Rod

General

AIS surgery differs substantially from adult deformity surgery, meaning desirable rod biomechanical properties may also differ.

The theory of "stress shielding" (where high-stiffness rods decrease physiological stress on bone, potentially leading to resorption) may apply to elderly populations, but has not shown clinical relevance in AIS surgery.

Stiffer constructs, minimally weakened by intraoperative contouring, likely best achieve primary objectives of curve correction and sagittal restoration.

This may explain the increasing popularity of CoCr and higher-diameter rods.

High-quality studies are needed to draw firm conclusions.

Biomechanical Properties

The key biomechanical properties of spinal rods are:

Yield strength (or yield stress)

is defined as the stress point at which permanent deformation occurs.

Stiffness (or rigidity)

is the extent to which a rod resists deformation when a force is applied.
Influenced by the

Material's Young’s modulus of elasticity
Rod's diameter.

An increase in rod radius significantly increases stiffness (to the 4th power of the change in radius).

For example, changing from a 5.5 mm to a 6.35 mm diameter rod increases bending stiffness from 5.17 EI (Nm²) to 9.18 EI (Nm²).
Larger the rod diameter the better.

Larger rod diameters do not always guarantee better outcomes;

A study comparing 5.5 mm and 6.35 mm rods found no significant difference in coronal or sagittal plane correction.

Excessive stiffness can lead to implant pullout or failure by exceeding the strength of the bone–implant interface.

Surgeons must balance rod stiffness with deformity characteristics and bone quality.

Note:

It is crucial to understand that laboratory measurements of these properties may not directly translate to clinical reality.
Additionally, rod properties can vary significantly between manufacturers, even for the same material.

Rod Material

Common rod materials used

Stainless Steel (SS) and Ultrahigh Strength Stainless Steel (UHSS):

Have higher stiffness than Ti but generally lower yield strength than Ti.
SS and UHSS tend to show more MRI artefacts than Ti or CoCr.
Ideal for complex deformity corrections.
High stiffness can pose challenges in osteopenic patients due to increased stress at anchor points.

Titanium alloy (Ti)

Generally characterised by high yield strength but lower stiffness compared to SS, UHSS, and CoCr.
Pros

high biocompatibility,
corrosion resistance,
MRI compatibility

Showing the least amount of artefacts.

Suitable for patients with poorer bone quality.
May potentially lower surgical site infection (SSI) rates by reducing bacterial glycocalyx formation.

Cons

High flexibiity reduces it use as deformity correction rod.

Used frequently as support rod after the deformity has been corrected

Cobalt chromium (CoCr)

Recently introduced,
Very high stiffness and relatively lower yield strength compared to Ti,
but a higher yield strength than SS.
It shows less MRI artefacts than SS but more than Ti.
Studies found CoCr rods provided significantly better thoracic kyphosis correction than titanium (Ti) rods in both short-term (0–3 months) and long-term (≥24 months) follow-ups.

Biomaterial	Elastic modulus (Young’s modulus) (GPa)	Yield strength (MPa)	Fatigue strength (MPa)
CoCr alloys	200–300	300–2,000	207–950
Titanium alloys	110–116	485–1,034	300–389
Stainless steel	190	792	241–820

Comparison of Corrective Forces and Deformation:

Serhan et al.

CoCr and UHSS provided 42% higher corrective forces than Ti due to their higher stiffness.
However, 90% of Ti rods retained their original shape after being removed from a synthetic spine model, compared to 77% for UHSS, 63% for SS, and 54% for CoCr, indicating Ti's higher yield strength.

Lamerain et al.

found CoCr rods resulted in significantly better coronal curve correction, reduced loss of correction, and better kyphosis restoration compared to SS rods in AIS patients.

Angelliaume et al.

reported similar coronal correction for Ti and CoCr but better kyphosis restoration with CoCr.

Effect of Contouring (Notch Effect):

Intraoperative bending of rods can introduce cracks or dents, reducing the rod's endurance limit.
Slivka et al.

CoCr's endurance limit to be at least 25% higher than UHSS, SS, or Ti in repetitive bending.

Noshchenko et al.

Highest "springback" (yield strength) in Ti rods.

Wedemeyer et al.

Ti could withstand higher strains than SS before failure.

Burger et al.

Ti rods lost correction at 6° per year, significantly more than SS rods, when stored to mimic physiological conditions.

Rod profile

Clinical Study Findings on Diameter:

Huang et al. and Liu et al. found no difference in coronal curve correction between 5.5-mm and 6.35-mm Ti rods.
However, Liu et al. and Abul-Kasim reported significantly better kyphosis restoration with 6.35-mm rods.
Fletcher et al. found a higher percentage of patients had normal kyphosis with 6.35-mm rods (72%) compared to 5.5-mm rods (47%).

Noncircular Rods:

Cui et al. found that a square cross-sectional rod increased axial stiffness by about 2.5% and reduced maximum stress by up to 22% compared to a circular rod of the same cross-sectional area.
Gehrchen et al. reported 9% higher coronal curve correction with "beam-like" (noncircular) rods compared to circular rods in AIS patients, with no significant difference in kyphosis restoration.

Hybrid Rods or Constructs

Iliac accessory rod technique-Berlin 2025

Aim

To increase the construct's strength and rigidity and reduce the rate of instrumentation failure, specifically rod fracture.

Principles

The accessory rods provide additional iliac fixation points, which theoretically leads to a greater dispersal of stress loads across the lumbosacral junction, meaning there are 3 or 4 fixation points instead of just 2.

Target Reinforcement at osteotomy levels, as these areas are known to have higher rod strain.

Technique

Standard Fixation of bilateral thoracolumbar and S1 pedicle screws at the required levels.

Iliac Fixation (Quad-Iliac Strategy): two iliac screws on each side.

Patient anatomy may sometimes only allow for three instead of four iliac screws.
To accommodate both iliac screws, a larger iliac crest tricortical wedge resection is performed.
Screw Placement:

The first iliac screw is placed using standard technique, with its position confirmed via obturator outlet (teardrop) and obturator inlet (sciatic notch) fluoroscopic views.
The second screw is placed either rostral or caudal to the first, following a similar trajectory.
S2 alar iliac screws may be substituted for traditional iliac screws, though a true iliac trajectory for better accessory rod spacing.

Primary Rod Placement:

A lateral connector of sufficient length is placed on the more rostral iliac screw to line up with the respective S1 screw tulip.
The primary rod on each side is then connected from this lateral connector to the top of the construct.
All compression and distraction maneuvers are completed, and all screw caps are fully tightened on the primary construct.

Accessory Rod Preparation and Placement:

Additional lateral connectors are placed in the caudal iliac screws. These connectors must be long enough to reach either medial or lateral to the primary rod.
The accessory rods are then placed on each side, spanning from these lateral connectors rostrally to the upper lumbar/lower thoracic spine.

Rod positioned medial to the primary rod if possible.

To allow for greater space for grafting materials posterolaterally.

The accessory rods are secured to their respective primary rods via side-to-side connectors.
Anchoring Points: Each accessory rod anchors caudally to an independent iliac bolt via a lateral connector, and attaches rostrally to the primary rod via a side-to-side connector.
Rostral connection: Variable but typically extends to the T12 to L1 level

Principle:

Crosses the thoracolumbar junction
Remains at least two vertebral levels distal to the Uppermost Instrumented Vertebra (UIV) to avoid increasing rigidity at the top of the construct.

Rod Material and Diameter: Cobalt chromium of 6.0-mm-diameter rods.
Laterality: The accessory rods were placed bilaterally in the majority of patients (86.6%).

Cons

The addition of the extra iliac bolts requires approximately 10 minutes, and the addition of both accessory rods adds an estimated 10–15 minutes to the total operative time.

Proximal Junctional Kyphosis (PJK)

Some studies suggest that gradually reducing stress at the proximal level (Soft landing) of the construct could lower PJK rates.

Lange et al. found that cerclage wires at the proximal segment reduced rigidity by about 60% compared to all-pedicle screw constructs.
Facchinello et al. and Thawrani et al. reported lower stiffness at the upper instrumented level with proximal hooks, reducing force on anchors.
Cahill et al. suggested that a transition rod with a proximal decrease in diameter could reduce disc angulation and implant stress.
Ohrt-Nissen et al. showed that double transition rods improved kyphosis restoration in AIS surgery, though its effect on PJK rates is not yet known.

Shape-Memory Metal (SMM) Rods

Some argue that the stiffness of implants is not always clinically relevant and advocate for dynamic flexible rods to reduce peak stress and prevent PJK or adjacent disc degeneration.

Nitinol (nickel-Ti) is an SMM characterised by its ability to recover from significant deformation and return to a preconditioned shape when heated (e.g., to body temperature).

This property allows SMM rods to apply a progressive and constant corrective force, counteracting the low viscoelasticity of tissue responsible for rod flattening.

Clinical studies have shown satisfactory results:

Wang et al. used SMM rods temporarily to correct deformity before replacement with rigid rods.
A recent randomised clinical trial found no difference in coronal or sagittal parameters at 5-year follow-up between SMM rods and standard rods, concluding SMM rods were safe and efficient for AIS surgery.