You can absolutely have a bone density test after a knee replacement, and in many cases it is more important than ever. The metal implant does complicate results if it falls within the scan field, but clinicians have several well-established ways around this problem. The standard DXA scan remains the go-to tool, though the site measured and the software used may need adjusting depending on whether you have a single replacement or both knees done.
How a Metal Implant Interferes with DXA Results
A DXA scan works by passing two low-energy X-ray beams through your body and measuring how much each beam is absorbed. Bone absorbs more than soft tissue, and the difference is used to calculate bone mineral density. The problem is that a metal knee implant absorbs X-rays far more aggressively than bone does. When a standard scan sweeps across a replaced knee, the machine cannot tell the difference between the implant and your actual bone. Any portion of the implant that remains in the image gets read as if it were native bone, which inflates the density reading for that region.1PubMed Central. DEXA overestimates bone mineral density in adults with knee replacements
Research quantifying this effect found that limbs with knee implants showed roughly 12% higher bone mineral density compared with the person’s other, un-operated leg when standard analysis software was used. That sounds like a modest error, but in practice it meant the vast majority of patients who actually had low bone density in that leg went undiagnosed. Among those who truly had low leg BMD, about 95% would have been missed; among those with low total-body BMD, about 80% would have slipped through undetected.1PubMed Central. DEXA overestimates bone mineral density in adults with knee replacements That is a clinically meaningful gap, because a false-normal reading could mean osteoporosis treatment never gets started.
Standard Sites Still Work Fine
The most common bone density test measures the hip and lumbar spine, not the knee. If your knee replacement is your only implant and you have no metal hardware in your hip or spine, those standard scan sites are completely unaffected by the knee prosthesis. The technologist positions you on the table, the scanner passes over your hip and lower back, and the metal in your knee never enters the picture. For the purpose of diagnosing osteoporosis and deciding whether treatment is needed, hip and spine readings are the primary measurements clinicians rely on anyway.
Where things get tricky is with total-body DXA scans, which image you from head to toe. A total-body scan inevitably passes over the replaced knee, and unless the technologist or the analysis software specifically accounts for the implant, the whole-body density number gets pulled upward. If you have bilateral knee replacements, the distortion doubles. This does not mean a total-body scan is off the table, but it does mean extra steps are needed during analysis.
Manual Artifact Correction Makes a Real Difference
The software that comes with modern DXA machines includes a manual artifact correction tool. After the scan is captured, the technologist can outline the metal implant on the image and exclude it from the density calculation. When this correction was applied in the study mentioned earlier, the overestimation caused by the implant was effectively eliminated.1PubMed Central. DEXA overestimates bone mineral density in adults with knee replacements A separate study looking at athletes with surgical hardware in the knee found that differences in density readings between scan types narrowed substantially once the manual artifact correction tool was used to exclude the hardware from the image.2Journal of Clinical Densitometry. Total-body versus knee-specific dual-energy X-ray absorptiometry scans for assessing bone mineral density surrounding the knee joint in collegiate athletes with and without an anterior cruciate ligament reconstruction
The catch is that manual correction requires an experienced technologist who knows exactly where the implant boundaries are, and not every clinic applies it by default. If you are having a total-body DXA and you have a knee replacement, it is worth asking your provider whether the analysis will use artifact correction. This is especially relevant if you have implants in both knees, since the cumulative distortion on a total-body scan becomes harder to ignore.
Why Monitoring Bone Density After a Knee Replacement Matters
Having a knee replacement does not just create a scanning challenge. It also changes the bone around the implant in ways that make density monitoring more important. When a metal prosthesis is fixed to your femur and tibia, the implant starts carrying loads that your bone used to handle alone. The bone, no longer bearing those forces, can gradually thin out in a process called stress shielding. This is most pronounced in the first couple of years after surgery, though research suggests it may continue beyond that window.3PubMed. Stress shielding after total knee replacement may cause bone resorption in the distal femur
A systematic review found that stress shielding is a critical concern in the early postoperative period and highlighted the importance of implant design and fixation methods in reducing the bone loss that follows.4PubMed Central. The relationship between stress shielding, bone density changes and implant migration, failure and fracture after total knee arthroplasty: A systematic review The bone around your implant needs to stay strong for the prosthesis to remain stable over its lifetime. Weakened bone raises the risk of a periprosthetic fracture, which is a break that occurs right at the edge of the implant, and of aseptic loosening, where the prosthesis gradually separates from the surrounding bone without any infection being involved.5PubMed Central. Baseline bone mineral density and bone turnover in pre-operative hip and knee arthroplasty patients
This is the irony of the situation: the people who most need careful bone density tracking around the knee are the same people whose scans are hardest to interpret accurately at that site. It is part of why clinicians often rely on the hip and spine for systemic osteoporosis screening while using specialized approaches when they need to see what is happening right around the prosthesis.
Periprosthetic DXA Scans
When the clinical question is not “do you have osteoporosis in general” but “what is the bone doing right next to your implant,” a specialized version of DXA called periprosthetic scanning can be used. This involves positioning the scanner specifically around the replaced knee and using dedicated software algorithms to measure bone density in defined zones adjacent to the prosthesis. Early research confirmed that DXA can precisely detect small bone mineral changes around a total knee replacement and could serve as a feasible method for monitoring implant health over time.6PubMed. Measurement of bone density around total knee arthroplasty using fan-beam dual energy X-ray absorptiometry
In practice, periprosthetic DXA is used more often in research settings and large orthopedic centers than in routine community clinics. A prospective study of 69 patients tracked bone density changes in the distal femur after cemented total knee replacement, scanning patients within a week of surgery and again at three, six, and twelve months.7PubMed. Periprosthetic femoral bone loss after total knee arthroplasty: 1-year follow-up study of 69 patients This kind of serial measurement helps surgeons understand how much bone remodeling is normal after the operation and when a patient might be losing bone faster than expected. Whether cemented or cementless fixation was used also matters: a study comparing the two methods found that the cemented group experienced more periprosthetic bone density loss in the medial, lateral, and anterior regions than the cementless group over two years.8Journal of Bone and Joint Surgery. Cementless Versus Cemented Tibial Fixation in Posterior Stabilized Total Knee Replacement
CT-Based Alternatives and Metal Artifact Reduction
DXA is not the only option for measuring bone density around metal hardware. Quantitative CT, which uses a standard CT scanner with a calibration phantom to calculate volumetric bone density, can be applied around implants. The challenge is that metal causes severe streak artifacts on CT images, distorting the picture of nearby bone. But newer techniques are steadily improving the situation.
Dual-energy CT, which acquires images at two different X-ray energy levels, can generate virtual monoenergetic images that significantly reduce the halo of artifact around metal. Researchers have demonstrated that this approach reduces artifacts from titanium, stainless steel, and cobalt-chrome implants, leading to clearer depiction of surrounding structures.9PubMed. Metal artifact reduction by dual energy computed tomography using monoenergetic extrapolation When applied specifically to total knee replacements, combining polychromatic images with iterative metal artifact reduction algorithms produced substantially better image quality compared with standard reconstruction.10PubMed Central. Comparison of the Quality of Various Polychromatic and Monochromatic Dual-Energy CT Images with or without a Metal Artifact Reduction Algorithm to Evaluate Total Knee Arthroplasty
A pilot study evaluating orthopedic metal artifact reduction algorithms together with virtual monoenergetic imaging in QCT protocols confirmed that these combined approaches can improve both the image quality and the accuracy of density measurements near implants.11Springer Nature / PubMed Central. Effect of different artifact reduction methods for quantitative computed tomography measurements of bone mineral density: a pilot study These are not widely available tools in every radiology department yet, but they are increasingly part of the toolkit at major medical centers. CT-based methods also deliver more radiation than DXA, though the doses are still in the range of a simple chest X-ray.12PubMed. Radiation exposure in bone mineral density assessment
Radiation Exposure Is Not a Concern
Some people worry about the radiation involved in bone density testing, particularly if they need repeated scans to monitor changes after surgery. Standard DXA delivers an extremely low dose, on the order of a few microsieverts, which is less than the background radiation you absorb in a typical day. Fan-beam DXA technology, which uses a wider beam for better resolution, does give a somewhat higher dose, but it is still quite small. QCT involves more radiation but remains comparable to a basic chest X-ray.12PubMed. Radiation exposure in bone mineral density assessment For the vast majority of patients, the clinical benefit of knowing their bone density status far outweighs any radiation risk, even with annual or biannual follow-up scans.
Other Factors That Can Skew Your Results
Metal implants are not the only thing that can throw off a DXA reading. Major changes in body composition, particularly significant weight gain or weight loss, can introduce errors because of how DXA handles the ratio of fat to lean tissue. The system can only resolve two tissue types at a time using its two energy levels, so it makes assumptions about the fat-to-lean ratio in the soft tissue overlying bone. When those assumptions break down, as they do at extremes of body weight, the density numbers can shift in ways that do not reflect actual changes in bone.13PubMed Central. Simulated increases in body fat and errors in bone mineral density measurements by DXA and QCT
This matters for knee replacement patients because many are older adults who may also be dealing with weight fluctuations related to reduced mobility before surgery and increased activity afterward. If you have a series of DXA scans tracking your bone health over time and your weight has changed substantially between measurements, that context is worth mentioning to your clinician so the numbers can be interpreted with the appropriate caution.
Bone Density Before Surgery
If you are reading this because you are planning a knee replacement rather than recovering from one, there is a good case for getting a bone density test before the procedure. The quality of the bone that will anchor your implant has a direct effect on how stable the prosthesis will be. One study found a clear relationship between preoperative bone density and how much the tibial component shifted after surgery in uncemented implants: too-low density was associated with more migration and subsidence.14PubMed. The effect of the preoperative bone quality on the fixation of the tibial component in total knee arthroplasty Knowing your bone density status before surgery lets your surgeon plan accordingly, whether that means choosing a cemented fixation method, adjusting implant design, or starting bone-protective medication in advance.
Preoperative screening also establishes a baseline that makes post-surgical monitoring far more useful. Without a “before” number, it is hard to know whether a later reading represents bone that was already thin going in or bone that has deteriorated since the implant was placed. Researchers have emphasized that osteoporosis and abnormal bone metabolism may be significant factors in arthroplasty outcomes, increasing the risk of both loosening and fracture around the implant.5PubMed Central. Baseline bone mineral density and bone turnover in pre-operative hip and knee arthroplasty patients
Medications That Protect Bone After Knee Replacement
For patients found to have low bone density before or after a knee replacement, treatment with bone-protective medications can make a real difference. A narrative review covering osteoporosis in the setting of knee arthroplasty concluded that antiresorptive medications have been shown to be beneficial after surgery.15PubMed Central. Osteoporosis in the setting of knee arthroplasty: a narrative review Bisphosphonates, the most commonly prescribed class of antiresorptive drugs, work by slowing down the cells that break down bone.
A study of women over 65 who had total knee replacements found that bisphosphonate treatment for one year prevented the early drop in hip bone density that typically follows the surgery, regardless of whether the patient had been formally diagnosed with osteoporosis. The researchers concluded that this could help prevent later hip fractures in elderly patients undergoing knee replacement.16PubMed Central. Bone mineral density changes after total knee replacement in women over the age of 65 A randomized controlled trial looking at the ipsilateral hip (the hip on the same side as the knee surgery) found that without bisphosphonate treatment, hip bone density dropped after the operation, while in the group receiving bisphosphonates there was no obvious reduction at any follow-up point.17International Journal of Surgery Open. The influence of bisphosphonate on bone mineral density after total knee arthroplasty: A Randomised Controlled Trial
The hip density drop after knee surgery catches some people by surprise. It happens partly because of the reduced weight-bearing and changed gait mechanics during the recovery period. The hip on the surgical side takes on different loads for weeks or months while you heal, and bone responds to those altered forces. If you already had borderline bone density going in, this temporary dip could push you into a higher fracture-risk category at a time when a fall could be particularly damaging. This is one more reason why bone density testing around the time of knee replacement can be genuinely useful rather than just a formality.
What to Tell Your Doctor and Technologist
If you have a knee replacement and are scheduled for a bone density test, the single most important thing is to make sure the facility knows about your implant before you arrive. For a standard osteoporosis screening, the hip and lumbar spine measurements will not be affected by a knee prosthesis, but if a total-body scan is ordered, the technologist needs to apply artifact correction during the analysis step. If both knees have been replaced, this becomes even more critical, since the error compounds.
You should also mention if you have any other metal in your body: a hip replacement on either side, spinal fusion hardware, or metal plates and screws from prior fractures. Each piece of hardware potentially affects a different scan region, and the technologist needs a complete picture to choose the right sites and the right analysis approach. In rare cases where both hips and the spine all contain metal, your provider may need to rely on forearm DXA or a CT-based method to get a clean reading. The forearm is a recognized alternative DXA site precisely because it is rarely affected by orthopedic hardware.
None of these situations means you cannot be tested. They just mean the test needs a little more thought in how it is set up and interpreted. The metal in your joints does not make bone density testing dangerous, inaccurate by default, or pointless. It just makes the conversation between you and your medical team slightly more important than it would be otherwise.