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Pediatric Complications: Growing Rod Treatment
Introduction
Early-onset scoliosis (EOS) is defined as scoliosis with onset earlier than age 10 years and is classified based on age, etiology, major curve, degree of kyphosis, and curve progression. The characteristics of the deformity guide treatment options available.
Stabilizing the spine and minimizing progression of spine and chest deformity must be balanced with the requirement of continued thoracic growth needed for pulmonary development. There are three types of growth-friendly surgical strategies used in the treatment of EOS: distraction-based, growth-guidance, and compression-based instrumentation.
Distraction-based techniques and implants have undergone many advancements ; however, the underlying principle of applying a distractive force across the apex of deformity that anchors at the top and bottom of the construct is unchanged. Distraction-based instrumentation is indicated for children with progressive scoliosis who have substantial growth remaining and have failed conservative management. Distraction-based techniques can be performed with either traditional growing rods (TGRs), which require a surgical procedure for expansion, or magnetically controlled growing rods (MGCRs), which contain a magnetically driven expansion mechanism that allows for noninvasive expansion in the outpatient clinic. Both TGR and MCGR have proven to be effective at correcting spinal deformity while allowing continued growth and pulmonary development; however, complication rates with both are high, affecting approximately one-third to one-half of patients. This chapter will review the associated complications of growing rod treatment for EOS as well as strategies to help minimize these complications.
Complications With Growing Rods
TGR and MCGR techniques can be performed with either a single rod or dual rods. The proximal and distal foundations are fused with hooks or pedicle screws as anchors. Proximal and distal anchors are typically anchored to the spine but can also be anchored to the ribs proximally or to the pelvis distally (usually for neuromuscular etiologies). TGRs use either tandem (in-line) or side-to-side connectors. MCGRs do not require connectors. For both single- and dual-rod constructs, lengthening procedures are scheduled approximately every 6 months. Once maximum distraction has been achieved, or spinal growth/thoracic development has been completed, definitive fusion is usually performed with removal of growing rods implants and posterior spinal fusion with segmental instrumentation.
Due to the multiple surgical procedures required for the TGR technique, there is a high complication rate reported in the literature, ranging from 0.47 to 2.62 complications per patient. Each subsequent surgical procedure increases the risk of complication by 24%.
Implant-Related Complications
Implant-related complications are the most common complications with TGR and MCGR. The risk of rod fracture is approximately 15% over the course of treatment. Risk factors for rod fracture include number of rods used, rod diameter, size and location of connectors, ambulatory status of the patient, and prior rod fracture. Ambulatory status and prior rod fracture are not modifiable by the treating surgeon. Single-rod constructs, smaller diameter rods, and smaller tandem connectors all have an increased risk for fracture. Most rod fractures occur above or below tandem connectors in TGR. There is no consensus on which rod material is least prone to fracture, with varied reports in the literature. To decrease the risk of implant-related complications, the treating surgeon should consider dual-rod constructs with higher diameter rods. If possible, stainless steel should be avoided, and titanium or cobalt-chromium rods should be utilized to reduce the risk of infection. When deciding on placement of tandem connectors, the surgeon should be cognizant of stress risers and attempt to place these connectors at areas of low stress to further minimize fracture risk (the thoracolumbar junction is typically a high motion area and would be an area of high stress to avoid if possible).
Successful distraction of the growing spine relies heavily on the proximal and distal anchors. The operating surgeon has some options in terms of anchor location as well as anchor type. TGR and MCGR can be anchored via pedicle screws, pedicle hooks, laminar hooks, or rib hooks proximally and via pedicle screws and laminar hooks on the spine distally, as well as S hooks or rigid screws into the sacrum/pelvis when sacropelvic fixation is planned. Anchor complications include rib fracture, spine fracture, loss of fixation, and in rare cases neurologic complications.
Several studies have investigated strategies to optimize anchor strength and longevity in growing rods. A biomechanical study demonstrated that four pedicle screws implanted in two adjacent vertebral bodies provided the strongest construct for growing rods. Others have highlighted that the spinal level plays a role in what type of anchor can be accepted; typically, the T5 and T7 pedicles tend to be smaller and easily breached, resulting in a 25% decrease in distraction force. If hooks are utilized, there is better resistance to failure on the lumbar lamina compared to the thoracic lamina. Pedicle morphology may dictate whether a pedicle screw can be utilized, and although four pedicle screws have been found to be the strongest anchor construct, similar distraction force can be achieved between laminar hooks and pedicle anchors. When ribs are utilized for anchoring, however, 50% of the distraction force is lost versus spine anchoring.
Appropriate planning can help optimize anchor placement and strength. Pedicle screws have been shown to be the strongest anchor for growing rods; however, they carry a higher risk of neurologic injury during placement versus alternative anchors. Spinal canal encroachment rates have been reported to be as high as 52% with upper pedicle screw pullout. This risk can be minimized by utilizing a less medial/convergent trajectory when placing pedicle screws to decrease the risk of neurologic complication if pullout does occur. If treating a patient with a complex deformity and atypical pedicles, the treating surgeon should consider the use of navigation and/or robotic assistance to improve accuracy of pedicle screw placement. Navigation has demonstrated safety and efficacy in the placement of pedicle screws in young children and those with congenital spinal deformities. When caring for a patient with poor bone quality or significantly rigid deformity, a two-staged approach, in which the anchors are placed in the primary stage then a subsequent procedure is done for rod placement and initial distraction, can be considered to decrease anchor complication. By allowing sufficient time for anchor site fusion prior to distraction, the risk of anchor complication can be significantly decreased, while achieving the intended deformity correction.
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