The Insall-Salvati ratio is a measurement used by healthcare professionals to assess the vertical position of the patella, or kneecap, relative to the tibia, which is the shin bone. This ratio helps determine if the patella is sitting too high or too low within the knee joint. You might encounter this specific measurement mentioned in radiology reports following knee imaging, such as X-rays or magnetic resonance imaging (MRI) scans.
How the Ratio is Calculated
Calculating the Insall-Salvati ratio involves taking two measurements from a side-view (lateral) image of the knee. These measurements are typically obtained when the knee is flexed at approximately 30 degrees.
The first measurement, known as the patellar tendon length (LT), is taken from the inferior pole of the patella down to its insertion point on the tibial tuberosity. The second measurement, the patella length (LP), represents the greatest pole-to-pole length of the patella itself. The Insall-Salvati ratio is then determined by dividing the patellar tendon length by the patella length (Ratio = LT / LP).
Interpreting the Results
The Insall-Salvati ratio provides insight into the patella’s vertical alignment. On standard plain radiographs, a ratio between 0.8 and 1.2 is considered within the normal range.
When the calculated ratio is greater than the normal range, typically above 1.2 on X-rays or 1.5 on MRI, the condition is termed “patella alta.” This indicates a high-riding kneecap, positioned higher than its expected anatomical location.
Conversely, if the ratio falls below the normal threshold, usually less than 0.8 on X-rays or 0.74 on MRI, it is referred to as “patella baja.” This signifies a low-riding kneecap, positioned lower than its typical position within the knee joint.
Clinical Significance
Healthcare providers pay attention to the patella’s position because an abnormal height can impact knee mechanics and lead to various issues. Patella alta is associated with increased patellar instability, meaning the kneecap is more prone to shifting or dislocating from its groove. This elevated position can reduce the contact area between the patella and the femoral trochlea, especially during early knee flexion, which contributes to instability and can cause pain at the front of the knee.
Patella alta is also linked to other conditions such as recurrent patellar subluxation, patellofemoral pain syndrome, and specific growth-related conditions like Osgood-Schlatter disease or Sinding-Larsen-Johansson disease. Patella baja can lead to different challenges. This condition is associated with a restricted range of motion in the knee and can cause discomfort or a grinding sensation, known as crepitus, within the joint.
Patella baja is observed following knee injuries or surgical procedures, including anterior cruciate ligament (ACL) reconstruction or total knee replacement. It may also signal other underlying concerns such as quadriceps tendon rupture or certain neuromuscular disorders. Understanding these positional abnormalities helps guide appropriate management strategies for knee health.