A unique pediatric problem with the use of certain implanted medical devices is that they either interfere with growth or do not grow as children grow. The approved labeling for a number of orthopedic and other implants describes them as not indicated for individuals with growing bones or skeletal, skull, or other aspect of growth that is less than 90 percent of adult levels.
Some devices or their accessories or the procedures for their use are designed to take children’s growth into account. For example, when surgeons first began to insert the drainage catheter for cerebrospinal fluid shunts into the abdomen, they used tubing just long enough to enter the peritoneal cavity. As children grew these catheters had to be replaced with longer ones. Recent experience suggests that even infants can tolerate a peritoneal catheter long enough to accommodate growth to adulthood . Cardiac pacemaker leads are also implanted so that some significant amount of growth can be accommodated.
Given the risk and discomfort of replacing an implant as a child grows and given the restrictions on the use of certain devices that interfere with growth, implants that can “grow” with a child have obvious appeal. Growing children who have bone cancers removed from their limbs and prosthetic devices inserted have faced repeated surgeries to replace or expand the device to accommodate growth. FDA recently approved a device that can be expanded without surgical intervention. As described by FDA, the device employs “a coil that fits around the patient’s leg that produces an electromagnetic field. The EMF induces an electrical current and subsequent heating of an internal wire [in the implant]. The generated heat softens a polymer locking ring, allowing a slow expansion of an internal compressed spring. The spring expansion pushes the spring housing and femoral housing apart, thus increasing the overall length of the implant”. According to the manufacturer’s webpage, FDA has cleared the device for distal femur and proximal tibia implants, but implants for the humerus, proximal femur, and total femur are only available so far under compassionate use guidelines.
To cite another orthopedic example, pediatric orthopedists treating children with leg fractures have increasingly used flexible titanium nails that support the leg as the bone heals but also provide flexibility for growing bones. For children between the ages of approximately 6 and 12, the technique avoids some of the disadvantages of alternative treatments with either a body cast and traction or certain rigid nailing techniques. This technique has not been associated with problems of arrested growth in the trochanter or osteonecrosis of the head of the femur that have sometimes been reported with rigid nailing techniques.
Interest in another kind of device, the resorbable implant, is particularly strong among those who treat children with certain craniofacial and orthopedic deformities. These implants are adequately rigid to support repair or reconstruction of a deformity for several months, but they then disappear without requiring removal or replacement and without appreciably interfering with a child’s growth. In a statement to the committee, the American Academy of Pediatrics pointed to metal craniofacial fixation devices that create problems with children that are not seen in adults. AAP cited “thinning of scalp leading to annoying prominence of the device . . . subcutaneous migration of screws . . . [and] intracranial migration of the devices”. In the latter process, the device has been engulfed by the child’s growing skull such that “within a few years plates and screws were sometimes found inside the dura resting in the substance of the brain,” a location for which they clearly were not intended.
Until recently, only the results of short-term studies of resorbable implants were available, but investigators have now reported on a combined prospective and retrospective multisite analysis of nearly 2,000 patients under 2 years of age treated over a 5-year period with the same type of device. They found a lower rate of devicerelated complications requiring reoperation than for metal devices and low rates of adverse events. Consistent with a characteristic of device innovation, they noted that “the specific types of plates and screws used evolved over the study period from simple plates, meshes, and threaded screws to application-specific plates and threadless push screws whose use varied among the involved surgeons”.
In an arena that holds potentially broad promise, the emerging field of tissue engineering is exploring the development of devices such as heart valves or skin that become populated by the patient’s living cells . Such devices might grow as young patients grow and also avoid or limit immunocompatibility or biocompatibility problems that are often seen with currently used materials.
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