What is a Fabella Bone? - Definition, Anatomy & Syndrome

Instructor: Charity Hacker

I am a nursing instructor with over 20 years of nursing experience and a Masters Degree in Nursing Education.

Everyone knows about the skeletal system and its purpose in supporting life. But how many know about the role of the fabella bone? In this lesson, we will discover its definition, anatomy, and lean about the syndrome that is caused by it.

Introduction

Oh! Ouch!!! Your teenage son has been playing a lot of sports and has started to complain of pain behind his left knee. He has been to the doctor, had some imaging, and is now sitting in the office of the orthopedic surgeon. The doctor tells you that he thinks it is the fabella bone and that he may have Fabella Syndrome.

What is the fabella bone and what is Fabella Syndrome? We are going to find out.

Definition and Anatomy

The Meriam-Webster Medical Dictionary defines the fabella bone as 'a small fibrocartilage ossified in many animals and sometimes in humans in the tendon of the gastrocnemius muscle, behind one or both of the femoral condyles'. Obviously, this is very technical so we are going to break it down into everyday language.

  • Fibrocartilage means a structure that is part fibrous, like a tendon or ligament and part cartilage. It is often found at a transition points between muscles and bones.
  • Ossified means to become hard and/or turn into bone.
  • Gastrocnemius is the largest muscle of the calf. It is attached at one end to the femur, the upper legbone, and the other to the Achilles of the heel.
  • Femoral condyles are the parts of the bone at either end that stick out and kind of look like knuckles.

So, let's recap, a fabella bone is formed when a small formerly somewhat soft tissue has turned into bone. It is located in the top of the calf muscle just before it attaches to the lower end of the femur bone, behind the two bumps that look like knuckles.

The fabella bone is the small white dot on the left side of the picture, behind the end of the femur.
Picture of fabella bone.

Here's the exciting, or not so exciting news as you will see later, about the fabella bone. Only some people actually have one, and even less people have two. It is estimated that a third of the population have a fabella bone, and only half of those people have a fabella behind each knee.

Fabella Syndrome

We have established that the general population does not have a fabella bone. But what about Fabella Syndrome? Among those who have a fabella bone, some are unlucky enough to have pain associated with it. Fabella Syndrome is the pain associated with the fabella bone. Additionally, the syndrome is more prominent in the teenage population.

Signs and Symptoms

The pain usually shows up when the leg is fully extended. It is characterized by sharp pain and local tenderness that increases in intensity upon extension. The pain can also be present with rigorous exercise or sports, flexion, and when crossing the legs.

Other complications may be present in the effected leg if the fabella is close to the common fibular nerve, a large nerve in the leg. These may include numbness or tingling, foot drop, and an altered gait (walking pattern) in which the foot must be lifted higher than normal off the ground to prevent dragging.

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