The only classification of Müller-Weiss disease (MWD) is based primarily on Méary's talo-first metatarsal angle. It describes increasing sag of the medial longitudinal arch with greater degrees of compression and fragmentation of the navicular. Purportedly, the talar head pushes the subtalar joint into varus and drives the medial pole of the navicular medially, as it protrudes inferiorly and laterally. Its authors stipulated heel varus as a pre-requisite, coining the term 'paradoxical pes planus varus' to define heel varus and flatfoot as hallmark deformities of the condition.
We measured Méary's and Kite's talocalcaneal angles, heel offset, anteroposterior thickness of the navicular at each naviculocuneiform (NC) joint, medial extrusion of the navicular and calculated percentage compression at each NC joint in 68 consecutive feet presenting with MWD. Morphology and activity at the various peri-navicular joints were studied using SPECT-CT in 45 feet.
Inverse relationships between Méary's angle and degree of navicular compression reach statistical significance at NC2 but not at NC3. Strong correlation exists between medial extrusion and percentage compression at NC2 and NC3. Medial extrusion is significantly greater on the affected side in unilateral cases and on the more compressed side in bilateral cases. Significant inverse relationships exist between Kite's angle and percentage compression at both NC2 and NC3 and degree of medial extrusion of the navicular. No correlation was detected between Kite's angle and either heel offset or Méary's angle. Varus heel offset was present in only 33% of cases. The combination of heel varus and negative Méary's angle was present in just 26% of cases, the commonest combination being heel valgus with sagging at 56%.
Our findings confirm part of Maceira's hypothesized pathomechanism of MWD. Reductions in Kite's talocalcaneal angle confirm that lateral and inferior protrusion of the talar head causes increasing compression and medial extrusion of the navicular. However, such shift of the talar head does not always lead to heel varus. As such, we caution against universal advocacy of lateral displacement calcaneal osteotomy, as the heel is not always in varus in MWD.