Coccygeal instability refers to pathological, excessive mobility of the coccyx, the terminal part of the spinal column, at its articulation with the sacrum (sacrococcygeal joint), beyond the normal range of movement that allows slight displacement during sitting or defecation.
It is usually caused by trauma, such as a fall in a seated position with direct impact on the area, by laxity or damage of the ligaments that normally stabilize the sacrococcygeal joint, as well as by repetitive mechanical stress, such as prolonged sitting at work on hard surfaces.
This instability can lead to excessive flexion or displacement of the coccyx during sitting, causing chronic pain in the region (coccydynia), which is usually aggravated by sitting, especially on hard surfaces, and relieved by standing or leaning the body forward.
Diagnosis includes clinical examination, as well as dynamic radiographic evaluation in sitting and standing positions, which allows comparison of the position of the coccyx and assessment of the degree of displacement between the two positions.
Treatment is usually conservative and includes the use of special pressure-relieving cushions, medication for pain management, physical therapy, and, in persistent cases, corticosteroid injections, while surgical removal of the coccyx (coccygectomy) is reserved for rare cases resistant to conservative treatment.