The effective dose in millisievert (mSv) is the risk-based metric for radiation exposure. A millisievert is defined as "the average accumulated background radiation dose to an individual for 1 year, exclusive of radon, in the United States." 1 mSv is the dose produced by exposure to 1 milligray (mGy) of radiation.

The radiation dose for a conventional shoulder X-ray series is reported as 0.01 mSv (see this link)

The effective doses from diagnostic CT procedures are typically estimated to be in the range of 1 to 10 mSv (100 to 1000 times the dose of plain shoulder x-rays). It has been suggested that such exposures may be associated with a small but increased radiation-related risk of mortality (see this link).

In Comparison of Simulated Low-Dose and Conventional-Dose CT for Preoperative Planning in Shoulder Arthroplasty the authors state, "Computed tomography (CT) scans have become increasingly popular in preoperative planning for reverse and anatomic total shoulder arthroplasty. These scans are often used in conjunction with software that allows surgeons to plan bone preparation and optimal implant position. While the use of shoulder CT scans can improve glenoid-component positioning, there is the undesirable side effect of ionizing radiation exposure."

They reported the volumetric CT dose index (CTDI) expressed in milligrays for 18 patients having shoulder CT scans obtained for preoperative arthroplasty planning, all exceeding CTDI of 10 mGy.

Out of concern for the risks of radiation exposure, they sought to evaluate the utility of simulated low-dose CT images for preoperative planning using manual measurements and common preoperative planning software using reduced radiation doses (RD) levels of 75%, 50%, and 25%.

They found that at all radiation dosage levels evaluated, the preoperative planning software successfully segmented the CT images. Semiautomated software measurement of 25% RD images was within tolerances in 99.1% of measurements; for 50% RD images, within tolerances in 96.3% of measurements; and for 75% RD images, within tolerances in 100% of measurements. Manual measurements of 25% RD images were within these tolerances in 95.1% of measurements; for 50% RD images, in 98.8% of measurements; and for 75% RD images, in 99.4% of measurements.

They concluded that simulated lower-dose CT images were sufficient for reliable measurement of glenoid version, glenoid inclination, and humeral head subluxation by preoperative planning software as well as by physician-observers. And that the adoption of low-dose techniques in preoperative shoulder CT may lower the risk of radiation exposure for patients undergoing shoulder arthroplasty, without compromising image quality.

Comment: Even with a 25% reduction in radiation dosage, it is evident that the CT scans used in preoperative planning subject the patient to substantially greater radiation exposure than for plain radiographs. Two questions arise:

1. For shoulders with straightforward anatomy, such as that in one of the case examples from the article shown below, how much increased value to the patient is there in three dimensional CTs in comparison to standardized plain radiographs (see this link)?

2. Since the purpose of 3D CT planning is to achieve a certain component position, is a second CT scan required to document the degree to which the preoperative plan is realized? Alternatively, a preoperative and postoperative set of plain films seems adequate to the task in the great majority of cases (see this link).

Future clinical research will be required to determine the appropriate use criteria for 3D CT planning, i.e. which patients with shoulder arthritis benefit sufficiently from 3D CT planning to offset its increased risk and cost.

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