A bone density test is possible even with a knee replacement, but the metallic implant requires careful adjustments to the standard procedure. Dual-Energy X-ray Absorptiometry (DEXA), the most common method for measuring bone health, is still the preferred tool, though the metal hardware affects the scan’s accuracy in the immediate area. Professionals use specific protocols and alternative measurement sites to ensure the patient receives an accurate assessment of their overall bone mineral density (BMD). Understanding how the metal interferes and knowing which parts of the body can reliably be measured is key to diagnosing conditions like osteoporosis.
Understanding Dual-Energy X-ray Absorptiometry (DEXA)
DEXA is the standard test used to measure bone mineral density and diagnose conditions such as osteopenia and osteoporosis. This non-invasive procedure utilizes two low-dose X-ray beams that pass through the body. The machine calculates density based on how much energy is absorbed by the bone. The standard regions for assessment are the lumbar spine and the hip, which are the most common sites for fragility fractures.
The results are reported using two main metrics: the T-score and the Z-score. The T-score compares an individual’s BMD to that of a healthy young adult of the same sex; a score of -2.5 or lower defines osteoporosis. The Z-score compares the patient’s bone density to people of the same age, sex, and size, which helps determine if an underlying medical condition is contributing to bone loss.
How Metallic Implants Interfere with Test Accuracy
The presence of a metallic implant, such as a knee replacement, interferes with the DEXA scan by creating a measurement error known as a metal artifact. The dense metal absorbs X-rays disproportionately compared to bone or soft tissue. This excessive absorption leads the DEXA software to misinterpret the area immediately surrounding the implant.
This misinterpretation results in a falsely elevated reading of the bone mineral density in the affected region. For example, a titanium hip prosthesis can lead to an increase in reported bone mineral content. This overestimation can be significant, potentially leading to the misclassification of a patient’s bone health and causing existing osteopenia or osteoporosis to go undiagnosed.
The error is highly localized; only the bone directly adjacent to the metal is affected by the artifact. The interference is caused by the metal’s high density, which completely blocks the X-ray beam in certain spots, making the data unreliable. Therefore, measuring the bone density of the implanted knee or the immediate surrounding bone will yield an inaccurate result.
Adjusting the Testing Protocol for Patients with Knee Replacements
When a patient has a knee replacement, the DEXA procedure is modified to ensure an accurate bone density measurement is obtained. The most common adjustment is to use the opposite, non-implanted hip as the primary measurement site. Since bone density loss is a systemic condition, the bone health of the contralateral hip is an excellent predictor of the overall skeleton’s condition.
If the patient has replacements in both knees, or if the hip area is otherwise compromised, the forearm (radius) becomes the alternative measurement site. The forearm is a standard site for bone density assessment and is unaffected by lower-body hardware. Measuring the one-third distal radius provides a reliable reading, especially when a central site measurement is not possible.
Technicians also employ specific positioning techniques to minimize the risk of the implant’s metal causing scatter or interference. The affected area is excluded from the final analysis, but the rest of the body can still be measured effectively. Focusing on unaffected, representative skeletal sites ensures the DEXA scan remains the preferred diagnostic tool.
Alternative Screening Methods for Bone Density Assessment
When a DEXA scan is not feasible or is unreliable due to extensive hardware, alternative imaging methods are available. One alternative is Quantitative Computed Tomography (QCT), which provides a three-dimensional, volumetric measurement of the bone. QCT is useful because it measures the density of the trabecular (spongy) bone, which is often more sensitive to early bone loss than the cortical bone measured by DEXA.
Peripheral QCT (pQCT) is another option used to measure the bone density of the extremities, such as the forearm or tibia, offering a precise localized assessment. These CT-based methods involve a higher radiation dose than DEXA, but they offer valuable information when the standard test is obstructed. Quantitative Ultrasound (QUS) is a non-ionizing method that measures bone density, typically at the heel or finger, and can predict fracture risk.
In addition to imaging, blood tests that measure bone turnover markers (BTMs) provide supplementary information about the rate of bone breakdown and formation. While not a direct measure of density, these markers help doctors monitor treatment effectiveness and provide a comprehensive view of the patient’s bone metabolism. These methods ensure that bone health can be properly evaluated in patients with metallic implants.