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