Surgical fixation with absolute stability
|Surgical fixation with absolute stability|
If a fracture is bridged by a stiff splint, its mobility is reduced and
little displacement occurs under functional load. Although stiffness of the
implants contributes to reducing the mobility
of the fracture, the only technique which will effectively abolish motion at the fracture site is interfragmentary compression.
Absolute stability abolishes deformation (strain) of repair tissue at the fracture site during physiological loading and results in direct bone healing. Reduction of strain to a level below the critical level will reduce stimulation of bone formation, causing the fracture to heal without visible callus.
- In a low-strain environment bone heals directly by osteonal remodeling—the same homeostatic mechanism that exists for normal physiological bone turnover.
This process is also called primary bone healing. It is much slower than
healing by callus formation and so the implant must not only provide and
maintain absolute stability for a
prolonged period of time, it must also be strong enough to resist fatigue failure during the prolonged healing period.
Direct bone healing is not the primary goal of this fracture fixation method
but rather an unavoidable consequence of using a technique that obtains and
maintains a perfect anatomical
reduction. Anatomical reconstruction is the true goal of surgery in articular fractures and in some diaphyseal fractures such as the forearm.
- Disturbance of bone biology or vascularity is far more serious than delayed union or nonunion resulting from a highstrain environment because the fixation is too flexible.
It takes much more experience and greater skill to treat a complication due to disturbed vitality than to fix a simple reactive (hypertrophic) nonunion, which just needs enhanced mechanical stability.
|Mechanics of techniques of absolute stability|
- Absolute stability is achieved by using a compressive preload and friction.
Compression maintains close contact between two fragments, provided compression at the fracture site exceeds the traction forces acting at the fragment ends. Studies in sheep showed that compressive preload (static compression) does not produce pressure necrosis, neither in lag screws nor in plates compressing in an axial direction . Even overloaded bone does not undergo pressure necrosis provided overall stability is maintained.
Stabilization by application of compression. The compressive preload prevents displacement of the fracture fragments and results in absolute stability as long as the compression produced is greater than any traction produced by function.
When fracture surfaces are pressed against each other, friction is produced. Friction counteracts shear forces that act tangentially, so sliding displacement is avoided. Shearing stems, in most cases, from torque applied to the limb, and this is more important than forces acting perpendicular to the long axis of the bone. The amount of resistance to shear displacement depends on the compression-induced friction and the geometry of the surfaces in contact (interdigitation). For smooth bone surfaces the normal forces produce somewhat less than 40% of friction. Rough surfaces allow a firm fixation and interdigitation of the fragments, which additionally counteracts displacement due to shear forces.
Stabilization by application of compression producing friction. As long as the amount of friction is greater than the force that tends to displace the fracture along the fracture plane, absolute stability is maintained. Screw fixation of a plate relies on the same principle.
The lag screw is an implant which stabilizes a fracture by compression alone. The lag screw is applied to have purchase only in the remote cortex, and approximation between the thread and the head of the screw results in interfragmentary compression between the cortices. The fracture located between the far and near cortices is thereby compressed, and absolute stability is obtained by preload and friction.
In vivo experiments have shown that lag screws produce high loads of force (>_ 2,500 N), and such forces are maintained over a period which exceeds the time required for fracture healing. Compression produced by a lag screw acts optimally from within the fracture, in contrast to compression produced by plates.
|Photoelastic model showing the compression exerted upon an oblique osteotomy. The lag screw produces forces of 2,500–3,000 N.|
In vivo experiments have shown that lag screws produce high loads of force (> 2,500 N), and such forces are maintained over a period which exceeds the time required for fracture healing. Compression produced by a lag screw acts optimally from within the fracture, in contrast to compression produced by plates.
There are two disadvantages of compression fixation by lag screws alone. Lag screws provide high compression force, but the lever arm of such compression is, in most instances, too small to resist functional loading. This applies to both bending and shearing because the area of compression is small when viewed from the center of the screw. Thus, in diaphyseal fractures, lag screws must always be combined with a plate that protects them from these forces (= protection plate, previously called neutralization plate). The other disadvantage of lag screw fixation is its lack of tolerance to single overload. When a screw thread strips, it loses its compressive action and is unable to recover its function. This is a contrast to plate fixation where loss of function of a single screw may be compensated by the rest of the fixation.
- Lag and plate screws must not be tightened to a level where they start to give way. With this mode of application, the bone threads are partially damaged and/or the screws are plastically deformed and may fail.
The more the screw is tightened, the greater the risk that it will fail.
Either the bone thread strips or the metal breaks with complete loss of
function. It is especially important to consider
this when titanium screws are used, as titanium provides little prewarning to the surgeon who may apply too much torque. Titanium screws are only slightly weaker than steel screws, but their ductility (plastic deformation before rupture) is low.
A fracture fixed with one or more lag screws results in fixation without motion (absolute stability), but in general such fixation tolerates only minimal loading. A splint bridging the fracture site can reduce the load placed on the screws. Therefore, lag screws are usually combined with plates acting as splints to protect the screw by reducing shear or bending forces. The term protection plate (earlier called neutralization plate) refers to a plate functioning in this way.
A plate can be used to function in five different ways:
- tension band;
A plate may be applied to one side of a fracture and then tensioned (by using eccentric placement of screws in the plate or by using the articulated compression device) to compress the bone (and fracture) along its long axis. This is only effective in simple transverse or short oblique fractures. However, when a straight plate is applied to a straight bone, this will produce compression underneath the plate with slight distraction (tension) of the opposite cortex.
Compression with a straight plate. This photoelastic picture shows that by applying tension to the plate, compression of the plated bone segment can be produced. Thus, compression acts within the bone along its long axis. Such compression is effective only in transverse fractures. With a straight plate, there is only compression in the near cortex, underneath the plate.
This is not a stable situation. Overbending of the plate, so there is a small gap between the plate and the bone at the level of the fracture, will achieve compression of both the near and far cortex and produce absolute stability.
Compression with a prebent plate. Symmetrical compression may be achieved by prebending the plate. The slightly curved plate is applied to the bone surface with the middle part elevated. When the screws are tightened, the far cortex opposite to the plate is compressed as well.
A plate may be placed on the tension side of the bone to act as a tension band. When the bone is loaded, the plate converts tension into compression at the far cortex and produces absolute stability.
A buttress plate is used in the metaphyseal areas. A buttress is a construction that resists axial load by applying force at 90° to the axis of potential deformity. Under such conditions, the plate initially carries full functional load. It can be used to provide absolute stability and is often combined with lag screws.
A bridging plate is used in multifragmentary fractures. It is used to fix only the two main fragments and restore length, alignment, and rotation. There is minimal disturbance of the fracture site and no fixation of further fragments. This technique always provides relative stability with healing by callus formation.
The locking compression plate (LCP) can be used to function in the five different modes described above. Thus, the LCP can be used to provide absolute or relative stability. It resembles a LC-DCP but has combination holes. The smooth part of the dynamic compression unit allows insertion of conventional screws so the plate can be used in the same way as a DCP or LC-DCP. The threaded part of the combination hole allows locking head screws to be inserted to produce a mechanical coupling between the plate and the screw. For multifragmentary fractures, the LCP can be used as a standard bridging plate. However, if locking head screws are used for the entire fixation, the plate is not compressed against the cortex and acts like an external fixator. This is the internal fixator principle. It provides relative stability with minimal interference with the blood supply to the fracture.
- When using the LCP, it is essential that the surgeon understands the different functions of the plate and knows how to use this device to achieve the goals of surgery. Careful preoperative planning is essential and must include the order of screw insertion, which can fundamentally alter the biomechanical function of this device.
Circular external fixators, as developed by Ilizarov, allow complete control
of length, alignment, and rotation of a fracture. These devices can be used to
provide absolute stability. The same principle applies when circular frames are
used to treat hypertrophic nonunions, where the provision of absolute stability
will allow rapid fracture union. Circular frames can
also be used to apply compression across oblique fractures, but this requires careful planning and a more complex frame design. The frame adjustments that allow compression in different planes are difficult to calculate but computer programs are now available to aid the surgeon in achieving this goal.
|Mechanobiology of direct or primary fracture healing|
Bone healing is different in cortical and cancellous bone. The basic elements correspond qualitatively, but as the vascularity and the volume to surface ratio are very different, the speed and reliability of healing is generally better in cancellous bone.
In the diaphysis, absolute stability is achieved by means of interfragmentary compression to maintain the fracture fragments in permanent apposition. Pain will subside and allow for early functional treatment within a few days of surgery.
Radiologically, only minor changes can be observed: Under absolutely stable
fixation, there is minimal visible callus formation or none at all . The
fact that the fragment ends are
closely approximated means that only a fine line can be seen on x-rays. This renders the judgment of fracture healing difficult. A gradual disappearance of the fracture line with
trabeculae growing across this line is a good sign, while a widening of the gap is a sign of instability. The surgeon judges the progress of healing by the absence of radiological signs of irritation, such as bone resorption or the formation of a cloudy “irritation” callus, as well as by clinical symptoms, such as the presence or absence of pain and swelling.
The histological sequence of healing under conditions of absolute stability:
- In the first few days after surgery there is minimal activity within bone near the fracture site. The hematoma is resorbed and/or transformed into repair tissue. The swelling subsides while the surgical wound heals.
- After a few weeks, the Haversian system starts to remodel the bone internally as visualized by Schenk and Willenegger (Fig 1.2-13; 1.2-14) . At the same time, gaps between imperfectly fitting fragment surfaces—if stable— will start to fill with lamellar bone, the orientation of which is transverse to the long axis of the bone.
- In subsequent weeks, the cutter heads of the osteons reach the fracture and cross it wherever there is contact or only a minute gap . The newly formed osteons crossing the gap provide a kind of microbridging or interdigitation.
Fractures in cancellous bone
Fractures around the metaphysis have a comparatively large fracture surface with good vascularity. This offers the opportunity of good fixation in terms of bending and torque, and thus these fractures tend to be more stable, and healing occurs more rapidly. Radiological evaluation is somewhat impeded by the complex 3-D structure of trabecular cancellous bone. The main histological activity seen in fracture healing of cancellous bone occurs at the level of the trabeculae. Healing—due to the larger surface per volume—is likely to occur faster than in cortical bone. Because vascularization of cancellous bone is better than in cortical bone, necrosis is less likely to occur.
The advantage of absolute stability is that it maintains perfect
reduction of the articular surface and allows early functional
rehabilitation. The disadvantages are that internal Haversian remodeling starts
late and takes a long time and that the absence of any movement at the fracture
gap does not stimulate callus formation. Therefore, the implant alone must
stable fixation initially and for a longer period than fractures treated with relative stability.
Recovery of blood supply
Absolute stability also has positive effects on the blood supply. Under stable conditions, blood vessels may cross a fracture site more easily. Despite the deleterious effects of surgical procedures used to achieve absolute stability, once obtained, it supports the repair of blood vessels.
The effect of stability on revascularization. The osteotomy of a rabbit tibia has been reduced and stably fixed. As early as 2 weeks after complete transection of the bone and medullary cavity the blood vessels have reconstituted and are functioning, as this angiography at 14 days shows.
In plate fixation, the comparably large contact area (footprint) of
conventional plates is considered a disadvantage. Bone tolerates mechanical
loading quite well and protects its inner
blood vessels from being affected by it. The blood vessels entering bone from the periosteal and endosteal sides are, however, very sensitive to any external contact. When plates are placed onto the bone surface, they are likely to disturb the periosteal blood supply. In conventional plating, part of the stability is obtained by friction between the plate and bone, which requires a minimum area of contact. Extensive and continuous contact between any implant and bone results in circumscribed areas of bone necrosis in the cortex directly underneath the plate. This may lead to temporary porosis of the bone and, exceptionally, to sequestration. Recent studies have shown that reduction of the implant-bone interface may improve resistance to local infection and enhance fracture healing.
Bridging plate. The plate spans a critical fracture area and is fixed only near its two ends. Thus, periosteal contact at the fracture site that could impede circulation is avoided and there is the possibility of placing a bone graft under the bridge.
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