Stapling across physes of the long bones has been accepted as an effective method for treating limb malalignment in young children for over 50 years [20, 21]. Around the same time, the potential benefits were discovered for the spine. Animal studies using a rat tail model confirmed the ability to modulate vertebral growth plates with skeletal fixation devices [22]. In 1951, Nachlas and Borden [23] were initially optimistic about their ability to create and correct lumbar scoliosis in a canine model using a staple that spanned several vertebral levels. Many of the dogs exhibited some correction, and some of the animals exhibited arrest of their curve progression. Some of the staples failed because they spanned three vertebrae. The enthusiasm for this new treatment was lost after the application of their stapling technique in three children with progressive scoliosis that yielded poor results. Other investigators have, similarly, been dissatisfied with convex stapling as a means of controlling progressive scoliosis.

Results for humans with congenital scoliosis were presented as early as 1954 [24], but the results were disappointing. The scoliosis correction was limited because the children had little remaining growth, and the curves were severe, with considerable rotational deformity. Some staples broke or loosened, possibly because of motion through the intervertebral disks. While the concept of stapling the anterior vertebral end plates/physes for growth modulation and curve stabilization seemed sound, the staples designed for epiphyseal stapling about the knee were prone to dislodging in the spine because they were not designed to function across the intervertebral disk and accommodate to the movement of the functional spinal unit.

In 2003, our institution published the results of a patient cohort that had undergone anterior VBS for moderate adolescent idiopathic scoliosis (AIS) with a newly designed staple [25]. The significance of the study was demonstrated by 87 % of the curves that were maintained using the stapling technique.

Recent work has shown the efficacy of anterior growth modulation in animals. Despite the successful use of staples for epiphysiodesis of long bones in angular deformity, staples for growth modulation around the spine were not nearly as successful. The obvious issue was that staples designed for the long bones were prone to dislodge in the spine because they were not designed for movement which occurs in the spine.

Medtronic Sofamor Danek (Memphis, Tennessee) designed staples using nitinol, a shape memory alloy, which have 510(k) approval from the FDA specifically for fixation in the anterior spine within a single vertebral body or for fixation of hand and foot osteotomies. These staples are unique in that the prongs are straight when cooled but clamp down into the bone in a “C” shape when the staple returns to body temperature, thus providing secure fixation. The nitinol staple described in this text is considered “off-label” by the FDA. Nitinol is a biocompatible shape memory metal alloy composed of 50 % nickel and 50 % titanium. The temperature at which the staples will undergo the shape transformation can be controlled by the manufacturing process. Injury to surrounding tissues through the transformation temperature has not been seen in animal or human experience with cervical spinal fusions.

Nitinol has a very low corrosion rate and has been used in orthodontic appliances. Implant studies in animals have shown minimal elevations of nickel in the tissues in contact with the metal; the levels of titanium are comparable to the lowest levels found in tissues of titanium hip prostheses, and titanium is considered a biologically safe implant material. No method of sterilization used in operating rooms has been shown to have any effect on the metal’s properties. Although sensitivity to nickel occurs in a very low percentage of the population, it is not anticipated to occur through the use of the nitinol staple. The crystal structure in nitinol is different than the small amount of nickel crystal structure in stainless steel such that the nickel does not leach out in nitinol compounds as it can on occasion with stainless steel. The nitinol staple has been tested in a goat scoliosis model applied across a disk space by Braun et al. [26] and has been shown to be safe and have utility for arresting iatrogenic curves of less than 70° in the goat.

In 2003, Betz and colleagues [25] reported on the use of the nitinol staples in 21 skeletally immature patients with AIS. Indications for the procedure were either brace noncompliance, often due to psychosocial reasons, or curve progression despite bracing. They found the procedure to be safe and effective, with the results comparable to the expected results of bracing. In 2005, this same group [27] reported on 39 patients and their increased experience with the procedure. Stabilization of the curve was seen in 87 % of those patients older than 8 years at the time of stapling who had a curve of 50° or less with at least 1 year of follow-up. No curve less than 30° at the time of stapling progressed more than 10° at follow-up.

In 2010, Cuddihy et al. reported a retrospective study comparing VBS to bracing for patients with moderate idiopathic scoliosis using identical inclusion criteria [28]. In this comparison of two cohorts of patients with high-risk (Risser 0–1) moderate idiopathic scoliosis (measuring 25–44°), the results of treatment of smaller thoracic curves (25–34°) by VBS were statistically better than the results seen with bracing (82 % versus 54 %, respectively, p = 0.05) when the cohorts were adjusted for mean age (10.5 years). For thoracic curves measuring 35–44°, the results were poor in both groups. The results of lumbar VBS and bracing were similar for curves measuring 25–44°. This study suggests that VBS can be used as an alternative or adjunct to bracing for patients with certain curve sizes who are noncompliant with bracing (Figs. 43.2 and 43.3).

Another study with 2-year outcome data consisted of 41 curves (26 thoracic, 15 lumbar) [29]. Thirteen patients had both curves stapled. The mean age was 9.4 years. Curves decreasing by greater than 10° were considered “improved.” Curves within 10° of their preoperative measurement were considered “no change,” and those progressing greater than 10° were considered “worse.” Success was defined as “improved” or “no change.” Thoracic curves measuring less than 35° had a 79 % success rate. Curves measuring less than 20° on first standing radiograph had an 86 % success rate. In patients with thoracic curves greater than 35°, 6 of 8 progressed past 50°. Seventy-one percent of patients with hypokyphosis showed improvement to a normal sagittal profile. One patient demonstrated worsening of kyphosis associated with coronal progression. Lumbar curves had an overall 87 % success rate, with only one patient with a preoperative curve of 40° progressing to 50°. Five patients lost greater than 10° of lordosis, but the final lumbar lordosis remained in the normal range. Complications were minimal, with a mean blood loss of 214 cc. A recent study by Auriemma et al. (unpublished data) reviewed the results of 63 patients who underwent VBS between the ages of 7 and 15 years for moderate idiopathic scoliosis (average Cobb, 30°). Of the 24 patients (36 curves) who reached skeletal maturity, defined as Risser grade 4 or 5, 71 % (12/17) of thoracic and 89 % (17/19) of lumbar curves were treated “successfully,” defined as either greater than 10° improvement in Cobb angle or within 10° of preoperative curve magnitude.

VBS has shown superior outcomes when curves correct to less than 20° on first standing radiographs [29, 30]. Subanalyses of lumbar curves from these studies revealed higher success rates in curves that corrected to less than 20° on first standing radiograph (88.9 %) than those that did not (83.3 %) [29, 30]. As a result of these findings, the treatment algorithm for VBS has been transitioning from stabilization of preoperative curves to maintenance of intraoperative correction by the addition of postoperative nighttime bracing. Recently, a cohort of AIS patients with moderate (20–45°) thoracolumbar/ lumbar (TL/L) curves underwent VBS and adjuvant postoperative bracing (unpublished data). TL/L Cobb angle significantly improved from a mean of 34° preoperatively to 21° at a minimum of 2-year follow-up. Lateral trunk shift also improved from a mean of 1.8 cm preoperatively to 0.5 cm at most recent follow-up. Although health-related quality of life outcomes have not correlated with trunk shift [31], trunk imbalance has been shown to negatively impact self-image and is a major clinical concern of patients.

Based on this review, we have altered our strategy for when to use staples alone and when to use additional strategies, as follows: if the thoracic curve measures 35–45° and does not bend below 20°, then we will offer vertebral body tethering or a posterior hybrid distraction implant, a unilateral VEPTR, or a growing rod in addition to stapling (Fig. 43.4). If on the first standing radiograph the curve does not measure below 20°, we will brace the child until the curve measures less than 20°.