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Orthopaedic
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Updated :19/06/2005
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Mechanical environment of fracture healing: It is useful to remember the biomechanical environment of fracture healing. We now know that the mechanical environment at that the fracture site has a major influence on fracture healing.The role of mechanical environment in modulating fracture healing is clearly seen in rigid internal fixation where healing takes place by primary bone healing, and secondary healing in non-rigid immobilisation. The bone healing process is a remarkably complex healing process beginning with the stage of inflammation and progressing gradually through the stages of soft and hard callus formation eventually to bone healing. This capacity for healing is remarkable in that mature adult bone can reconstitute itself by forming original tissue. Bone healing takes place in two ways: primary bone healing takes place by direct cortical remodelling. Secondary bone healing takes place by a combination of intramembranous and endochondral ossification. Optimal healing, however, depends on duration, rate, timing and type of mechanical influence. Bone is formed by osteoblasts that are adapted to the very low strains of over 1% change in length. Osteoblast synthesis and proliferation is stimulated at uniaxial strain of between 0.3% and 2.8%. It is known that limited inter-fragmentary movement of 0.2 mm to 1 mm is optimal for fracture healing, resulting in promotion of callus and increase in rigidity. Excessive movement, on the other hand, prolongs fracture healing. Researchers have identified that tissue strain of 2% is suitable for primary bone healing and secondary bone healing takes place at tissue strain of 2-10%. Strian of 10-100% results in fibrous tissue formation and 100% strain to non-union. This is known as Perren's theory.For this stimuli to be effective, it has to be applied early. Carter and Blenman, on the other hand, theorised that vascular supply is the primary factor to determine tissue formation. Following an acute fracture, there is significant strain at the fracture site which is bridged by granulation tissue , which can withstand high strain. They stabilise the mechanical environment and form a scaffold on which cartilage and eventually bone form. It is postulated that primary mechanical function of the callus is t reduce inter-fragmentary strain to a level at which mature bone could bridge the fracture gap. Initially, there is intramembranous ossification proximal and distal to the fracture site with fibrous connective tissue in the fracture gap and cartilage in the surrounding area. Subsequently, there is endochondral ossification of the external callus which allows osseous bridging first at the outside.This periosteal bridge creates a load transfer path through the external callus which reduces strain at the fracture gap, allowing cartilage differentiation and ossification. This allows uniform end to end loading at the fracture site which in turn takes the strain off external callus-leading to its remodelling. Measurement of stiffness can be an objective criteria of fracture healing. Granulation tissue has a Young's modulus of 0.5 MN/m2 and mature bone 20,000 MN/m2. Bending stiffness of between 7-15 N-m/degree is claimed to be a reliable indicator of fracture healing. Mode of fracture fixation: Cortical screw: pullout strength of the cortical screw can be increased by : Cancellous screw: Cannulated screws: generally have less pullout strength because they have relatively large root diameter which reduces bone contact between screw threads. Lag screw: is used to directly provide inter fragmentary compression. It can produce up to five times the compression of a plate. In order to get ideal interfragmentary compression, one needs to Screw fatigue: screws are subjected to considerable loading and can fail by fatigue. This risk can be reduced by The torsional stiffness of a screw is related to the smallest diameter, which is usually the core diameter of the screw. It is proportional to fourth power of core diameter. Most bone screw threads are asymmetrical: §
flat on the upper and rounded
underneath §
helps to provide a wide surface
on the pulling side and little frictional resistance on the underside §
results in more torque to pull
two objects together and waste less force on overcoming friction during
screw insertion. §
the amount of thread in contact
with bone determines the pull out strength of the screw §
a deeper thread is useful in weak
cancellous bone to capture more bone between the threads and increase pull
out strength Not all bone needs tapping : §
hard cortical bone needs tapping §
screw can be inserted non-tapped
in cancellous bone to compress
and pack more bone between threads and increase pull out strength A plate
has different uses in orthopaedic preactice: §
static compression device ( DCP) §
a buttress ( in tibial plateau ) §
a tension band ( in asymmetric
loading situations) §
a neutralisation device ( when
used with lag screw) §
a bridging device ( to restore
length and alignment where anatomical restoration is not possible due to
comminution). Plate is fixed with screws to provide friction with the bone to counteract loading at the fracture site. However,
bone until bone is strong enough to take the entire load.
Plate: are used as internal splints to hold the fractrured bone fragments in allignment , to allow compression between fracture ends and load transfer. Correct application of plate requires:
Long bones are loaded eccentrically, resulting in a tension side and a compression side. Position of the plate on the tension side helps to counteract the load compressing the fragments.
Successful creation of a stable bone-plate construct
also depends on §
respecting the local soft tissues
and damaging them as less as possible §
long bone fracture frequently
results in significant disruption of local blood supply. §
This is further damaged during
plate insertion ( dissection, periosteal stripping ). §
New generation of LISS ( Less
invasive skeletal stabilization) and PC Fix ( Point contact fixation)
plates acknowledge this fact. These are locking compression plates (LCP). §
Conventional plating provides
stability by rigid friction between plate and bone provided by the screws.
This requires dissection, plate position against the bone and bi-cortical
screw purchase. Vascularity is disrupted beneath the plate. §
LCP allows screws to lock onto
slots on the plates themselves. Plates can be positioned away from the
bone. Unicortical purchase is sufficient. §
LCP is more like an internal
external fixator placed closer to the bone axis and thus minimizing moment
arm of bending. §
Flexible bridging in LCp fixation
allows callus fixation. §
LCP mode of fixation is
recommended in comminuted meta and diaphyseal fractures, peri-articular
fractures, osteoporotic fractures, peri-prosthetic fractures.
Ref.
An intramedullary nail is
an internal splint which stabilises long bone fractures. They neutralize varus/valgus forces and
allow axial compression. This can result in shortening in comminuted
fracture if nail is not locked. §
They have a "clover
leaf" cross-section to maintain good contact with endosteal bone. §
Nails may be either solid or
hollow, slotted or non-slotted. §
Slotted nails have less torsional
stiffness than non-slotted ones. Shear flow is interrupted at the open
edge and can not support shear stress. §
Solid nails are stronger than
hollow ones. Slotted nails are weaker than non-slotted ones. §
Hollow nails are less stiff in
bending than solid ones, although this may be altered by changing the wall
thickness. §
Stiffness and strength are
related to the diameter of the nail, stiffness in bending is proportional
to the diameter raised to the fourth power and the strength in bending
varies with the third power of the diameter. Simply stating, stronger
nails are more stiff. §
Strength of a nail determines its
resistance to fatigue failure. §
Stiffness is related to working
length of the nail and its moment of inertia. §
Bending and torsional stiffness
is inversely proportional to working length ( cf. a long pencil is easier
to break than a short one) §
The length of a nail that
transmits load from one fragment of a fractured bone to the other is known
as the working length. §
Excessive stiffness of the nail
can give rise to stress shielding.
§
External fixator-fracture complex
is stable but non-rigid. §
There will be movement (or
strain) in the construct on movement. §
Strains, along the long axis of
the bone, are thought to be a good stimulus for healing bone and encourage
new bone formation (see mechanical environment of fracture healing). §
Excess or no strain may inhibit
healing. It is therefore felt that ideally stability should be sufficient
to permit a little axial strain. Frame rigidity is affected by many factors:
Joint replacement is a great success story of modern orthopaedic surgery. However, there are years' of research behind a successful joint replacement prosthesis. A successful prosthesis has to fulfill certain criteria. It should:
In order to achieve these goals, we need to know in detail about the biomechanics of a particular joint. An important reason why early artificial prostheses failed was because of the lack of understanding of the biomechanics of the joint in question. Hip and knee are the joints widely and successfully replaced. The results from replacement of shoulder, elbow, ankle, wrist have not matched the results from hip or knee replacement.
Shoulder: Hemiarthroplasty is more successful and more commonly performed. It can restore ADL to a reasonable degree. But it cannot give pain-free overhead activity. TSR is more promising, but has a high rate of glenoid loosening.This is because of poorly functioning rotator cuff and poor glenoid bone stock in these patients. In the absence of a functioning rotator cuff, the humeral component tends to be pulled upward and anteriorly by the unopposed action of deltoid. Glenoid is normally devoid of enough bone to allow secure fixation. Rotator cuff disease would tend to complicate the problem. Shoulder movement would result in excessive ant-sup humeral migration and resulting edge loading of the glenoid component and eventual loosening. A successful humeral component design needs to reproduce normal anatomy as close as possible. This means offsetting the head compared to the shaft and proper soft tissue balance. Humeral component design has evolved much. Initial designs offered little choice. This would result in improper soft tissue balancing. The head would be too large ( an overstuffed joint) or too loose. New models are modular, offering more choice. Biomechanically, an all PE threaded peg gives better anchorage to a glenoid component. Because of the problems with stemmed models, surface replacement models without the restriction of a stem has been tried. Main principle is minimal bone removal and cementless fixation.
Elbow: important bio-mechanical considerations prior to a successful TER is that it is not a simple hinge joint, it has a carrying angle, it is a load bearing joint-subject to significant shear and rotatory force and the joint reaction vector is normally directed posteriorly. Early linked constrained prostheses failed because they treated elbow as a simple hinge joint. the resulting high shear and rotatory force transmission at the bone-cement interface was responsible for its failure. Linked semi-constrained prostheses are still in use. They are known as " sloppy hinge" as the prosthesis allows some degree of varus-valgus and rotatory movement. Unlinked semi-constrained prostheses used to be resurfacing type.This promises reduction in stress transfer to the interface at the price of reduction in stability. Medullary fixation gives more stability, but you need good bone stock.
Ankle: arthrodesis is still indicated in many cases and widely practiced. This is because of the low success of the first generation constrained ankle prostheses. Arthrodesis makes the ankle stiff. Loss of ankle motion gives rise to abnormal gait pattern, low gait velocity and poor gait efficiency. Compensation is sought by increased movement in the small joints. Many patients with subtalar and mid-foot arthritis would not have the give for compensation after a fusion. Ankle is not a simple hinge joint. It also has changing joint axis. Initial constrained designs treated it as such and failed due to the high torsional and shear forces transmitted to the bone-cement interface. These designs were also cemented; requiring large bony resection. This would leave the metal supported on unsuitable soft cancellous bone. The new generation designs like the star and agility are uncemented and semi-constrained. This ensures good bone stock and less force transmission to the prosthesis-bone interface.
With the increase in longevity of succeeding of generations has come the need to develop reliable joint prostheses. It would be ideal if we could replace damaged joint with auto or allograft. Until that dream becomes a reality, we would have to depend on man-made materials to maintain ADL as we grow older. Artificial joint replacement has revolutionised many lives and the practice of orthopaedics. However, there are still nagging problems with mismatch of elastic moduli, interfacial stability etc. So far, we have mainly depended on cement fixation for implant stability. Then came the idea of bioactive fixation. Example of this technique is the use of uncemented hydroxyapatite coated femoral stem in THR. But this has not resulted in greatly increased implant survivality; it still does not address the issues of elastic moduli mis-match, wear debris reaction. This is where biocomposites come to the scene. The advantage of a composite material is that we can combine the desirable properties of two metals and control mechanical properties. Bioenert composite like carbon has wide industrial application. They are strong, light weight and has low elastic modulus. But they fail by delamination on cyclic loading. Carbon can also elicit chronic inflammation. If these issues can be resolved then they could be a good material to use.
Bioactive materials: every foreign material implanted in the body is bio-active;it induces a foreign body reaction. However, some promote bone formation in various ways. They are classifiable according to their mode of action. However, this is a complex biologic reaction and the classification is somewhat arbitrary. They do not usually have one mode of action.
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plate and screw
