What You Should Know About Planal Dominance And Pronated Feet
It is unfortunate that the terms pes planus and flatfoot are so ingrained in the medical literature because they concentrate attention on only one component of a very complex deformity. Smith and Ocampo described a classification for pes “pronatus” based on an earlier work by Borelli and Smith that identified the dominant plane of the deformity.1,2 Although it was originally designed for surgical procedure planning, it is equally ideal for non-surgical treatment.
Dating back to the 1970s, biomechanical theory of the pronation syndromes concentrated almost exclusively on pronation of the subtalar joint. Recognizing forefoot varus was the first step in developing the understanding of pronation as a multiplanar deformity. However, emphasis on forefoot varus as a coronal or frontal plane deformity ignored the fact that it also represents a sagittal plane deformity.
The current understanding of pronation emphasizes talocalcaneonavicular (peritalar) hyperpronation or subluxation. However, even this may be oversimplistic because it does not take into consideration variability in rearfoot anatomy. In order to facilitate optimal treatment, one must have a strong understanding of the plane of dominance of deformity in each case.3
It is necessary to consider each of the planes of the deformity separately and collectively in order to intervene appropriately. Inversion and eversion of the calcaneus are markers of movement in the coronal or frontal plane. The examiner uses the bisections of the calcaneus and the distal tibia to measure the range of motion as the subtalar joint is pronated and supinated.
Physicians may evaluate the transverse plane component by noting the medial bulge produced by the uncovered talar head and the percentage of talar head coverage on an AP weightbearing radiograph.
One can clinically evaluate the sagittal component by the height of the “medial arch.” It is almost impossible to assign a specific numerical measurement to this clinical observation. Evaluation of a weightbearing lateral radiograph will identify failure or “fault” along the medial column. The apex may be at the talonavicular joint, the cuneonavicular joint, the metatarsocuneiform joint or may be equally represented at all three of these locations.
The subtalar joint axis determines the movement characteristics of the rearfoot. If the subtalar joint axis is close to the weightbearing plane, the majority of movement will be in the coronal or frontal plane. This means that most of the rearfoot movement will be inversion and eversion of the calcaneus. If the subtalar joint axis is close to vertical, rearfoot movement will be almost exclusively in the transverse plane. If the axis forms an angle of 30 degrees to the weightbearing plane, there will be twice the amount of inversion and eversion compared to transverse plane motion. If the axis forms an angle of 60 degrees to the weightbearing plane, there will be twice as much transverse plane motion compared to inversion and eversion.
Sagittal plane deformity seems to be independent of the subtalar joint axis. It is more correctly regarded as structural instability along the medial column that is initially flexible and subsequently becomes rigid through adaptive remodeling due to longstanding abnormal forces placed on the developing bone.