Intramedullary Lengthening Devices

and Mark T. Dahl2



(1)
Department of Orthopedic Surgery, University of California – Irvine, Orange, CA, USA

(2)
Limb Length and Deformity Correction Clinics, Gillette Children’s Specialty Healthcare and University of Minnesota, St. Paul / Minneapolis, MN, USA

 



Keywords
Bliskunov® Albizzia® ISKD® Fitbone® Phenix® PRECICE®



Background



Elimination of the External Fixator


It has long been the goal of reconstructive surgeons to eliminate the external fixator altogether and use some method of internal fixation to achieve distraction osteogenesis. The transcutaneous implants used to secure the frame to the limb create portals of entry for bacteria into deeper tissues and bone. This can cause not only pain and inflammation but also risks prolonged or even permanent infection of the bone or surrounding soft tissues. Ilizarov, however, was generally opposed to such notions because he always contended that new bone formation depended upon both periosteal and endosteal blood supply, that is, blood entering the cortical bone from its exterior surface as well as its marrow blood vessels.

Research by Delloye et al. [1], however, showed that the periosteal blood supply was more important for distraction osteogenesis than was preservation of bone marrow vascularity. He did so by filling the marrow with absorbable bone wax in some of their experimental animals but not in others. Bone formed during distraction, provided that the proper conditions of stability, latency interval, and rate and rhythm of distraction were observed. This research, more than any other, confirmed that surgeons could employ an elongating intramedullary nail to create new bone in a widening distraction gap.

The basic principles of the Ilizarov method, however, must be maintained. Indeed, since reaming and nail insertion destroys the endosteal blood supply, careful preservation of the periosteal blood supply (by gentle elevation before osteotomy) is even more critical during intramedullary lengthening than with external fixation lengthening.

However, performing what Ilizarov and his co-workers call a “sparing corticotomy” is not necessary with IM lengthening. During this type of osteotomy, the tip of the chisel or osteotome stays within the cortex and avoids transecting the endosteal blood vessels. It’s technically challenging to do so because the far cortex cannot be reached by the surgeon’s sparing chisel and must therefore be cracked by counterrotation of the fragments or by prying apart the bone through the accessible cortices. Because reaming destroys the marrow vessels, protecting them is not necessary with IM lengthening techniques .


Ratcheting Devices



Bliskunov Intramedullary Nail


The first intramedullary fixation devices used for distraction osteogenesis were all driven by mechanical ratchet systems. The Bliskunov® intramedullary nail fit in the femur like a normal trauma nail but had an extension at the top end connected via a universal joint to a rod that bolted to the pelvis [2]. Rotating the hip in relation to the pelvis twisted the rod, which turned a mechanism within the nail, elongating it (Fig. 4.1).

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Fig. 4.1
The Bliskunov® intramedullary nail. Note the connection of the ratcheting mechanism to the pelvis. Copyright 2016 NuVasive

The fundamental problem with this implant was that it limits hip mobility while in place. It is still used in Eastern Europe as of this writing.


Albizzia Intramedullary Nail


The Albizzia® intramedullary nail possessed a similar elongating ratchet mechanism but eliminated the connection to the pelvis [3]. The nail resembles the telescoping leg of a camera tripod. Elongation occurs by counterrotating one end of the bone, which rotates the inner telescopic rod in relation to the outer cylinder, while both elements of the device are locked in their respective fragments of the bone (Fig. 4.2).

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Fig. 4.2
The Albizzia® intramedullary nail. Copyright 2016 NuVasive

The basic problem with this device, which appears to successfully elongate the bone during the initial phases of distraction, is that gradual stiffening of the newly formed regenerate bone in the distraction gap makes counterrotation of the fragments increasingly more difficult. For this reason, some patients must be taken back to the operating room to crack apart the regenerate bone forming in the distraction gap so that elongation can proceed .


Internal Skeletal Kinetic Distractor


The ISKD® (Internal Skeletal Kinetic Distractor) was developed by Dean Cole M.D. of Orlando, Florida [4]. As with the other mechanical limb lengthening devices, a ratchet mechanism elongates the telescopic nail. Unlike the Albizzia nail, which requires 30 degrees of counterrotation to engage the racket mechanism, the ISKD requires only a few degrees of rotation to elongate the device. In ordinary walking, planting the foot on the ground provides sufficient rotation power to lengthen the nail (Fig. 4.3). Indeed, this has proven to be a problem with the device, which can lengthen too rapidly if the patient does too much walking [5]. Excessive elongation can occur between clinic visits, even if they are a week apart.

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Fig. 4.3
The ISKD® intramedullary nail. Copyright 2016 NuVasive

In early 2015, the manufacturer Orthofix Inc. removed the ISKD® from the market although it may return.


Rotating Spindle Devices


The next category of the intramedullary lengthening nails includes those that are elongated by a rotating threaded spindle that gradually separates the telescopic portions of the nail, each interlocked into its respective fragment of the bone. Each of these devices requires an external power source that can transfer energy or motive power into the nail.


Fitbone® Intramedullary Nail


The Fitbone® , a German-made intramedullary lengthening nail, incorporates an electric motor to turn the spindle [6]. Rather than place batteries within the nail to power the motor, the device utilizes a subcutaneous antenna attached by wire to the motor (Fig. 4.4). This antenna generates electric current through induction by a corresponding electric current passing through a pad placed on the outside of the skin adjacent to the antenna. The current causes the motor to turn, rotating the spindle (Fig. 4.5).

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Fig. 4.4
The Fitbone® intramedullary nail and remote controller. An induction coil is placed on the skin over the corresponding subcutaneous induction coil connected to the nail’s internal electric motor. Copyright 2016 NuVasive


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Fig. 4.5
(a) The Phenix intramedullary lengthening nail. Copyright 2016 NuVasive. (b) Handheld magnet used to power the nail

The Fitbone® has been successfully employed to lengthen long bones in many musculoskeletal conditions, including post-trauma shortening, congenital anomalies, and short stature. At present, the motor turns in only one direction, allowing lengthening but no compression. The FDA has recently cleared the nail for marketing in the United States.


Phenix® Intramedullary Nail


The other category of threaded spindle devices consists of those motorized by an internal rotating magnet responding to an external magnetic field pressed against the body. The Phenix® nail, developed in France by Soubeiran, is just such a device [7].

The external component is a handheld solid magnet. Simply moving the magnet circumferentially around the limb rotates the internal magnet. The device has been in development for many years and has not yet been cleared for marketing in the United States by the FDA (Fig. 4.5).


PRECICE® Intramedullary Nail


The most recent threaded spindle intramedullary lengthening implant, and the only one that is cleared for marketing in the United States, is the PRECICE® intramedullary lengthening nail, manufactured by NuVasive Specialized Orthopedics (formerly Ellipse Technologies). A small magnet in the nail rotates in response to two rotating external rare earth magnets. (The internal magnet can rotate either clockwise or counterclockwise, thereby lengthening or compressing the device.) The two external magnets are housed in a computer-controlled, electrically powered assembly. The external magnets, however, are not electromagnets; instead they are composed of rare earth elements and are thus always “on” (Fig. 4.6).

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Fig. 4.6
The PRECICE® intramedullary nail in the femur and tibia. Copyright 2016 NuVasive

The PRECICE® intramedullary nail and external remote controller (ERC) are cleared for use by the FDA for bone lengthening. Preliminary reports describing small series of patients treated with the PRECICE® nail have confirmed its reliability and accuracy [8] (Figs. 4.7 and 4.8).

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Fig. 4.7
(Right) The external remote controller (ERC) powers the PRECICE® intramedullary nail. The surgeon programs the device with a lengthening prescription. (Left) The ERC contains two rare earth magnets to, when rotated, cause the small magnet within the nail to turn, elongating the nail via three 1:4 planetary gears. Copyright 2016 NuVasive


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Fig. 4.8
A patient positioning the ERC over the magnet inside the nail. Marking the limb during surgery helps with location. Future ERCs will automatically use the internal magnet’s field to aid location. Copyright 2016 NuVasive


Transferring Energy Through Soft Tissues


All rotating spindle of intramedullary lengthening nails currently available depends on external energy sources to power the internal rotor. None have internal batteries. As a result, just as the light from a bulb decreases in intensity proportional to the square of its distance from the source, so too does electrical and magnetic energy diminish with distance. This limits the possible distance from the source to the receiver. The Fitbone® nail employs a subcutaneous induction coil as a receiver antenna, with the transmitter being another coil pressed against the skin opposite the internal one. Wires connect the internal coil to the motor in the nail. Placing the antenna too deeply in the tissues would inhibit its function. The maximum distance between the two antennae should not exceed 1.0 cm (Fig. 4.9).

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Fig. 4.9
With the Fitbone nail , the transmitting and receiving antennae should be no more than 1.0 cm apart. The receiving antenna is usually placed in the subcutaneous fat, but in thin patients with little subcutaneous fat, the antenna is placed beneath the circular investing fascia. Copyright 2016 NuVasive

The same consideration applies to the PRECICE® and Phenix® implants, both depending on an internal magnet rotating in response to an external magnet field. The interval between the exterior power source and the internal rotating magnet contains not only skin and subcutaneous fat as with the Fitbone® antenna but also muscle and cortical bone. None of these tissues interfere with the transmission of magnetic flux, but all of them occupy space, pushing the external magnet way from the device.

With the large diameter PRECICE® nail, the magnets in the external remote controller (ERC) must be about 50 mm (~2.0 in.) from internal magnet to power it predictably. In the lower leg, the tibia is an anterior subcutaneous bone, so the ERC magnets, when placed on the front of the limb, are always reasonably close to the nail. The femur, however, is a deeply seated bone, surrounded by bulky muscles. There will thus be some individuals whose thigh circumferences are too great to permit use of the implant .

Fortunately, however, most of the adult population can be successfully treated with the nail, especially if the external device is pressed firmly into the flesh in bulky individuals (Fig. 4.10).

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Fig. 4.10
(a) With bulky patients, the magnet within the nail may be too far from the external magnets to function properly. (b) Pressing downward on the ERC may bring it within range of the implant. Copyright 2016 NuVasive


Correcting Deformities with Intramedullary Lengthening Nails


At present, the ability to make substantial deformity corrections with lengthening intramedullary nails is rather limited. Too much manipulation of the osteotomy site risks retarding regenerate bone formation in the distraction zone—a region whose intramedullary blood supply has already been compromised by reaming for the nail. Simple corrections of rotational malalignment with the nail in place are appealing, but excessive rotation at the osteotomy site, beyond 15–20°, may slow maturation of new bone formation especially in the tibia.

Correction of angulation at the distraction zone should also be restricted, with knowledgeable surgeons limiting angular correction to 10–15° in the femur and 5–10° in the tibia.

In some situations, it may be possible to correct a deformity at one end of a bone while simultaneously lengthening the same bone at the other end. In this way, a wedge resection corrective osteotomy, which always shortens a bone, can be neutralized by concomitant elongation. However, the periosteal elevation required to expose the bone, remove a wedge, and apply a plate, when combined with reaming for an intramedullary nail, will devitalize the bone between the nail and the plate, leading to delayed union or non-union. Therefore, these maneuvers should be performed at opposite ends of the bone. Future developments might change this perspective .

The subject of deformity correction requires an understanding of deformities in three-dimensional space, a topic best covered in Paley’s monograph “Principles of Deformity Correction” [9].


Achondroplasia and Related Conditions


The discovery by Prof. G. A. Ilizarov that distraction osteogenesis would result in predictable bone formation in a widening distraction gap resulted in the development of a program at his institution in Siberia to treat achondroplasia and similar dwarfing conditions. Indeed, since the clinical departments at his institute are organized by treatment strategies (rather than pathologies), achondroplasia is managed by surgeons in the “Department of Symmetrical Dwarfs.”


Achondroplasia


Achondroplasia is the most common and easily recognized dwarfing condition. It is characterized by rhizomelic shortening, meaning that proximal segments are relatively shorter than the distal segments. Thus, the humeri are proportionally shorter than the forearm bones, and the femora are proportionally shorter than the tibia and fibula. Even within the hands and feet, the shortening is more pronounced in the metacarpals and metatarsals than in the phalanges, with the tips of the fingers and toes nearest to normal length.

Likewise, there is a disproportionate relationship between the way bones of the skull and face enlarge during normal childhood growth. This results in a proportionately overlarge forehead and apparent constriction of the central part of the face. Thus, all individuals afflicted with achondroplasia have a similar facial appearance.

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Nov 5, 2018 | Posted by in RHEUMATOLOGY | Comments Off on Intramedullary Lengthening Devices
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