Current Concepts In Medial Column Hypermobility
The impact of medial column hypermobility on foot function and deformity development has gained significant attention in the past few decades.1 It has been associated with pes planus, metatarsus primus adductus, hallux valgus, midfoot arthritis, metatarsalgia, plantar plate injury and lesser metatarsal stress fractures. Dudley Morton, an anatomist, introduced the concept of hypermobility.2 The so-called “Morton’s foot” includes hypermobility, equinus and a short first metatarsal.3
While the existence of hypermobility is generally accepted, its definition remains an area of controversy. In simple terms, most would agree hypermobility is an excess of first ray motion in the sagittal plane. Although hypermobility is often used to describe the sagittal plane, it may affect any plane. Morton’s theory suggests hypermobility may functionally translate into a loss of medial column stability and the first ray may become ineffective in resisting the vertical ground reactive force.3 Accordingly, the weightbearing loads are transferred laterally to the lesser metatarsals.
The term first ray insufficiency better describes the lesser metatarsal overload due to the first ray incompetence. Early clinical signs of first ray insufficiency include the absence of callus under the first metatarsal head and/or the presence of callusing under the lesser metatarsal heads. The second metatarsal typically takes the brunt of this force and a discrete callus may be present. Later stages may include metatarsalgia, plantar plate injury, hammertoe formation and lesser metatarsal stress fracture. Advanced first ray insufficiency may result in midfoot collapse and arthrosis.
Medial column motion is the combined motion of the talonavicular joint, naviculocuneiform joint, first intercuneiform joint and the first metatarsocuneiform joint (MCJ). Therefore, hypermobility does not pertain only to the first MCJ. Moreover, hypermobility is often inappropriately used synonymously with instability. While the first ray may be hypermobile, it may be a functionally stable unit in the propulsive phase of gait due to engagement of the windlass mechanism. Palladino believes a nonhypermobile first ray can be dynamically unstable with pronation.4 Nonetheless, the overall stability of the medial column is dependent on a complex interaction between the skeletal structure and dynamic function of the ligaments and musculature.
First ray motion is triplanar.5 The first ray is composed of the first metatarsal and medial cuneiform. The axis is deviated 45 degrees from the frontal and sagittal plane, and is almost parallel with the transverse plane. This orientation results in triplanar motion in which sagittal plane motion is associated with equal frontal and transverse plane contributions. However, with pronation, it has been postulated that the first ray axis shifts and favors adduction, and the dominant planes of deformity accordingly become the transverse and sagittal planes.
Morton stated that “the first metatarsal segment is the most important element of medial stability in the foot.”3 During gait, the foot functions as a mobile adapter and as a rigid lever. The mobile adapter function is needed at heel contact to absorb the force of impact. Throughout stance, the foot must gain stability to act as a rigid lever for propulsion. One achieves this through external rotation of the leg, supination of the subtalar joint and locking of the midfoot. As body weight passes over the foot, the arch must resist the vertical load and maintain its stability for the propulsive phase of gait.