Z Effect in Proximal Femoral Nail (PFN)

Background

Z Effect

• The fracture undergoes varus collapse and the proximal fragment tilts
• With cyclic loading, the inferior lag screw (which can slide freely within the nail) migrates laterally because it has poor bony purchase in the neck
• This transfers all load to the superior locked anti-rotation screw, which is forced medially through the femoral head
• Ultimately, the superior screw can cut out medially into the acetabulum — catastrophic failure

Reverse Z Effect

Radiographic Appearances

Z effect: Superior screw migrates medially (cutting into femoral head), inferior lag screw migrates laterally (backing out)

Reverse Z effect (image b): Superior lag screw migrated laterally (backed out) 6 months post-op, with maintained fracture union

Summary Table

Risk Factors

• Unstable intertrochanteric fractures (reverse oblique, comminuted, lack of medial cortical support)
• Poor bone quality / osteoporosis
• Inadequate fracture reduction before nailing
• Nail toggling within the femoral canal (toggling = nail not snugly seated)
• Lack of bony purchase in the femoral neck for the inferior lag screw
• High tip-apex distance (TAD > 25 mm)

Prevention

1. Adequate fracture reduction before nail insertion — restoring medial cortical support minimizes toggle
2. Proper screw placement — center-center position in the femoral head, TAD < 25 mm
3. Use of single-screw designs (e.g., PFN-A with a helical blade, or PFNA) — eliminates the two-screw differential migration problem entirely; the blade provides rotational stability without a second screw
4. Ensure inferior screw has good bony purchase in the femoral neck
5. Use of long nails for fractures with distal extension to minimize nail toggling

Treatment

Clinical Significance

Proximal Femoral Bionic Nail (PFBN)

Background & Concept

Theoretical Basis — Zhang's N-Triangle Theory

• Ward's triangle identifies the area of least bone density in the femoral neck, bounded by tensile and compressive trabecular systems
• Zhang's N-triangle theory extends this by proposing that internal fixation of the proximal femur must simultaneously address both tensile and compressive stress trajectories to restore physiological load balance
• Multiple overlapping stable triangular areas are formed by the fixation construct, mimicking the natural trabecular architecture — hence the term "bionic"
• The theory also addresses the lever arm problem: in conventional nails, the lever point of the construct is far from the anatomical fulcrum, increasing the moment arm and shear stress at the fracture. The PFBN's triangular support structure reconstructs the lever point closer to the anatomical support point, shortening the moment arm

Design & Components

Panel A: PFBN implant and screw components. Panels B–D: Radiographic configurations of PFBN and comparative fixation constructs

• PFN — two parallel screws (prone to Z effect)
• PFNA — single helical blade (no cross-support)
• InterTAN — two interlocking screws but no trochanteric lateral support arm

Biomechanical Advantages

1. Triangular support structure — the cross-screw design forms stable triangular fixation areas, distributing both compressive and tensile loads along the trabecular lines
2. Reduced stress concentration — FEA studies show 9.36%–59.32% reduction in maximum von Mises stress at the implant compared to PFNA, InterTAN, and Gamma nail (PMID 38938983)
3. Greater trochanter anchoring — the lateral support screw locks into the greater trochanter, providing a lateral buttress against varus collapse
4. Shorter effective moment arm — the construct's lever point is closer to the anatomical fulcrum of the hip joint
5. No Z effect — the cross-locked screw design prevents differential independent migration of the two screws (unlike classic PFN)
6. Improved fracture interface stability — FEA shows lower fracture interface displacement (0.21 mm) vs PFNA (0.24 mm) and DHS+DS (0.25 mm)

Indications

• Unstable intertrochanteric fractures (AO/OTA 31-A2 and 31-A3)
• Reverse obliquity intertrochanteric fractures (31-A3) — where conventional nails perform poorly
• Subtrochanteric fractures — biomechanical studies support its use
• Basicervical femoral neck fractures — finite element analysis demonstrates superiority (PMID 38053203)
• Particularly suited to elderly, osteoporotic bone where implant failure rates are high

Clinical Evidence

Meta-Analyses (2025)

• PFBN: significantly shorter fracture healing time and earlier weight-bearing vs both comparators
• PFBN: longer surgery duration than PFNA (no difference vs InterTAN)
• No significant difference in blood loss, hospital stay, Harris Hip Score, or complication rates
• GRADE: moderate to high certainty evidence

Original Clinical Study

• Mean op time: 90 min (A2: 83 min, A3: 105 min)
• Harris Hip Score at 12 months: 82.6 ± 4.6
• No major implant failures reported

Advantages vs Conventional Nails

Limitations & Disadvantages

• Longer operative time — the cross-screw assembly is technically more demanding
• Slightly more blood loss intraoperatively vs PFNA
• Relatively new implant — limited long-term follow-up data; most studies are retrospective, single-centre, from China
• Learning curve — assembly of the interlocking cross-screw mechanism is unfamiliar to most surgeons trained on PFNA
• Limited availability outside China currently

Summary