How To Address Key Biomechanical Issues With Second MPJ Injuries
Q: What biomechanical factors are associated with the pathomechanics of injury to the second MPJ?
A: Dr. Clough maintains that injury to the second MPJ is first and foremost a problem with insufficiency of the first ray. If the windlass mechanism is not engaging properly and the first metatarsal is not plantarflexing into the ground to accept adequate weightbearing, Dr. Clough says then the second metatarsal will be overloaded.
Not only is the first ray capable of excessive dorsal displacement but Dr. Clough says it is also capable of significant plantar displacement with proper foot function. If the first metatarsal head remains in a dorsally displaced position due to inadequate function of the first MPJ, he says this results in the inevitable overload of the second MPJ.
“This of course is the case with functional hallux limitus, which is present in a great number of feet in clinical practice,” says Dr. Clough. He says research has shown that hallux limitus is present in 62 percent of asymptomatic feet and adds that the condition is present in close to 80 percent of the feet he sees in clinical practice.1
“It is a pervasive problem that needs to be dealt with for effective therapeutic outcomes,” emphasizes Dr. Clough.
Dr. Bouché says one must consider biomechanical factors such as ankle equinus, a hypermobile medial column with or without bunion deformity, and/or a prominent second metatarsal that is either long or plantarflexed.
Dr. Kirby says the vast majority of second MPJ injuries are actually injuries to the plantar plate, which is a fibrocartilaginous structure plantar to the lesser MPJs and which is continuous with the plantar fascia. He explains that the plantar plate is subject to large magnitudes of tensile forces from the plantar fascia and is also subject to large magnitudes of compressive forces from ground reaction force (GRF) on the forefoot during weightbearing activities. This greatly increases the risk of the plantar plate developing partial or complete tears within its structure, according to Dr. Kirby.