Porous 3D Printed Titanium Cages in Anterior Cervical Discectomy and Fusion are Associated With Less Subsidence,Improved Maintenance of Segmental Lordotic Correction,and Similar Clinical Outcomes as Allograft

Abstract

Study-Design

Retrospective chart review.

Objectives

Investigate radiographic and clinical outcomes of 3D printed titanium cages (3DTC) vs allograft in patients undergoing Anterior cervical discectomy and fusion (ACDF).

Methods

Consecutive series of patients undergoing ACDF with 3DTC were compared to patients using corticocancellous allograft. Cage subsidence, fusion status, sagittal alignment, and patient-reported-outcomes. Radiographic evaluation was performed on the closing intraoperative x-ray and compared to films at 6-weeks, 6-months, and 1-year. Cage subsidence was calculated based on the amount of settling into superior and inferior endplates compared to the intraoperative x-ray. Fusion was assessed based on < 1 mm of flexion/extension motion. Sagittal alignment parameters and patient-reported-outcomes were measured.

Results

Seventy six-patients/(120 levels) in 3DTC group and 77-patients/(115 levels) in allograft group were evaluated. No significant differences were noted in patient demographics, level fused or the number of levels fused between the groups. The most common level fused was C5-6. 3DTC had a significantly lower subsidence rate at all-time points as compared to allograft (P < .001). 3DTC maintained segmental lordosis better than allograft at all-time points including 1-year postop (P < .001). No significant differences were noted in fusion rate for 3DTC vs allograft at 6-months (P > .05). There were no significant differences in patient-reported-outcomes.

Conclusion

3D printed titanium cages had similar patient-reported outcomes and fusion rates as allograft, but less subsidence at all-time points. 3D printed titanium cages better maintained the segmental lordosis at the operative level at all-time points. Although longer term evaluation is needed, based on these results, 3DTC appear to be viable graft options for ACDF that better maintain disc space height and improve segmental lordotic interbody correction.

Keywords

cervical anterior cervical discectomy and fusion subsidence titanium allograft

Introduction

Anterior cervical discectomy and fusion (ACDF) is commonly performed for a variety of cervical conditions such as myelopathy, radiculopathy, and kyphosis correction. Successful spinal fusion depends on multiple factors, including host healing potential and stability of the mechanical environment. In addition, graft type plays a central role, with material type and footprint being important factors.

Postoperative evaluation of these patients focuses on clinical improvement as well as radiographic parameters, such as bony fusion, subsidence, and sagittal alignment. Although these radiographic parameters are not always correlated with clinical outcomes, they are frequently utilized as objective measures of successful surgery.^1-3 Achievement of bony fusion is important as it may lead to quicker improvement of arm and neck pain as well as reduce the risk for settling or kyphosis.^4,5 Anterior cervical discectomy and fusion cage subsidence rates are noted to be as high as 21%.¹ Cage subsidence, when it occurs, can reduce foraminal height and cervical lordosis, which may lead to poor results.^6-8 Furthermore, failure of fusion and pseudoarthrosis may be correlated with worse outcomes as well.^2,9 As such, techniques that promote bony fusion and reduce subsidence are overall clearly desirable in ACDF surgery.

The addition of anterior plate fixation, as an adjunct to ACDF, has been shown to lead to improved fusion rates, better maintenance of sagittal alignment, and decreased graft subsidence.^10-12 Similarly, interbody device selection is another variable that may affect fusion and subsidence with resultant effects on clinical outcomes. Allograft, titanium, and Polyetheretherketone (PEEK) are all commonly used for interbody devices. While studies comparing PEEK vs allograft, as well as PEEK vs titanium, do exist, there remains a paucity of evidence regarding differences in subsidence, fusion rates, and clinical outcomes between titanium and allograft interbody devices.^11,13

The ideal graft for use in ACDF remains to be identified. Recently, porous 3D printed titanium cages (3DTC) have been introduced as an option. Purported advantages over PEEK cages include a highly porous architecture mimicking cancellous bone which may promote osteoblastic ingrowth while maintaining structural strength. They have shown to induce greater osteoblast differentiation of stem cells as compared to PEEK, have a favorable bone-implant contact surface, and superior osteointegration with the surrounding bone.¹⁴ Additionally, the relatively larger footprint compared to standard bone allografts may lead to better load sharing and less subsidence.

The purpose of this study was to investigate radiographic and clinical outcomes of 3DTC vs allograft in patients undergoing ACDF. We hypothesized that titanium interbody devices will have less subsidence, increased fusion rates, improved maintenance of sagittal alignment, and improved clinical outcomes when compared to allograft implants.

Materials/Methods

A retrospective chart review was performed after an approval from the institutional review board via an expedited review (IRB00117066) and exempt from the need of an informed consent from the patient. Patients over the age of 18 years and having undergone an ACDF based on the CPT codes (22548, 22551, 22552, 22554, 22585) between 2014 and 2020 were included. Patients undergoing a corpectomy or combined anterior and posterior procedures were excluded. Based on the search, a consecutive series of all patients undergoing ACDF with 3DTC (Stryker Tritanium C; Kalamazoo, MI) during that timeframe were selected. The control group consisted of an equal number of patients during the same timeframe with consecutive medical record numbers using fresh frozen machined corticocancellous allograft (VG2; Lifenet, Virginia Beach, VA).

All patients underwent a standard Smith-Robinson surgical approach to the cervical spine using the same ACDF technique and instrumentation using the same anterior cervical plating system. Demineralized bone matrix (Grafton DBM putty) supplemented the 3DTC or allograft in all cases.

Radiographic evaluation was performed on the closing intraoperative lateral x-ray, then compared to postoperative films at 6 weeks, 6 months, and 1 year. Cage subsidence was calculated based on the amount of settling into the superior and inferior endplates compared to the intraoperative x-ray (Figure 1). Fusion was assessed based on < 1 mm of flexion/extension motion on x-rays at 6 months and 1 year, in the absence of implant failure and lucent lines.^10,15 Three observers performed the measurements independently, and the measurements were normalized based on the magnification at each time point. Inter-rater reliability was measured. Patient reported outcomes included the 36-item Short Form Health Survey (SF36), Visual Analogue Pain (VAS) Pain, Electronic Quality of Life questionnaire (EQOL), and Neck Disability Index (NDI). Prior literature has demonstrated high inter-observer reliability for sagittal alignment parameters.^16,17 Therefore, 1 observer performed measurements on sagittal alignment parameters including C2-C7 Cobb angle, C2-C7 sagittal vertical axis (SVA), and Cobb angle of levels fused (Figure 2).

Figure 1.

Radiographic images highlighting measuring method for subsidence. Images (A) & (C) represent radiographs taken intraoperatively and images (B) and (D) represent radiographs at 1 year postoperative follow up. (1 = C2 body width and 2 = measurement of disk height).

Figure 2.

(A) Cobb angle of level fused measured from superior end plate of upper vertebral body to inferior endplate of lower vertebral body of level fused. (B) Red line indicates C2-C7 Cobb angle, orange line indicates C2-C7 sagittal vertical axis.

Theory/Calculation

Statistical Analysis

After radiographic review statistical analysis was conducted using IBM SPSS Statistics, Version 25.0 (Armonk, NY: IBM Corp.). Intraclass correlation coefficient and Cronbach’s alpha were utilized to compare agreement and reliability between reviewers for all radiographic measurements. Categorical data was analyzed using Fisher’s exact testing for comparison between cage types. Continuous variables were compared using the student’s t-test for variables that fit the normal distribution and using the Mann-Whitney U test for variables that do not follow the normal distribution. An alpha coefficient of .05 was set to determine statistical significance between groups.

Results

Seventy six patients (120 levels) in the 3DTC group and 77 patients (115 levels) in the allograft group were evaluated (Table 1). No significant differences were noted in patient demographics with respect to age, sex, race, BMI, smoking status, or Charlson Comorbidity Index (CCI) (Table 2). The most common level fused was C5-6 (Table 3). No significant differences were noted with respect to the level fused.

Table 1.

Number of single-level, two-level, and 3-level ACDF in patient cohort.

Number of levels fused	Number of patients		P-value
Number of levels fused	Titanium	Allograft	P-value
1	34	42	.30
2	40	32
3	2	3
Total	76	77

Table 2.

Detailed demographics of the patients included in the study.

	Cage type		P-value
	Titanium	Allograft	P-value
Total patients N	76	77
Total levels N (%)	120 (51.1%)	115 (48.9%)
Age^b	58.1 ± 10.4	57.6 ± 12.3	.62
Sex^a M:F	1:1	5:8	.08
Race^a N (%)			.80
White	94 (78%)	91 (79%)
Black	25 (21%)	22 (19%)
Asian	1 (1%)	2 (2%)
BMI^b	29.6 ± 6.9	30.3 ± 6.6	.20
Current smoker^a N (%)	17 (14%)	12 (10%)	.43
Hypertension^a N (%)	29 (24%)	33 (29%)	.46
Anxiety/depression^a N (%)	16 (13%)	15 (13%)	1.00
Charlson comorbidity index (CCI)^b	1.66 ± 1.3	1.54 ± 1.39	.34

^aAnalysis conducted using student’s t-test.

^bAnalysis conducted using Fisher’s exact test.

Table 3.

Highlighting the distribution of the levels fused in the study.

	Cage type
	Titanium	Allograft	P-value
C 3-4	7	8	.49
C 4-5	18	18
C 5-6	57	51
C 6-7	35	38
C 7-1	3	0

Subsidence

3D printed titanium cages had a significantly lower amount of subsidence at all time points as compared to allograft at 6 weeks (.79 ± .47 mm vs 1.31 ± .63 mm), 6 months (1.28 ± .72 mm vs 2.29 ± .99 mm), and 1 year (1.80 ± .96 mm vs 2.82 ± 1.07 mm) (P < .05) (Figure 3). Allograft was found to subside more than 3DTC by .52 mm at 6 weeks, 1.01 mm at 6 months, and 1.02 mm at 1 year. No significant differences were noted in fusion rates for 3DTC vs allograft at 6 months (58.0% vs 66.3%, P = .43) and 1 year (83.3% vs 88.3%, P = .08) (Figure 4). There were also no significant differences in patient reported outcomes with respect to Euro QOL, NDI, VAS Pain, and SF36 scores (Table 4, Figure 5).

Figure 3.

Subsidence based on the interbody graft type at time points 6 weeks (.79 mm vs 1.31 mm P < .001), 6 months (1.28 mm vs 2.29 mm P < .001), and 1 year (1.80 mm vs 2.82 mm P < .001) postoperative. The subsidence rate for the titanium cage was significantly lower at all times points as compared to allograft.

Figure 4.

Fusion rate based on the interbody graft type at 6 months (58.0% vs 63.3% p .43) and 1 year (83.3% vs 88.3% p .08) postoperative. The fusion rate for the titanium cage was comparable to allograft at both time points.

Table 4.

Table listing the patient reported outcomes following an ACDF.

	Cage type		P-value
	Titanium	Allograft	P-value
Subsidence
6 Weeks	.79 ± .47	1.31 ± .63	<.001
6 Months	1.28 ± .72	2.29 ± .99	<.001
1 YR	1.80 ± .96	2.82 ± 1.07	<.001
Fusion
6 Months	58.0% Fusion	63.3% Fusion	.43
1 YR	83.3% Fusion	88.3% Fusion	.08
Euro QOL
Preop	.61 ± .23	.69 ± .50	.82
Δ6 Weeks	.18 ± .32	.69 ± .51	.71
Δ6 Months	−.18 ± .36	.03 ± .64	.17
Δ1 YR	−.02 ± .22	−.16 ± .92	.22
Neck Disability Index (NDI)
Preop	45.9 ± 16.2	42.8 ± 18.9	.25
Δ6 Weeks	.65 ± 19.5	6.6 ± 18.5	.06
Δ6 Months	−22.1 ± 27.9	−23.9 ± 27.3	.60
Δ1 YR	43.3 ± 32.5	44.4 ± 38.6	.91
VAS
Preop	6.35 ± 1.73	6.42 ± 1.88	.40
6 Weeks	4.49 ± 2.55	3.77 ± 2.32	.06
Δ6 Weeks	2.38 ± 2.72	2.8 ± 2.45	.30
Δ6 Months	2.35 ± 2.88	2.86 ± 2.61	.32
1 YR	4.71 ± 2.03	3.85 ± 2.52	.07
Δ1 YR	2.56 ± 2.71	2.85 ± 3.1	.69
SF36
General health
Preop	44.3 ± 10.0	46.7 ± 9.3	.15
Δ6 Weeks	.9 ± 9.6	−1.2 ± 11.9	.94
Δ6 Months	−1.9 ± 13.1	−5.0 ± 13.9	.24
Δ1 YR	.4 ± 10.6	1.7 ± 12.2	.69
Physical component score
Preop	32.8 ± 9.2	36.7 ± 10.1	.09
Δ6 Weeks	3.0 ± 11.4	1.9 ± 11.0	.16
Δ6 Months	3.2 ± 10.7	6.4 ± 7.8	.24
Δ1 YR	1.6 ± 10.4	6.8 ± 8.8	.05
Mental component score
Preop	45.5 ± 11.7	45.5 ± 12.3	.94
Δ6 Weeks	7.5 ± 15.6	6.4 ± 15.3	.91
Δ6 Months	6.5 ± 15.7	6.9 ± 15.4	.90
Δ1 YR	6.5 ± 10.2	2.5 ± 13.2	.15

There were no significant differences noted between the 2 interbody graft types at all time points with respect to patient reported outcomes.

ICC IntraOp C2: .864 (95% CI: .831-.887) Cronbach Alpha: .949.

ICC IntraOp EP Height: .571 (95%CI: .501-.637) Cronback Alpha: .800.

Figure 5.

Patient reported outcome scores (A) NDI, (B) Pain, (C) Euro Quality of Life, and (D) SF36 at preop, 6 weeks, 6 months, and 1 year. No significant difference at any time points P > .05.

Intraclass Correlation Coefficient was .864 (95% CI: .831-.887), and Cronbach Alpha was .949, suggesting a high degree of measurement reliability between the 3 observers and excellent internal consistency for C2 measurements (Table 4). There was significantly less number of levels in the 3DTC group with greater than 2 mm of subsidence as compared to the allograft group at all time points (Table 5).

Table 5.

Table listing the percentage of patients with greater than 2 mm of subsidence at the respective time points.

	Cage type
	Titanium	Allograft
6 Weeks	9.24%	44.44%
6 Months	18.42%	72.81%
1 Year	31.82%	81.93%

Overall C2-7 Lordosis

3D printed titanium cages was associated with significantly greater overall C2-C7 lordosis at 1 year postoperatively when compared to allograft (11.65 ± 9.20 vs 8.8 ± 9.66, P < .04) (Table 6). The change in overall C2-7 lordosis vs preoperative demonstrated a trend toward greater improvement with 3DTC, but this was not significant (Table 6). There were no significant differences in C2-C7 SVA when comparing 3DTC and allograft (Table 6).

Table 6.

Comparison of sagittal alignment parameters between titanium and allograft interbody devices from preoperative imaging through 1-year postoperative imaging.

	Cage type		P-value
	Titanium	Allograft	P-value
C2-C7 Cobb
Preop^a	7.94 ± 10.43	6.28 ± 10.87	.26
POD0^a	13.02 ± 10.00	13.66 ± 10.26	.63
ΔPOD0	5.12 ± 6.67	7.48 ± 8.08	.09
6 Weeks^a	12.32 ± 9.32	11.09 ± 9.41	.31
Δ6 Weeks	4.17 ± 6.27	5.01 ± 7.48	.38
6 Months^a	12.30 ± 9.23	10.09 ± 10.16	.09
Δ6 Months	4.46 ± 6.52	4.34 ± 7.51	.91
1 Year^a	11.65 ± 9.20	8.80 ± 9.88	.04
Δ1 Year	4.75 ± 6.59	2.95 ± 8.20	.13
C2-C7 SVA
Preop^a	27.74 ± 11.77	29.83 ± 12.91	.22
POD0^b	25.38 ± 13.97	27.11 ± 11.97	.21
6 Weeks^b	28.00 ± 11.97	30.73 ± 13.47	.09
6 Months^b	28.97 ± 12.29	31.27 ± 12.55	.24
1 Year	30.30 ± 12.51	33.52 ± 13.18	.09
Level fused Cobb
Preop^a	−1.01 ± 5.94	−.51 ± 6.57	.55
POD0^a	5.03 ± 5.08	5.11 ± 5.89	.92
ΔPOD0	5.91 ± 4.59	5.65 ± 5.07	.70
6 Weeks^a	4.69 ± 4.88	3.26 ± 5.51	.04
Δ6 Weeks	5.48 ± 4.53	3.78 ± 4.94	.01
6 Months^a	4.77 ± 5.15	2.74 ± 5.55	<.01
Δ6 Months	5.48 ± 4.85	2.95 ± 5.15	<.001
1 Year^a	4.78 ± 4.96	2.33 ± 5.68	<.01
Δ1 Year	5.25 ± 5.14	2.02 ± 4.50	<.001

Δ, the Cobb angle measured at postoperative time point minus the Cobb angle measured at preoperative visit.

^aCalculated using student’s t-test.

^bCalculated using mann-whitney U test.

Preop, Pre-operative; POD0, Post-operative Day 0; SVA, Sagittal Vertical Axis.

ICC Preop C2C7Cobb .985 (95% CI:0.957-.995) Cronbach’s Alpha .992.

ICC Preop C2C7SVA .992 95% CI:0.978-.997) Cronbach’s Alpha .996.

ICC Preop CobbLevelFused .978 (95%CI: .940-.992) Cronbach’s Alpha .983.

ICC 6 weeks C2C7Cobb .991 (95% CI: .978-.997) Cronbach’s Alpha .996.

ICC 6 weeks C2C7SVA .984 95% CI:0.958-.994) Cronbach’s Alpha .992.

ICC 6 weeks CobbLevelFused .915 (95%CI:0.793-.966) Cronbach’s Alpha .956.

Segmental Alignment at the Operative Level

Both groups started out with slight segmental kyphosis at the operative level (Table 6; -1.01^o for 3DTC, -.51^o for allograft, P = .22) that then improved into lordosis on POD 0 (5.03^o for 3DTC, 5.11^o for allograft, P = .92). The amount of change in segmental lordosis from preoperative to postoperative at the operative level was similar for both groups on POD 0 (Table 6; 5.91^o increased lordosis for 3DTC vs 5.65^o for allograft, P = .70). However, the initial increase was relatively maintained for 3DTC at all time points postoperatively, whereas allograft lost much of the initial correction over time, such that by 1 year postop, allograft had lost 3.63^o (64%) of the initial correction whereas 3DTC had only lost .66^o (11%). Therefore, the increase in segmental lordosis compared to preoperative was significantly greater for 3DTC than for allograft at 6 weeks (5.48 ^o vs 3.78 ^o, P = .01), 6 months (5.48 ^o vs 2.95 ^o, P < .001), and 1 year (5.25 ^o vs 2.02^o, P < .001).

Discussion

In the present study, we compared clinical and radiographic outcomes after ACDF using 3D-printed titanium cage (3DTC) vs corticocancellous allograft and found that 3DTC had a significantly lower amount of subsidence at every time point up to 1 year postoperatively. The differences in subsidence averaged approximately .5 mm at 6 weeks, then increased to approximately 1 mm at 6 months and 1 year (Figure 3). In addition, whereas both groups initially had similar increases in segmental lordosis at the operative level, 3DTC lost only 11% of the initial correction by 1 year postop (5.25^o more segmental lordosis than preoperative), while allograft lost approximately 64% of the initial correction by 1 year (2.02^o more segmental lordosis than preoperative). With regards to patient-reported outcomes, we found no difference in Euro QOL, NDI, VAS Pain, and SF36 outcomes scores between 3DTC and allograft devices at 6 weeks, 6 months, and 12 months (Table 4, Figure 5). When comparing fusion rates, we found no difference between the 2 implants (Figure 4).

Generally desired goals of ACDF surgery include neural decompression, disc height restoration, and maintenance or improvement in segmental lordosis. A variety of intervertebral grafts options are available, but the ideal graft remains to be identified. The literature has shown an incidence of subsidence between 0% and 83% for all cervical intervertebral grafts.^1,18 Although prior studies have demonstrated only a weak correlation between amount of subsidence and differences in clinical outcomes,^2,18 subsidence is generally not a desirable outcome, as it causes disc height loss which may lead to a recurrence of neuroforaminal stenosis or worsening of sagittal alignment.

Prior studies have reported the subsidence rate of allografts in ACDF to be between 5% and 43%.^19,20 Less subsidence for 3DTC devices may occur for several reasons. One potential reason for greater subsidence of allograft vs 3DTC in our study may be due to their larger physical dimensions when compared to most commercially available allografts. The usual allograft footprint measures 14 × 11 mm, while 3DTC are available in larger sizes, up to 17 × 14 mm. An implant with a larger footprint distributes contact forces at the graft-implant interface over a larger area, which, in turn, generates less force per unit area between the vertebral endplate and graft, potentially leading to less subsidence of 3DTC vs smaller footprint allografts. A larger footprint likely also explains why 3DTC better maintained the initial segmental lordotic correction achieved at the time of surgery.

An additional purported advantage of newer titanium cages is a roughened surface, which provides a favorable bone-implant interface, boosts production and differentiation of osteoblasts, downregulates excessive production and activity of osteoclasts, and increases the production of both osteogenic and angiogenic markers.²¹ Surface topography is reported to play a key role in the formation of new bone as a roughened interface mimics the hierarchical structure of bone on the implant surface.²¹ Purely cortical allografts do not possess this type of surface porosity. The allograft used in the present study consisted of a fresh frozen cortico-cancellous “sandwich,” and so it did possess a partially porous structure. However, prior literature has demonstrated that freeze-dried dense cancellous allografts, despite their porosity and possessing similar compressive strength to that of cervical endplates, have high rates of graft resorption of up to 53%.²² The authors proposed that while the porosity of dense cancellous allografts may theoretically improve bony ingrowth, it may also lead to a more rapid and complete resorption by osteoclasts that outpaces bony ingrowth.²² Such resorption of the graft itself is unlikely to occur with implants like 3DTC that are not made of bone.

To our knowledge, subsidence and fusion rates with titanium cages have not been previously compared to allograft options. When compared to PEEK cages in an ovine model, a lumbar version of the 3DTC used in this study resulted in increased bony ingrowth and stiffer constructs overall.²³ However, a systematic review by Noordhoek et al failed to conclusively demonstrate differences in subsidence between PEEK and titanium cages, though both seemed to outperform PMMA cages, which the authors felt should be avoided in ACDF surgery when aiming to avoid subsidence.¹

Despite similar clinical outcomes and fusion rates between our 2 groups, subsidence may still have long term effects in patients undergoing ACDF surgery. Over time, subsidence could lead to worsening of sagittal alignment.²⁴ As patients age, subsidence at the operative level combined with age-related loss of lordosis at adjacent levels, can predispose to global cervical malalignment. Prior literature has shown significant subsidence to occur at both 6 and 12 months with a variety of interbody devices, although the clinical impact has yet to be clearly demonstrated.^1,7,11,24,25 This coincides with the findings of the present study.

In order to determine the effect of subsidence on overall cervical alignment, we also measured C2-C7 Cobb angle, C2-C7 SVA, and segmental lordosis at the fusion level for each patient included in the study. 3D printed titanium cages was associated with significantly greater overall C2-C7 lordosis at 1 year postoperatively when compared to allograft (11.65° vs 8.80°, P = .04; Table 6).

Furthermore, when comparing preoperative to postoperative segmental lordosis at the operative level, 3DTC better maintained the correction achieved vs allograft at all time points (Table 6). Both 3DTC and allograft had similar increases in segmental lordosis on post-op day 1 (5.91^o for 3DTC vs 5.65^o for allograft; Table 6). However, over time, 3DTC maintained that initial correction significantly better than did allograft, such that by 1 year, the increase in segmental lordosis compared to preoperative at the operative level was 5.25° for 3DTC vs 2.02° for allograft (P < .001), leading to a difference of 3.23° between the implants. Given that subaxial segmental lordosis is estimated to range from approximately 1 to 5° per level in asymptomatic adults depending on the cervical level measured,²⁶ a difference in 3.23° is relatively large. Although this difference did not correlate with superiority in clinical outcomes at time points up to 1 year, additional lordosis is generally a radiographically desired outcome, especially given that the patients in this study began with a segmental kyphosis ranging .51 to 1.01° at the operative level. Multi-year follow up is necessary to determine whether the improved segmental alignment obtained by 3DTC is durable and affects clinical outcomes over time.²⁵

Anakwenze et al compared changes in sagittal alignment parameters between total disc replacements (TDR) and ACDF.²⁷ Through review of 89 TDR and 91 ACDF patients, they found restoration and maintenance of lordosis in both TDR and ACDF, but ACDF resulted in better lordosis correction at the operative level.²⁷ Multiple prior studies have suggested that kyphosis after ACDF may be associated with adjacent segment disease, poor functional recovery, or axial neck pain.^28,29 Our study, however, is the first to compare sagittal alignment parameters in 3DTC vs allograft interbody devices.

The strengths of this study include a large number of patients in both groups, robust follow up, and consistency of the data available for each patient. There were 3 independent reviewers who assessed each patient individually. There was excellent internal consistency as indicated by the high intraclass correlation coefficient and Cronbach Alpha. The limitations of the study include its retrospective nature with associated selection bias. However, there were no differences between the groups with respect to preoperative characteristics, number of levels fused, or the distribution of levels fused. Another limitation is the length of follow-up; longer term follow up would allow for evaluation of the effects of the loss of segmental lordosis in the allograft group. This would allow for determination of whether a costlier implant as the 3DTC is indeed a better alternative given its similar outcomes and fusion rates. Another limitation of the study is measurement error while assessing for subsidence and pseudoarthrosis on radiographs. One concern with metal interbody implants is the potential for image distortion with MRI. However, we have been able to obtain MRI images postoperatively without substantial artifactual distortion (Figure 6).

Figure 6.

Postoperative T2 MRI (A) sagittal and (B) axial in a patient who had undergone ACDF with 3DTC. Note that the neural elements are well visualized without significant artifact.

In conclusion, 3DTC had similar patient-reported outcomes and fusion rates as allograft, but it demonstrated less subsidence and better maintained correction of segmental lordosis at all time points. Although longer term evaluation is needed, 3DTC appear to be a viable graft option for ACDF that better maintains disc space height while simultaneously better improving segmental lordosis.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iD

Hardeep Singh

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