The Anatomy of the Fibula

The fibula is the slender long bone that is attached next to and a little below the tibia (shinbone). It bears very little body weight. The fibula provides lateral stability for the lower leg and acts as a tie rod to increase the range of motion for the ankle, especially lateral and medial rotation of the foot. It is the thinnest of all the long bones compared to its length.

The word fibula is Latin for "the brooch" and many believe it is named that way because when paired with the tibia anatomically, it forms the safety pin look of an ancient brooch.

Stress fractures of the fibula can occur with repetitive stress impact exercises like running. The pain of stress fractures in the fibula is similar to other common running injuries, making it difficult to correctly diagnose.

Since it does not bear significant weight, the fibula is often used as a donation site for bone grafts to repair bony structures in other parts of the body. Grafts can be removed from the fibula and replaced with implants to maintain the stability of the shaft.

Anatomy

There are four types of bone in the human body: long bones, short bones, flat bones, and irregular bones. The fibula is a long bone, meaning that it is longer than it is wide. Long bones have trabecular (spongy) bone on the ends and compact (dense) bone along the shaft. Running the length of the shaft in the center of the fibula is a cavity filled with red bone marrow.

There is also red bone marrow in the trabecular bone at both ends. Separating the trabecular and compact bone is an epiphyseal plate (growth plate). The epiphyseal plate is the location where new bone is formed until the bone is fully matured at adulthood.

Structure of the Fibula

The average fibula is about 390 millimeters (mm) (15.4 inches) long in adult men and about 360 mm (14.2 inches) long in adult women. There are three distinct types of fibula shapes when viewed as a cross-section along the shaft: triangular, quadrilateral, and irregular. Each fibula can contain more than one type of cross-section shape, and the combinations differ between males and females. The fibula is the most slender long bone in the body as a ratio of width to length.

Location of the Fibula

The fibula is located on the lateral (outside) of the tibia, slightly posterior (to the back) and offsets a little below. The proximal (top) end of the fibula is articulated with the lateral condyle of the tibia, just below the knee. That is called the proximal tibiofibular joint. The fibula does not make up any part of the knee joint.

The distal (bottom) end of the fibula articulates with the tibia in a depression called the fibular notch and that is called the distal tibiofibular joint. Even more distally, the fibula articulates with the talus at the talofibular joint, which forms part of the ankle joint called the lateral malleolus and can be felt externally as the hard bump on the outside of the ankle.

The fibula is connected to the tibia through a web of connective tissue that runs nearly the entire length of the fibular shaft. The proximal tibiofibular joint is held in place with the lateral fibular collateral ligament.

Function

The fibula provides lateral stability to the lower limb and to the ankle joint. It also articulates with the tibia and the talus to allow for additional range of motion during rotation of the ankle.

A natural fibula in an otherwise healthy individual does not bear any significant body weight. There are several muscles of the leg, including some from the upper leg, that attach along the entire length of the fibula to include both ends and the shaft. The progression of the cross-section shapes of the shaft from triangular to irregular is driven by the insertion points of muscles and ligaments.

Associated Conditions

The most common significant medical conditions of the fibula are fractures. Trauma to the fibula can be caused by a single episode of significant force or repetitive high-impact exercise forces such as those related to running.

Stress Fractures of the Fibula

Repetitive high-impact exercise such as running and jumping can lead to stress fractures in the proximal third of the fibula. One study of military recruits in Korea had a 1.9% incidence of proximal fibula stress fractures during basic training.

Ankle Fractures of the Distal Fibula

At the distal end of the fibula where it articulates with the talus, it creates the lateral malleolus. Because the lateral malleolus is so prominent, it presents a vulnerable spot for an external force to potentially fracture the ankle, including the fibula. Also, extreme twisting motions of the ankle can lead to spiral fractures of the fibula at the distal end.

While not specifically a fracture, ankle injuries can also disrupt the ligaments and bones of the tibiofibular syndesmosis, separating the tibia from the fibula at the point where they articulate, the fibular notch.

Proximal Fibula Tumors

Pain in the proximal (closest to the knee) end of the fibula should be evaluated by a physician. In rare circumstances, tumors can occur in the proximal fibula. Tumors do not happen in the fibula with any greater frequency than they do in other parts of the skeleton, but they are often overlooked because it is a difficult area to examine. A study of proximal fibula tumors found that pain significantly increased the possibility of a malignant tumor and warranted a visit to the doctor.

Tibialization of the Fibula

In certain rare cases of complete fractures (the bone is completely broken into more than one part) of both the fibula and the tibia, the fibula can heal faster because it has more relative blood flow than the tibia. If not addressed, the fibula can harden and thicken to become more like the tibia. If that happens, the tibia might not grow back together at all. That is known as a non-union of the tibia. Tibialization of the fibula is a limb salvaging procedure that can be done when there is severe bone loss and damage to the tibia.

The Fibula as a Bone Donor Site

Because of its lack of weight-bearing (except for tibialization of the fibula as described above) and dense vascularization, the fibula is a primary donor site for bone grafts to repair the mandible and a few other sites.

Rehabilitation

Depending on the condition being treated, treatment and rehabilitation of the fibula can take different forms.

Surgical Repair of the Distal Fibula

Complete fractures and orthopedic injuries to the distal fibula, including those of the tibiofibular syndesmosis, often require surgical repair and fixation with screws and plates. Sometimes, the fixation is temporary and will be removed after the bones have begun to heal. Post-surgery, the patient might be required to keep weight off the injured leg for six to eight weeks. This is often accomplished with the use of a walking cast.

Rehabilitation After Bone Donation from the Fibula

When used as a donor site for a flap of bone to repair or rebuild the mandible, the donor section of the fibula is replaced with an osseointegrated implant—an artificial section of bone. In most cases, there need to be two donor sites on the fibula, but both sites come from the same bone.

The use of home exercises and physical therapy can increase mobility and function after donating bone from a leg.

Rehabilitation from Stress Fractures

Minor repetitive injury fibular stress fractures are usually treated without surgery. These are closed, incomplete fractures of the fibula that will heal on their own as long as the activity is stopped. Due to the proximal pain associated with certain types of stress fractures, it is important to have it evaluated by a physician to rule out the possibility of tumors. Rehabilitation is likely to include rest and avoiding weight-bearing for a week or more. This can usually be accomplished with the use of crutches.