Evidence-Based Practice: How to Choose Between Cannulated Screws, Crossed Fully Threaded Screws, and FNS in Femoral Neck Fractures?

Femoral neck fractures can be treated with a variety of internal fixation techniques. Among the most representative options are parallel cannulated screw fixation, crossed fully threaded screw fixation, and the Femoral Neck System (FNS).

To make the right clinical decision, surgeons must first understand the biomechanical principles behind each technique, as well as their indications and limitations.

The fundamental principle of treatment is simple:

Make the right choice first, then execute it precisely.

Every internal fixation procedure begins with selecting the appropriate implant—essentially choosing the mechanical support strategy behind it. The goal is to balance two critical factors:

• Promote fracture healing through controlled compression

• Prevent complications by maintaining fracture length and resisting shear forces

1. Core Principles, Advantages, and Limitations (Biomechanical Logic)

1.1 Parallel Cannulated Screws (CCS/PTS) – The “Controlled Sliding” Strategy

Principle:
Cannulated screws are typically arranged in an inverted triangular configuration. Their partially threaded design allows the femoral head to slide along the screw shaft, creating a sliding compression effect.

Advantages:

• Minimally invasive

• Preserves soft tissue and blood supply—critical for femoral head viability

• Simple and cost-effective

Critical Limitation:

• Poor resistance to vertical shear forces

In Pauwels Type III fractures (vertical fracture lines), axial loading can lead to:

• Varus collapse (“sliding failure”)

• Femoral neck shortening

• Reduced abductor lever arm → postoperative limping

1.2 Crossed Fully Threaded Screws (FTS / Crossed Configuration) – The “Length Preservation” Strategy

Principle:

• Fully threaded screws (length-stable design):
  Engage both the femoral head and neck, eliminating sliding and locking fracture length

• Crossed screw configuration (Alpha or “F-technique”):
  Mimics a truss structure, adding a screw oriented nearly perpendicular (≈90°) to shear forces, significantly improving mechanical stability

Advantages:

• Superior resistance to shear forces compared to parallel screws

• Significantly reduces femoral neck shortening (supported by meta-analyses)

Limitations:

• No dynamic compression → risk of nonunion if bone resorption occurs

• Technically demanding

• Risk of vascular injury or cortical perforation if poorly executed

1.3 Femoral Neck System (FNS) – The “Angular Stable Armor” Strategy

Principle:
FNS combines the strengths of Dynamic Hip Screw (DHS) and cannulated screws, featuring:

• Bolt-in-barrel design

• Locking plate system

• Controlled sliding (≤20 mm) + strong angular stability

Advantages:

• Best shear resistance among minimally invasive techniques

• Prevents varus collapse

• Lower nonunion and revision rates in young patients with unstable fractures

• Preserves minimally invasive benefits

Limitations:

• Higher implant cost

2. Clinical Decision-Making: How to Choose the Right Fixation?

Avoid a “one-size-fits-all” approach. Decisions should be based on:

• Pauwels angle (biomechanical environment)

• Patient physiological age

2.1 When to Choose Parallel Cannulated Screws (CCS)

• Garden Type I/II (non-displaced fractures)
  Stable environment → minimally invasive fixation is sufficient

• Pauwels angle < 30° (horizontal fracture)
  Low shear force → sliding compression promotes healing

• Elderly patients (not candidates for arthroplasty)
  Lower surgical tolerance → simpler, less invasive option preferred

  2.2 When FNS (or DHS) Is Mandatory

  • Pauwels angle > 50° (vertical/shear-dominant fractures)
    ⚠️ Absolute contraindication for cannulated screws
    → FNS provides essential angular stability

  • Young patients

    • High functional demands

    • FNS reduces:

      • Nonunion risk (OR = 0.53)

      • Femoral neck shortening

    • Better long-term outcomes despite higher cost

      2.3 When to Consider Crossed Fully Threaded Screws

      • Limited budget + Pauwels Type III fracture
        → Use crossed fully threaded screws (Alpha technique)
        → A cost-effective alternative to FNS

      • Patients requiring strict length preservation (e.g., athletes)
        → Fully threaded screws are optimal

3. Evidence-Based Conclusions (Meta-Analyses 2023–2024)

• FNS vs CCS

  • Higher union rate

  • Better resistance to shortening

  • No increased risk of AVN

• Fully threaded vs partially threaded screws

  • Less femoral neck shortening

  • No increased nonunion risk

  • Lower fixation failure rate

4. Quick Summary

• Stable fractures + good biomechanics → Cannulated Screws

• Unstable fractures or young patients → FNS

• Limited budget but need stability → Crossed Fully Threaded Screw

5. Common Misconceptions Clarified

Misconception 1: “Partially threaded screws are more physiological”

Truth:
Only valid for compression-dominant fractures (Pauwels I).
In shear-dominant fractures, they worsen instability.

Misconception 2: “Larger implants damage blood supply”

Truth:
Clinical data show FNS reduces AVN risk (OR = 0.49).
Stability matters more than implant size.

Misconception 3: “Crossed screws are too complex”

Truth:
The Alpha technique is relatively easy to learn and highly effective.

Misconception 4: “FNS advantages are just marketing”

Truth:
Meta-analysis (>1300 patients, 21 studies) confirms real clinical benefits.

Misconception 5: “All fixation methods are equal in elderly patients”

Truth:

• Displaced fractures → arthroplasty preferred

• Non-displaced fractures → crossed fixation shows advantages

6. Clinical Insight

In femoral neck fracture management, measuring the Pauwels angle is as critical as measuring displacement in pelvic fractures—it directly determines fixation success.

20th century breakthrough: mastering fracture compression
21st century advancement: knowing when NOT to allow sliding

7. Core Treatment Principles

1. Respect biomechanics
   → Vertical fractures require shear-resistant constructs

2. Respect biology
   → Stability enables revascularization of the femoral head

3. Respect clinical reality
   → Adapt strategies based on skill level and cost constraints

Final Thought

There is no universally “best” fixation method—only the most appropriate tool for a specific biomechanical environment.

Choosing the right implant is not just a technical decision—it is a core competency of modern orthopedic surgeons.