Degenerative Pathology of the Coxa Pedis – A New Pathological Entity

When a patient – most often a post-menopausal woman – complains of worsening medial tarsalgia with progressive evolution to pes planus, the instant diagnosis is of so-called posterior tibial tendon dysfunction.¹ Generally defined as posterior tibial tendon dysfunction syndrome in the framework of degenerative pes planus in the adult, the literature has reported extensively on this disease by focusing on clinical symptoms and proposing aetiopathogenetic theories and various therapeutic options since the late 1980s.^2–5 The clinical pictures described progressively evolve towards the collapse of the plantar arch, deformity with eversion of the foot and the onset of degenerative arthropathy of the subtalar and talonavicular joints. Surgical investigations for suspected posterior tibial muscle tendinopathy intra-operatively found that the anatomical continuity of the tendon was preserved, and documented the presence of regressive aspects in the fibrocartilaginous component of the acetabulum pedis (spring ligament), with anatomical and pathological pictures ranging from malacia to tearing along the entire thickness. The literature has reported similar lesions in the same ligament in cases of pathological tearing of the posterior tibial tendon.^6–8 They have also been found experimentally⁹ in acquired pes planus in the adult with a deformed talonavicular joint. Balen and Elms¹⁰ considered the frequent magnetic resonance imaging (MRI) findings (92%) of spring ligament lesions associated with posterior tibial lesions, and also reported¹¹ finding a possible spring ligament lesion in patients with a normal posterior tibial tendon.

Our previous paper (2003)¹² reported 13 cases of patients who, appearing for surgery with a clinical diagnosis of degenerative posterior tibial tendonopathy, were surgically found to have either minor or major structural degenerative involvement of the spring ligament with macroscopic evidence of tendon integrity, besides moderate and most likely secondary tenosynovitis. A case record study revealed a frequent and interesting association between the accessory navicular bone and glenoid structure lesions, thus theorising a direct microtraumatic process.

Anatomical References
The acetabulum pedis ¹³ is the acetabulum component of the coxa pedis.¹⁴ It presents a mosaic-like structure ^{15, 16} with a panniculus adiposus in the centre and a skeletal component (navicular, anterior subtalar joint) with interposed glenoid fibrocartilage (see Figure 1) on which the superomedial and plantar branches of the spring ligament converge (spring ligament). The superomedial calcaneonavicular ligament has a triangular cartilaginous articular facet that is articulated with the corresponding medial articular surface of the talar head. Elastin fibres can be found in the spring ligament.¹⁴ The glenoid’s strengthening elements are the superficial deltoid ligament fibres that are attached to the sustentaculum and the recurrent tract of the posterior tibial tendon. ¹² The glenoid is laterally completed by the calcaneonavicular branch of the bifurcate ligament (or Chopart’s ligament). The acetabulum of the coxa pedis is functionally modular owing to the presence of the glenoid ligamentous component; this allows talar head axis variations while opening and closing the foot’s kinetic chain.^15,17,18

The role played by the sole glenoid component in maintaining the plantar arch is hard to establish due to the significant interaction between muscular function, especially of the posterior tibial muscle, the congruence of skeletal parts and capsuloligamentous constraint. ^19–21 In experimental conditions,²² the sole section of the posterior tibial muscle does not modify the plantar arch of a foot that bears an axial load, hence the need to perform large sections of the medial capsuloligamentous structures of the talonavicular joint to generate deformities that are comparable to the everted pes planus. ⁷ Other experimental studies ^20,21 have shown that the major mechanical contribution to the stability of the longitudinal plantar arch is provided by the plantar fascia, followed by the short and long plantar ligaments and by the spring ligament. While the ligamentous structures guarantee plantar arch stability with a silent electromiograph during double limb support, the muscular structures act when the foot is loaded.¹⁸ Posterior tibial tendon tearing or dysfunction causes a loss of active stabilisation during inversion of the mid-foot joint with subsequent inability to turn the foot into the rigid lever needed for the push-off. The medial tarsal capsuloligamentous structures (deltoid ligament, superomedial and plantar branches of the spring ligament) carry a greater load induced by destabilisation caused by calcaneal valgism and increased talar pronation. During the push-off phase,the gastrocnemius–soleus complex partly loses its action on the metatarsal region and, by inducing talonavicular hyperextension, maintains calcaneal valgism due to its lateral action and progressive retraction. The evolution leads to deformity with valgus eversion of the foot typical of degenerative pes planus in the adult.