C2 vs C3 as the Upper Instrumented Vertebra in Posterior Cervical Fusion: Effects on Junctional Mechanics and Midterm Outcomes

Abstract

Background

Selection of the upper instrumented vertebra (UIV) in posterior cervical decompression and fusion remains variable, and its influence on junctional mechanics and clinical outcomes is not fully defined. Extending fixation to C2 may offer mechanical advantages but introduces additional operative complexity. This study compared radiographic alignment, proximal junctional complications, and patient-reported outcomes between constructs terminating at C2 vs C3.

Methods

A retrospective cohort analysis was conducted on consecutive patients undergoing posterior cervical fusion from 2021 to 2023 with distal fixation at T1. Patients were stratified by UIV: C2 or C3. Demographic, operative, and radiographic data were collected. Cervical alignment parameters, proximal and distal junctional complications, and patient-reported outcomes (visual analog scale, Neck Disability Index, and 36-item Short Form physical component score and mental component score) were evaluated at baseline and at 6 and 24 months. Group comparisons used t and χ² tests, with significance set at P < 0.05.

Results

A total of 104 patients were included (C2: n = 63; C3: n = 41) with comparable baseline demographics and clinical characteristics. Estimated blood loss was higher in the C2 group. Both groups demonstrated significant postoperative improvement in cervical alignment and patient-reported outcomes through 24 months, with no significant differences between groups for visual analog scale, Neck Disability Index, or physical component score. Constructs ending at C3 exhibited a significantly larger UIV – UIV + 1 angle and higher rates of proximal junctional failure (9.7% vs 0%), whereas distal junctional complication rates and overall revision rates were similar.

Conclusions

Termination at C2 reduces proximal junctional complications and results in a smaller transition angle without compromising midterm clinical outcomes; however, it involves greater operative burden. UIV selection should be individualized, favoring C2 when anatomical feasibility and junctional risk profiles support cranial fixation. Longer-term prospective studies are needed to determine whether mechanical advantages translate into durable clinical benefit.

Clinical Relevance

These findings suggest that selecting C2 rather than C3 as the upper instrumented vertebra may reduce proximal junctional complications in posterior cervical fusion extending to T1, although the greater operative burden of C2 fixation should be considered when individualizing surgical planning.

Level of Evidence

3.

Introduction

Posterior cervical decompression and fusion (PCDF) remains a primary surgical strategy for multilevel degenerative stenosis, cervical myelopathy, and fixed deformity, with well-established improvements in pain and functional outcomes; however, the procedure still carries meaningful risks of wound complications, readmissions, and reoperations influenced by construct design and patient characteristics.^1,2 Among the key technical decisions, selection of the proximal endpoint of the fusion construct significantly impacts biomechanical loading, complication profiles, and long-term durability. Choosing C2 as the upper instrumented vertebra (UIV) provides the advantage of stronger fixation options, including pedicle or pars screws compared with lateral mass screws, but necessitates heightened vigilance to vertebral artery anatomy and soft-tissue preservation.³ Conversely, terminating the construct at C3 may simplify exposure and avoid the risks associated with C2 yet may confer increased proximal junctional susceptibility depending on alignment, anchor type, and overall construct mechanics. These considerations highlight that the proximal end of the construct is not merely a technical preference but a modifiable factor that directly influences safety and alignment durability.^2,4,5

Prior studies demonstrate associations between cervical sagittal alignment patterns and both proximal and distal junctional failure, and a systematic review suggests that UIV selection at C2 vs C3/C4 may differentially affect proximal junctional angles and related complications.^5,6 Although recent multicenter reports describe comparable patient-reported outcomes through 24 months regardless of UIV, emerging evidence indicates potential differences in junctional complications and immediate postoperative alignment that could influence anchor selection, reduction maneuvers, and soft-tissue management.^6,7 Additionally, contemporary value-based frameworks increasingly weigh UIV choice against complications and resource utilization in elective PCDF, reinforcing its importance in preoperative planning.⁶

In this cohort with uniform distal fixation at T1 and standardized perioperative protocols, we compared constructs terminating at C2 vs C3, evaluating perioperative complications, radiographic alignment across key cervical parameters, and patient-reported outcomes over 24 months. We hypothesized that selecting C2 as the UIV reduces proximal junctional complications without compromising sagittal alignment or functional recovery, addressing a frequent yet unresolved decision point in posterior cervical fusion planning.

Methods

Participants

This retrospective cohort study reviewed consecutive patients who underwent posterior cervical spine fixation between 2021 and 2023 for degenerative cervical canal stenosis and cervical myelopathy in our academic hospital. We followed standardized perioperative protocols and contributed identified patient data to a central database. Patients were separated into 2 groups based on the level of the UIV: constructs terminating at C2 and constructs terminating at C3. Constructs were terminated at C2 when bone quality was poor, imbalance was present, or cranial anchoring was deemed necessary for stability, and were terminated at C3 when C2 anatomy posed a risk (high-riding vertebral artery, narrow pedicle, or when operative burden was a concern).

Inclusion and Exclusion Criteria

Included patients were those who underwent a posterior cervical fixation construct extending distally to T1 for degenerative stenosis or cervical myelopathy with at least 24-month follow-up and available pre- and postoperative clinical documentation and standing radiographic studies sufficient for evaluation. Patients were excluded if the underlying pathology was traumatic, deformity, or tumor related; if they had a history of prior cervical spine instrumentation; or if congenital cervical anomalies were present. Additional exclusions included age younger than 20 years, incomplete radiographic or clinical follow-up, documented substance dependence or sensitivity to nonsteroidal anti-inflammatory drugs, patient refusal, or death from nonsurgical causes prior to outcome assessment.

Surgical Technique and Postoperative Considerations

Preoperative planning incorporated high-resolution computed tomography (CT) with 3-dimensional reconstruction and standing lateral cervical radiographs to assess pedicle and lateral mass anatomy, vertebral artery course, and sagittal alignment. Instrumentation plans were defined in advance, including screw type and dimensions for C2 (pedicle or pars) and for C3 to C7 (lateral mass), with predetermined alternatives in cases of unfavorable anatomy. Neuromonitoring with somatosensory and motor-evoked potentials was used in all cases, and fluoroscopy was available throughout the procedure.

All procedures were performed by 2 fellowship-trained spine surgeons. Patients were positioned prone on a radiolucent table with head fixation in a Mayfield clamp. Cervical alignment was maintained neutral to slightly lordotic based on the preoperative plan, and gentle traction was applied if required to restore alignment. A midline approach was used to expose the posterior elements from the planned UIV (C2 or C3) to T1, with attention to preserving muscular attachments to C2 and C1 whenever feasible. Decompression via laminectomy or foraminotomy was performed according to the clinical indication for neural element decompression. C2 fixation utilized pedicle or pars screws, while C3 fixation utilized lateral mass screws.

All fusion constructs were extended to T1, and local autograft obtained from laminectomy bone was supplemented with allograft or graft extenders when required. Precontoured rods were used to recreate lordosis and secured with controlled reduction maneuvers under fluoroscopic confirmation. Meticulous hemostasis was achieved with bipolar electrocautery and topical agents, and drains were placed at the discretion of the surgeon. Wound closure was performed in a layered fashion. Postoperatively, patients underwent early mobilization, followed a multimodal analgesia regimen, received pharmacological venous thromboembolism prophylaxis per protocol, and avoided heavy lifting for approximately 3 months.

Data Collection and Outcome Measures

Demographic and perioperative characteristics collected included age, sex, body mass index, smoking status, diabetes, American Society of Anesthesiologists classification, bone mineral density (T score), estimated blood loss, extension of laminectomy, and postoperative hospital stay. Radiological outcomes included C2 to C7 sagittal vertical axis (SVA), C2 to C7 lordosis, T1 slope, thoracic inlet angle, C0 to C2 alignment, and the UIV – UIV + 1 angle, measured preoperatively, early postoperatively, and at last follow-up. Complications recorded included proximal junctional failure (PJF; increase of more than 10° in the C2 slope accompanied by neurological symptoms at the C2–C3 level), distal junctional kyphosis, distal junctional failure (DJF), postoperative infection, and revision surgery. Definitions followed standard radiographic criteria, including a ≥10° change in proximal or distal junctional Cobb angle compared with preoperative films and the presence of structural failure or associated symptoms when classified as failure. Patient-reported outcomes included pain intensity (visual analog scale), Neck Disability Index (NDI), and 36-item Short Form [SF-36] physical and mental component scores, measured preoperatively and at 6 and 24 months.

Statistical Analysis

Data were analyzed using SPSS version 15 (Chicago, Illinois, USA). Continuous variables were expressed as mean ± SD and compared between groups using independent samples t tests. Categorical variables were reported as frequencies and percentages and analyzed using χ ² or Fisher’s exact test as appropriate. Changes in radiological and clinical outcome measures over time were compared between groups using repeated-measures comparisons at baseline, early postoperative, and final follow-up time points. A P value < 0.05 was considered statistically significant for all tests.

Results

A total of 104 patients met the inclusion criteria, including 63 patients in the C2 group and 41 patients in the C3 group. Baseline demographic and clinical characteristics were comparable between groups, with no significant differences in age, sex distribution, body mass index, smoking history, diabetes prevalence, American Society of Anesthesiologists classification, bone mineral density (T score), or duration of follow-up (Table 1). The extent of posterior decompression was similar between groups (P = 0.49), with C3 to C6 laminectomy representing the most common pattern in both cohorts. The incidence of postoperative C5 palsy did not differ between the C2 and C3 groups (7.9% vs 7.3%, P = 0.99), nor did the rate of other postoperative neurological deficits (P = 0.44). Estimated blood loss and operation time were significantly higher in the C2 group compared with the C3 group (273 ± 53 cc vs 242 ± 59 cc; P < 0.01 and 176 ± 81 minutes vs 144 ± 53 minutes; P = 0.02, respectively). Length of hospital stay showed a nonsignificant trend toward longer hospitalization in the C2 cohort (2.4 ± 1.2 vs 1.9 ± 1.2 days; P = 0.06). Rates of postoperative infection and revision surgery were similar between groups.

Table 1.

Baseline characteristics of patients included in the study.

Variables	C2 Group (n = 63)	C3 Group (n = 41)	P
Age, y	64.8 ± 7.5	63.2 ± 9.4	0.33
Sex, n, men/women	40/23	24/17	0.68
BMI	24.9 ± 3.2	24.2 ± 3.1	0.29
Duration of follow-up, mo	31.8 ± 4.6	33.2 ± 5.6	0.17
History of smoking	14 (22.2)	14 (34.1)	0.25
DM	19 (30.1)	9 (21.9)	0.37
ASA classification	2.3 ± 0.9	2.0 ± 0.8	0.13
T score	−1.4 ± 0.5	−1.3 ± −0.5	0.21
EBL, cc	273 ± 53	242 ± 59	<0.01
Operation time, min	176 ± 81	144 ± 53	0.02
Length of hospital stay, d	2.4 ± 1.2	1.9 ± 1.2	0.06
Extension of laminectomy			0.49
C3–C5	13 (20.0)	4 (9.7)
C3–C6	40 (63.5)	30 (73.1)
C4–C5	5 (7.9)	4 (97)
C4–C6	5 (7.9)	3 (7.3)
No. of patients with postoperative C5 palsy	5 (7.9)	3 (7.3)	0.99
Neurological deficit other than C5 palsy	1.9 ± 0.5	1.8 ± 0.5	0.44

Abbreviations: ASA, American Society of Anesthesiologists; BMI, body mass index; DM , diabetes mellitus; EBL, estimated blood loss.

Note: Data presented as n (%) or mean ± SD.

Radiographic Outcomes and Junctional Complications

Preoperative cervical alignment parameters, including C2 to C7 SVA, C2 to C7 lordosis, T1 slope, thoracic inlet angle, C0 to C2 alignment, and C2 to C3 segmental angle, were not significantly different between groups (Table 2).

Table 2.

Comparison of radiologic outcome and complication profile of patients with constructs ending at C2 vs C3.

Variable	C2 Group (n = 63)	C3 Group (n = 41)	P
C2–C7 sagittal vertical axis, mm
Preoperative	27.6 ± 11.9	24.5 ± 8.2	0.11
Early postoperative	29.3 ± 11.4	34.5 ± 14.2	0.04
Last follow-up	28.2 ± 10.3	36.4 ± 8.8	<0.01
C2–C7 lordosis, °
Preoperative	9.8 ± 8.8	11.2 ± 6.1	0.18
Early postoperative	14.7 ± 6.8	10.7 ± 5.5	<0.01
Last follow-up	15.2 ± 6.6	12.7 ± 5.9	0.04
T1 slope, °
Preoperative	25.3 ± 7.4	27.9 ± 6.7	0.07
Early postoperative	34.4 ± 6.8	36.8 ± 11.7	0.18
Last follow-up	34.5 ± 9.8	37.2 ± 13.3	0.24
Thoracic inlet angle, °
Preoperative	74.8 ± 15.9	70.1 ± 16.8	0.15
Early postoperative	71.9 ± 10.5	71.0 ± 16.0	0.74
Last follow-up	76.7 ± 18.9	72.6 ± 14.8	0.24
C0–C2 angle, °
Preoperative	31.1 ± 15.5	32.8 ± 11.9	0.55
Early postoperative	34.2 ± 13.9	40.9 ± 13.5	0.01
Last follow-up	33.3 ± 11.8	44.7 ± 9.0	<0.01
C2–C3 segmental angle, °
Preoperative	3.6 ± 1.5	3.0 ± 1.5	0.10
Early postoperative	3.8 ± 1.4	6.3 ± 2.1	<0.01
Last follow-up	3.7 ± 1.6	7.0 ± 2.3	<0.01
PJF	0 (0%)	4 (9.7%)	0.02
DJK	4 (6.3%)	3 (7.3%)	0.57
DJF	1 (1.5%)	0 (0%)	0.39
Length of hospital stay, d	2.4 ± 1.2	1.9 ± 1.2	0.06
Postoperative infection	5 (7.9%)	4 (9.7%)	0.73
Revision surgery	4 (6.3%)	4 (9.7%)	0.70

Abbreviations: DJF, distal junctional failure; DJK, distal junctional kyphosis; PJF, proximal junctional failure.

Note: Data presented as n (%) or mean ± SD.

Postoperatively, distinct alignment patterns emerged. The C3 group demonstrated a significantly larger C2 to C7 SVA compared with the C2 group at both early postoperative evaluation (34.5 ± 14.2 mm vs 29.3 ± 11.4 mm; P = 0.04) and final follow-up (36.4 ± 8.8 mm vs 28.2 ± 10.3 mm; P < 0.01). In contrast, C2 to C7 lordosis was greater in the C2 group at early postoperative assessment (14.7° ± 6.8° vs 10.7° ± 5.5°; P < 0.01) and remained significantly higher at last follow-up (15.2° ± 6.6° vs 12.7° ± 5.9°; P = 0.04).

Upper cervical compensatory changes were more pronounced in the C3 cohort. C0 to C2 angle increased significantly in the C3 group compared with the C2 group at both early postoperative assessment (40.9° ± 13.5° vs 34.2° ± 13.9°; P = 0.01) and final follow-up (44.7° ± 9.0° vs 33.3° ± 11.8°; P < 0.01). Similarly, the C2 to C3 segmental angle was significantly larger in constructs terminating at C3 at both early postoperative (6.3° ± 2.1° vs 3.8° ± 1.4°; P < 0.01) and final follow-up (7.0° ± 2.3° vs 3.7° ± 1.6°; P < 0.01), reflecting a steeper proximal transition angle.

With respect to junctional complications, PJF occurred exclusively in the C3 group (4 patients, 9.7% of whom 3 patients underwent revision surgery) and was not observed in the C2 group (P = 0.02). Rates of distal junctional kyphosis (6.3% vs 7.3 %; P = 0.57), DJF (1.5% vs 0 %; P = 0.39), postoperative infection, and revision surgery did not differ significantly between groups. In this regard, in the C3 group, 3 patients underwent revision surgery due to PJF, while 1 patient required revision for persistent myelopathic symptoms necessitating additional decompression. In the C2 group, 2 patients required revision surgery due to deep infection, 1 underwent revision for further cervical decompression, and 1 required revision for DJF.

Patient-Reported Outcomes

Patient-reported outcomes were comparable between groups at baseline. Both C2 and C3 constructs resulted in substantial improvement in pain, disability, and health-related quality of life at 6 and 24 months (Table 3). Visual analog scale and NDI scores improved significantly in both cohorts, with no statistically significant differences between groups at any postoperative time point, although NDI scores trended lower in the C2 group at final follow-up (P = 0.06). SF-36 physical component scores improved similarly in both groups without between-group differences. The C3 group demonstrated a modestly higher SF-36 mental component score at 24 months (P = 0.057), the clinical relevance of which remains uncertain.

Table 3.

Comparison of outcome measurements between patients in the C2 group and those in the C3 group.

Variables	C2 Group (n = 63)	C3 Group (n = 41)	P
Pain intensity (VAS)
Baseline	65.1 ± 14.0	64.4 ± 16.4	0.37
6-mo follow-up	32.4 ± 12.5	36.3 ± 10.9	0.10
24-mo follow-up	35.8 ± 10.8	33.2 ± 13.4	0.27
Neck Disability Index
Baseline	62.0 ± 12.8	63.5 ± 14.9	0.59
6-mo follow-up	33.9 ± 8.4	36.8 ± 9.8	0.10
24-mo follow-up	38.7 ± 8.3	43.0 ± 12.7	0.06
MCS (SF-36)
Baseline	42.6 ± 12.2	41.3 ± 9.8	0.58
6-mo follow-up	39.8 ± 9.5	38.6 ± 9.5	0.54
24-mo follow-up	39.2 ± 10.2	43.0 ± 8.2	0.05
PCS (SF-36)
Baseline	27.7 ± 6.7	30.4 ± 9.1	0.08
6-mo follow-up	44.7 ± 9.2	43.2 ± 8.6	0.39
24-mo follow-up	43.6 ± 8.0	46.1 ± 10.2	0.17

Abbreviations: MCS, mental component score; PCS, physical component score; SF-36, 36-item Short Form; VAS, visual analog scale.

^aValues are represented as mean ± SD.

Illustrative Cases

To illustrate representative operative strategies and radiographic outcomes, selected cases from each cohort are presented. These cases demonstrate typical preoperative pathology, surgical constructs terminating at either C3 or C2, and corresponding postoperative alignment. One case further illustrates the development of PJF and the subsequent need for revision surgery, reflecting the type of complication observed in the C3 cohort.

Case 1

A 67-year-old patient presented with progressive myelopathic symptoms associated with multilevel cervical spinal canal stenosis. Preoperative magnetic resonance imaging demonstrated significant stenosis from C3 to C6 with loss of normal cervical lordosis. The patient underwent posterior cervical decompression and instrumented fusion from C3 to T1. Postoperative radiographs demonstrated appropriate implant placement and restoration of cervical lordosis (Figure 1).

Figure 1.

Pre- and postoperative images of the patient in case 1: a 67-year-old with multilevel cervical spinal canal stenosis. (A and B) Preoperative magnetic resonance images. (C and D) Anteroposterior and lateral radiographs after posterior cervical decompression and instrumented fusion from C3 to T1.

Case 2

A 70-year-old patient with cervical myelopathy was found to have multilevel stenosis extending from C3 to C6 on preoperative imaging. The patient underwent posterior decompression with C3 to T1 fusion. Early postoperative radiographs confirmed satisfactory alignment and appropriate positioning of instrumentation (Figure 2).

Figure 2.

Pre- and postoperative images of the patient in case 2: a 70-year-old patient with multilevel stenosis extending from C3 to C6 as shown on the magnetic resonance image (A). The patient underwent posterior decompression with C3 to T1 fusion, with postoperative radiographs shown (B and C) .

Case 3

A 65-year-old patient presented with neck pain and cervical myelopathy. Preoperative imaging demonstrated multilevel stenosis from C3 to C6, and the patient underwent laminectomy with posterior fusion from C3 to T1 (Figure 3). At 18-month follow-up, the patient developed symptomatic PJF, requiring revision surgery (Figure 4). This case illustrates the type of proximal junctional complication observed in the C3 cohort.

Figure 3.

Pre- and postoperative images of the patient in case 3: a 65-year-old who presented with neck pain and cervical myelopathy. (A) Preoperative magnetic resonance image revealed multilevel stenosis from C3 to C6. The patient underwent laminectomy with posterior fusion from C3 to T1, as shown in the (B) postoperative radiograph.

Figure 4.

Eighteen-month follow-up images of the patient in case 3 (A and B) show the presence of proximal junctional failure. (C) Postrevision radiograph shows resolution.

Case 4

A 62-year-old patient presented with cervical myelopathy and multilevel cervical canal stenosis. Preoperative CT angiography demonstrated a high-riding vertebral artery, which increased the risk of standard pedicle instrumentation at C2. Therefore, C2 pars screw fixation was used, and the patient underwent posterior decompression and fusion extending to C2. Pre- and postoperative imaging demonstrating the construct are shown in Figure 5.

Figure 5.

Pre- and postoperative images of the 62-year-old patient in case 4. The patient had a high-riding vertebral artery (A) and underwent posterior decompression and fusion extending to C2, as shown in the radiograph (B).

Discussion

In this multicenter cohort, constructs terminating at C3 demonstrated a significantly larger proximal transition angle and a higher incidence of PJF compared with constructs ending at C2, while both groups achieved comparable midterm improvements in pain, disability, and physical health. Although extending fixation to C2 was associated with greater operative burden, including higher blood loss, it conferred a measurable mechanical advantage at the cranial junction, reflected by more favorable alignment parameters and the absence of PJF in this group. It should be noted that despite the significant difference in cervical radiological parameters after surgery, there was no significant difference in PRO between the 2 groups.

These findings reinforce the concept that UIV selection is a modifiable surgical factor that meaningfully influences junctional biomechanics, even when distal fixation strategies and decompression extent are standardized. Prior large database studies and institutional series have consistently demonstrated substantial complication and reoperation rates following PCDF, with construct design emerging as a key determinant of junctional outcomes.1–3,5,8,9 Radiographic studies report adjacent segment pathology rates of approximately 3.4% at 1 year and 5.9% at 2 years, with longer-term incidence estimates ranging from 20% to 30%.^10–12 A systematic review including 1215 patients showed that constructs terminating at C2 were associated with lower rates of proximal adjacent segment disease compared with C3/C4 termination (1.2% vs 3.0%; OR 0.36), although overall reoperation rates remained similar.⁵ Our results are consistent with this literature and further support the biomechanical advantage of C2 anchorage. Several series have reported reduced proximal clinical adjacent-segment pathology and lower rates of PJF when fixation extends to C2.¹³ In a multicenter comparison, Pinter et al demonstrated significantly lower PJF in C2 to T2 constructs compared with C3 to T1 constructs, along with higher fusion rates in the C2 cohort.⁷ Collectively, these data suggest that while short-term patient-reported outcomes may not differ substantially by UIV level, junctional durability may be more sensitive to proximal construct termination.

Biomechanical Considerations

The UIV – UIV + 1 angle serves as a clinically meaningful radiographic surrogate for the abruptness of stiffness transition at the cranial end of a posterior cervical construct. Larger transition angles amplify bending moments and shear forces at the adjacent mobile segment and have been associated with increased risks of proximal junctional kyphosis and failure.4,5,7,14 In the present study, constructs ending at C3 demonstrated a substantially larger proximal transition angle and greater upper cervical compensation, accompanied by a higher incidence of PJF.

Unlike the thoracolumbar spine, traditional definitions of PJK and PJF are not directly transferable to the upper cervical region due to differences in anatomy and compensatory behavior.⁵ From a mechanistic perspective, fixation to C2 appears to moderate upper cervical hyperextension, producing a smoother sagittal transition between the fused and mobile segments. This interpretation is supported by our findings of smaller C0 to C2 and C2 to C3 angles in the C2 group at both early and final follow-up. Prior work has shown that excessive C2 to C3 disc angulation and elevated C2 slope are associated with adverse outcomes and impaired compensatory capacity after cervical deformity correction.^15,16 Pinter et al defined PJF as a >10° increase in C2 slope accompanied by neurologic symptoms at C2 to C3 and demonstrated reduced upper cervical hyperextension when C2 was selected as the UIV.⁷ C2 fixation also benefits from denser bone stock and more robust anchorage through pedicle or pars screws, allowing more effective load sharing compared with subaxial lateral mass fixation.^17,18 However, multiple studies have shown that the type of C2 screw does not significantly influence fusion rates or complication profiles, suggesting that the principal benefit of C2 termination lies in construct-level mechanics rather than screw selection alone.^19,20

Procedural Considerations and Patient Selection

Technical execution remains central to mitigating junctional complications. Preservation of the posterior tension band, meticulous soft-tissue handling, limited disruption of semispinalis and suboccipital attachments, and deliberate rod contouring are consistently emphasized strategies to reduce junctional stress.^1,2,21 Avoiding excessive cranial dissection, preventing overcorrection that induces focal kyphosis, and ensuring gradual sagittal transitions are particularly important when constructs terminate in the upper cervical spine.

Despite its mechanical advantages, C2 fixation introduces distinct anatomical challenges, including vertebral artery proximity and variable pedicle morphology, which increase technical complexity and may contribute to greater blood loss, as observed in this study.^3,22 Advanced preoperative planning with CT angiography and 3-dimensional reconstruction can improve safety, and intraoperative navigation may further reduce risk in select cases, although high-level comparative data remain limited.^23,24

Consistent with prior comparative studies and meta-analyses, our findings suggest that the radiographic and biomechanical benefits of C2 termination may precede measurable differences in patient-reported outcomes.^4,5,7 This divergence underscores the importance of individualized decision-making. Bone quality, baseline sagittal alignment, extent of fusion, and cervicothoracic balance should all inform UIV selection. Contemporary value-based frameworks tend to favor C2 instrumentation in patients with osteoporosis, sagittal imbalance, long-segment constructs, revision surgery, or constructs crossing the cervicothoracic junction, where stopping at C3 may increase proximal junctional risk.²⁵ Conversely, subaxial termination remains reasonable when C2 anatomy is unfavorable or operative risk outweighs potential mechanical benefit.^6,14 Adjunctive strategies, including transitional constructs or supplemental fixation, may help mitigate risk when stopping at C3 is necessary^2,7

Limitations

Not all studies have demonstrated a clear advantage to C2 extension, and some long-term cohorts report equivalent radiographic and clinical outcomes between C2 and C3 termination, reflecting variability in surgical technique, implant choice, and follow-up duration.^26,27 The retrospective design of the present study, surgeon-directed UIV selection, and relatively low event rates limit causal inference and statistical power. In addition, midterm follow-up may not capture late junctional failures, and heterogeneity in definitions across studies remains an important challenge.

Conclusion

In this retrospective analysis, constructs terminating at C3 were associated with a steeper UIV – UIV + 1 transition angle and higher rates of PJF, while both C2 and C3 fixation strategies produced comparable midterm clinical improvements. Extension of fixation to C2 appears to mitigate junctional complications by providing stronger cranial anchorage and a more favorable biomechanical load transition, albeit at the cost of increased technical complexity and operative burden. Accordingly, UIV selection should be individualized: fixation to C2 may be preferred in patients at higher risk for junctional failure when anatomy permits safe instrumentation, whereas subaxial termination remains a reasonable option when C2 anatomy or operative risk outweighs its potential mechanical benefit.