Patella Fractures

Patella fractures make up about 1% of all fractures. They test your understanding of the extensor mechanism, the tension band principle, and the decision between fixation and partial patellectomy. The examiner wants to hear you discuss biomechanics and early motion.

Who. The distribution is bimodal: young patients injured by high energy, and the elderly after falls. Males outnumber females 2:1, with a peak age of 20-50 years, and a direct blow often produces an isolated injury.

Mechanism. Three patterns of loading break the patella:

• Direct blow - a dashboard injury or a fall on the knee
• Indirect - a forceful eccentric quadriceps contraction, as in landing
• Combined - a direct blow with muscle contraction, the most common

The bone. The patella is the largest sesamoid in the body: triangular, apex inferior, 3-7cm long and 2-3cm thick. The articular surface has a larger lateral facet, a medial facet, and an odd facet on the far medial edge. It provides smooth gliding and carries the thickest articular cartilage in the body, 5-7mm at the centre.

The attachments. The quadriceps tendon inserts on the superior pole and the patellar tendon arises from the inferior pole. On either side run the retinacula: medially the VMO and MPFL, laterally the vastus lateralis and fibres of the iliotibial band.

The blood supply. The superior and inferior genicular arteries form an extraosseous anastomotic ring. Vessels enter mainly on the anterior surface and at the inferior pole, which is why the anterior surgical approach is safe.

What the patella does. Sitting in the extensor mechanism, it moves the extensor axis anteriorly and lengthens the lever arm, increasing the extensor moment by 30-50% and improving quadriceps efficiency. It also:

• Increases the contact area, distributing force and reducing patellofemoral stress; the contact area changes with flexion
• Covers the trochlea, protecting the femoral condyles from direct trauma
• Gains medial and lateral support from retinacular tension

Patellofemoral contact forces reach 3-7 times body weight during stair descent, the eccentric loading of stairs and slopes.

The tension band principle. When the quadriceps pulls, the anterior surface of the patella is the tension side, pulled apart, and the articular surface is the compression side. A wire placed anteriorly, on the tension side, converts those tensile forces into compressive forces at the articular surface during knee flexion, closing the fracture gap and promoting healing: the AO/Pauwels principle. Two consequences follow:

• Motion increases the compression, so the construct is dynamic and early range of motion is encouraged
• It works only if the cortex on the compression side, the articular cortex, is intact

By pattern. Each pattern carries its own treatment.

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By the AO/OTA system. The type follows the extent of articular involvement.

History. The questions that shape management:

• Mechanism - direct blow, fall, dashboard
• Ability to walk after the injury
• Knee swelling - a haemarthrosis if the fracture is intra-articular
• Previous knee surgery or patellar problems
• Occupation and sport, for the return to activity

Examination. A palpable gap at the patella indicates displacement, and a tense effusion is the haemarthrosis. Check the skin, as the fracture may be open, and whether the retinacula are intact: can the patient extend despite the fracture?

Radiographs. AP and lateral are first line, taken with the knee fully extended for accurate assessment:

• Lateral - the most important view: displacement (the gap between fragments), articular step-off at the patellofemoral joint, and sometimes the haemarthrosis as a joint effusion
• AP - the pattern (transverse, vertical or comminuted), the width of the patella, a bipartite patella, and osteochondral fragments after a dislocation
• Skyline (Merchant) - the patellofemoral articular surface, helpful for vertical fractures and chondral injury, though it may be difficult acutely because of pain

Bipartite patella. Present in 1-2% of the population, usually as a superolateral fragment. Rounded margins, no haemarthrosis and comparison with the other knee, where it is often present too, separate it from a fracture. It is occasionally symptomatic without trauma.

CT. Used when the radiographs leave doubt and to plan a reconstruction, with 3D reconstructions helpful for operative planning:

• Comminuted fracture - reconstruction versus partial patellectomy
• Articular depression - assesses step-off better than a radiograph
• Pre-operative planning - fragment size, number and position, and the integrity of the posterior cortex
• High-energy mechanism - an associated tibial plateau injury

Patellar height. "Restore patellar height" is a hollow instruction unless you can measure it, and it is the number that tells you whether a comminuted-pole repair, a partial patellectomy or a healed tendon has left a functionally normal extensor mechanism. Three classic lateral-radiograph ratios measure it:

• Insall-Salvati - patellar tendon length divided by patellar bone (pole-to-pole) length. Normal is roughly 0.8 to 1.2; a high ratio is patella alta, a low ratio patella baja (infera). It is independent of knee flexion angle, but because it uses the patella's own length as the denominator it is unreliable after a fracture that shortens the patella or after a partial patellectomy
• Blackburne-Peel and Caton-Deschamps - reference the articular surface to the tibial plateau instead of patellar length, so they are the better choice once the patella has been altered by injury or excision

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Using it in theatre. When you perform a partial patellectomy or repair a comminuted inferior pole, advance and repair the patellar tendon so the operated patella sits at the same height as the uninjured contralateral side, compared on a template or contralateral film. Over-tightening creates an iatrogenic patella baja, with stiffness, anterior knee pain and loss of flexion; under-tensioning leaves an extensor lag.

Using it afterwards. A new patella alta after fixation signals patellar tendon rupture or fixation pull-out. Progressive baja signals over-shortening or contracture.

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The decision. The extensor mechanism comes first, then the radiograph. Any one of these is an operative indication:

• Loss of active extension - a failed straight leg raise, regardless of displacement
• Displacement (gap) over 2-3mm
• Articular step over 2mm

Non-operative treatment is reserved for the fracture with displacement under 2mm, an articular step under 2mm and intact active extension, in a reliable patient who will attend for weekly radiographs initially, because secondary displacement occurs. Whatever the treatment, the aim is to restore the extensor mechanism and articular congruity and to avoid a step-off greater than 2mm.

Algorithm

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Hardware removal is the most common early complication of tension band wiring: up to 50% of patients require removal, mainly for K-wire prominence. Counsel them before surgery about a likely second operation. Screw fixation may have a lower removal rate.

Tension band fixation is designed for early active motion, and prolonged immobilisation leads to stiffness and poor outcomes.

Most patients achieve satisfactory outcomes with proper treatment, and modern fixation generally preserves extensor function well.

Prognostic factors. A simple transverse pattern, minimal articular comminution, an anatomic reduction, early range-of-motion rehabilitation and a compliant patient favour a good result. A comminuted pattern, significant articular damage, malreduction with a step-off over 2mm, prolonged immobilisation, and an open fracture or infection count against it.

The long term. Patellofemoral osteoarthritis is the main long-term problem, and total knee arthroplasty may eventually be required for severe post-traumatic arthritis.