Anatomy, Bony Pelvis and Lower Limb, Distal Tibiofibular Joint (Tibiofibular Syndesmosis)

A syndesmosis is a fibrous joint characterized by two adjacent bones connected by ligamentous structures, including a strong interosseous membrane (IOM). [1] The distal tibiofibular syndesmosis, which is comprised of two bones, the distal tibia and distal fibula, and four stabilizing ligaments, the anterior-inferior and posterior-inferior tibiofibular ligaments (AITFL and PITFL), the inferior transverse ligament (ITL), and the interosseous ligament (IOL), a distal continuation of the interosseus membrane. The syndesmosis is responsible for the integrity of ankle mortise. The syndesmosis is susceptible to compromise during high ankle sprains, ligament tears, and ankle fractures. This article will highlight the anatomical structure and function of the distal tibiofibular syndesmosis as well as its clinical and surgical significance.

The syndesmotic articulation of the distal tibiofibular joint occurs between the convex surface of the distal tip of the fibula and the concave fibular notch of the distal tibia. The stability of the syndesmosis is crucial to proper dynamic ankle and lower extremity function. The osseous portion of the distal tibiofibular syndesmosis consists of the tibia and fibula, and the four connecting ligaments include the anterior inferior tibiofibular ligament, posterior inferior tibiofibular ligament, inferior transverse ligament, and the interosseus ligament.[2] The primary function of the distal ligaments of the tibiofibular syndesmosis is to prevent the fibula from displacing (laterally, and/or anteriorly/posteriorly) from its groove in the tibia.[3] The ankle syndesmosis resists pathologic external rotational forces that attempt to laterally/externally rotate the talus, thereby displacing and pushing the fibula away from the tibia.

The anterior-inferior tibiofibular ligament originates from the longitudinal tubercle on the anterior surface of the lateral malleolus and attaches on the anterolateral tubercle of the tibia (Chaput fragment). It functions to hold the tibia and fibula tightly together and prevents excessive fibular displacement and external talar rotation. The posterior-inferior tibiofibular ligament originates at the posterior tibial tubercle (Volkmann fragment) and attaches to the posterior lateral malleolus. It functions alongside the anterior-inferior tibiofibular ligament to hold the tibia and fibula together. Slightly deep to the posterior-inferior tibiofibular ligament is the inferior transverse ligament, which serves to create a labrum that deepens the articular surface of the tibia posteriorly, helping to increase joint stability. The interosseus ligament lies at the distal end of the interosseous membrane and acts as a spring to prevent excessive separation of the tibiofibular mortise during ankle dorsiflexion and heel strike.

The limb buds of the embryo typically begin forming around 4 to 5 weeks after fertilization, as activation of mesenchymal cells of the lateral plate mesoderm cause a limb bud to start growing. The limb bud formation depends on many factors, which include the apical ectodermal ridge (AER), HOX genes, retinoic acid, sonic hedgehog (SHH) genes, and the zone of polarizing activity (ZPA). The AER forms via a thickening of the distal ectodermic border and produces signals that initiate rapid undifferentiated growth of the mesenchyme of the underlying progress zone, beginning limb formation. Retinoic acid acts to initiate transcription factor production and growth of the limbs by upregulating HOX genes and SHH genes, which then leads to synchronized limb outgrowth along the anteroposterior, dorsoventral, and proximo-distal axes of growth. Additionally, the T-box transcription factor Tbx4 is expressed in the development of the lower limbs and produces a growth factor, Fgf10, which acts at the AER to stimulate mitosis and further limb growth. The ZPA is a collection of cells that lie close to the AER on the posterior aspect of the limb and moves distally with the AER as the limb grows. The ZPA releases retinoic acid as well, which controls the development and growth of the limb along the anteroposterior axis. SHH and Fgf products promote HOX gene expression, leading to more complex limb polarization and regional formation of the developing limb. Errors in HOX gene expression, as well as T-box gene expression, can lead to malformations of limbs or changes in limb morphology.[3][4]

The blood supply to the distal leg includes the anterior and posterior tibial arteries, both branches of the popliteal artery originating from the femoral artery and external iliac artery proximally. The blood supply to the syndesmotic region itself, however, has not been extensively studied. The studies published on the topic have described anatomical variations in the vasculature supplying blood to the anterior aspect of the tibiofibular syndesmosis. Different variants of blood supply to the anterior syndesmosis seen in the literature include[5][6]:

• Branches stemming primarily from the perforating branch of the peroneal artery
• Branches from the peroneal artery, supplemented by smaller vessels from the tibial artery
• Supply from the tibial artery predominantly with smaller branches from the peroneal artery

Blood supply to the posterior aspect of the distal tibiofibular syndesmosis has less variation. In most cases, the blood supply to the posterior ligaments of the syndesmosis is supplied solely by the peroneal artery. In rarer instances, branches from the tibial artery also supplemented the vascular supply to the area.[5]

Lymphatic vessels of the lower extremity divide into superficial and deep vessels, which further subdivide into medial and lateral groups. These vessels drain into the popliteal, deep or superficial inguinal, external iliac, or lumbar (aortic) nodes.[4]

The nerves in the area of the tibiofibular syndesmosis include the deep peroneal (fibular) nerve, tibial nerve, and sural nerve.[7][8] The peroneal nerve is a sciatic nerve branch that begins where the sciatic nerve splits into the common tibial and common peroneal nerves, usually at the level of the lower thigh.[9][10] The peroneal nerve further splits into a superficial branch that innervates muscles in the lateral compartment of the leg, and the deep branch that innervates muscles in the anterior compartment of the leg and the dorsum of the foot. The tibial nerve runs posterior to the tibia and supplies the deep muscles in the posterior leg.[4]