Addressing The Role of Equinus In Stage II Adult-Acquired Flatfoot

Recognizing equinus as a key deforming force in those with stage II adult-acquired flatfoot, this author offers a primer on anatomical considerations, pertinent diagnostic pearls and insights on conservative and surgical treatment options, including percutaneous Achilles tendon lengthening.

The role of ankle joint equinus in the pathology of adult-acquired flatfoot has long been a subject of debate. Many physicians believe that intra-operative release of equinus contracture is a necessary component when surgically addressing flatfoot. However, others rarely, if ever, address the deformity. That said, the majority of current literature recognizes the importance of ankle joint equinus as a major deforming force in those with a symptomatic adult-acquired flatfoot.

The general definition of ankle joint equinus is less than 10 degrees of dorsiflexion relative to the leg. More specifically, researchers have shown that equinus exists in 96 percent of patients with biomechanically induced foot pain and pathologic equinus remains one of the most underdiagnosed and undertreated pedal conditions.¹ Much of the early literature exploring surgical management options for equinus discussed treatment of spastic contractures in children as well as patients with cerebral palsy. The earliest described surgical procedures had a high rate of recurrence, significant weakness and loss of function.

Pertinent Anatomical Considerations
The function of the triceps surae complex is to decelerate the forward motion of the tibia during the stance phase of gait as well as enhance plantarflexion and propulsion. The gastrocnemius is the largest, most superficial muscle in the posterior compartment. It originates proximal to the medial and lateral condyles of the posterior femur, thereby crossing the knee, ankle and subtalar joints. The medial head is larger, thicker and extends more distally.

The soleus muscle belly lies deep to the gastrocnemius, and is broad and flat. It originates from the posterior aspect of the tibia and fibula.²

The plantaris muscle originates from the lateral femoral condyle and its tendon inserts medially and anteriorly to the Achilles tendon on the calcaneus. The plantaris, like the gastrocnemius, contributes to ankle joint plantarflexion and knee joint flexion. Proximally, the Achilles tendon begins as a broad aponeurosis. The soleal component rotates toward the medial side as the gastrocnemius rotates laterally toward its distal insertion.

During normal heel strike, internal rotation of the tibia causes the hindfoot to evert and the talus to plantarflex. The midfoot and forefoot follow the hindfoot and abduct, thus causing the medial arch to flatten. The tibialis posterior contracts eccentrically, acting as a shock absorber and decelerating the internal rotation of the tibia and plantarflexion of the talus. The posterior tibial tendon gets assistance from the plantar fascia and spring ligament at this stage in an attempt to prevent flattening of the medial arch.

As the heel strikes the ground and the foot moves toward midstance, the anterior muscle group transitions from concentric to eccentric contraction. This prevents the foot from slapping the ground at midstance.

As the body’s center of gravity moves over the foot and approaches heel rise, the tibia rotates externally as the ankle joint dorsiflexes. The subtalar joint, influenced by the pull of the posterior tibial tendon, begins to invert into a slight varus position. This causes reconstitution of the medial arch and locking of the midtarsal joint, therefore creating a rigid lever for propulsion. This complex motion of collapsing and reconstituting the medial arch can inherently lead to failure in the face of pathological equinus.³ A tight triceps surae decreases the stabilizing effect of the posterior tibial tendon, subsequently allowing faulting of the midtarsal joints and the medial column.  

In the presence of ankle joint equinus, the body attempts to compensate distally and/or proximally for the lack of ankle dorsiflexion.⁴ Proximally, in a patient with equinus contracture, the center of gravity shifts posteriorly, potentially leading to low back strain, hip and knee pain, calf cramping, and muscle fatigue. Clinically, lumbar lordosis, hip flexion and genu recurvatum may result.⁴ When it comes to symptoms secondary to these proximal compensations, one can often attain relief via surgical correction of the ankle joint equinus.

Distal compensation for the limitation of ankle joint dorsiflexion typically occurs through abnormal pronation at the subtalar and midtarsal joints. In the rectus foot, the subtalar joint axis is parallel to the Achilles tendon. In the pronated foot, the subtalar joint axis is more perpendicular. Subtalar joint pronation unlocks the distal structures and the forefoot compensates by dorsiflexing on the rearfoot. When there is uncompensated equinus, there is minimal motion across the subtalar joint and midtarsal joint, and often no heel contact during gait. Common clinical findings in these cases include forefoot tylomas and digital contractures.

This is most often evident in patients with neuromuscular disease and spasticity, or “true toe-walkers.” The fully compensated patient exhibits a hypermobile flatfoot with maximal eversion through the midtarsal and subtalar joints. In contrast, the partially compensated foot is characterized by a pronated subtalar joint, minimal midtarsal joint motion, forefoot eversion and early heel off during the gait cycle.⁴

Who Is The ‘Typical’ Patient With Stage II Adult-Acquired Flatfoot?
Traditionally, the profile of a patient with typical symptomatic stage II adult-acquired flatfoot is that of an overweight, middle-aged female though limited studies are available to support this data. The prevalence of stage I and II posterior tibial tendon dysfunction (PTTD) in women over 40 years was 3.3 percent in the United Kingdom in 2009.⁵ Associated conditions such as concomitant diabetes, hypertension, rheumatoid arthritis and seronegative arthropathies may be significant in this patient population. Also consider occupation, lifestyle and activity level, footwear choices, past trauma, prior treatments and the presence of obesity.

Many patients relate having a flatfoot structure since childhood with it only recently becoming symptomatic. Few can identify a specific incident or traumatic event that triggered their symptomatology. Prolonged standing or walking typically triggers symptoms and some patients note that their feet are “‘turning out” or “turning over” when they walk. Of great importance relative to the presence of equinus is that some patients identify a loss of strength when “pushing off” or decreased ability and coordination while attempting to run. One should perform a comprehensive examination of the patient with a symptomatic stage II adult-acquired flatfoot, taking into account complaints such as medial pain, arch fatigue, ankle pain or proximal leg pain.

Essential Diagnostic Insights
One should employ a systematic approach to ensure a thorough examination of the affected lower extremity. This includes non-weightbearing joint motion including ankle joint dorsiflexion (Silfverskiold exam); weightbearing examination in relaxed stance and neutral or “reduced” position, and checking single and double limb heel rise capabilities; and a thorough gait examination.⁶ During the gait examination, take special care to identify any proximal compensations for ankle equinus including genu recurvatum, lumbar lordosis and forward postural position.⁷

Specific to the examination of ankle joint equinus, physical examination in the flatfoot patient must include a thorough examination of the gastrocsoleus complex. It is clinically accepted that the ankle joint requires 10 degrees of dorsiflexion during the stance phase of gait for normal ambulation.

The Silfverskiold test is a reliable indicator of the presence of equinus. The patient extends the knee with the subtalar joint neutral to slightly supinated, the windlass mechanism activated and the ankle joint maximally dorsiflexed. Repeat the test with the knee and hip flexed to 90 degrees, again measuring maximum ankle joint dorsiflexion. The examiner must answer the following questions. Is ankle dorsiflexion unchanged with knee flexion? Is ankle joint dorsiflexion increased with knee flexion? Is there a soft end feel? Is there a bony end feel?

These answers will help to classify the subtype of ankle equinus and guide the treatment plan. Adequate dorsiflexion with knee flexion represents contracture of the gastrocnemius only as flexing the knee joint eliminates the influence of the gastrocnemius muscle. Inadequate dorsiflexion with knee flexion and knee extension represents contracture of the entire triceps surae versus an osseous equinus. Consider both the subtype and severity of equinus in selecting an appropriate surgical procedure. Isolated gastrocnemius contracture is amenable to gastrocnemius recession while gastrocsoleus equinus requires tendo-Achilles lengthening, either open or percutaneous.

When one performs radiographic examination of the adult-acquired flatfoot, there are specific findings suggestive of concomitant ankle joint equinus. Ankle joint equinus prevents dorsiflexion of the talus within the ankle mortise. Therefore, the remainder of the foot will dorsiflex, abduct and evert beneath the talus with subsequent subluxation at the subtalar and talonavicular joints. Sagittal plane radiographic findings include increased talar declination and decreased calcaneal inclination angles as well as naviculocuneiform joint faulting. Anterior-posterior radiographs demonstrate an increased dorsoplantar talocalcaneal angle and decreased talonavicular congruency. In early stage flatfoot, a dorsal first metatarsocuneiform exostosis is often visible, with plantar joint gapping.

A Guide To Conservative Treatment Options
Treatment of the stage II adult-acquired flatfoot includes custom foot orthoses, ankle braces and custom, short articulated ankle foot orthoses. Treatments aim toward improving alignment of the medial longitudinal arch, correcting forefoot abduction and toward functionally shortening the posterior tibial tendon and potentially additional supporting ligamentous structures.^8,9 Note that selection of a solid ankle foot orthosis may be counterproductive in that weakness of the plantarflexors may worsen over time.^8,9

I have found that concomitant ankle equinus, if untreated, may reduce the effectiveness of the aforementioned treatment options for the stage II adult-acquired flatfoot. Conservative measures for the treatment of equinus begin with intensive physical therapy, including a comprehensive stretching program. One may employ orthotics, casting, night splints and additional bracing in recalcitrant cases.

A four- to six-week course of gastrocsoleus stretching can increase available ankle joint dorsiflexion between 5.4 to 11 percent in comparison to ankle joint dorsiflexion prior to exercise.^10-13 Studies have specifically addressed the effectiveness of such stretching programs in middle-aged and elderly females, who have the greatest prevalence of symptomatic stage II adult flatfoot, and in whom soft tissue and muscle quality would be of greater concern.^14-16 This patient population was also able to achieve a statistically significant increase in ankle joint dorsiflexion in four to eight weeks.^14-16

I employ a weightbearing eccentric stretching protocol in which I advise patients to place their toes on the lowest step in their home and do a sustained heel drop, holding the stretch for at least 15 to 20 seconds. They follow this with a 15 to 20 second rest in neutral position. Patients perform three to four repetitions three to four times daily for four to six weeks. I advise the patient that a bent knee technique may be less painful initially.    

When Equinus Requires Surgery
The surgical management of ankle equinus is a widely debated topic and procedure selection is often based on surgeon preference as there is no consensus regarding the superiority of a single procedure. In addition, equinus correction is rarely indicated as a single procedure in the treatment of adult-acquired flatfoot. There are various recommended osteotomies, tendon balancing procedures and arthrodesis procedures for correction of the adult-acquired flatfoot and the correction of ankle joint equinus is a powerful adjunct to these procedures.

Extraarticular osteotomies such as the Evans calcaneal osteotomy, the posterior calcaneal displacement osteotomy and the Cotton osteotomy serve to maintain joint function while improving structural alignment. Various arthrodesis techniques, including medial column fusions (naviculocuneiform and metatarsocuneiform joints), subtalar arthrodesis, talonavicular arthrodesis and triple arthrodesis provide increased stability and predictable rates of fusion.¹⁷

In my experience, gastrocnemius recession offers acceptable cosmesis and minimizes perceived weakness. However, this procedure requires increased operating time and is indicated only for mild to moderate contractures. One may perform gastrocnemius recession via an open or endoscopic approach.

Surgeons may also perform tendo-Achilles lengthening (TAL) via a percutaneous or open technique. While TAL is efficient, technically straightforward and may achieve large amounts of required length, final lengthening is sometimes unpredictable and it may markedly decrease posterior muscle group strength. Achilles tenotomy is usually not preferable in the adolescent and adult flatfoot due to persistent weakness and muscle atrophy, but surgeons frequently perform the procedure in infants and patients with Charcot.

The Vulpius and Strayer techniques employ a transverse recession of the gastrocnemius fibers just distal to the muscle belly.¹⁸ Identify the plantaris muscle medially. One can transect this muscle later if contracture persists despite a full release of the gastrocnemius fibers. Then inspect the medial and lateral edges of the aponeurosis for a subtle separation of the gastrocnemius and soleal fibers. Dorsiflex the foot and transect the superficial gastrocnemius fibers, leaving the deep soleal fibers intact. It is very important to control incisional depth carefully so as not to penetrate the muscle fiber layer. The transverse recession, on average, will provide lengthening of 2.5 cm or less.

Following open gastrocnemius recession, perform closure in layers. Close the paratenon and deep fascia as a single layer with absorbable sutures. During closure, periodically dorsiflex the foot to preserve the gliding mechanism. After subcutaneous closure with a running absorbable stitch, proceed to subcuticular skin closure with Steri-Strips.

What You Should Know About Functional Outcomes
The concept of posterior muscle group lengthening as an adjunct to the correction of the pediatric or adult-acquired flatfoot remains controversial. The literature is widely variable concerning which technique is superior and produces the most reliable results while minimizing overall morbidity.

A common critique of the gastrocnemius recession is the belief that permanent weakness and disability can be expected, even after intensive physical therapy. Sammarco and colleagues followed 40 patients after gastrocnemius recession for an average of 25.4 months.¹⁹ These patients had an average increase in ankle joint dorsiflexion from -3.5 degrees to 15.3 degrees and American Orthopaedic Foot and Ankle Society (AOFAS) scores improved from 62.3 to 79.5. There was an average of 74 percent plantarflexion strength in comparison to the non-operative limb with only one patient having persistent perceived weakness. An interesting finding from the study was that long-term results — from six months post-op to 18 months post-op — demonstrated improved peak strength and fatigue resistance.

Rush and colleagues explored the morbidity of the open gastrocnemius recession, and followed 126 patients for an average of 19 months.²⁰ Nine of 126 patients experienced post-surgical complications with the most common being scar complications (six of 126) while three patients experienced sural nerve paresthesias or pain. Two patients had wound dehiscence, two had soft tissue infection and one patient developed chronic regional pain syndrome (CRPS). No patients had a discernable limp or calcaneus gait, and no patients had a persistent decrease in muscle strength. Xou and coworkers also demonstrated a progressive increase in plantarflexion strength after gastrocnemius recession at one year postoperatively in comparison preoperatively to the ipsilateral limb.²¹

Advantages of the open gastrocnemius recession include faster healing and recovery time with less risk of overcorrection. Disadvantages include that it is indicated only for mild to moderate contractures, precise dissection is required and OR time may be increased secondary to positioning.

Additional Insights On Percutaneous Tendo-Achilles Lengthening
There have been modifications to percutaneous and open TAL procedures over the course of time to accommodate new knowledge of anatomy and biomechanics. Achilles tendon torsion has been the topic of much debate, particularly with the use of percutaneous techniques.

Examining percutaneous Achilles tendon lengthening, Hoffman and Nunley compared the “medial-lateral-medial” approach to a “lateral-medial-lateral” approach on 22 cadaveric limbs.²² They found no statistically significant difference in achievable dorsiflexion between these two approaches. Of note, all limbs had post-procedure dissection and the sural nerve was partially cut in one limb.²²

Pinney and coworkers further examined the chance for neurovascular injury when employing a percutaneous technique.²³ After using the standard Hoke triple hemisection to perform TAL in 15 cadaveric specimens, surgeons performed dissection to measure the accuracy of cuts and proximity of critical structures. Ideally, each cut should section 50 percent of the tendon body. On average, post-procedure accuracy measured an average of 50 percent sectioning at the middle cut, 60 percent at the proximal cut and 55 percent at the distal cut. Several critical structures were lying less than 1 cm from the cut portion of the tendon. At the level of the proximal cut, the flexor hallucis longus tendon and tibial nerve sat medial 5.7 mm and 8.3 mm respectively. At the level of the middle cut, the flexor hallucis longus was lying 9.1 mm medial and the sural nerve was 7.9 mm lateral.

Advantages to the percutaneous TAL include short operating time, ease of positioning, minimal scarring and skin closure may not be necessary. Additionally, percutaneous techniques are more desirable in immunosuppressed patients, patients with diabetes and those with vascular impairment. Disadvantages include a risk of iatrogenic damage to surrounding structures, adhesions, prolonged edema, persistent muscle weakness, a greater risk of overlengthening/calcaneus gait, or complete rupture secondary to overzealous transaction at each interval.²⁴ In addition, difficulty may arise in significantly obese patients or revisional cases when one cannot easily palpate the tendon, and uneven transection may result.

In Conclusion
Ankle joint equinus plays a significant role in the pathogenesis of adult-acquired and pediatric flatfoot. Contracture of the gastrocnemius or gastrocnemius-soleus complex may encourage valgus pull of the hindfoot, and accelerate midfoot collapse and forefoot abduction. The surgical management of ankle equinus is a widely debated topic and procedure selection is often based on surgeon preference as there is no consensus regarding the superiority of a single procedure. A thorough physical examination and evaluation of biomechanical and radiographic findings will help to ensure appropriate procedural choice. We should recognize the presence of ankle joint equinus as a contributing factor not only to the stage II adult-acquired flatfoot but to a variety of other forefoot and midfoot pathologies as well.

Dr. Meszaros is board certified in foot, reconstructive rearfoot and ankle surgery, and is a Diplomate of the American Board of Podiatric Surgery. She is a Fellow of the American College of the Foot and Ankle Surgeons. Dr. Meszaros is in private practice in Oberlin, Ohio.

References

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