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Principles of fracture fixation in orthopaedic trauma surgery.

Fractures are common injuries with an incidence of 3.6 fractures per 100 people per year. The lifetime fracture prevalence exceeds 50% in middle-aged men and 40% in women over the age of 75 years (Donaldson et al 2008). The management of fractures takes up more time and resources in emergency theatres than any other type of injury (Tambe et al 2005). This review describes the principles of successful fracture management and how fractures heal. This is followed by a description of the implants commonly used for fracture repair and the biomechanical principles that guide their use.

Successful fracture management

Successful fracture management aims to achieve early fracture union without significant complications. It is important to restore patients to a pre-fracture level of function. Fractures need to be reduced to a satisfactory position if they are displaced significantly. Following this the fractures need to be held or stabilised in a satisfactory position to allow fracture healing. In most cases, fracture stabilisation can be achieved using non-operative measures e.g. plaster casts, slings or straps. In some cases however surgical fixation or a prolonged period of traction is needed to stabilise a fracture until union is achieved.

Surgical fixation of fracture fragments can give sufficient stability to permit immediate joint movement, earlier weight-bearing, shorter hospital stay and early return to work and other activities. Early active pain-free mobilisation enhances articular cartilage nutrition by the synovial fluid. It also allows a rapid return of normal blood supply to bone and soft tissues. Early weight-bearing decreases post-traumatic osteoporosis by restoring the equilibrium between bone resorption and bone formation. Surgical fixation prevents many of the disastrous sequelae associated with the non-operative management of fractures with traction, and the associated prolonged periods of immobility, pressure sores, infections and deep vein thromboses. It is these findings that have shaped fracture fixation surgery over the last 30 years and there has been a shift from non-operative management of these fractures to a greater propensity for surgical fixation (Broos & Sermon 2004). This has been promoted by a higher patient expectation and a desire for earlier return to activities of daily living, and facilitated by the design of better surgical devices for fracture fixation.

Bone healing

Depending on the biological and mechanical environment, potentially contributed to by the surgical fixation devices, bone can heal in two ways (Rahn 1987). The first is indirect bone healing, afforded by relative stability of the fixation device. This begins with haematoma and inflammation followed by callus formation that remodels into woven or lamellar bone. This is achieved by any device which holds the bone sufficiently to maintain length and prevent rotation and angular deformity, but still allows a low level of displacement or micro-movement between fracture fragments thus providing a mechanical signal that stimulates the biological repair processes. Gross motion between fracture fragments leads to non-union and fibrocartilage tissue formation and should be avoided. Examples of fixation devices that provide relatively stable fixation include locked intra-medullary nails, plates used as bridging plates and external fixators as discussed below.

The second method is direct bone healing, and is facilitated in an environment of absolute stability provided by the fixation device. Direct healing results from internal remodelling that bypasses callus formation and allows contact healing between two vascular bone surfaces. This can be achieved by a device that prevents fracture surfaces displacing under physiological load. This requires anatomical reduction and inter-fragmentary compression. Inter-fragmentary compression is best achieved by a lag screw, or for large, dense bones, compression plate fixation as discussed below. The design and use of various surgical fixation devices relies on an understanding of bone healing and the loads and forces they are subjected to.

Different implants used in fracture fixation surgery

Fractures vary in their pattern, and bones vary in their size, structure and strength. As a result, a wide variety of fixation devices has been and continues to be developed. More recent implants are made of biocompatible materials and have a low profile resulting in reduced rates of soft tissue irritation. They can also be contoured to suit individual anatomy. The successful application of these fixation devices lies in a thorough understanding of the individual fracture personality, its biology and the underpinning biomechanics of the fixation device.

In addition to understanding how the mechanical environment of fracture fixation shapes fracture healing, it is crucial to recognise how the type of implant affects the biology of the fracture healing. It is thought that adequate blood supply to bony fragments of the fracture through the soft tissues is of utmost importance for fracture healing to progress (Ruedi & Murphy 2000). Hence any fracture fixation surgery which further violates the blood supply in already tenuous devascularised bony fragments will adversely affect healing. This is one of the main reasons why external fixation is preferred where there is significant soft tissue damage, rather than internal fixation and its associated soft tissue dissection.

Any fixation device used in trauma surgery must be biologically inert and not give rise to toxic reactions or local inflammatory changes (Gotman 1997). It is also important to avoid any electrolytic degradation and corrosion by avoiding implant of different materials at the same site. Metals currently used include stainless steel (316L), alloys of chromium, cobalt and molybdenum, and titanium (Ti-6Al-4V). Corrosion resistance of these materials relies on their passivation by a thin surface layer of oxide. Stainless steel is the least corrosion resistant and less inert, but easier to machine and cheaper. The cobalt chrome alloys are biologically very inert but are difficult to machine adding to their cost. Titanium has the favourable combination of high strength, light weight, high fatigue strength and low modulus of elasticity, and is biomechanically closer to bone than 316L stainless steel. This allows production of thinner plates and stronger intra-medullary nails.

More recently there has been interest in the use of biodegradable polymers e.g. polylactic acid (Hughes 2006). These have the advantage of being more flexible resulting in greater weight-bearing by the bone thus avoiding stress shielding, and they do not need to be removed. Their disadvantages include limited strength and risk of adverse biological response, and these have hindered the more widespread use of these materials.

Numerous devices are available for the fixation of fractures and these include pins, wires, screws, plates, intra-medullary nails and external fixators.

Pins

There is a variety of pins used in trauma surgery for the fixation of fractures (Kurup et al 2008). The pins may be smooth or threaded and are available in a number of sizes. Threaded pins provide additional stability as they minimise sliding of bone fragments, but are also more difficult to take out. Among the most commonly used are Kirschner wires, commonly called K-wires, and Steinman pins.

K-wires can be used to hold fragments of bone prior to rigid fixation, and for percutaneous pinning of small bone fractures. They are commonly used in hand and wrist fractures (Malik et al 2010). At least two wires should be used and they should not be parallel (Figure 1). They lack sufficient mechanical stability for use as the primary fixation device in long weight bearing bones. Steinman pins are commonly used to apply skeletal traction and are attached to traction devices.

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Wires

In combination with pins or screws, wires can be used to create a tension band that converts distractive muscular forces to compressive forces at the fracture site. Wires are frequently used for the fixation of olecranon (Figure 2) and patella fractures. They are also used to reattach bone fragments e.g. greater trochanter or olecranon after elective osteotomies (Nork et al 2001).

Tension band wires require tightening to achieve equal tension, and this can result in wire breakage or cut-through of bone. Many companies now manufacture wire tensioning and twisting instruments. The use of wires also relies on making holes for passing the wires and this can be associated with complications. Wires in the form of cerclage wires are also used to deal with periprosthetic fractures, for fixation of bone fragments and plate onto bone.

Screws

A variety of screws are used in trauma surgery (Mudgal & Jupiter 2006). A screw converts rotational or torsional forces to compressive forces. Screws can be cancellous or cortical, self tapping or non-tapping, partial or fully threaded, locking or non-locking. They can be of different sizes from 1.5mm to 8.0 mm, based on the site of their application. The screw essentially consists of the screw head--the bulbous end and the part engaged by the screwdriver, and the shank or core which can be of variable diameter and sizes. The distance between the threads is called the pitch. The factors that affect the holding power of the screw are its outer thread diameter, the thread configuration and thread length. The choice of screw depends on the type of bone, the fracture configuration, the associated implant used and the surgeon's preference.

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Screws can be used in isolation to hold two fragments of bone together. If screws are used in isolation, an attempt should be made to insert them perpendicular to the fracture line to avoid shear forces (Figure 3). If the fixation relies on the lag principle, the hole in the proximal bone fragment is over-drilled to avoid engagement of the threads and this allows compression between the distal fragment engaging the screw threads and the screw head on the proximal fragment. Screws are commonly used in combination with plates (Figure 4) and nails or rods (Figure 5). Screws can be inserted using a torque screwdriver that allows a controlled and constant force to be applied.

Plates

Plates have screw holes that are countersunk to avoid prominence of the screw head. A variety of plates is available that can be used for fracture fixation to provide both relative stability and absolute stability (Egol et al 2004). The way a plate is used determines its mechanical function. In the neutralisation mode, the plate functions to reduce the forces on an inter-fragmentary compression or lag screw. In the buttress mode, the plate acts to support the fracture fragment, preventing displacement from forces. Plates can be used as bridging plates to span gaps where bone has been lost due to trauma or disease. In the compression mode, the screws are inserted offset from the centre and on tightening the bone fragments move closer together providing inter-fragmentary compression.

Locking plates have threaded holes for the screw head to engage in and provide more durable fixation that is better for osteoporotic bone (Cornell et al 2003). These locking plate systems avoid the need for bicortical fixation. The majority of these plates are made of stainless steel or titanium.

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A plate is considered to provide relative stability if it allows appreciable inter-fragmentary movement under functional load. Fracture healing under conditions of relative stability takes place by callus formation. Bridging of the fracture with a stiff splint reduces mobility of the fracture fragments, which allows minimal displacement under functional load. A plate that provides rigid fixation e.g. with inter-fragmentary compression, provides absolute stability of the fracture fragments, diminishes the strain at the fracture site and allows direct healing without callus formation.

Rigid fracture fixation with plates and screws still has an important place and is desirable for fractures that involve an articular surface. Articular fractures require exact anatomic reduction and stable fixation to avoid development of abundant callus. This is important because incongruity of the joint surface and presence of callus formation at the articular surface lead to patient discomfort and often the development of early and progressive osteoarthritis (Dirschl et al 2004).

The use of plates does rely on a significant exposure that can compromise a blood supply that is already disrupted from the fracture and possible associated soft tissue injury. If a plate is in total contact with a bone surface, it can act like a plug and stop radial perfusion, leading to necrosis of the bone. If the bone plate contact is reduced, as is increasingly the case with the newly developed plates, there will be less disruption to this radial perfusion of the bony fragment (Miclau & Martin 1997).

Intra-medullary nails

Intra-medullary devices act as an internal splint to stabilise long bone fractures (Hohaus et al 2008) as shown in Figure 5. They neutralise varus and valgus forces, and allow axial compression. This can result in shortening in comminuted fracture if the nail is not locked. The locking option of a nail allows better rotational control. Nails can be solid or hollow, slotted or nonslotted. Solid nails are stiffer in bending, although these properties may be altered by changing the wall thickness. Reaming of bone allows better fixation of the nail and improves the transfer of load from the bone to the nail and stability (Pape & Giannoudis 2007). Slotted nails have less torsional stiffness than non-slotted ones.

Stiffness and strength are related to the diameter of the nail, and the strength of a nail determines its resistance to fatigue failure. The bending rigidity of the nail is proportional to the radius to the fourth power, thus one large nail is stiffer than multiple smaller nails.

External fixation

Another method of fracture stabilisation is with an external fixation (Ziran et al 2008) as shown in Figure 6. This involves percutaneously placed transosseus pins and/or wires secured to external scaffolding in the form of bars and rings providing support and stabilising a fractured bone. A large number of variables determine the stability provided by the external fixator construct, including the number, location and diameter of the pins and connecting rods as well as the orientation of the construct.

External fixators cause less disruption to the soft tissues, and better preserve the blood supply to bone and periosteum (Claes et al 1999). This makes fixators a useful tool in open fractures enabling satisfactory soft tissue management in the acute trauma setting with skin contusions and open wounds. They are also useful in periarticular fractures as they allow restoration of the articular surface by ligamentotaxis and early mobilisation (Khan & Fahmy 2006). One major complication with the use of external fixators is associated pin site infections. External fixators have a wide scope of use in trauma surgery as quick, life saving temporary fracture stabilisation devices in the multiple trauma patients.

Conclusion

Fractures are common injuries and there has been a greater trend over the last few years from non-operative management with traction to early surgical fixation. There are a number of fracture fixation devices available for the fixation of fractures and these devices could be used in a number of different ways depending on the nature of the fracture and associated injuries. Bone can heal in two ways: the first is indirect bone healing afforded by the relative stability of locked intra-medullary nails, bridging plates and external fixators. The second method of bone healing is direct bone healing, and is facilitated in an environment of absolute stability provided by lag screws and compression plating.

Provenance and Peer review: Commissioned by the Editor; Peer reviewed; Accepted for publication September 2009.

References

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Tambe A, Rodriguez JC, Monk J, Chen C, Calthorpe D 2005 Orthopaedic trauma care: a two-tier system? Injury 36 771-774

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Mamun Al-Rashid

MRCS

Specialist Registrar, Trauma and Orthopaedics, King George Hospital, Goodmayes

Wasim Khan

MBChB, MSc, MRCS, PhD

Academic Orthopaedic Registrar, UCL Institute of Orthopaedics and Musculoskeletal Sciences, Royal National Orthopaedic Hospital, Stanmore

Kishan Vemulapalli

FRCS

Consultant Orthopaedic Surgeon, Trauma and Orthopaedics, King George Hospital, Goodmayes

No competing interests declared

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Correspondence address: Mr W Khan, UCL Institute of Orthopaedics, Royal Orthopaedic Hospital, Stanmore, HA7 4LP. Email: wasimkhan@doctors.org.uk
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Title Annotation:CLINICAL FEATURE
Author:Al-Rashid, Mamun; Khan, Wasim; Vemulapalli, Kishan
Publication:Journal of Perioperative Practice
Article Type:Report
Geographic Code:4EUUK
Date:Mar 1, 2010
Words:3047
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