The Hip: Preservation, Replacement and Revision

Classification of Patellar Fractures




- Discussion:
    - 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 
         contracted quadriceps.
    - 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.

- Osteochondral Frx:

- 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
         marginal fractures.
    - 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



Original Text by Clifford R. Wheeless, III, MD.

Last updated by Data Trace Staff on Friday, September 9, 2011 9:14 am