Normal Patellar Nose Size on Radiography and MRI
The patellar nose refers to the inferior apex of the patella, a bony projection visible on lateral knee radiographs and MRI. Accurate measurement of this structure is important because an elongated or proportionally prominent patellar nose has been associated with patellar instability and abnormal patellofemoral mechanics. Quantifying its size helps guide surgical planning and risk stratification in patients with anterior knee pain or recurrent dislocation.
Normal Reference Values
| Orientation | Location | Measurement |
|---|---|---|
| Lateral | Ratio | >0.25 |
| Lateral | Length | >9 cm |
Clinical Significance
Two key measurements define an abnormal patellar nose: a ratio greater than 0.25 (patellar nose length relative to total patellar length) and an absolute length greater than 9 mm on lateral projection. When these thresholds are exceeded, the patellar nose may act as a mechanical obstacle within the trochlear groove, predisposing to lateral patellar maltracking and instability episodes.
An enlarged patellar nose can impinge on the anterior fat pad or the trochlea, contributing to inferior patellar impingement syndrome. It is also a recognized anatomical feature in the constellation of findings associated with objective patellar instability, alongside trochlear dysplasia, increased tibial tubercle–trochlear groove (TT-TG) distance, and patella alta. Isolated patellar nose enlargement is uncommon; its significance is typically interpreted alongside these co-existing morphological variants.
- Patellar instability / recurrent lateral dislocation
- Inferior patellar impingement on the anterior fat pad
- Hoffa fat pad fibrosis secondary to chronic impingement
- Patellofemoral osteoarthritis with inferior osteophyte formation
- Post-traumatic patellar apex avulsion or enthesopathy
Reference: Escala JS, Mellado JM, Olona M et al. Objective patellar instability: MR-based quantitative assessment of potentially associated anatomical features. Knee Surg Sports Traumatol Arthrosc. 2006;14(3):264-72.
Imaging Notes
On lateral knee radiography, the patellar nose is measured with the knee in approximately 30° of flexion. The total patellar length and the inferior nose segment are drawn along the long axis of the patella; the ratio is calculated by dividing nose length by total patellar length. Ensure a true lateral projection without rotation to avoid foreshortening artifacts that could falsely alter the ratio.
On MRI, sagittal proton-density or T1-weighted sequences provide optimal bony cortical definition for measurement. The same ratio and absolute length criteria apply. MRI additionally allows assessment of associated soft-tissue changes such as fat pad edema, chondral lesions at the patellar apex, and concomitant trochlear morphology—making it the preferred modality for comprehensive preoperative evaluation of patellar instability.