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Paul Banaszkiewicz Paul Banaszkiewicz Section Editor, Segment Author
Edward Edward Jeans Segment Author
  • Biomechanics of fracture fixation is a very important orthopaedic topic that crosses over a number of viva stations such as basic science and trauma.
  • It is essential for surgeons to have an excellent working knowledge of the biomechanics of the implants they use in everyday practice for fracture fixation. This is so to minimise the risks of mechanical failure of the construct occurring.
  • Although we routinely use screws, plates, nails and external fixation in everyday trauma practice it is surprising how we take this usage for granted without necessarily fully understanding the biomechanical properties of their use.
  • In recent years there has been a change of emphasis from mechanical to biological fixation of fractures.
  • This aims to produce the best biological conditions for healing rather than absolute rigid stability.
  • Achievement of accurate precise stable fixation requires a fairly extensive operative dissection and increases the risk of delayed healing, infection and non-union.
  • With biological fixation there is avoidance of precise anatomical reduction and extensive soft tissue dissection.
  • Restore length and alignment
  • Decrease pain
  • Assist rehabilitation
  1. Mobilise
  2. Restore joint function
  • Achieve fracture union
  • Prevention of complications
  • The resistance to action or to change.
  • Although inertia has no units of measurement, the amount of inertia a body possesses is directly proportional to its mass.
  • Moment of inertia: resistance of a body at rest capable of rotatory motion.
  • Resistance to bending, twisting, compression or tension of an object is a function of its shape.
  • Relationship of applied force to distribution of mass (shape) with respect to an axis.
  • Three types:
  1. Mass moment of inertia
  2. Area moment of inertia
  3. Polar moment of inertia


BS7BIOMECHANICSFRACTURE1.jpg

Figure 1. Common cross sectional shapes and moments of inertia

  • Where m = mass and r = distance from axis of rotation.

Mass moment of inertia

  • The resistance to a change of state (a speeding up or slowing down) during rotation.
  • Intuitively, this would appear to be dependent on the mass of the object and the way the mass is distributed. In fact, the effect of distribution of mass is dominant as the following formula indicates:

I = m.r2

m = mass of the body, r = radius of gyration (perpendicular distance to centre of the mass)

  • Depends on the distribution of material around the axis of rotation rather than the total mass of the body.
  • The further the mass is distributed from its centre of rotation, the larger the mass moment of inertia.

Area moment of inertia

  • Resistance offered by a structure when placed under a bending load.
  • Depends on the shape of its cross-section.
  • Formulae differ depending on the different geometric cross-sections used.
  • The area moment of inertia takes into account the fact that structures are more resistant to bending when the cross-sectional material is distributed further away from the neutral axis.
  • A long bone, with its tubular shape, is aptly designed uniformly to resist bending in all directions and in addition has its mass located circumferentially at a distance from the neutral axis, thus providing a high area moment of inertia and high resistance to bending.

Polar moment of inertia

  • Polar moment of inertia: rigidity or strength of a rod or tube against torsional stress.
  • Moment of inertia proportional to r4.
  • Increase in radius by callus greatly increases moment of inertia which increases stiffness and strength.

BS7BIOMECHANICSFRACTURE2.jpg

Figure 2 Polar moments of inertia relating to radius 4

  • In bone healing:
  • Callus formation around the periphery of a fracture increases the second moment of area (I) and the polar moment of inertia (J) of a bone, thus maximising the strength and stiffness of the bone in bending and torsion during healing.

BS7BIOMECHANICSFRACTURE3.jpg

Figure 3 Moments of inertia relating to radius 4.Strength and rigidity are significantly increased when the callus is located over the periosteal surface, compared to within the medullary canal

  • A screw is a device that converts a torsional force into a compressive force across a fracture site.
  • Complex structure with a four part construction: head, shaft, thread and tip.

BS7BIOMECHANICSFRACTURE4.jpg

Figure  4.Components of a screw 

Components of a screw

Screw head

  • Serves as the attachment for the screwdriver.
  • Countersink = conical area under head.
  • Hexagonal head recess design is most popular because:
  • It avoids slippage of screwdriver and thus head distortion.
  • It allows for better directional control during screw insertion.
  • The torque is spread between six points of contact.

Screw shaft

  • This is the smooth link between head and thread.
  • The “run out” is the transitional area between shaft and thread. This is the area screws break.

Screw thread

  • The standard orthopaedic screw has a single thread (more threads increase the rate of advancement, but produces less compression for the same energy).
  • Core/root diameter = the narrowest diameter.
  • The cube of the root diameter is proportional to the torsional strength of the screw.
  • Outer/thread diameter = across the maximum thread width.
  • The larger the outer diameter the greater the resistance to screw pullout.
  • Pitch= the distance between adjacent threads.
  • Cortical screws have small pitch and cancellous screws have large pitch.
  • The stronger the bone the smaller the pitch.
  • Lead = the distance the screw advances with each turn.
  • The smaller the lead the greater the mechanical advantage of the screw.
  • Cortical screws have a smaller lead than cancellous screws.
  • Pitch and lead = incline of a ramp. A barrel travels a shorter distance on a steeper incline before it gets to the top, but it is harder to push it up the ramp.
  • Thread design:
  • 'V' profile – produces shear and compression forces.
  • Buttress profile – produces compression forces only.
  • Shear forces promote bone resorption, reducing pullout strength.
  • Thread length:
  • Partially threaded screws are designed for lagging cancellous bone.
  • 80% of the screw's grip is determined by the thread on the near cortex and 20% on the purchase at the far cortex.

BS7BIOMECHANICSFRACTURE5.jpg

Figure 5. Screw tip

Screw tip

  • Blunt tip of self-tapping screw – cortical.
  • Fluted to act as a cutting edge and transport bone chips away.
  • The sharpness, number and geometry of flutes determine its effectiveness.
  • Blunt tip of non-self-tapping screw – cortical.
  • The rounded tip allows for more accuracy and direction into a pre-tapped hole.
  • More “effective torque” is obtained from pre-tapping ® increased interfragmentary compression.
  • Corkscrew tip – cancellous screw.
  • Compresses trabecular bone and produces compression by overshooting the pre-drilled hole.
  • Trocar tip
  • Doesn't have a flute, thus displaces bone as it advances.

Screw insertion

Drilling

  • Heat generation:
  • Bone heated to >45°C leads to osteocyte necrosis, deactivation of alkaline phosphatase and degradation of collagen-hydroxyapatite bone. This results in permanent alterations in the mechanical properties.
  • Causes:
  • Dull drill bit – also causes crushing of bone and small local fractures.
  • Time
  • Thick bone
  • Excessive thrust and speed
  • Dry bone
  • No drill sleeve ® drill wandering
  • Good drilling practice:
  • Straight, sharp drill bit with three flutes and cutting angle of >70°
  • Clean the tip frequently
  • Start slowly and maintain the drilling angle
  • Use a drill sleeve
  • Simultaneous saline irrigation

Tapping

  • Allows precision placement when placing screw obliquely (lag).
  • Less torque lost in overcoming friction at the bone-screw interface.
  • Less force required. = Less likelihood of losing fracture position.
  • Self-tapping screws = quicker, less instruments, tight fit, same holding power as pre-tapped screw.

Lag screws

  • Involves placement of one or more screws across a fracture or osteotomy site to produce interfragmentary compression.
  • Achieved by over-drilling the near cortex.
  • The ideal position is perpendicular to line of fracture, but this does not provide axial or rotational stability. Therefore, should try and use more than one screw with the other screw perpendicular to the long axis of the shaft.

Biomechanics of screw fixation

  • To increase strength of the screw and resist fatigue failure:
  • Increase the inner root diameter
  • To increase pull-out strength of screw in bone:
  • Increase outer diameter
  • Decrease inner diameter
  • Increase thread density
  • Increase thickness of cortex
  • A greater density of the bone it is placed into
  • A longer embedded length of screw shaft
  • To improve screw purchase consider embedding the largest diameter screw possible into the bone of the greatest density over as long a purchase length as possible.
  • Interference with healing during implant insertion.
  • Stiffness of implant alters strain at the fracture site
  • Failure of implant:

Race between fracture repair and implant failure.

  • Fatigue failure is a common mode of failure of orthopedic implants and fracture fixation constructs.

Three main fracture fixation devices

  • Plates and screws
  • IM nails
  • External fixators

Plates

  • Bone plates are like internal splints holding together the fractured ends of the bone.

Types of plates

  • Regardless of their length, thickness, geometry, configuration and types of hole, all plates may be classified in to four groups according to their function.
  1. Neutralisation plates
  2. Compression plates
  3. Bridging plates
  4. Buttress plates

Advantages

  • Versatile
  • Offer the option of anatomical reduction of the fraction with stability for early function of muscle–tendon units and joints.

Disadvantages

  • Invasive technique
  • Higher bending moment than nails
  • Load bearing
  • Risk of bone refracture following their removal
  • Stress protection and osteoporosis beneath a plate
  • Plate irritation
  • Very occasionally immunological reaction

Neutralisation plates

  • Function as load-sharing devices.
  • Protect (neutralise) a direct (lag screw) fixation.
  • These plates are placed across a fracture, already reduced and compressed by lag screws, to neutralise the effect of bending, rotational, and axial forces on the fracture site.
  • No compression at the fracture site.
  • Absolute stability, direct fracture healing.

BS7BIOMECHANICSFRACTURE6.jpg

Figure 6. Neutralization plate used commonly in fractures with butterfly or wedge-type fragments after interfragmentary screw fixation of the wedge portion of the fracture.

One third tubular plate

  • Plates have the form of one third of the circumference of a cylinder.
  • Low rigidity (1mm thick).
  • Oval holes – axial compression can be achieved.
  • Uses – lateral malleolus, distal ulna, metatarsals.
  • Limited stability.
  • The thin design allows for easy shaping and is mainly used on the lateral malleolus and distal ulna.
  • The oval holes allow for limited fracture compression with eccentric screw placement

Compression plates

  • The fracture fragments are driven together, compressing them. This improves stability, allows primary bone healing with minimal calleous formation and improves the plates’ resistance to bending fatigue failure.
  • Low profile with combination holes.
  • Can eccentrically drill in order to compress.

BS7BIOMECHANICSFRACTURE7.jpg

Figure 7. Compression plate

BS7BIOMECHANICSFRACTURE8.jpg

Figure 8. When the head of the screw advances downwards towards the bone surface,the screw and the fragment of bone it is attached to slide towards the centre of the plate. The fracture surfaces are driven together which creates a significant compressive force across the ends of the fracture.

  • Compression can be:
  • Static: does not change with time
  • Dynamic: periodic partial loading and unloading

BS7BIOMECHANICSFRACTURE9.jpg

Figure 9.Axial compression result from the interplay between screw hole geometry and eccentric placement of the screw in the screw hole.The screw hole is like a portion of an inclined and angled cylinder

  • Compression plates cancel out torsional, bending, and shear forces and create compression across the fracture site through specially designed self-compression holes in the DCP design. Torsional loads are resisted by the frictional force and interlock between the ends of the fracture components.
  • Fracture fixation provides absolute stability and healing is by direct fracture healing with no callous formation. The fracture gap is reduced.

Limited-contact dynamic compression plate (LC-DCP)

  • Represents a design change to overcome problems with DCP
  • Structure:
  1. Structured undersurface
  2. Undercut screw holes
  3. Trapezoid cross-section

Advantages

  • Improve blood circulation by minimising plate–bone contact.
  • More evenly distribution of stiffness through the plate.
  • Allows small bone bridge beneath the plate.
  • The trapezoid cross-section of the plate results in a smaller contact area between plate and bone.
  • The plate holes are uniformly spaced, which permits easy positioning of the plate.
  • Undercuts plate holes; undercut at each end of the plate hole allows 4° tilting of screws both ways along the long axis of the plate. Lag screw fixation of short oblique fractures is thereby possible.

Bridging plates

  • These plates are used to span a comminuted unstable fracture or bone defect in which an anatomical reduction and rigid stability of the fracture cannot be restored by fracture reduction.
  • This concept combines the relative stability provided by the plate with the preservation of natural fracture biology to achieve rapid callus formation and fracture consolidation.
  • The fracture fixation in this situation provides relative stability. Fractures treated with this more flexible fixation heal through secondary or indirect fracture healing with callus formation.

Buttress plates (antiglide plates)

  • These are load-bearing devices that act to counter shear forces at a fracture site by converting them to compressive axial forces.
  • These plates are placed at the apex of the fracture; the plate–screw construct acts as a load-bearing device.
  • Force applied to the bone is perpendicular to the surface of the plate.
  • Plate has a large surface area.
  • The plate strengthens (buttress) a weakened area of cortex.
  • The plate prevents the bone from collapsing during the healing process.
  • The fixation to the bone should begin in the middle of the plate, i.e. closest to the fracture site on the shaft. The screws should then be applied in an orderly fashion, one after the other, towards both ends of the plate.

BS7BIOMECHANICSFRACTURE10..png

Figure 10. Buttress plate In fractures with a vertical fracture line, a buttress plate is necessary to counteract the vertical shear forces. The buttress plate prevents proximal displacement of the fragment.The buttress plate is added to enhance the stability and to counter axial load on the fracture, especially in osteoporotic bone.

Locking plates

  • Conventional plating relies on the axial force from the applied screw torque and the friction between the plate and bone to resist loading.
  • Locked plates do not require compression between the plate and bone due to the locking nature of threads.
  • By firmly fixing the screw to the plate, the plate screw construct acts as a fixed-angle device; with the screws functioning as threaded locked bolts.
  • Similar to the bars of an external fixator, plates are not applied directly to the bone, thereby providing elastic fixation, which facilitates fracture union through secondary bone healing with callus formation.
  • Free of the need to apply the plates directly to the bony surface, the locked plate creates a more biological approach to the management of fractures, allowing for indirect reduction using minimally invasive percutaneous plating techniques.
  • Indications for use:
  • Osteoporotic bone
  • Revising a fixation
  • Small distal end segment (i.e. the fracture is close to a joint)
  • Complex periarticular fractures
  • Comminuted metaphyseal or diaphyseal fractures
  • Periprosthetic fractures
  • Principles of using locking plates:
  • First reduced the fracture as anatomical as possible.
  • Cortical screw should be used first in a fracture fragment.
  • If the locking screws have been put, use of a cortical screw in the same fragment without loosening and retightening of the locking screw is not recommended.
  • If a locking screw is used first avoid spinning of plate.
  • Unicortical screws causes no loss of stability.
  • With osteoporotic bones bicortical screws should be used.
  • In the comminuted fractures screw holes close to the fracture should be used to reduce stain.
  • In the fracture with small or no gap the immediate screw holes should be left unfilled to reduce the strain.

Tension band plate

  • Plate on tension side.
  • Eccentrically loaded.
  • Compresses the fracture.
  • A tension band plate converts tensile forces to compression forces on the convex side of an eccentrically loaded bone.

BS7BIOMECHANICSFRACTURE11(a).jpg BS7BIOMECHANICSFRACTURE11(b).jpg

Figure 11(a) and (b) Tension band plate

Reconstruction plates

  • Can be bent and twisted in two dimensions.
  • Decrease stiffness than DCP.
  • Should not be bent more than 15°.
  • Used where the exact and complex contouring is required, e.g. pelvis, distal humerus, clavicle.
  • Reconstruction plates are thicker than third tubular plates but not quite as thick as dynamic compression plates.
  • Designed with deep notches between the holes, they can be contoured in three planes to fit complex surfaces, as around the pelvis and acetabulum.
  • Reconstruction plates are provided in straight and slightly thicker and stiffer precurved lengths.
  • As with tubular plates, they have oval screw holes, allowing potential for limited compression.

IM nails

  • IM nails stabilise a fracture by acting as an internal splint, forming a composite structure in which both implant and bone contribute to fracture stability.
  • The load sharing property of IM nails is fundamental to their design feature.

Advantages

  • Compared to plating indirect technique of closed reduction and closed fixation.
  • It is a zero moment device as it is positioned along the neutral axis of the bone.
  • Compared to plating wide healing.

Disadvantages

  • Suited only to certain fracture locations.
  • Difficulties with fractures near the end of long bones.

Definitions

  • Solid or hollow – hollow over a guide wire.
  • Hollow can be open (slotted) or closed.
  • Proximal and distal interlocking screws.
  • Improve stability by resisting torsional and axial forces.
  • Static or dynamic.
  • Dynamic allows axial compression with early WB.
  • Cephalomedullary nail – screw or blade into the femoral head. This improves stability such as with unstable proximal femoral fractures (sub-trochanteric).

Generations of nails

  • First generation:
  • Primarily acts as splints, minimal rotational stability, primarily relies on close fit to bone.
  • Second generation:
  • Improved rotational stability due to the use of locking screws.
  • Third generation:
  • Nails with various designs to fit anatomically as much as possible, to aid the insertion and stability.

Nail biomechanics

  • Stability determined by:
  • Nail design
  • Number and orientation of locking screws
  • Distance of locking screw from fracture site
  • Reaming or non-reaming
  • Quality of bone

Nail design

  • Factors contributing to biomechanical profile include:
  • Material properties
  • Cross-sectional shape
  • Diameter
  • Curves
  • Length and working length
  • Ends of the nail

Material properties

Modulus of elasticity

  • Titanium alloy is nearest cortical bone.
  • Stainless steel is twice that of titanium.

Cross-sectional shape

  • Determines bending and torsional strengths

Nail diameter

  • Nail diameter affects:
  • Bending rigidity D 3
  • Torsional rigidity D4

Nail curves

  • Governs how easily a nail can be inserted as well as bone/nail mismatch, in turn influences stability of fixation of nail in bone.

Nail length and working length

  • A. Total nail length – anatomical length.
  • B. Working length – length between proximal and distal points of firm fixation to bone.
  • Length of a nail spanning fracture site from its distal point of fixation in proximal fragment to proximal point of fixation in distal fragment.
  • Unsupported portion of nail between two major fragments.

Area moment of inertia

  • Area moment of a solid nail = π ´ r4/4.
  • If diameter = 10 mm, r = 5 mm, area moment = 490.
  • If nail is hollow with 10 mm diameter and 2 mm thickness.
  • Area moment = π (r1r2)/4 = 427.
  • Therefore a solid nail of 10 mm diameter is slightly stronger in bending than a hollow nail having 10 mm diameter and 2 mm wall thickness, but the solid nail is much heavier than the hollow nail.
  • If a 2 mm thickness hollow nail is made using the same material used to make the solid nail, diameter will be 16 mm.
  • Area moment will now be 2198.
  • Rigidity is therefore 4.5 times greater than that of a solid nail.

BS7BIOMECHANICSFRACTURE12.jpg

Figure 12. Moments of inertia relating to hollow IM nails

Polar moment of inertia

  • Polar moment of inertia (solid nail) J = π ´ r4/2.
  • Same example with solid nail diameter = 10 mm, r = 5 mm.
  • Polar moment of inertia (solid nail) J = π ´ r4/2 = 981.25.
  • Polar moment of inertia of a hollow nail constructed with diameter 16 mm.
  • J = π ´ 84 = 6430 (6.6 times greater).
  • As such a hollow nail is much stronger than a solid nail, i.e. the further the material is spread away from the neutral axis, the greater is its resistance to bending and torsional forces.

Biomechanics of external fixators

Advantages

  • Versatile and used for:
  • Bone transport
  • Limb lengthening
  • Angular correction
  • Soft tissue reconstruction
  • Contractures
  • Serve many mechanical functions.
  • Especially suited for open fractures and fractures with significant soft tissue injury.
  • Apply quickly.
  • Technically easy to perform.
  • Can adjust at later date if needed.
  • Soft tissues minimally disturbed.
  • Allows access to wounds.
  • Joints can be mobilised.
  • Can dynamise
  • Easy to remove

Disadvantages

  • Pin site infection
  • Mal-union
  • Patient compliance needed.
  • Psychological

Types of external fixator

  • Simple pin fixator
  • Unipolar
  • Bipolar
  • Clamp pin fixator
  • Ring fixator

Pin size

  • Stiffness proportional to (radius),4 so small changes in pin diameter greatly increases stiffness. This is the most significant factor affecting stability of the fixator.
  • Larger pins provide more rigid fixator, providing less bending stress at the bone–pin interface and lower rate of loosening.
  • However, the hole size acts as a stress riser; if the pin diameter is greater than 30% of the size of the bone the risk of fracture greatly increases.

BS7BIOMECHANICSFRACTURE13.jpg

Figure 13. Factors affecting construct stiffness

Number of pins

  • Two pins per fragment to prevent rotation, usually in same plane.
  • A third pin in the fragment adds slightly to the resistance to bending and axial deviations, but little to rigidity.

Pin location

  • Avoid zone of injury or future ORIF.
  • Pins close to fracture as possible.
  • Pins spread far apart in each fragment.
  • Deformation perpendicular to the plane of two pins is best controlled with a third pin in that perpendicular plane. A third pin (C) out of plane of two other pins (A and B) stabilises that segment.

Bone frame distance

  • The closer the frame is to the bone the more stable is the construct.
  • Allow room for oedema and wound care (2–3 cm).
  • External fixator stability increased by:
  • Increasing the pin diameter.
  • Increasing the number of pins.
  • Increasing the spread of the pins.
  • Multiplanar fixation.
  • Reducing the bone-frame distance.
  • Predrilling and cooling (reduces thermal necrosis).
  • Radially preload pins.
  • 90° tensioned wires.
  • Stacked frames.

BS7BIOMECHANICSFRACTURE14.png

Figure 14 Left. When a gap is left on the cortex opposite that to which the plate is attached, bending of the plate at the fracture site can cause the plate to fail rapidly in bending..Right. Compressing the fracture surfaces not only allows the bone cortices to resist bending loads, but the frictional contact and interdigitation helps to resist torsion

  • Failure mode
  • Fracture location

Reason for failure

  • Overload
  • Bone fracture site
  • Implant screw hole
  • Screw thread
  • Small size implant
  • Unstable reduction
  • Early weight bearing
  • Fatigue
  • Bone fracture site
  • Implant screw hole
  • Screw thread
  • Early weight bearing
  • Small size implant
  • Unstable reduction,fracture non union
  • Corrosion
  • Screw head-plate hole
  • Bent area
  • Mismatch of alloys
  • Screw over tightening
  • Scratches during insertion
  • Loosening
  • Screw
  • Wrong screw choice
  • Osteoporotic bone

Table 1. Failure modes of fracture – fixation devices

Failure mode

Fracture location

Reason for failure

Overload

Bone fracture site

Implant screw hole

Screw thread

Small size implant

Unstable reduction

Early weight bearing

Fatigue

Bone fracture site

Implant screw hole

Screw thread

Early weight bearing

Small size implant

Unstable reduction,fracture non-union

Corrosion

Screw head-plate hole

Bent area

Mismatch of alloys

Screw over tightening

Scratches during insertion

Loosening

Screw

Wrong screw choice

Osteoporotic bone


Screw failure

  • The countersink of a screw is either conical or hemispherical.
  • The undersurface of a cortical screw should be centred and perpendicular to the plate hole.
  • If the screw is set to any other angle the undersurface does not adapt well to the plate hole and its wedge shape will create undesirable high forces and uneven contact that predisposes to corrosion.
  • Both factors weaken the screw and may led to screw failure.
  • A screw may break at the run out during insertion if it is incorrectly centred over the hole or is not perpendicular to the plate.
  • Usually it breaks with spiral configuration indicating failure under torsional load.
  • A screw may break if:
  • During insertion if applied torsional load exceeds its torsional strength.
  • When pilot whole is too small.
  • Not tapped in hard bone.
  • Due to lack of lubrication.
  • High stress develop in screw when there is significant resistance to insertion causing screw to shear at a cross-section and leave a part lodged in bone.

Plate failure

  • The most common failure of plate is fatigue failure. Fatigue failure of plate is inevitable if healing fails to occur.
  • Improper application of plates and poor technique are other causes of plate failure.

Biomechanics of plate failure

  • If a gap is left on the side opposite the plate the fracture site can become a fulcrum around which the plate bends under the combined compression and bending loads that will occur with axial loading.
  • Compressing the fracture ends significantly improves the stability of the construct. It allows the bone cortices to resist bending loads with the frictional contact and interdigitation helping to resist torsion. The fracture gap to heal is smaller.

BS7BIOMECHANICSFRACTURE15.png

Figure 15. The application of a plate on the compressive as opposed to the tensile side of a bone subjected to bending causes a gap to open on the opposite side of the plate during functional loading.

  • Plates are vulnerable to bending failure because plates are relatively thin and easy to bend with low moments of inertia.

BS7BIOMECHANICSFRACTURE16.png

Figure 16. The greater the span or distance of a beam is between its supports, the lower its stiffness will be, and the more it will deform under load in bending and torsion. 

Fracture gapping

  • Fracture gapping can predispose to plate failure and occurs if:
  • A segment of bone missing at the fracture site.
  • The plate is not properly contoured to the bony surface before application, e.g. overbending of the plate.
  • The plate is applied to the compression side of the fracture.
  • Inaccurate reduction.
  • Placing the plate so that an empty screw hole is located over the fracture will significantly increase the potential for fatigue fracture of the plate.
  • The plate material around a hole will have higher material stresses than occur in the solid regions of the plate.
  • Around a hole, the force acts through a smaller cross-sectional area so the material stresses must be higher.
  • The stiffness of the construct is one of the important factors in determining fracture healing by affecting fracture-site motion.
  • Stiffness is determined by the material used and thickness of the plate.
  • For conventional compression plate screws should be placed as close together across the fracture site as possible. By comparison locking plates require some relative motion for fracture healing and it is not as critical to insert screws as close together.

BS7BIOMECHANICSFRACTURE17.png

Figure 17. The moment arm of the force will be longer in the case of the more distal fracture and therefore the moment acting at the fracture site on the implant will be larger

Implant failure in interlocking nailing

  • Associated with either the insertion of a small diameter nail or use of an interlocking nail for a very proximal or distal shaft fractures.
  • Plastic deformation (bending) of the IM rod mainly occurs with nails that are less than 10 mm in diameter.
  • Early dynamisation, especially of subisthmal fractures, is associated with increased risk of developing a valgus deformity at the fracture site.
  • Bending of the nail at the fracture site usually occurs as an early complication caused by premature wet bearing, lack of adequate support, or deficient material (nail) strength.
  • Bent distal screws may result from early weight bearing if the screws are too close to the fracture site.

IM breakage

  • If the same force acts on IM rods placed in femora with more proximal (left) or more distal (right) fractures, the moment arm of the force will be longer in the case of the more distal fracture, and therefore the moment, acting at the fracture site, on the implant, will be larger.
  • For the more distal fracture, the high stress region, close to the fracture site, is also significantly closer to the distal locking screw holes, which are significant stress risers.

BS7BIOMECHANICSFRACTURE18 .png

Figure 18 If the screw is not well supported by trabecular bone but mainly by cortex, then its stiffness and strength decrease with the third power of its length between cortices. If the screw length doubles, the deformation of the screw under the same load increases by a factor of eight

 IM rod and locking screw breakage

  • As the distal end of the femur flares rapidly, the length of the locking screw required to cross-lock the rod can be quite variable.
  • If the screw is not well supported by trabecular bone but mainly by cortex, then its stiffness and strength decrease with the third power of its length between cortices. If the screw length doubles, the deformation of the screw under the same load increases by a factor of eight.
  • Screws experience four-point loads.
  • Screws experience significantly higher forces if the screw is closer to the fracture or where there is not cortical contact at the fracture site.

Perren’s strain theory1

  • Perrenintroduced the importance of strain in fracture healing.
  • Fracture gap strain is defined as the relative change in the fracture gap (ΔL) divided by the original fracture gap (L).
  • Tissue cannot be produced when strain conditions exceed the tissue strain tolerance.
  • It is accepted that cortical bone can tolerate only 2% strain. Rigid internal compression fixation, which minimises strain, will lead to primary or direct fracture healing.
  • Lamellar bone can tolerate up to 10% strain, and when this relative stability is present, the fracture heals with callus or secondary fracture healing.
  • Fracture healing will not occur when the strain at a fracture gap exceeds 10%.
  • Comminuted fractures can tolerate more motion than simple fractures, since in a comminuted fracture the overall motion is shared among many fracture gaps.
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References

  • 1. Perren SM. Evolution of the internal fixation of long bone fractures. Bone & Joint Journal 2002; 84(8): 1093–1110.