Surgical Technique for Single Growing Rods (Fig. 5.1)

Although the dual growing rod has been shown to be a more stable construct in general, the single-rod technique continues to be utilized for some patients with EOS. ⁴ – ⁶ In cases where the patient is small and the skin is tenuous, the rod may be prominent over the convex side of the curve, the single-rod technique provides a lower profile construct. As with all growing rod techniques, meticulous preoperative planning and attention to detail is paramount to obtaining a good outcome.

This is our preferred technique: The patient is brought to the operating room, connected to neuromonitoring, and placed under general anesthesia. Prophylactic antibiotics are administered prior to the start of the procedure, and the back is prepped and draped in sterile fashion. Typically, a midline skin incision is made over the portion of the spine to be instrumented. With use of meticulous hemostasis, the fascia is exposed at the levels of the proximal and distal foundations. We then expose one spinous process proximal and one distal, and mark them with a clamp. Intraoperative imaging is utilized to identify the preplanned foundation sites. Once identified, the two to three adjacent vertebrae are exposed subperiosteally to be used as the foundation on the concave aspect of the curve. The fascia between the proximal and distal foundations is preserved, and a rod is passed on the concave side subfascially, typically from distal to proximal. Once this is complete, work begins on the distal foundation site utilizing a pedicle screw construct, as we believe this provides a more solid distal foundation. The proximal foundation consists of a claw construct, with an over-the-top laminar hook proximally and a sublaminar hook in the next distal vertebrae. This enables compression between the hooks once the rod has been inserted. On occasion, this may be extended one level distal. At times, if a hook construct is unable to be utilized one can also consider an all-pedicle-screw construct for the proximal foundation. The size of the rod is determined by the size of the patient. In most small children, a 4.5- to 5.5-mm titanium rod is used, whereas in older children, a larger diameter rod may be utilized. When measuring the rod, typically allow for an additional 5 to 6 cm to compensate for initial distraction, and allow enough extra length to enable it to be expanded at subsequent surgeries. The rod is contoured to the desired sagittal plane. Once inserted, it is grasped with rod holders and rotated to correct and align the sagittal plane. The upper foundation is typically tightened first and the rod is distracted distally. There should be 4 to 5 cm of rod extended below the distal hook or screw to be used for subsequent lengthenings. The wound is irrigated with normal saline solution and closed in layers.

Although this is our preferred technique, different variations of techniques have been developed over time, including those using anchors bilaterally of the proximal and distal foundations connected with a cross-link, creating a more stable foundation and using only one rod along the concavity of the curve. This technique may provide better control of spinal rotation than would be provided by a one-sided foundation. This is an interesting technique that may be considered when utilizing a single-rod technique.

Surgical Technique for Dual Growing Rods (Fig. 5.2)

The setup is similar to that described above for a single growing rod. The index procedure can typically be performed through one or two midline incisions. ⁷ , ⁸ Clamps are placed on the spinous processes once they are exposed and once the levels have been verified under image intensification. Meticulous technique is used to avoid a broad exposure and risk spontaneous fusion. Exposures at the foundations are the only locations used to perform a subperiosteal dissection.

The upper foundation is generally placed between the levels of T2 to T4. In a typical dual-rod construct, we utilize at least four anchors (hooks and screws) for each founda tion for maximum stability. Our preference is to use four hooks at the upper foundation. Harrington′s ¹ preference is a supralaminar hook for the superior location and a sublaminar hook under the lamina, creating a bilateral claw construct. Occasionally, in patients with a small transverse diameter of the spinal column, there is concern about crowding the spinal canal, causing canal stenosis. In these cases, the hooks can be staggered over two to three levels. If this is the case, one must remember that the hooks may block the use of a transverse connector at the proximal foundation level. In that event, the connector can be placed just caudad to the lower hooks. Although our preference is hooks, occasionally hooks are difficult to place secondary to the patient′s anatomy, and in those cases we utilize a pedicle screw construct. Mahar et al ⁸ demonstrated an increased stability of pedicle screws over hooks alone. We therefore always use transverse connectors when using proximal hooks. In the event that proximal pedicle screws are warranted, care must be taken with placement, especially in hyperkyphotic patients, as cutout into the spinal canal can be catastrophic.

The caudal foundation is typically instrumented with four pedicle screws. The lowest instrumented vertebra (LIV) of the foundation tends to be the most proximal horizontal lumbar vertebra. However, the goal is to avoid instrumenting to the pelvis in ambulatory patients. It is important to create a strong, stable foundation. Therefore, a combination of local bone autograft and cancellous chips allograft with demineralized bone matrix is used to augment the bony fusion across the foundation sites. Once the foundations have been adequately prepared, a low-profile implant system is chosen. These are often two 4.5-mm titanium or stainless steel rods. The rods are measured and cut into four segments. The two proximal rods (one for the left and one for the right) are contoured with the appropriate kyphosis. The two distal rods are contoured with the appropriate lumbar lordosis. One proximal and one distal rod are connected with a straight box connector that usually spans the thoracolumbar junction. Appropriate contouring may help to correct the sagittal plane deformity and restore it to a more normal profile in a flexible curve. However, one must be careful to avoid extreme sagittal correction, as this may lead to anchor failure especially in stiffer curves. At this point, the rods are passed subfascially from the caudal to cephalad direction while palpating the tip of the rod so as to avoid penetration into the pleural cavity. The rod is secured to the anchors. As previously mentioned, if an all-hook construct is utilized at the cephalad foundation, it is strongly recommended to use a cross connector. The cross connector is not necessary if using a four-pedicle-screw construct.

When a tandem connector is used for periodic lengthening, the majority of lengthenings are completed by loosening the screw of the upper rod. Therefore, only a short segment of the lower rod is secured to the tandem connector. The segment of the rods entering the connector should not be contoured, as the connector is straight and this will create a stress riser at the transition and also may interfere with smooth lengthening. The tandem connectors are placed in the least prominent position of the thoracolumbar region to achieve a low-profile construct. If set screws are used with tandem connectors, they can be positioned either medially or laterally. The benefit of placing the screws medially is that they can be accessed more easily in a less invasive fashion during future lengthening procedures. However, lateral placement may have a lower profile in a very small child. At this point, the initial correction and lengthening can be performed. Extreme care must be taken so as not to over-distract, leading to immediate implant or neurologic complication. During the initial surgery, care is taken so as not to maximally distract across the deformity. We tend to wait until the first distraction when the foundation fusions are more mature, and then prefer not distracting more than 2 cm because of concern about inadvertently causing a neurologic injury.

Traditional growing rod lengthening procedures are typically performed every 6 to 9 months. In healthy children, this can be performed on an outpatient basis or during a short hospital stay. However, children with significant medical comorbidities may require an inpatient or even an intensive care unit stay. We choose to use intraoperative neuromonitoring for all growing rod procedures including lengthenings. Sankar et al ⁹ conducted a multi-center retrospective review of 782 growing rod surgeries (252 growing rod implantations, 168 implant exchanges, and 362 lengthenings) in 252 patients. They identified neuromonitoring changes in two patients during primary implantation, one during implant exchange and one during lengthening. Furthermore, Akbarnia et al ¹⁰ reported a case of delayed neurologic deficit after a rod exchange procedure despite normal somatosensory evoked potentials (SSEPs), Hoffman reflexes, and electromyograms (EMGs) intraoperatively. The patient made a full recovery after shortening of the rod.

For dual-rod lengthening, a small midline incision is made centered between the tandem connectors at the site of the rod gap. The skin incision is made to the depth of the tandem connector prior to working laterally, so as to maintain the integrity of the overlying skin and soft tissue coverage. Both connectors are exposed, and the upper set screws are loosened. The rod on the side of the spine needing the most length, usually the concavity, is distracted first. A rod holder is placed on the rod, and the lengthening over the rod is performed by distracting between the rod holder and the connector. The same technique is used for lengthening of the contralateral side. Typically, the distraction is performed to match the first side. As with the index procedure, care must be taken not to over-distract the rods during lengthenings.