12.13 Principles of intramedullary nailing

The term intramedullary nail is used to describe a group of implants that are placed within the medullary canal of a long bone. Intramedullary nails form a construct with the bone and act to resist deformity. Nails may be used to prevent fractures, control fractures acutely, or in post-trauma reconstruction. Intramedullary nailing is the treatment of choice for many diaphyseal fractures and modern nail design has extended their application to include some metaphyseal fractures.

It is over a century since the first descriptions of intramedullary nailing. The technique was refined and popularized by the German surgeon Kuntscher who inserted his first femoral nail in 1939. He also introduced intramedullary reaming and, shortly before his death, described an implant that was the forerunner of the interlocking nail. It was Herzog who, in 1950, first introduced tibial nailing after bending the proximal part of a nail. Intramedullary nailing was further popularized through the 1970s and 1980s and has now become an indispensable tool for the orthopaedic surgeon.

Today, various flexible nailing systems are available with a wide range of applications, especially in the immature skeleton.

Nails may be either solid or hollow. The latter are designed to be introduced over a guide wire. Hollow nails were traditionally open section (slotted) but are now more commonly closed section. Both solid and hollow nails have holes proximal and distal to allow the insertion of interlocking bolts. These bolts greatly increase construct stability by resisting torsional and axial forces. In the proximal femur, many nails provide an option to interlock by inserting a screw obliquely into the femoral head. These so-called ‘cephalomedullary’ nails are useful in the treatment of subtrochanteric fractures or ipsilateral neck and shaft fractures.

Interlocking may either be static or dynamic. Static locking provides a fixed relationship between the nail and the proximal and distal portions of the bone. Dynamic locking is produced via an elongated interlocking hole within the nail that allows axial compression while maintaining torsional stability.

The intramedullary nail and bone form a composite structure that is subject to load. The mechanical characteristics of this composite depend on the characteristics of the nail as well as the quality and integrity of the bone. The implant is central in the bone, close to the mechanical axis of the limb, which means it is optimally positioned to resist bending forces.

Intramedullary nails do not, nor are they intended to, abolish movement at the fracture site. This motion stimulates callus formation that bridges the fracture, gradually assuming more of the load and reducing stress upon the implant. The nail and interlocking bolts must be sufficiently durable to survive until fracture healing occurs. If, however, they are excessively rigid they will inhibit the stimuli required for fracture healing.

The strength of an intramedullary nail is a property of its cross-sectional geometry and the implant material. The rigidity of a cylindrical intramedullary nail in bending and torsion is proportional to the fourth power of the radius. Therefore, for a given amount of material, the further the material is distributed from the bending or torsional axis, the stiffer the structure becomes.

Producing a slot in a hollow nail reduces torsional strength by two-thirds, with little effect on the bending stiffness. The slot provides the nail with some radial flexibility, which decreases hoop stresses during insertion and increases contact area between the bone and implant after insertion.

Intramedullary nails are usually made of stainless steel or titanium alloys. The lower modulus of elasticity of titanium alloys has the theoretical advantage of less stress shielding of the bone, but no differences in healing or complication rates have been demonstrated. The geometry of the nail, specifically the diameter and wall thickness, are more significant factors for fracture healing.

The working length of a nail is the distance between the proximal and distal points of secure fixation to bone. With small-diameter interlocking nails without diaphyseal fit, this is usually the distance between the interlocking screws (Figure 12.13.1). The torsional rigidity of an implant is inversely proportional to the working length, and the bending rigidity is inversely proportional to the square of the working length.

Fatigue fracture is the usual mode of failure for intramedullary nails. Failure is usually associated with delayed or non-union, particularly in unstable fracture patterns where the bone contributes little to the overall construct. Nail design is an important factor that relates to the frequency of nail breakage. Defects in fabriscation may also contribute to breakage. Smaller interlocking bolts used with smaller nails are more prone to fatigue failure. In studies examining interlocking intramedullary nailing in tibial fractures, smaller nails inserted without reaming had a higher rate of interlocking bolt failure. This problem is attributed to the size of locking bolt, and is not exclusive to unreamed tibial nails. Locking bolts used in 8- or 9-mm tibial nails are significantly weaker than the bolts used in femoral nails and larger tibial nails (≥10mm). Laboratory testing has shown that the number of cycles to fatigue failure of a 5-mm bolt would be expected to be more than 20 times greater than for a 4-mm bolt.

Reaming of the medullary canal was first employed by Kuntscher to provide a better contact area between the nail and bone, thus improving the construct’s stability. Interlocking bolts and smaller diameter nails can now provide much of this stability, but there are several other advantages to reaming. After reaming, a larger diameter nail can be passed with less chance of nail incarceration, fracture propagation, and implant failure. The morselized bone formed by reaming may also stimulate fracture healing.

Concerns about both the systemic and local effects of reaming have led some to advocate the use of unreamed nails. The pressure and abrasion from a reamer destroys medullary contents and acts as a piston, displacing medullary contents both locally and systemically. Reaming has, therefore, been implicated in fat embolism syndrome, respiratory distress syndrome, and multisystem organ failure. Recent studies have failed to confirm this link. Locally, reaming damages the endosteal blood supply, but, when compared to unreamed nailing, no ill effects have been demonstrated.

The arterial supply to the diaphyses of long bones is from the nutrient, metaphyseal, and periosteal arteries. The venous drainage of bone consists of thin-walled vessels running in the centre of the medullary cavity. The central two-thirds of the diaphyseal cortex receives its blood supply from the high-pressure endosteal circulation, and the outer third of the cortex is supplied by the low-pressure periosteal system. Cortical blood flow is, therefore, centrifugal. The direction of blood flow is reversed after a fracture, resulting from suppression of the intramedullary flow and increased vascularity of the periosteum.

Reaming damages the nutrient arterial circulation, but the clinical significance of this damage is uncertain. The periosteal circulation is only able to supply only the outer one-third of the cortex. Necrosis of the inner two-thirds of the cortex has been demonstrated in animals following reaming. A rapid regeneration of the nutrient system has been demonstrated with loose-fitting nails. Where the nail directly contacts the cortex, osteoclasts must remove necrotic bone to create a channel for vessels. At 12 weeks, the restoration of endosteal circulation is complete. The extent of revascularization varies and is influenced by the extent of original damage. Revascularization is facilitated by vessels, originating from the surrounding soft tissues, which traverse the external callus to reach the necrotic cortex. After reaming, there is an early periosteal vascular proliferation and hyperaemia, which is associated with periosteal new bone formation.

In an effort to reduce problems associated with reaming, some thought has gone into designing a new generation of reamer. The RIA (Reamer Irrigator Aspirator, Synthes) is a tool that irrigates while reaming, simultaneously aspirating the resulting mix of reamings and fluid. The RIA is a single-use instrument, with a steep cutting angle, designed to ream to the required diameter in a single pass, thus reducing operative time. The intramedullary pressures while reaming are significantly reduced, and throughout much of the process are negative. A reduction in local heat generation has also been demonstrated. Overall the RIA seeks to reduce the physiological impact of reaming both locally and systemically. The RIA is also useful for canal debridement after infected nail removal and harvesting bone graft from the femoral canal.

Intramedullary nailing is ideally suited to treating diaphyseal fractures, particularly in the femur and tibia. Nailing provides good stability via a minimally invasive technique, minimizing soft tissue disruption in the zone of injury. Avoiding further damage to the tissues surrounding the fracture site enhances revascularization and aids early bridging callus formation. The strength of the bone–nail construct will often allow for early weight bearing, accelerating rehabilitation.

Statically locked reamed intramedullary nailing is the treatment of choice for diaphyseal fractures of the femur. Reamed intramedullary nailing is the treatment of choice for displaced tibial shaft fractures and can safely be performed in all but the most severe open fractures. Locked intramedullary nailing of the humerus is indicated in patients with pathological fractures, otherwise open reduction and plate fixation is the preferable operative fracture treatment.

Other indications for intramedullary nailing include fixation of subtrochanteric, pertrochanteric, or supracondylar femur fractures, stabilization of arthrodeses, and as a guide for bone transport. Intramedullary nails are also indicated for fracture prophylaxis with metastatic bone tumours, and for tumour-like conditions, e.g. fibrous dysplasia.

Small diameter devices such as Ender’s nails were traditionally used multiply to fill the medullary canal or were pre-bent to provide three-point fixation. These devices did not perform well in adult long bones, especially the femur and tibia, as they did not provide sufficient stability for healing.

Elastic or flexible nailing systems are now very much the preserve of paediatric long-bone fractures. These nails are bent prior to their insertion and are used in opposing pairs. When appropriately placed, the elastic recoil of these nails provides adequate internal splintage to permit fracture healing in the aforementioned bones.

This technique is further discussed in the Paediatric Trauma section (Section 14).

In order to successfully place an intramedullary nail, patient positioning in the operating room is vitally important. Starting and interlocking points must be accessible, fractures reduci-ble, and importantly all parts of the bone exposable to image intensifier.

The starting point is the where the nail first enters the bone and must be considered when positioning the patient. The point may be on-axis, such as the piriformis fossa for many anterograde femoral nails. Alternatively an off-axis starting point may be necessary, e.g. tibial nail, or desirable, e.g. trochanteric starting point femoral nail. Off-axis starting points increase forces required for insertion and may deform the fracture particularly if it is close to the starting point.

Limbs may be placed on traction or left free; both techniques have their pros and cons. The obese patient may provide difficulties; particularly with anterograde femoral nail insertion. Preoperative planning, appropriate implant selection and attention to the previously mentioned points will contribute greatly to the overall success of the procedure.

It is desirable to reduce the fracture by closed means before passing a guide wire or solid nail across the fracture under image guidance. Sometimes fracture reduction is impossible by closed means and open nailing is indicated. There are several methods that may assist in closed reduction.

Tibial fractures can usually be reduced by closed manipulation and held, if necessary, with percutaneously applied reduction clamps. The use of the traction table is often sufficient to reduce femoral fractures, particularly with a fresh injury. Alternatively the femoral distractor may be used to provide traction and maintain reduction. The seldom-used F-tool applies indirect two-point forces, in the plane of deformity, and can be helpful in completing reduction. Shanz screws may be inserted temporarily to aid fracture reduction. They should be placed close to the fracture site and be unicortical on the side of nail insertion. Attachment of a universal chuck and T-handle aids manipulation.

Intramedullary nails are only well suited to reducing diaphyseal fractures. Diaphyseal-metaphyseal junction fractures are prone to predictable angular deformity with nailing. There is a tendency for oblique fractures to displace giving the appearance of the nail exiting the proximal fragment early and entering the distal fragment late. For successful fixation of these fractures the nail must be centralized within the short segment, using the correct entry point and ensuring appropriate nail tip position. Transmedullary blocking or ‘Poller’ screws may be used in conjunction with intramedullary nails to control this deformity. These screws can effectively extend the tight fit of the diaphysis and control the relationship between bone and nail. A single screw placed in the metaphyseal segment on the concave side of the deformity will allow the nail to reduce that deformity (Figure 12.13.2). The screw forms a third point of fixation together with the nail tip or entry point, and the point of diaphyseal contact. If there is insufficient anchorage of the nail tip then a second blocking screw is inserted contralaterally to the first.

Rotational malreduction is one of the most common problems to go undetected in the operating room. In one study 28% of patients who had undergone femoral nailing, in a high volume trauma unit, had a residual rotational deformity of greater than 15 degrees as determined by computed tomograhpy scan. These patients were also more likely to have ongoing functional problems.

Clinical assessment or ‘eyeballing’ is not particularly sensitive and other techniques may need to be employed. Accurate assessment of rotation requires direct comparison with the normal limb. When performing intramedullary nailing without a traction table, it may be useful to prepare, and leave uncovered, the unaffected limb to allow this comparison. In the femur, radiological signs can be used to avoid malrotation. A comparison of lesser trochanter profiles with the patella facing anteriorly can be used before final locking. A less sensitive sign of malrotation is a sudden change of shaft diameter or cortical thickness on fluoroscopy. Changes may, however, be quite subtle, particularly in areas with a more rounded cross-section.