Use of supplemental rods in long posterior sacropelvic fusions for adult spinal deformity

Introduction

Operative treatment for adult spinal deformity (ASD) can improve pain, disability, and health-related quality-of-life but is associated with high rates of complications (1-6). A common complication after ASD surgery is rod fracture (RF) and/or pseudarthrosis (PA), which is a leading cause of revision ASD surgery (1,7-10). The reported rates of RF are quite variable, which could be because of heterogenous patient cohorts, different surgical techniques [e.g., pedicle subtraction osteotomy (PSO)], and a wide range of study follow-up rates and durations. In addition, earlier ASD studies with follow-up ≤2 years likely underestimated the true rate of RF (1,11). A recent multicenter study with long-term follow-up demonstrated that most RFs occur >2 years after index surgery, and that nearly 50% of operated ASD patients have ≥1 RF within 8 years of surgery (12).

Symptomatic RF can result in substantial pain, disability, and potential loss of deformity correction, which may warrant revision ASD surgery (13-15). Over the past decade, spinal deformity surgeons have increasingly utilized supplemental rods to provide biomechanical support to protect against primary RF (16,17). Utilization of supplemental rods generally involves configuring the additional rod(s) to support the primary rods at sites of disproportionate implant fatigue stress, which include 3-column osteotomy (3CO) or adjacent levels, multiple levels of posterior column osteotomies (PCOs), and the lumbosacral junction or caudal lumbar spine (16,17). Some examples of supplemental rods intended primarily for biomechanical protection are satellite rods, iliac accessory rods, delta rods, and secondary rods (Figure 1) (18-20).

Figure 1 Examples of supplemental rods used in adult spinal deformity surgery for the purpose of biomechanical reinforcement to reduce risk of primary rod fracture/pseudarthrosis. (A) Satellite rods are short, deeply recessed rods that are typically connected directly to pedicle screws across a pedicle subtraction osteotomy. Satellite rods are not connected to the primary rods to avoid severe angular bending which may reduce risk of implant fatigue across the pedicle subtraction osteotomy. (B) An accessory rod is not directly connected to pedicle screws but is connected to primary rods via side-to-side connectors. Accessory rods can be configured medial or lateral to the primary rod and function to bolster areas of high primary rod strain such as the lumbosacral junction. (C) Iliac accessory rods are similar but are anchored distally to the pelvis via traditional iliac or S2-Alar-Iliac bolts. (D) A delta rod is an accessory rod with proximal and distal attachments to the primary rod such that the center portion of the delta rod is not angulated/weakened. (E) A secondary rod is directly connected to pedicle screws and is typically shorter than the primary rod.

Another function of supplemental rods is to facilitate deformity correction via intraoperative maneuvers involving compression or distraction of the supplemental rod (21-24). Examples include distraction of a kickstand rod and compression of intercalary rods which serve as “rails” for the rail technique (Figure 2) (21-26). Kickstand rods facilitate coronal correction via distraction maneuvers against the pelvis on the side ipsilateral to truncal shift (23-25). Kickstand rods may also confer biomechanical protection of the primary rods; however, if no true distraction for coronal correction was performed, these are better termed “iliac accessory rods” (20). In contrast to kickstand rod distraction, the tie rod technique involves compression of an iliac accessory rod on the side contralateral to truncal shift, which functions to pull the C7 plumb line towards midline while correcting coronal malalignment (26). Finally, compression of intercalary accessory rods via the rail technique can correct kyphotic deformity in controlled fashion using construct-to-construct closure across an osteotomy (22).

Figure 2 Examples of supplemental rods used to facilitate correction of adult spinal deformity. (A) Intraoperative compression of intercalary rods via the “rail” technique can help correct kyphotic deformity in controlled fashion using construct-to-construct closure across an osteotomy. (B) Kickstand rods facilitate coronal correction via distraction maneuvers against the pelvis on the side ipsilateral to truncal shift.

Nomenclature for supplemental rods in ASD constructs has been inconsistent, and various classifications have been proposed to improve communication. Since then, novel uses for supplemental rods have been introduced, which we think warrants an update in contemporary supplemental rod nomenclature. As such, our objective was to review and describe the most comprehensive, up-to-date nomenclature for supplemental rods. Also, we provide an update on how supplemental rods may confer biomechanical reinforcement and facilitate adult thoracolumbar deformity correction. The review focuses on supplemental rod constructs for ASD operations involving long posterior fusions to the sacropelvis. Supplemental rods for adult cervical deformity are beyond the scope of this review. Most long posterior instrumented fusions to the sacropelvis for ASD that include supplemental rods do so primarily to provide biomechanical support at high stress locations. A less common application of supplemental rods is to help facilitate deformity correction via intraoperative compression or distraction maneuvers, which will be discussed later towards the end of this review.

Incidence rates and clinical impact of RFs

The etiology of RF after ASD surgery may be related to increased instrumentation fatigue in the setting of PA and high biomechanical loads and shear forces (27,28). Risk of RF also increases with the severity of bend in the rod (29). Common RF sites after long posterior instrumentation to the sacropelvis include the 3CO/PSO, adjacent levels to a 3CO/PSO, and the lumbosacral junction/caudal lumbar spine (13,30,31). Intrinsic mechanical properties of the rod implant material, such as stiffness and notch sensitivity, may also impact the occurrence of RF; notably, the rate of RF was demonstrated to be lower with cobalt chrome compared to titanium alloy or stainless steel (7,32).

The reported RF rates in the ASD literature are quite variable, ranging from 9.0% to 43.1% (7,11-13,19,20,30,33-37). The overall rate of RFs was 20.3% when analyzing available RF data from selected studies in Table 1. Of note, use of supplemental rods demonstrated a significant reduction in the pooled RF rate (12.1% vs. 34.0%, P<0.001). The wide range of reported RF rates may be due to heterogenous patient cohorts and different study follow-up (Table 1). For example, Smith et al. reported a low occurrence of RFs in a multicenter International Spine Study Group consecutive ASD series with minimum 1-year follow-up (11). The authors demonstrated that RF occurred in 9.0% of the cohort at a mean of 14.7 months (range, 3–27 months) after index surgery (11). In their conclusions, Smith and colleagues suggested that with further additional follow-up, the RF rates would likely be even higher (11). More recently, Sardi and colleagues reviewed the Adult Symptomatic Lumbar Scoliosis-1 (ASLS-1) database and reported that 38.8% of patients had ≥1 RF with a median follow-up of 5.1 years (interquartile range, 3.8–6.6 years) (12). For these ASLS-1 patients with RF, the median time to RF was 3.0 years (interquartile range, 1.9–4.54 years), and 73% occurred more than 2 years after index surgery (12). The authors then reported that the estimated Kaplan-Meier RF rates at 2, 4, 5, and 8 years after surgery were 11%, 24%, 35%, and 49%, respectively (12).

Treatment strategies for RF after ASD surgery depend on factors that include patient symptomatology and, in some patients, associated loss of deformity correction (13,14,30,35). For many patients, RFs are detected incidentally on routine radiographic follow-up even sometimes after apparent solid fusion (35). Asymptomatic RFs can be unilateral or bilateral and are managed nonoperatively in many patients (14,30,35). Patients with symptomatic RF most commonly report back or buttock pain. Severe pain radiating distally to the lower extremities and neurological deficits may occur but are not common (14). Regarding loss of deformity correction, Yamato et al. demonstrated significant deterioration in post-RF radiographic alignment for symptomatic RF patients (versus asymptomatic) (14). Revision surgery may be warranted when symptomatic RFs are associated with substantial pain, disability, and/or malalignment (13,14,30,35). In addition to the detrimental health impact on patients and need for reoperation, the management of symptomatic RFs also represent a significant economic burden (15,38). An estimate of the cost of spinopelvic fixation failures (including RF/PA at L4-S1, iliac screw failure or set screw dislodgement, iliac screw prominence, sacroiliac joint pain) after ASD surgery was reported to be nearly $1 billion over five years (38). Zuckerman et al. estimated that revision surgery for RF/PA at L4-S1 costs approximately $77,000±$10,000 (38).

Nomenclature and classification of supplemental rods in ASD

Nomenclature for supplemental rods in ASD constructs has been inconsistent and highly variable. In an effort to improve communication, Ramey et al. proposed a novel classification for supplemental rod constructs (39). The authors defined primary rods, secondary rods, satellite rods, accessory rods, kickstand rods, and delta rods (39). The current review expands on prior supplemental rod nomenclature as more recent publications have introduced novel supplemental rod configurations (20,21,39).

Primary rod: the primary rod is typically the longest rod of an ASD construct that is connected to pedicle screws/segmental fixation and typically spans from the lower-most instrumented vertebra to the upper-most instrumented vertebra. There are usually two primary rods, one on each side of the construct (39).

Secondary rod: like the primary rod, a secondary rod is directly connected to pedicle screws is typically shorter than the primary rod. Secondary rods can be connected to the primary rod via side-to-side connectors (39). In 2006, Shen et al. described a novel “four-rod” technique in which primary and secondary rods were connected to alternating pedicle screws spanning the lumbopelvic junction in a young patient undergoing sacrectomy for chordoma (40). Their surgical technique involved alternating pedicle screw trajectories to allow screw offsets for placement of four distinct longitudinal rods. Of note, the authors did not describe use of side-to-side connectors to connect primary and secondary rods (40). Later in 2018, Shen et al. described their series with the “dual construct” technique in 36 adults undergoing complex spinal reconstructions. All rods were 5.5 mm diameter titanium alloy. At minimum 2-year follow-up, Shen and colleagues reported RFs in three patients, none of which required revision surgery (18). Additional examples of supplemental rod constructs for ASD that involve primary and secondary rods are shown in Figure 3.

Figure 3 These examples demonstrate hybrid cobalt chrome and titanium alloy transitional constructs. In each example, the primary rod is the longest rod spanning from the pelvis to the UIV. Like the primary rod, secondary rods are connected directly to pedicle screws but are typically shorter in length. Secondary rods can be connected to primary rods via side-to-side connectors. A potential benefit of this technique is that transitional ‘hybrid’ constructs can be implemented by utilizing less biomechanically rigid rod material at the UIV (e.g., titanium alloy), while using more rigid rod material (e.g., cobalt chrome) distally across high stress areas in the lumbar spine such as the lumbosacral junction, 3-column osteotomy, or multilevel posterior column osteotomies. Less rigid rod material at the UIV may potentially mitigate risks of proximal junctional kyphosis/failure in long segment posterior instrumented fusions. In each example, bilateral primary rods are comprised of 5.5 mm titanium and the shorter secondary rods are comprised of 5.5 mm cobalt chrome. UIV, upper instrumented vertebra.

Satellite rod: a satellite rod is connected directly to pedicle screws adjacent to a 3CO and not connected to the primary rods. Gupta et al. pioneered this 4-rod technique using satellite rods for closure of a 3CO in a controlled fashion to help prevent sagittal translation, while conferring increased biomechanical support across the high stress osteotomy site (19). Satellite rods are not connected to the primary rods in an effort to avoid severe angular bending which may reduce risk of implant fatigue across the 3CO (19,29). Examples of satellite rods are demonstrated in Figures 4,5.

Figure 4 (A) This is a 75-year-old male with iatrogenic flatback deformity after multiple prior lumbar fusions. (B) This example includes intraoperative X-rays demonstrating controlled closure of an extended L4 pedicle subtraction osteotomy using satellite rods attached to pedicle screws at L3 and L5. (C) Postoperative standing films demonstrate restoration of lumbar lordosis without evidence of rod fracture or proximal junctional kyphosis/failure.

Figure 5 Satellite rod technique for lumbar pedicle subtraction osteotomy (PSO). (A) This example involves a 65-year-old patient with history of T12-S1 posterior fusion who presented with pseudarthrosis and bilateral rod fractures at L2-L3. The patient underwent operative management with T4-pelvis posterior instrumentation and L3 PSO with satellite rods. Satellite rods are deeply recessed and span the L3 PSO with direct attachment to bilateral pedicle screws at L2 and L4. (B) The 1-year postoperative films demonstrate physiological restoration of lumbar lordosis with significant improvement in sagittal alignment. PSO, pedicle subtraction osteotomy.

Accessory rod: an accessory rod is not directly connected to pedicle screws but is connected to primary rods via side-to-side connectors. Accessory rods can be configured medial or lateral to the primary rod and function to bolster areas of high primary rod strain (Figure 6). Accessory rods are likely the most common supplemental rod constructs given their ease of application and supporting biomechanical literature for 3CO, multilevel PCO, and reducing RFs/PA at the lumbosacral junction (10,13,27,39,41). Variations of accessory rods can be configured to connect two separate constructs (intercalary accessory rod), span 3CO or multilevel PCO without bending/contouring the accessory rod (delta accessory rod), and distally anchored to the pelvis (iliac accessory rod) (20,21,42).

Figure 6 Accessory rod for support across multilevel posterior column osteotomies and the lumbosacral junction. This is a 64-year-old patient with kyphoscoliosis who underwent deformity correction with T4-pelvis posterior instrumented fusion and placement of an unilateral accessory rod attached medial to the primary rod with side connectors located at T11-T12 and below S1. (A) Preoperative standing X-rays. (B) Postoperative standing X-rays. (C) Close-up view of postoperative X-rays.

Intercalary accessory rod: An intercalary rod is a type of accessory rod that connects two main rods above and below a 3CO (Figure 7) (21). The intercalary rod configuration can be utilized for construct-to-construct shortening or closure of a 3CO via the rail technique (Video 1) (21,22).

Figure 7 Bilateral intercalary accessory rods for closure of L2 extended pedicle subtraction osteotomy. (A) This is a 63-year-old male patient with severe iatrogenic flatback deformity who underwent deformity correction with use of bilateral intercalary rods to facilitate controlled closure of an extended pedicle subtraction osteotomy. The final construct also includes bilateral delta accessory rods spanning the osteotomy site. This example is further described in Video 1. (B) Intraoperative photographs. (C) Intraoperative X-rays.

Delta accessory rod: a delta rod is an accessory rod with proximal and distal attachments to the primary rod such that the center portion of the delta rod is not angulated/weakened (Figures 7,8). Biomechanically, the straight axial support of delta rods may confer protection across segments with high strain (39). Berjano et al. originally described delta rods spanning and supporting 3CO sites (42). Other authors have reported use of delta rods during revisions after failed primary ASD surgery (26,42).

Figure 8 Delta accessory rod spanning multiple posterior column osteotomies. (A) This is a 49-year-old female with kyphoscoliosis and multiple previous spine surgeries culminating in L4-S1 instrumented fusion. (B) She underwent operative management with T8-pelvis posterior instrumentation with multilevel posterior column osteotomies (T12-L5) and transforaminal lumbar interbody fusions (L2-L3, L3-L4). (C) The right-sided delta rod (red dashed line) is contoured to facilitate attachment to the primary rod via side connectors. Together with the left-sided iliac accessory rod, both these supplemental rods bolster the primary rods across multiple posterior column osteotomies and the lumbosacral junction.

Iliac accessory rod: an iliac accessory rod can be configured to span high stress areas in the lumbar spine (e.g., lumbosacral junction or multilevel PCO). Iliac accessory rods are anchored distally to the pelvis (traditional iliac or S2-alar-iliac) and are connected proximally to the primary rod via side-to-side domino connectors near the thoracolumbar junction (Figures 9,10) (20). Recently, Berlin et al. reported that the rate of RFs was 2% (incidental RF in one patient) among 50 ASD patients surgically treated with constructs involving iliac accessory rods with median 2-year follow-up (20). In contrast to kickstand rods, no distraction maneuver for coronal correction is performed when using iliac accessory rods (23-25).

Figure 9 Bilateral iliac accessory rods (lateral to primary rods) for support across multilevel posterior column osteotomies and the lumbosacral junction. This is a 4-rod construct with bilateral iliac accessory rods for support of the lumbosacral junction and multilevel lumbar posterior column osteotomies. The bilateral iliac accessory rods span the lumbosacral junction and are positioned lateral to the primary rods. Both iliac accessory rods have staggered proximal attachments to potentially facilitate a “smoother” biomechanical transition from a more rigid 4-rod to 2-rod construct, and then to the adjacent non-instrumented spine. (A) Preoperative standing X-ray. (B) Postoperative standing X-rays.

Figure 10 Bilateral iliac accessory rods (medial to primary rods) for support across an extended pedicle subtraction osteotomy. (A) This is a 71-year-old man with iatrogenic lumbar flatback and adjacent level disease following L3-S1 posterior instrumented fusion. (B) He was treated with revision T10-ilium posterior instrumented arthrodesis, L1-L3 posterior column osteotomies, and L4 extended pedicle subtraction osteotomy. Bilateral iliac accessory rods were placed medial to primary rods for reinforcement across osteotomy levels. (C) Intraoperative X-rays demonstrating iliac bolt placement and the pedicle subtraction osteotomy.

Kickstand rod: a kickstand rod is like an iliac accessory rod, but it serves an additional function to help facilitate coronal correction (Figure 11). The kickstand rod is attached distally to an iliac screw (often more lateral along the iliac crest) on the side ipsilateral to truncal shift. Distraction of the kickstand rod against the pelvis can generate increased torque for coronal alignment correction (23-25). In contrast to kickstand rod distraction, the tie rod technique involves compression of an iliac accessory rod on the side contralateral to truncal shift, which functions to pull the C7 plumb line towards midline while correcting coronal malalignment (26).

Figure 11 Kickstand rod for coronal correction. Distraction of the left-sided kickstand rod can help facilitate coronal correction. This is a 63-year-old female with multiple prior spine surgeries who underwent T10-pevis posterior instrumented arthrodesis, posterior column osteotomy at L1, transforaminal lumbar interbody fusion at L1-L2, left-sided kickstand distraction for coronal correction, and placement of a right-sided accessory rod located medial to the primary rod. There are a total of four rods spanning the lumbosacral junction for biomechanical support. Global coronal malalignment improved from 13 cm to 1.5 cm. Thoracolumbar major curve improved from 35° to 10°. (A) Preoperative standing X-rays. (B) Postoperative standing X-rays.

Supplemental rods for 3CO/PSO

Operative treatment of ASD with 3CO/PSO is associated with high rates of RF and/or PA (8,10,11). Tang et al. demonstrated that primary RF after PSO may be related to the severity of rod contouring needed to maintain angular correction created by the osteotomy (29). As such, the use of supplemental rods in ASD (e.g., Gupta’s satellite rods) likely began as an attempt to increase biomechanical strength and reduce primary RF/PA near the PSO (19,39).

Earlier studies used the terms “satellite rods” and “accessory rods” interchangeably (43,44). However, current nomenclature defines satellite rods as recessed short rods connected directly to pedicle screws adjacent to the PSO, while not being connected to the construct’s primary rods (like a satellite orbiting a larger celestial body) (19,21,39). Gupta and colleagues first described the technical nuances and differences between satellite and accessory rods in the setting of PSOs for ASD correction (19,45). The authors popularized the satellite rod technique for controlled closure of a PSO (mitigate risk of sagittal translation) while conferring additional biomechanical stability since four rods spanned the PSO (2 satellite rods, 2 primary rods) (39,45). Moreover, use of satellite rods could potentially avoid severe angular contouring of the primary rods, which weakens the rod and increases risk of RF/PA (19,29,45). Examples of satellite rod constructs for lumbar PSO are depicted in Figures 4,5.

In contrast to Gupta’s satellite rods, accessory rods are not directly connected to pedicle screws but are instead connected to the primary rod via side connectors above and below the PSO level (10,21,46). Buell et al. demonstrated that accessory rods configured across an extended lumbar PSO significantly reduced the occurrence of RF/PA compared to traditional 2-rod constructs (10). In a different study, Hyun and colleagues also demonstrated significant reduction in RF and revision for PA at the 3CO site in ASD patients with supplemental accessory rods compared to traditional 2-rod constructs (46). Multiple clinical and biomechanical studies support the use of accessory rods, which can be configured medial or lateral to primary rods, and typically span areas of high primary rod strain near the osteotomy (10,43,44,46-49). In spite of these biomechanical studies, we acknowledge that there is a lack of high-level evidence (e.g., randomized control trial) regarding superiority of satellite rods vs accessory rods for ASD correction.

Berjano et al. described delta rods spanning and supporting 3CO sites (42). A delta rod is a type of supplemental accessory rod with bends only at its proximal and distal attachments to the primary rod such that the center portion of the delta rod is not angulated and weakened (Figure 7) (26,42). The straight, central portion of the delta rod may confer additional biomechanical protection at the PSO site (42). Delta rods can be configured to span other high stress areas such as multiple levels of posterior column osteotomies (Figure 8) (17).

Supplemental rods for multilevel posterior column osteotomies

Supplemental rods can also support ASD constructs with high primary rod strain due to multilevel posterior column osteotomies (13,50). In 2015, Palumbo et al. described use of “outrigger rods” for areas of high stress such as multilevel transforaminal lumbar interbody cage placements (50). Current nomenclature would classify these “outrigger rods” as accessory rods since the supplemental rod is attached to the primary rod using side connectors (16,17,39,50). More recently, Rabinovich et al. reported significantly reduced incidence of RFs at minimum 2-year follow-up in a cohort operated ASD patients, with many treated using multilevel PCOs spanned by supplemental accessory rods (13). Accessory rods can be configured using side connectors to attach medially or laterally to the primary rod above and below the levels of PCOs (16,17,39). Berlin et al. described a technique for the accessory rod to be anchored distally to an additionally iliac bolt (i.e., iliac accessory rod) (20). Examples of bilateral iliac accessory rods spanning multilevel osteotomies are demonstrated in Figures 9,10.

A potential complication associated with the use of supplemental rods is that stiffening spinal instrumentation constructs may increase risk of proximal junctional kyphosis. Despite this biomechanical risk, some authors have studied this potential issue and reported no clinical complications related to proximal junctional kyphosis. Ye et al. performed a multicenter analysis of ASD patients and demonstrated that patients receiving supplemental rods did not have increased incidence of proximal junctional kyphosis (51).

Supplemental rods for the lumbosacral junction

The high biomechanical loads and shear forces at the lumbosacral junction, which can make fixation and stabilization difficult, are associated with high risks of RF/PA after long posterior instrumentation to sacropelvis (27,28,52,53). Additional supplemental rods spanning the lumbosacral junction may provide greater fixation stability compared to traditional dual-rod constructs (40,54,55). In 2006, Shen et al. described the use of “dual constructs” which utilize primary and secondary rods connected to alternating medial and lateral offset pedicle screws for a total of four independent rods spanning the lumbosacral junction (18,40). Of 36 patients treated using “dual constructs” and minimum 2-year follow-up, three (8.3%) had RFs and none required revision surgery (18). In a separate study, Merrill et al. demonstrated a significant reduction in RF/PA at the lumbosacral junction after ASD surgery with supplemental rod constructs (3 or 4 rods spanning the lumbosacral junction) compared to traditional dual-rod constructs (27). Various supplemental rods can be utilized to support the lumbosacral junction and include accessory rods, iliac accessory rods, and secondary rods (13,16-18,20,40).

Jung et al. reported that interbody fusion at the lumbosacral junction could significantly prevent occurrence of RF at this high stress location (53). Godzik et al. demonstrated that use of anterior lumbar interbody fusion (ALIF) and accessory rods with transforaminal lumbar interbody fusion (TLIF) significantly reduced lumbosacral rod strain in long segment cadaveric models (41). Notably, the authors reported no additional benefit when combining ALIF with supplemental rods and speculated that this may be related to the larger footprint of the ALIF and possible additional stabilization from anterior fixation (41). Collectively, this may suggest that supplemental rods for long posterior instrumentation to sacropelvis are more protective in posterior-only operations when anterior interbody support is not utilized (41,53).

Kickstand rods and intercalary rods for deformity correction

Most supplemental rods are intended for providing biomechanical support at high stress locations susceptible to primary RF and/or PA. However, some supplemental rods provide an additional function to help facilitate intraoperative deformity correction in the coronal plane (e.g., distraction of kickstand rods) or the sagittal plane (e.g., compression of intercalary rods) (22-25,56).

The kickstand rod is anchored distally to a more laterally offset iliac bolt and can be connected to the primary instrumentation near the thoracolumbar junction (23-25,56). When configured ipsilateral to the side of truncal shift, intraoperative distraction of the kickstand rod creates powerful torque to improve global coronal malalignment (23-25,56). An example of a kickstand rod for coronal correction is included in Figure 11. If the supplemental rod anchored to the laterally offset iliac bolt is placed contralateral to the side of truncal shift, then compression of the tie rod can create a force to pull the C7 plumb line towards midline (26).

Intercalary rods are a type of accessory rod that connect two separate constructs via side-to-side domino connectors (Figure 7) (21,39). Intraoperative compression techniques using intercalary rods can provide controlled construct-to-construct closure across a 3CO or PSO using the rail technique (Video 1) (22). In this video, we demonstrate how compression of intercalary rods can facilitate construct-to-construct closure across a PSO in an elderly male patient with iatrogenic flatback deformity. Another indication for intercalary rod compression and construct-to-construct closure includes revision surgery for proximal and distal junctional failures (22). The intercalary rods may be replaced with linked rods if the primary rods for each construct begin to overlap during construct-to-construct shortening (21). Of note, El Dafrawy et al. described linked rods across a 3CO in their classification system for supplemental rods (21).

Strengths and limitations

The strengths of this review include its thorough and detailed descriptions of current supplemental rod constructs for ASD. In this review, we have provided numerous pictorial examples that may help surgeons adopt consistent nomenclature to improve communication and future clinical study. However, this review has several limitations. First, there is a lack of rigorous statistical comparisons among the various supplemental rod constructs when assessing primary RFs or deformity correction. Also, many of the reviewed literature were not randomized controlled trials, and so we suggest utilizing supplemental rod constructs based on surgeon experience and careful patient selection.

Conclusions

Long posterior instrumentation to the sacropelvis is associated with high rates of primary RF and PA. Supplemental rod configurations likely began as an attempt to lower primary RFs/PA across 3-column osteotomies. Common examples include Gupta’s satellite rods (unattached to primary rods) and accessory rods (attached to primary rods) including delta accessory rods. Currently, accessory rods are likely the most common supplemental rod configuration and are frequently utilized for multilevel posterior column osteotomies and the lumbosacral junction. When attached distally to an iliac bolt, accessory rods are termed iliac accessory rods (distinct from kickstand rods) and have been shown to significantly lower the occurrence of primary RF. We suggest that deformity surgeons consider utilizing supplemental rods to provide biomechanical support to areas of high stress such as 3-column osteotomies, multilevel posterior column osteotomies, and the lumbosacral junction. In select patients, surgeons may consider supplemental rods to facilitate deformity correction. Examples include kickstand rod distraction for coronal correction and intercalary rod compression for sagittal correction. Future studies with long-term follow-up are needed to compare the effectiveness between supplemental rod configurations.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was commissioned by the editorial office, AME Surgical Journal for the series “Adult Spinal Deformity: Principles, Approaches, and Advances”. The article has undergone external peer review.

Peer Review File: Available at https://asj.amegroups.com/article/view/10.21037/asj-25-57/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://asj.amegroups.com/article/view/10.21037/asj-25-57/coif). The series “Adult Spinal Deformity: Principles, Approaches, and Advances” was commissioned by the editorial office without any funding or sponsorship. M.C.G. served as the unpaid Guest Editor of the series. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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