The coccyx (also known as the tailbone) is the terminal part of the vertebral column. It is comprised of four vertebrae, which fuse to produce a triangular shape. In this article, we will discuss the anatomy of the coccyx – its structure, bony landmarks, ligaments, and clinical relevance.
The coccyx arises embryologically as the skeletal remnant of the caudal eminence that is present from weeks 4-8 of gestation. This eminence subsequently regresses, but the coccyx remains. Initially, the four coccygeal vertebrae are separate, but throughout life they fuse together to form one continuous bone. There is considerable variation in structure between individuals. One common variant is failure of the first coccygeal vertebra (Co1) to fuse, remaining separate throughout adult life. In some individuals, there can be one more or one less coccygeal vertebra, giving the individual a coccyx with 5 or 3 vertebrae respectively.
The coccyx consists of an apex, base, anterior surface, posterior surface and two lateral surfaces. The base is located most superiorly, and contains a facet for articulation with the sacrum. The apex is situated inferiorly, at the terminus of the vertebral column. The lateral surfaces of the coccyx are marked by a small transverse process, which projects from Co1. The coccygeal cornua of Co1 are the largest of the small articular processes of the coccygeal vertebrae. They project upwards to articulate with the sacral cornua.
The coccyx articulates with the sacrum at a fibrocartilaginous joint called the sacrococcygeal symphysis. Movement here is limited to minor flexion and extension which occurs passively, for example during defecation and labour.
The sacrococcygeal symphysis is supported by five ligaments:
One of the key functions of the coccyx is as an attachment point for various structures. The gluteus maximus attaches to the coccyx, as does the levator ani muscle, which is a key component of the pelvic floor. The anococcygeal raphe is a thin, fibrous ligament which runs from the coccyx and helps support the position of the anus.
An abrupt fall onto the buttocks, for example falling off a chair, can fracture the coccygeal vertebrae. This condition is normally managed with conservative care, although severe fractures may require in-patient treatment.
Coccydynia refers to a sensation of general discomfort around the coccyx, and has a wide range of causes. Childbirth may be a cause, as the stretching of pelvic floor muscles during labour puts pressure on their attachment to the coccyx, causing pain. Blunt trauma can contribute, as can poor posture when cycling or rowing, leading to irritation of the bone. Normally coccydynia is an acute condition, although if the pain lasts for more than 3 months it is considered chronic, and requires more specialised treatment.
A sacrococcygeal teratoma is a tumour of the coccyx that is thought to derive from the embryological primitive streak. Mostly they are benign, although 12% of the time they are malignant and life-threatening.
Knowledge of the anatomy of the coccyx is important for informed and effective management of related injuries and medical conditions. There is considerable variation in the structure of the coccyx, which must be taken into account when assessing individual cases. The coccyx provides attachment points for a number of structures, and has a number of ligaments that provide support and stability to the sacrococcygeal joint. Being aware of the clinical relevance of the coccyx is essential for medical professionals, in order to properly assess and diagnose patients with coccyx-related conditions such as fractures and coccydynia, and to help identify life-threatening conditions such as sacrococcygeal teratomas.
The coccyx, often referred to as the tailbone, is a triangular bony structure located at the inferior terminus of the vertebral column. It consists of three to five articulated vertebrae. In the adult human body, the coccyx is formed from four vertebrae. These vertebrae are connected by ligaments and discs, and articulate through sacrococcygeal symphysis and synchondroses. The coccyx also serves as the attachment point for the sacrococcygeal ligaments and the coccygeus muscle.
The base of the coccyx is situated most superiorly and has a facet for articulation with the sacrum. The apex of the coccyx is positioned inferiorly and is at the end of the vertebral column. The lateral surfaces of the coccyx show small transverse processes which project from the first coccygeal vertebra, commonly referred to as Co1. The coccygeal cornua of Co1, which are small articular processes, are the largest cornua out of the four coccygeal vertebrae. The cornua project upwards to articulate with the sacral cornua.
The coccyx articulates with the sacrum at a fibrocartilaginous joint known as the sacrococcygeal symphysis. Movement within this joint is limited to minor flexion and extension which occurs passively during activities such as defecation and labor.
The sacrococcygeal symphysis is supported by five ligaments:
The coccyx serves an essential role in providing attachment points for various structures. The gluteus maximus and the levator ani muscle, which is a crucial component of the pelvic floor, both attach to the coccyx. The anococcygeal raphe, a thin and fibrous ligament, is attached to the coccyx and helps support the position of the anus.
An abrupt fall onto the buttocks, such as falling off a chair, can lead to a fracture of the coccygeal vertebrae. Generally, fractures like this are managed through conservative care, although severe fractures may require in-patient treatment.
Coccydynia is an umbrella term to describe the sensation of general discomfort around the coccyx. It can have many causes, such as childbirth, blunt trauma, or poor posture when cycling or rowing, all of which can lead to irritation of the bone. Usually, coccydynia is an acute condition, however if the pain persists for over three months, it is considered chronic. In this case, more specialised treatment is required.
A sacrococcygeal teratoma is a tumour which develops from the coccyx. It is thought to originate from the embryonic primitive streak. The majority of these tumors are benign, however in 12% of cases they are malignant and life-threatening. These tumors are most common in newborns, occurring at a frequency of 1/35,000 live births.
