What is indicated by the inability to flex the knee to 90 degrees?

The inability to flex the knee to 90 degrees can be indicative of significant underlying injuries to the knee joint, such as a ligament injury or a fracture. When a ligament is injured, especially the anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL), it can lead to biomechanical instability and swelling around the joint, which inhibits the range of motion.

Similarly, a fracture, particularly around the knee area (such as a femoral condyle fracture or a patellar fracture), can cause pain, swelling, and mechanical blockage that prevents the knee from bending adequately. Together, these factors contribute to the observed limitation in flexion.

Both ligament injuries and fractures correlate with acute knee conditions that are serious and warrant further evaluation, possibly including imaging studies. The presence of either injury type is a plausible reason for restricted knee flexion at this level, ultimately supporting the conclusion that both conditions are significant indicators in this scenario.