1. Preexisting flatfoot deformity
2. Pronated position of the subtalar joint creates increased friction and gliding resistance of the posterior tibial tendon
3. Posterior tibial tendon gradually attenuates and ruptures
4. Pronated subtalar joint creates increased mobility of the forefoot on the rearfoot, increasing strain on the supportive ligaments
5. Sequential ligament rupture occurs beginning with the spring ligament and followed by the long and short plantar ligaments as well as the superficial and deep deltoid ligaments
6. Progressive flatfoot deformity occurs and is characterized by hindfoot valgus, lowering of the medial longitudinal arch and forefoot abduction
Addressing The Biomechanics Of Stage II Adult-Acquired Flatfoot
With overload of the posterior tibial tendon, gradual attenuation and eventual rupture occur. Researchers have demonstrated that in early stage II adult-acquired flatfoot deformity, measurable weakness of the foot occurs with these tendon changes.14 The remaining medial ankle flexors, the flexor digitorum longus (FDL) and flexor hallucis longus (FHL), are unable to compensate for weakness of the tibialis posterior in providing necessary hindfoot inversion and forefoot adduction during gait.15,16 Before significant ligamentous rupture occurs, strengthening and rehabilitation of the weakened posterior tibial muscle/tendon unit can be of significant benefit to prevent the progression of deformity. Kulig and colleagues demonstrated this in a prospective study in which patients with stage II adult-acquired flatfoot deformity had significantly decreased pain and improved function after a 12-week program of progressive eccentric resistance exercise of the tibialis posterior muscle combined with custom functional foot orthotic therapy.17
What Emerging Research Reveals About The Role Of The Spring Ligament
Progressive weakness and attenuation of the posterior tibial tendon will place overload on the ligamentous structures supporting the ankle and hindfoot. Detailed magnetic resonance imaging studies have documented the critical role of ligament rupture with the progression of adult-acquired flatfoot through stage II and III deformity.18,19 The most important structural failure attributed to progression of deformity involves the spring ligament.
Early on, clinicians suspected that deformity in the adult-acquired flatfoot could not be attributed solely to rupture of the posterior tibial tendon.7,20,21 Other cadaver studies revealed the importance of the spring ligament in the creation of the flatfoot deformity.22,23 Jennings and Christensen elegantly demonstrated the fact that rupture of the spring ligament cannot be overcome by the posterior tibial tendon, and evaluation of the adult-acquired flatfoot must take the integrity of this structure into account.24 Deland and co-workers have also identified the superficial deltoid ligament as well as the plantar tarsometatarsal ligaments as other key ligaments that sequentially rupture in the progression of adult-acquired flatfoot.18 However, rupture of the spring ligament has gained considerable consensus as the key structure which, when ruptured, leads to the significant postural change of alignment one sees with adult-acquired flatfoot.25
Where Forefoot Supination Comes Into Play
In 2011, a panel of orthopedic surgeons published a detailed classification system of stage II adult-acquired flatfoot deformity, which focused on changes in forefoot alignment that correspond with ligament failure.26 With attenuation of the spring ligament as well as other plantar ligaments of the medial column, forefoot supination or “supinatus” deformity will develop. This is the result of progression of valgus positioning of the hindfoot, which puts reciprocal varus loading against the forefoot. One can visualize this supinatus deformity by correcting the hindfoot to neutral and looking at the alignment of the plantar surface of the metatarsal heads relative to a bisection of the calcaneus. As stage II deformity progresses, supination deformity of the forefoot becomes fixed and rigid. As further ligaments attenuate and rupture along the plantar surface of the midfoot and tarsometatarsal joints, clinicians will note the deformity on standing radiographs along with elevation of the first ray and abduction of the forefoot.
A Closer Look At Relevant Gait Studies And The Diagnostic Value Of The Heel Rise Test
Numerous gait studies have been conducted on patients with adult-acquired flatfoot, particularly those with stage II deformity.27-29 These studies have shown postural changes that one would not see in asymptomatic feet that are flat and pronated. Instead, stage II adult-acquired flatfoot deformity consistently shows a combination of excessive hindfoot eversion, lowering of the medial longitudinal arch and significant abduction of the forefoot.6,30