Understanding the radiographic anatomy of the patella to avoid placement of intra-articular implants

To map the articular margins of the patella and quantify an "at-risk" zone for intra-articular implant placement during fixation of patella fractures.

This was an observational cadaveric study utilizing eight fresh frozen adult knee specimens. Radiopaque wire was applied to the articular margins of the medial and lateral patellar facets and the center apical ridge. Fluoroscopic images were obtained and examined to identify radiographic "at-risk" zones for intra-articular surgical implants.

Radiographically, the medial and lateral facets were deep to the central ridge on all specimens. The articular margin of the lateral facet was on average 11.4 mm (range 5.9-14.5 mm, SD 2.8 mm) anterior to the apex of the center ridge. The articular margin of the medial facet was on average 7.45 mm (range 2.0-11.6 mm, SD 3.5 mm) anterior to the apex of the central ridge. On average, the depth of the medial and lateral facets occupied 31.5% and 53.1% of the total patellar depth (measured from the anterior cortex to the central articular ridge).

The articular margins of the medial and lateral facets of the patella are difficult to distinguish radiographically. When viewed on a lateral radiograph, the surgical "at-risk" zones of the medial and lateral patella exist between 33% (one-third) and 50% (one half) of the total patellar depth when measured from the articular surface. Implants placed within this zone have the potential to be intra-articular.