Congenital Subtalar Synostosis
1. Definition—Congenital mal-deformation, “Joined together (failed to separate)” (see Table 1-1, Chapter 1)
a. Congenital failure of segmentation/separation of the talus and calcaneus
i. with translational valgus alignment of the calcaneus under the talus
ii. usually associated with fibula hemimelia syndrome with
• Hypoplasia of the entire lower extremity in relation to the other lower extremity
• Genu valgum
• Often, cruciate ligament deficiency
• Valgus-oriented ball-and-socket ankle joint
• Often, lateral ray deficiency of the foot (Figure 6-21)
2. Elucidation of the segmental deformities
a. Forefoot—supinated or neutral (if neutral in relation to the valgus hindfoot, the entire foot is pronated and the 5th MT head does not touch the ground in weight-bearing. see Basic Principle #13, Figure 2-18B, Chapter 2)
i. There is usually complete absence of 1 to 3 lateral rays of the foot with associated absence of cuneiform bones.
b. Midfoot—neutral
i. Absence of cuneiform bones if lateral rays of the forefoot are absent
ii. The cuboid is always present, though sometimes fused to the calcaneus.
c. Hindfoot—translational valgus (without eversion)
i. There is a synchondrosis (that eventually undergoes metaplasia to a synostosis) between the talus and calcaneus, with the calcaneus laterally positioned under the talus, thereby creating a congenital translational valgusdeformity of the hindfoot. There is no rotational deformity of the foot. The talonavicular joint is usually well-aligned. The thigh–foot angle is neutral (see similarities to translational valgus overcorrection of the subtalar joint in clubfoot, Chapter 5).
ii. The synchondrosis/synostosis sometimes extends to the calcaneocuboid joint ± the talonavicular joint.
d. Ankle—valgus
i. Ball-and-socket ankle joint in valgus alignment/orientation (Figure 6-22)
Figure 6-21. A. Type I fibula hemimelia syndrome with severe translational valgus alignment of a congenital subtalar synostosis and coincident severe valgus orientation of a ball-and-socket ankle joint. B. Absence of the lateral ray is apparent. C. Severe pes planus is seen.
3. Imaging
a. Simulated standing or standing AP, lateral, and Harris axial views of the foot (Figure 6-23)
b. AP, lateral, and mortis of the ankle (Figures 6-22 and 6-23)
4. Natural history
a. Most often, there is no pain or functional disability. The associated congenital ball-and-socket ankle joint malformation is an excellent adaptation to congenital lack of subtalar motion, which is why a congenital subtalar synostosis does so much better than a surgically created subtalar arthrodesis. The ankle cannot convert to a ball-and-socket joint later in life, or even later in childhood.
b. In cases with exaggerated valgus alignment of the “talocalcaneal” bone, pain is related to the deformity. There may be lateral hindfoot impingement pain, medial collateral ligament stretch pain, and/or pain under the plantar–medial surface of the laterally displaced heel pad.
5. Nonoperative treatment
a. Accommodative shoe wear, often with a shoe filler if there are absent rays
Figure 6-22. Incomplete fibula deficiency with congenital subtalar synostosis. A. Valgus deformity of the hindfoot. The scar was created by surgery to lengthen the peroneal tendons, obviously without benefit. B.AP x-ray of the ankle shows partial fibula hemimelia and a ball-and-socket ankle joint in valgus alignment. Congenital subtalar synostosis is always associated with a ball-and-socket ankle joint.
Figure 6-23. A. Harris axial x-ray showing a congenital synchondrosis of the subtalar joint (white arrow) undergoing metaplasia to a synostosis in a 16-year-old boy with hypoplasia of the lower extremity. The subtalar synchondrosis/synostosis is in valgus alignment. B. AP ankle x-ray shows a ball-and-socket joint with severe valgus orientation. The laterally translated calcaneus articulates with the lateral malleolus (white arrow). C. AP x-ray of the foot shows that the subtalar synostosis extends to the navicular (white arrow indicates the site of the talonavicular synostosis). There is normal axial alignment of the medial column of the foot. The lateral ray of the foot is absent, and there are only 2 cuneiform bones adjacent to the cuboid. D. Lateral x-ray shows a flatfoot and a dramatic C-sign of Lateur (yellow C) (see Talocalcaneal Tarsal Coalition, Figure 5-48, Chapter 5).
6. Operative indications
a. Failure of nonoperative treatment to relieve
i. lateral hindfoot impingement pain
ii. medial collateral ligament stretch pain
iii. pain under the plantar–medial surface of the laterally displaced heel pad
7. Operative treatment with reference to the surgical techniques section of the book for each individual procedure
a. Posterior calcaneus displacement osteotomy (see Chapter 8)—perform this for lateral hindfoot impingement pain, medial collateral ligament stretch pain, and/or pain under the plantar–medial surface of the laterally displaced heel pad
b. Preliminary correction of the ankle valgus deformity with medial distal tibia guided growth with a screw (see Chapter 8) or a distal tibial valgus corrective osteotomy (see Chapter 8)—perform this for lateral hindfoot impingement pain, medial collateral ligament stretch pain, and/or pain under the plantar–medial surface of the laterally displaced heel pad if the ball-and-socket ankle joint is in severe valgus alignment.
i. If the forefoot is pronated in relation to the hindfoot, the posterior calcaneus displacement osteotomy (see Chapter 8) is performed after the ankle deformity is improved and the plane of the forefoot is perpendicular to the tibia.
ii. If the forefoot is supinated in relation to the hindfoot, the posterior calcaneus displacement osteotomy (see Chapter 8) is performed in conjunction with a medial cuneiform plantar flexion osteotomy (see Chapter 8) after the ankle deformity is improved.
c. Possible Syme amputation (see Chapter 7, and Figure 6-7 in this chapter)—perform this as treatment for severe fibula hemimelia syndrome