Current Concepts Review Understanding the foot’s functional anatomy in physiological and pathological conditions: the calcaneopedal unit concept I. Ghanem1,2 Moreover, the calcaneopedal unit and the talo-tibiofibular A. Massaad1 complex are interdependent; rotation of the latter produces A. Assi1 morphologic modifications inside the former and vice versa. M. Rizkallah2 Purpose This paper is a review article of this concept and of its A. J. Bizdikian1 physiopathological applications. R. El Abiad2 Cite this article: Ghanem I, Massaad A, Assi A, Rizkallah 3 R. Seringe M, Bizdikian AJ, El Abiad R, Seringe R, Mosca V, Wicart P. 4 V. Mosca Understanding the foot’s functional anatomy in physiolog- P. Wicart5 ical and pathological conditions: the calcaneopedal unit concept. J Child Orthop 2019;13:134-146. DOI: 10.1302/1863- 2548.13.180022 Abstract Background A thorough review of the available orthopaedic Keywords: calcaneopedal unit; foot biomechanics; literature shows significant controversies, inconsistencies and talocalcaneonavicular joint sparse data regarding the terminology used to describe foot deformities. This lack of consensus on terminology creates confusion in professional discussions of foot anatomy, patho- Introduction anatomy and treatment of deformities. The controversies ap- ply to joint movements as well as static relationships between A thorough review of the available orthopaedic litera- the bones. ture shows significant controversies, inconsistencies and sparse data regarding the terminology used to describe Description The calcaneopedal unit (CPU) is a specific ana- foot deformities. This lack of consensus on terminology tomical and physiological entity, represented by the entire creates confusion in professional discussions of foot anat- foot excepted the talus. The calcaneus, midfoot and forefoot omy, pathoanatomy and treatment of deformities. The are solidly bound by three strong ligaments that create a unit controversies apply to joint movements as well as static that articulates with the talus. The movement of the CPU is relationships between the bones. complex, as it rotates under the talus, around the axis of Hen- The terms varus and valgus are often used to describe ke that coincides with the talo-calcaneal ligament of Farabeuf. deformities in the forefoot and the hindfoot. They are fre- This calcaneopedal unit is deformable. It is compared with quently used interchangeably with the terms supination a twisted plate, able to adapt to many physiological situa- and pronation, inversion and eversion, and adduction and tions in standing position, in order to acheive a plantigrade abduction. This is mostly due to the multiple biomechanical position. theories and concepts that have been applied to the foot.1-5 Before attempting to bring consistency and reason to 1 Faculty of Medicine, University of Saint-Joseph, Beirut, Lebanon 2 Orthopedic Surgery Department, Hôtel-Dieu de France Hospital, the terminology, a good understanding of foot mechanics University of Saint-Joseph, Beirut, Lebanon is mandatory. 3 Orthopedic Surgery Department, Cochin University Hospital – In 1803, Scarpa6 proposed an analogy between move- University of Rene Descartes, Paris, France ments in the hindfoot and those in the hip joint, thereby 4 Department of Orthopedic Surgery, Seattle Children’s Hospital, developing the concept of the ‘acetabulum pedis’. He saw Seattle, Washington, USA 5 Orthopedic Surgery Department, Hôpital Necker-Enfants Malades, similarities in the way the femoral head rotated within the University of Paris Descartes, Paris, France acetabulum of the pelvis and the talar head/talus rotated within the ‘acetabulum’ of the subtalar joint, which he Correspondence should be sent to I. Ghanem, MD, Hôtel-Dieu defined as the articular surfaces of the navicular and the cal- de France Hospital, A. Naccache Avenue- Achrafieh, University of 6-10 Saint-Joseph, Beirut, Lebanon. caneus as well as the spring ligament. Around 25 years E-mail: [email protected] later, Delpeche11 reported on the movement of the calca- THE CALCANEOPEDAL UNIT CONCEPT neus under the talus when the foot is adducted: “...when Functional anatomy of the normal foot the foot moves inward, the calcaneus rolls as a cylinder and swings at the same time, almost horizontally under the The CPU is made up of the calcaneus, the midfoot and the posterior part of the astragalus (talus); in such a way that forefoot (Fig. 1a), solidly bound by the calcaneocuboid the inferior aspect of the first one of these two bones (i.e. ligament, the bifurcate ligament (Y ligament of Chopart) the calcaneus) rotates inward, and its anterior part slides and the inferior calcaneonavicular ligament (glenoid or in the same direction, under the head of the second (i.e. spring ligament) (Figs 1a and 1b). It represents a func- the astragalus)...”. In Duchenne de Boulogne’s book, Phys- tional unit that articulates with the talus through four iologie des Mouvements,12 published in 1867, he devoted a articular components: the posterior, middle and anterior 14 chapter of 200 pages to ‘the foot’. In it he describes, in a subtalar, and the talonavicular joints. The talus does not very clear manner, the different muscular activities in the belong to the foot CPU; it rather belongs to the same func- foot with the corresponding joint movements. He described tional unit as the tibia and fibula, with which it forms the 4,5,12,15 a certain rotation of the foot under the ‘astragalus (talus) talo-tibiofibular complex (Fig. 1a). and the leg’.12 These early great thinkers and authors laid This concept, in addition to an oblique division of the foundation for the second wave of pioneering work on the foot, is complementary to the traditional and com- foot biomechanics lead by Méary in 1967 and Queneau in monly used separations of the foot: 1) the transverse 1977. 13 Their research and writings led to the concept of division between hindfoot, midfoot and forefoot; and 2) and the terminology for, the calcaneopedal unit (CPU). the longitudinal division between the medial arch (three The CPU is a specific anatomical and physiological entity, medial rays) and the lateral arch (two lateral rays). This represented by the entire foot excepted the talus. The cal- concept goes along with the ‘acetabulum pedis’ concept 6,9 caneus, midfoot and forefoot are solidly bound by strong of Scarpa where the articular surface of the navicular, ligaments that create a unit that articulates with the talus. the anterior and middle articular facets of the calcaneus The movement of the CPU is complex, as it rotates under and the dorsal part of the spring ligament form a socket the talus around the axis constituted by the talo-calcaneal (or acetabulum) in which the talar head articulates. In ligament of Farabeuf. Additionally, CPU is deformable. The this ball and socket concept, it is the socket (acetabulum bones that constitute it move to small degrees in relation to pedis) that rotates three-dimensionally around the ball 6,7,9,10 each other in order to maintain a plantigrade foot in stance. (talar head). Moreover, it seems to have a close interdependence with The CPU has 3D mobility around an oblique axis (the the talo-tibiofibular unit, so that any movement of the latter Henke axis) that is, on average, 41° dorsiflexed and 23° inter- 14-16 produces an adaptive or compensatory movement and/or nally rotated. This imaginary axis enters the talus at the deformity of the former. Although this concept is old and upper medial aspect of its neck, passes through the deep very useful for the understanding of the foot mechanics, we part of the talocalcaneal interosseous ligament and emerges 15-19 did not find any detailed description about it in the ortho- at the posterior lateral aspect of the calcaneus (Fig. 2). paedic literature of the last 60 years, with exception of an The combination of the four joints mentioned above article written by Seringe and Wicart in 2013.9 rotating three-dimensionally around the non-orthogo- The purpose of this paper is to review the anatomical and nal Henke axis creates movements that give the subtalar biomechanical aspects of the CPU concept, to use them in joint complex the appearance of an enarthrosis, or ball the analysis of some foot deformities, and to explore them and socket joint, although it is not truly a ball and socket in the planning of treatment, taking into consideration the joint like the hip or shoulder. Its proximity and coordinated ultimate functional position of the foot, i.e. standing. movements with the pure dorsiflexion-plantar flexion ankle Fig. 1 The calcaneopedal block is formed by the calcaneus, the midfoot and the forefoot; it articulates with the talus (talo-tibiofibular unit) and is bounded by the calcaneocuboid ligament (a), the Y ligament of Chopart and the calcaneonavicular ligament (b). J Child Orthop 2019;13:134-146 135 THE CALCANEOPEDAL UNIT CONCEPT Fig. 2 (a) Transverse view of the talocalcaneonavicular joint (TCN, talocalcaneonavicular axis); (b) sagittal view of the talocalcaneonavicular joint. The TCN passes through the interosseous talocalcaneal ligament (TCL) (CNL, calcaneonavicular ligament). joint further create the false appearance of a true enarthro- Medial and lateral arches interdependence law sis in the hindfoot. The subtalar joint complex would best The medial and lateral arches interdependence law stip- be described as a constrained ball and socket joint that ulates that every time the fifth metatarsal is displaced rotates three-dimensionally and simultaneously