Motorized Intramedullary Lengthening Nail for Limb Reconstruction

Introduction

The remote-controlled magnetic intramedullary lengthening nail is a disruptive technology that has changed the surgical approach to limb lengthening and reconstruction surgery. Its availability and use have led to a new paradigm for limb reconstruction with a decreased need for external fixation and the advent of new approaches for complex limb reconstruction.

The primary indications for bone lengthening and deformity correction are congenital and posttraumatic deformity and leg-length discrepancy (LLD), including malunion and growth arrest. Patients include children, adolescents, and adults. Distraction osteogenesis (DO) has been successfully used over the past half-century to lengthen bone. Critical to successful DO is an optimal rate and rhythm of distraction and ideal stability, for which external fixation has been a reliable tool and the gold standard. This has become known as the Ilizarov method. However, the disadvantages of external fixators are well known and include pin-tract infections, skin pain, soft tissue tethering, and joint stiffness. Integrated fixation techniques combining internal and external fixation, such as lengthening over a nail (LON), lengthening and then nailing (LATN), lengthening and then plating (LAP), and bone transport over a nail (BTON), were implemented to minimize the time in external fixation.

Despite these incremental advances, bone lengthening with a fully implantable device is desirable to entirely avoid external fixation. However, mechanical integrity and accurate control of distraction with such an implant are mandatory. Early designs of the internal lengthening nail (ILN) were telescopic intramedullary nails lengthened by a ratchet mechanism requiring mechanical rotation of the limb. Although reports suggest a satisfactory experience, there is unpredictable nail distraction, causing significant complications and mechanical failures.

New remote-controlled ILNs have recently become available that are very accurate and predictable. One design employs an electric motor embedded in the telescopic rod, activated by intermittent transcutaneous transmission of radiofrequency waves to an implanted antennae/receiver that converts these waves into an electrical impulse discharged via a connecting cable. The second design, now most widely used in the United States, is a telescopic rod with a magnetic actuator drive mechanism, activated by a handheld external electromagnetic activator. These new types of ILNs have reliable mechanisms that allow for an accurate and well-controlled distraction rate and rhythm.

There are several nail types available, allowing for varied surgical approaches. The femur can be approached antegrade through a piriformis or greater trochanteric entry or can be entered retrograde. The tibia is approached antegrade. Although the ILNs are primarily used for DO bone lengthening, the magnetic ILN can also be used for compression of nonunions. Creative off-label uses of the commercially available nails have included humerus lengthening, retrograde tibia through an ankle/subtalar arthrodesis, and bone transport over a plate. Additional nail designs are in development, including humerus, ankle fusion/retrograde tibia, and bone transport nails.

Fundamental Principles

One should use an ILN rather than external fixation for long bone lengthening when possible and safe because the patient experience is better with an ILN. Patients with open growth plates are usually not considered for nailing because the entry may damage the physis. The intramedullary canal must be patent and without active infection.

In contrast to external fixation, the ILN can only be used for axial lengthening or compression but cannot be used for gradual deformity correction. If the angular and/or translational deformity can be safely corrected acutely, then the ILN may be used. Acute correction of angulation and translation in the coronal and sagittal planes and rotation in the axial plane may be accomplished with the assistance of adjunct techniques, including fixator-assisted nailing (FAN) and blocking screws ( Fig. 27.1 ). Once the deformity has been corrected and the ILN is in place, DO may follow, but a longer latency and slower distraction is advisable because of the increased degree of periosteal disruption during acute deformity correction.

When choosing the nail diameter, one should implant the largest nail possible considering the size of the intramedullary canal. Flexible canal reaming 1.8 to 2.0 mm above the ILN diameter is advisable so that the nail can slide through the bone without impingement. The excessive reaming of cortical bone to enlarge the canal is to be avoided. The magnetic ILN diameters of 8.5 mm, 10.7 mm, and 12.5 mm allow the patient to bear 30, 50, and 70 pounds, respectively, based on biomechanical testing done by the manufacturer.

The osteotomy location is determined based on optimal bone quality, the apex of deformity or natural bow of the bone, and optimal stability of the bone nail construct after implantation and after distraction. Damaged sclerotic bone is best avoided for osteotomy because the regenerative potential is diminished. The ILN does not have a bow. If the natural anterior bow of the bone is to be spanned by the nail, then the osteotomy would need to be positioned to enable straightening of the bone through the osteotomy site, which commonly occurs with antegrade femur lengthening.

The choice of nail length is tied to the desired osteotomy location. During distraction of the telescopic nail, the thicker segment of the nail will be gradually pulled out of the moving segment. It is important to have the thick part of the nail across the regenerate bone and several centimeters into the far segment. Ideally, we plan to have 5 cm of thick nail (TN) in the far segment. The starting length of the magnetic nail has 30 mm of thin protruding nail (TPN) at the end. The shortest nail length (SNL) that should be used = distance to the osteotomy level + desired lengthening + TN + TPN ( Fig. 27.2 ).

There are predictable deformities that occur during bone lengthening, such as valgus and flexion during tibial lengthening and varus and flexion during femur lengthening. If the diameter of the bone is larger than the nail at the osteotomy site, unwanted deformity will likely occur during lengthening unless blocking screws are utilized. The desired location of blocking screws is usually in the concavity of the anticipated or corrected deformity adjacent to the osteotomy site. The reverse rule of thumb, described by Muthusamy and colleagues, is also a useful technique to determine the optimal placement of blocking screws. FAN and blocking screws are often needed in the retrograde femur and tibial techniques and will be further elaborated in those sections. The placement of external fixation pins and rotational markers must be out of the path of the ILN.

Lower extremity lengthening often requires the judicious use of soft tissue releases. The iliotibial band and lateral anterior thigh fascia are usually released during femur lengthening to prevent knee contracture and patella subluxation. The gastroc-soleus recession (GSR) is often needed during tibial lengthening to prevent ankle equinus contracture. Prophylactic fasciotomy of the leg is advisable during acute correction of tibial deformity. Prophylactic peroneal nerve release may be needed during acute correction of tibia valgus, especially if there is localized scarring.

The magnetic ILN contains a rare-earth magnet. While this is implanted, magnetic resonance imaging (MRI) scans must be avoided. Routine removal of hardware after mature bony union is advisable.

Preoperative radiographic studies should include a full-length standing radiograph of both legs with a lift used under the foot to accurately measure deformity and localize the difference in length ( Fig. 27.3A ). A lateral standing radiograph of the femur and/or tibia also should be obtained. All radiographs should be made with a magnification marker to facilitate accurate measurement of the length and diameter of the femur and/or tibia. The LLD should be assessed in terms of location, with the discrepancy superior to the knee (femur) considered separate from that inferior to the knee (see Fig. 27.3B ). Differences in foot height can be considered together with the tibia. The anatomic-site options for lengthening of the lower limb bone are in the femur and/or tibia. In general, the bone or bones with the deformity/shortening are approached. If there is a contraindication for surgery in one bone, the other long bone may be approached, but this would result in knee heights being uneven. In the femur, there are three options: antegrade via a piriformis entry, antegrade via a trochanteric entry, and retrograde via the knee. In the tibia, the antegrade approach via the proximal tibia is used with an infrapatellar or suprapatellar (semi-extended) method.

At the end of surgery, the internal magnet is localized with fluoroscopy and marked on the skin with a permanent marker so that the external remote controller (ERC) may be optimally placed for distraction ( Fig. 27.4 ). Patients are encouraged to re-mark the magnet location every few days so that it does not fade. The connection between the ERC and the magnet in the nail is critical for success. The ERC should be pointed straight to the knee or ankle in case of femur or tibia lengthening, respectively. Furthermore, the ERC should be pressed firmly on the skin to minimize the distance to the magnet. A trial distraction of a fraction of a millimeter may be done in the operating room to confirm that the distraction mechanism is working properly ( Fig. 27.5 ).

During postoperative follow-up every 2 weeks, the patient is examined for neurologic function and adjacent joint range of motion. Calibrated radiographs are examined for distraction length and regenerate quality (see Fig. 27.5 ). The rate of distraction is adjusted accordingly. For example, if the bone formation is poor and the joint motion is stiff, the distraction rate should be slowed.

The motorized magnetic ILN mechanism can be used to both lengthen and shorten. This allows shortening if there has been overlengthening. It also allows compression of regenerate or nonunion.

The indications, planning, surgical techniques, and postoperative regimen for antegrade and retrograde femur lengthening, tibial lengthening, and nonunion compression are discussed in the following sections ( Table 27.1 ; Box 27.1 ).

Antegrade Femur Lengthening

Indications and Preoperative Planning

Antegrade femur lengthening is indicated for patients with a femoral LLD who have an open intramedullary (IM) canal with a large enough diameter to fit the IM nail ( Fig. 27.6 ). Angular deformity with the apex from the proximal meta-diaphyseal femur to the mid-diaphysis and rotational deformity can be corrected acutely using this approach. In adults, a piriformis or trochanteric entry can be used. In adolescents, a trochanteric entry should be used to avoid injury to the blood supply of the femoral head. The level of the osteotomy is planned preoperatively and should be performed at the apex of deformity in the coronal and sagittal planes. The IM nail is straight (no anterior bow as in trauma nails) and will correct angular deformity after osteotomy at the apex of deformity. The nail length is chosen using the SNL analysis to make sure there is adequate stability after lengthening, with at least 5 cm of the thick part of the nail remaining in the distal segment at the end of distraction ( Fig. 27.7 ).

Surgical Technique

The patient is positioned supine on a flat radiolucent table, and a bump is placed under the ipsilateral buttock. Split sheets are used to drape the surgical extremity and provide adequate exposure of the buttock. The C-arm is positioned on the opposite side.

At the planned osteotomy level, a 1-cm incision is made, the periosteum is elevated to make a pocket for the reamings, and multiple drill holes are made in a transverse fashion (see Video 27.1 ). This first step of the osteotomy also serves to vent the IM canal. The hip is adducted, and a 2.4-mm Steinmann pin is inserted into the canal through the piriformis fossa or greater trochanter. A 2-cm incision is made over the guidewire. A 12-mm cannulated acorn reamer is used over the guidewire to enter the IM canal, and this is then replaced with a long beaded flexible guidewire inserted into the canal to the distal end of the bone. Flexible reamers are used to ream the IM canal 2 mm larger than the diameter of the nail. The reamings egress through the drill (vent) holes. Steinmann pins or temporary external fixation pins are placed in the proximal and distal bone segments away from the nail tract to mark rotation. If there is rotational deformity, the pins are placed with the desired axial plane angular divergence. The motorized ILN is then inserted to a level 1 cm proximal to the osteotomy site ( Fig. 27.8 ). An osteotome is used to complete the osteotomy, and the nail is passed across the osteotomy site, correcting any angular deformity. The femur is rotated around the nail to ensure that the osteotomy is complete.

If there is a malunion, a FAN technique may be needed. The osteotomy would be done at the apex of deformity, and the straightened bone would be stabilized with a fixator. Fixator pins must be out of the path of the nail. Reaming is then done, followed by insertion of the ILN.

Distal locking screws are inserted first using a freehand technique. The optimal rotation is set, and the proximal interlocking screw is inserted via the jig. The order in which the locking screws are inserted is at the discretion of the surgeon. However, the benefit of inserting the proximal locking screw through the jig after the distal screws have been inserted is that the optimal rotation is fine-tuned without the challenge of freehand insertion of the distal screws. Iliotibial band release can be performed at this point; however, this step may be omitted for small lengthenings (e.g., femur lengthening of less than 2 cm).

Retrograde Femur Lengthening

Indications

The retrograde method for femur lengthening is popular and has not created any knee joint problems that have been disclosed in lengthening nail publications. There are many indications for pursuing a retrograde approach for ILN insertion. Correction of distal femoral deformity through a distal osteotomy site often requires a retrograde nail to control the distal fragment and achieve an accurate alignment ( Fig. 27.9 ). Subsequent lengthening further destabilizes the construct by pulling the bone into flexion, which can be minimized with a retrograde nail and blocking screw placement ( Fig. 27.10 ). The distance between the internal and external magnets is critical for the nail to lengthen. In obese patients, an antegrade nail will place the intramedullary magnet in the proximal thigh, which may be too far away from the skin surface for the external magnet to control it. A retrograde ILN places the internal magnet in the distal thigh, where the tissues are thinner and the external magnet can function effectively, making obesity a relative indication for retrograde insertion. Retrograde nailing is recommended in cases where there is an obstacle in the proximal femur, including a hip prosthesis, hip deformity that is not being corrected, dysplasia that places the hip at some risk for subluxation, excessive buttock fat, and implanted nerve or bladder stimulator, to name a few. Preexisting hip abductor weakness may be a relative indication for retrograde nailing.

Contraindications to retrograde ILN are few. An open distal femoral physis is a relative contraindication to retrograde nailing. Excessive obesity with too great a distance between internal and external magnets in the distal thigh would result in nondistraction. A narrow canal diameter may preclude the use of the ILN. The need for an osteotomy closer than 7 cm (approximately) to the knee joint line will make the distal fragment a challenge to control with the ILN and is best treated with a plate. The presence of a total knee replacement prosthesis would make retrograde nailing complicated.

Preoperative Planning

Preoperative planning for retrograde ILN insertion begins with analyzing a 51-inch standing radiograph including hip and ankle. The degree of femoral deformity and total limb deformity are measured ( Fig. 27.11A ). The tendency for lateralization of the mechanical axis needs to be incorporated into the total angular correction (see Fig. 27.11B ). The level of the osteotomy needs to be selected based on several factors: bone-healing capacity at a particular site, distance from the joint line (or notch), location of the apex of the deformity, and need for translation (see Fig. 27.11C and D ). The ideal nail length is then calculated. This requires measuring the proposed path of the nail in the diaphysis starting from the osteotomy site and extending proximally. The length from the osteotomy site needs to include the tip of the nail (3 cm), a critical length of the thicker portion of the nail (5 cm), and the intended regenerate length ( x cm). For example, if the limb length discrepancy is 3 cm, then the length of the nail beyond the osteotomy would need to be 30 + 50 + 30 = 110 mm. The distance from the osteotomy to the femoral notch is then added to this sum for the total minimum SNL. The nail can be longer than this SNL, but its length is limited by the proximal femur bow. A lateral radiograph of the femur is required to mark the proximal femoral sagittal bow and measure the longest possible nail length. Any length nail that falls between the shortest and longest nail length is appropriate. The width of the nail is selected as well. The narrowest portion of the femoral canal on anterior-posterior (AP) and lateral radiographs is measured. The IM canal must be over-reamed at least 1.8 to 2 mm over the diameter of the nail. One must decide which diameter nail to use based on how large a reamer can be passed safely. If the canal measures 10 mm and the surgeon feels a 13-mm reamer can be inserted safely, then a 10.7-mm diameter nail can be used. We subscribe to the guideline that a minimum thickness of 5 mm of cortex needs to be preserved circumferentially. Larger-diameter nails are stronger and take larger locking bolts, allowing for more weight bearing; have more powerful magnets, allowing for a larger distance from the external magnet during adjustments; and are more rigid, resisting bending at the lengthening site. Once the ideal nail is selected, then it needs to be ordered. Consider ordering two identical nails as well as one size longer and one size shorter. The path of the nail is then templated, and the location of blocking screws to maintain or prevent deformity is decided. Large procurvatum deformities may require a prophylactic peroneal nerve release.