Dual-Energy X-ray Absorptiometry (DEXA scan) is the standard method for measuring bone mineral density (BMD). This test uses two distinct X-ray beams to calculate the mineral content in bone. The primary purpose is to diagnose or assess the risk of osteoporosis, a condition characterized by weakened, brittle bones susceptible to fracture. Since osteoporosis is a silent disease often revealed only after a fracture, regular screening is a standard part of health management for many older adults. The presence of large metallic orthopedic hardware, such as a total knee replacement, complicates the test’s accuracy.
The Direct Answer
Yes, a patient with a total knee replacement can safely undergo a bone density test using DEXA technology. The scan is non-invasive and uses a small amount of radiation, posing no risk to the metallic implant or the patient. The challenge lies in the reliability and interpretation of results near the prosthetic knee.
The large metal component makes measuring BMD in the knee and surrounding bone unreliable. The DEXA scan cannot accurately assess the bone health of the replaced knee itself. Therefore, the procedural focus shifts entirely, requiring the technician to scan alternative anatomical sites to obtain valid data and assess the patient’s overall skeletal health.
How Metallic Implants Interfere
The interference caused by a metallic joint replacement is a direct consequence of how DEXA technology differentiates between bone and soft tissue. DEXA measures density by analyzing the differential absorption of two X-ray energy levels. Metal absorbs nearly all the X-ray energy, far more than bone, which creates a metal artifact.
This massive absorption causes the implant area to appear as a “bright spot” on the scan. The software incorrectly interprets this as extremely high bone mineral content (BMC), artificially inflating the density reading and leading to a falsely elevated or unreadable result. This inflation can cause significant errors in measurement.
Since the DEXA machine cannot distinguish bone from the high-density metal component, the scan of the affected limb is compromised. Consequently, any BMD measurement taken directly over the knee replacement is clinically meaningless. Technicians are trained to recognize this artifact and must exclude the affected area from the final analysis and report.
Scanning Procedures for Patients with Joint Replacements
To overcome metal interference, the technician must select alternative measurement sites distant from the implant. For routine DEXA scans, the standard sites are the lumbar spine and the hip. When a patient has a knee replacement, the most important alternative sites for assessing systemic bone health are the non-replaced hip and the lumbar spine.
The International Society for Clinical Densitometry guidelines recommend prioritizing the lumbar spine and the non-affected hip for diagnosis and monitoring. If the patient has had a total replacement in both knees, or if the lumbar spine is compromised by severe degenerative disease, the next standard site is the forearm. Specifically, the non-dominant forearm is scanned to measure BMD at the distal radius, providing a reliable measure of cortical bone density.
The results from these alternative sites provide the patient’s T-score, which compares their bone density to that of a healthy young adult. This systemic measurement accurately reflects the patient’s overall skeletal health, independent of the localized implant. Focusing on these standardized areas ensures the DEXA scan remains a highly effective tool for detecting osteoporosis and assessing fracture risk in patients with joint replacements.
The Importance of Systemic Bone Health Monitoring
Monitoring bone density is important because osteoporosis is a systemic skeletal disease, affecting the entire body, not just the operated joint. Low BMD increases the risk of fragility fractures in other areas, such as the spine or the non-replaced hip. Many patients undergoing total knee arthroplasty have pre-existing undiagnosed osteoporosis, which can negatively impact surgical outcomes.
Poor bone quality can lead to complications such as aseptic loosening, subsidence of prosthetic components, or periprosthetic fractures around the artificial joint. Therefore, the modified DEXA test is an important step in the long-term management of the patient’s health. Identifying and treating low bone density allows physicians to implement therapies that reduce the risk of future fractures and enhance the longevity of the joint replacement.