Review Question - QID 5821

The question describes a knee with patella baja, which would likely be seen in the presence of a quadriceps tendon rupture.

The measurements described allow you measure an Insall-Salvati Index (ISI). The ISI is the ratio of the patellar tendon length to the patellar height (distance from the superior pole to the inferior pole). These measurements give a ratio of 0.75 which indicates the presence of patella baja. Normal values range from 0.8 to 1.2. A lower ratio is diagnostic of patella baja, while a higher ratio is diagnostic of patella alta. The ISI as well as some other measures of the patella's position within the trochlea are important when evaluating conditions such as patellar instability or patellofemoral pain.

Insall and Salvati present their original paper describing the anatomy of the knee with respect to patellar positioning. They note that previous methods of determining patellar positioning was either too complicated, or too subjective. They found the length of the patellar tendon and the height of the patella to be approximately equal, and saw no variations greater than 20%. Thus they proposed the ISI with "normal" values ranging from 0.8 to 1.2.

Shabshin et al. present a study making similar calculations as the ISI but done on sagittal MRI rather than lateral radiographs. They found a slightly higher degree of variation, and defined "normal" as ranging from 0.79 to 1.52. They also noted females tended to have higher ratios on average compared to males.

Aglietti et al. compared various measurements of the patellofemoral joint in a group of asymptomatic knees compared to knees with recurrent patellar subluxation. The normal knees were found to have an ISI of about 1, average congruence angle of -8, and an average Q angle of 15°. The subluxating knees had higher ISIs (average 1.23), higher congruence angles (average +16), and a higher Q angles (average 20°).

Illustration A shows how to measure and calculate the ISI. Illustration B shows measurements of the patellofemoral joint that may help identify presence of trochlear dysplasia and patellar maltracking.

Incorrect Answers:
Answer 1: Osteochondritis dissecans does not lead to alterations of the ISI and thus would not cause patella baja.
Answer 3: An inferior pole patellar sleeve avulsion would lead to patella alta and an ISI greater than 1.2.
Answer 4: A disruption of the medial patellofemoral ligament (MPFL) would lead to patellar instability and abnormalities of other measures, such as the patellar tilt.
Answer 5: Nail-patella syndrome is a genetic disorder often associated with small or absent patellae. The measurements given are not diagnostic of this condition.