The tibia, or the shin bone, is a long bone articulating superiorly with the inferior articulating surfaces of the femur, and with the talus inferiorly. The tibia hangs down the medial surface of the talus and forms the medial malleolus. It is the second largest bone in the body after the femur and its size results from the heavy weight distribution that is required of it. The tibia is a long bone, which means it has a hard outer surface known as compact bone, with a mesh-like interior called cancellous bone, designed to take pressure from multiple angles. It is also useful to note that the cancellous bone contains bone marrow that produces red blood cells.
Articulations with Fibula
There is a special articulation with the fibula bone that runs parallel to the tibia. Compared to the tibia, the fibula is the same length but thinner. This suggests a difference in the weight bearing demands of the two bones. Some sources say there is a 10% weight bearing responsibility placed on the tibia, other sources say there is no weight bearing pressure on the fibula.
The two articulations of the tibia and fibula are the superior and inferior tibiofibular joints:
Superior Tibiofibular Joint
The superior tibiofibular joint is a synovial plane joint, with the transverse joint line spanning the lateral tibial condyle and the medial fibular head. The capsule is thickened anteriorly and posteriorly and joins with the anterior ligament (passing obliquely from the fibular head to the tibial condyle), relating closely to the tendon of biceps femoris. The posterior ligament is thicker and oblique, projecting between the posterior fibular head and lateral tibial condyle.
Inferior Tibiofibular Joint
The inferior tibiofibular joint is a syndesmosis joint, just above the ankle region. This joint lies between the medial distal end of the fibula and the concave fibular notch region of the tibia. The distal end of the joint has a 4 mm separation from a prolongation of the ankle joint. There is no fibrous capsule surrounding this joint but there is a flat band of ligament named the anterior tibiofibular ligament which descends laterally between the two leg bones.
The posterior tibiofibular ligament is stronger than the anterior, running in a similar fashion to its anterior counterpart but having a distal band named the inferior transverse ligament. Finally, the interosseous ligament is a continuous ligament with the leg bones, possessing many short, strong bands. This is the strongest support for the two bones. All these ligaments are important stability factors.
At the proximal end of the tibia lie the tibial condyles. The tibial condyles carry the body weight transmitted by the femur. To ease the pressure, there are pads of fibrocartilage known as menisci attached to these condyles. The condyles swing down posteriorly to allow for flexion and are palpable anteriorly.
The menisci divide the intercondylar areas into anterior and posterior sections, resembling an hourglass shape. The anterior section has an attachment site for the menisci (via the meniscofemoral and transverse ligaments) and the anterior cruciate ligament. The posterior section does a similar job but attaches to the posterior cruciate ligament.
The tibial tuberosity is a big landmark on the tibia as it provides an attachment surface for the quadriceps muscles, via the quadriceps tendon. This is prone to inflammation due to the tension exerted. The tuberosity is located anterior to where to condylar surfaces meet. There is also a bursa, the infrapatellar bursa, that separates the tibial tuberosity from the skin.
Muscle & Menisci Attachments
The menisci of the knee are attached to the medial and lateral condyles of the proximal tibia by the meniscotibial (coronary) ligaments. The quadriceps tendon attaches to the superior part of the tibial tuberosity. The medial and lateral tibial condyles provide attachment to the semimembranosus and biceps femoris muscles respectively, with the third hamstring muscle, the semitendinosus, attaching just anteriorly to the medial condyle attachment point. The tendons of the semitendinosus muscle group together with the tendons of the gracilis and sartorius muscles form the pes anserinus.
The tibialis anterior muscle originates from the proximal two-thirds along the lateral surface of the tibia, with the extensor digitorum longus originating behind the tibialis anterior, along the midline. Posteriorly, the soleal line houses the soleus, with the popliteus originating just proximal to the region, in a roughly triangular area. Finally the tibialis posterior and flexor digitorum longus originate just distal to the soleal line.