

C. REMODELLING STAGE

· It takes months to years.
· It consists of osteoclastic and osteoblastic activities that
results in replacement of immature disorganized bone
with a mature organized bone.

· The medullary canal gradually reforms.
There is resorption of bone from the convex surface
and new bone formation on the concave surface.

IMPLANTS &  PROSTHESIS in ORTHOPAEDICS

STRESS-SHARING DEVICE

· It permits partial transmission of load across the fracture site.

· This results in micromotion at the fracture site, thus inducing secondary healing with callus formation.

· Eg:- casts, rods and intramedullary nails.

STRESS- SHEILDING DEVICES

· It shields the fracture site from stress by transferring stress to the device.

· The fracture ends of the bone are held under compression and there is no motion at the fracture site.

· Hence resulting in primary bone healing without callus formation.

· Eg:-compression plattinG

BONE SCREWS

A bone screw is used for internal fixation more often than any other implant

Though it appears as a simple device, it has a great deal of complex design.

It has four functional parts: Head, Shaft, Threads and Tip.

HEAD

· The screw head serves as an attachment for the screw driver.

· Is essential while removing and insertion of the screw.

· The undersurface of the screw is the countersink.

SHAFT

The shaft is the smooth part of the screw between the head and the thread

RUN-OUTS

It is the spot where the shaft ends and the thread begins.

THREAD

It is wrapped around the core which provides the main support of

the screw.

PITCH

It defines the distance between the adjacent threads

LEAD

The distance the screw will advance with each turn, the lead is therefore equal to the pitch.

PRINCIPLE

It provides interfragmentary compression which improves mechanical stability of internal fixation between bone fragments by minimizing the effect of torsion, shear and bending forces.

FUNCTION

It is used either to fasten plates or similar devices on to the bones or as lag screws, to hold together fragements of bones.

TYPES OF SCREWS

CORTICAL SCREWS

· It is a machine type of screw.

· The threads are smaller (in diameter) and are closely placed ( lower pitch).

· The core diameter is relatively large and provides the   necessary strength.

· The smaller pitch increases the holding power.

· Threads are cut in the pilot hole before the screw is inserted.

· The elastic reaction vital to hold the bone surfaces together, comes from elastic deformation of the bone rather than the screw.

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· Advantage : more engagement of screw threads in to the bone is possible because taps provide four cutting flutes, micro- motion of bone is less likely to occur, therefore these procedures better hold the screw.

Disadvantages: it require extra step in operative procedure and because rather smooth track is established, it is more likely to loosen by backing out when it is subjected to cylindrical; stress.

CANCELLOUS SCREW

· It is a modified wooden type of screw.

· It has larger threads and a higher pitch as compared to the cortical screws.

· The core diameter which is smaller than the shaft, provides a greater surface area for purchase of the screw threads on the bone.

· It is inserted in to an untapped pilot hole.

Uses: used as fastening devices such as plates in metaphyseal and epiphyseal areas

Advantages:

i. Holding power in fine trabecular bone is more.

ii. Tapping is not usually required because cancellous

bone is fairly soft and easily deformed.

iii. As the screws penetrate, it compressesthe bone to

either side, thereby increasing the bone density in thE

immediate vicinity of the thread, this improves the

holding power.

iv. Typically they have smooth shank in poetion

immediately adjacent to screw head so that an

automatic lag effect occurs without having to overdrill

the near cortex

Disadvantage: when used without a plate they are more

prominent

CANNULATED SCREWS

· It is used for precise insertion in metaphyseal or epiphyseal site over a guide wire.

· This reduces the problem of having to remove and reposition an incorrectly placed screw.

· The guide wire also maintains the reduction and controls the fracture fragments.

· Cancellous cannulated screws come in large and small sizes.

· Large screws are used to fix fractures of the femoral neck, femoral condyle and tibial plateau.

· Small cannulated screws are used for distal radius, distal humerus, distal and proximal tibia, carpals and scaphoid.

Advantages: it needs less soft tissue dissection.

Disadvantages: screws are weaker than non cannulated screw particularly in small fragment size, they break more easily when removal is attempted

THE HERBERT SCREW

· It is an specialized implant to achieve intrafragmentary compression.

· In this unique device there is no head and threads are present at both the ends of the screws with a pitch differential between the leading and trailing screws.

Principle: intrafragmentary compression is achieved the differences in the threads.

LAG SCREW

It is the most effective way to achieve compression

between two bony fragments.

· It pulls the fragments together producing pressure at the

fracture line.

· Compression between the fracture fragments increases

the friction force so that interfragmentary motion is less

likely and therefore strengthens the structure.

· It achieves this by producing purchase on the distal

fragment while being able to turn freely in the proximal.

Lag screw principle:

1. The screw must glide freely through the near fragment and engage only the far fragment.

2. Whenever a screw crosses a fracture line it should be inserted as a lag screw.

3. Two small screws produce a more stable fixation than one large screw.

· The lagging technique can be applied to virtually all of screws. In diaphyseal fractures a cortical screw is applied as a lag screw.

· In epiphyseal or metaphyseal fractures cancellous screws are applied.

· To effect maximal interfragmentary compression, lag screw must be inserted in to the centre of the fragments and at right angles to the fracture plane.

  Uses: in communited fractures, in metaphyseal area, to achieve inter fragemental compression & stability.

Disadvantages: it does not provide great deal of strength, if while doing screw is inserted at an acute angle to fracture plane then as it is tightened it introduces a shearing moment and tends to displace the fragements causes loss of reduction.

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NAILS

INTERLOCKING NAILS

The intramedullary nailing techniques which are in common use today. They are derivd mainly from Gerhard kuntsher.

Biomechanics: interlocking nails act as internal splints, serving as load sharing devices stabilizing fracture fragements and maintaining alignment while permitting slight bending during functional activities, a thicker nailmay not allow bending by allowing the movement of the adjacent joints, rehabilitation is concurrent with treatment and stress shielding is minimal.

FRACTURE HEALING FOLLOWING NAILING

The peripheral circulation is generally maintained however, the

remaining process causes additional damage.

It has been observed that the small vessels grow in to the existing gaps

between the bones and the nail is an astonishingly short period of

time from where they penetrate in to the neighbouring

malperfused cortical bone and initiate endosteal bone formation.

Tilt nails gives the best results.

The healing of well done closed nailing and the shaft of tibia or the femur depends on the fracture geometry and the level of fracture healing in a biological process helped by mechanical stability.

INDICATIONS

1) All closed fractures of tibia

2) Aseptic non union

3) Pathological fractures

4) Deformity correction

5) Septic non union

6) Open fractures up to grade 3 tibial diaphyseal fractures

7) Limb lengthening procedures

8) Arthrodesis

Advantages:

Fractures fixed with intramedullary nails displayed higher

values for blood flow in the whole bone and at the fracture

site which remained elevated for longer time than these

managed with rigid plate fixation.

ü For the weight bearing bones, intramedullary nailing is a

fixation because the location of the rod in intramedullary

canal virtually guarantees proper axial alignment

Disadvantages

ü The size of the intramedullary canal may limit the size of the

nail that can be used, this limits the bending strength of the

nail unless extensive reaming performed.

ü Intramedullary nails particularly reamed nails interfere with

the endosteal blood supply, which makes up to 90% of

vascular supply to diaphysis of long bone.

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TYPES OF NAILS

1. REAMED NAILS

The classic reamed nail is hollow open section nail of kunscherz. Reaming provides precise fit for nail in intramedullary canal, thereby reducing the incidence of nail in correction and improving the stability of the fixation.

Reaming permits the use of longer nails which are stronger than the smaller ones.

2. Non reamed nails

Single, non reamed, non locking nails have been designed for most of long bones including femur, tibia, humerus and foreram bones.

Single non reamed nails are easy to insert and associated with improved preservation of endosteal blood supply and rapid revascularization.

There disadvantages include an increase likelihood of impaction during driving and because smaller nails must be used.

3. Locking nails

They have single non locking nails absolute. The only advantage of non locking single nails are their simplicity and low cost.

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•4. Specialized nails

These are based on locking principle;

i. Gamma nails: developed in a short designs for fixation of intratrochanteric fractures. Now they are available in long devices that function like reconstructive nail.

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•PINS

•STEINMENN PINS

Steinmann pins are rigid stainless steel pins of varying lengths, 4-

6mm in diameter. After insertion a special stirrup is attached to

the pin. The bohler stirrup allows the direction of the traction to

be varied without turning the pin in the hole.

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They are now a days threaded rather than the ones that are

smooth, smooth pins tend to loosen rapidly, so they slip in and

out, leading to soft tissue infection or osteomylitis of bone

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Indications

     They are mainly used for traction through the femur, tibia, calcaneus

Complication

· Pin traction infection

· Ligamentous damage

· Damage to epiphyseal growth plates when used in children

· Depressed scars

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DENHMANS PIN

The denhman pin is identical to Steinmann pin accept for a

short raised threaded length situated towards the end.

This threaded portion engages the bone cortex and reduces

the risk of pin sliding.

This type of pin is particularly suitable for use in cancellous

bone such as calcaneus or osteoporotic bone.

NEUFELD PIN

It is advantageous in elderly, medically unstable patients

with impacted and non displaced femoral neck fractures.

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•TENSION BAND WIRING

If a fracture is to unite, it requires mechanical stability, which is obtained by compression of the fracture fragments.

· Conversely, distraction or tension interferes with fracture healing. Therefore, tension forces on a bone must be neutralized or, more ideally, converted into compression forces to promote fracture healing.

· This is especially important in articular fractures, where stability is essential for early motion and a good functional outcome.

· In fractures where muscle pull tends to distract the fragments, such as fractures of the patella or the olecranon, the application of a tension band will neutralize these forces and even convert them into compression when the joint is flexed.

· Similarly, a bone fragment can be avulsed at the insertion of a tendon or ligament.

· Examples include the greater tuberosity of the humerus the greater trochanter of the femur, or the medial malleolus.

Here, too, a tension band can reattach the avulsed fragment, convert tensile force into compression force allowing

immediate motion of the joint.

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•EXTERNAL FIXATORS

In the management of limb injuries, external skeletal fixations,

wide variety of applications, now has a firm place in the

armamentarium of technique available to trauma surgeon.

External fixations is a method of immobilizing fractures by means

of pins passed through the skin and bone.

In external fixation a minimum of metal exists inside the tissues,

and the fracture elements are will realigned, distracted or

compressed.

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INDICATIONS:

1. Compound fractures

2. Closed fractures with severe associated soft tissue injuries;

compartment syndrome

3. Limb injuries requiring plastic and vascular procedures.

4. Stress shielding device to protect internal fixation

5. Infected non unions

6. Poly traumatized patients

7. Selected fractures of the pelvis

There are two types main types of external fixators: Pin fixator

and Ring fixator

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PIN FIXATOR

· They are applied quickly to stabilize most diaphyseal fractures.

· Also wound access is adequate for management of soft  tissue injuries.

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Disadvantages:

1. The fracture needs to be reduced before constructing

the frame.

2. The presence of a fixed bar, limits adjustability of the

frame to control angulatory and rotatory deformities,

3. It does not allow axial loading at the fracture site.

4. There is high incidence of delayed union and non union.

5. Not suitable for ankle and pelvis fractures.

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RING FIXATOR

· In this mode of external fixator, the frames have a major role to play in complex reconstructions.

· These frames replicate the structure of a long tubular bone and are somewhat like exoskeletal.

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The bone is stabilized by tensioned wires acting like an  elastic band.

· The frame gives stability for the fracture.

•Fracture healing is better than in the pin fixator as weight bearing produces micromovements that favour faster healing

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DISADVANTAGES:

1. They are heavy and cumbersome

2. It is a time consuming procedure

3. There is a risk of neurovascular damage as the pins and wires transverse the entire thickness of the bone.

4. Oedema is a commoner occurrence in unilateral frames.

•USES

1. Progressive deformity correction

2. Limb lengthening and

3. Management of non union.

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Basic components of an external fixator

· Bone screws or pins

· Clamps

· Couplings

· Central body

· Compression-distraction system

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1. The pin (schanz screw, half pins)

· The stabilizing hold on a bone segment is obtained through a

specialized bone pin that does not pass much beyond the far

cortex

· This pin has threads at one end and a rounded tip at the

other end.

· The half pin is a main stay of the external fixator. It is a

modified cortical screw and it is only used as a hold fast, it

does not exert intrafragmentary compression as cortical

screws.

· A Steinmann pin is also used which passes through the bone.

2. The clamp

It provides a connection between the pins and the other components of the fixator.

· There are two types, in the first type an indiviual pin is fastened to the frame by a single pin tube articulation.

The second type can attach a group of pins together and attach them to the main frame.

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3. The central body

· The central body, a connecting rod or a tube is the mian structure of the fixator.

· Increasing the number of rods, increases the rigidity of the frame.

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4. Compression-distraction system

· The compression- distraction assembly can be fixed to the

main structure in special circumstances.

· These devices are useful to apply compression at the

fracture surface or bone interface.

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5. Frames

· The three dimensional structure built with the components of a fixator system is called a fixator frame, construct, or a  fixator configuration.

· Types of frame are: unilateral, unilateral uniplanar, unilateral biplanar,bilateral, bilateral uniplanar, bilateral biplanar, modular.

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Unilateral uniplanar frame

· It is a best suitable stabilization frame in regions where the local topography, anatomy and functional considerations make the erection of the double frame or a triangulated assembly impossible.

· The stiffness of this frame in the saggital plane is is higher than in the bilateral uniplanar.

· The increased stiffness neutralizes most bending forces which tend to cause displacement of the fragements.

· Indications: The frame is useful in stabilising humeral shaft, the ulna and radius and fracture femur and tibia.

·

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Advantage: Walking is greatly facilitated and the patient can square or sit crossed legged, fewer skin entry holes reduce the possibility of bacterial contamination and the number of scars.

· Disadvantages: There is no possibility of improving th reduction alignment once the frame is completed, also there is vulnerability of the anterior tibial crest; should infection occur the strongest portion of the tibia can be severly damaged

Unilateral biplanar frame

· It is the most stable of the unilateral frames and it is well for

the treatment of tibial fractures since a large surface of that

bone is subcutaneous.

· Pins are inserted at various positions without going through

muscles, tendons, nerves or vessels.

A unilateral biplanar frame is useful for prolonged application of the fixator in the presence of bone loss or severe soft tissue damage

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Bilateral uniplanar frame

· Depending upon the stability produced by the lag screws, the bilateral uniplanar frame is either applied with a axial compression or used simply to neutralize bending and shearing forces.

· In the presence of a bone defect or severe communition, one

cannot apply compression for fear of producing shortening.

· The stability of this frame is improved by prestressing  the Steinmann  within each main fragement.

· The symmetry of the bilateral uniplanar assembly has offers certain mechanical advantages over the unilateral uniplanar frame, it almost completely eliminates lateral movements of the fragments and it allows for uniform distribution of stress on the cortices and the external structure of the frame.

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Bilateral biplanar frame

· It offers greater torsional stability than other frames, with

only a few additional pins.

· The frame is useful in the presence of a large bony defect

and in achieving arthrodesis of the knee and elbow.

· This configuration neutralizes the bending movements in the

ventrodorsal or saggital plane, which is of great advantage

in the postoperative mobilization of the lower extremity after

arthrodesis of the knee joint.

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Modular frame

· Unilateral uniplanar frame requires pins to be placed in a particular order and does not allow any variation to accommodate soft tissue conditions nor does it permit secondary correction without new pin placement.

· In a modular frame which is a modification of the unilateral uniplanar frame, the pins are inserted as local condition demand.

· The modular frame gives unnprecendented freedom of pin placement and permits the positioning of pins in different planes according to the anatomy and nature of soft tissue damage.

· An example of the usefulness of this frame is the external fixation of the humerus, where damage to the radial nerve is avoided by applying the pins in two planes at right angles to each other.

BONE PLATES

Bone plates are like internal splints holding together the fracture

ends of the bone.

The bone plates can be classified in to four groups

1. Neutralization plates

2. Compression plates

3. Buttress plates

4. Condylar plates

NEUTRALIZATION PLATES

· A neutralization plate acts as a ‘bridge

PRINCIPLE:

1. It transmits various forces from one end of the

bone to the other, bypassing the area of the

fracture.

· Functions:

1. Acts as a mechanical link between the healthy

segments of bone above and below the fracture,

such a plate does not produce any compression at

the fracture site.

2. Used in combination with a lag screw also

counteracts the torsional, bending, and shearing

forces that tend to disrupt the screw, allowing

mobilization of the extremity.

3. The most common clinical application of the

neutralization plate is to protect the screw fixation

of a short oblique fracture or a communited

fracture of a long bone.

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COMPRESSION PLATES

· A compression plate produces a locking force across a fracture site to which it is applied

   This effect occurs according to newton’s third law(action and reaction are equal and opposite).

·  The plate is attached to a bone fragement. It is then pulled across the fracture site by a device, producing tension in the plate. As a reaction to this tension, compression is produced at the fracture site across which the plate is fixed.

   The direction of the compression force is parallel to the

plate.

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The role of compression plate

1. Compaction of the fracture to force together the interdigitating spicules of bone and increase the stability of the construct.

2. Reduction of the space between the bone fragments to decrease the gap to be bridged by a new bone.

3. Protection of the blood supply through enhanced fracture stability.

4. Friction, which at the fracture surfaces resists the tendency of the fragements to slide under torsion or shear. This is advantageous as plates are not particularly effective in resisting torsion.

•BUTTRESS PLATE

 FUNCTIONS

1. The mechanical function of this plate is to strengthen (buttress) a weakened cortex.

2. It prevents the bone from collapsing during the healing process.

3. It is designed with a large surface area to facilitate wider distribution of the load.

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PRINCIPLE

1. In order to prevent shearing at the fracture site or displacement of the fracture fragments bringing about widening of the articular surface, it is necessary to apply a plate from the diaphysis across the outer surface of the metaphyseal-epiphyseal fragment.

2. Such a plate acts as a buttress or retaining wall. A buttress plate applies a force to the bone which is perpendicular to the flat surface of the plate.

A buttress plate is used to maintain the bone length or to

support the depressed fracture fragments.

· Commonly used in fixing epiphyseal and metaphyseal fractures.

· There are two types of buttress plates T-plate or L-plate

·T-plate is used for fixation of the distal radius and tibial plateau, also used to fix fractures of the tibial pilon and the distal humerus.

CONDYLAR PLATE

Its main application is in the treatment of intra articular distal femoral fractures.

FUNCTIONS:

1. It maintains the reduction of the major intra articular fragments hence restoring the anatomy of the joint surface.

2. It also rigidly fixes the metaphyseal components to the

diaphyseal shaft, permitting early mobility of the extremity.

3. It can function as both the neutralization plate as well as the buttress plate.

· A condylar plate is used to fix a proximal femoral osteotomy and intercondylar fracture of the femur

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Discussion:
    - Gilberty & Bateman in 1974, reported use of bipolar prosthesis;
    - rationale was that erosion and protrusion of acetabulum would be less because motion is present between metal head & polyethylene socket (inner
             bearing), as well as between metallic cup & acetabulum (outer bearing), since cup is not fixed in bone;
    - theory that distribution of shear forces between the inner and outer bearings will spare acetabular surface from wear and erosion;
             - acetabular wear is diminished through reduction of total amount of motion that occurs between the acetabular cartilage and metallic outer shell
                    by the interposition of a second low-friction interbearing within the implant;
    - because of compound bearing surface, bipolar designs provide greater overall range of motion than either unipolar designs or conventional THR;
    - made available with a 22 or 32 mm diameter head;

Recent modifications:
             - axis of metallic and polyethylene cups are now eccentric so that with loading of hip, metallic cup rotates laterally than medially, and thus avoids
                    fixation in varus position and avoids impingement of head on edge of cup, which causes frx of poly bearing insert and dislocation;

- Indications:
    - femoral neck fracture:  
           - due to the risks of osteolysis, many orthopaedist are reluctant to insert bipolar components and instead insert unipolar componenets:
    - salvage procedure in revision THR surgery:
           - indications:
                  - massive acetabular deficiencies which do not permit secure fixation of the acetabular component;
                  - hip instability from deficiency of abductors is a relative indication because bipolar component is intrinsically more stable than fixed component;
                  - if patient has no abductor musculature, bipolar implant provides added stability against dislocation compared w/ THR;

•Relative Contra-indications:
    - DDH:
           - further technical point is that a bipolar prosthesis may not do well if the acetabular souricil angle is greater than 15 deg;
                  - most dysplastic acetbuli will have a high sourcil angle;
           - in these cercumstances the acetabulum should be reamed deeply, and if necessary bone grafted superiorly;

Complications:
    - degree of inner bearing motion decreases over time;
    - acetabular migration:
           - bipolar implants in revision acetabular surgery usually migrate, especially in soft bone such as that in rheumatoid arthritis or against bone graft;
           - migration and pain are occasional problems with bipolar implants in young patients requiring total hip replacement;
    - osteolysis:
           - this is any especially prevelant complication due to the thinness of the polyethylene bearing surface (between the inner and out components);
           - bipolar components shed twice as many polyethylene particles as are found in fixed acetabular components;
           - young active patients are at high risk for osteolysis, and in these patients a 22 mm head should be used

•Hip resurfacing

•Total - resurfacing arthroplasty Charnley 1948

     using Teflon bearings – poor wear

1960 –Townley – metal on polyurethane articulation – high wear – later replaced by metal on polyethylene

1967 – Muller – metal on metal bearings

1970 – Gerhard – metal on Vitallium

1st Generation poly fails: thin poly & large femoral heads

•1988- Weber – Metasul bearing – high Co-Cr alloy

•1991: Heinz Wagner- 2nd gen.  Hip resurfacing with titanium shell & metasul inlay

Contemporary hip resurfacing

1.Bearing surface containing high carbon Co – Cr alloy

2. Cementless acetabulum with press fit component without screws

3. Cemented femoral component

4. Diameter approximates diameter of native femoral head & designed to avoid notching of femoral neck.