Causes and Mechanisms of Common Coccydynia

Role of Body Mass Index and Coccygeal Trauma

Spine, volume 25, number 23, 2000, 3072-3079


Study design. A total of 208 consecutive coccydynia patients were examined with the same clinical and radiologic protocol.

Objectives. To study radiographic coccygeal lesions in the sitting position, to elucidate the influence of body mass index on the different lesions and to establish the effect of coccygeal trauma.

Summary of Background Data. A protocol comparing standing radiographs and radiographs subsequently taken in the painful sitting position in coccydynia patients and in controls has shown two culprit lesions: posterior luxation, and hypermobility. Obesity and a history of trauma have been identified as risk factors for luxation.

Methods. Dynamic radiographs were obtained. The body mass index was compared with the coccygeal angle of incidence, sagittal rotation of the pelvis when sitting down, and the presence and time of previous trauma. The patients with the newly described lesions were examined after an anesthetic block under fluoroscopic guidance.

Results. Two new coccygeal lesions are described (anterior luxation and spicules). Obesity was found to be a risk factor. The body mass index determines the way a subject sits down, and lesion patterns were different in obese, normal-weight, and thin patients (posterior luxation: 51%, 15.2%, 3.7%; hypermobility: 26.5%, 30.3%, 14.8%; spicules: 2%, 15.9%, 29.6%; normal: 16.3%, 32.6%, 48.1%, respectively; P < 0.0001). Trauma affected the type of lesion only if it was recent (< 1 month before the onset of coccydynia), in which case the instability rate rose from 55.6% to 77.1%. Backward-moving coccyges were at greatest risk of trauma.

Conclusions. This protocol allows identification of the culprit lesion in 69.2% of cases. The body mass index determines the causative lesion, as does trauma sustained within the month preceding the onset of pain.

Extended abstract by Jon Miles:

This study examined the causes of coccyx pain in 208 patients using dynamic (sit/stand) x-rays. When an abnormality of a coccyx was identified, the site of the abnormality was injected with lidocaine anesthetic to check whether it was the source of the pain. If the injection caused a 75% reduction in pain or more, then it was concluded that the abnormality was the cause of the pain.

The causes of pain were found to be:


This was best detected using dynamic (sit/stand) x-rays. Instability was of three types:

  1. posterior luxation (partial dislocation of the coccyx backwards when sitting) - 40% of instabilities
  2. anterior luxation (partial dislocation of the coccyx forwards when sitting) - 10% of instabilities
  3. hypermobility (excessive flexing of the coccyx forwards and upwards when sitting) - 50% of instabilities

Posterior luxation was most common in obese (fat) patients. This is probably because obese people: (1) often drop themselves into a chair, rather than lowering themselves, and (2) don't rotate the lower part of the spine forwards when sitting down, so their coccyxes point downwards more than forwards. Sitting causes an increase in pressure in the tissues in front of the coccyx, pushing the coccyx backwards.

Hypermobility and anterior luxation usually occur in thinner patients. This is probably because thin people usually rotate the bottom of the spine forwards when sitting, so the pressure on the coccyx is forwards and upwards.

Spicule or bony spur

This is usually invisible on x-rays, but may be seen on MRI images. The spur points backwards at the tip of the coccyx, and can always be felt through the skin. There is usually a corresponding pit in the skin above the spur, suggesting that the spur has been present since before birth. It is suggested that some cases in the medical literature described as a pilonidal sinus (a pit associated with a hair follicle) in this position may in fact be the pit corresponding to a spicule.

Spicules only caused pain in thin patients, who do not have any padding of fat over the spur.


Only if an accident or childbirth had occurred within a month before the pain started did it appear that the trauma was the origin of the instability of the coccyx.

Body-mass index (BMI) is a measure of your weight/height ratio. It is your weight in kilograms divided by the square of your height in metres.

For instance, if your height is 1.82 metres, the divisor of the calculation will be 1.82 x 1.82 = 3.31

If you weigh 70.5 kilograms, then your BMI is 70.5 / 3.31 = 21.3

If you only know your weight and height in old units, then:

A BMI of 19 - 25 is considered ideal, 25 to 30 is overweight and over 30 is obese.

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