AO/OTA 33 — Distal Femur (Supracondylar)

Overview — Distal Femur Fractures

Distal femur fractures (supracondylar and intercondylar fractures) present in a bimodal distribution: high-energy injuries in young adults (road traffic accidents, sports) and low-energy fractures in osteoporotic elderly patients (particularly those with periprosthetic fractures around a knee arthroplasty). They represent approximately 4–7% of all femoral fractures and carry significant morbidity from stiffness, malunion, non-union, and post-traumatic arthritis. The AO/OTA classification using code 33 (3 = femur; 3 = distal segment) is the standard classification for distal femur fractures, dividing them into extra-articular (33-A), partial articular (33-B), and complete articular (33-C) types — providing a comprehensive and reproducible description that guides surgical planning.

Anatomy and deforming forces: the distal femoral metaphysis is bounded by the medial and lateral femoral condyles; the knee joint is formed by the femoral condyles articulating with the tibial plateau and the patella (trochlear groove); the anatomical axis of the distal femur is in approximately 6° of valgus relative to the mechanical axis; distal femoral fractures produce characteristic deformity — the distal fragment is displaced posteriorly (by the pull of the gastrocnemius, which originates from the posterior condyles) and rotated into flexion; the popliteal artery runs closely posterior to the distal femur and is at risk in displaced distal femoral fractures
Why distal femur fractures are challenging: (1) the articular surface involves the weight-bearing condyles — even small articular step-offs (>2 mm) accelerate post-traumatic arthritis; (2) the distal fragment is short — fixation in the distal fragment is inherently limited by the small amount of bone available for screw purchase; (3) the surrounding soft tissues (popliteal fossa posteriorly) limit the surgical approaches; (4) the bone is often osteoporotic in elderly patients — standard screws lose purchase; (5) the deforming forces (gastrocnemius flexion of the distal fragment) must be counteracted during fixation; locking plate technology has revolutionised distal femoral fracture fixation by providing angular stability in osteoporotic bone through locking screw-plate interfaces

AO/OTA 33 Classification

AO Type	Description	Subtypes	Key Features	Treatment
33-A — Extra-articular	Fractures of the DISTAL FEMORAL METAPHYSIS that do NOT involve the articular surface; the fracture is in the metaphysis between the shaft and the condyles; the condylar articular surface is intact; these are supracondylar (extra-articular) fractures only	A1 = Simple metaphyseal fracture (single fracture line in the distal metaphysis — transverse, oblique, or short spiral; no comminution); A2 = Metaphyseal fracture with butterfly fragment (a third fragment in the metaphysis — butterfly or wedge pattern); A3 = Comminuted metaphyseal fracture (multiple metaphyseal fragments without articular involvement; the most complex A-type)	The articular surface is intact; the fracture is `above` the condyles; the condyles are intact as a unit but displaced from the shaft; the distal fragment (condyles + metaphyseal fragment) is typically displaced posteriorly and into flexion by the gastrocnemius	Distal femoral locking plate (DFLP) or a retrograde femoral IM nail (retrograde nailing — from the knee into the distal femur); DFLP is preferred when the fracture is very distal (too short for a retrograde nail) or when there is metaphyseal comminution; retrograde IM nail is useful for periprosthetic fractures around a hip arthroplasty stem (when an antegrade nail cannot bypass the stem)
33-B — Partial articular (unicondylar)	Fractures involving PART of the articular surface — only ONE condyle is fractured; the other condyle AND the metaphysis are intact; the fracture separates one condyle from the intact femur; this is the Hoffa fracture type (coronal plane fracture of the femoral condyle)	B1 = Lateral condyle sagittal fracture (vertical split of the lateral condyle — a coronal plane shear fracture; the `Hoffa fracture` pattern); B2 = Medial condyle sagittal fracture (vertical split of the medial condyle — medial Hoffa fracture); B3 = Coronal plane fracture of the posterior condyle (the Hoffa fracture proper — a coronal plane fracture shearing off the posterior portion of a condyle; the fragment includes the posterior articular surface; ONLY visible on the LATERAL X-ray — invisible on the AP view)	B3 (Hoffa fracture): the posterior condyle shears off in the coronal plane from a high-energy valgus or direct impact; INVISIBLE on AP X-ray (the fragment is posterior — the AP view sees only the end-on view of the fracture line); VISIBLE on the lateral X-ray and CT; missed if lateral X-ray is not carefully assessed; CT is mandatory for all suspected Hoffa fractures; the fragment carries the posterior articular surface (posterior weight-bearing zone) and MUST be fixed to restore articular surface	ORIF is mandatory for all displaced 33-B fractures (articular surface involvement); the articular surface must be anatomically restored (<2 mm step-off); Hoffa fractures (B3) — approach: lateral (for lateral Hoffa) or medial (for medial Hoffa) or combined; lag screws from posterior to anterior (through the condyle from posterior to anterior — the screw head must be countersunk below the posterior articular surface); the posterior condyle fragment must be rigidly fixed to prevent further shearing under load
33-C — Complete articular (intercondylar)	Fractures involving the ENTIRE articular surface — BOTH condyles are fractured and separated from each other AND from the femoral shaft; the intercondylar fracture splits the condyles apart AND there is a metaphyseal fracture separating the condylar block from the shaft; the hallmark of 33-C is the intercondylar split (`Y` or `T` fracture pattern) — both condyles are separated from each other AND from the shaft	C1 = Simple articular + simple metaphyseal (intercondylar split + simple metaphyseal fracture — no comminution; the `pure Y-fracture`); C2 = Simple articular + complex metaphyseal (intercondylar split + comminuted metaphyseal zone); C3 = Complex articular + complex metaphyseal (comminuted intercondylar split + comminuted metaphyseal zone — the most severe pattern; the condyles are themselves comminuted)	The intercondylar split is the diagnostic hallmark of 33-C — it is visible as a `Y` or `T` shape on the AP X-ray; the condyles are pulled apart medially and laterally by the collateral ligaments; the individual condyles may also be rotated around their long axes by the attached collateral ligaments; CT is mandatory to characterise the articular comminution and plan fixation (how many condylar screws are needed and where the plate should be positioned)	ORIF for most 33-C fractures; the standard sequence: (1) reduce and fix the intercondylar split FIRST (restoring the single condylar block with a lag screw from medial to lateral or lateral to medial — this `provisionally` restores the articular surface); (2) then fix the condylar block to the shaft with a distal femoral locking plate (DFLP) or retrograde IM nail; the sequence is critical — the articular reduction is performed first before the shaft alignment is addressed; primary TKA or distal femoral replacement (megaprosthesis) for severe comminuted C3 fractures in elderly osteoporotic patients with pre-existing knee OA

The Hoffa Fracture (33-B3) — Detail

The Hoffa fracture: a coronal plane shear fracture of the posterior femoral condyle (lateral or medial); the posterior condyle is sheared off in the coronal plane; the fragment includes the posterior articular surface; CRITICAL: the Hoffa fracture is INVISIBLE on the AP X-ray and may only be seen on the LATERAL view (where the posterior fragment is visible overlapping the posterior condyle); CT is mandatory for ALL patients with distal femur injuries where a Hoffa fracture is suspected; frequency: lateral condyle Hoffa fractures are more common (70%) than medial; the Hoffa fracture is associated with AO 33-C fractures (the intercondylar fracture may have a posterior Hoffa component in addition to the sagittal condylar split — creating a `trifragmentary` pattern)
Fixation of Hoffa fractures: screws placed from posterior to anterior (posterior approach or mini-open posterior approach) through the condyle; the screw head is countersunk below the posterior articular surface; alternatively, posterior-to-anterior screws can be placed percutaneously under fluoroscopic guidance; anti-glide plates placed on the posterior condyle surface are an alternative for very comminuted fragments; the Hoffa fragment must be RIGIDLY fixed — any motion at the Hoffa fragment creates further shear forces and risks non-union

Fixation Options — Locking Plate vs Retrograde Nail

Fixation	Advantages	Disadvantages	Best Indications
Distal Femoral Locking Plate (DFLP)	Angular stability from locking screws (essential in osteoporotic bone); multiple distal locking screws possible even in very short distal fragments; can address articular comminution directly (medial screw for intercondylar split); allows simultaneous management of shaft and articular components through one construct	More invasive than retrograde nail; higher wound complication rate; requires adequate distal bone for locking screw purchase (min 2 cm of distal bone needed for secure locking); non-union rate with DFLP is ~10–15%	Complex articular fractures (33-C); very distal metaphyseal fractures (too short for nail); periprosthetic fractures with a knee arthroplasty in situ (nail may not be possible); articular comminution requiring multiple fixation screws
Retrograde Femoral IM Nail	Less invasive (percutaneous); preserves periosteal blood supply; load-sharing (biomechanically superior to plate bridging in shaft comminution); lower wound complication rate; faster healing in some series	Limited distal fixation (only 2–3 distal locking screws possible); not suitable for complex articular fractures (cannot directly fix the intercondylar split or Hoffa fragment); not suitable for very short distal fragments (<2 cm); requires the patellofemoral joint to be intact (nail enters through the intercondylar notch)	Simple metaphyseal fractures (33-A); periprosthetic femur fractures around a total hip arthroplasty stem (antegrade nail cannot be used); bilateral femoral fractures (reduces blood loss vs bilateral plating); selected 33-C1 fractures after intercondylar lag screw fixation

Exam Pearls

AO 33: A = extra-articular (supracondylar, no condylar involvement; A1 simple, A2 wedge, A3 comminuted); B = partial articular (unicondylar; B1 lateral condyle, B2 medial condyle, B3 Hoffa posterior coronal); C = complete articular (bicondylar + shaft; C1 simple, C2 comminuted metaphysis, C3 comminuted articular + metaphysis)
Hoffa fracture (33-B3): posterior coronal plane condyle fracture; INVISIBLE on AP X-ray — only seen on lateral view; CT mandatory; fixed with posterior-to-anterior lag screws countersunk below the articular surface; most commonly lateral condyle; associated with 33-C fractures
33-C surgical sequence: fix intercondylar split FIRST (lag screw medial to lateral) to create a single condylar block → THEN fix condylar block to shaft with DFLP or retrograde nail; the articular surface must be restored before shaft alignment is addressed
Gastrocnemius deformity: pulls distal fragment posteriorly + into flexion; must overcome this deformity at reduction; the knee is placed in flexion during nailing/plating to relax the gastrocnemius; reduction aids include a bump under the distal thigh and a bump under the femoral shaft
Locking plate vs retrograde nail: DFLP = complex articular, very distal, osteoporotic; retrograde nail = simpler fractures, periprosthetic around THA, bilateral fractures; locking plates provide angular stability through locking screw-plate interface (essential in osteoporotic bone where standard cortical screws lose purchase)
Popliteal artery: runs posterior to the distal femur; at risk in displaced distal femoral fractures, particularly in posterior displacement; assess distal pulses and ABI in all distal femur fractures; any absent or asymmetric pulse = urgent CT angiography
Periprosthetic distal femur fractures (around TKA): the distal femur is weakest at the anterior femoral notch of the femoral component; fractures occur here in low-energy falls in the elderly; managed with DFLP (above the prosthesis — multiple distal condylar locking screws above the femoral component) or retrograde nail through the femoral component intercondylar box (if the box size is adequate); distal femoral replacement (megaprosthesis) for highly comminuted fractures with poor distal bone stock

AO/OTA 33 — Distal Femur (Supracondylar)

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