Correction of a severe coronal malalignment in adult spinal deformity using the “kickstand rod” technique as primary surgery

Abstract

Objective

Adult spinal deformity (ASD) is a growing healthcare issue due to the aging population. A satisfying spine balance in both sagittal and coronal planes is achieved through surgery. Only few studies about the coronal alignment correction with the kickstand rod were reported in the literature, until now. The aim of the present study was to describe clinical and radiological outcomes of the Kickstand rod (KR) technique in a series of ASD patients with severe coronal malalignment after 1 year of follow-up.

Material and methods

Six patients affected by ASD with severe CM who underwent surgery between 2018 and 2019 were retrospectively analyzed. The mean follow up was 14 months. All patients had posterior-only approach with long pelvic-thoracic fixation according to the Kickstand rod technique.

Results

Postoperative alignment and pain numerical rating scale scores significantly improved. No instrumentation complications occurred. A coronal alignment improvement from a mean of 163 mm preoperatively to a mean of 32 mm postoperatively was observed.

Conclusion

KR technique appears to be a safe and efficient way for coronal and sagittal imbalance correction in ASD patients. Although technically demanding, by using this technique good and stable radiological and functional outcomes are achieved especially in selected patients.

1. Introduction

Adult spinal deformity (ASD) is one of the most disabling diseases and represents a widespread condition considering the global population aging.^1,2 The incidence of this condition in fact increased up to 32% in adults and 60% in the elderly.³ Furthermore the quality of life (QoL) of patients suffering from ASD seems to be lower than patients affected by other debilitating chronic diseases such as arthritis, chronic lung disease, congestive heart failure, and diabetes.² Many conservative alternatives should be used to improve these patients QoL (e.g. pain therapy, physical therapy).^4,5 However, when nonoperative management fails, surgery may be a valid option. The aim of the surgical intervention in ASD patients include the sagittal and coronal spinal balance correction, the eventual decompression of the neural structures and the promotion of a solid spinal fusion.⁶ Surgical efforts have been directed principally to the sagittal spinopelvic parameters correction in the last decade, whereas the coronal malalignment (CM) was often underestimated. This last condition is defined as a lateral displacement of the C7 plumbline more than 4 cm from the central sacral vertebral line (CSVL).^7,8

Moreover, recent studies suggest that CM has a great impact on pain, disability and functional outcomes.⁹ Operative CM treatment is still a challenging surgical issue since this deformity is often associated with sagittal imbalance that requires tridimensional corrective maneuvers.¹⁰ Many techniques have been suggested as surgical solutions over the years, however the topic is still controversial in the Literature.11, 12, 13 Kickstand rod (KR) technique is a novel surgical procedure developed for the CM correction in patients affected by ASD with sagittal imbalance.^14,15

This operative procedure involves the implantation of a supplementary rod, called kickstand, onto the side which the trunk has shifted. Dual headed screws or side-to-side connectors are used to join the KR to the thoracolumbar implants. Screws inserted in the ilium provide a distal foothold to the kickstand rod. When the system is intraoperatively distracted, it spreads powerful coronal corrective forces on the thoracolumbar tract pushing on the iliac wing.

Only a few studies about the coronal alignment correction with the KR were reported in literature until now14, 15, 16, 17, 18, 19, 20, 21

The aim of the present study was to describe clinical and radiological outcomes of the KR technique in a series of patients affected by ASD with severe CM after at least 1 year of follow-up.

2. Material and methods

2.1. Study setting and design

The present investigation represents an Institutional Review Board-approved retrospective study. The radiological and clinical outcomes of patients undergone ASD corrective surgery through thoraco-lumbar-pelvic open posterior fixation in our institution (single-surgeon team) between 2018 and 2019 were analyzed. All patients included in the present study were clinically and radiographically evaluated at months 1, 3, 6 and 12 and annually after surgery.

2.2. Participants and eligibility criteria

All patients suffering from ASD with severe CM and surgically treated at our institution between December 2018 and November 2019 were potentially eligible for the study.

Inclusion criteria were: (I) the presence of CSVL > 4 cm; (II) the use of “Kickstand rod technique”; (III) a complete clinical and radiological data set; (IV) a minimum follow-up of 12 months.

Exclusion criteria were: (I) Preoperative bone density (studied by Dual Energy X-Ray Absorptiometry, DEXA) with t-score < 2.0; (II) Neoplastic diseases; (III) spinal infections; (IV) Rheumatic diseases with ossification of the posterior longitudinal ligament (e.g. ankylosing spondylitis); (V) previous thoracolumbar surgery; (VI) combined posterior and anterior or lateral spinal surgery.

2.3. Surgical technique

All procedures were performed under general anesthesia in a prone position with neuromonitoring during the whole surgical procedure. A posterior epispinous skin incision was performed. After preparing the pedicle to instrumentation, trans-pedicle screws were placed with free-hand technique.²² Then, after the iliac crests exposition the iliac screws were placed. The KR distal screw was placed on the side of the truncal shift. Either Multiple Posterior Column Osteotomy (PCO) or alternatively the placement of interbody cages (with TLIF technique, usually at L5-S1 level) were carried out to allow the fractional curve correction and to improve the lumbar lordosis.^16,17 The construct was finally completed with two titanium (5.5 mm) rods properly modeled under continuous neurophysiological control until the deformity correction was achieved (derotation maneuvers and segmental correction). According to the KR technique, the support CrCo (5.5 mm) rod on the side where the patient is shifted was placed, locked in the KR iliac screw (figure x) and connected to the main rod with a connector between T10 and T12 vertebra. A fourth support rod, connected with 2 or 3 connectors to the main rod on the opposite side was usually placed. Under fluoroscopic guidance, the coronal correction obtained was checked. Afterwards we proceeded with the laminae preparation and the autologous bone chips addition to promote a stable secondary posterior fusion. Finally the closure of the muscle, the subcutaneous tissue and the skin was performed. Usually 2 subfascial drains were placed. The drains were removed on the second postoperative day. After surgery all patients treated wore a fiberglass thoracolumbar corset over a period of three months after surgery. The patients were encouraged to stand and walk on the second postoperative day.

2.4. Variables

The primary outcomes were the rod fracture rate after a follow up period of 12 months and the correction of CM. The secondary outcomes were the coronal and sagittal radiographic parameters change and the clinical and functional outcomes at the last follow-up visit.

2.5. Radiological outcomes

Preoperative, immediately postoperative and 12 months postoperative X-Ray images (Antero-posterior AP and Lateral whole spine in standing position, Fig. 1) were retrieved and reviewed, using a dedicated workstation (Advantage Windows Workstation; GE Medical Systems, Milwaukee USA). The following parameters were measured: Pelvic Index (PI), Pelvic Tilt (PT), Sacral Slope (SS), Lumbar Lordosis (LL, from L1 to S1), PI-LL mismatch, Thoracic Kyphosis (TK, from T5 to T12), CM, Coronal Cobb (CC) of major thoracolumbar/lumbar curve. The CM type was categorized according Obeid et al. classification.¹⁰

Fig. 1.

Preoperative (A,C) and 12 months follow up (B,D) full spine standing radiographs of the patient 1 showing the correction of coronal balance and no instrumentation failure.

2.6. Clinical and functional evaluation

The clinical evaluations were performed preoperatively, 6 and 12 months after surgery, using a ten-points itemized visual analogue scale (VAS) for leg (VAS-l) and back (VAS-b) pain, and the Oswestry Disability Index (ODI) score, and Short Form 36 questionnaire (SF36), Physical component score (PCS). Intraoperative and postoperative complications were also recorded.

2.7. Statistical analysis

Analyzed data were presented as mean ± standard deviation for continuous variables and frequency and percentages for categorical variables The paired samples t-test or Wilcoxon signed-rank test, when appropriate, was used to analyze the study outcomes. An alpha value of 0.05 was set for statistical significance. SPSS calculation software (SPSS Inc, Chicago, IL) was used for data analysis.

3. Results

3.1. Participants

Six patients, (4F; 2M) were included in the present study. The mean age was 72.2 ± 1.9 years. The mean BMI was 25.8 ± 0.9. The mean follow up was 14 ± 2.5 months. Demographic characteristics of patients were reported in Table 1, Table 2. An explicative case was reported in Fig. 1, Fig. 2.

Table 1.

Surgical case series.

Patient	Sex	Age	BMI	CM (cm)	Obeid CM type	Major curve	Major curve apex	Level fused	Screw implanted	Rods implanted	Side of Kikstand Rod	Cages	Osteotomies	Blood loss (ml)	Complication	Follow up (months)
1	F	76	24,2	26	Type 1A	46°	L2-L3	D4-Ileum	29	4	Concavity		PCO L1-L4	320	Postoperative Ileus	18
2	F	71	26,8	17	Type 1A	37°	L3-L4	D4-Ileum	30	3	Concavity	TLIF L5-S1	PCO L2-L5	400	Need of blood transfusion	16
3	M	74	27,1	14	Type 1A2	29°	L3-L4	D2-Ileum	31	4	Concavity		PCO L2-L5	280	–	14
4	F	69	26,2	21	Type 2A	52°	L1-L2	D4-Ileum	28	4	Convexity	TLIF L5-S1	PCO L1-L4	420	Superficial wound infection	12
5	M	72	25,9	8	Type 2A2	41°	L2-L3	D4-Ileum	30	4	Convexity		PCO L1-L4	300	–	12
6	F	71	24,7	12	Type 1A	58°	L2-L3	D2-Ileum	30	3	Concavity	TLIF L5-S1	PCO L2-L5	430	Need of blood transfusion	12

BMI: Body Mass Index; CM: Coronal Malalignment; PCO: Posterior Column Osteotomy; TLIF: Transforaminal Lumbar Interbody Fusion.

Table 2.

Demographics features.

Demographics
Number of patients	6
Age	72.2 (±1.9) y
Sex	M:2; F:4
BMI	25.8 (±0.9)
Diabetes	1
Smokers	3
Osteopeniaa	1
Estimated blood loss in ml	458.3 (±56.3) ml
Operative duration in minutes	327.6 (±72.1) m
Length of stay in days	9.7 (±3.9)
Local intrawound vancomycin powder	6 patients

^aPatients with t-score < 2.5 were excluded. Patients with t-score <2.0 typically receive treatment prior to surgery.

Fig. 2.

Preoperative (A,B,C) and 12 months postoperative clinical photos (D,E,F) of the patient 1.

3.2. Surgical data

There were no intraoperative complications recorded, excluding a change in electrophysiological potentials that occurred during corrective maneuvers in a single case, however without clinical consequences. The mean intraoperative blood loss was 458.3 ± 56.3 ml. In all patients enrolled multiple PCO were performed at the lumbar level to restore lumbar lordosis. (Table 1). Three intersomatic cages at L5-S1 level were implanted in 3 patients using TLIF technique. Concerning the postoperative complications: 2 patients needed blood transfusion due to hemoglobin value less than 8 g/dl; 1 patient had a superficial wound infection treated with 2 weeks of antibiotic therapy; 1 patient experienced a paralytic ileus self-resolved on the fifth postoperative day. After 12 months of follow up, no implant loosening or rods fractures were observed. Surgical data were resumed in Table 1.

3.3. Radiographic findings

According to Obeid et al. classification, we found the type 1 A in 3 cases, the type 1A2, 2A and 2A2 in 1 patient.

The radiographic data changed as follow on the sagittal plane: SVA from 9.2 (±5.6) cm preoperative to 2.9 (±2.3) cm (p = 0.001), PT from 33.7 (±5.2)° to 26.1 (±5.8)° (p = 0.023), SS from 24.2 (±4.9)° to 31.4 (±6.1)° (p = 0.034), LL from 31.5 (±6.1)° to 49.9 (±7.4)° (p < 0.001), PI-LL mismatch from 22.5 (±7.4)° to 6.7 (±4.5)° (p < 0.001), TK from 24.5 (±12.2)° to 37.6 (±9.3)° (p = 0.004).

Concerning the coronal plane radiographic parameters we recorded the following changes: CM from 16.3 (±6.4) cm preoperative to 3.2 (±2.6) cm postoperative (p < 0.001), Major T/L curve (Cobb angle) from 48.8 (±10.4)° to 10.3 (±4.9)° (p < 0.001), Fractional L/S curve to 17.2 (±9.7)° to 8.7 (±4.7)° (p = 0.003), Thoracic curve from 12.9 (±10.2)° to 8.6 (±5.2)° (p = 0.047). The main radiographic data were resumed in Table 3.

Table 3.

Radiographic and clinical outcomes.

	Pre operative	Post operative (12 months of FU)	P value
Sagittal plane parameters
SVA (cm)	9.2±5.9	2.9±2.3	0.001
PI (°)	52.3±6.2	52.7± 6.4	>0.05
PT (°)	33.7±5.2	22.1±5.8	0.023
SS (°)	24.2±4.9	31.4±6.1	0.034
LL (°)	31.5±6.1	49.9±7.4	<0.001
PI-LL mismatch	22.5±7.4	6.7±4.5	<0.001
TK (°)	24.5±12.2	37.6±9.3	0.004
Coronal plane parameters
CM (cm)	16.3±6.4	3.2±2.6	<0.001
Major T/L curve (°) (Cobb angle)	48.8± 10.4	10.3±4,9	<0.001
Fractional L/S curve (°) (Cobb Angle)	17.2± 9.7	8.7±4.7	0.003
Thoracic curve (°) (Cobb angle)	12.9±10.2	8.6±5.2	0.047
Clinical Outcomes
VAS back	7.9±2.1	3.8±2.7	0.021
VAS Leg	4.8±3.4	3.6±2.1	>0.05
ODI (%)	49± 10	28±8	0.003
SF36	31.1±7.2	47.1 ± 6.7	0.001

CM: Coronal Malalignment; L/S: Lumbo/Sacral; LL: Lumbar Lordosis; ODI: Oswestry Disability Index; PI: Pelvic Index; PT: Pelvic Tilt; SF36: Short Form 36; SS: Sacral Slope; SVA: Sagittal Vertical Axis; T/L: Thoraco/Lumbar; TK: Thoracic Kyphosis; VAS: Visual Analogue Scale.

3.4. Clinical and functional outcomes

The VAS value for back pain improved from a pre-operative score of 7.9 (±1.4) to a value of 3.8 (±2.7) (p = 0.021) at the 12 months evaluation. The VAS value for leg pain improved from a pre-operative score of 4.8 (±3.4) to a value of 3.6 (±2.1) (p > 0.05) after 12 months of follow up. The ODI improved from a pre-operative score of 49 (±10) to a 12 months postoperative score of 28 (±8) (p = 0.003). The SF36 improved from a pre-operative score of 31.1 (±7.2) to a 12 months postoperative score of 47.1 (±6.7) (p = 0.001).

4. Discussion

4.1. Overview of the literature

Coronal imbalance (CI) represents a hot topic in ASD patients. In fact, recent studies^10,19 showed that CM correction allows a resolution of painful symptoms such as rib-iliac conflict pain or gait disturbance.

In 2018 Makhni et al. described a new surgical technique for CM correction using for the first time the so-called KR on a 62-year-old patient with osteoporosis.¹⁴ Subsequently, they published a case series of 24 patients showing a mean pre-operative CI of 63 mm compared to an average postoperative CI value of 47 mm, close to achieving normal spinal alignment following the procedure.¹⁵ No peri or postoperative complications were reported.

Afterwards, this innovative technique was described by other authors. In 2020 Buell et al. reported a case series of 19 patients with a correction of the CI from 80 mm to 10 mm due to KR technique. Three patients underwent reoperations: one patient due to proximal junctional kyphosis and implant failure; one because of wound dehiscence; one case of coronal overcorrection.¹⁷

Redaelli et al. in 2019 reported a case series of 4 female patients (mean age 64 years-old) affected by severe postoperative coronal imbalance who underwent revision surgery.¹⁹ These patients underwent combined KR and Tie Rod (TR) techniques surgery. The first pushes with distraction on the concave side, while the second pulls with compression on the convex side. The mean surgical correction of CM was 35 mm (range from 20 to 52 mm). In particular, the mean correction for the KR technique was 26 mm and 43 mm for TR technique. All of the patients improved their preoperative disability. The mean follow-up was 19 months. No complications were reported.

Martini et al. in 2020 retrospectively analyzed 77 ASD patients who underwent revision surgery.²⁰ Five patients underwent the KR technique. They concluded that surgical techniques such as KR or TR have been shown to be effective in improving stiffness and primary stability of the construct.

Finally, Fiani et al. recently published a case report and a review of the literature for a total of 45 patients, reporting a mean preoperative CI value of 64.16 mm and a mean postoperative CI value of 26.83 mm.²¹ This made for a coronal correction of 37.3 mm.

It emerges that the KR technique is a valid option to correct coronal deformity in revision surgery. However to date only a few authors in the Literature described the KR as a primary surgery for the CM correction.

4.2. Biomechanical consideration

ASD are 3-dimensional deformities, so a proper correction on the sagittal, axial and coronal plane is needed. Surgery aims to achieve patient-specific alignment targets in order to restore optimal coronal and sagittal radiographic parameters. Despite advances in surgical techniques and implant's biomaterials there are still instrumentation failures. Among these the rods fractures that often worsen the clinical outcome and that require reoperation. The optimal construct configuration should be implemented case by case. Its stability could depend on patients and surgery related factors such as: the number of screws implanted, the tension band integrity, the discal degeneration, the need for anterior support, the spinal coronal and sagittal balance.²³ Thoracolumbar junction (TLJ) and lumbosacral junction (LSJ) stabilization plays a significant role in construct stability and durability. In fact, primary stability loss in the TLJ and LSJ could lead to pseudarthrosis and, consequently to implant failures.¹⁸

Based on biomechanical and clinical studies, it is well known that multiple-rods constructs are useful, especially in sites of increased instability (e.g. PCO, multiple laminectomy or 3-column osteotomies).²⁴ Actually, additional rods reduce the range of motion (ROM) of the analyzed segments and decrease the risk of pseudarthrosis and rod fractures. Nevertheless, multiple rods constructs do not seem to increase the overall stiffness of the implant, distributing forces over the whole construct.²⁵

That said, the KR technique spreads loading forces from a segment proximal to the TLJ down to the iliac wing on the side where the trunk is shifted. Moreover, it leads to major primary stability on the coronal plane compared to traditional two-rod constructs that prevent the rods breakage and the pseudarthrosis rate after ASD corrective surgery.

In our series the KR was connected to the main rod between T10 and T12 vertebra on the side where the trunk is shifted; this allows to bypass the TLJ and to reduce the load acting on it by downloading forces directly to the pelvis. In addition, in all treated patients only multiple PCO at lumbar level were performed with the advantage of avoiding the Pedicle Subtraction Osteotomies (PSO). It has been shown that the latter involve an enormous rate of mechanical complications due to the heavy breaking forces that weigh on the rods and screws.²⁶

In support of these considerations, up to now, no instrumentation failure was reported in the Literature when the KR technique was performed.

Concerning the rod's biomaterials, the choice depends on the surgeon's objectives. Usually in ASD patients corrective surgery the aim is to achieve as much stability as possible. It is well known that Cobalt chromium (CoCr) rods compared with titanium (Ti) rods with similar diameter have a higher rigidity, stiffness and yield strength.²⁷

These properties suggest us to prefer CoCr rod for the KS rod and Ti-rods for the principal construct in ASD corrective surgery.

Iliac screw loosening after corrective surgery in ASD patients is also a frequent complication which reduces interbody fusion causing potential progression to pseudarthrosis and pain: recent studies assessed its rate at 27.8%.²⁸

The bear loading forces are constrained on the KR distal iliac screw, therefore its orientation is crucial. In fact, as much as the direction of the screw moves from the direction of the loading forces, as much the torque which acts on the screw increases (Fig. 3 a–c). When the angle between the direction of the forces and the direction of the screw becomes close to perpendicular, the torque becomes higher and consequently the iliac screw loosening could occur (Fig. 3a). Otherwise, when bearing forces are parallel to the screw there is no torque (Fig. 3b). In our series, we insert the iliac screw of the KR as parallel as possible to the loading forces in all patients treated with this technique (Fig. 3c).

Fig. 3.

The torque is a vector quantity which causes an object to acquire angular acceleration. The magnitude of the torque is τ = F * r * senα, where F is the bearing force downloaded by the kickstand rod, r is the length of the moment arm (length of the screw) and is the angle between the force vector and the moment arm (c). The direction depends on the force direction on the axis. When the angle between the force vector and the moment arm is 90°, there is the highest possible torque because sen90° is 1 (a). When is 0, there is no torque because sen0 is 0 (b).

Moreover, the KR screw should be inserted as lateral as possible to the primary iliac screw requiring 4–5 cm dissection proximal and lateral to the posterior superior iliac spine (Fig. 4). In fact, a wider distance from the main construct and the KR holdfoot generates powerful distraction forces.¹⁷ Careful must be paid when the KR iliac screw is inserted because it could lead to the ilium fracture as the bone is thinner (Fig. 4).

Fig. 4.

The iliac screw entry point in KR technique should be about 4–5 cm proximal and lateral the posterior and superior iliac spine, as shown in the figure in light green (a, posterior view; b, axial view). Around this area, the red dot points out the ideal iliac screw entry point. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

4.3. Our experience

In our series of patients we observed a statistically significant change of coronal and sagittal balance parameters. In particular we found a coronal alignment improvement from a mean of 163 mm preoperatively to a mean of 32 mm postoperatively, and this correction was preserved until the last 12 months-follow up evaluation. Clinical and functional parameters also (VAS leg, VAS back, ODI and SF36) improved significatively after surgery.

Although other techniques have been described for the ASDs correction in the sagittal and coronal planes (e.g. anterior and lateral procedure or posterior osteotomies such as 3-column osteotomies, vertebral column resection and asymmetrical pedicle subtraction osteotomies), these appear to be burdened with a high rate of peri and post-operative complications (increased blood loss, neurological deficit, implant failure).²⁹ It is also known that lateral lumbar interbody fusion surgery in addition to posterior surgery (MIS or open) could improve the immediately postoperative coronal and sagittal parameters correction and overall QoL^30,31

However, it should be reserved for selected cases that do not require major corrections on the coronal plane or an extended posterior open surgery. Therefore, in the present investigation we have deliberately excluded patients treated with a double approach (lateral lumbar interbody fusion and posterior surgery) to verify the stability and efficacy of posterior surgery combined with the KR. Furthermore, the patients' selection plays an important role. In our opinion, the use of the KR technique allows the best results in patients with flexible scoliotic curves that can only be corrected with multiple PCOs. This is the reason why we excluded patients with ankylosing spondylitis or ossifying pathologies of the spine who generally have a rigid scoliotic curve, which often require PSO or Vertebral Column Resection (VCR) to be corrected, with great overload and higher risks of implant failure.

Although in the Literature high mechanical and neurological complications rate were reported after surgical treatment of ASD,²⁶ in our series, no major mechanical complications like rods fractures, implant mobilization, junctional kyphosis or neurological deficits were observed at the last follow up.

4.4. Limitations

Notwithstanding, our study had some limitations. First of all the present investigation is a retrospective study, and this it does not allow comparison with patients in whom the KR has not been used. Moreover, due to the rareness of cases in which the KR technique could be used, the number of patients enrolled is small. Last but not least, the 12 months follow-up is relatively short to rule out late mechanical complications. Therefore, further comparison studies with larger case series and longer follow-up are necessary.

5. Conclusion

The KR technique is effective in coronal plane correction in ASD surgery and guarantees stable clinical and radiographic results after 12 months of follow up.

Furthermore, multiple rod constructs in long instrumentation allows to increase the stability of the implant, reducing the short-term mechanical complication without limitations in the deformity correction. However, a careful patient selection (preferably flexible scoliotic curve) could further improve the outcomes.

Funding

None.

Availability of data and material

The data used to support the findings of this study are available from the corresponding author upon request.

Compliance with ethical standards

All procedures performed were in accordance with the 1964 Helsinki declaration. This research has been approved by the IRB of the authors' affiliated institutions. Written informed consent for scientific purposes and clinical data collection was obtained according to institutional protocol.

CRediT authorship contribution statement

Luca Proietti: Conceptualization, Methodology, Writing – original draft. Andrea Perna: Conceptualization, Methodology, Writing – original draft. Calogero Velluto: Methodology, Writing – original draft, Conceptualization, Supervision, Writing – review & editing. Amarildo Smakaj: Methodology, Writing – original draft, Conceptualization, Supervision, Writing – review & editing. Maria Beatrice Bocchi: Writing – original draft, Writing – review & editing. Caterina Fumo: Writing – original draft, Writing – review & editing. Luca Fresta: Writing – original draft, Writing – review & editing. Francesco Ciro Tamburrelli: Methodology, Writing – original draft, Conceptualization, Supervision, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no conflict of interest.