Patella — Pattern-Based (AO/OTA 34)

Overview — Patella Fractures

The patella is the largest sesamoid bone in the body, embedded within the extensor mechanism of the knee. It functions to increase the mechanical advantage of the quadriceps muscle by displacing the patellar tendon further from the axis of knee rotation — increasing the lever arm and thus the extension force. Patella fractures account for approximately 1% of all skeletal injuries and result from two main mechanisms: direct impact (dashboard injury, fall onto the knee) and indirect avulsion (eccentric quadriceps contraction — most commonly when landing or stumbling). The AO/OTA classification (code 34 — patella) provides the most comprehensive description of patellar fracture patterns, while clinical decision-making rests primarily on the assessment of extensor mechanism integrity and fracture displacement.

The extensor mechanism: the key functional unit; consists of the quadriceps femoris muscle → quadriceps tendon → patella → patellar tendon → tibial tuberosity; ANY disruption of the extensor mechanism produces the same clinical sign: inability to perform a straight-leg raise against gravity; assessment of the extensor mechanism is the FIRST and MOST IMPORTANT step in managing a suspected patella fracture; if the patient can perform a straight-leg raise (SLR) → extensor mechanism intact → non-operative management is considered; if the patient CANNOT perform a SLR → extensor mechanism disrupted → surgical repair is indicated
Patellar vascular supply: the patella has a rich blood supply entering from multiple directions (anastomotic ring from the geniculate arteries — superior, inferior, medial, and lateral geniculate arteries — providing a complete peri-patellar anastomosis); this rich supply means isolated patellar fractures rarely cause avascular necrosis; however, disruption of the posterior periosteal blood supply (from over-aggressive periosteal stripping during ORIF) can occasionally cause osteonecrosis of a pole fragment

AO/OTA 34 Classification

AO Type	Description	Subtypes / Pattern	Extensor Disruption Risk	Treatment
34-A — Extra-articular (peripheral)	Fractures at the PERIPHERY of the patella that do NOT involve the main articular surface; includes marginal fractures, avulsions from the patellar poles (superior or inferior pole avulsions), and sleeve fractures in children	A1 = Avulsion fracture (small peripheral bony fragment — superior pole from quadriceps tendon; inferior pole from patellar tendon); A2 = Vertical marginal fracture (a longitudinal split at the lateral margin — from a direct blow that does not involve the main articular weight-bearing surface); A3 = Comminuted peripheral fracture (peripheral comminution without central articular involvement); the inferior pole avulsion fracture (`bipartite patella` variant in adolescents) is a specific 34-A injury	Variable — inferior pole avulsions (from patellar tendon traction) disrupt the extensor mechanism; superior pole avulsions (from quadriceps traction) also disrupt it; lateral marginal fractures (34-A2) typically do NOT disrupt the extensor mechanism	Assess SLR: if intact → non-operative; if disrupted (inferior/superior pole avulsion) → surgical repair (tension band wiring around 2 K-wires, or suture fixation of the pole avulsion to the patellar body); inferior pole avulsions in skeletally immature patients may represent a `sleeve fracture` (the cartilaginous inferior patella plus a small bony flake avulse with the patellar tendon) — these require ORIF
34-B — Partial articular (incomplete)	Fractures that involve PART of the articular surface but the patella is NOT completely transected; includes vertical fractures that split the patella longitudinally without separating it transversely, and osteochondral fractures from acute patellar dislocation	B1 = Vertical fracture without separation (a sagittal split through the patella; the retinaculum on both sides is intact; the extensor mechanism is maintained); B2 = Osteochondral fracture (a shear fracture of articular cartilage ± subchondral bone from the medial patellar facet — classic in acute patellar dislocation as the patella relocates and the medial facet shears against the lateral femoral condyle; a loose osteochondral fragment is created); B3 = Partial articular with comminution	B1 (vertical) = usually intact extensor mechanism; B2 (osteochondral) = extensor mechanism intact but intra-articular loose body requires removal	B1 — non-operative if extensor intact and <3 mm displacement; ORIF if >3 mm displacement or articular step-off; B2 (osteochondral) — the loose fragment must be removed (arthroscopically in most cases) to prevent further cartilage damage; if the fragment is large and has viable bone, open fixation (pin or screw) is preferred to preserve articular surface; osteochondral fragments from patellar dislocation are a classic cause of haemarthrosis after acute dislocation
34-C — Complete articular (transverse/comminuted)	Fractures that COMPLETELY transect the patella or are highly comminuted; the entire patella is involved; these are the most common operatively significant patellar fractures — the classic `displaced transverse patellar fracture` is a 34-C injury; includes transverse fractures and stellate (star-shaped) comminuted fractures from direct impact	C1 = Transverse simple fracture (a complete transverse break of the patella — typically at the junction of the middle and distal thirds; MOST COMMON operatively treated patellar fracture pattern; the proximal and distal fragments are separated by the pull of the quadriceps proximally and the patellar tendon distally); C2 = Transverse + additional fragment (a transverse fracture with a third fragment — either a polar fragment or a fragment from one side); C3 = Highly comminuted (stellate or multi-fragmentary — multiple fragments radiating from the impact point; difficult to reconstruct)	Extensor mechanism is DISRUPTED for displaced transverse fractures; the proximal quadriceps pulls the proximal fragment superiorly while the patellar tendon holds the distal fragment — the two fragments separate, tearing the retinaculum on both sides	ORIF for displaced (>2–3 mm) 34-C fractures; tension band wiring (TBW) — 2 parallel K-wires longitudinally through the patella (or 2 cannulated screws) + a figure-of-eight tension band wire circling the K-wires anteriorly; the TBW converts the tensile pull of the extensor mechanism into compressive force at the articular surface; or screw + tension band (modified tension band); highly comminuted fractures (C3) — partial or complete patellectomy is considered if reconstruction is not feasible; every attempt should be made to preserve as much patellar tissue as possible

Tension Band Wiring (TBW) — The Biomechanical Principle

The tension band principle: the tension band wiring technique is the biomechanical gold standard for patellar fracture fixation; the principle applies to any fracture that is distracted by eccentric tension forces: a WIRE (or suture) is placed on the TENSION side of the fracture (the anterior surface of the patella, since the quadriceps/patellar tendon create a posterior-compressive and anterior-tensile load during knee flexion); when the knee is flexed and the quadriceps contract, the anterior wire is placed under tension → this tension is transmitted to the articular surface as COMPRESSION; the tension-band acts as a dynamic fixation — the harder the muscle pulls, the tighter the fracture is compressed; the classic TBW uses 2 × 1.6 mm K-wires placed parallel through the patella longitudinally + a figure-of-eight 18-gauge wire looped around the K-wires anteriorly and through the quadriceps and patellar tendons
Post-operative care after TBW: the knee is allowed to begin ROM (range of motion) within 48 hours of surgery; early motion is important to convert the tension band into compression during flexion; K-wires frequently migrate proximally and cause skin irritation over the proximal patella (the most common complication requiring K-wire removal at 12 months); the K-wires and wire are typically removed electively once the fracture has healed (12–18 months)

Bipartite Patella vs Fracture

Bipartite patella: a developmental variant in which a secondary ossification centre at the superolateral pole of the patella fails to unite with the main patellar body; present in approximately 2–3% of the population; bilateral in 50% of cases; the accessory ossicle is smooth-bordered, rounded, and has corticated margins; it is located at the SUPEROLATERAL pole (the most consistent location); the superolateral position is pathognomonic — a fracture at the inferomedial or polar regions is a fracture, not a bipartite variant; clinically, bipartite patella is usually asymptomatic but may become painful (symptomatic bipartite patella) from repetitive stress in athletes; management of symptomatic bipartite: activity modification, physiotherapy, occasional surgical excision of the fragment or fusion if refractory
Distinguishing bipartite from fracture: bipartite — smooth corticated margins; superolateral location; bilateral (check contralateral knee); chronic appearance; no periosteal reaction; fracture — sharp edges; irregular margins; associated soft tissue swelling; acute history; tenderness; any location on the patella; MRI shows bone marrow oedema at a fracture but not typically at a bipartite variant (unless symptomatic)

Exam Pearls

AO 34: A = extra-articular/peripheral (avulsions, poles); B = partial articular (vertical split, osteochondral from patellar dislocation); C = complete articular (transverse — most common operative fracture; comminuted); assess extensor mechanism (SLR) first
Extensor mechanism = SLR test: if patient can straight-leg raise → mechanism intact → non-operative for undisplaced fractures; if cannot SLR → mechanism disrupted → ORIF (or TBW); the most important clinical test in patellar fracture assessment
Tension band wiring: anterior K-wires + figure-of-eight anterior tension band wire; converts tensile extensor force into compressive force at the articular surface; dynamic fixation — compression increases with knee flexion; early ROM is essential to activate the compression mechanism
34-B2 osteochondral fracture: from acute patellar dislocation; medial patellar facet shears against lateral femoral condyle during relocation; creates loose osteochondral fragment (haemarthrosis); arthroscopic removal or open fixation if large viable fragment
Bipartite patella: superolateral pole; smooth corticated margins; bilateral in 50%; 2–3% of population; distinguish from fracture by location (superolateral = bipartite; any other location = fracture), corticated margins, and bilateral appearance; MRI confirms no marrow oedema
K-wire migration: most common complication of TBW; K-wires migrate proximally through the quadriceps tendon, causing proximal knee pain and skin irritation; elective removal at 12 months after confirmed union
Sleeve fracture of the patella (children): a paediatric variant where the cartilaginous inferior pole of the patella (with a small bony flake) avulses with the patellar tendon; the bony component is small but the cartilaginous avulsion is large — the X-ray underestimates the size of the avulsion; MRI or arthrogram shows the true extent; ORIF is required to restore patellar height and extensor mechanism function

Patella — Pattern-Based (AO/OTA 34)

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