Richie

Articles 2

New Perspectives on Lateral Ankle Instability

By: Douglas H. Richie, Jr., D.P.M

Introduction

Despite its frequent involvement in injury, the ankle joint remains poorly understood from a mechanical standpoint. The interdependent movement of the foot and ankle joints as well as the nature and orientation of their joint axes continues to be debated - with significant erroneous information published on these subjects in the medical literature.

The role of the foot - in its entirety - continues to be overlooked in proposing mechanisms of ankle injury and preventive interventions. Yet, the foot has been recognized as the dominant lever, rather than the leg, in causing ankle injuries. In 1929, Cotton wrote:

"it is much more simple to consider...major ankle injuries not as dislocations of the tibia, but as dislocations of the foot, as the foot plays a major role in the matter of reduction..." (1)

Later, Kelikian wrote:

"The talus links two unequal levers - the leg and the foot. In dorsiflexion of the foot, it becomes locked within the tibiofibular socket and serves as a part of the leg; in plantarflexion, it moves with the foot. Undoubtedly, the tibia is more effective as a lever than the calcaneus, which serves as a terminal gear of the shorter lever or the foot. For practical considerations, mainly because it can be maneuvered in attempts of closed reduction, injuries of the ankle mortise are ascribed to the leverage action of the foot and are classified accordingly." (2)

This paper will focus on lateral ankle sprains in discussing three primary areas:

1) The role of movement coupling of the pedal joints in transmitting force to the ankle.

2) Mechanical versus functional instability: results of recent research.

3) The role of passive support vs. dynamic defense mechanisms in preventing lateral ankle injury. Incidence of Injury

Lateral ankle sprains are the most common injury affecting athletes, accounting for up to 25% of all time lost from participation in sports. (3, 4) Lateral ankle ligament sprains comprise 85% of all ankle sprains while eversion sprains of the deltoid ligament comprise 5% of sprains and syndesmosis sprains comprise 10% of these injuries. (5, 6, 7)

Mechanism

Most inversion ankle sprains occur when the foot is plantarflexed at the ankle. (8, 9) This position of the foot is postulated to most likely cause injury because:

1) The plantarflexed talus with its narrow posterior body thrust forward between the malleoli has less stability than in its dorsiflexed position. (2)

2) The anterior talofibular ligament and the calcaneal fibular ligament are under greatest mechanical strain in a plantarflexed position. (10)

Conversely, the position of the ankle in full dorsiflexion has been observed to reoppose the ends of a divided anterior talofibular ligament as well as providing maximal stability against anterior drawer stress. (11) The dorsiflexed ankle is in its close-packed position, i.e., when its joint surfaces are fully congruent, contact areas maximal, and the joint relatively compressed by tension of the tendo-Achilles. (12) In the unloaded ankle, the primary restraint of internal rotation is the anterior talofibular ligament, and the primary restraint of inversion is the calcaneofibular ligament. (13) Studies performed by McCullough and Stormont applied axial load to the dorsiflexed ankle and determined that the articular surfaces of the ankle joint, not the ligaments, provide primary stability against inversion. In both studies, however, the force applied to the ankle through the subtalar joint was in one plane only, i.e., pure inversion, eversion, or pure internal-external rotation.

A knowledge of foot kinematics reveals that pure inversion or pure internal rotation could not be applied through movement coupling of the foot to the ankle. Such pure single plane movements of the foot simply do not occur. Cass and co-workers recognized this fact in their study applying axial load to a dorsiflexed ankle and then allowing simultaneous inversion/eversion and external/internal rotation of the talocrural and subtalar joints. (14) In this study, talar tilt did not occur with sectioning of either the anterior talo- fibular or calcaneal fibular ligaments alone. However, sectioning of both ligaments caused an average of 20° of talar tilt. External rotation of the tibia on the talus increased 13° by sectioning of both the anterior talofibular and the calcaneal fibular ligament. No testing was performed on a plantarflexed ankle.

Pedal Movements and Ankle Sprains: Controversy and Misunderstanding

Of the thirteen tendons that cross the ankle, none insert on the talus. Without muscle attachments, the talus is moved, via ligament attachment, by the bones of the foot lying distal. A subluxatory force applied distally on the foot will have a longer lever arm for action. Conversely, when a force is applied to the calcaneus or, more distal, to the metatarsals during weightbearing, the various pedal joints including the tarsometatarsal, midtarsal and subtalar joints can independently move, thus dampening transmission of force directly to the talus.

So overlooked is the role of the entire foot in the mechanism of lateral sprains that researchers commonly disarticu- late the foot at the tarsus when setting up experimental cadaver models reproducing lateral ligament injury conditions. (12, 13, 14) Authorities on the subject designate the calcaneus as transmitting primary inversion force and internal rotation force to the talus. (2, 12, 13, 14) Yet, in situations where the calcaneus would be the primary transmitter of force, it would also have to be the primary weightbearing bone of the foot - which would only occur during the heel strike phase of walking or running gait. At heel strike, the ankle is in a dorsiflexed, stable position - close-packed with the talus firmly lodged in the tibiofibular socket. Experimental protocols that utilize the calcaneus as the primary or solitary deforming force on the ankle fail to recognize the biomechanics of the foot and the position of the foot when lateral ankle ligament injury is most likely.

As presented and documented earlier, it is the plantarflexed ankle that is most vulnerable to inversion ankle sprains. Clinical situations commonly attributed to precipitating an ankle sprain include landing from a fall, landing from a jump on an opponent's shoe, stepping off a curb and stepping down stairs. (3, 4, 5, 6, 7, 8, 9) In each of these situations, forefoot contact would precede rearfoot (calcaneal) contact, and the forefoot, through a gearing mechanism and movement coupling sequence, would sublux the ankle joint.

Pure inversion or internal rotation of the foot cannot occur within the normal ranges of pedal joint motion. The primary joints of the foot - midtarsal and subtalar - move upon hinge axes that are obliquely oriented to the three cardinal body planes so that pure movement in one plane is not possible. (15, 16) Inman described ankle joint movement about a pure hinge axis that was oriented in an oblique fashion, slightly deviated from the transverse and frontal planes. (17) When the forefoot moves in a direction of plantarflexion, it is also accompanied by movement into internal rotation and inversion. This triplane movement is known as supination. (18)

During walking gait, Root identified several anatomic deformities of the foot that caused compensatory supination of the midtarsal joints and the subtalar joint: a plantarflexed first ray, a forefoot valgus and a short gastroc-soleus complex inhibiting heel strike. (18) However, orthotic strategies to limit supination compensation for these deformities are based on a theory of linear gait where heel strike precedes forefoot loading. (19)

Podiatric physicians, physical therapist and pedorthists have used functional foot orthoses based on the Root theory to provide stability for the athlete and prevent inversion injuries the ankle. (20, 21, 22) The efficacy of such devices has never been verified in a controlled randomized cross-over study. (23) In sports where forefoot contact precedes rearfoot contact, the effect of functional foot orthoses on such movements has remained poorly understood.

Efforts to study the combined kinematics of the pedal joints in closed chain (weightbearing) situations has led to confusing, if not conflicting, data. (24, 25, 26, 27) Without a knowledge of the triplane motion of the midtarsal and subtalar joints, and without careful classification of foot types and foot deformities in experimental material, researchers have designed flawed experimental methods leading to erroneous conclusions. For example, Lundberg and co-workers studied displacement of various pedal joints during weightbearing movements of pronation and supination. A changing joint axis orientation, rather than a stationary constant axis, was reported by Lundberg but actually resulted from experimental design error. (28) Singh and co-workers elegantly disputed theories of multiple-variable axes of the foot and ankle joints. (29) Using a mechanical method to locate the axis of the talocrural joint, a single constant axis of rotation was found, verifying previous findings reported by Inman. (17)

To summarize, it must be recognized that foot movement initiates an ankle sprain. In an inversion sprain, the calcaneus is not the sole transmitter of force to the talus. Supination moment applied to the forefoot initiates movement coupling that is transmitted proximal to the navicular, cuboid and calcaneus. The pedal joints rotate upon axes that allow triplane movement - not pure single plane movement. Passive restraint of an inversion sprain is provided by the articular surface of the talocrural joint and by the lateral collateral ligaments. Plantarflexion of the foot diminishes the articular stability of the ankle because of the anatomic configuration of the tibiofibular socket and the body of the talus. Experiments to determine the significance of restraint provided by these passive structures have often failed to reproduce the movement coupling sequence of the foot in three planes - hence, have provided conflicting data. More recent studies demonstrate that dynamic muscular action far exceeds passive anatomic structures in preventing lateral ankle sprains.

Mechanical Vs. Functional Instability

Clinicians and researchers have classified lateral ankle instability as being either mechanical or functional or both (29, 30). Mechanical instability involves an anatomic aberration such as a disruption of one or more collateral ligaments of the ankle. Biomechanical deformities such as tibial varum, rearfoot varus, or forefoot valgus create mechanical instability. Functional instability has been described by Konradsen as "recurrent sprains and/or a feeling that the ankle gives way, resulting from damage to mechanoreceptors in the lateral ligaments or muscle/tendons with subsequent partial de-afferentiation of the proprioceptive reflex." (30)

Mechanical instability can cause functional instability. Not all ankles that are mechanically unstable actually function in an unstable fashion. (41) Damage to the sensorimotor control system of the ankle has been implicated as the primary cause of recurrent inversion injuries. (31, 33, 34, 35)

Dynamic Defense - Peroneal Reaction Timing

Protective activation of the peroneal muscles (peroneal reaction time) has shown significant delay in patients with chronic lateral ankle instability. Konradsen and Ravn found a significant mean delay of 17 milliseconds in peroneal reaction time in patients with unstable ankles compared to patients with stable ankles. (30) Brunt and coworkers found a 13 millisecond difference in patients with previous Grade II ankle sprains compared to healthy subjects (36).

A contradiction to these findings can be found in two separate studies published by Isakov and Nawoczenski (31, 37). Both studies failed to show a significant decrease in peroneal reaction time in patients with sprained ankles vs. uninjured patients. However, these studies used unilateral inversion stress conditions while studies by Karlsson and Brunt used bilateral perturbation on tilt platforms.

In terms of proprioception, several studies have shown that ankle ligamentous injury will lead to significant distortion of joint position sense. Glencross and Thornton found significant differences between sprained and nonsprained ankles in an active positioning task (34). Konradsen studied 44 patients with clinical Grade II to III first-time ankle inversion sprains. (41) A significant loss of ankle joint position sense was found in the sprained ankle that persisted twelve weeks after injury. Interestingly, Konradsen found no increased peroneal reaction time on the injured side compared to the non-injured side at 3, 6, and 12 weeks after injury. In comparing to previous work performed on subjects with chronic ankle instability, Konradsen concluded that a first time ankle sprain does not compromise peroneal reaction time.

Dynamic Defense - Sequence of Activation

Considerable research has been conducted on the compensatory response to humans to perturbations occurring during gait. (36, 38, 39) The rapid response of lower extremity musculature to such perturbations has been measured on tilting platforms and treadmills during both static stance and gait. (30, 36, 40, 41) The reflex mechanism of the body as a whole attempts to fulfill one primary requirement: to maintain the body's center of mass over the feet.

Sudden displacement of the foot or feet activates a sequence of muscle firing that is dependent upon central generators and programs interacting with peripheral reflexes. (42) Afferent information is provided by proprioceptive, visual, vestibular, and auditory systems.

The programmed leg muscle response varies according to the direction of perturbation and the phase of gait or alignment of the foot on the ground. Unilateral foot and leg displacement evoke a bilateral response pattern with a similar latency of onset on both sides (approximately 55 milliseconds). (43) Both proximal and distal activation of muscles occur in the legs with activation of agonist and antagonist groups. In cases of unilateral displacement of the foot (sprain), a rapid contraction of the muscles of the contralateral occurs to provide a stable base of support. (44) The mediation of afferent input and coordination of bilateral response is controlled by spinal interneuronal circuits, which themselves are under control of the cerebrum and cerebellum. (42)

Proprioceptive afferent input is provided by receptors located in muscles, tendons, joints, and other tissues. The monosynaptic stretch reflex involves muscle spindle receptors connective Ia nerve fibers as well as Golgi tendon organs connecting to Ib fibers (45). During rapid perturbation such as tripping or falling, monosynaptic reflexes are absent and compensation occurs as a result of transmission along Group II and III afferent fibers from secondary muscle spindles. (46) These connect through a polysynaptic reflex system to generate an appropriate response. A central program as well as supraspinal influences interact in a complex manner which is poorly understood. The contribution of vestibular and visual input to these reflexes is minimal. (42) Gravity and pressure on the joints and on the plantar skin surface of the feet may be critical to these reflexes. Experiments during weightlessness in space and during emersion in water shows a compromise in peripheral afferent input and increased reliance on visual and vestibular cues. (42) Otherwise, the vestibular system is primarily involved during falls with stabilizing the head and, along with visual input, compensating body sway. (40) At the same time, leg muscle EMG activity occurring after a fall is predominantly induced by proprioceptive reflex rather than vestibular input. (42)

In terms of lateral foot and ankle perturbations, several studies have evaluated the timing and power of neuromuscular response in the lower leg muscles. Konradsen tested ten subjects with mechanically stable ankles walking and standing on a trap door apparatus capable of suddenly inverting the foot 30° in the frontal plane. (68) Peroneal latency (time for initial peroneal EMG activity) was 54 milliseconds. This reflex latency was significantly faster when the foot was placed in an inverted position rather than an everted position prior to sudden inversion on the trap door.

A significant faster reflex time was found in the peroneal muscles compared with the quadriceps and hamstring, leading Konradsen to conclude that these later muscle reflexes rely on a centrally coordinated program. The peroneal reflex appeared more reliant on peripheral receptors in the tendons or muscles because of the shortened reaction time occurring when the foot was placed in an inverted position rather than everted position.

Konradsen also studied electromechanical delay, which is the time necessary to generate eversion moment after initial EMG activity is noted in the peroneal musculature. This delay of 72 milliseconds must be added to the peroneal reaction time of 54 milliseconds before sufficient muscle tension is developed to prevent inversion. At least 126 milliseconds will thus pass before true protective muscle activity develops - too long of a delay before the ankle can invert to the point of ligament failure. In this study, it took only 80 milliseconds for subjects to invert 30° on a trap door. This concern about peroneal reaction delay was presented by Isakov and Nawoczenski. (31, 37)

Karlsson studied peroneal reaction times in subjects with unstable chronic lateral ankle instability. (29) The reaction time was significantly shorter in stable ankles compared to unstable ankles (68.8 milliseconds vs. 84.5 milliseconds, p < 0.001). Interestingly, when the unstable ankles were taped, the reaction time improved significantly (p < 0.05). Not all ankles responded the same to tape; the most unstable ankles responded best. This interaction between passive external supports augmenting the dynamic defense mechanism has been extensively studied.

Passive External Support

Many studies have been conducted on the effects of tape and braces to prevent and treat ankle sprains. Athletic tape has been reported to effectively prevent ankle sprains in athletes. (47, 48, 49, 50, 51) Other studies have shown either no reduction of injury (52) or a hindrance of athletic performance with taping of the ankles. (53, 54, 55) Until recently, most studies on the effectiveness of tape to resist ankle inversion were carried out in non-weightbearing conditions. (48, 56, 57, 58, 59, 60)

Manfroy and co-workers studied the effects of tape and prewrap on healthy subjects under weight bearing conditions and found that tape significantly improved ankle eversion moment (resistance to inversion). (61) However, after 40 minutes of exercise, this protective benefit of tape was lost. Other studies have verified that tape can lose its mechanical strength as soon as ten minutes after exercise. (48, 49, 60, 62, 63, 64)

The effect of footwear in providing passive support for the ankle has been studied. Garrick not only found protective benefit from taping but also a lowering of risk when athletes combined tape with a high top shoe. (7) Rovere found that a lace-up brace was more effective than tape in preventing ankle sprains in collegiate football players and that combining a brace with a low-top shoe was better than a high top shoe (65).

The effectiveness of tape may not be dependent on its mechanical effect on the ankle. Karlsson studied 20 patients with mechanically unstable ankles verified by stress radiography utilizing the Telos device. (29) When these ankles were taped, no reduction of instability could be measured with stress radiography. However, peroneal reaction time was significantly shortened when the unstable ankles were taped. Overall, peroneal reaction time was significantly shorter in stable vs. unstable ankles. Karlsson concluded that tape helps patients with unstable ankles by facilitating proprioceptive and skin sensory input to the central nervous system. Glick also found that tape improved peroneal reaction time in patients with significant talar tilt. (49) In patients with stable ankles, tape caused no improvement of peroneal reaction time.

As with previous studies on tape, studies on the use of ankle braces have been primarily performed on non-weight bearing subjects or on cadaver models that had the foot disarticulated. Greene studied a semi-rigid ankle brace in non-weight bearing human subjects to determine passive restriction of ankle joint range of motion. (57) In this study, the semi-rigid orthosis was more effective than the tape in limiting inversion, both during and after exercise. Gross has published three studies testing various ankle braces utilizing a Biodex device to measure passive inversion or eversion in non-weight bearing human subjects. (58, 59, 66) Shapiro studied five cadaver ankles to determine the effects of taping and bracing. (14) A Materials Test Systems device determined passive inversion force, moment and stiffness. The cadaver specimens were mounted on a platform with only the calcaneus contacting and with the forefoot removed at the tarsometatarsal joints. The braces and tape provided a more than two times improvement of resistance to inversion.

Ashton-Miller and Manfroy designed a special testing apparatus to measure functional eversion ankle strength of human subjects under full weight bearing conditions in a neutral position and at 32° of plantarflexion. (61, 67) Ashton-Miller utilized this unipedal strength test to measure maximal voluntary resistance to inversion moment developed by 20 healthy adult men in various conditions, including with ankle taping in place, or any one of three different ankle braces. A 3/4 high top shoe increased eversion strength significantly at 0° of plantarflexion (5.9 N-m increase) and at 32° of plantarflexion (3.3 N-m increase). Ankle taping or bracing increased inversion resistance significantly, although no differences were found between taping and any of the three braces. In any shoe, ankle taping or bracing improved inversion resistance by 7.8% at 0° plantarflexion and by 4.6% at 32° plantarflexion.

Of interest in this study was the finding that increased muscular eversion moment developed as the ankle was more plantarflexed, yet effectiveness of passive support provided by ankle braces decreased as the ankle was plantarflexed. Ashton-Miller calculated that at 15° of inversion, the ankle evertor muscles isometrically developed an eversion moment up to six times larger than that developed by a 3/4 high athletic shoe alone. (67) This eversion muscle strength was also three times larger than that developed passively by tape or any one of three popular ankle braces. In a plantarflexed, inverted ankle, the activated and powerful contraction of the peroneal muscles provides a dynamic defense mechanism that is far more effective than any combination of footwear, taping or bracing.

This dynamic defense mechanism appears most effective when the ankle is in its most vulnerable position: plantarflexed and inverted. Peroneal latency is significantly shorter when the ankle is placed in inversion compared to eversion. (68) Active tension in the peroneal musculature is more than 63% greater in an inverted ankle compared to a neutral ankle. (67) When the ankle is plantarflexed 32°, the peroneal muscles generate 73% more power than in a neutral ankle position. (67) These findings suggest a length-tension phenomenon as well as an augmentation of the stretch reflex.

Activating the Dynamic Defense Mechanism

Experimental research suggests that the peroneal latency added to electromechanical delay in the peroneal musculature does not appear to be capable of protecting the ankle from sudden inversion force when tripping, falling, or landing on uneven terrain. In evaluating previous research, this delay would exceed 150 milliseconds. (31, 68) However, these tests were conducted on subjects in a standing position with lower extremity muscles at rest. In the walking, running, or jumping conditions, humans demonstrate pre-activation of lower extremity muscles prior to foot touch down. (69) Preactivation of lower leg muscles prior to ground contact increases segmental reflex activity and stretch velocity. (70, 71, 72) Preactivated peroneal muscles, with fully activated cross bridges of contractile units prior to foot touch down, would provide significantly greater muscle force upon touch down without significant time delay. (73) A higher rate of tension rise will occur during stretching as the foot inverted due to eccentric lengthening contractions that occur in the peroneal musculature. The force per active fiber ratio is greater during eccentric muscular contractures than during concentric conditions. (74)

Plyometric contractions involve a stretch-shortening sequence which combine eccentric and concentric contractions. (75) The force developed from stretch-shortening is greater than in an isometric contraction. (76) In normal running, stretch-shortening determines muscle stiffness and accounts for the spring-like elastic properties of muscle during landing, push-off, and acceleration of the body. (73) Higher brain centers may regulate muscle stiffness prior to touch down when perceiving changes of terrain or surface hardness. (77)

Vestibular and auditory cues may be linked to pre-contraction of lower leg muscles during tripping and falling. (78) Melvill Jones and Watt demonstrated that human subjects deprived of visual input when dropped from a height required a minimum of 74 milliseconds to activate lower leg muscles and prepare for impact. (79) Effective build up of muscle tension could not occur until at least 102 milliseconds. Falls from heights under 5 cm, occurring in less than 100 milliseconds time, resulted in insufficient activation of lower leg musculature in human subjects. Thus, Melvill Jones actually measured sharper impact force on the feet of human subjects falling less than 10 cm compared to those falling from heights greater than 17 cm. Falls above 18 cm, taking 190 milliseconds, were required to fully activate a protective shock absorbing reaction. Some protection is formed from reflex occurring in the otolith apparatus, but requires a height of at least 7.5 cm to activate this reflex. Winter has shown that the foot passes as close as 5 mm to the ground during the swing phase of walking gait. (87)

Higher impact force applied to the ankle has greater potential to cause an ankle sprain. A force of one body weight applied more than 3.4 cm medial to the midline will cause an ankle sprain. (67) A four BW force need only be applied to 0.85 cm medial to the midline for a sprain to occur.

From their data on muscle eversion power in a night isometric condition, Ashton-Miller and coworkers calculated the potential effect of precontracted muscle action prior to ground contact on a 15° inverted surface. (67) The resulting eccentric contraction would increase muscle force from 35.8 to 68.0 N-m. Total equivalent muscle force based on the lever arm of the peroneal longus and brevis was calculated at 2533 Newtons - enough to rupture or tear the peroneal tendons or avulse the styloid process of the fifth metatarsal base.

Validation of Prevention Meaures

Scientific data have validated the use of passive external support to prevent ankle sprains. (7, 47, 65, 48) These supports include high top shoes, tape, lace-up braces, and semi-rigid ankle braces. Further scrutiny of laboratory and clinical studies reveal that some of the scientific conclusions can indeed be challenged.

Tape has not been shown to improve the mechanical stability of the ankle joint. (29) Tape appears to lose its restraining effect after ten minutes of exercise. (48, 49, 60, 62, 63, 64) The clinical benefits of taping, demonstrated in several studies, appears to be enhancement of proprioceptive input from the ankle to the spinal cord, shortening peroneal reflex time. (29, 49)

Ankle braces have also shown efficacy of stabilizing the ankle both in clinical trials and laboratory testing. (57, 58, 59, 65, 66) Most clinical trials have lacked adequate controls or have failed to use proper selection criteria of human subjects. One such study recently reported the results of an 8-year prospective trial on 300 intercollegiate football players. (80) Players who had their ankles taped on a daily basis suffered 72% of all sprains out of the experimental group while players wearing a customized semi-rigid ankle brace suffered 27.2% of the ankle sprains. Laboratory testing of ankle braces must be scrutinized from a weight bearing vs. non-weight bearing situation as well as passive vs. dynamic testing conditions. Many studies have failed to use the entire foot in a plantarflexed position as a deforming force on the ankle joint when establishing inversion stress on the talus.

Based on the kinematics of the foot as a primary structure transmitting force to the ankle, an effective ankle brace must stabilize the entire foot, including the forefoot, to have adequate leverage for support. This notion has been verified in two separate studies of a new ankle brace containing an orthotic foot plate attached to a semi-rigid ankle support. (81, 82) This brace improved recovery from a syndesmosis sprain and also showed better ankle inversion resistance than a traditional lace-up brace.

An ideal ankle brace, while supporting the rearfoot and forefoot, should also facilitate proprioception and augment the dynamic defense mechanism. Contact of the brace to the plantar surface of the foot as well as the medial and lateral aspect of the ankle and distal tibia and fibular can improve contact area for spacial feedback and cutaneous afferent receptor activation. (83)

Preventive programs that emphasize disc training have been shown to decrease rates of ankle sprains in athletes. (84, 85) Disc training appears to enhance eversion power of the peroneal muscles by decreasing input to the antagonistic tibialis anterior and tibialis posterior muscles during sudden inversion perturbation. (86)

Summary

A review of the kinematic and experimental insight into mechanical vs. functional instability of the ankle has been presented. Significant areas of misunderstanding and controversy have also been explored. In spite of conflicting data, a number of consistent findings appear to have been validated:

1) The ankle joint connects two levers: the leg and the foot. The role of the entire foot in the mechanism of ankle injury continues to be overlooked in clinical and scientific investigations.

2) Experimental models testing the mechanism of ankle sprains and the efficacy of ankle braces often fail to utilize the entire foot rotating about the triplane axes of the midtarsal and subtalar joints. Invalid studies have applied pure inversion-eversion or pure internal-external rotation, which are movements not possible above the axes of the midtarsal and subtalar joints. Other studies utilized a foot that is disarticulated at the tarsometatarsal articulation, overlooking the role of the forefoot in creating a retrograde subluxatory force at the ankle.

3) Single lateral collateral ligament disruption does not cause mechanical instability of the ankle joint. Double ligament disruption will lead to significant inversion instability and rotational instability of the tibio-talar articulation.

4) Mechanical instability does not always cause functional instability. First-time ankle sprains with ligament disruption do not have delayed peroneal reaction time or significant strength loss of the peroneal musculature. Only position sense appears affected in patients with a first-time lateral collateral ligament disruption.

5) Stumbling reactions in man have been extensively studied. Complex muscle firing patterns, stimulated by stretch reflexes and cutaneous receptors appear essential components of the defense mechanism.

6) Peroneal reaction time is not rapid enough for a person standing at rest to resist a sudden inversion force at the ankle joint. The average peroneal reaction time is 130 milliseconds. The time for an ankle to reach a state of 40° of inversion is 100 milliseconds.

7) Experimental models studying peroneal reaction time fail to account for pre-activation of the muscles of the lower leg prior to impact. Pre-activation of lower leg muscles prior to impact or inversion allows shortening of peroneal reaction time and greater power due to eccentric-concentric plyometric contraction.

8) As the ankle plantarflexes, passive eversion restraint of braces and taping diminishes while dynamic support strength of the musculature increases significantly.

9) Falls of greater than 17 cm are required to fully active lower leg muscle defense against inversion force. During walking, the foot passes as close as 5 mm to the ground during swing phase. The average curb height or stair height in a home is 15 cm.

10) Passive external support, in the form of semi-rigid ankle braces, have proven superior to taping in preventing ankle sprains in athletes. The ideal ankle brace will: support the forefoot and rearfoot, resist midtarsal joint supination, provide a long lever arm of support under the entire foot, provide contact to the cutaneous and mechanoreceptors of the plantar aspect of the foot as well as providing medial and lateral contact to the surfaces of the ankle joint to enhance proprioceptive feedback.
New Perspectives On Lateral Ankle Instability

By Douglas H. Richie, Jr., D.P.M.

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