Keys To The Biomechanical Evaluation Of The Symptomatic Adult-Acquired Flatfoot
Supination lag test. With the patient in a supine position and feet hanging in space off the end of the exam table, ask the patient to actively invert or supinate his or her feet. This is an open chain, active inversion movement of the ankle and hindfoot. Measure the ability of the feet to cross the midline of the body. With rupture of the posterior tibial tendon, the foot will not cross the midline.23 As deformity increases and rigidity of the hindfoot occurs, there will be no observed movement of the symptomatic foot in the direction of inversion. This will differentiate a Stage II versus a Stage III deformity.
Staging of forefoot supinatus. As Stage II deformity increases, significant change will occur with frontal plane inversion of the forefoot and deformity in the medial column, which can provide a new sub-classification of Stage II as proposed by Myerson and cited by Haddad and colleagues.24 As the hindfoot moves into a valgus alignment during gait, the forefoot will invert in reciprocal fashion to keep the metatarsals flat on the ground. Furthermore, as ligaments progressively rupture in the flatfoot, deformity will occur in the medial column, causing a dorsiflexion alignment. As the forefoot inverts on the rearfoot and as the medial column dorsiflexes, an acquired forefoot varus deformity will occur. This is also known as forefoot supinatus. The severity of this deformity and ease of reduction will allow a sub-classification of Stage II.
To make an assessment of forefoot supinatus, evaluate the patient with him or her in a supine position, which is much easier for the patient than lying prone. In some cases, it may be more accurate to evaluate forefoot to rearfoot alignment with the patient in a prone position. The examination begins with the examiner holding the foot and grasping the heel, and moving the hindfoot to neutral or vertical. Then evaluate the frontal plane alignment of the forefoot to the rearfoot with the ankle dorsiflexed and then plantarflexed.
Myerson describes five classifications of Stage II AAF based upon this exam.9 In every case, one can correct the hindfoot to a vertical or neutral position.
A. Hindfoot valgus without residual forefoot supinatus.
B. Flexible forefoot supinatus. Inversion alignment of the forefoot becomes apparent when one corrects the hindfoot to neutral position, but the deformity reduces when the ankle plantarflexes.
C. Fixed forefoot supinatus. Inversion alignment of the forefoot does not reduce with ankle plantarflexion. The deformity is a frontal plane inversion across the midtarsal joint.
D. Forefoot abduction. This occurs at the tarsometatarsal joints, primarily at the first tarsometatarsal joint. One must determine this deformity by radiographic examination.
E. Medial ray instability. Ligament rupture leads to dorsiflexion instability of the first ray and can occur at the talonavicular joint, the naviculocuneiform joint or the medial cuneiform-first metatarsal joint, or any combination thereof. As with type C, this deformity does not reduce with plantarflexion of the ankle but the varus alignment of the forefoot is the result of dorsiflexion of the first ray rather than inversion of the entire forefoot.