ACTA CHIRURGIAE ORTHOPAEDICAE ET TRAUMATOLOGIAE ČECHOSL.,
82, 2015, p. 389 - 397
The use of cerclage was condemned for decades based on the erroneous assumption that the cerclage would strangulate bone circulation. We understand today that most of the failures attributed to vascular strangulation were due to a lack of understanding of the biomechanics and biology of fixation and the application thereof. Figs 1 and 2 are examples of recent successful applications of cerclages to fulfill different functions in fracture fixation. The cases shown are extracted from the ICUC® database of continuous, complete, unchanged and audited recordings. In the following the basic and special mechanics and biomechanics as well as biological aspects and application are dealt with in so-called one page papers.
The successful use of cerclage requires attention understanding of the mechanical limitations. In the following the most relevant characteristics are discussed with spring back and loose-lock as new elements to be considered in internal fixation.
The cerclage consists of a loop that encircles bone fragments with or without additional splinting implants like plates, nails or shaft of a prosthesis. The loop usually consists of a wire with a knurled connection, or of a cable with a crimped connection. We will not consider straps here.
The cerclage wires are made of annealed steel that is characterized by its ductility, i.e. large deformation before breakage occurs. This deformability is important when the knurled connection of a wire loop is produced. The deformability of cables made of titanium, with its excellent tissue compatibility and larger flexibility but smaller plastic deformation, depends mainly on the stranded structure of the cable.
Traction produced: Fig. 3 demonstrates the effect of wire diameter compared to cable on the amount of traction produced. In respect to the latter the strength of the wire connection depends on the type of application (spring back or plastic deformation). The plastic deformation on application of the knurl has a strong effect on remaining that is active traction after application (Fig. 4). The data on fatigue is crucial, here again the cable stands out (Fig. 5). The effect cutting and bending down will be dealt with in "special aspects" (Figs 11-13).
The strength determines the maximal load that can be applied without breakage or irreversible deformation. Strength plays an important role in cerclage fixation. The strength of the wire loop is limited by the weaker element of wire breakage or unwinding of the knurled connection. The knurled connection of a wire loop may unwind under load either elastically when not properly tightened, or plastically under excessive load. The cables and their crimp connection are stronger.
The stiffness determines the amount of deformation under load. Stiffness depends upon material and especially structure.
The elongation of a solid steel wire under traction is small but unwinding of the knurl results in a loose-lock situation (see below). With its low bending stiffness the cable adapts better to the shape of the bone cross section and helps to keep fragment tips in place (Fig. 6). The elongation under load of a cable is somewhat larger than the one of a solid wire. This depends on its stranded structure and on the larger flexibility of the titanium when compared to solid wire made of steel.
The lever arm determines the momentum which a given force exerts. The larger the lever arm the stronger the effect of the loop. Therefore, cerclage loops need to be well spaced. This demand collides with the need to avoid the tip of a fragment end in order to prevent its breakage; a balanced compromise is therefore needed (see also Fig. 6).
Long spiral fractures provide large leverage while leverage of short oblique fractures is small. Therefore, the cerclage provides good strength in the former. A point to consider is that the leverage of fragmented bone is much smaller than in simple fractures. (Fig. 7)
The bone is an important partner when considering cerclage fixation. The function of the cerclage loop relies not only on the mechanical characteristics of the loop, but also on those of the bone. While cortical bone is strong and resists, the pressure exerted by a cerclage loop may cut into spongy bone.
The shape of bone fragments fixed by cerclage plays an important role. When considering the strength of a cerclage fixation the weaker element is often the bone due to its small cross section near the tip of a fragment end. Therefore, the cerclage loop should not be positioned in the region near the tip of a fragment end. A distance of about 1 cm from the (full cortical thickness) tip of a fragment end to the position of the cerclage is a rule of thumb.
We call "loose-lock stability" a third type of stability beside absolute stability, i.e. compressed fragment contact, and elastic stability, i.e. a (small) gap allowing reversible displacement under load. Loose-lock needs consideration especially in respect to cerclage fixation. In conditions where there is a loosely applied or loosened cerclage loop the fragment can displace under load with little resistance until the loop "engages" and rigidity limits further displacement. This type of fixation is also typical for locked nailing where the locking screw engages after a certain play within the transverse hole in the nail. A loose-lock stability also occurs when biological loosening at an interface between implant and bone allows some play at the interface. Loose-lock stability exerts an important effect on healing because the range of loose displacement may allow induction of bone repair while the locked range prevents too large a deformation (strain) of the repair tissue and thus prevents nonunion due to excessive strain.
Some mechanical aspects of cerclage require special attention to gain full advantage when reducing and fixing fractures. The goal is to apply and maintain enough traction to keep the fracture fragments aligned and in a stable position in relation to each other (Fig. 8). Failures which were previously attributed to strangulation of blood supply can often be traced back to improper use and therefore insufficient mechanical performance of the cerclage.
When considering the use of cerclage instead of lag screws we consider the fact that multiple fragments do not lend themselves to fixation by lag screws which are independent from plates,
The cerclage offers limited strength and is often applied in a way that results in unstable fixation from the outset. The result is further loosening due to bone surface resorption induced by micro-motion (3, 13).
The following elements need to be considered and appropriate action needs to cope with them:
Thus if the wire is tightened within the elastic range only the cerclage loop is loose at the start, which produces a loose-lock fixation
The cerclage technique offers substantial help for specific situations (like periprosthetic fractures) when applied to provide maintained stable fixation and to avoid surgical trauma (7).
Cerclage can offer substantial help in the reduction and fixation of fractures. It can fill certain gaps left by the existing instrument sets, particularly in the treatment of periprosthetic fractures. For decades the use of cerclage was condemned because it would "strangulate blood supply". Is this fact or myth? The following addresses aspects of blood supply as well as of biological loosening of the cerclage.
Cerclage was a technique in frequent use in fracture management in the early years. It seemed obvious that a long spiral fracture, for example, would profit from simple transverse loops pushing the fragments together. Thus, not only reducing but permanently stabilizing with cerclage appeared to be a favorable solution. Still the results of internal fixation depending on cerclage alone were all too often unsatisfactory. Lack of mechanical strength and secondary instability due to biologically induced instability were the main shortcomings. The poor results were often attributed to strangulation of blood supply in spite of earlier observations (9, 15).
Recent observations question the all too easily voiced and accepted theory of vascular strangulation. Histology (Fig. 18) demonstrates that cerclage loops applied so as to avoid gross soft tissue stripping exert no relevant strangulation. The blood vessels might be squeezed by the cerclage loop at entry into the bone. Due to the radial orientation of the blood vessels entering along muscle fibers (Fig. 16) this effect is small. Any implant to bone contact impedes the blood supply to the bone locally causing "contact damage" (4). For wires and cables the contact is less than a millimeter wide and its effect is superficial and mitigated by diffusion (Fig. 18).
The reason why the blood supply is not strangulated in spite of a closely fitting wire or cable is explained by the fact that the blood vessels are not oriented along the bone but radially (Fig. 16). Therefore, the cerclage loop has a minimal effect on periosteal blood supply.
When cerclage wires are tensioned and fixed with a knurl without special care there is often an elastic spring back producing a so-called "loose-lock stability". The resulting micro movements at the interface between the bone and the wire induce bone surface resorption. (Fig. 17).
Disregarding the myth of vascular strangulation of blood supply, the cerclage offers help (7) in demanding situations like periprosthetic fractures. It is important to avoid spring back of the connecting knurl to prevent induction of biological loosening that will result in gross instability.
To benefit from the advantages offered by cerclages the technology of their application demands understanding, attention and skills. Understanding the rationale of different applications and their effect on fracture healing is a prerequisite for success. (For more information [ctrll + double-click] the following links: Link for cerclage movie: https://www.dropbox.com/s/Ojcsnbfkmhwztfg/CER-CLAGE150327.mov?dl=0)
Cerclages are best suited to long oblique and spiral fractures where they take advantage of a large lever arm. They are exceptionally effective when applied to spiral fractures with a butterfly fragment at a location where all three fragments can be included within the loop. Under ideal circumstances this allows perfect reduction and anatomic contact of the three fragments in one action (Figs 19 and 20). Under other circumstances cerclages enable at least solid fragment contact by approximation rather than precise reduction. Atraumatic reduction, avoiding tissue damage, is realized using the "AO-cerclage passer".
Cerclages may be used for reduction and may remain in place for fixation. Their contribution is restricted by limited strength either due to unwinding of the connecting knurl, or due to breakage of the wire. To gain the best advantage tensioning during application is important. Using cables (Fig. 22) instead of wires alleviates these disadvantages. The cable is stronger, more flexible and is able to reliably install and maintain tension. The limited strength of the cerclage loop does not allow its use as an exclusive (isolated) implant. In turn, when protected through additional load sharing splints such as the stem of a prosthesis, a plate or a nail, the cerclage offers valued help (6). It is important to realize that a plaster cast cannot protect a cerclage, because the plaster cast is only loosely coupled to bone. Such coupling allows a range of bending deformations. The wire breaks before the cast function engages and would protect. The additional plaster cast adds inertia and with it loading. Some possible problems deserve attention:
The attached video clip visualizes the major issues regarding procedures. Tissue damage may occur on application due to stripping of the periosteum if the conventional technology for passing the wire around the bone and catching its tip results in extensive tissue displacement (Fig. 24). To avoid the latter a cerclage passer consisting of two semicircles instead of one (Fig. 25) exists to facilitate tissue-preserving application especially in the femur. Its basic concept evolved in the MIO group of the Technical Committee of the AO Foundation. This application is often less traumatizing than the application of free lag screws. To close the loop solidly under traction a twisted knurl is frequently applied. The wire is elasti-cally deformable, therefore, when the knurl is applied by twisting, the knurl will exhibit a strong tendency to elastically spring back (Fig. 21). This results in loosening of the wire and in a "loose-lock" instability. Spring back may be avoided by twisting the knurl exceeding the elastic limit of the steel and plastically deforming the knurl. Cutting off and bending down the knurl to lay flat against the surface are the next steps that may diminish or eliminate the tension in the wire. Cutting is best done with a tightening movement. Bending down has a very different effect according to the direction of bending. The optimal direction of bending carries forward the twisting movement. Using crimped cables instead of twisted wires avoids the above-mentioned problems and are therefore an important advantage.
Cerclage may be used as a temporary reduction tool or as an efficient supplementary fixation. Safe use requires avoiding the pitfalls listed to take advantage of the possibilities offered.
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