It is however useful to divide the anterior to posterior dimension of the intervertebral foramina (IVF) (4) into four equal units. A posterior displacement of up to ¼ of the IVF is graded as Grade 1, ¼ to ½ as Grade 2, ½ to ¾ as Grade 3, ¾ to total occlusion of the IVF as Grade 4. Alternatively, a measurement of the amount of displacement can also made by measuring the bone displacement in millimetres.Retrolistheses can be caused by injury and the resulting instability of the connecting soft tissues especially ligaments, discs, muscles, tendons and fascia. They may also involve muscles through spasm as a result of nerve malfunction due to pressure caused by the posterior displacement of the vertebra encroaching on the contents of the IVF. The IVFs contents include spinal (sensory and motor) nerves, arteries, veins and lymphatic vessels which cater to the nutritional and waste removal needs of the spinal cord.Degenerative spinal changes are often seen at the levels where a retrolisthesis is found. These changes are more pronounced as time progresses after injury, and are evidenced by end plate osteophytosis, disc damage, disc narrowing, desiccation and disc bulging. “A retrolisthesis hyperloads at least one disc and puts shearing forces on the anterior longitudinal ligament, the annular rings, nucleus pulposus, cartilage end plates and capsular ligaments. The bulging, twisting and straining tissues attached to the endplates pull, push and stretch it. It is worsened with time, becoming irreversible.”  This is the etiology of degenerative joint disease.
Crutches can be used for ambulation while providing adequate rest for the affected limb and minimizing the haematoma formation. Ice therapy for 15 to 20 minutes every 30 to 60 minutes is useful to reduce the skeletal muscle blood flow by 50%. Aggressive limb physiotherapy is not recommended at this stage to prevent the worsening of symptoms. After 48 to 72 hours, range of motion exercise can be introduced as long as the range of motion is not painful. If the lesion becomes more mature, active range of motion and resistance strengthening exercises are useful in maintaining joint function.Surgical excision is reserved for those who failed the non-surgical treatment, those with intolerable pain, compression of the neurovascular structures, or limitation of the range of motion of the joint which affects the activities of daily living. Surgery is only performed after 6 to 18 months following injury because surgery does not alter the bone maturation process. If a surgery is performed too early, it may predispose to recurrence. However, the optimum timing for surgery and the rate of recurrence following early surgery is controversial because some studies have shown that early surgery does reduce the rate of recurrence.For those who had total hip replacement or total hip arthroplasty, postoperative single low-dose radiation with 3 weeks of oral indomethacin regimen will be preventive  for heterotopic ossification. Radiation therapy is also effective in preventing recurrence in those who had done operative excision of heterotopic ossification of the elbow. References
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Radiograph showing myositis ossificans