Internal fracture fixation Flashcards
What are the major forces acting on a fracture?
What are the principles of biological osteosynthesis?
- Indirect reduction techniques
- Fracture stabilization using bridging implants
- Limited reliance on secondary implants
- Limited, if any, use of bone grafts
Describe the various gauge diameters and relative tensile strengths of orthopedic wire
What is tensile strength of a wire dependent on?
The cross sectional area (pi x radius squared). Therefore small increase in diameter has a significant effect on tensile strength
In how many directions will a hemicerclage counteract forces?
One direction only - must decide on direction in which to wrap the wire based on this
How many loops are required for a stable twist knot cerclage?
1 - 1.5
What are the general principles of cerclage application?
2 or more cerclage, 0.5 a diameter of the bone apart, perfectly reconstructed bony column. If to be used as sole fixation with IM pin must have long oblique fracture 2.5-3 times the diameter of the bone.
The resting tension of cerclage drops below 30N after how much of a collapse in the bony column?
1% collapse
How much does pushing a twist knot flat after twisting reduce tension?
45-90%
What is the function of a skewer pin?
Prevent shearing of short oblique fractures when secured with a cerclage.
Describe the differences in initial tension and load before loosening for the twist, single loop and double loop cerclage
What is the size of the three available K-wires?
0.035, 0.045, 0.065 inch corresponding to 0.9, 1.1, 1.6mm
How does the area moment inertia of interlocking nails and orthopedic plates differ?
The AMI of interlocking nails is calculated by radius to the fourth power, plates are calculated by thickness to the third power
How much of the medullary canal is ideally filled by an intramedullary pin?
70%
What is stacked pinning, and does it have any biomechanical benefit?
Use of multiple small pins rather than a single large pin. Has been suggested to improve rotational stability, but biomechanical evaluation has not shown any significant advantage.
Is there a biomechanical advantage to Rush pinning over cross pinning?
No, stability is greater with the cross pinning technique.
How much of the medullary cavity is ideally filled with an interlocking nail device?
75-80% (avoid going larger than 90% to prevent iatrogenic fracture)
What are the three types of interlocking nail?
Regular, angle stable, inverse
In which direction of bending are interlocking nails weakest?
Parallel to the long axis of the nail cannulation.
Label the following image.
What is the benefit of using bolts over screws in an interlocking nail device?
Bolts are better able to resist bending (increased core diameter due to absence of threads and higher AMI).
Is the AMI of a 8mm ILN greater than a 3.5mm DCP?
Yes - 6.8 times the AMI of a regular 3.5 DCP, and 3.5 times the AMI of a broad DCP.
What are the main biomechanical differences between the AS-ILN and the regular ILN?
AS-ILN eliminates the slack that was experienced with the regular ILN (particularly severe in torsion [33 degrees of slack]). Hourglass shape of the AS-ILN also increases the AMI of the implant and reduces stress risers at the implant/bolt interface
How does the torsional stability of an inverse ILN compare to a 2.4mm LCP?
Reduced torsional stability, with evidence of slippage of the locking mechanism of the bolts.
What are the advantages and disadvantages of reaming during placement of an interlocking nail?
Advantages: allows placement of a larger interlocking nail.
Disadvantage: damage to the medullary blood supply (potentially greater risk of infection, fat embolism, and delayed healing times).
What size nails are recommended based on patient weight?
5-15 kg: 3-4mm nail.
15-30 kg: 6mm
Up to 40 kg: 7mm
8-10 mm nails: large to giant breed dogs.
How far should locking devices be placed from the fracture gap in an interlocking nail?
1-2 bone diameters (shorter distances equal to or greater than the nail diameter may be possible for the AS-ILN).
What are methods to prevent missing of the distal bolt holes when placing an ILN?
(1) proximal locking devices should be placed first.
(2) the leg should be placed on a Mayo stand to provide stability.
(3) the alignment guide should not be leveraged while drilling.
(4) a system with an adjustable alignment/drilling guide that can be moved closer to the bone should be used.
(5) use of conical bolts or bolts that link the aiming guide temporarily to the bone should be considered.
(6) pulsed, light, and steady drilling should be employed, without bending the drill bit and avoiding application of any force to the alignment guide.
(7) use of a drill bit with a sharp point at the tip may prevent skidding.
(8) the bone should be drilled under visual guidance and using a “feeler-pin.”
What is the recommended insertion point for interlocking nail placement in the tibia?
Cranial to the intermeniscal ligament, at a point midway between the tibial tuberosity and medial collateral ligament in the frontal plane.
How can AS-ILN be applied to fractures of the distal third of the humerus?
Shortening of the distal end of the nail and slight protrusion into the supratrochlear foramen.
What is the entry point for interlocking nails in the humerus?
Junction of the crest of the greater tubercle and greater tubercle.
What landmark should proximal locking devices be placed distal to when using ILN to repair fractures of the humerus?
Tricipital line because the specifics of bone and muscle anatomy of the proximal humerus may otherwise increase the risk of fracture in this area.
What is the major complication rate associated with the use of interlocking nails?
Regular: 4-23%
Angle stable: 0% (missing the distal screw hole reported in <1% of cases with AS-ILN.
What are the primary differences between cortical and cancellous screws?
Cortical screws have less pitch (distance between threads) and less depth to threads compared to cancellous screws.
What is the difference between locking and regular cortical screws?
Locking screws have finer pitch and less depth to increase core diameter to better resist bending forces (less risk of screw pull-out with locking systems)
Name the following implants.
What determines screw pull-out strength?
The outer diameter of the screw and the strength of the bone.
What determines the bending strength of a screw?
The core diameter.
Ideally how long is an oblique fracture to allow lag screw placement?
1.5 times the diameter of the bone
What is the optimal tightness of a screw?
70% of stripping strength. Stripping torque is influenced by the thickness and quality of the bone engaged and the design of the screw threads.
How does the strength of a 3.5mm Synthes locking screw compare to a 3.5mm cortical screw?
Twice as strong, because the core diameter of the locking screw is 2.9mm, compared to 2.4mm for the cortical screw.
What are the biomechanical differences between stainless steel and titanium implants?
Titanium may have better fatigue resistance but it is not as stiff or strong as stainless steel.
Is a 3.5mm DCP broad or 4.5mm DCP regular plate stronger?
3.5 mm broad as uses the same bar stock but has smaller screw holes.
Identify common screw types
What is generally the weakest point of a conventional plate/screw construct?
The shear strength of the bone screw interface
What are the two plate sizes available for veterinary cuttable plates?
1.5/2.0, or 2.0/2.4/2.7mm plates
Are reconstruction plates or DCP plates stronger?
DCP plates. V-notches in the reconstruction plates allow easy contouring but make them significantly weaker. They are available in 2.0, 2.7, and 3.5 mm sizes.
What size acetabular plates are available?
2.0 and 2.7mm
What is the degree of angulation of the undersurface of a carpal arthrodesis plate?
5 degrees.
What is the core diameter of various cortical and cancellous screws?
Name the following plates.
Name the following plates.
Is the LC-DCP a stronger plate than the DCP?
No. It has been designed to reduce stress concentrations risers at the screw hole so may be better able to resist fatigue, however reducing the AMI of the solid section of the plate means that it is not a stronger construct.
What are advantages of the LC-DCP over the regular DCP?
- Less stress concentration at individual screw holes.
- Stress is distributed over the whole length of the plate rather than at the screw holes during contouring.
- Compression can be achieved in either direction regardless of the fracture location (no solid central section of the plate).
- Screws can be angled at a greater degree.
What are the goals of biologic osteosynthesis?
Functional alignment, relative fracture stability, promoting of an optimal biologic environment for healing.
What are the different methods of plate application?
Compression, neutralization, bridging, buttress (specifically for transcortical defects in metaphyseal regions), elastic plate osteosynthesis
What are some variables that determine the stability of a plated fracture?
What is the difference between locking and conventional plates in method of failure?
Conventional plates typically fail in sequential screw pull-out (the shear strength between the bone and screw is the weakest part of the construct). When axial compressive forces overcome frictional forces between the plate and bone, shear stresses are propagated along the screw/bone interface. This leads to sequential screw loosening. Increased tolerance to loosening can be achieved by increasing the outer diameter of the screw.
Locking plates typically fail through catastrophic implant failure or failure of a large segment of bone. Shear stress created during axial compression is converted to compressive stress at the screw-bone interface (due to the angle stable nature of the construct). Bone is much more resistant to compressive rather than shear loads, requiring a failure of a large area of bone for screw pull-out to occur.
What determines the frictional force between the plate-bone interface?
Screw torque and coefficient of friction. Generally the screw with the largest torque experiences the largest load.
Where does a significant stress riser exist in locking implants?
The screw-plate interface. May be minimized by reducing plate-to-bone offset.
What are some guidelines for application of locking plates?
- Long plate span (>3x length of the fracture).
- Limiting screw to hole ratio to <0.5
- Limiting distance between the plate and hole to <2mm.
- Leaving at least 2-3 screw holes empty over the fracture defect (controversial).
How do you calculate the AMI of a screw?
Radius to the fourth power
What is the minimum number of screws for a locking construct?
1 bicortical, 1 monocortical. Third screw if placed close to fracture gap will increase axial stiffness. Fourth screw will increase torsional stiffness regardless of location.
How much greater insertional torque can be achieved with a StarDrive (locking implant) rather than a hexagonal screw head?
65% greater insertional torque.
Name the following implants.
What is the aim of buttress plating?
To negate compression and shear forces within the metaphyseal region. Used to specifically describe repair of transcortical defects within the metaphyseal region.
What is the recommended plate span ratio for bridge plating?
Comminuted fractures: 2-3, simple fractures: 8-10.
Low plate to screw density (0.5-0.4) and higher plate to bone length ratios are also recommended.
What is the goal of elastic plate osteosynthesis?
Preservation of the weak bone-screw interface that is present in juvenile animals. This is through use of flexible fixation (long, thin, compliant plates with long working lengths) that more closely mimics the biomechanical properties of immature bone.
Only recommended in animals <5-6 months of age.
What are some factors that may contribute to development of complications following bone plating?
Extensive soft tissue dissection, disruption of the fracture hematoma, periosteal necrosis secondary to plate compression, iatrogenic trauma with interfragmentary implants.
What is degree of angulation is possible using the polyaxial locking system?
15 degrees, 10 degrees for the PAX, Securos system
How much of the medullary canal should be filled by a IM pin in a plate rod construct?
35-40%. One study found a 20% decrease in plate strain with each increase of 10% of canal filling. Greater than 50% canal filling may result in a too-stiff construct that minimizes axial micromotion.
According to Butare-Smith 2021 in Vet Surg, which of the following cerclage configurations was best able to resist cyclical loading?
1) Twist
2) Single loop
3) Double loop
Double loop cerclage
According to Paulick 2021 in Vet Surg, which of the following 2 constructs were least resistant to cyclic loading in a feline ilial fracture model?
1) ALPS-5
2) ALPS-6
3) 2.0 LCP
4) FIXIN 1.9-2.5 series
5) DCP 2.0
ALPS-5 and DCP
According to Raleigh 2021 in Vet Surg was the biomechanical performance of an interlocking thread screw or buttress thread screw greater in simulated repair of a lateral humeral condyle fracture?
The interlocking thread screw had less condylar fragment rotation at failure compared to a buttress lag screw, and resisted greater loads than a positional buttress screw.
In a study by Nabholz 2019 in VCOT, when using a Targon interlocking nail, which bone was associated with a high rate of damage to the neurovascular structures during percutaneous placement in a cadaveric model?
The humerus. Placement in this bone was not recommended based on this study. Percutaneous placement was safe in the tibia and femur.
What 3 miniature locking plate systems used to fixate radial and ulnar fractures in toy breed dogs are shown here?
What is the locking mechanism of the screws in the conical coupling (FIXIN) plate system?
The low-profile stainless steel
CCP has titanium alloy bushings that thread into the screw
holes in the plate. The Morse taper fit design creates the locking
mechanism via friction, micro-welding and elastic deformation
between the bushings and titanium screws
What two locking plate systems are depicted in the following image?
- Polyaxial locking plate system
- LCP
In a study by Kaczmarek 2020 in VCOT which of the following locking plate systems had the highest mean amount of thread connection?
1) LCP
2) Polyaxial locking system with monoaxial screws
2) Polyaxial locking system with polyaxial screws
In an additional study by the same author in the same year, under biomechanical testing in a fracture gap model were PLS or LCP plates stronger? Did PLS plates with monoaxial or polyaxial screws perform in a similar fashion?
LCP had the greatest thread connection, with polyaxial locking systems with polyaxial screws the least.
The LCP was 30% stronger than the PLS plates.
Polyaxial and monoaxial screws in the PLS plate perform in a similar fashion under loading (despite only partial contact of the screw head and plate).
What are the currently available polyaxial locking systems, and what degree of screw angulation can be achieved with each?
Currently available systems are the PAX (Securos, US), VetLOX (UK) and PLS (Germany).
The PAX and
VetLOX are made of titanium and represent ‘cut-in’ locking
mechanism which allows multi-directional screw insertion
up to 10° off axis. In these systems, sharp threads of the
titanium screws cut into the softer plate.
The PLS system is made of 316 stainless steel which represents ‘point
loading thread-in’ locking mechanism. The PLS plates feature
partially threaded plate holes, which allow maintenance of
the locking mechanism with screws inserted polyaxially up
to 15° off axis. The threads inside the plate hole are in five
positions at regular intervals.
What implants are depicted in this image from Bird 2021 in VCOT?
Notched head T-plate and LCP
How did the biomechanics of the 2.0mm notched head T-plate and 2.0mm LCP differ in a study by Bird 2021 in VCOT?
The LCP was stiffer and had less strain than the NHTP.
Increasing the working length of both plates reduced construct stiffness and increased plate strain.
In a study by Marturello 2021 in VCOT, which of the following implants had the largest bending compliance and angular deformation when used in a feline surrogate bone model?
1) I-loc 3
2) Targon 2.5
3) LCP 2.0
LCP 2.0 (but the Targon implant had the lowest failure moment on testing once implants were removed, which may increase the risk of secondary fracture following implant removal).
In a study by Kaczmarek 2022 in VCOT, what was the effect of increased angle of screw insertion on screw push-out strength for both a 3.5 LCP and 3.5 polyaxial locking system?
Decreased push-out strength was observed for both systems (although values for the PLS remained well above likely biomechanical loads in vivo).
Increasing screw torque improved push-out strength for the LCP but not PLS.
In a study by Palierne 2022 in VCOT, what was the impact of adding an additional locking screw (two v. three bicortical locking screws) in a fracture gap construct?
Addition of a third screw increased number of cycles to failure by 13%, and increased load to plastic deformation by 24%.
What 3 ESF pins are depicted?
Tapered run out, positive profile, and negative profile.
In a study by Park 2022 in VCOT, did placement of ESF pins beyond the trans cortex and then reversed back into correct position affect pull-out strength?
Yes, for both tapered run out and positive profile pins bidirectional placement was associated with reduced pull-out strengths. No difference was observed for negative profile pins but pull-out strengths were reduced in general.
What plating system is depicted here?
Liberty Lock polyaxial locking system (allows up to 15 degrees of screw angulation).
In a study by Evans 2024 in VCOT, what was the effect on the biomechanical properties of increasing working length on a 2.0mm LCP in a fracture gap model? What was the effect of plate-to-bone offset?
Increasing working length resulted in reduced stiffness in bending and torsion, as well as increased strain.
Increased offset reduced torsional but not bending stiffness.
In a study by Gallaher 2019 in Vet Surg, what was the effect of 8-weeks of post-operative carprofen on osteotomy healing in experimental dogs?
Delayed osteotomy healing based on radiographs and decreased stiffness and maximum stress compared to dogs that received no carprofen.
No difference in dogs that received no carprofen vs. two weeks of carprofen.
