Summary

The patella is the largest sesamoid bone in the body, functioning as a fulcrum to increase the mechanical advantage of the quadriceps extension by 30%. Fractures occur via Direct Trauma (Dashboard injury -> Comminuted) or Indirect Trauma (Violent contraction -> Transverse). The management hinges entirely on the integrity of the Extensor Mechanism, tested by the ability to perform an active Straight Leg Raise (SLR). If the SLR is absent, the retinaculum is torn, and surgery is mandatory. Transverse fractures make up the majority and are treated with Tension Band Wiring (TBW). [1,2,3]

Key Facts

• The Mechanic: The patella displaces the tendon anteriorly, lengthening the moment arm. Treating a patella fracture with a Patellectomy reduces extension power by 50% and is a salvage option only.
• Bipartite Patella: A normal variant (Saupe Type III - Superolateral) seen in 8% of the population. It is bilateral in 50% and has smooth, sclerotic margins. Do not fix it!
• Tension Band Principle: The logic of TBW is to convert the distractive force of the quadriceps into a compressive force at the fracture site during flexion. The wire creates a beam on the tension side (anterior), so when the muscle pulls, the fracture compresses.
• Sleeve Fracture: A unique paediatric injury where the cartilage "sleeve" avulses from the bone. X-rays may look normal or show a tiny fleck of bone ("Fleck Sign"). Missed sleeve fractures lead to permanent disability.

Clinical Pearls

"The SLR is Binary": Can they lift their heel off the bed with a straight knee? Yes = Intact (Non-op). No = Ruptured (Surgery). Pain inhibition can mimic failure—aspirate the hematoma and inject Lidocaine to be sure.

"The Open Joint": A superficial laceration over a fractured patella is an Open Joint until proven otherwise. The prepatellar bursa is thin. Use the Saline Load Test to check for communication.

"Hardware Pain": Warn patients that 50% will need the metalware removed after a year because it rubs on the skin when kneeling.

                  PATELLA FRACTURE
                         ↓
              OPEN FRACTURE / FLOATING KNEE?
             ┌────────────┴─────────────┐
            YES                        NO
             ↓                         ↓
      URGENT SURGERY/ABx     EXTENSOR MECHANISM INTACT?
                           (Straight Leg Raise Possible?)
                           ┌───────────┴────────────┐
                          YES                       NO
                   (Non-Displaced)             (Displaced/Disrupted)
                           ↓                        ↓
                     NON-OPERATIVE               SURGICAL
                   (Cylinder Cast)             (Fixation)
                           ↓                        ↓
                  6 WEEKS EXTENSION          FRACTURE PATTERN?
                                           ┌────────┴────────┐
                                      TRANSVERSE        COMMINUTED
                                          ↓                 ↓
                                     TENSION BAND     CERCLAGE WIRE /
                                       WIRING            SCREWS

Indications

• Intact Extensor Mechanism (SLR +).
• Displacement <3mm.
• Step-off <2mm.
• Vertical fractures (almost always stable).

Protocol

• Immobilization: Hinged Knee Brace locked in Extension (0°).
• Weight Bearing: Full weight bearing allowed (locked brace). The axial load actually compresses the joint (good).
• Progression: Start passive flexion 0-30° at 2 weeks. 0-90° at 6 weeks.
• Duration: 6-8 weeks until union.

Principle: Tension Band Wiring (TBW)

• Concept: Converts a distracting force into a compressive force.
• Mechanism: A figure-of-8 wire loop lies on the anterior (tension) surface. When the knee flexes and quads pull, the wire prevents distraction, forcing the posterior fracture surfaces to compress.
• Requirements: Intact anterior cortex. Transverse pattern.

Techniques

1. Modified TBW: 2 parallel 1.6mm K-wires + 18G Figure-of-8 wire. Standard of care.
2. Cannulated Screws + Wire: Modern variation. Screws replace K-wires. Stronger construct, less migration. The wire is passed through the hollow screws.
3. Cerclage Wiring: For comminuted fractures (Purse-string). Encircles the equator of the patella.
4. Partial Patellectomy: For comminuted polar fractures. Remove the fragments -> Reattach tendon to remaining bone (Krackow suture through bone tunnels).
5. Total Patellectomy: Salvage for shattered bone. Significant disability (loss of moment arm).

Hoshino et al (Biomechanics)

• Cannulated screws with tension band are biomechanically superior to K-wires. Less likely to back out.

Lazaro et al (Outcomes)

• Total patellectomy results in poor functional scores. Every effort should be made to save the patella (ORIF).

Saupe Classification (Bipartite Patella)

• Type I: Inferior pole (1%).
• Type II: Lateral margin (20%).
• Type III: Superolateral (75%).

The Injury

You have cracked your kneecap. It is like the pulley in a crane. Because it is broken, the rope (muscle) cannot lift the jib (your leg).

The Operation

We use a loop of wire to tie the bone fragments together. The clever part is that the tighter your muscle pulls, the tighter the bone is squeezed together, helping it heal.

Recovery

• Walking: You can walk immediately with the leg straight.
• Bending: We limit this for 6 weeks to protect the repair.
• The Wire: The wire is right under your skin. It often hurts when you kneel on it later. About half of patients ask us to take it out after a year. This is a small, simple operation.

Q1: Explain the Tension Band Principle. A: A tension band converts tensile force on the convex side of a curved bone (or the distraction side of a fracture) into compressive force at the fracture interface. This works because the fracture is eccentrically loaded. The implant must be placed on the Tension side (Anterior surface for Patella, Posterior surface for Olecranon).

Q2: What is the Saupe Classification? A: Classification of Bipartite Patella.

• Type I: Inferior.
• Type II: Lateral.
• Type III: Superolateral (Most common - 75%).

Q3: Describe a "Sleeve Fracture". A: A paediatric avulsion fracture where the thick articular cartilage sleeve is pulled off the bony nucleus. Because the fragment is mostly cartilage, it is invisible on X-ray (or shows a tiny "fleck" of bone). Diagnosis is clinical (Palpable gap + Loss of SLR). Management is urgent surgery to reattach the sleeve, otherwise the bone will not grow.

Q4: A patient has a "Floating Knee". What is this? A: Ipsilateral fracture of the femoral shaft and tibial shaft. The knee joint is "floating" between two breaks. It indicates high energy trauma and has high rates of associated vascular injury and fat embolism.

(End of Topic)