Anterior cervical discectomy and fusion remains the definitive surgical standard for treating cervical disc degeneration and radiculopathy. In recent years, spinal implant design has evolved rapidly toward zero-profile, integrated devices. Consequently, spine surgeons frequently evaluate whether two-screw internal fixation provides equal stability compared to traditional four-screw constructs. Biomechanical stability and surgical ease represent critical considerations in this ongoing debate. A landmark prospective randomized controlled trial now provides vital clarity on how screw configuration directly influences the rate and speed of interbody fusion.

Biomechanical Rationale for Two-Screw Internal Fixation

Modern spinal arthrodesis relies on a delicate balance between mechanical stabilization and physiological load distribution. Traditionally, surgeons selected four-screw locking configurations to maximize immediate biomechanical rigidity across treated motion segments. However, excessive structural stiffness can trigger significant stress shielding, which potentially delays natural osteogenesis. When an anterior implant carries the full mechanical load, the underlying bone graft experiences reduced physiological compression.

In contrast, two-screw internal fixation creates a slightly less rigid construct that permits micro-motion. This micro-motion transfers compressive loads through the interbody spacer according to Wolff's law. Therefore, active mechanical stimulation encourages osteoblast recruitment and accelerates trabecular bone formation. Finite element investigations demonstrate that two-screw devices maintain adequate multidirectional stability while facilitating graft loading. In addition, simpler implant geometry reduces mechanical interference within the intervertebral disc space. Consequently, this biological environment provides optimal conditions for earlier bridging trabeculae without causing construct destabilization.

Study Design and Patient Allocation in the Trial

The prospective randomized controlled trial enrolled 80 adult patients diagnosed with symptomatic single-level cervical spondylosis. The investigative team conducted the trial between October 2021 and April 2023 at a tertiary spine center. Researchers randomly allocated participants into two parallel cohorts: a four-screw fixation group and a two-screw fixation group. Ultimately, 77 patients successfully completed all clinical assessments and radiological evaluations throughout the full study period.

Importantly, baseline demographic characteristics and disease severity remained balanced across both treatment groups. Evaluators assessed primary osseous fusion at three, six, and twelve months postoperatively using standardized thin-cut computed tomography. Furthermore, secondary outcome metrics captured patient-reported outcome measures alongside surgical complications. These secondary measures included the Japanese Orthopaedic Association score, Neck Disability Index, and visual analog scale assessments for neck and arm pain. Thus, the rigorous prospective methodology effectively eliminated confounding surgical and demographic variables.

Accelerating Osseous Fusion Rates at Six Months

The trial's primary endpoint delivered compelling evidence regarding the temporal pattern of spinal healing. At six months postoperatively, the two-screw cohort achieved an impressive osseous fusion rate of 69.2 percent. In contrast, the four-screw cohort demonstrated a significantly lower fusion rate of only 42.1 percent. This substantial difference achieved statistical significance with a p-value of 0.022.

However, long-term radiographic evaluations revealed that this difference gradually equalized over time. By three months, neither cohort demonstrated widespread solid fusion, showing statistically comparable initial rates. Similarly, by the twelve-month follow-up, both surgical groups achieved high, equivalent overall fusion success. Therefore, the primary benefit of the two-screw construct centers specifically on fusion velocity rather than final arthrodesis rates. This accelerated bone healing reduces the temporal window during which hardware nonunion or pseudarthrosis might develop. Consequently, patients achieve reliable radiographic stability significantly earlier in their rehabilitation phase.

Patient-Reported Clinical Outcomes and Functional Recovery

Radiological success must always correlate with meaningful patient recovery. Throughout the prospective study, both cohorts demonstrated remarkable, statistically significant improvements across all functional scoring metrics. Specifically, patients reported major decreases in both cervical and radicular pain on visual analog scales immediately after decompression.

Furthermore, functional disability scores showed parallel trajectories between the two groups. The Neck Disability Index scores dropped substantially, reflecting restored daily activity and enhanced physical capability. Similarly, Japanese Orthopaedic Association scores increased consistently, confirming sustained neurological recovery from baseline myelopathic symptoms. Most importantly, researchers noted no statistically significant differences in these scores between the groups at any follow-up visit. Therefore, despite achieving faster early osseous union, the two-screw cohort maintained equivalent functional and subjective clinical efficacy. Both surgical interventions provided reliable, profound symptom relief for patients suffering from degenerative cervical disease.

Procedural Efficiency and Safety Profiles

Surgical instrumentation simplicity translates into practical operative advantages. Placing two divergent screws requires fewer instrument passes than inserting four separate anchoring screws. Consequently, surgeons experience streamlined operative workflows, reduced retraction pressure, and potentially shortened surgical durations. Prolonged esophageal retraction during anterior instrumentation frequently causes postoperative dysphagia and local soft tissue swelling.

Moreover, safety evaluations during the trial revealed no significant differences in adverse events between the two groups. Neither cohort experienced elevated rates of catastrophic hardware migration, screw backout, or construct failure. In addition, rates of transient dysphagia, wound infection, and postoperative hematoma remained low and evenly distributed. Some surgeons historically feared that two-screw constructs might increase graft subsidence or provoke segmental kyphosis. However, the trial demonstrated that properly sized two-screw implants maintain sagittal alignment and intervertebral disc height effectively. Thus, two-screw fixation combines proven procedural safety with streamlined surgical execution.

Strategic Clinical Implications for Spine Surgeons

These clinical findings provide clear, practical guidance for practicing spine specialists and neurosurgeons. For routine single-level cervical spondylotic disease, choosing a two-screw integrated construct delivers distinct mechanical and temporal advantages. By encouraging earlier biological fusion through controlled load sharing, surgeons safely accelerate patient healing trajectories.

Nevertheless, clinical judgment remains crucial during implant selection. Patients with severe osteopenia, severe segmental instability, or multi-level pathology may still warrant traditional multi-screw rigid stabilization. However, for standard single-level indications, the evidence robustly supports two-screw fixation as a superior strategy for rapid arthrodesis. Furthermore, reducing foreign metal hardware inside the cervical spine promotes optimal long-term biomechanical harmony. In conclusion, adopting two-screw instrumentation provides spine surgeons with an evidence-based approach that optimizes procedural speed, patient safety, and bony union.

Why does two-screw internal fixation achieve faster fusion than four-screw fixation?

Two-screw constructs provide sufficient stability while reducing excessive rigidity. Consequently, the bone graft experiences greater physiological compressive loads according to Wolff's law. This favorable mechanical environment minimizes stress shielding, stimulates early osteoblast recruitment, and accelerates bridging trabecular bone formation across the adjacent vertebral endplates during early recovery.

Do clinical outcomes differ between two-screw and four-screw internal fixation?

No, patient-reported clinical outcomes do not differ significantly between the two techniques. Both cohorts achieve comparable, substantial improvements in neck and arm pain, Neck Disability Index scores, and Japanese Orthopaedic Association scores. Furthermore, complication rates, including dysphagia and hardware displacement, remain statistically equivalent across both surgical groups.

Are twelve-month fusion rates different between the two fixation methods?

No, final fusion rates at twelve months are statistically indistinguishable between both groups. While two-screw fixation achieves significantly higher fusion rates at six months, the four-screw cohort eventually catches up. Therefore, the primary clinical advantage of two-screw fixation lies in accelerated fusion velocity rather than final union success.

Disclaimer: This content is for informational and educational purposes only and does not constitute medical advice, diagnosis, or treatment. Healthcare professionals should exercise their independent clinical judgment, and patients should consult a qualified doctor regarding medical concerns. Refer to the latest local and national guidelines for clinical practice.

References

1. Huang CY et al. Comparison of Two-Screw Versus Four-Screw Internal Fixation on Fusion Speed in Anterior Cervical Discectomy and Fusion : A Prospective Randomized Controlled Trial. Spine (Phila Pa 1976). 2026 Oct 15. doi: 10.1097/BRS.0000000000005334. PMID: 40123565.
2. Zhang KR, Sheng XQ, Wang XJ, et al. Biomechanical comparison of two-screw and four-screw zero-profile fusion devices in anterior cervical discectomy and fusion using finite element analysis. Scientific Reports. 2026;16(1):51154. doi:10.1038/s41598-026-51154-1.
3. Nambiar M, Mobbs RJ, Rao PJ. Zero-profile implants for anterior cervical discectomy and fusion: a systematic review and meta-analysis. Global Spine Journal. 2020;10(5):645-656. doi:10.1177/2192568219864383.