- classified according to both the mechanism of injury and morphology.
- two major mechanisms of injury:
- direct and indirect trauma;
- Direction Trauma:
- patella may be fractured by direct blow during fall onto knee or when it hits dashboard in an MVA;
- because of small amount of prepatellar soft tissue & direct contact with the distal aspect of femur posteriorly,
nearly all of force of direct blow is delivered to patella;
- such direct trauma frequently causes considerable comminution, but often there is little displacement of frx fragments.
- w/ certainty, articular cartilage of contact area is damaged by this mechanism of injury.
- Indirection Trauma:
- indirect trauma that causes fractures can be due to jumping or, more frequently, to unexpectedly rapid flexion of the knee against fully
- natural anatomy and biomechanics of knee, as previously described, create tension, three-point bending, and compressive strains in
patella that exceed values sufficient to cause a fracture.
- frx resulting from indirect injury tend to be < comminuted than those from direct trauma, but they are displaced and are often transverse.
- articular cartilage is less damaged than with direct trauma.
- Combined Mechanism:
- most patellar fractures occur as a result of a combination of direct and indirect trauma.
- rarely does anyone hit a dashboard w/ relaxed quadriceps.
- in addition, Thompson et al clearly demonstrated that direct blows to patella of magnitudes < those sufficient to cause patellar frx
predictably damage contacting articular cartilage of patella & femur & that early biochemical and histological changes after such blows
are consistent with the initiation of post-traumatic osteoarthrosis.
- Transverse Frx:
- fractures that occur in medial-lateral direction are called transverse.
- these fractures are usually in central or distal third of the patella.
- Vertical Fractures:
- are in superiorinferior direction, and they are rare.
- frx of edge of the patella that do not extend across patella and that are not associated with disruption of extensor mechanism are called
- displaced frxs are those w/ articular incongruity (step-off) of more than two mm or separation of fragments of more than 3 mm;
- frx w/ multiple fragments are called comminuted fractures;
- some comminuted fractures can be characterized as stellate fractures;
- some transverse frx also demonstrate comminution of one or both poles