Polar screw in intramedulary nailing

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I now have comprehensive information to answer the question. The term "polar screw" is another name for the Poller screw (also called a blocking screw) in intramedullary nailing. Let me compose a thorough answer.

Polar / Poller (Blocking) Screw in Intramedullary Nailing

Terminology

The polar screw is the same concept as the Poller screw or blocking screw — the three names are used interchangeably in orthopedic literature. The term "Poller" comes from the German word for a bollard (a post used to direct traffic), which captures exactly what the screw does mechanically. "Polar" is a phonetic/regional variant of the same term.

Concept & Rationale

In metadiaphyseal fractures, the medullary canal is flared and wide proximally and distally. An IM nail in this region has excessive room to shift, which allows the fracture to malalign. The nail tends to follow the path of least resistance — often toward the deformity — rather than remaining centered in the medullary canal.

"Fractures that occur in the metadiaphyseal region are not intrinsically stable with the use of an intramedullary nail... The most frequently used technique is the application of a blocking or Poller screw." — Rockwood and Green's Fractures in Adults, 10th ed.

Mechanism of Action

The screw acts as an artificial cortex — an interference device that narrows the effective medullary diameter and directs the nail into the correct alignment:

Placed on the concave side of the deformity (where you do NOT want the nail to go)
When the nail encounters the screw, it pushes the bone segment in the direction needed to correct the deformity
Conceptually: "Place the screw where the nail should NOT be"

Two functional modes:

Mode	Timing	Purpose
Guiding	Before/during nail insertion	Directs the reamer and nail trajectory to prevent malalignment
Containment	After nail is in place	Narrows the medullary space to prevent the bone from toggling around the nail

Indications

Proximal tibial fractures — First described by Krettek et al. for the tibia. Common deformity is valgus and apex-anterior; blocking screw placed posterior and lateral in the metaphyseal segment.
Distal femur fractures — Retrograde nailing; blocking screws placed on either side to create a "pseudocanal."
Proximal femur / subtrochanteric fractures
Any periarticular long-bone fracture treated with an IM nail where the nail sits in a wide flared segment.

Technical Details

Can be placed before reaming (to guide the trajectory of the reamer and nail) or after nail insertion (as containment screws)
Placed relatively close to the fracture to maximally affect alignment
K-wire or drill bit can be used temporarily in the same role before committing to a definitive screw
On both medial and lateral sides simultaneously → creates a pseudocanal for extreme periarticular nailing

Classic example — proximal tibia valgus deformity:

Deformity: valgus + apex anterior
Blocking screw: placed posterior AND lateral in the proximal segment
This forces the nail to stay anterior and medial → corrects both planes

Classic example — distal femur varus deformity (retrograde nail):

Blocking screw placed on the medial side of the nail in the distal fragment
Acts as a medial "wall" → corrects varus

Biomechanical Effect

Effectively reduces the working length of the nail in the wide segment
Prevents toggle in the coronal plane (adding a second interlocking screw prevents sagittal rotation around a single transverse screw)
Together with multiple interlocking screws, significantly improves construct stability in metaphyseal regions

Summary

The polar/Poller/blocking screw is a simple but powerful adjunct to IM nailing in metaphyseal fractures. It converts an unstable wide-canal situation into a mechanically controlled one by acting as an artificial cortex — directing the nail trajectory and preventing postoperative malalignment. The key rule: place it on the concavity of the deformity / where the nail should not go.

— Rockwood and Green's Fractures in Adults, 10th ed. — Miller's Review of Orthopaedics, 9th ed.

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