Sunday, November 20, 2011

Anterior Leg, Part 1: The Supporting Cast

Since we've covered some lateral knee and thigh structures and are due for more leg posts, I thought we'd dip down and visit the lower leg today-- specifically the anterior side. While this our the first visit to this area, it's not the first time we've mentioned the most prominent muscle here-- the tibialis anterior. This muscle is a star! Its body sweeps down the lateral surface of the tibia and stands out strikingly in foot dorsiflexion. Its tendon is even more visible on the antero-medial ankle as it courses down to the medial side of the foot. We've actually observed the tibialis anterior tendon before, in The Dorsal Foot: How Do I Love Thee? Let Me Count Your Tendons.

The tibialis anterior muscle was going get top billing in this post until it occurred to me that describing it was next to impossible without a thorough explanation of its supporting cast, the lower leg bones. So we'll examine those today and move on to a more detailed explanation of tibialis anterior next time.

The two bones of the lower leg are the tibia and the fibula. (Not fibia!) It's easy to distinguish these two bones from one another: The tibia is the wider of the two and lies more medial. It's the second longest and strongest bone in the human body (after the femur.) The fibula is the narrower of the two and lies on the lateral side. The tibia supports most of the weight placed on the lower leg, but the fibula breaks more often-- usually at its distal end-- because it's so thin. 

In the above diagram, the structures labeled in green are bony surface landmarks, which means they come right up under the surface of the skin and are often visible and/or palpable there. Notice that everything on the medial side has earned landmark status. Have you ever noticed that the medial side of your lower leg is much bonier than the lateral side? This is because the entire medial side of the tibia is completely exposed; nothing covers it other than skin, a little adipose tissue, and a thin layer of connective tissue.

The tibia, unlike other long bones, is not cylindrical in form. If cut transversely across its middle, its cross section would look more like a rounded triangle than a circle. The point of this triangle that faces anteriorly forms the anterior crest of the tibia, a long ridge down its anterior side. There are flat surfaces on either side of this crest, one lateral to it and one medial to it. The lateral surface of the tibia is not a surface landmark because it is almost entirely obscured by the tibialis anterior muscle. The medial surface of the tibia, however, is completely exposed. The tibia's medial surface and its anterior crest together form what we think of as the shin. And its complete exposure is what makes shin bumps so painful.

As we can see in the diagram above, the tibia's anterior crest and medial surface come right to the surface of the body, while its lateral surface is covered by the tibialis anterior muscle. Note also that the fibula is entirely embedded in muscle at this point (and along most of its length.) The only parts of the fibula that show on the surface of the human body are the head (at its proximal end) and the lateral malleolus (at its distal end.)

The above photo shows the appearance of these bony structures (among others) on the surface. Notice the entire medial tibia shows on the medial leg. (This will be more clear in the next post when we'll observe photos of the medial tibia juxtaposed against the tibialis anterior muscle.) We can also see the tibial tuberosity, a small bump just inferior to the patella, and the patellar ligament, which runs from the patella to the tibial tuberosity. We can also clearly see the medial malleolus of the tibia, which appears as a bump on the medial side of the ankle. Notice also that all we can see of the fibula on the lateral side are both ends of it-- the head proximally and the lateral malleolus distally.

Incidentally, this photo also shows lateral knee tendons (those of the iliotibial band and the biceps femoris muscle) which were discussed in The Lateral Knee: A Change of Scenery, and the lateral ankle tendons that were discussed in A Lateral Ankle Tendon: Peroneus Longus or Peroneus Brevis? Please visit these links for further information.

The tibia and the fibula provide the structural foundation for the muscular anatomy of the lower leg. Most of the lower leg muscle tissue is posterior to these bones, and we'll get to that soon. But next time, we'll take a good thorough look at the leading lady of the anterior leg, the lovely tibialis anterior! There might even be a sneak preview on the Human Anatomy for the Artist Facebook page. I'll get the popcorn and save you a seat down front.

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