CASE 30

By: Dr. Waleed Althobaity, MD

HISTORY: 26-year-old female with recent injury.

What are your findings?

What is your diagnosis?

What are the primary cause and the predisposing risk factors?

FINDINGS:

Axial T2 weighted MR images (A & B) of right knee joints at the level of patellofemoral joint demonstrate bony irregularity and edema at medial aspect of patella at site of medial patellar retinaculum insertion. There is also bone marrow edema at lateral femoral condyle. Shallow and severely dysplastic trochlear groove is identified with high grade patellar chondropathy.

DIAGNOSIS:

Transient lateral patellar dislocation and relocation.

Risk factors:

The primary cause is trauma.

Predisposing risk factors:

• Congenitally shallow trochlear groove, any of which significantly increase a patient’s likelihood.

• Excessive lateral distance between the tibial tubercle and trochlear groove.

• Patella alta and baja.

• Laterally tilted patella.

• Generalized joint laxity.

PEARLS AND DISCUSSION:

Traumatic dislocation of the patella can be produced by a direct blow or an exaggerated contraction of the quadriceps mechanism.

When the patella spontaneously relocates after dislocation, bone marrow contusions of the medial patellar facet and lateral femoral condyle occur owing to impaction injury. In addition, chondral or osteochondral fractures may also be seen in one or both locations. Injury to the medial patellar retinaculum is typically seen. Medial retinaculum and medial patellofemoral ligament (MPFL) injuries range from sprains to complete disruption or avulsion.

The bone Marrow edema pattern is specific. Don’t forget to look for predisposing factors which are mentioned above.

FURTHER READINGS:

1. Bone and Joint Imaging, 3rd Edition. By Donald L. Resnick, MD, Mark J. Kransdorf, MD, Donald L. Resnick, MD and Mark J. Kransdorf, MD.

2. Diederichs G, Issever Ahi S, Scheffler S. MR Imaging of Patellar Instability: Injury Patterns and Assessment of Risk Factors. RadioGraphics 2010; 30: 961-981.

3. MR Imaging of Patellar Instability: Injury Patterns and Assessment of Risk Factors: Gerd Diederichs, MD, Ahi S. Issever, MD, and Sven Scheffler, MD