CORR Insights®: Loss of Mechanical Ankle Function Is Not Compensated by the Distal Foot Joints in Patients with Ankle Osteoarthritis

Where Are We Now?

In kinematics and kinetics research, it is very difficult to model the ankle and foot segments. Doing so in a realistic way is important, because researchers believe these approaches might improve clinicians’ abilities to provide orthotic devices or surgery that can improve gait in patients with a variety of disabling conditions [10, 12]. Although the foot has historically been modeled as a single rigid segment, we know that this concept is not nuanced enough to give us the biomechanical information we need when studying human motion. Early representations of the foot as a rigid structure have recently been replaced with multisegment models that can estimate motion separately for smaller segments of the foot [10].

Similar to a motion analysis of the knee or other large joints, a kinematic analysis of the foot (and more generally, gait studies of the foot) is especially limited by challenges with marker placement. Externally placed optic markers on the surface of the skin are easy to use but can generate artifact errors associated with skin movement. Models using intracortical bone pins or cine-radiographic models are more-precise, but have obvious disadvantages pertaining to their invasiveness and complexity. Movements in the sagittal plane occur mainly at the talocrural joint, and those in the transverse (horizontal) plane mainly occur around the rear of the foot or another axis, and these movements could be measurable. However, it is not easy to measure united subtalar-midfoot-hindfoot movements.

Multisegment foot models are further challenged by decisions about the number of foot segments to include, which bony landmarks to use, how many optical markers to attach, and how to distribute those markers [10]. When the patient is wearing shoes, the situation is even more complicated [3]. We have little information to guide us regarding relevant measures of reliability such as intertrial, intersession, and interexaminer variability in multisegment kinematic analyses of the foot [6]. A systematic review found that many different foot models have been reported, but there is inadequate evidence to support their clinical use [7]. Several proposals for improvement have been made to overcome this situation, such as reporting standards for the transparency of methods or protocols for comparisons [1, 2, 9, 12]. Eerdekens et al. [8] give new insight into such topics with their recent article in this issue of Clinical Orthopaedics and Related Research®.

This group [8] performed a kinematic and kinetic study to evaluate movement of the ankle and foot segments, comparing patients with ankle osteoarthritis with those without this diagnosis. Participants underwent a three-dimensional (3-D) gait analysis, during which a multisegment foot model (the Rizzoli foot model [11]) was used to quantify the following variables in the ankle: the Chopart, Lisfranc, and first metatarsophalangeal joints during the stance phase of walking; joint ROM and joint rotational force (moments); and joint power generation and absorption.

They found patterns of stiffness and power generation in patients with ankle osteoarthritis that they attributed to ankle stiffness that resulted in difficulty protecting the midfoot from the forces of gait [8]. This is important because it helps to explain some of the secondary arthritic changes in the foot we see in patients with ankle arthritis; this indicates potential future therapeutic interventions such as a rehabilitation program to improve the generation of ankle joint power during the push-off phase in plantar flexor training.

Where Do We Need To Go?

Although a technical commentary on motion analyses is more than what most readers might wish to read here (and so I will not provide one), it is nonetheless important to realize that the accuracy of the measurements made in this kind of research heavily depends on the performance of the instrument being used (including elements such as camera resolution) and on a large number of experimental conditions, including the definition of the segment, biomechanical model, angular calculation algorithm, whole marker setting, and actual measurement [9]. As such, future studies that vary those conditions will be important to validate the findings of Eerdekens et al. [8].

In any kinetic study, variables such as walking speed also affect the calculation results. A study of combinations of kinematics and kinetics, which include a force plate, surface electromyography, or other measurement systems such as pressure or inertial sensors, has investigated foot dynamics [10]. One of the serious problems of a motion analysis is, I think, that although researchers are able to obtain a set of calculated values, they often fail to confirm the accuracy and validity of their results. For that reason, future studies should compare different foot models and measurement systems. Simulation experiments linked to clinical data may also be valuable.

Eerdekens et al. [8] mentioned using whole lower-limb power absorption during the stance phase. If a pain-mediated stiffness strategy is a possible compensatory mechanism, research targets might expand a multisegment foot model to a lower-limb model to measure various segments simultaneously [4]. However, I expect that in-foot segmentation analyses will continue to be used for the time being, because it is unnecessary to increase the number of segments and complicate the analysis before the foot problem is solved.

As for joint stiffness, an interesting recent article [13] found that stiffness of the foot depends on not only the longitudinal arch but also the transverse arch. In the foot, the material properties of the inter-metatarsal tissues and the mobility of the metatarsals may additionally influence the longitudinal stiffness of the foot and thus the curvature-stiffness relationship of the transverse tarsal arch. This finding seems to be related to the evolution of bipedalism. In-foot dynamics and biomechanical studies have had interesting results in terms of impact absorption and propulsion in the foot during bipedal walking [5, 12].

How Do We Get There?

While these kinds of biomechanical analyses give us some insight into the function of the foot and help us to develop gait analysis techniques that may someday be practical clinical tools, my greatest concern at the moment is that too many studies have accepted the findings of motion analysis research without verification. For example, apart from an analysis in the sagittal plane, even in a large joint other than the foot, the clinician needs to be very careful when considering the data of adduction or abduction and rotation [7]. In the future, if we proceed with 3-D motion measurement, in which the measurement accuracy is often uncertain, we need to verify the reproducibility and accuracy of these methods. We also need to compare multiple models (such as cine radiographs and precise kinetic sensors) to one another, as has been done before, but only to a limited degree [9].

What is happening in the clinical scenario is the sum of the movement of the nearby joints that try to compensate for limitations of the involved joint’s movement because of the lesion’s progression [5]. Therefore, fusion of each joint is similar to that in a "knockout mouse" model, which is a good example for calculating the motion contribution of each joint. And if there is unilateral ankle or foot involvement, we could compare the pathologic status of the involved side with that of opposite side in the same patient. By accumulating such data, the dynamic mechanism of the foot will become clearer, and the findings of this kind of research will be more useful for clinical decision-making.