The foot structure; the Ankle Joint.

The ankle joint can be described as a saddle-shaped lower end structure of a long bone (tibia and fibula). Its inferior transverse ligament encloses the superior aspect of the body of the talus (the trochlea). It is the joint that first receives the transient impact that travels through the tibia in gait or other movement. It alternates in both form and function to receive load as a shock absorbing mechanism and to propel significant leverage based force during fast locomotion. The subtalar and ankle joint act like a mitered hinge. The tibial surface forming the superior dome of the ankle is concave sagittally, is slightly convex from side to side, and is oriented about 93° from the long axis of the tibia (it is higher on the lateral than the medial side). The upper articulating surfaces of the talus appear to match closely that of the cavity formed by the tibiofibular mortise. The superior part of the body of the talus is wedge-shaped. It is about one-fourth wider in front than behind, with an average difference of 2.4 mm; anteriorly a minimal difference of 1.3 mm and a maximal difference of 6 mm. From front to back, the articular surface spans an arc of about 105°. This surface contour, having a smaller diameter medially than laterally, has been compared to a section or rostrum of a cone. The primary motion of the ankle joint is dorsiflexion-

plantarflexion. Its axis of rotation is obliquely oriented with respect to all 3 anatomic planes with ankle dorsiflexion and tibial internal rotation being associated with subtalar eversion (pronation) whereas the ankle plantarflexion and tibial external rotation are associated with subtalar inversion (supination). The axis extends from anterior, superior, and medial to inferior, posterior, and lateral as it is passing through the inferior tips of the malleoli. It is at angles of 93° with respect to the long axes of the tibia and about 12° to the joint surface. However, rather than a true single ICR, the ankle has been noted to have multiple instant centers, all of which

arc of ankle rotation, the center may be displaced anywhere from 4 to 7 mm. The oblique

orientation of the axis of rotation to the sagittal, coronal, and transverse planes, translation of the talus in the mortise can occur in all 3 directions. The talus has been observed in vitro to rotate easily in the ankle mortise implying relative movement between the malleoli. Because the trochlea is wider anteriorly than posteriorly, it has been suggested that lateral play of the talus within its mortise occurs only when the ankle is in plantarflexion. Subtalar motion has been described as screw-like influencing the flexibility of the transverse tarsal joint. Others suggest that instability exists in dorsiflexion, while others yet believe that with intact ligaments

translation occurs only in the sagittal direction. These differences can be explained by behavior of more that 100 ligaments and by the roles played by the subtalar joint, the kinematic chain of the hindfoot, and the muscles that traverse this area in transmitting forces across this area during plantarflexion and dorsiflexion. The talus is unique because this bone has seven articulations that connect it to four other bones, it lies between the foot and the leg, and contains no muscular attachments. The stability of the talus and its articulations, therefore, relies heavily on the ligamentous attachments and musculotendinous complexes that traverse the talus and attach distally. Therefore the main characteristic of ankle joint is its strong passive stability attributed to a variety of factors. First is the bony stability provided by contact of the trochlea with the tibial plafond. Second are the medial and lateral cartilaginous slightly concave surfaces that articulate with the two malleoli. Third are the ligamentous connections between the tibia, fibula, talus, and calcaneus. Ankle stability increases during weight bearing and depends more on articular surface congruency.

The tarsometatarsal joints are relatively mobile and intrinsically stable joints that produce the arch-like configuration allowing wide range of motion at the first metatarsophalangeal joint with gliding during most of its range and jamming artful extension. The medial longitudinal arch functions as a beam and a truss.

The mode of foot/ankle mobility and muscular control is the most significant determinant of both limb stability and body progression (137).

Standing barefooted we load our heels with 2/3 of the load while the other third is loading the forefoot. During walking and at the early phase of stance, the center of pressure moves from the posterolateral heel rapidly across the midfoot, a phenomenon coupled with the firing of the anterior tibial musculature to slow foot plantarflexion and prevent foot slap. Then, at mid-and late stance the posterior calf musculature fires, propelling the body over the foot towards toe-off phase where the hallux bears the most pressure.

At higher speeds/rates of mobility it is the intrinsic mechanical properties that provide control rather than the neuromuscular control system. The force at the ankle joint can reach magnitudes up to six times body weight during walking and thirteen times BW during running. The heel fat pad is a very effective shock absorbing mechanism and it has been shown that high heels, or narrow shoes, narrow toebox, can lead to altered foot mechanics that ultimately result in foot deformities and pain.

5.4 The Spine

The spine is composed of a series of vertebrae (seven cervical, twelve thoracic, five lumbar, the sacrum and coccyx) and intervening soft tissues such as intervertebral discs, ligaments and muscle attachments. It provides important functions including support of the body structure and

protection of vital tissues such as the spinal cord, nerves and arteries. Yet it is flexible and allows mobility to the torso. Several studies have described the passive and active range of motion of spinal segments differently (Table 4). The motion of the spine is usually analyzed through consideration of motion segments which are comprised of two adjacent vertebrae with the intervertebral disc and other intervening soft tissues. These structures, also called functional spinal units (FSU), move with 6 DOF, however, the motion is quite complex due to 6 articulate faces between the two bony segments and attachment of multiple ligaments and muscles. Normally simultaneous translations and rotations of FSU are coupled in the analyses (138). Creep, relaxation and hysteresis are the three prevailing viscoelastic characteristics of the intervertebral discs (15). At high rates of loading the discs serve as a shock absorber with compression strength being higher from upper cervical to lower lumbar levels. Although this is true for most joints, vibrational properties of spinal segments have attracted particular attention as they are thought to be related to injury and pain. The spinal cord exhibits some longitudinal elasticity but very poor axial translation. These translational forces are the ones primarily associated with neurological injury. Apparently muscular support is of vital importance is stability. If for example bilateral facet resection exceeds 50% the significant increase in annulus stresses may occur. Seatbelts can adequately protect front seat occupants against frontal impact in an airbag equipped vehicle (14).

Several models of the spine have been developed (13, 14). A nonlinear finite element model of lumbar spine segment L3-L5 is shown in figure 19. The effects of upper-body mass, nucleus injury, damping, and different vibration frequency loads were analyzed for the whole body vibration (WBV) using this model. Anterior regions of L3-L5 segment show small vibration amplitudes, but posterior regions show large amplitudes. The posterior regions of intervertebral discs of lumbar spine are easy to injury during long-term WBV compared with anterior regions. The vibration of the human spine is more dangerous to facets, especially during WBV approximating a sympathetic vibration, which may lead to abnormal remodeling and disorders of lumbar spine.