What organization put forth the principles of fixation?
Arbeitsgemeinschaft fur Osteosynthesisfragen (AO) --> literally means "the working group for the study of osteosynthesis
What are the 4 tenants of bone fixation aka AO principles?
1. anatomic reduction
2. rigid internal fixation to provide stability
3. atraumatic surgical technique to minimize tissue damage & aid in healing
4. early ROM to decrease healing time & prevent atrophy & other complications associated with prolonged immob
Why are cerclage wires used in conjunction with K-wires?
to achieve fixation called TENSION BANDING
What is tension banding? What are some examples?
-when there is dynamic pull on frx fragment thus using pull to achieve compression by converting a distraction force of the tendon to a compressive force.
Examples:
1. styloid process frx of the 5th met where peroneus brevis pulls on the frx fragment
2. t
What is an example of dynamic compression?
- TENSION BANDING is an example of DYNAMIC COMPRESSION when the biomechanical load adds to the compression
What is the lag principle?
-compression is achieved between the most PROXIMAL threads that are DISTAL to the frx line pushing back against to the undersurface of the head of the screw
-this causes compression to be achieved along the part of the screw that is not purchasing the bon
What is the difference between cortical and cancellous screws?
Cortical screws are FULLY THREADED & have narrow distance bt the threads (thread pitch)
Cancellous screws are PARTIALLY THREADED & threaded (1/2 or 1/3 threads) & have wide pitch
When are cortical screws used?
when distal end of the screw from the insertion point will purchase cortical bone
When are cancellous screws used?
when distal part of the screw ends in cancellous bone
What are cannulated screws & why have they become popular?
-cannula of the screw is a small hole in center of screw to allow for screw to be threaded over the pin, helping to ensure proper alignment of the screw --> makes it easier to insert
-SLIGHT DISADVANTAGE: there is a slight weakening of the screw due to de
What are cloverleaf plates used for?
-distal tibia & pilon fractures
What are hook plates used for?
-lateral malleolus fractures (Danis-Weber A)
What is the purpose of a neutralization plate?
-plate used to simply buttress the fracture
What is the purpose of a bridge plate?
-to span a gap or large comminuted area
How are plates used to create DYNAMIC COMPRESSION?
-by using ECCENTRICALLY shaped hole & inserting the screw off center in the hole instead of a hole that is round with the screw placed within the center of the hole
Why were LIMITED CONTACT PLATES (LCP) developed?
-to have new osseous contact between the holes to limit unnecessary pressure applied to the bone --> decrease potential for pressure necrosis to the periosteum & the bone which supplies the vascularity to promote healing --> designed to prevent stress shi
What is the purpose of LOCKING PLATES?
-there are threads within holes in the plate & corresponding threads under the head of the screw or on the head of the screw --> this allows the screw to LOCK into the plate so the PLATE & SCREW can function as a SINGLE UNIT decreasing how many components
How are intermedullary nails used?
-inserted within medullary canal
-usually used in rearfoot & ankle arthrodesis procedures
-inserted & interlocking screws are used to maintain the position of the nail to prevent movement
-nails used to achieve compression across the fusion site when WB s
What type of metal can be allowed for MRI?
TITANIUM
What are risk factors that affect fixation methods?
-bone density/osteoporosis
-pathology that affects bone quality
-corrosion of implant when different metals are placed in close proximity to each other
What fixation are indicated in osteoporotic bones?
-pins or external fixation
-locking plates
-fill drill hole with bone cement prior to screw fixation
What is percutaneous fixation?
-when fixation device pierces the skin & stabilizes the frx while having a portion of the fixation device remain outside the body
How should x-rays be used to assess for proper alignment?
1. maintaining the frx or osteotomy position in the bone to ensure the proper purchase is noted
2. all threads are beyond the frx line to ensure proper compression is achieved
3. screw head is flush against the bone or a washer is employed
4. there is no
What effect does metallic fixation create on CT?
STARBURST effect making the reading more difficult --> obscures some of teh area aroudn teh fixation device
CTs are NOT CI'ed when metal is present
What imaging modality is CI if metallic fixation is used?
MRI bc of very strong magnet presence
IF TITANIUM is used, then MRI is okay to use
What is Vassal Principle?
-if the primary frx is stabilized, then the secondary frxs maintain themselves in the reduced position due to the presence of ligamentous & other soft tissue attachments
-not every frx requires rigid internal fixation with compression or even needs to be
How are wires and pins inserted into frx site?
-inserted using a wire driver
-can be drilled across the frx
-in case of hammertoe frx, can be retrograded across the site as well
What is the technique for inserting CORTICAL SCREWS?
1. overdrill the near cortex to the frx line
2. drill pilot hole to far cortex
3. countersink the near cortex to allow for the screw head to sit flush against the bone
4. measure the depth of the bone (depth gauge)
5. tap the bone which inserts the thread
What is the general rule to achieve proper plate stability?
3 or 4 cortices should be purchased on either side of the frx in the forefoot & 5 or 6 should be purchased in the more proximal bone
How are interfragmentary screws used to achieve compression with plates?
-by placing the screw external to the plate or passing through one of the plate holes --> screw must follow the proper over-drilling technique to achieve compression if it is a cortical screw
What is the general rule for using an external fixator?
-fixator is applied AFTER achieving the desired correction of the procedure, albeit a frx reduction or realignment osteotomy
-exception: doing callus distrcction for gradual deformity correction --> pins are inserted & the frame part of teh device is then
What must mechanically occur to prevent fixation device failure?
-preload must EXCEED functional load
-preload: compression across the frx site by fixation device
-functional load: load the frx site is subjected to post-operatively
What is an example of cyclic load?
-weightbearing that is dependent upon amt of weight, length of time force is applied, strength of implant, strength & stability of bone & frx orientation
Where do screws typically break?
-at the RUN-OUT, junction between screw shank & thread pattern
When can stress shielding occur?
-where the pressure fromt he plate on the periosteum can result in decreased vascularity to the bone under the plate over time thus leading to weakening of the bone from decreased osseous density
What is the FIXATION OF CHOICE?
Intermedullary nail fixation -- generally has good outcomes
What is the "Windshield wiper" effect?
-when interlocking screws are not used or they fail -- allows the distal end of the intermedullary nail to migrate back & forth within the medullary canal
Why is there a high rate of amputation in the distal tibial?
osteomyelitis secondary to poor vascularity and difficulty in success of antibiotics