Differential Diagnosis for Falling and Injuring Knee while Running Outside

Single Most Likely Diagnosis

• Medial collateral ligament (MCL) sprain: This is a common injury in runners, especially when they fall or experience a sudden twist on their knee. The MCL is one of the most frequently injured ligaments in the knee, and its location on the medial (inner) aspect of the knee makes it prone to injury from valgus stress (a force that pushes the knee inward from the outside).

Other Likely Diagnoses

• Meniscal tear: The menisci are cartilage structures in the knee joint that can be damaged by a sudden twist or fall. Running outside increases the risk of uneven terrain, which can lead to such injuries.
• Anterior cruciate ligament (ACL) sprain: Although less common than MCL sprains, ACL injuries can occur from non-contact mechanisms like landing from a jump or sudden stops, which might happen when running outside.
• Patellofemoral pain syndrome: This condition, also known as runner's knee, is characterized by pain around or behind the patella (kneecap). It can be exacerbated by activities like running, especially on uneven surfaces.

Do Not Miss Diagnoses

• Fracture (e.g., tibial plateau fracture): While less common, a fracture is a critical diagnosis that must not be missed. A fall onto the knee can result in a fracture, especially in older individuals or those with osteoporosis.
• Dislocation: Though rare, a knee dislocation is a serious injury that requires immediate medical attention. It can occur from high-energy trauma, such as a fall from a significant height onto the knee.
• Vascular injury: Direct trauma to the knee can potentially injure the nearby blood vessels, leading to serious complications if not promptly addressed.

Rare Diagnoses

• Osteochondritis dissecans: This condition involves a piece of cartilage and a thin layer of the bone beneath it becoming detached from the end of the bone because of inadequate blood supply. It can be caused by trauma and might present with knee pain after a fall.
• Peroneal nerve injury: The peroneal nerve wraps around the lateral aspect of the fibular head and can be injured in severe knee trauma, leading to foot drop or other neurological deficits.