How Common Are Extra Ribs Or Sesamoid Bones?

These anatomical variations are quite common and are the main reason why the 206-bone count is a standard, not a rule.

• Sesamoid Bones: These are small, independent bones that are embedded within a tendon, typically where the tendon passes over a joint.
  • Commonality: They are extremely common. The patella (kneecap) is the largest sesamoid bone and is included in the 206 count.
  • "Extra" Sesamoids:Accessory sesamoid bones are very common, especially in the tendons of the feet (near the big toe) and hands (near the thumb). The fabella, a small sesamoid in the back of the knee, is present in 10-30% of people. These small "extras" are usually not included in the standard 206 count.
• Extra Ribs (Supernumerary Ribs): This is less common than extra sesamoids but not rare.
  • Cervical Rib: This is the most well-known type. It's an extra rib that grows from the lowest neck vertebra (C7). It occurs in about 1 in 150 people (or 0.5% to 1% of the population). In most cases, it causes no symptoms, but it can sometimes compress nerves or blood vessels, leading to thoracic outlet syndrome.
  • Lumbar Rib: A less common variation is a small, extra rib on the first lumbar vertebra.