Static Ankle Joint Equinus. Toward a Standard Definition and Diagnosis

Abstract

Equinus is characterized by reduced dorsiflexion of the ankle joint, but there is a lack of consensus regarding criteria for definition and diagnosis. This review examines the literature relating to the definition, assessment, diagnosis, prevalence, and complications of equinus. Articles on equinus and assessment of ankle joint range of motion were identified by searching the EMBASE, Medline, PubMed, EBSCOhost, Cinahl, and Cochrane databases and by examining the reference lists of the articles found. There is inconsistency regarding the magnitude of reduction in dorsiflexion required to constitute a diagnosis of equinus and no standard method for assessment; hence, the prevalence of equinus is unknown. Goniometric assessment of ankle joint range of motion was shown to be unreliable, whereas purpose-built tools demonstrated good reliability. Reduced dorsiflexion is associated with alterations in gait, increased forefoot pressure, and ankle injury, the magnitude of reduction in range of motion required to predispose to foot or lower-limb abnormalities is not known. In the absence of definitive data, we propose a two-stage definition of equinus: the first stage would reflect dorsiflexion of less than 10° with minor compensation and a minor increase in forefoot pressure, and the second stage would reflect dorsiflexion of less than 5° with major compensation and a major increase in forefoot pressure. This proposed definition of equinus will assist with standardizing the diagnosis and will provide a basis for future studies of the prevalence, causes, and complications of this condition. (J Am Podiatr Med Assoc 100(3): 195–203, 2010)

Search Strategy

This article reviews the literature on ankle joint equinus and devices that measure ankle joint range of motion, identified from searches of the EMBASE, Medline, PubMed, EBSCOhost, Cinahl, and Cochrane electronic databases. The searches were not limited by date or language and used various combinations of the following key words: equinus contracture, ankle joint, range of motion, measur, apparatus, tool, device, and instrument. This search strategy yielded 214 articles published between January 1, 1988, and October 1, 2008 (inclusive). The abstracts of these articles were read to select articles that were directly relevant to the topic of this review, and the reference lists of these selected articles were examined to identify additional relevant articles that had not been detected by the electronic search. This strategy provided 39 articles for inclusion in this review.

Ankle Joint Motion

The term ankle joint refers to the articulation of the talus between the lateral and medial malleoli. The movements of the ankle joint are dorsiflexion and plantarflexion, with the axis of rotation of the joint travelling obliquely downward and laterally, generally in line with the center of the tips of the lateral and medial malleoli.[1–3] As a result of the position and alignment of the axis of rotation, the movement about the ankle joint was considered to be triplanar with a single degree of freedom[3,4] such that when moving the ankle joint from a neutral position through 25° of dorsiflexion there is a corresponding 4° of supination and 6° of external rotation.[3,4] Mattingly et al[1] used magnetic resonance imaging to examine the rearfoot during nonweightbearing movement of the ankle and found that the movement of the talus was biplanar rather than triplanar. Mattingly et al[1] did not specify which two planes the ankle joint moved through but implied that there was negligible eversion of the talus at the ankle joint.

Definition and Diagnosis of Equinus

Although it is generally accepted that static ankle joint equinus reflects a reduced range of dorsiflexion at the ankle joint, there is no consensus regarding what magnitude of reduction in dorsiflexion is required to constitute this condition. This lack of consensus has resulted in practitioners using a wide range of dorsiflexion limitation for diagnosis.[5] Sobel et al[6] suggested that to have a diagnosis of equinus, patients should have less than 0° of dorsiflexion (ie, no movement beyond plantargrade), whereas Orendurff et al[7] recommended a cutoff value of 5°.

On the other hand, DiGiovanni et al[8] recommended a cutoff value of less than 10° of dorsiflexion, which is in accord with the need for at least 10° of dorsiflexion to achieve normal gait and prevent potential increased loading of the forefoot during locomotion.[7,9,10] This latter suggestion is consistent with the more recent recommendation of Meyer et al[11] that rather than basing a diagnosis of equinus on a particular range of motion for dorsiflexion, a diagnosis should be confirmed when there is a reduction in dorsiflexion of a magnitude that increases tension on the Achilles tendon and loading on the forefoot. Although basing a diagnosis of equinus on a limitation of 10° of dorsiflexion would be expected to increase forefoot pressure during locomotion, there is no evidence that this will lead to the development of foot or lower-leg abnormalities. Furthermore, although a cutoff point of 10° may increase forefoot loading during locomotion, Orendurff et al[7] suggested that a 5° range of dorsiflexion be used for the diagnosis of equinus because they found that forefoot pressure was greater in patients with less than 5° of dorsiflexion compared with patients with more than 5° of dorsiflexion (P < .05). Thus, there is support for limiting dorsiflexion to either 5° or 10° as the basis for a diagnosis of equinus, with a 10° cutoff point being based on gait changes, which can increase forefoot loading, and a 5° cutoff point being based on a greater level of forefoot loading. However, although greater limitations in range of motion for dorsiflexion increase forefoot loading, the threshold reduction that leads to an increase in forefoot pressure sufficient to promote the development of foot or lower-leg abnormalities is not known, and a diagnosis of equinus should be based on criteria that will ultimately lead to the development of foot or lower-leg abnormalities.

Therefore, without any prospective studies to determine the effects of different levels of restriction of dorsiflexion on forefoot loading, and the longer-term effects of this loading on foot health, it is difficult to define a particular range of motion below which a definition of equinus can be justified. There is an urgent need for prospective studies to examine the relationships between limitations in range of motion for dorsiflexion and the subsequent development of foot or lower-leg abnormalities. In the absence of definitive data, if using a valid and reliable tool, we propose based on the evidence currently available, a two-stage definition: stage 1 would reflect dorsiflexion of less than 10°, indicating minor compensation and minor increased forefoot pressure, and stage 2 would reflect dorsiflexion of less than 5°, indicating major compensation and major increased forefoot pressure.

Measuring Ankle Joint Range of Motion

Although the lack of a universally accepted definition of equinus poses some difficulty for diagnosis, this is further complicated by the fact that there is no standardized method for measuring dorsiflexion range of motion at the ankle joint. Traditionally, in a clinical setting, ankle joint range of motion is measured by the clinician dorsiflexing the patient’s foot passively while aligning one end of a goniometer with the fibula and the other end with the fifth metatarsal bone. However, this method introduces a variety of potential sources of error. The positioning of the knee can affect the range of motion achieved because if the knee is flexed during the assessment it reduces the effect of the gastrocnemius muscle on limiting the range of motion, with the range of motion being limited by tension in the soleus muscle, bony limit, or the end of range of the ligaments. Whether the patient is sitting up or lying down and whether the subtalar joint is maintained in a neutral position can also affect the measurement of dorsiflexion recorded.[12,13] In a sitting position, the range may be limited by neurovascular tissues,[12,13] and if the midfoot or rearfoot is allowed to move, the measurements will be compromised.[5]

Other major sources of error include the potential to incorrectly align the goniometer with the fibula and the fifth metatarsal and the axis of the goniometer with the axis of movement and the application of nonuniform torque, resulting in a greater or lesser degree of dorsiflexion.[14] Backer[15] compared goniometry measurements of ankle joint range of motion with radiographic measurement and found that clinical goniometer measurement overestimated range of motion, particularly when range of motion was limited. It was suggested that midfoot and rearfoot movement caused the overestimation in goniometric measurements, ie, if the foot is allowed to move at its many joints it will dorsiflex and increase the range of motion measurement without any additional movement of the ankle joint. Martin and McPoil[14] recently reviewed 11 studies that examined the reliability of manual goniometry and found good intrarater reliability for the measurement of ankle joint range of motion (intra-class correlation coefficient [ICC], 0.72–0.99) but poor interrater reliability (ICC, 0.29–0.81). Evans and Scutter[16] found very poor reliability for intrarater and interrater goniometric assessment of ankle joint range of motion. Evans and Scutter[16] reported that the ICC for intrarater reliability ranged from 0.12 to 0.78 with the knee extended and from 0.12 to 0.13 with the knee flexed. Interrater reliability was worse, with ICCs ranging from 0.03 to 0.05. Thus, although manual goniometry may be useful in some clinical contexts, its reliability is questionable, which limits its applicability for use in research.

Recognizing the limitations of manual goniometry, a variety of mechanical devices have been developed in an effort to standardize the assessment of ankle range of motion and improve reliability; 12 such devices were found in this database search.[11,17–26] Although some of these devices may be useful, they are not without their limitations, not the least of which is that they all use static nonweightbearing measurements to predict weightbearing dynamic function, and the validity of this is questionable. Other limitations are discussed herein.

Criteria

Several characteristics should be considered when selecting equipment to assess ankle joint range of motion. An appropriate device would ideally require only a single operator to use, be relatively inexpensive to build or purchase, be portable, align its axis of rotation with the axis of rotation of the ankle joint, reduce or remove any potential for parallax error when reading measurements, facilitate the adoption of standardized body and limb positions, hold the foot securely in a neutral position throughout range of motion, and provide valid and reliable measurements.

Number of Operators

Many clinicians and researchers work independently, making it desirable for any device to be able to be used by a single operator. The publications that describe the devices in the literature have not made it clear whether the devices can be operated by a single user[17,19]; however, it seems that in most devices this is the case, except for those developed by Moseley[18] and by Scharfbillig and Scutter.[25] Moseley’s device uses a camera to record range of motion as a second operator applies torque to move the ankle through range of motion. It may be possible to reduce the number of operators required by mounting a camera that automatically records images. The device developed by Scharfbillig and Scutter[25] requires two operators because the inclinometer that records the change in angle of the ankle joint cannot be seen by the operator who has to apply the torque to move the ankle through its range of motion. Again, this tool could be modified by moving the inclinometer to a position where it can be seen by the operator applying the torque or by using an inclinometer that automatically records the change in angle.

Portability

Portability of measurement devices is desirable to allow data collection across a variety of sites and in the field. Many of the existing devices meet this requirement, but some are too large or too heavy to be easily transported.[17,19,20] This lack of portability is a significant disadvantage that considerably reduces the utility of these instruments.

Axis of Rotation

For any device to provide valid measures, its axis of rotation must align with the axis of rotation of the ankle joint. Most devices described in the literature have attempted to achieve this by aligning their axis of rotation with the lateral malleolus, which approximates the axis of rotation of the ankle joint[2,3]; however, they use a coronal axis, which allows for sagittal plane movement only. Moreover, most devices have a fixed axis of rotation that does not accommodate for differences in limb size. Only the device designed by Rao et al[26] has an adjustment that allows for the axis of rotation to be moved to better align with the ankle axis when assessing limbs of different sizes and shapes.

Reliability

It is essential that any device should produce reliable (ie, reproducible) measures. Reliability measures have been reported for most devices, and, for the most part, reliability has been high. This is particularly the case for interrater reliability, which far exceeds the interrater reliability that can be achieved using manual goniometry. Indeed, the device designed by Harvey et al21 reported the lowest ICC for interrater reliability (ICC = 0.84), whereas that designed by Mayhew et al[20] reported the highest (ICC = 1.00). All of the devices reported good intrarater reliability, with ICC values ranging from 0.77 to 1.00.[17,20] Thus, it seems that most devices described in the literature have good intrarater and interrater reliability and, in particular, provide values that are considerably superior to those that can be achieved using manual goniometry. Furthermore, the variability in measurement with these devices is small, with the standard error of the measurement reported by Scharfbillig and Scutter[25] to be 0.63°, by Orendurff et al[7] to be 0.8°, and by Meyer et al[11] to be 0.60°. This low level of variability allows for precise estimations of differences in range of motion. If the clinician or researcher is using a custom-built tool with a standard error of the measurement less than 1°, this will provide sufficient sensitivity to discriminate between the 5° and 10° cutoff points of the proposed two-stage definition of equinus. This will minimize misdiagnosis and facilitate more accurate evaluation of the effects of any treatment plan on ankle joint range of motion.

Position of Body, Leg, and Foot

Achieving good reliability is, in part, related to the ability to standardize positioning of the body, leg, and foot during assessment to prevent compensatory movement or differences in muscle length, which may affect range of motion. Although standardization of body position can be achieved by having a volunteer sit or lie down, lying down is preferable because sitting up may apply tension on neurovascular tissues.[12,13]

Standardization of the positioning of limbs has been attempted with the assistance of braces or padding, but perhaps the best approach to standardizing the position of the leg being assessed was incorporated into the devices designed by Meyer et al[11] and Weaver et al.[23] Both of these devices attach directly to the leg along the line of the tibia, increasing the likelihood of being able to accurately achieve a neutral (90° to tibia) starting position. The device developed by Weaver et al[23] attaches to the anterior aspect of the tibia, eliminating the effect of muscle and fat. All of the other devices require estimation of the neutral starting point, a potential source of error. The devices designed by Meyer et al[11] and Weaver et al[23] also use heel cups to reduce rearfoot movement during assessment. This assists in ensuring the correct starting position and reducing compensatory movement in the leg and foot during assessments, which should assist in providing valid and reliable results. Indeed, in support of this, the reliability reported by Weaver et al[23] and by Meyer et al[11] was as good as, if not better than, that reported for the other devices. However, although the use of heel cups can reduce rearfoot movement, none of these devices fix the foot securely enough to prevent all compensatory joint movement, and this may potentially lead to some overestimation of ankle joint range of motion.

Torque Application

In addition to standardizing the body, leg, and foot positions during assessment, it is also important that standardized torque be applied to the ankle to achieve valid and reliable estimates of range of motion. All of the devices described in the literature attempted to apply standard torque. The device that seems to be best designed to apply constant, standardized torque is that developed by Meyer et al.[11] This device uses a crank that has an electronic digital display of the torque being applied at the joint axis to dorsiflex the ankle. Use of this digital display would allow for a more consistent application of torque and avoid parallax error, which might occur if a mechanical display were used. Other devices have also attempted to avoid parallax error by using electronic force and angle measurement components,[20–22,24] but many devices do use mechanical readouts and alignments, which are subject to parallax.[11,17–19,23,25,26] However, although there is the potential for parallax to introduce error into the assessment of ankle range of motion, no studies, to our knowledge, have specifically examined the magnitude of this error.

The magnitude of the torque applied is of paramount importance when testing range of motion. The devices identified in this review had a wide range of torques applied (6–25 Nm).[19,26] It is likely that increased torque will result in increased passive range, but it may be argued that the torque applied when measuring functional dorsiflexion should be similar to that experienced in normal gait. The most commonly used torque is 15 to 17 Nm[18,20,24–26]; we believe that this is the torque that best replicates that experienced in gait. It may also be argued that the amount of torque applied should change depending on an individual’s height, weight, and foot length. In an effort to address differences in foot length, some devices have allowed for the application of torque through a footplate, thus negating any effects of differences in foot length. Ideally, an appropriate torque would be applied that replicates an individual’s ground reaction forces during locomotion, but methods for determining such torque require sophisticated and expensive gait analysis equipment and specialized expertise to operate and so is beyond the scope of most clinical practices. In the absence of such equipment and expertise, it is proposed that a standard torque needs to be applied to compare findings from different studies. Because 16 Nm has been used frequently in the past, it is suggested that this be used as a starting point for other studies and for use in clinical practice.

Cost

For any device to be adopted widely, it would be favorable for its cost of purchase or construction to be inexpensive. From the published literature describing the various devices for assessing ankle range of motion, it is difficult to determine how expensive the devices would be to purchase or build, with the only cost of purchase provided by Mayhew et al[20] for a commercial multijoint isokinetic dynamometer that was used to assess ankle joint range of motion ($25,000 US). Thus, sufficient data are not available to comment on the costs associated with the construction of devices for the assessment of ankle range of motion.

Of the devices described in the literature reviewed, that designed by Meyer et al[11] rates highly on most criteria set by the authors and rates best overall compared with the other devices in this evaluation, suggesting that this device is potentially the most appropriate for use in clinical and research applications. Selection of a tool will vary depending on the situation, requirements, and resources of individual clinicians and researchers.

Causes of Equinus

The etiology of equinus is poorly understood, but a variety of potential causes have been identified and discussed in the literature. The reduction in dorsiflexion associated with equinus can be caused by tonic contraction or congenital shortening of the gastrocnemius or soleus muscle, adaptive shortening of these muscles, bony block, or joint stiffness.[10,27,28] A range of neurologic abnormalities can affect the functioning of the ankle (eg, cerebral palsy and brain injury) and may, therefore, potentially contribute to the development of equinus. However, a discussion of all of these potential neurologic abnormalities is beyond the scope of this review, which instead focuses on the contribution of congenital deformities of the foot and ankle and the roles of aging and type 2 diabetes in the development of equinus.

Congenital Equinus

Sobel et al[6] conducted a prospective study in 60 idiopathic toe walkers aged 1 to 15 years and found that 46% had no dorsiflexion beyond plantargrade and that all had less than 8° of dorsiflexion, thus directly relating reduced ankle joint range of motion to altered dynamic function. Sobel et al[6] found that most toe walking in their sample was caused by a short Achilles tendon and that toe walking resolved with time in most participants. However, despite toe walking resolving with time, the range of dorsiflexion did not increase, indicating that some structural compensation in the foot occurred in some subjects that eventually allowed the heel to touch the ground during walking.

Acquired Equinus

There is evidence to suggest that equinus can develop as part of the aging process because the range of dorsiflexion has been found to be lower in the elderly. Grimston et al[29] compared ankle joint range of motion in young (14–16 years old) and old (70–79 years old) male and female volunteers (n = 120) and found that 14- to 16-year-olds had 29% greater ankle joint range of motion compared with 70- to 79-year-olds, with the greatest difference evident in females. The lesser ankle joint range of motion in older people is most likely due to increased passive resistive torque and passive elastic stiffness. Gajdosik et al[30] measured passive resistive torque and elastic stiffness in a cross-sectional study of 20- to 39-, 40- to 59-, and 60- to 84-year-old women (n = 81) using an isokinetic dynamometer to produce dorsiflexion and found that both of these parameters were reduced with age, as was range of motion for dorsiflexion. The younger women had, on average, 26° of dorsiflexion, the middle-aged women had 23° of dorsiflexion, and the older women had 15° of dorsiflexion. The authors suggested that the reductions in range of motion, passive resistive torque, and elastic stiffness were most likely due to reduced elasticity of ligaments, fascia, and skin. In a similar study in men aged 20 to 35 years and 65 to 80 years (n = 17), Allinger and Engsberg[19] evaluated ankle joint range of motion and reported that both groups had similar range of motion for dorsiflexion but that the older men had reduced plantarflexion, eversion, inversion, abduction, and adduction. However, in this study, skin markers were used to identify landmarks, which were recorded with video cameras, and movement of the skin over the bony landmarks during performance of the ankle movements may have confounded the ability to validly assess range of motion.

Kim et al[31] compared cross-sectional slices of muscle tissue using electron microscopy in young and old rats (n = 344) and found that age-induced sarcopenia was associated with muscle weakness, a reduced muscle fiber number, a reduced muscle fiber cross-sectional area, and increased muscle connective tissue content. The higher connective tissue content in the muscles of older rats exhibited a 3.7-fold greater extramycocyte (ie, connective tissue) area, and this tissue was found to contain more collagen (9.7%) compared with the muscle connective tissue of younger rats (3.7%). The increased connective tissue content in the muscle of older rats, particularly the higher collagen content, would be expected to contribute to greater passive tension and reduced extensibility, which could contribute to reductions in joint range of motion.

Type 2 Diabetes and Equinus

Type 2 diabetes is associated with increased oxidative stress and increased glycation of proteins, and this may contribute to a reduction in joint range of motion.[32,33] Markers of oxidative stress (protein carbonyls) were compared in soleus and plantaris muscles of nondiabetic and diabetic rats, and higher plasma glucose concentrations in diabetic rats were associated with greater oxidative stress and more oxidative damage of muscle proteins.[32]

A study[33] on human tissue has shown that higher glucose levels in patients with diabetes are also associated with increased glycation of proteins. Glycation of connective tissue proteins induces structural changes in tendons, including increased density of collagen fibrils, decreased fibrillar diameter, and abnormal fibril morphological features, which can contribute to the shortening of muscles and reduce their compliance (ie, stretch).[34,35] Connective tissue glycation in patients with type 2 diabetes would be expected to potentially contribute to reductions in joint range of motion, and, indeed, several studies[6,36,37] that have compared joint range of motion in patients with diabetes and controls have found reduced range of motion in diabetic patients. Komatsu et al[37] compared range of motion in large joints in 72 adolescents with diabetes and 46 nondiabetic controls and reported reduced range of motion in the diabetic group associated with higher glycated hemoglobin levels and accumulation of advanced glycation end products. Similarly, in a small study that specifically examined associations between diabetes and range of motion in the ankle, Salsich et al[38] compared ankle range of motion in an age-matched sample of 17 people with diabetes and 17 without diabetes and reported a reduced range of motion for dorsiflexion in patients with diabetes compared with controls. In a similar, but much larger, study, Duffin et al[36] compared range of motion in the ankle and foot joints of 302 adolescents with diabetes and 51 nondiabetic controls and found that patients with diabetes had less range of motion in the subtalar joint and in the first metatarsophalangeal joint compared with nondiabetic controls. Duffin et al[36] also reported double the prevalence of hammer and claw toes in patients with diabetes compared with controls, indicating that the changes in joint range of motion in the foot associated with diabetes were also associated with the development of foot abnormalities.

Therefore, the damage to proteins that can be caused by the oxidative stress and glycation associated with diabetes likely adversely affects connective tissue structures around joints, leading to reduced range of motion in a variety of joints, including the ankle. These changes in range of motion seem to predispose patients to the development of equinus and potentially to the development of foot and lower-leg abnormalities.

Prevalence of Equinus

Without a universal definition of equinus, or a standardized method for the assessment of ankle range of motion, it is difficult to establish the prevalence of this condition. This point was emphasized in a review by Digiovanni et al,[5] who indicated that most of the literature reporting on the prevalence of equinus was based on observational or anecdotal evidence, making it difficult to estimate the true prevalence of the condition.

DiGiovanni et al[8] used an equinometer to examine ankle joint range of motion in patients with foot pain (n = 34) and in controls without foot pain (n = 34) matched for weight, age, and sex. Using ankle dorsiflexion of less than 10° as the criterion for equinus, DiGiovanni et al[8] reported that the prevalence of equinus in the group with foot pain was 65% compared with 24% in the control group and suggested that the prevalence in their control sample was generalizable to the population. However, the sample used in this study was limited to service veterans and their spouses, and the findings are, therefore, unlikely to be generalizable to the broader population.

Thus, at present, few studies have assessed the prevalence of equinus, with most based on observational or anecdotal evidence. Moreover, no reliable data are available on which a valid estimate of the prevalence of equinus can be based. Further work is required to establish the prevalence of this condition.

Complications of Equinus

During the stance phase of gait, when the knee extends immediately before heel lift, the gastrocnemius and soleus muscles work eccentrically to decelerate the forward momentum of the leg over the foot,[10,35] and if the range of dorsiflexion is limited during this stance phase, loading of the forefoot can be increased.[7,39] Thus, equinus may be associated with increased forefoot loading during locomotion.

Without a minimum of 10° of dorsiflexion at the ankle, when the leg and trunk move over the foot during locomotion, the heel will lift prematurely, which will increase forefoot pressure, or the foot will pronate excessively to compensate. It would be expected that the greater the limitation in dorsiflexion, the greater the loading on the forefoot. This proposition was supported by findings from Orendurff et al,[7] who reported that peak forefoot pressure was increased during gait in people with limited dorsiflexion. Indeed, the magnitude of the reduction in dorsiflexion was inversely related to forefoot pressure such that a greater limitation in dorsiflexion was associated with greater loading of the forefoot during locomotion. Subsequently, DiGiovanni et al[8] indicated that individuals who lacked the ability to dorsiflex the foot as a result of equinus developed pain in the midfoot, the forefoot, or both. This development of pain was most likely related to the increased forefoot loading reported by Orendurff et al[7] and excessive repetitive compensation at the midfoot and forefoot joints.

More recently, Kelly et al[40] proposed that equinus is commonly associated with structural breakdown of the foot and, if left unaddressed, could lead to unstable and inefficient gait, which would increase the risk of foot deformity and negatively affect walking ability.

Apart from increasing forefoot loading and increasing foot pain, evidence suggests that equinus may also be associated with an increased risk of ankle sprain, with a recent meta-analysis of 13 studies[41] reporting that reduced ankle joint range of motion was a predictor of ankle sprain. Ankle joint stability increases when the foot is dorsiflexed owing to the tapered nature of the talus, allowing it to fit more tightly between the tibia and fibula heads. Thus, a reduced ability to dorsiflex would reduce ankle stability during gait, predisposing to a risk of ankle sprain.

Conclusions

Equinus is defined as a reduction in the range of motion for dorsiflexion at the ankle, but there is no consensus regarding the magnitude of reduction required to constitute a diagnosis. Various levels of reduction in dorsiflexion have been suggested as constituting equinus, with 0°, less than 5°, and less than 10° of dorsiflexion being proposed as thresholds for diagnosis. Range of motion for dorsiflexion of less than 5° has been shown to significantly increase forefoot pressure during gait compared with dorsiflexion of 5° to 10°, and we recommend a two-stage definition of equinus, with stage 1 reflecting less than 10° of dorsiflexion and stage 2 reflecting less than 5° of dorsiflexion. Assessment of range of motion with sufficient precision for accurate diagnosis should be possible provided that the practitioner is using a reliable tool, with a low standard error of the measurement similar to that for devices already reported in the literature. There are no data indicating the longer-term effects of impaired dorsiflexion range of motion and increased forefoot pressure, and long-term prospective studies are required.

Apart from the lack of a standard definition for the diagnosis of equinus, there is also no standardized method for measuring ankle joint range of motion. Ankle joint range of motion has previously been measured primarily using manual goniometry, but in recent times a variety of purpose-built devices have been described and evaluated in the literature that provide better reproducibility of assessment. There remains no consensus on which device is most appropriate for use, as all have limitations that reduce their utility. In particular, they all provide nonweightbearing measures of ankle joint range of motion, which may limit their ability to predict dynamic function. Nevertheless, the device designed by Meyer et al[11] seems to be the one that meets the most desirable criteria for such a device and so may be the most useful for clinical and research applications of those currently available; however, it may not be the most appropriate in all situations.

Without a standard definition of equinus or a standard method for assessing ankle range of motion, it is difficult to determine the prevalence of this condition or to investigate its etiology. Nevertheless, there is some evidence that equinus may be congenital or acquired, with some suggestion that its prevalence may increase with age and in response to complications of diabetes.

Further research should aim to standardize the method of assessment of ankle range of motion and develop a standardized definition for the diagnosis of equinus. This will facilitate investigation of the true prevalence of this condition and its etiology and allow for assessment of the adverse health effects of the condition and the efficacy of potential treatments.