How To Address Key Biomechanical Issues With Second MPJ Injuries
Q: In terms of orthotic therapy, what are the specific orthotic requirements and prescription criteria for offloading the second MPJ?
A: One should use a balanced/negative impression cast and cast out supinatus deformity if it is present, according to Dr. Bouché. In addition, he advises using a deep heel seat, a moderate medial arch fill, no lateral arch fill (to capture the lateral arch fully), a forefoot extension with a sub-second metatarsal cutout and/or metatarsal cookie. One may use additional strategies depending on the severity of the pronated foot deformity. Dr. Bouché says options may include a medial skive, inverted orthoses or a medial extended rearfoot post.
For the past 15 years, Dr. Kirby has used the following orthosis modifications to achieve very good therapeutic results in treating second MPJ pathology. He uses a 3/16-inch polypropylene plate with a standard rearfoot post, minimal medial arch fill and a 2 to 3 mm medial heel skive in the orthosis to increase the supination forces on the foot and redirect ground reaction force (GRF) toward the lateral forefoot. He says one should also make the anterior orthosis edge with an abrupt 3/16 inch drop-off or an “internal metatarsal bar” to reduce the GRF on the metatarsal heads.
In addition, Dr. Kirby notes that clinicians should also order the anterior orthosis edge so the orthosis shell parabola extends distally to all of the metatarsal necks and the orthosis is much longer under the distal second metatarsal shaft. He calls this a “capsulitis modification.” He uses a full length, 1/8-inch neoprene topcover along with a 1/8-inch korex forefoot extension plantar to the first, third, fourth and fifth metatarsal heads. Sometimes he combines this with a metatarsal pad sandwiched between the top cover and orthosis shell to facilitate further reduction of the GRF on the second MPJ.
During the casting of the orthotic, Dr. Clough advocates maximum dorsiflexion of the first MPJ to plantarflex the first ray and allow the first ray to bear weight when one is dispensing the orthotic. As he explains, this maneuver will reduce forefoot supination and eliminate the need to correct an inverted forefoot deformity. Intrinsic or extrinsic balancing of a forefoot supination will always result in jamming of the first MPJ and he says one should avoid this in all situations in which this deformity is reducible. Minimal arch fill is necessary to slow down any eversion velocity over the foot structure and provide pressure to the base of the first metatarsal, according to Dr. Clough.
Re-establishing the first ray function is critical for an orthotic to be effective and Dr. Clough feels applying a Cluffy wedge is a good solution as it pre-stresses the hallux in dorsiflexion and allows proper first MPJ motion to occur. He says this reliably overcomes a functional hallux limitus. As the first MPJ dorsiflexes, he says the first metatarsal plantarflexes and helps offload the second MPJ.
Other orthotic modifications like a reverse Morton’s extension or a kinetic wedge rely on increased pressure underneath the lesser metatarsals and decreasing weightbearing underneath the first metatarsal to improve the range of joint motion for the second MPJ, says Dr. Clough. He notes this is counterintuitive if one is trying to decrease weightbearing on the second MPJ. Further, he says it is critical to establish normal first metatarsal weightbearing pressure to overcome second MPJ forefoot pathology but still enable the coupling mechanism of rearfoot supination to occur in a timely fashion.
Dr. Bouché is a Staff Podiatrist at The Sports Medicine Clinic in Seattle. He is a Fellow of the American Academy of Podiatric Sports Medicine, a Diplomate of the American Board of Podiatric Surgery and a Fellow of the American College of Foot and Ankle Surgeons.
Dr. Clough in practice at the Foot and Ankle Clinic in Great Falls, MT. He is the inventor of the Cluffy Wedge. He is a Diplomate of the American Board of Podiatric Surgery. He can be reached at firstname.lastname@example.org
Dr. Kirby is an Adjunct Associate Professor in the Department of Biomechanics at the California School of Podiatric Medicine at Samuel Merritt College. He is the Director of Clinical Biomechanics at Precision Intricast Inc.