Low bone density can be a problem for anyone receiving any type of endoprotheses in the knee, hip, shoulder, or back. However, low bone density for ADR patients does not disqualify them for surgery, because bone ceramic material can be injected into the vertebral body to support the endoprothesis. However, despite all precautions bad bones can be a problem in about 3 out of every 500 hundred cases, leading to subsidence. Microfractures and other defects of the bones are not always detected by tests or perioperative examination.
Bone Density Testing
Bone density testing has been historically used for determining osteoporosis and osteopenia, the precursor to osteoporosis. In ADR surgery, some spine surgeons use bone density measurements to assess the risk of subsidence of the disc into the bone, although in this author’s opinion bone density testing should be used by all spine surgeons. Subsidence also occurs in fusion.
Bone density testing, when properly done, is about 80% accurate in assessing osteoporosis and osteopenia. The results are reported as a T score. A T score is a statistical term denoting where the patient lies on the normal curve of distribution. A T score of 0 means that the patient is exactly average for the patient’s gender, height, weight and age. A T score of +1 means the patient is one standard deviation above the average person for bone density. A T score between -1 and -2.5 is interpreted as osteopenia and scores lower than -2.5 are interpreted as osteoporosis.
At it's heart both the DXA (low radiation) and/or the QCT scanner (high radiation) look for the amount of minerals per a given volume. The measuring unit is kg/m3. Slices are measured as g/cm2. Of course, the findings are normalized to the population group of the patient. Reference data exists on the femur, lumbar, hip, total body and wrist. None exist for the cervical or thoracic spine.
A popular myth is that bone density should always be done in the area of the operation, but this is not necessarily true and shows a lack of understanding of how the test works. For example, they suggest a lumbar patient should have a lumbar bone density test. While it is acceptable to do the test in the lumbar spine (and ProSpine does lumbar testing) caution must be exercised with the resulting numbers, because patients with DDD have shorter spines than normal and can create a false high result. Bone density measurements in the hip are entirely acceptable.
Bone density is surprisingly consistent throughout life, except for many women past the age of fifty, who are a high risk group for osteoporosis, or at least that is the conventional wisdom. ProSpine testing has shown in its patient population a much higher risk for men, than previously thought.
Proper operation of the scanner by the technician is also required. It is not merely a question of turning on the switch. Judgments are made by the technician about where each level begins and ends.
Perioperative Examination of the Bone
An experienced surgeon exams the conditions of the bones during surgery. The machine tests are just indicators.