The ankle joint – consisting of the tibial plafond, the medial malleolus, and the lateral malleolus – is a highly congruent joint with three articulations:
- Tibiotalar joint
- Talofibular joint
- Tibiofibular joint
The mortise of the tibia and fibula articulates with the talus. Dorsiflexion is combined with eversion of the foot, while plantar flexion is combined with inversion.
The distal tibial plafond and the medial malleolus are the articulating surfaces of the mortise. The distal tibial plafond has a concave shape with an average medial angle of 22° ± 4° (mean ± standard deviation) and shows a slight varus.1 The mean distal tibial angle (the angle between surgical axis of the tibia with the tibial tuberosity and the plafond of the tibia) is 92.4° ± 3.1°, with a range of 84° to 100°. Women have a ca. 2.2° smaller tibiotalar angle than men. Also, the angle diminishes from 92.4° to 91.5° with increasing age.2 However, the measured medial distal tibial angle differs between whole lower leg radiographs and mortise views of the ankle, and this distinction should be clearly noted during clinical evaluation for the preoperative planing.3
From a sagittal view, the posterior edge of the distal tibial plafond is slightly lower than the anterior edge, resulting in a mean anterior distal tibial angle of 83° ± 3.6°, with a range of 76° to 07°, as shown by Magerkurth et al.4 In the same radiological study, tibial coverage was been found to be 88.1° ± 6.7°. Frigg et al later found that decreased tibial coverage is a risk factor for chronic ankle instability.5
Another aspect of ankle integrity relevant to total ankle replacement (TAR) is the bone strength at the distal tibia.6 Using an osteopenetrometer to measure the tibial and talar bone strength at 2-mm sections, Hvid et al^6^ showed the talus has significantly higher bone strength than the tibia (ca. 40%). However, in both bones the strength rapidly decreased with the depth of section. Furthermore, Hvid et al showed that the bone strength distribution is not equal in the distal tibia, with the lowest values measured in posterior-medial areas.6 The section of 4 mm has been defined to be critical to withstand the compressive forces, which should be considered at bone cuts for TAR.6 This finding has been confirmed by Kofoed in his study showing that only 1 to 1.5 cm of the proximal tibia present a solid subchondral bone.7
The fibula terminates at the lateral malleolus, which has a pyramid-like shape with three surfaces:
- Lateral, which is convex and subcutaneous
- Medial, which is in contact with the incisura fibularis of the tibia
- Posterior, upon which the tendons of the peroneus brevis and longus lie
The distal tibiofibular joint is also flexible with limited articulation. Several studies have demonstrated movement of the fibula during range of motion (ROM) of the ankle joint.8,9 These have shown that during passive plantar flexion and dorsiflexion the fibula has an axial rotation relative to the tibia and mediolateral translation.9 It has been observed that the distal end of the fibula has more axial rotation than the proximal end during ankle ROM.9 Congruency of the distal tibiofibular articulation has high biomechanical importance. Minor changes of fibula position (eg, 2 or more millimeters of shortening, lateral shift, or 5 or more degrees of external rotation of fibula) may cause a significant increase of intra-articular pressures in tibiotalar joint.10
The talus forms the convex side of the ankle joint. It articulates with the tibia superiorly and medially, and with the fibula laterally. The talus has a conically shaped surface with a smaller medial radius and medially directed apex. In the sagittal plane, the talar trochlea has a wedge shape that articulates to the tibial plafond. The average width of the talar trochlea is 29.9 ± 2.6 cm anterior, 27.9 ± 3.0 cm in the middle, and 25.2 ± 3.7 cm posterior.11 In general, the talus body is wider in males than in females.11
The internal architecture of the talus is unique and reflects its biomechanical properties. Compact bone in the talus is non-uniform, with thicker regions over the posterior calcaneal facet, medial malleolar facet, and lateral malleolar facet.12 Thinner regions of compact bone exist over the trochlear surface and head of the talus.12 The trabeculae of the talus can be resolved into two sets:
- The first set is deep to the medial part of the trochlear surface and from the anterior third of the lateral trochlear surface. Lamellae of this set are directed anteriorly towards the head and neck of the talus.
- The second set is deep to the posterior two-thirds of the trochlear surface and descends onto the posterior calcaneal facet.12
The neck of the talus includes sagittal plates extending from the body to the head. This internal architecture may explain the distribution and transmission of forces through the talus during gait.12,13 Furthermore, it has been shown that the internal trabecular alignment may change due to cartilage degenerationm which should be considered in ankles with degenerative changes.14
The talus does not have tendon insertions; therefore, blood supply is limited to that obtained from blood vessels in the tarsal tunnel, deltoid ligament, and sinus tarsi.15-17 This lack of direct blood delivery may make the talus susceptible to avascular necrosis, especially if blood supply is lessened as a result of surgery.
Ligaments and Soft Tissues
Stability of the ankle is due to bony and ligamentous structures. The medial deltoid ligament complex providing the primary source of stability to the ankle joint, and it consists of both superficial and deep components. The lateral ligament complex is responsible for lateral stability of the ankle joint. It consists of three major ligaments - the anterior talofibular ligament (ATFL), the calcaneofibular ligament (CFL), and the posterior talofibular ligament (PTFL). The ligaments of the ankle syndesmosis include the anterior talofibular, posterior talofibular, and interosseous ligaments.
Thirteen tendons cross the ankle joint.
Nerves and Blood Vessels
Five nerves, two arteries, and veins cross the ankle joint. The sural nerve is lateral to the Achilles tendon.
The structures are divided into the posterior, medial, lateral, and anterior groups. The structures that cross the posterior ankle include:
The structures that cross the medial ankle include:
The structures that cross the anterior ankle include:
The structures that cross the lateral ankle include:
- Peroneus tertius
- Peroneus longus
- Peroneus brevis
- Superior retinaculum
- Lesser saphenous nerve
- Sural nerve
- Inman VT. The Joints of the Ankle, Baltimore, Md., Williams & Wilkins, 1976.
- Knupp M, Ledermann H, Magerkurth O, Hintermann B. The surgical tibiotalar angle: a radiologic study. Foot Ankle Int 2005;26:713-6.
- Stufkens SA, Barg A, Bolliger L, Stucinskas J, Knupp M, Hintermann B. How should the medial distal tibial angle be measured? Foot Ankle Int, accepted for publication, 2011.
- Magerkurth O, Knupp M, Ledermann H, Hintermann B. Evaluation of hindfoot dimensions: a radiological study. Foot Ankle Int, 27:612-616, 2006.
- Frigg A, Frigg R, Hintermann B, Barg A, Valderrabano V. The biomechanical influence of tibio-talar containment on stability of the ankle joint. Knee Surg Sports Traumatol Arthrosc 2007;15:1355-62.
- Hvid I, Rasmussen O, Jensen NC, Nielsen S. Trabecular bone strength profiles at the ankle joint. Clin Orthop Relat Res 1985;199:306-125.
- Kofoed H. Cylindrical cemented ankle arthroplasty: a prospective series with long-term follow-up. Foot Ankle Int 1995;16:474-9.
- Bozkurt M, Yavuzer G, Tonuk E, Kentel B. Dynamic function of the fibula. Gait analysis evaluation of three different parts of the shank after fibulectomy: proximal, middle and distal. Arch Orthop Trauma Surg 2005;125:713-20.
- Bozkurt M, Tonuk E, Elhan A, Tekdemir I, Doral MN. Axial rotation and mediolateral translation of the fibula during passive plantarflexion. Foot Ankle Int 2008;29:502-7.
- Thordarson DB, Motamed S, Hedman T, Ebramzadeh E, Bakshian S. The effect of fibular malreduction on contact pressures in an ankle fracture malunion model. J Bone Joint Surg Am 1997;79:1809-15.
- Hayes A, Tochigi Y, Saltzman CL. Ankle morphometry on 3D-CT images. Iowa Orthop J 2006;26:1-4.
- Athavale SA, Joshi SD, Joshi SS. Internal architecture of the talus. Foot Ankle Int 2008;29:82-6.
- Pal GP, Routal RV. Architecture of the cancellous bone of the human talus. Anat Rec 1998;252:185-93.
- Schiff A, Li J, Inoue N, Masuda K, Lidtke R, Muehleman C. Trabecular angle of the human talus is associated with the level of cartilage degeneration. J Musculoskelet Neuronal Interact 2007;7:224-30.
- Gelberman RH, Mortensen WW. The arterial anatomy of the talus. Foot Ankle 1983;4:64-72.
- Kelly PJ, Sullivan CR. Blood supply of the talus. Clin Orthop Relat Res 1963;30:37-44.
- Mulfinger GL, Trueta J. The blood supply of the talus. J Bone Joint Surg Br 1970;52:160-7.