Spring Ligament (Plantar Calcaneonavicular Ligament) Strain
The most likely strained structure is the spring ligament (Option B), which serves as the main static supporter of the medial longitudinal arch and is characteristically injured in painful flatfoot deformity with medial sole pain. 1
Anatomical and Clinical Rationale
The spring ligament complex is the primary static stabilizer of the medial longitudinal arch of the foot, positioned between the calcaneum and navicular to support the talus. 1, 2 In this soldier's case, the combination of:
- Painful flat feet after prolonged weight-bearing activity (thousands of kilometers of marching)
- Acute pain localized to the medial aspect of the sole
- History of repetitive loading in military service
These findings are pathognomonic for spring ligament strain or insufficiency. 1, 2
Why Not the Other Options
Achilles Tendon (Option A)
- The Achilles tendon inserts on the posterior calcaneus and produces pain at the posterior heel or lower calf, not the medial sole. 3
- Achilles pathology does not typically cause flatfoot deformity or medial arch pain. 3
Flexor Retinaculum (Option C)
- This structure maintains the tarsal tunnel on the medial ankle, superior to the medial malleolus. 2
- Injury produces tarsal tunnel syndrome with neurologic symptoms (numbness, tingling), not mechanical arch pain. 2
Extensor Retinaculum (Option D)
- Located on the anterior/dorsal ankle, this structure restrains extensor tendons. 3
- Injury would cause anterior ankle pain, not medial sole pain or flatfoot. 3
Pathophysiology in Military Personnel
Military recruits and active-duty personnel have documented increased incidence of lower extremity overuse injuries from repetitive loading. 3 The spring ligament complex fails under chronic repetitive stress, leading to:
- Progressive loss of medial arch support 1, 2
- Acquired adult flatfoot deformity 2, 4
- Medial plantar pain from ligamentous strain 5, 2
The spring ligament is particularly vulnerable because it bears substantial load during the stance phase of gait, and thousands of kilometers of marching create cumulative microtrauma. 1, 2
Clinical Significance
Isolated spring ligament injuries are rare but can occur with chronic overuse. 2 More commonly, spring ligament pathology occurs secondary to posterior tibial tendon dysfunction, though the question stem describes isolated medial sole pain without mentioning posterior medial ankle symptoms typical of PTT pathology. 1, 2
The spring ligament complex includes three components, with the superomedial calcaneonavicular ligament being the most clinically relevant and commonly injured. 2 Injury to this structure results in loss of talar head support and progressive flatfoot deformity. 1, 4
Common Pitfall
Do not confuse spring ligament pathology with plantar fasciitis—while both cause medial plantar pain, plantar fasciitis typically produces heel pain that is worst with first steps in the morning, whereas spring ligament injury causes midfoot medial arch pain that worsens with prolonged weight-bearing and is associated with visible flatfoot deformity. 1, 2