Introduction 

Tarsal coalition is defined as an abnormal connection between two or more bones in the foot. This link, made of bone, cartilage, or fibrous tissue, frequently causes limited mobility, pain, and, in many cases, a stiff flatfoot deformity. The disorder usually affects the tarsal bones, including the calcaneus, talus, and navicular bones. While many cases are congenital and present at birth, clinical symptoms do not often appear until adolescence, when these abnormal connections begin to change and impair joint movement.

Tarsal coalition has generally been thought to be a congenital defect, but new data reveals that it can also be caused by genetic predisposition or acquired environmental causes such as trauma, infections, and inflammatory disorders. Understanding the causes of tarsal coalition is essential for effective diagnosis, management, and treatment.

Congenital causes of tarsal coalition

Failure of mesenchymal segmentation

During normal foetal development, the foot bones form from a cartilaginous precursor generated from mesenchymal tissue. This tissue is designed to divide and differentiate into separate bones. An error or malfunction in this segmentation process causes a lasting link between the future tarsal bones, resulting in a coalition.¹ This failure is believed to occur during the sixth and eighth weeks of gestation and is often limited to the tarsal bones. Because this coalition frequently originates as a fibrous or cartilaginous link, it is asymptomatic in early childhood. Only when the coalition ossifies during puberty can symptoms like foot pain and stiffness arise.

Timing and type of ossification

The time of symptom development varies according to coalition type. Fibrous coalitions (syndesmosis) present symptoms early, but cartilaginous (synchondroses) and bony (synostoses) coalitions do so later when they stiffen. Calcaneonavicular coalitions commonly ossify between years 8 and 12, while talocalcaneal coalitions ossify between ages 12 and 16.²

Types of congenital coalitions

Congenital tarsal coalitions can be classified into numerous categories, with the most prevalent being: Calcaneonavicular Coalition: This kind has an unusual relationship between the calcaneus and the navicular bone. Patients frequently report lateral foot discomfort, reduced subtalar mobility, and stiff flatfoot deformity.³ Talocalcaneal Coalition: This kind of connects the talus and calcaneus bones, usually at the middle facet of the subtalar joint. This combination may also cause peroneal muscular spasms, which contribute to a painful, stiff flatfoot.⁴ Less Common Coalitions: Examples include talonavicular, cuboid-navicular, and calcaneocuboid alliances. These are uncommon but should be examined in complicated foot pain instances.

Genetic factors in tarsal coalition

Evidence for hereditary transmission

Tarsal coalition is commonly seen in families, indicating a significant hereditary component. The syndrome is frequently inherited in an autosomal dominant manner, which means that a single defective gene passed down from one parent is sufficient to induce the illness.¹ As a result, family history plays a significant role in diagnosis. Surprisingly, several investigations have discovered bilateral coalitions in over half of instances, indicating a genetic relationship.⁵ Genetic predisposition may not directly cause the coalition, but it may raise the risk of aberrant bone segmentation during foetal development.

Genetic syndromes associated with tarsal coalition

Tarsal coalitions are linked to a number of hereditary disorders that cause improper bone growth or ossification: Fibrodysplasia Ossificans Progressiva (FOP) is a rare hereditary illness in which muscle and connective tissue gradually change into bone, occasionally impacting the tarsal bones at the beginning of the disease.⁶ Multiple Synostoses Syndrome is a hereditary illness characterised by extensive joint fusion, particularly in the hands, elbows, and feet. Tarsal coalition is a typical phenotypic trait of this species.⁷ Symphalangism: Another genetic disorder that causes congenital joint fusions in the fingers, toes, and tarsal bones.⁸ These diseases are frequently associated with abnormalities in genes related to development such as NOG, GDF5, and HOXD13, which control bone growth, segmentation, and joint creation.

Molecular and genetic pathways

Advances in molecular biology have identified distinct signalling pathways associated with bone and cartilage growth. Disruptions in routes include: FGF, BMP, and TGF-β have been related to aberrant bone fusion and growth defects, such as tarsal coalition.⁹ These discoveries could give rise to future genetic screening or focused therapeutic possibilities.

Acquired causes of tarsal coalition

Though less prevalent than congenital or hereditary reasons, acquired tarsal coalitions can occur later in life as a result of trauma, inflammation, or surgical intervention.

Trauma-induced coalition

Foot and ankle injuries, particularly serious fractures or dislocations of the tarsal bones, can alter the articular surfaces and cause aberrant healing. The body's natural mending system can occasionally result in heterotopic ossification, which is the formation of bone in soft tissue, leading to a coalition.¹⁰ Even modest trauma, if repeated, can cause variations in proximal bone structure over time, resulting in a coalition in susceptible people.

Inflammatory and degenerative conditions

Chronic inflammatory illnesses such as rheumatoid arthritis, psoriatic arthritis, and juvenile idiopathic arthritis can damage articular cartilage and cause bone bridging between joints. Chronic inflammation is known to increase osteophyte development and synostosis, especially in weight-bearing joints such as the foot.¹¹ Furthermore, degenerative joint problems in older individuals may have comparable results, particularly if combined with a biomechanical imbalance.

Infections

Infections like osteomyelitis (bone infection) and septic arthritis can degrade cartilage and bone matrix, causing aberrant bone healing processes. During the healing process, these alterations might cause a fibrous or bony coalition among neighbouring tarsal bones.¹² Prompt treatment of foot infections is critical to avoiding this consequence.

Surgical or Iatrogenic causes

Surgical treatments on the foot, such as fractures, realignment, or deformity repair, can occasionally induce tarsal coalition without aim. Bone grafts, for example, positioned excessively near neighbouring tarsal bones, or excessive stabilisation with screws or plates may cause fusion over time.¹³ This emphasises the value of meticulous surgical planning and postoperative surveillance.

Summary

Tarsal coalition is a multifaceted condition that can result from congenital segmentation defects, inherited genetic effects, or acquired triggers such as trauma and illness. While symptoms are sometimes missed in early infancy, they most typically appear throughout adolescence and can have a substantial impact on mobility and quality of life. If not addressed, the condition can progress to long-term concerns such as persistent pain, altered gait, and secondary musculoskeletal difficulties such as knee, hip, or back problems caused by poor mechanics.

Understanding the underlying aetiology, whether congenital, genetic, or acquired, is critical for proper diagnosis and appropriate treatment. Clinicians are now more prepared than ever to detect and manage this disorder by combining modern imaging methods, genetic screening, and individualised therapy protocols. Healthcare experts may greatly improve results for people with tarsal coalition by detecting it early, developing personalised treatment approaches, and continuing to explore genetic mechanisms. Continued patient education and multidisciplinary treatment are also critical in enabling patients to control their symptoms and live an active, healthy lifestyle.