When it comes to the load-bearing joints of the lower limb, the foot is the least understood. This stems from the fact that its size is a major barrier to quality scientific investigation but is also partly due to the the misconception that its function is simple. While we may believe we know a great deal about the biomechanics of the foot and ankle, in reality, it is relatively uncharted territory compared to the knee and hip. The foot is far from simple as it comprises hundreds of different ligaments and bony structures and scores of articulations. Its role in weightbearing ambulation is critical. In the next few decades, the foot and ankle will be a major topic investigated by biomechanics researchers and this will present some major challenges for these researchers as well as podiatrists.
How The Ankle Fits Into The Biomechanics Equation
The ankle of course plays a significant role in enabling the body to move in the sagittal plane over the weightbearing foot and initiating the next step in walking. However, an important but often overlooked characteristic of the ankle is its capacity for frontal and transverse plane motion. This is critical. When we clinically observe movement of the heel relative to the leg, we interpret these motions with the knowledge that they occur at a combination of the ankle and subtalar joint, not just the subtalar joint. In one of the first studies to quantify the transverse plane motion at the ankle joint, McCullough and Burge described approximately 17.5 degrees of motion in eight cadavers.1 Siegler, et. al., described a mean of 26.5 degrees of transverse plane motion at the ankles of 15 unloaded cadavers.2 The discrepancy between these figures is perhaps an indication of the importance of loading on the range of transverse plane motion at the ankle. In their invasive, in vivo study of eight subjects, Lundberg, et. al., found an average of 8.4 degrees of transverse plane ankle motion during 30 degrees of external leg rotation.3 Our own work on “walking” cadaver specimens has demonstrated 10 degrees (SD 4.7 degrees) of transverse plane ankle motion during a simulated stance phase.4 These data demonstrate that the ankle is clearly capable of transverse plane motion. Therefore, the ankle not only “transfers” a transverse plane moment to the subtalar joint but undergoes transverse plane motion itself. The ankle may be potentially as important to this mechanism as the subtalar joint. In our in vivo study, we predicted up to 20 degrees of transverse plane ankle motion in some individuals.5 In our more recent “walking” cadaver study, we reported an average of 10 degrees of transverse plane motion at the ankle and an average of only 8.1 degrees at the subtalar joint (mean of 13 cadavers), suggesting the ankle moves the most in the transverse plane.4 The capacity for frontal plane motion at the ankle will, like the subtalar joint, enable the accommodation of angulation of the foot to the floor at heel strike. It also facilitates subtalar joint movement by enabling the talus to move relative to the tibia as it moves relative to the calcaneus. Understanding this complex capacity for motion at the ankle broadens our understanding of the role of support structures at the ankle. Leardini, et. al., present an excellent summary of ligament structures and function at the ankle.6 Ligaments are critical to guiding the motion at the ankle and rearfoot complex. It is not solely the role of the articular surfaces as many believe. There has been a good deal of quality research capturing the kinematic capabilities of the ankle. Lundberg described the ankle joint axes of rotation during dorsiflexion and plantarflexion of the foot using a hinged plate. The complexity of the motion is reflected in the triplanar and changing orientation of the axes, although it consistently passes through both malleoli.7-8 The ankles in Lundberg's work displayed a pattern that suggested distinct dorsiflexion and plantarflexion axes when the foot was dorsiflexed and plantarflexed. Plantarflexion axes were inclined downward in a medial direction or were close to the horizontal when compared to the dorsiflexion axes, which tended to have a downward inclination in a lateral direction. Data is not available for the ankle during walking due to the practical difficulties in identifying the talus. However, we have recently undertaken a cadaver-based study using a mechanism that loads the cadaver tibia and we applied loads through residual leg tendons to simulate the walking cycle and thus enable the cadaver foot to “walk.” In that study of 13 cadavers, on average, the ankle showed 21 degrees of sagittal plane motion, 15.3 degrees of frontal plane motion and 10 degrees of transverse plane motion. The average motion pattern is illustrated in “Motion Of The Talus Relative To The Tibia” above. This work has provided some of the most detailed descriptions of foot kinematics to date.4
Current Concepts And Questions About The Subtalar Joint
The complex three-dimensional orientation of the subtalar facets is indicative of its complex kinematics. Motion between the calcaneus and talus produces motion in all three body planes. However, the ratio of motion differs both during the range of motion and between patients. Van Langelaan presented one of the most detailed analyses of subtalar joint kinematics.9 He assessed cadavers during external rotation of the tibia and found the angulation of the axis to the sagittal plane to vary between 5.4 degrees and 32.3 degrees (mean 23.5 degrees). He also found that the angulation of the axis to the transverse plane varied between 23.2 degrees and 56.4 degrees (mean 41.9 degrees) in his sample of 10 cadavers. In their in vivo, invasive research, Lundberg, et. al., described subtalar motion during various single plane movements on a hinged platform (dorsiflexion/plantarflexion, adduction/adduction, inversion/eversion).10 Their work illustrates the capacity for the joint to behave in a variety of different ways depending upon how the foot as a whole is moving. However, the articulations between the talus and calcaneus are such that the motion is highly guided by the facets plus the tight interosseous ligament. Overall, the motion is almost always that of pronation and supination. What remains unexplained is why the motion characteristics vary between individuals the way they do and what effects these variations may have on the ankle and midtarsal joints, in which the talus and calcaneus play an integrated role. As the talus is inaccessible, it is not entirely clear how the subtalar joint moves during walking. However, there are many descriptions of how the heel moves relative to the leg during walking. While these descriptions might be used to indicate subtalar motion, this would assume that only sagittal plane motion occurs at the ankle and we have already made it clear this is not the case.11-14 In broad terms, the heel pronates relative to the leg immediately after heel strike and either remains in the same position or supinates between the forefoot hitting the ground and the middle of mid-stance. Then it supinates relative to the leg during propulsion until the toes leave the ground. In our recent cadaver simulations of stance phase, the calcaneus dorsiflexes, everts and abducts (externally rotates) relative to the talus during the first half of stance. After the middle of stance, the calcaneus plantarflexes, inverts and adducts (internally rotates) relative to the talus. The mean motion data from 13 cadavers is illustrated in “Motion Of The Calcaneus Relative To The Talus” above.4
Understanding The Midtarsal Joint And First MPJ
The midtarsal joint is a functional joint rather than an anatomical joint. The midtarsal joint is formed by the combination of talonavicular and calcaneocuboid joints, and one assumes these two separate anatomical joints function together in synchronicity. There is strong clinical and experimental evidence that one can conceptually consider the joint across the middle of the foot as a single functional unit.15 Viewing these joints as one functional joint makes sense as one can clinically identify and manipulate the articulation while one cannot do so with the separate joints. The midtarsal joint has a considerable freedom of movement in all directions. This is principally due to the ball and socket nature of the talonavicular joint, which has a greater freedom of movement than the calcaneocuboid joint. There are few descriptions of its motion during gait partly because the joint has, until recently, been difficult to identify using current motion capture techniques. Many researchers have also combined the midtarsal joint with more distal bones and articulations. Recent research by Findlow has provided the first description of midtarsal joint motion (see “Assessing Midtarsal Joint Motion” below). The first metatarsophalangeal joint (MPJ) is the source of considerable deformity and clinical presentations. Unfortunately, we know precious little of the biomechanics of the joint but we do have some indications as to the kinematics of the joint during walking (see “A View Of Metatarsophalangeal Joint Motion” below). We have little information of the biomechanics of the soft tissue complexes of the joint including the details of the role of the sesamoid bones. These are critical in the mechanics of the soft tissue forces applied across the first metatarsophalangeal joint. Any study of these to date has been cadaver based and involved false loading or movement strategies as opposed to investigating these structures during walking conditions. We hope the use of cadaver-based walking simulators will help overcome these issues on the future.
Pertinent Insights From Simulated Walking Cadaver Experiments
When it comes to the existing research on the biomechanics of the foot and ankle, there is a considerable dearth of information regarding the articulations of the midfoot, those articulations between the navicular, cuboid, cuneiforms, metatarsal and the toes. We typically consider the joints of the middle part of the foot to be entirely rigid, moving little or not at all. Our recent research reveals the inadequacy of this concept and that all parts of the foot move and are important in the foot performing its biomechanical function.4 During our simulated cadaver walking experiments, the motion between the cuneiforms and navicular, and the cuboid and cuneiforms was greater than we had anticipated. For example, during stance, the range of motion between the medial cuneiform and navicular joint averaged 11.4 degrees, 8.3 degrees and 4.5 degrees in the sagittal, frontal and transverse planes respectively. The metatarsals moved 2 to 8.9 degrees relative to each other in the three body planes. However, the motion of the metatarsals relative to their proximal structures (cuneiforms and cuboid) was not consistent between metatarsals. The motion between metatarsals four and five and the cuboid was consistently greater (5.1 to 12.9 degrees) than the motion between the other metatarsals and their cuneiforms (4.6 to 7.7 degrees). Given this data and the evident intermetatarsal motion, one could argue against the concept of a tarsometatarsal joint, in which all five metatarsals function in tandem. Our data reveal that the motion among the navicular, cuneiforms, cuboid and five metatarsals is likely to be of clinical significance and demonstrates that all these joints have an important role in the overall kinematic function of the foot. For example, based on our data, sagittal plane motion between the navicular and talus, navicular and medial cuneiform, and medial cuneiform and the first metatarsal (the medial column of the foot), totaled 29.2 degrees during stance. Clearly, these joints have a role in the sagittal plane progression of the body over the weightbearing foot, and it is not the sole function of the ankle and subtalar joints. Furthermore, the motion between the cuneiforms and navicular (9.8, 8.5 and 10.5 degrees for the medial, central and lateral cuneiforms respectively in the sagittal plane) was comparable to, or in some cases exceeded, the motion between the talus and navicular (12.2 degrees) and the motion between the calcaneus and cuboid (9.8 degrees).
Raising Other Key Questions And Unresolved Biomechanical Issues
Several other aspects of foot and ankle biomechanics are critical to a complete understanding. When it comes to the intrinsic muscles under the foot, little is known of their role due to their small physical size, their location beneath the plantar fascia and their layered arrangement. While they clearly have a role in creating plantarflexion moments across the major joints of the foot, the presence and effects of variation between feet, and when and what forces they produce are largely unknown. One could say the same thing about the peroneal muscles. They too have proven difficult to evaluate using EMG due to the size and proximity to each other but the very different insertions of longus and brevis are indicative of complex and different functions. Much of the research into the foot and ankle has involved subjects walking barefoot. Of course, this is rarely the reality for patients and how shoes affect foot function is also poorly understood. We also don't have a strong understanding of how and why the foot moves within the shoe and how different shoe designs affect the biomechanics of the foot. Inevitably, the foot moves inside the shoe and a description of heel motion is not synonymous with a description of the motion of the heel of the shoe. Also, there is likely to be anterior/posterior sliding and significant frictional forces under the forefoot. These issues have also not been quantified in relation to shoe design. In another important and unresolved issue, we are not sure whether the mechanical function of the foot is determined by the forces in and motion of the proximal structures of the limb (tibia, femur, pelvis and their muscles) or whether it is the foot that controls the proximal limb. Bellchamber and van den Bogert presented the most clear investigation of whether the foot controls proximal rotations.16 Using an inverse dynamic technique, they calculated the power flow associated with transverse plane tibial rotation from the foot to the tibia for both walking and running. Their results show that, during walking, axial motion of the tibia drives rearfoot motion between the time periods of 5 to 20 percent and 60 to 100 percent of stance. During running, the same pattern is present with slightly different timing. When running, the tibia drives the rearfoot between 10 to 25 percent and 70 to 100 percent of stance. During the remaining periods of stance phase, it was not possible to establish whether the foot was driving axial tibial motion or vice versa. Given these concepts, we are currently undertaking work to evaluate the potential for different gluteal and hamstring recruitment patterns to influence foot motion. To date, we have been largely unable to measure shear forces in different areas of the foot. Therefore, any biomechanical tissue model, related to ulcer formation for example, has been unable to account for these forces in detail. (Of course, one can measure shear forces using a force platform but this relates to the entire foot and how these shear forces are distributed is unknown.) We have recently been involved in the development of shear sensors for this purpose. However, the work has revealed several fundamental difficulties. One of the primary obstacles is the fact that the sensor must have at its surface the same frictional and compression properties as the insole and the shoe touching the rest of the foot. If it does not, the sensor will adversely affect (usually elevate) the vertical forces under that area of the foot and, of course, alter the shear forces we intend to measure. From what we understand, major in-shoe pressure measurement manufacturers have also tried and failed to address this problem.
In many respects, the foot remains the last unknown link in the lower limb chain. The hip and knee have been well researched and at the stage where quality joint replacements can be made, it is clear that our understanding of their function is strong. The same cannot be said of the foot. We understand the movement of the four major articulations in the foot (the ankle, subtalar, midtarsal and metatarsophalangeal joints). However, we know little of how the small midfoot joints move and how the musculature within the foot acts. Our own research has confirmed the complexity of rear-, mid- and forefoot kinematics, demonstrating that the articulations distal to the talonavicular and calcaneocuboid joints can contribute significantly to the overall kinematic function of the foot. However, this has only served to confirm how little we actually know. Previous emphasis on the importance of the rearfoot joints, particularly the ankle and subtalar joints, has neglected more distal articulations. There is a wealth of other issues that require more research. We know little of where and what effect shear forces have on the foot. It is not clear what role the foot performs as part of the lower limb. Does it simply move as dictated by the proximal structures or can it be more influential in controlling the lower limb? Little is truly understood about how shoe design and fitting affects the foot biomechanics nor how these factors could be used as interventions or adjuncts to other interventions such as foot orthoses and joint replacements. With so many fundamental issues unresolved, there is great potential for significant change in our understanding of foot and ankle biomechanics as the research continues over the coming decades. Dr. Nester is a Senior Research Fellow at the Centre for Rehabilitation and Human Performance Research at the University of Salford in the United Kingdom. Mr. Findlow is part of the podiatry academic faculty and member of the Centre for Rehabilitation and Human Performance Research at the University of Salford in the United Kingdom. Mr. Liu is a Research Fellow at the Centre for Rehabilitation and Human Performance Research at the University of Salford in the United Kingdom. Dr. Ward is affiliated with Iowa State University. She practices in Perry, Iowa. Dr. Cocheba is affiliated with Iowa State University. He practices near Seattle. For related articles, see “ACloser Look At Studies In Gait Analysis”and “Secrets To Biomechanical Considerations In Static Stance” in the August 2005 issue of Podiatry Today or “Reconsider Biomechanical Causes In Heel Pain Cases” in the November 2002 issue. Also be sure to check out the archives at www.podiatrytoday.com.
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