ME 518
Lecture
19 : Fracture Fixation
TOPICS
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- Goals:
- Constraints:
- Method should take into account mechanisms
which enhance bone healing
- Loading in compression vs. tension
- Weight-bearing vs. non-weight-bearing
- However, best loading mechanism for fracture
healing is not understood and studies have yielded conflicting
results
- Typical: apply some load to damaged tissue
to induce growth and prevent resorption, but not too much that
it damages bone cells
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- Simplest and most versatile implant
- Less than 3/32 inch diameter for classificati
on as wire
- Used to hold fragments of bone together
- Facial fractures
- "Shattered" bones
- Reattachment of greater trochanter in
hip rep lacements if fracture distal to typical osteotomy location
- Long oblique or spiral fractures
- Benefits:
- Problems:
-
- Diameters greater than 3/32 inch, unthreaded
- Used in cases where plating is difficult
or o ther means of achieving stability are not possible
- Pins implanted percutaneously into bone
segments and connected/fixed using an external fixator
(See Figure 0)
- Pins removed after fracture healing has
occured
- Benefits:
- Problems:
- Used alone to connect bone fragments or
in co njunction with plates
- Head designs typical of other screw types
- Two variations of screw design -
- Self-tapping: cuts its own threads as
it is s crewed in
- Non-self-tapping: requires pre-tapping
of hol e
- Pull out strengths equal for both designs
of screw
- Two variations of thread design (See Figure 1)-
- V-thread and Buttress thread
- Pull-out strength of screws is directly
dependent on the diameter of the screw
- Larger screws have a higher pull-out strength
- Further design differences have little
effect on pull-out strength
- Tissue immediately adjacent to screw may
necrose and resorb initially
- Due to initial trauma of inserting the
screw
- Once screw is firmly fixed, tissue ingrowth
into the threads occurs
- If micromotion exists between the screw
and t he surrounding bone, a fibrous capsule will form between
the two materials
- Prevents ingrowth of bone and optimal
fixation
- Loading of bone should be prevented until
firm fixation has occured to prevent micromotion
- Screws should be countersunk into a bone
or plate to minimize stress concentrations at the junction of
the head and shaft
- Can result in corrosion or failure of
the screw
- Screws may be removed following fracture
healing or left in place
- Dependent on surgeon and application
- Benefits:
- Problems:
-
- Means of internal fixation and direct
mechanical stablization
- Many designs with varying screw placement
and cross-sectional shape (See Figure 2)
- Different designs posess different structural
properties, including strength and stiffness
- Some plates designed to induce compression
of bone segments during fixation (See Figure 3)
- May be self-compressing due to seating
of screws
- Also may involve the use of a "jack"
to induce compression before final tightening
- Benefit of induced compression is still
being debated
- Bending moments experienced by bones,
due to muscle forces, generally greater than the bending strength
of bone plates
- Greatest resistable bending moment measured
at proximal femur ranges from 25 - 130 N*m
- Immobilization required until sufficient
heal ing has occurred to allow the bone to bear a portion of the
load
- Bending strength can vary significantly
based on plate design (See Figure 4)
- The effect of screw holes on stress concentrations
within the plate must be considered
- The placement of a plate on a bone is
critical in regards to the load-bearing capabilities of the system
(See Figure 5)
- Fractured bones can withstand compressive
loading but not tensile
- Plates should be positioned to provide
tensile support to the bone during normal loading conditions
- Bone plates are often removed after fracture
fixation
- Benefits:
- Problems:
-
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- Variations on plates can also be sued to fix maxillofacial
fractures which often involve thinner segments of bone
- Titanium mini-plates with non-linear geometries or more flexible
meshes can be used (See Figure 6)
Fracture Fixation - Trabecular
Bone
- More difficult to provide mechanical stability
in fracture locations that are highly trabecular
- Generally fixed with a combination of
plates, screws, bolts, and nuts (See Figure 7)
- Large amount of material required for
fixation
- Fixation of trabecular bone with nails
alone may be possible if the trabecular bone is sufficiently dense
- Fixation of a femoral neck or intertrochanteric
(lower) bone fracture may be accomplished with a hip nail
- Combined with a plate to provide better
stability (See Figure 8)
- Cross-sectional shapes of the nail vary,
but are designed to prevent rotation of the nail
(See Figure 9)
- Hip nails generally involve a mechanism
to induce compression at the fracture zone by tightening a screw
- Benefits:
- Problems:
-
- Used for fractures of long bones
- Inserted into the medullary canal
(See Figure 10)
- Should fit snugly and have some elastic
recoil to prevent rotation and slippage
- More resistant to bending than cortical
bone plates, but more susceptible to torsional loading
- May destroy intramedullary blood supply,
but does not affect periosteal supply
- Can be inserted through a small incision
near the proximal or distal end of the bone
- Device designs vary predominantly in their
cross-sectional shape (See Figure 11)
- Clover-leaf, diamond, cross
- Shapes have varying resistances to bending
an d torsion for a given length and "diameter"
- All stiffnesses less than that of the
bone itself
- Benefits:
- Problems:
-
- New, experimental technique published
in Science in 1995
- Mixture of mono- and tri-calcium phosphates,
calcium carbonate, and sodium phosphate
- Can be injected into fracture sites to
provide some mechanical stablization
- Bone fragments or segments can be held
in pla ce to allow for traditional casting
- Reduces or eliminates necessity of using
impl anted fixation devices
- Especially good for trabecular fractures
- Is gradually resorbed and replaced with
healt hy tissue
- Mineral paste hardens to provide initial
fixa tion in 10 minutes
- Compressive strength approximately 10
MPa
- Within 12 hours, mineral is fully cured
- Compressive strength 55 MPa
- Tensile strength 2.1 MPa
- Also good for filling bony defects resulting
from surgery, implant revision, implant removal (ie. screws),
etc.
- Benefits:
- Problems:
-
- Used to fill gaps in native bone due to
surgery or injury
- Can be chips of trabecular bone or segments/strips
of cortical bone
- A new material known as Grafton is demineralized
bone matrix, either in gel or fabric form
- Gel can be used to fill in the gaps that
exist when using bone chips (See Figure 13)
- Forms continuous interface between composite
graft and host bone
- Fabric can be applied in non-load bearing
situations to act as inductive surface
-
- Difficult due to percutaneous positioning
of implant, with the external portion existing in the hostile
oral environment
- Must withstand significant cyclic loading
- Compression, shear, and torsion during
chewing
- Dentures -
- Supported by gums and underlying bone
- Lack stability, aesthetically poor, can
result in resorption of jaw bone
- Benefits:
- Problems:
-
- Fixed within the jaw bone
- Consist of a self-tapping root implant
which is positioned into the alveolar bone (See Figure 14)
- After being covered by soft tissue, the
bone and implant are allowed to heal for about 14 months to ensure
good stability
- A crown is then placed on the exposed
end of the root implant to provide a more natural looking tooth
- Root implants have been made from metals,
ceramics, and polymers with little difference in success rates
- Benefits:
- Problems:
-
- Park, J.B., Biomaterials Science and Engineering, Plenum
Press, New York, 1984.
- Radin, E., Practical Biomechanics for the Orthopaedic Surgeon.
- Mermer, R.W., Orban, R.E., Jr. "Repair of Orbital Floor
Fractures with Absorbable Gelatin Film," Journal fo Cranio-Maxillofacial
Trauma, 1(4):30-34, 1995.