Understanding The Biomechanics Of Equinus

Given that equinus is a factor in many foot and ankle conditions, it is imperative to be knowledgeable in the biomechanical aspects of the condition. Accordingly, this author discusses the biomechanical compensations that can occur with equinus, keys to evaluation and current concepts in treatment.

Physicians have implicated ankle joint equinus in multiple foot pathologies. These pathologies include plantar fasciitis, shin splints, Achilles tendinopathy, metatarsalgia, metatarsal stress fractures, pes planus, anterior and posterior tibial tendonitis, Charcot neuroarthropathy, hallux valgus, hallux rigidus, ankle instability, forefoot ulceration, and lesser digit deformities.^1-4 Equinus is a unique pathology in that a patient will very rarely present with a chief complaint of “ankle joint restriction” but rather will present with symptoms of one of the aforementioned secondary diagnoses. Therefore, when treating these foot and ankle problems, it is important to recognize the presence and influence of ankle joint restriction.    

While many definitions of equinus have surfaced, we can define equinus simply as insufficient ankle joint dorsiflexion for normal gait, resulting in lower extremity compensation, pathology or a combination of both.⁵ Reported normal values of ankle joint dorsiflexion are varied with ranges as large as -10 to +22 degrees.² Despite this variability, authors generally agree that a normal gait requires more than 10 degrees of dorsiflexion with the knee extended.^5-7    

Charles and colleagues proposed a two-stage definition of equinus based on observations of functional range of motion, where greater than 10 degrees of motion is associated with no compensation and normal forefoot pressures.⁶ Stage 1 equinus with available dorsiflexion between 5-10 degrees is associated with minimal gait compensation and slightly increased forefoot pressures. Stage 2 equinus with available dorsiflexion of less than 5 degrees is associated with significant gait compensation and increased forefoot pressures, leading to a greater incidence of pathology.    

Compensation injuries due to equinus are well documented in the literature. DiGiovanni and coworkers found that individuals presenting with forefoot or midfoot pathology had significantly less passive ankle joint dorsiflexion than healthy individuals.¹ Armstrong and colleagues, among other authors, recommended lengthening of the Achilles tendon to reduce forefoot pressure in patients with diabetes at risk of forefoot ulceration.⁸ Similarly, Sgarlato and coworkers reported relief of calf and foot pain, keratomas and hallux valgus pain following Achilles tendon lengthening.⁹    

Equinus has previously been classified in multiple ways, including via etiology, apex of equinus, spastic and non-spastic forms. Simpler classifications have focused on osseous versus soft tissue causes of joint restriction.¹⁰ Soft tissue equinus includes isolated gastrocnemius equinus and gastrocnemius soleus (gastroc-soleus) equinus. Each of these varieties can be either spastic or non-spastic. Researchers have long recognized spastic equinus as having an association with toe walking, leading to the development of Achilles tendon lengthening as one of the oldest recorded orthopedic procedures.^1,2,4,5,11    

Presently, the most commonly reported form of ankle joint restriction is gastroc-soleus equinus, which results from a shortened conjoined tendon of the gastrocnemius and soleus as they form the Achilles tendon. Osseous forms of equinus may be due to tibiotalar exostosis or talar neck exostosis, syndesmotic dysfunction, pseudoequinus or any combination of the above.¹¹

Recognizing The Biomechanical Compensations That Occur With Equinus

The triceps surae is composed of the gastrocnemius and soleus muscles within the superficial compartment of the posterior calf. The muscles combine to form the Achilles tendon with final insertion on the calcaneus. The plantaris is the third muscle in the superficial compartment and has variable insertions. The gastrocnemius and plantaris are three-joint muscles as they cross the knee, ankle and subtalar joints. The soleus is a two-joint muscle, crossing only the ankle and subtalar joints. The tibial nerve innervates the triceps surae as a whole.^1-3,11    

When it comes to ankle dorsiflexion deficiency during the swing phase of gait, one may compensate for this through a steppage gait to allow adequate toe-ground clearance as clinicians may see in patients with foot drop. Stance phase compensation can occur in multiple forms. Researchers have described the stance phase of gait itself to include three phases of propulsion.¹² The first phase, heel rocker, begins with heel strike. The ankle joint is slightly plantarflexed as loading and deceleration occurs. In the second phase, ankle rocker, the lower extremity pivots over the loaded foot and the ankle joint begins to dorsiflex, allowing for forward pivotal motion. This second phase requires the greatest amount of ankle dorsiflexion just before heel lift when the knee is fully extended. The third phase, metatarsal rocker, begins as the heel lifts off the weightbearing surface and the ankle joint remains dorsiflexed through midstance until the limb pivots over the metatarsophalangeal joints in preparation for toe off.    

Limitation of ankle joint dorsiflexion, specifically in the ankle rocker portion of stance phase, will cause posterior displacement of the body’s center of gravity, thereby limiting forward pivotal motion. This forces the lower extremity to compensate to realign the center of gravity. Without some form of compensation, a shortened stride length will result due to inadequate range of motion for normal gait.^11,12 In simple terms, if there is a restriction of motion at the ankle joint, the motion must then occur at adjacent joints either proximal or distal to the ankle.    

Proximal compensation through increased range of motion at the knee or hip occurs to bring the center of gravity forward to facilitate pivotal motion over the planted foot. Genu recurvatum from hyperextension at the knee and lumbar lordosis with flexion at the hip and knee each facilitate this forward shift of the body’s center of gravity.^4,11 While these compensatory mechanisms restore pivotal motion over the restricted ankle, they can result in pathologic processes proximally. Researchers have previously correlated both knee dysfunction and chronic low back pain with the restriction of ankle joint dorsiflexion.^7,12,13    

Distal compensation alters the mechanics of the foot to bring the foot into a more functional position with respect to the leg. Metatarsalgia, forefoot calluses, forefoot ulceration and numerous other forefoot pathologic processes are the hallmark of inadequate equinus compensation. Researchers have shown distal compensation in the sagittal plate to occur through dorsiflexion at the naviculocuneiform joint, leading to midfoot hypermobility, spring ligament attenuation and pes planus.^2,3,12 As midfoot compensation becomes more severe, abduction of the forefoot and dorsiflexion of the metatarsocuboid joints may lead to further midfoot instability and result in midfoot arthritis.^2,12 The most severe form of this progressive midfoot collapse occurs in Charcot neuroarthropathy, resulting in rocker bottom deformity due in part to contribution from ankle equinus in the patient with diabetes.    

Distal compensation at the tarsometatarsal joint may contribute to hallux valgus through hypermobility of the first ray. Research has shown that increased tension of the Achilles tendon interferes with the stabilizing effect of the peroneus longus on the first ray, further increasing the deforming forces in favor of metatarsus primus varus and hallux valgus.³    

Without compensation through distal joint dorsiflexion, the foot may compensate through early heel rise or “bouncing gait.” The most extreme form of this gait pattern occurs in idiopathic toe walking, which results in early and prolonged loading of the metatarsal heads, contributing to forefoot pathology and metatarsalgia.^2,4 Additionally, anterior compartment muscles may be recruited to overcome the tight posterior muscles, contributing to shin splints or hammertoe deformity through extensor substitution.^2,12

Key Insights On Evaluating Equinus

Measurement of ankle joint range of motion occurs with various instruments, including goniometers, dynamometers, radiographs and inclinometers. However, there is inconsistency and a lack of standardization in the literature regarding accurate measurement, as well as questionable inter-rater and intra-rater reliability of these measurements.^4,6,14,15 Specialized devices for the measurement of ankle dorsiflexion have recently emerged. While they show good validity and reliability, these devices can be cost-prohibitive and subject to availability.^14,16    

The most commonly described method of ankle joint dorsiflexion measurement is with the Silfverskiöld test, which measures the angle between the lateral aspect of the foot in relation to the lateral aspect of the leg.^11,17 In regard to goniometer measurements, you have one arm along the long axis of the fibula with the hinge just distal to the lateral malleolus, and the other arm along the lateral aspect of the heel.⁷ Hold the subtalar joint in neutral or slight supination to lock distal compensatory mechanisms, and maximally dorsiflex the ankle joint. It is important to measure the lateral aspect of the heel and not the forefoot because pronation and midfoot collapse may yield inaccurate measurements.¹⁸ Repeat this test with the patient’s knee extended and flexed. This helps differentiate gastrocnemius and gastroc-soleus equinus as the gastrocnemius crosses both the ankle and knee joints while the soleus only crosses the ankle joint.⁷ If limitation of dorsiflexion is present with both the knee extended and flexed, one must then rule out osseous equinus with use of radiographs. Osseous equinus may also present with a “hard stop” at end range dorsiflexion.    

Due to the questionable reliability of the Silfverskiöld test, authors have described several other tests, including prone, supine, weightbearing and non-weightbearing measurement techniques.^14,15,20    

The weightbearing lunge test has shown good intra- and inter-rater reliability for both experience and inexperienced raters.^19-21 Patients perform this test facing a wall with the foot pointed straight forward and the knee bending forward just until it contacts the wall, all while maintaining the heel on the ground. The patient slides the ankle being measured away from the wall until the heel can no longer remain in contact with the ground. One can take measurement of either the distance between the wall and the longest toe by inclinometer measurement on the tibia, or with goniometer measurement between the fibula and the weightbearing surface. The reported normal distance between the wall and longest toe is 9-10 cm, but this will vary depending on patient height and length of the lower extremity. However, researchers have reported good correlation between angular and distance measurement methods in the weightbearing lunge test regardless of these variables.²⁰

What You Should Know About Managing Equinus

Much has been written on treatment for the various forms of equinus. The goal of any therapy is to regain adequate ankle joint motion while minimizing compensatory mechanisms. Conservative treatment for soft tissue equinus takes the form of gait training for shortened stride length, stretching, casting, physical therapy, heel lifts and botulinum toxin (Botox, Allergan) injection.^4,22    

Physicians have done botulinum toxin injections in conjunction with serial casting.^22,23 However, botulinum injection has been controversial due to potential side effects and questionable effectiveness in managing ankle contracture in long-term follow-up. Engström and coworkers recently reported no difference between children ages 5 to 15 years treated with serial casting with or without botulinum toxin.²³ They concluded that botulinum toxin injection does not alter the outcome of treatment and therefore one should not routinely use it.    

A recent publication documented successful non-invasive isolated serial casting for treatment in an idiopathic toe walker.²⁴ While this study was limited to a case study in an 18-month-old, range of motion improvements occurred for 12 months of follow-up, suggesting that isolated serial casting is indeed a viable treatment option for children. Similarly, Fox and colleagues reported improvement of gait and function in 66 percent of children ages 2 to 14 who had isolated serial casting for the treatment of idiopathic toe walking.²⁵ This suggests that serial casting can be beneficial at least up to adolescence.    

Stretching exercises are the mainstay of conservative treatment of equinus and it is commonplace for physicians to prescribe stretching regimens for the initial management of various foot and ankle disorders. Yoon and Park described the correlation among overpronation, equinus and chronic low back pain. They found that calf muscle stretching exercises three times per week in addition to improving ankle joint range of motion also improved chronic low back pain and lumbar flexibility.¹³    

Surgical correction for equinus must focus on the underlying etiology. In osseous equinus, this requires resolution of the osseous impingement through exostectomy or joint arthroplasty. In cases in which osseous impingement is the presumed source of equinus, it is important to realize that soft tissue adaptation may have occurred, resulting in a concomitant soft tissue equinus. At the time of tibiotalar exostectomy, it is worthwhile to evaluate the presence of soft tissue contractures and release them as needed.    

Multiple authors have described different surgical procedures for soft tissue contractures, including neurectomy, gastrocnemius release at proximal or distal levels, Achilles tendon advancement, and Achilles tendon lengthening.^8,9,11,26-35 An in-depth review of these techniques is outside the scope of this article. Clinical examination is crucial to preoperative planning and procedure selection to appropriately treat any given equinus contracture.    

Despite its potential shortcomings, the Silfverskiöld test holds value in differentiating gastrocnemius equinus from gastroc-soleus equinus, which can then guide procedure selection. In this regard, the Baumann procedure holds value as a stepwise surgical approach to equinus release.³² This procedure is an intramuscular lengthening of the gastrocnemius and soleus at their aponeuroses. The surgeon will initially incise the gastrocnemius aponeurosis. If there is an incomplete release, one can also incise the soleus aponeurosis to achieve maximum lengthening.

In Conclusion

Ankle equinus can have detrimental effects on gait and lead to multiple pathologies. Most patients and even some physicians may regard equinus as only a “secondary finding” when in fact it may be the underlying etiology in many situations. Understanding the biomechanics of ankle joint restriction is critical to proper conservative and surgical management of equinus and its associated compensatory injuries.    

Dr. Clifford is a Research Director at the Franciscan Foot and Ankle Institute in Federal Way, Wash. He is affiliated with Franciscan Orthopedic Associates in Federal Way, Wash., and Franciscan Foot and Ankle Associates in Burien, Wash. He is an Associate of the American College of Foot and Ankle Surgeons.

References

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