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High Failure Rates of a Unilateral Posterior Peri-Apical Distraction Device (ApiFix) for Fusionless Treatment of Adolescent Idiopathic Scoliosis

Stadhouder, Agnita MD1,a; Holewijn, Roderick M. MD, PhD2; Haanstra, Tsjitske M. PhD3; van Royen, Barend J. MD, PhD1; Kruyt, Moyo C. MD, PhD4; de Kleuver, Marinus MD, PhD5

Author Information
The Journal of Bone and Joint Surgery: October 6, 2021 - Volume 103 - Issue 19 - p 1834-1843
doi: 10.2106/JBJS.20.02176
  • Open
  • Disclosures
  • Data Availability

Abstract

Adolescent idiopathic scoliosis (AIS) is characterized by a 3-dimensional deformity of the spine and trunk. Surgical correction and spinal fusion for patients with curves exceeding 45° to 50° at skeletal maturity results in reliable and good correction1,2. However, there is room for improvement. Recently, Bartley et al. reported a reoperation rate of 4.1% for the treatment of complications in a large cohort with >2 years of follow-up3. In the long term, sporting activity, physical function, and activities of daily living are negatively affected by the loss of spinal flexibility4-7. Functional outcomes have been reported to be significantly worse in surgically treated patients with scoliosis as compared with healthy controls8. Some authors have reported increased risks of adjacent segment disease, but its relation to fusion of the spine remains a topic of debate9-12. Therefore, a fusionless surgical technique that is less invasive and preserves spinal motion could provide further improvement in AIS treatment.

Currently, several fusionless surgical techniques based on either a convex tether principle or a concave distraction principle are available for skeletally immature patients with AIS. Both types of techniques aim at coronal curve correction by growth modulation. They do not address the rotational deformity, although some studies have shown an improvement of the angle of trunk rotation13,14. The anterior vertebral body tethering technique aims to modulate the remaining spinal growth by reducing growth on the convexity of the curve. Although this technique was promising initially, follow-up showed a high revision rate of 21% to 41%15, indicating that those implants require a scrutinous evaluation of indications and results16.

With the concave distraction technique, a posterior ratcheted rod is placed at the concavity and is connected to cephalad and caudad pedicle screws with polyaxial joints (ApiFix). With side-bending during regular exercises and possibly also with growth, the ratchet is supposed to lengthen, thereby correcting the curve in the coronal plane.

For both the tether and ratchet techniques, some remaining growth and related remodeling capacity of the tissues is required. Sufficient flexibility of the curve is required to facilitate correction both intraoperatively and during the postoperative physical exercises. Only 1 report on the distraction ratchet had been published at the start of our study; recently, 2 more studies, both retrospective case series, have been published 17-19. Additionally, an in vitro biomechanical study demonstrated that the decrease in range of motion was much less in association with this implant as compared with fusion20.

Materials and Methods

Study Design

The present study was a limited-efficacy, prospective, open-label study performed in a university hospital by a single surgeon embedded in a multidisciplinary scoliosis team. Approval was obtained from the institutional ethical review board of the VU University Medical Center Amsterdam, the Netherlands (15-223), and the study was registered at the Netherlands Trial Register (Trial NL5162 [NTR5302]). All subjects and their legal representatives were fully informed and interviewed to understand the study according to the Declaration of Helsinki. The study was performed in accordance with the Dutch national law/Dutch guidelines for new interventions in clinical practice21,22. The institution of a data safety and monitoring board (DSMB) was considered unnecessary as preliminary results of the device indicated a very low risk of serious adverse events. One author (M.K.) is involved in development of dynamic implants as an alternative to growing rods.

We were obliged to discontinue the study and make this publicly available if new information would indicate that the original considerations were no longer met according to the study protocol23.

Study Population

The inclusion criteria were (1) a diagnosis of AIS, (1) an age of 12 to 17 years, (3) skeletal immaturity (Risser stage 1 to 4); (4) a single structural curve (Lenke type 1 or 5); (5) a major Cobb angle of 40° to 55°, (6) reduction of the major curve to <35° on a supine lateral bending radiograph, and (7) apical vertebral rotation of <15° (Bunnell scoliometer).

Surgical Technique

Patients were positioned prone under general anesthesia. During exposure of the concave side of the spine, special care was taken not to damage bone, periosteum, and ligaments. One pedicle screw was inserted at the cephalad-end vertebra and 1 was inserted in the caudad-end vertebra (Cobb to Cobb) under fluoroscopic guidance. The expandable ratcheted rod was connected to the 2 pedicle screws. Mild device distraction was performed to ensure proper lengthening and to achieve initial curve correction. The patients were allowed unrestricted weight-bearing during normal daily activities. A representative of the manufacturer was present during all operations.

Two weeks after surgery, patients started an exercise protocol under the guidance of a trained physical therapist in accordance with the guidelines of the manufacturer. The exercises encompassed lateral bending and stretching movements to lengthen the expandable rod. Follow-up visits were scheduled at 2 and 6 weeks, 3 and 6 months, 1 year, and 2 years postoperatively.

Outcome Measures

Clinical parameters collected during surgery included the length of incision, blood loss, intraoperative complications, and duration of surgery (skin-to-skin time). Furthermore, the length of hospital stay and postoperative complications were recorded. The Cobb angles of the major and minor curves, thoracic kyphosis, and lumbar lordosis were measured on full-spine standing radiographs. All radiographic measurements were performed with use of Surgimap spine software (Nemaris)18. Rotational deformity was measured using the Bunnell scoliometer. For patient-related outcome measurements, the revised Dutch version of the Scoliosis Research Society 22-item patient questionnaire (SRS-22r) was used preoperatively and 12 and 24 months postoperatively24.

Statistical Analysis

A power analysis was performed based on the residual curve. However, due to termination of the study, this analysis is considered not relevant anymore. The focus of this paper was changed to the complications that we encountered in this prospective cohort study.

Source of Funding

ApiFix Ltd. supported this study with an institutional grant of €10,000. The payment was made directly to the institution as contribution to the costs for an interim analysis. No other funding was received, and the implants were billed and paid for by the institution in the standard fashion. ApiFix Ltd. had no influence on the data gathering, data analysis, or writing of this report.

Results

Due to an unacceptable high failure rate (7 of 20 patients) at an average of 12 months (range, 8 to 19 months) postoperatively, patient recruitment for the study was halted. Subsequently, 3 more patients showed implant failures at the time of follow-up. Therefore, further patient inclusion was definitively terminated. The current analysis is based on all 20 patients who were operatively managed between November 2015 and May 2018. The mean duration of follow-up (and standard deviation) was 3.4 ± 1.0 years (Tables I and II).

TABLE I - Patient Characteristics (N = 20)
Sex (no. of patients)
 Female 19
 Male 1
Age at surgery*(yr) 14.8 ± 1.4
Lenke type (no. of patients)
 1 14
 5 6
Risser stage at surgery* 2.5 ± 1.1
*The values are given as the mean and the standard deviation.

TABLE II - Surgical Outcome Measures (N = 20)*
Number of included vertebrae 5.1 ± 0.9
Surgical time (hr) 1.1 ± 0.2
Blood loss (mL) 42.5 ± 43.8
Length of incision (cm) 16.1 ± 1.9
Length of hospital stay (d) 2.3 ± 0.7
*The values are given as the mean and the standard deviation.

Follow-up

The patients who had removal of the implant (n = 6), revision to a new implant (n = 2), or revision to posterior spinal fusion (n = 2) were excluded from the final results presented here and in Tables III and IV. The remaining 10 patients were followed for a mean of 3.8 ± 0.9 years. In this group, the mean major curve measured 45.4° preoperatively, 31.4° at 2 weeks postoperatively, and 31.0° at the time of the latest follow-up. The mean minor curve measured 31.3° preoperatively, 26.1° at 2 weeks postoperatively, and 24.2° at the time of the latest follow-up. Despite postoperative exercises, no significant change in major or minor curve deformity was observed from 2 weeks postoperatively to the latest follow-up. No adding-on (curve progression cranial or caudal to the instrumentation), proximal junctional kyphosis, or distal junctional kyphosis was observed. Lumbar lordosis and thoracic kyphosis remained unaltered (Table III). No changes in the scoliometer rotation measures were observed (Table IV). Patient-reported outcome parameters are shown in Table V.

TABLE III - Cobb Angle in Patients with Implant in Situ (N = 10)*
Baseline (1) 2 Weeks (2) Latest Follow-up (3) 1 vs. 2 1 vs. 3 2 vs. 3
Major curve 45.4° ± 5.2° 31.4° ± 6.8° 31.0° ± 7.5° P < 0.001 P = 0.001 NS
Minor curve 31.3° ± 8.2° 26.1° ± 9.4° 24.2° ± 10.4° P = 0.030 P = 0.001 NS
*Repeated-measures analysis of variance was used to compare the effect of measurement. If a significant main effect for measurement was found, post-hoc Bonferroni-corrected t tests were used. Patients who underwent revision surgery were excluded from these results. NS = not significant.
The values are given as the mean and the standard deviation.

TABLE IV - Kyphosis, Lordosis, and Rotation in Patients with the Implant in Situ (N = 10)*
Baseline Latest Follow-up P Value
Lumbar lordosis 17.6° ± 7.2° 22.5° ± 8.8° 0.987
Thoracic kyphosis 55.4° ± 1.8° 55.4° ± 12.3° 0.183
Rotation (Bunnell scoliometer) 10.3° ± 2.4° 8.8° ± 3.5° 0.100
*Patients in whom revision surgery was performed were excluded from these results.
The values are given as the mean and the standard deviation.

TABLE V - Patient-Reported Outcome (SRS-22r)*
Baseline (N = 10) Latest Follow-up (N = 8) P Value
Function 2.7 ± 0.3 4.0 ± 0.3 <0.001
Pain 2.4 ± 0.6 4.3 ± 0.6 <0.001
Self-image 2.5 ± 0.5 4.4 ± 0.3 <0.001
Mental health 3.0 ± 0.3 4.4 ± 0.5 <0.001
Satisfaction with management 1.9 ± 1.2 4.5 ± 0.5 <0.001
Total 2.6 ± 0.4 4.3 ± 0.4 <0.001
*Patients who had revision were excluded from these results.
The values are given as the mean and the standard deviation.

At the time of writing, 11 serious adverse events in 10 patients have been recorded (including 2 events in a single patient) (Table VI). The events occurred at a mean of 21 ± 13.1 months (range, 8 to 50 months) after surgery. Six events were related to osteolysis of 1 of the pedicle screws, with 1 case involving breakage of the screw. Three patients had failure of the ApiFix device as result of breakage of a screw (1 patient), failure of the ratchet mechanism (1 patient), or breakage of a screw and the device (1 patient). Two implants were removed at the patient’s request because of continuing pain without any abnormal radiographic findings; 1 of those patients already had had a revision of the implant in the past. No abnormalities had been observed in those patients intraoperatively. Revision procedures were performed to (1) remove the implant and place a new implant at different levels, (2) remove the implant, or (3) remove the implant and convert to posterior spinal fusion. Culture specimens were obtained during 9 revision procedures, and 6 of those specimens were positive for Cutibacterium acnes (previously known as Propionibacterium acnes). Macroscopic metal particles (seen intraoperatively) and microscopic metal particles (seen on histological analysis) were observed in all patients who had revision, especially around the ratchet and ball-and-socket joints. Figure 1 shows details of the surgical strategy to resolve failures, and Figures 2-A and 2-B show radiographic findings in patients with and without an event.

TABLE VI - Summary of Serious Adverse Events*
Case Months After Initial Surgery Description Intervention Duration of Clinical Follow-up After Latest Intervention Outcome at Latest Follow-up, Including Possible Re-Intervention
1
9 • Proximal pedicle screw osteolysis
• Metallosis: yes
• Culture: not performed
Implant revision 11 months Initial recovery. Later occurrence of pain.
14 • Pain eci
• Metallosis: yes
• Culture: negative
Implant removal 12 months Some improvement of pain. Increase in major curve Cobb angle from 34° to 49°. Last 2 years, no follow-up.
2 14 • Proximal pedicle screw osteolysis
• Metallosis: yes
• Culture: C. acnes
Implant revision, oral antibiotics 27 months Implant in situ. Deformity remained unchanged with Risser 5.
3 12 • Persisting pain eci
• Metallosis: yes
• Culture: negative
Implant removal 27 months Increase in major curve Cobb angle from 22° to 34° with Risser 5.
4 8 • Distal screw breakage
• Metallosis: yes
• Culture: not performed
Revision to posterior spinal fusion 5 months Posterior spinal fusion in situ. Decrease in major curve Cobb angle from 36° to 20° with Risser 5. Last 2 years, no follow-up.
5 14 • Distal pedicle screw osteolysis
• Metallosis: yes
• Culture: C. acnes
Implant revision, oral antibiotics 31 months Implant in situ. Decrease in major curve Cobb angle from 49° to 32° with Risser 4. Mild persisting complaints.
6 19 • Proximal pedicle screw osteolysis
• Metallosis: yes
• Culture: C. acnes
Implant removal, oral antibiotics 14 months Unchanged major curve Cobb angle of 40° with Risser 4. New appointment after 2 years will follow.
7 10 • Proximal pedicle screw osteolysis and breakage of screw
• Metallosis: yes
• Culture: C. acnes
Implant removal, no antibiotics 14 months Minimally increased major curve Cobb angle from 39° pre-revision surgery to 43° with Risser 4.
8 22 • Failure of ratchet
• Metallosis: yes
• Culture: negative
Revision to posterior spinal fusion 12 months Posterior spinal fusion in situ. Decrease in major curve Cobb angle from 54° to 12°. Risser 5.
9 37 • Proximal pedicle screw osteolysis
• Metallosis: yes
• Culture: C. acnes
Implant removal, no antibiotics 6 weeks No follow-up radiograph yet.
10 50 • Breakage of distal screw and breakage of rod proximal
• Metallosis: yes
• Culture: C. acnes
Implant removal 8 weeks Minimally increased major curve from 33° with broken implant in situ to 38° after removal. Risser 5.
*Metallosis: metal particles observed macroscopically during surgery and/or microscopically on pathological evaluation.
eci = e causa ignota.

Fig. 1
Fig. 1:
Patient flowchart showing serious adverse events (SAEs) and surgical treatment.

Figs. 2-A and 2-B Radiographic findings for 1 patient who did not have a serious adverse event and 1 patient who did have such an event.

Fig. 2-A
Fig. 2-A:
A 14-year-old girl with a 42° Lenke 1A curve, instrumented from T5-T11, that was corrected to 26° at 6 months postoperatively and 30° at the time of the latest follow-up.
Fig. 2-B
Fig. 2-B:
A 15-year-old girl, with a 51° Lenke 5 curve, instrumented from T11-L3, that was corrected to 32° at 6 months of follow-up. The 12-month follow-up radiographs showed increase of the curve to 37° and osteolysis of the proximal screw that was confirmed with computed tomography (CT). Revision surgery was performed with a new implant from T10-L2.

To identify possible causes of the implant failures, we compared the 10 patients who underwent revision (because of osteolysis, screw/device breakage, or ratchet failure) with the 10 patients without implant failure. The variables that were studied included body weight, age, Risser stage, number of instrumented vertebrae, and several radiographic parameters (major [instrumented] curve magnitude at baseline, curve flexibility at baseline, magnitude of curve correction, and sagittal screw-rod angle [SSRA], defined as the angle between the screw and rod [optimal, 90°] [Fig. 3]). The SSRA is a proxy for the degree of residual mobility in the polyaxial bearings as these ball-and-socket joints allow a total of ±40° of mobility in each plane. We also looked at the sagittal inter-screw angle (SISA, defined as the angle between the pedicle screws in the sagittal plane [optimal, 0°, with the screws parallel to each other] [Fig. 3]). None of these parameters was significantly different between the patients with and without revision (Table VII).

Fig. 3
Fig. 3:
Standing full-spine lateral radiographs of the same patients shown in Figures 2-A and 2-B, demonstrating the sagittal screw-rod angle (SSRA, in blue) and the sagittal inter-screw angle (SISA, in red) for each patient.
TABLE VII - Sagittal Screw-Rod Angle (SSRA) and Sagittal Inter-Screw Angle (SISA) in Patients with and without Revision*
Mean Angle
SSRA
All patients (n = 20)
 Proximal screw 76° ± 5° (64°-83°)
 Distal screw 88° ± 7° (73°-99°)
Smallest angle screw, non-revision patients (n = 11) 77° ± 4° (71°-83°)
Smallest angle screw, revision patients (n = 9) 73° ± 4° (64°-80°)
Failed screw, revision patients (n = 9) 76° ± 7° (64°-89°)
SISA 16° ± 8° (4°-31°)
 All patients (n = 20)
 Non-revision patients (n = 11) 16° ± 7° (4°-30°)
 Revision patients (n = 9) 17° ± 8° (8°-31°)
*A patient with pain eci (e causa ignota) was considered a non-revision case as there was no construct failure.
The values are given as the mean and the standard deviation, with the range in parentheses.

Source of FundingApiFix supported the present study with an institutional grant of €10,000. The payment was made directly to the institution as contribution to the costs for an interim analysis. No other funding was received, and the implants were billed and paid for by the institution in the standard fashion. ApiFix had no influence on the data gathering, data analysis, or writing of this report.

Discussion

Fusionless options could provide further improvement of the surgical treatment of AIS by maintaining spinal motion and potentially reducing surgical morbidity. In the current prospective study, we investigated a peri-apical unilateral posterior distraction implant device (ApiFix) that was recently approved by the U.S. Food and Drug Administration (FDA)25. Because of the unacceptably high complication rate, the study was prematurely terminated. The FDA-approved device studied here (according to the manufacturer, a phase-2 design) has since been redesigned (phase-4 design) to encompass low-profile screws and to include a main-curve flexibility threshold of <30°. With respect to the serious failure mechanisms presented here, the difference between phases 2 and 4 can be considered irrelevant. Because of the preliminary termination of the study (with 20 of the intended 24 patients included) and the exclusion of the cases with serious adverse events (n = 10), we did not reach the intended power to draw firm conclusions concerning efficacy of curve correction. Nevertheless, based on the results of 10 patients, we found a final reduction of the main curve to an average Cobb angle of 31°.

In the first 2 years of the study, 8 serious adverse events (affecting 7 of 20 patients) occurred. The complications occurred at around 1 year postoperatively, and nearly all were related to the device. Osteolysis was the most frequent adverse event, suggesting a relationship with loading of the screws.

We have several hypotheses for this high failure rate. Although the ball-and-socket design of the implant allows a mobile connection of the rod to the pedicle screws, it does not preclude loading on the polyaxial joints and screws. As the aim of the device is to maintain a reduction of the scoliotic curve, considerable stresses will be present permanently as fusion is not intended. We postulate that the osteolysis is a consequence of excessive loading on the single screws at the end vertebrae and at the bone-screw interfaces, which continues as long as the spine continues to move20. After informing the manufacturer about the high rate of osteolysis, strengthening the proximal construct with 2 pedicle screws instead of 1 pedicle screw was suggested.

Furthermore, if the range of motion of the ball-and-socket joints is exceeded, the loading of the system will increase further. Therefore, the manufacturer suggests that the optimal angle between the pedicle screws in the sagittal plane (SISA) is 0° (i.e., parallel) and that the optimal angle between the screw and rod (SSRA) is 90° (i.e., perpendicular). Because of the sagittal profile of the spine and the scoliotic spinal anatomy, these angles cannot always be obtained, as reflected by a mean SSRA of about 76° for the cephalad screw and 88° for the caudad screw in all patients.

Besides the continuous stress, another cause of osteolysis around the pedicle screws could be related to the metallosis that was observed at the time of all revision procedures. This finding is probably a consequence of continuous loading and movement of the ball-joint components that are made of titanium. Hallab et al., in an extensive review, noted that the presence of metal wear debris negatively influences early osseointegration of posterolateral bone graft and also negatively influences an already established fusion mass26.

Five of the 6 patients with osteolysis had a positive culture for C. acnes (no cultures specimens were obtained from the first patient because infection was not suspected). In recent years, C. acnes has been recognized as an important pathogen that is strongly related to revision in spine surgery and shoulder arthroplasty27. In a study of a dynamic neutralization system (Dynesys; Zimmer), Lutz et al. also reported high rates of C. acnes colonization and related this finding to the lack of osseointegration of the pedicle screws due to continuous loads in the absence of spinal fusion28. The causative relation cannot be determined definitively: does C. acnes cause osteolysis, or do the dead space and metallosis provide a fertile environment for colonization28-30? However, since C. acnes infections following regular titanium instrumented spinal fusions for scoliosis are extremely rare at our institution (2 of 500 cases of spinal fusion), we believe that it would be very unlikely that the reason for the failures was a coincidental primary deep-wound infection.

To our surprise, no further curve correction was seen postoperatively, despite a rigorous supervised exercise program. Eleven of 20 patients did show some distraction of the ratchet, but this distraction did not improve the Cobb angle. The lack of postoperative correction goes against the underlying philosophy of the ratchet mechanism. We could not identify the cause of failure of further Cobb angle correction. The rod-ratchet mechanism was mildly distracted manually intraoperatively (in the presence of a representative of the manufacturer) to correct the scoliosis. The amount of distraction was based on surgeon assessment and was not standardized. We do not know whether patients were over-distracted or under-distracted or whether this factor influenced the postoperative situation.

On the basis of our findings, we concluded that the ApiFix device did not allow postoperative curve correction for which its ratchet mechanism is designed and that all but 1 of the 10 failures were classified as an unexpected serious adverse effect that was directly related to the implant design.

It should be recognized that the conventional surgical treatment of relatively mild scoliosis with spinal fusion generally allows very good deformity reduction, including apical derotation, with well-established good long-term results31-33. Therefore, we consider that the unfavorable findings of the present study on the fusionless approach with a posterior distraction device are important and should be carefully weighed against the possible benefits. The registry of at least 200 patients that was a condition for the recent FDA approval for ApiFix will be very valuable and essential to make this comparative assessment in the future.

In conclusion, the present report on the use of the ApiFix device for fusionless scoliosis surgery highlighted an unacceptably high rate of implant-related complications, and no curve correction or distraction of the ratchet was observed postoperatively. The design of the implant results in metal wear and probably induces high screw and screw-bone interface loads. These failure mechanisms could potentially be mitigated with more and stronger anchors (screws) and higher-quality mobile bearings that are less prone to wear. However, the fundamental design problem and lack of postoperative correction remain. All patients in the current cohort will be monitored closely. The local medical ethics committee and all included patients and their guardians have received written information about the complications and the decision to terminate further inclusions.

Data Sharing

A data-sharing statement is provided with the online version of the article (https://links.lww.com/JBJS/G539).

Note: The authors acknowledge Sayf S.A. Faraj for his assistance in including patients in this study.

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