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Abstract

Study Design

Retrospective cohort study.

Objectives

Imaging changes in the vertebral body after posterior lumbar interbody fusion (PLIF) are determined to be trabecular bone remodeling (TBR). This study aimed to investigate the influence of cage materials on TBR and segment stabilization in PLIF by studying image changes.

Methods

This was a retrospective study reviewing 101 cases who underwent one-level PLIF with three-dimensional porous titanium (3DTi) cages (53 patients) or polyether-ether-ketone (PEEK) cages (48 patients). Computed tomography images obtained 3 months, 1 year, and 2 years postoperatively were examined for TBR, vertebral endplate cyst formation as an instability sign, cage subsidence, and clear zone around pedicle screw (CZPS).

Results

No significant differences in the TBR-positivity rates were observed between the two cages at 3 months, 1 year, and 2 years postoperatively. However, all 3DTi cage segments that were TBR-positive at 3 months postoperatively showed no CZPS and fewer final instability segments than the TBR-negative segments (0% vs 9%). In contrast, although the PEEK cage segments that were TBR-positive at 3 months postoperatively were not associated with future segmental stabilization, those that were TBR-positive at 1 year postoperatively had fewer final instability segments than the TBR-negative segments (0% vs 33%).

Conclusions

The 3DTi cage segments with TBR 3 months postoperatively showed significant final segmental stabilization, whereas TBR at 1 year rather than 3 months postoperatively was useful in determining final segmental stabilization for the PEEK cage segments. The timing of TBR, a new osseointegration assessment, were associated with the cage material.

Keywords

posterior lumbar interbody fusion osseointegration trabecular bone remodeling vertebral endplate cyst three-dimensional porous titanium polyether-ether-ketone

Introduction

Titanium and polyether-ether-ketone (PEEK) are the most popular materials for interbody cages used in spinal fusion surgeries. Titanium is a popular implant material because of its excellent corrosion resistance and high biocompatibility.¹ However, because solid titanium has a much higher elastic modulus than bone, stress shielding and subsidence problems have been identified for intervertebral cages.¹ Recently, additive manufacturing techniques, such as three-dimensional (3D) printing,² have attracted attention because of their ability to improve the biomechanical properties of metal cages. Therefore, 3D porous titanium (3DTi) cages are currently the mainstream titanium cages used in clinical applications. In contrast, PEEK is a hydrophobic polymer with excellent properties, such as in vivo stability, fatigue resistance, and low elastic modulus, similar to bone.³ Therefore, it is widely used as an interbody cage material. However, PEEK is not a perfect material because it does not make direct contact with bone and has properties that can induce inflammatory tissue reactions.⁴ Nevertheless, PEEK cages have been continuously used for many years, providing satisfactory clinical results. Thus, despite their respective advantages and disadvantages, titanium and PEEK are currently popular primary materials for spinal interbody cages.

However, the clinical outcomes of posterior lumbar interbody fusion (PLIF) using 3DTi and PEEK interbody cages remain controversial. Although animal studies have shown that 3DTi achieves better osseointegration than PEEK,² several studies clinically comparing 3DTi and PEEK cages have found that segmental fusion and cage subsidence are comparable.^5,6 Segmental fusion and cage subsidence are influenced by multiple factors and are consequences observed after the PLIF segment is complete. Therefore, it is difficult to explain the influence of cage materials on PLIF segment stabilization using these parameters because both 3DTi and PEEK cages can usually achieve favorable clinical outcomes.

Intervertebral osseointegration after lumbar fusion surgery produces changes on computed tomography (CT) images that are thought to be trabecular bone remodeling (TBR)⁷ within the vertebral body away from the cage (Figure 1). Thus, attempts have been made to evaluate PLIF segment stabilization using TBR.⁸ TBR is expected to result from stresses on the vertebral endplates being concentrated in the interbody cages in the PLIF segment, which is partially consistent with the results of finite element models.^9,10 Theoretically, TBR is expected to be observed earlier in 3DTi cage segments than in PEEK cage segments. Alternatively, the implications of TBR may differ depending on the cage material. However, the difference in TBR has not yet been investigated between these cage materials. Thus, observing and understanding the changes in segments would provide a better understanding of the impact of cage materials in lumbar interbody fusion procedures. Therefore, this study aimed to investigate the imaging findings of patients who underwent one-level PLIF using 3DTi or PEEK cages up to 2 years postoperatively to understand the impact of cage materials on TBR and segment stabilization.

Figure 1.

Trabecular bone remodeling (TBR). TBR (white arrow) 1 year postoperatively in a PLIF segment using 3DTi cages. Coronal (A) and sagittal images (B). 3DTi, three-dimensional porous titanium.

Methods

Patient Population

This study was approved by the Institutional Review Board of the authors’ affiliated institutions. Additional written informed consent was waived because all interventions, including surgical procedures and pre- and postoperative imaging studies, were routinely performed. This was a retrospective study including patients who underwent one-level PLIF at a single institution between July 2016 and June 2018. A total of 151 consecutive patients were selected as the initial patient population. Of the 151 patients, 16 were excluded, including those whose surgery was not the initial surgery, those with severe scoliosis (Cobb angle >30°), those with severe vertebral endplate sclerosis that inhibited obtaining exact imaging findings, and those on hemodialysis. Additionally, 35 patients were excluded because of not performing postoperative CT evaluation. Finally, 101 patients were included in the analysis (Figure 2). The patients were divided into two groups: the 3DTi group (53 patients), which included patients who underwent PLIF using 3DTi cages (Tritanium PL, Stryker, Kalamazoo, MI, USA) from July 2017 to June 2018, and the PEEK group (48 patients), which included patients who underwent PLIF using PEEK cages (OIC PEEK, Stryker) from July 2016 to June 2017.

Figure 2.

Patient selection flowchart. PLIF, posterior lumbar interbody fusion; CT, computed tomography.

Surgical Procedure

PLIF was performed in a standard fashion. A bilateral pedicle screw (PS)-rod system and two interbody cages per segment were used in all patients. Bilateral or unilateral facet joints were resected as needed. After insertion of the PSs, temporary rod fixation was performed, and the disc was resected, taking care not to damage the vertebral endplates. The local bone was morselized and used as a bone graft before the insertion of the two cages. The two cages used were always of the same type, and the insides of the cages were filled with the same bone graft used in the intervertebral cavity. After cage insertion, compression manipulation was performed between PSs for final fixation. All patients wore a rigid brace for 3 months postoperatively.

Variables

Data, including patients’ age, sex, body mass index, comorbidities, and teriparatide administration, were investigated. CT values (Hounsfield units) were measured on CT axial images of the center of each vertebral body on the cephalic and caudal sides of the PLIF segment,¹¹ and the average value was recorded. Clinical outcomes were evaluated using the Japanese Orthopaedic Association (JOA) score.¹² The JOA scoring system (0-29 points) assesses three subjective symptoms (0-9 points), three clinical symptoms, such as neurological deficit (0-6 points), seven activities of daily living (0-14 points), and bladder function (−6 to 0 points).

Radiological evaluations were performed preoperatively, postoperatively, and 3 months, 1 year, and 2 years postoperatively. The lumbar lordosis angle and local lordosis angle, measured from the rostral endplate of the upper vertebra to the caudal endplate of the lower vertebra, were measured on standing lateral images, with positive values indicating lordosis. The difference in the local angle between postoperative and 2 years postoperatively was defined as correction loss. The local range of motion (ROM) was calculated from standing lateral forward- and backward-bending radiographs.

CT images were obtained using standard methods, and multiplanar reconstruction (MPR) images were produced. TBR was identified on coronal or sagittal CT-MPR images by comparison with immediate postoperative images. According to a previous report,⁷ TBR was defined as relatively thick, tilted trabeculae inside the vertebra that were not depicted on normal (immediate postoperative) images. TBR were present from the cage-vertebral endplate contact area and extended into the vertebral body. TBR was evaluated regardless of the size of the spread. Segmental bridging was defined on CT-MPR images as the presence of an intervertebral bone graft in contact and continuity with both cephalad and caudal vertebral endplates or vertebral body-to-vertebral body bridging by the new bone.

The formation of vertebral endplate cysts (VECs)^13,14 appearing in unstable interbodies was used as an imaging reference that contrasted with osseointegration. The appearance of VECs that were not present on previous imaging or an enlargement of VECs was defined as VEC-positive. Additionally, because the vertebral endplate in contact with the cages may show degeneration-like sclerosis, and small depression (local cyst)¹⁵ may be observed, this depression was also defined as a VEC in this study. Cage subsidence was defined as cage migration of more than 2 mm to the vertebral endplate compared with previous imaging studies. The absence of subsidence progression after previous imaging was defined as negative. Of note, no cage dropout from the intervertebral space was observed in this study. Clear zone around PS (CZPS) was defined as a translucent zone of 2 mm or greater width around the PS. CZPS was expressed as the number of PSs that met the definition (0-4). Segmental instability was defined as any of the following findings: CZPS of 4, intervertebral vacuum sign, or local ROM >5°.¹⁶ Of note, no pseudarthrosis that required revision surgery was observed.

Statistical Analysis

Data were presented as mean ± standard deviation for continuous variables or as numbers and percentages for categorical data. Statistical analyses were performed using R version 4.3.1 (The R Foundation; https://www.R-project.org) with Wilcoxon rank-sum and Fisher exact tests. A P value <.05 was considered statistically significant.

Results

Table 1 shows the patients’ characteristics. The number of male patients was significantly higher in the 3DTi group than in the PEEK group (74% vs 50%, P = .015). No significant differences were observed in other patients’ characteristics. L4-5 was the most common PLIF segment in both groups. No significant differences in vertebral CT values in the PLIF segment were observed between the two groups. Furthermore, no significant differences in pre- and postoperative radiographic parameters or clinical outcomes assessed by the JOA scoring system were observed between the two groups.

Table 1.

Patient Characteristics.

	3DTi N = 53	PEEK N = 48	P-Value
Age, year	62.1 ± 14.5	64.5 ± 11.5	.50
Sex, male	39 (74%)	24 (50%)	.015
BMI (kg/cm²)	24.9 ± 3.6	25.0 ± 5.2	.57
Diabetes	8 (15%)	8 (17%)	.83
Steroid	1 (2%)	3 (6%)	.34
Smoking	11 (21%)	7 (15%)	.42
Teriparatide	2 (4%)	1 (2%)	>.99
PLIF segment			.18
L1-2	0 (0%)	1 (2%)
L2-3	0 (0%)	4 (8%)
L3-4	4 (8%)	4 (8%)
L4-5	37 (70%)	31 (65%)
L5-S	12 (23%)	8 (17%)
CT value (HU)	142.5 ± 43.8	132.2 ± 42.6	.25
LL (°)
Preop	40.0 ± 12.9	39.5 ± 13.4	.71
Postop	39.6 ± 11.6	42.4 ± 11.9	.62
Final	41.7 ± 13.6	44.7 ± 13.0	.72
Local angle (°)
Preop	14.6 ± 6.8	12.5 ± 7.2	.20
Postop	19.5 ± 6.2	15.9 ± 7.3	.085
2 years postop	17.2 ± 5.9	14.5 ± 6.8	.10
Correction loss	2.3 ± 3.1	1.4 ± 2.9	.27
JOA score
Preop	14.9 ± 4.8	15.0 ± 4.5	.92
2 years postop	25.8 ± 2.4	26.3 ± 1.8	.75

Bold indicates statistically significant (P-value <.05).

3DTi, three-dimensional porous titanium; PEEK, poly-ether-ether-ketone; BMI, body mass index; CT, computed tomography; HU, Hounsfield unit; PLIF, posterior lumbar interbody fusion; LL, lumbar lordosis; JOA, The Japanese Orthopaedic Association.

At 3 months, 1 year, and 2 years postoperatively, no significant differences in TBR-positivity rates were observed between the two groups (Table 2). In both groups, TBR-positivity rates at 3 months were around 40%, and at 1 and 2 years were around 70% (Figures 3 and 4). The 3DTi group showed a significantly higher VEC-positivity rate at 1 year than the PEEK group (49% vs 23%, P = .023; Figure 3C). Conversely, the PEEK group showed a significantly higher incidence of cage subsidence at 1 year than the 3DTi group (42% vs 21%, P = .006; Figure 4C). All imaging findings did not differ significantly between the two groups 2 years postoperatively, with 51% of the 3DTi group and 52% of the PEEK group showing interbody bridging (P = .91) and 8% of the 3DTi group and 10% of the PEEK group showing segmental instability (P = .73).

Table 2.

Comparison of Imaging Findings by Cage.

	3DTi	PEEK	P-Value
	N = 53	N = 48	P-Value
3 months postop
TBR	19 (36%)	21 (44%)	.42
VEC	23 (43%)	20 (42%)	.86
Subsidence	12 (23%)	11 (23%)	.97
CZPS			.24
0	50 (94%)	48 (100%)
2	3 (6%)	0 (0%)
1 year postop
TBR	41 (77%)	33 (69%)	.33
Bridging	11 (21%)	17 (35%)	.10
VEC	26 (49%)	11 (23%)	.006
Subsidence	11 (21%)	20 (42%)	.023
CZPS			>.99
0	45 (85%)	42 (88%)
1	2 (4%)	2 (4%)
2	5 (9%)	4 (8%)
4	1 (2%)	0 (0%)
Vacuum	3 (6%)	4 (8%)	.71
2 years postop
TBR	39 (74%)	35 (73%)	.94
Bridging	27 (51%)	25 (52%)	.91
VEC	8 (15%)	4 (8%)	.29
Subsidence	4 (8%)	3 (6%)	>.99
CZPS			.93
0	46 (87%)	41 (85%)
1	1 (2%)	1 (2%)
2	5 (9%)	5 (10%)
4	0 (0%)	1 (2%)
Vacuum	2 (4%)	3 (6%)	.67
Instability^a	4 (8%)	5 (10%)	.73

Bold indicates statistically significant (P-value <.05).

3DTi, three-dimensional porous titanium; PEEK, poly-ether-ether-ketone; TBR, trabecular bone remodeling; VEC, vertebral endplate cyst; CZPS, clear zone around pedicle screw.

^aInstability is a segment with CZPS of 4 or vacuum-positive or local ROM >5°.

Figure 3.

3DTi group. (A) Postoperative coronal image. (B) 3 months postoperatively, TBR was observed (white arrow). (C) 1 year postoperatively, TBR (white arrow) and VEC (black arrow) were observed in both cages. (D) 2 years postoperatively, TBR (white arrow) continued, VEC disappeared, and cage subsidence (black arrow) was observed. 3DTi, three-dimensional porous titanium; TBR, trabecular bone remodeling; VEC, vertebral endplate cyst.

Figure 4.

PEEK group. (A) Postoperative coronal image. (B) 3 months postoperatively, TBR was observed (white arrow). (C) 1 year postoperatively, TBR (white arrow) and cage subsidence (black arrow) were observed in both cages. (D) 2 years postoperatively, interbody bridging (white arrowheads) was observed. PEEK, polyether-ether-ketone; TBR, trabecular bone remodeling.

Imaging findings were compared according to TBR positivity at 3 months by cage (Table 3). The comparison showed that, in the 3DTi group, all TBR-positive segments at 3 months had CZPS of 0 and significantly more segments with CZPS of 0 than the TBR-negative segments (100% vs 76% at 1 year postoperatively, P = .040; 100% vs 79% at 2 years postoperatively, P = .041). As a result, fewer segments showed final instability in the 3-month TBR-positive segments (0% vs 9%, P = .54). In the PEEK group, the 3-month TBR-positive segments had a significantly higher 1-year TBR-positivity rate. However, no significant differences in other findings were observed. Nevertheless, fewer segments also showed final instability in the 3-month TBR-positive segments (5% vs 15%, P = .37). In summary, segments that were TBR-positive at 3 months had significantly more findings suggestive of subsequent segmental stability in the 3DTi cages, but not in the PEEK cages.

Table 3.

Comparison of Imaging Findings by 3-Month TBR According to Cage.

	3DTi			PEEK
	3mo TBR-Positive N = 19	3mo TBR-Negative N = 34	P-Value	3mo TBR-Positive N = 21	3mo TBR-Negative N = 27	P-Value
3 months postop
VEC	6 (32%)	17 (50%)	.19	5 (24%)	15 (56%)	.027
Subsidence	3 (16%)	9 (26%)	.50	4 (19%)	7 (26%)	.73
CZPS 0	19 (100%)	31 (91%)	.54	21 (100%)	27 (100%)	>.99
1 year postop
TBR	18 (95%)	23 (68%)	.038	18 (86%)	15 (56%)	.025
Bridging	4 (21%)	7 (21%)	>.99	6 (29%)	11 (41%)	.38
VEC	9 (47%)	17 (50%)	.85	4 (19%)	7 (26%)	.73
Subsidence	1 (5%)	10 (29%)	.074	9 (43%)	11 (41%)	.88
CZPS 0	19 (100%)	26 (76%)	.040	18 (86%)	24 (89%)	>.99
Vacuum	1 (5%)	2 (6%)	>.99	1 (5%)	3 (11%)	.62
2 years postop
TBR	17 (89%)	22 (65%)	.060	18 (86%)	17 (63%)	.078
Bridging	12 (63%)	15 (44%)	.18	10 (48%)	15 (56%)	.59
VEC	2 (11%)	6 (18%)	.70	2 (10%)	2 (7%)	>.99
Subsidence	0 (0%)	4 (12%)	.28	1 (5%)	2 (7%)	>.99
CZPS 0	19 (100%)	27 (79%)	.041	18 (86%)	23 (85%)	>.99
Vacuum	0 (0%)	2 (6%)	.53	1 (5%)	2 (7%)	>.99
Instabilitya	0 (0%)	3 (9%)	.54	1 (5%)	4 (15%)	.37

Bold indicates statistically significant (P-value <.05).

3DTi, three-dimensional porous titanium; PEEK, poly-ether-ether-ketone; TBR, trabecular bone remodeling; VEC, vertebral endplate cyst; CZPS, clear zone around pedicle screw.

^aInstability is a segment with CZPS of 4 or vacuum-positive or local ROM >5°.

Comparison of imaging findings according to TBR positivity at 1 year by cage (Table 4) showed that, in the 3DTi group, the TBR-positive segments at 1 year had significantly more segments with CZPS of 0, fewer vacuum signs, and more bridging at 2 years than the TBR-negative segments. As a result, fewer segments showed final instability in the 1-year TBR-positive segments (5% vs 17%, P = .22). In the PEEK group, the 1-year TBR-positive segments had significantly fewer VECs at 1 year, more segments with CZPS of 0 at 2 years, fewer vacuum signs, and fewer segments showing final instability than the TBR-negative segments (0% vs 33%, P = .002). In summary, segments that were TBR-positive at 1 year had significantly more findings suggestive of segmental stability at 1 and 2 years in both cages.

Table 4.

Comparison of Imaging Findings by 1-Year TBR According to Cage.

	3DTi			PEEK
	1yr TBR-Positive N = 41	1yr TBR-Negative N = 12	P-Value	1yr TBR-Positive N = 33	1yr TBR-Negative N = 15	P-Value
1 year postop
Bridging	9 (22%)	2 (17%)	>.99	14 (42%)	3 (20%)	.13
VEC	19 (46%)	7 (58%)	.46	3 (9%)	8 (53%)	.002
Subsidence	6 (15%)	5 (42%)	.10	11 (33%)	9 (60%)	.082
CZPS 0	38 (93%)	7 (58%)	.010	31 (94%)	11 (73%)	.067
Vacuum	0 (0%)	3 (25%)	.009	0 (0%)	4 (27%)	.007
2 years postop
TBR	38 (93%)	1 (8%)	<.001	30 (91%)	5 (33%)	<.001
Bridging	25 (61%)	2 (17%)	.007	19 (58%)	6 (40%)	.26
VEC	5 (12%)	3 (25%)	.36	1 (3%)	3 (20%)	.084
Subsidence	2 (5%)	2 (17%)	.22	1 (3%)	2 (13%)	.23
CZPS 0	38 (93%)	8 (67%)	.039	31 (94%)	10 (67%)	.024
Vacuum	1 (2%)	1 (8%)	.40	0 (0%)	3 (20%)	.026
Instability^a	1 (2%)	2 (17%)	.12	0 (0%)	5 (33%)	.002

Bold indicates statistically significant (P-value <.05).

3DTi, three-dimensional porous titanium; PEEK, poly-ether-ether-ketone; TBR, trabecular bone remodeling; VEC, vertebral endplate cyst; CZPS, clear zone around pedicle screw.

^aInstability is a segment with CZPS of 4 or vacuum-positive or local ROM >5°.

Discussion

In this study, we investigated changes in CT imaging findings up to 2 years postoperatively in PLIF segments with 3DTi or PEEK cages. At 2 years, the incidence of TBR, VEC, and cage subsidence did not significantly differ by cage. Therefore, the imaging findings and clinical outcomes were comparable. However, 1 year postoperatively, a slight but definitive difference was observed between the 3DTi and PEEK cage segments. The 3DTi cage segments had a higher incidence of VEC formation, and the PEEK cage segments had a higher incidence of cage subsidence. Furthermore, the 3DTi cage segments with TBR 3 months postoperatively showed significant final segmental stabilization, whereas TBR at 1 year rather than 3 months postoperatively was useful in determining future segmental stabilization for the PEEK cage segments. Thus, the new measure of TBR allowed us to explore the relationship between cage material and osseointegration over time.

The clinical differences between the 3DTi and PEEK cages may be slight. No differences were observed in CT imaging findings between the cages at 2 years postoperatively and in spinal alignment or clinical assessment. The results of this study showed that the 3DTi cage segments that were TBR-positive in the early postoperative period (3 months) had significantly more findings suggestive of subsequent segmental stability, but not in the PEEK cages. This indicates that the early TBR-positive 3DTi cage segments establish osseointegration, whereas the early TBR in the PEEK cage segments may not. Furthermore, the positivity or negativity of TBR at 1 year postoperatively is a significant predictor of the fate of the segment at 2 years, regardless of cage material. Therefore, it may be too early to determine the establishment of PLIF stability for PEEK cage segments by TBR determination at 3 months postoperatively. A previous study that analyzed data from patients who underwent PLIF with 3D porous tantalum or titanium-coated PEEK cages showed an association between TBR positivity at 3 months postoperatively and segment stability at 1 year.⁷ However, early postoperative TBR positivity was significantly more common in porous tantalum cage segments.⁸ Thus, TBR positivity at “3 months postoperatively” was associated with final PLIF stability only in segments with “3D porous metal” cages. Additionally, a significant association was observed between TBR at 1 year and segment stability at 2 years, regardless of cage material. Thus, it is more reasonable to assume that TBR itself is a universal phenomenon and that the cage material influences the timing of osseointegration establishment (Figure 5).

Figure 5.

Illustration of vertebral bone responses to cage material. In the 3DTi cage segments (A–C), the contact between the vertebral endplate and titanium caused osseointegration and TBR. However, TBR and VEC were observed simultaneously during the conflict between forces that attempted to stabilize and destabilize the segment (B). As segment stabilization was established, the TBR developed, and the VEC shrank (C). In the PEEK cage segments (D–E), direct osseointegration did not occur because of the nonbioactivity of PEEK. TBR at 3 months postoperatively did not appear to be associated with osseointegration, but TBR at 1 year suggested osseointegration (E). Once segmental stabilization was established, TBR developed as in 3DTi (F). 3DTi, three-dimensional porous titanium; TBR, trabecular bone remodeling; VEC, vertebral endplate cyst; PEEK, polyether-ether-ketone.

In this study, VEC formation, a recognized predictor of delayed union and nonunion,¹³ was more common in 3DTi cage segments than in PEEK 1 year postoperatively. This finding contrasts with a previous report that found no significant difference in VEC formation between 3DTi and PEEK cage segments.¹⁷ This may be due to the expanded definition of VEC in this study. Stress-induced microfractures of vertebral endplates due to cage instability may be a possible cause of VEC formation.¹³ Therefore, the small depression with degenerative-like endplate sclerosis was treated as a VEC. Additionally, PEEK cage segments showed opposing rates of TBR (suggesting stability) and VEC (suggesting instability), as previously reported,⁷ whereas 3DTi cage segments showed no difference in VEC formation with or without TBR. Therefore, VECs in 3DTi cage segments may not only be suggestive of instability but also include findings specific to 3DTi cages. The 3DTi cages have an elastic modulus of 6.2 GPa,¹⁸ which is much smaller than that of solid titanium but still higher than that of pure PEEK (about 3-4 GPa)¹⁹. The difference in the elastic modulus between the bone and cage may be associated with cage subsidence.¹ It is possible to hypothesize that the higher elastic modulus of 3DTi cages than vertebral trabecular bone is a risk factor for endplate microfracture and induces VEC formation compared with PEEK. Therefore, the 3DTi cage segments may promote VEC formation with or without TBR. Therefore, further studies are needed to investigate the relationship between these imaging findings and eventual segmental stabilization.

Although PEEK can reduce the problem of cage subsidence compared with “solid” titanium cages because of its low elastic modulus,¹ the potential of PEEK and 3DTi cages to subside remains controversial. Comparisons of the two materials have shown either no significant difference in subsidence^5,6 or a greater risk of subsidence for PEEK.^20-22 Because these results may be qualified by the definition of subsidence and various biases, the results of more subsidence for PEEK cages than for 3DTi cages in this study should also be carefully considered. In contrast to the subsidence rate, the PEEK cage segments had a relatively lower incidence of VEC formation than the 3DTi cage segments. Therefore, the mechanism of PEEK cage subsidence may be different from that of the metal cage. A straightforward interpretation of the imaging findings indicates that PEEK cages can migrate to vertebral endplates without vertebral endplate damage that would form VECs. PEEK has low biocompatibility and does not allow direct osseointegration.² Therefore, even if the PEEK cage and vertebral endplate are in contact on CT imaging, an intervening tissue exists between them histologically. This form of connection not only does not have robust stability like the bridging state but also causes foreign body responses and chronic inflammation that promote fibrous encapsulation instead of bone binding.^19,23 Although how the cage will behave in response to this biological reaction is unknown, the potential risk of cage migration to the vertebral body may be triggered. Thus, PEEK cages, despite their advantage in material elastic modulus, are potentially more at risk of subsidence than 3DTi cages.

Paradoxically, TBR is not a finding that corresponds one-to-one with the establishment of intervertebral stability. As evidence, this series included a segment that was TBR-positive but also showed instability. It has been clinically shown that a metal ball insertion between vertebrae results in a high-density area on CT images.²⁴ Thus, some of the increased CT values of the trabecular bone adjacent to the intervertebral implant may be due to densification of the underlying bone due to subsidence.²⁴ This finding may clarify the coexistence of TBR and VEC at a higher rate in 3DTi cage segments. In other words, some findings determined to be TBR probably included densification of the underlying bone by the cage. However, because descriptors to accurately distinguish these findings are currently lacking, this confusion is a problem to be solved in the future. Furthermore, PLIF segments, like other bone tissues, are naturally subject to constant remodeling. Thus, no matter how strong the interbody stabilization findings are at the time of image acquisition, they are not likely to remain the same indefinitely. In short, the status of the PLIF segment should be judged comprehensively in conjunction with the other findings. Despite these challenges, TBR is a promising imaging finding for determining PLIF segment stability.

This study has some limitations. First, the number of cases was relatively small. Therefore, judgments based on P values may be underestimated because of the lack of statistical power. Second, the interpretation of TBR and other imaging findings may be subjective in its determination. Despite these limitations, the results of this study provide valuable insight into the interpretation of imaging results and evaluation of vertebral cage osseointegration for each of the two distinctive and universally used cage materials.

Conclusion

This study investigated changes in CT imaging findings up to 2 years postoperatively in one-level PLIF with 3DTi or PEEK cages. At 2 years, no significant differences were observed in imaging findings by cage. However, the 3DTi cage segments that were TBR-positive at 3 months postoperatively showed significant signs of segmental stabilization. The PEEK cage segments that were TBR-positive at 3 months postoperatively were not associated with future segmental stabilization. However, the PEEK cage segments that were TBR-positive at 1 year postoperatively were associated with future segmental stabilization. Thus, the timing and status of TBR, a new osseointegration assessment, were associated with cage material, which indicates a relationship between cage material and osseointegration over time.

Footnotes

Acknowledgments

We would like to thank Enago () for the English review.

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.

Ethical Statement

ORCID iDs

Naoki Segi

Hiroaki Nakashima

Shiro Imagama

References

Seaman

Kerezoudis

Bydon

Torner

Hitchon

. Titanium vs. polyetheretherketone (PEEK) interbody fusion: meta-analysis and review of the literature. J Clin Neurosci. 2017;44:23-29. doi:10.1016/j.jocn.2017.06.062

McGilvray

Easley

Seim

, et al. Bony ingrowth potential of 3D-printed porous titanium alloy: a direct comparison of interbody cage materials in an in vivo ovine lumbar fusion model. Spine J. 2018;18(7):1250-1260. doi:10.1016/j.spinee.2018.02.018

Kurtz

Devine

. PEEK biomaterials in trauma, orthopedic, and spinal implants. Biomaterials. 2007;28(32):4845-4869. doi:10.1016/j.biomaterials.2007.07.013

Toop

Gifford

Motiei-Langroudi

, et al. Can activated titanium interbody cages accelerate or enhance spinal fusion? a review of the literature and a design for clinical trials. J Mater Sci Mater Med. 2021;33(1):1. doi:10.1007/s10856-021-06628-1

Sultana

Hossain

Jeong

. Comparative analysis of radiologic outcomes between polyetheretherketone and three-dimensional-printed titanium cages after transforaminal lumbar interbody fusion. World Neurosurg. 2023;179:e241-e255. doi:10.1016/j.wneu.2023.08.056

Kim

Kwon

Park

. Comparison between 3-dimensional-printed titanium and polyetheretherketone cages: 1-year outcome after minimally invasive transforaminal interbody fusion. Neurospine. 2022;19(3):524-532. doi:10.14245/ns.2244140.070

Segi

Nakashima

Shinjo

, et al. Trabecular bone remodeling as a new indicator of osteointegration after posterior lumbar interbody fusion. Global Spine J. 2024;14(1):25-32. doi:10.1177/21925682221090484

Segi

Nakashima

Shinjo

, et al. Trabecular bone remodeling after posterior lumbar interbody fusion: comparison of three-dimensional porous tantalum and titanium-coated polyetheretherketone interbody cages. Global Spine J. 2023;21925682231170613. doi:10.1177/21925682231170613

Talukdar

Saviour

Tiwarekar

Dhara

Gupta

. Bone remodeling around solid and porous interbody cages in the lumbar spine. J Biomech Eng. 2022;144(10):101011. doi:10.1115/1.4054457

10.

van Rijsbergen

van Rietbergen

Barthelemy

, et al. Comparison of patient-specific computational models vs. clinical follow-up, for adjacent segment disc degeneration and bone remodelling after spinal fusion. PLoS One. 2018;13(8):e0200899. doi:10.1371/journal.pone.0200899

11.

Choi

Kim

Lim

. Diagnostic efficacy of Hounsfield units in spine CT for the assessment of real bone mineral density of degenerative spine: correlation study between t-scores determined by DEXA scan and Hounsfield units from CT. Acta Neurochir (Wien). 2016;158(7):1421-1427. doi:10.1007/s00701-016-2821-5

12.

Inoue

. Assessment to treatment for low back pain. J Jpn Orthop Assoc. 1986;60:391-394.

13.

Fujibayashi

Takemoto

Izeki

Takahashi

Nakayama

Neo

. Does the formation of vertebral endplate cysts predict nonunion after lumbar interbody fusion? Spine. 2012;37(19):E1197-E1202. doi:10.1097/BRS.0b013e31825d26d7

14.

Tanida

Fujibayashi

Otsuki

, et al. Vertebral endplate cyst as a predictor of nonunion after lumbar interbody fusion: comparison of titanium and polyetheretherketone cages. Spine. 2016;41(20):E1216-E1222. doi:10.1097/BRS.0000000000001605

15.

Sakaura

Ohnishi

Yamagishi

Ohwada

. Early fusion status after posterior lumbar interbody fusion with cortical bone trajectory screw fixation: a comparison of titanium-coated polyetheretherketone cages and carbon polyetheretherketone cages. Asian Spine J. 2019;13(2):248-253. doi:10.31616/asj.2018.0169

16.

Gruskay

Webb

Grauer

. Methods of evaluating lumbar and cervical fusion. Spine J. 2014;14(3):531-539. doi:10.1016/j.spinee.2013.07.459

17.

Makino

Takenaka

Sakai

Yoshikawa

Kaito

. Comparison of short-term radiographical and clinical outcomes after posterior lumbar interbody fusion with a 3D porous titanium alloy cage and a titanium-coated PEEK cage. Global Spine J. 2022;12(5):931-939. doi:10.1177/2192568220972334

18.

Khan

Parker

Bow

, et al. Clinical and cost-effectiveness of lumbar interbody fusion using Tritanium posterolateral cage (vs. Propensity-Matched Cohort of PEEK Cage). Spine Surg Relat Res. 2022;6(6):671-680. doi:10.22603/ssrr.2021-0252

19.

Buck

Lee

Gao

, et al. The role of surface chemistry in the osseointegration of PEEK implants. ACS Biomater Sci Eng. 2022;8(4):1506-1521. doi:10.1021/acsbiomaterials.1c01434

20.

Duan

Feng

Wang

Jiang

Huang

. Comparison of lumbar interbody fusion with 3D-printed porous titanium cage versus polyetheretherketone cage in treating lumbar degenerative disease: a systematic review and meta-analysis. World Neurosurg. 2023;183:144-156. doi:10.1016/j.wneu.2023.12.111

21.

Alan

Vodovotz

Muthiah

, et al. Subsidence after lateral lumbar interbody fusion using a 3D-printed porous titanium interbody cage: single-institution case series. J Neurosurg Spine. 2022;37(5):1-7. doi:10.3171/2022.4.SPINE2245

22.

Deng

Zou

Wang

, et al. Comparison between three-dimensional printed titanium and PEEK cages for cervical and lumbar interbody fusion: a prospective controlled trial. Orthop Surg. 2023;15(11):2889-2900. doi:10.1111/os.13896

23.

Olivares-Navarrete

Hyzy

Slosar

Schneider

Schwartz

Boyan

. Implant materials generate different peri-implant inflammatory factors: poly-ether-ether-ketone promotes fibrosis and microtextured titanium promotes osteogenic factors. Spine. 2015;40(6):399-404. doi:10.1097/BRS.0000000000000778

24.

Rundell

Isaza

Kurtz

. Biomechanical evaluation of a spherical lumbar interbody device at varying levels of subsidence. SAS J. 2011;5(1):16-25. doi:10.1016/j.esas.2010.12.001

Trabecular Bone Remodeling after Posterior Lumbar Interbody Fusion: Comparison of the Osseointegration in Three-Dimensional Porous Titanium Cages and Polyether-Ether-Ketone Cages

Abstract

Study Design

Objectives

Methods

Results

Conclusions

Keywords

Introduction

Methods

Patient Population

Surgical Procedure

Variables

Statistical Analysis

Results

Discussion

Conclusion

Footnotes

Acknowledgments

Declaration of Conflicting Interests

Funding

Ethical Statement

ORCID iDs

References