1.Physiopathology
Most patients present with chronic mid and hindfoot pain in the fourth or fifth decades, as the navicular collapses and secondary osteoarthrosis develops.
With further navicular collapse,
pes planovarus develops, and tibia is forced to external rotation, which may lead to altered biomechanics and osteoarthrosis of the knee.
Swelling and tenderness over dorsomedial aspect of the midfoot and heel varus can appear.
The medial longitudinal arch can be markedly flattened depending on the severity of the disease.
This combination of pseudohindfoot valgus and pes planus deformity is also known as paradoxical pes planovarus.
2 .
Radiologic Findings
Weight-bearing radiographs in the frontal and lateral projections remain the test of choice in the evaluation of MWS,
and its radiologic findings can be grouped by anatomic location.
-Ankle and Hindfoot
Lateral half of the navicular collapse producing a peritalar subluxation of the talar head,
resulting in hindfoot varus with loss of the cyme lime (S- shaped line seen on frontal and lateral radiographs composed of well aligned talonavicular and calcaneocuboid joint).
On the lateral view,
there is an impression of a bigger sinus tarsi as the talocalcaneal angle decreases.There is also retroposition of the fibula as the tibia externally rotates.
-Midfoot
As the lateral half of the navicular collapses, it becomes sclerotic and comma- or hourglass- shaped in appearance.
There is peritalar subluxation of the remaining medial aspect of the navicular bone. The later with the lateral subluxation of the talar head lead to a new articulation (talocuneiform),osteoarthrosis the talonavicular and naviculocuneiform joints,
and medial subluxation of the cuboid with respect to the calcaneus (the cuboid sign).
Fig. 1: Fig 1. Anteroposterior (a-c) and (d) oblique (focalized) weight-bearing radiographs of both feet and left foot.
References: Department of Radiology, Clínica INDISA; Santiago, Chile
Fig. 2: Fig 2: Weight bearing plain radiographs in posteroanterior, oblique and lateral view.
References: Department of Radiology, Clínica INDISA; Santiago, Chile
Fig. 3: Fig 3: Radiographs of both feet in oblique views (a-b)
References: Department of Radiology, Clínica INDISA; Santiago, Chile
-Forefoot
When the plantar arch fails,
the metatarsals adquire a parallel orientation and develop hypertrophic changes in the second metatarsal secondary to altered transmission of compressive forces through the second,
rather than the first,
metatarsal and tarsometatarsal joints (index minus sign).
The medial longitudinal arch can be markedly flattened depending on the severity of the disease.
This combination of pseudohindfoot valgus and pes planus deformity is also known as paradoxical pes planovarus.
4.
Sequence of events
There is a sequence of events caused by lateral compression force acting on a suboptimally ossified navicular that can be appreciated with both radiographs and cross-sectional imaging:
1.
Lateral compression of the navicular
2.
Lateral subluxation of the talar head
3.
Subtalar varus
4.
Fragmentation of the navicular into large medial and small lateral or dorsolateral fragments with extrusion (“listhesis navicularis”)
5.
Exposure of the lateral,
and sometimes the intermediate, cuneiforms with talocuneiform neoarticulation
6.
Plantar flexion of the talus leading to paradoxical pes planovarus.
5.
Imaging studies
Cross-sectional imaging can get additional information.
CT can assess bone arrangement for preoperative planning,
and MRI can detect bone marrow edema and early osteoarthritic changes and exclude other differential diagnosis,
such as stress fractures or infection.
Fig. 4: Fig 4: Ankle CT, multiplanar reconstructions
References: Department of Radiology, Clínica INDISA; Santiago, Chile
Fig. 5: Fig 5: Angle MR: Sagittal acquisitions in T2W (a) and STIR (b), coronal acquisitions in STIR (c), T1W (d), and PD SPAIR (e)
References: Department of Radiology, Clínica INDISA; Santiago, Chile
Fig. 6: Figure 6: Ankle MR. Sagittal acquisitions in STIR (a,b) and T2W (c), coronal acquisitions in STIR (d) and T2W (e)
References: Department of Radiology, Clínica INDISA; Santiago, Chile
With progressive lateral collapse of the tarsal navicular and distortion of the midfoot anatomy,
the perinavicular ligaments may undergo degeneration,
but the precise soft tissue pathology of MWS has not been described yet in the literature.
This reflects the emphasis that have been placed understanding the osseous changes in MWS,
but is important to comphehend that the failure at the lateral aspect of navicular reflects also damage in the lateral calcaneonavicular component of the bifurcate ligament and lateral calcaneonavicular capsule of the acetabulum pedis.