Nuclear Medicine Imaging: Orthopedic Prostheses

by Gagandeep Choudhary, MD and Phillip Kuo, MD, PhD

Orthopedic Prostheses

Orthopedic devices may produce troublesome bizarre images if one is not familiar with their effects. They can cause active bone formation with a "positive" bone scan; or, they can block photons from decay of radiotracer originating deeper in the body. Many of the devices and surgical constructs - spine struts, knee prostheses, hip prostheses, spinal fixation apparatus, skull plates, and large fracture fixation plates - have a characteristic appearance. They are readily explained if the patient's surgical history and radiographic studies are available for comparison (Patton, 1994).

Technetium MDP bone scans often show typical photopenic defects involving knee prostheses in the distal femur and tibial condyles (figure 3). FDG-PET  imaging is sometimes performed for evaluation of primary and secondary bone malignancies. Large prostheses, particularly large limb sparing prostheses, produce streak artifact and photopenia from scatter of the photons from the annihilation of positrons emitted from F-18 (figure 4). Typical prostheses placed for degenerative arthritis or after a hip fracture will show photopenia at the prosthesis site and periprosthetic FDG uptake (figure 5A; figures 5B, 5C, 5D).

Sodium fluoride (NaF) PET images may show intense radiotracer uptake in bone surrounding a prosthesis (figure 6). This represents active bony remodeling. This can last for years and is not pathologic. It often most intense in the first six months after surgery (Patton, 1994). Stable uptake consistent with ongoing reparative bone remodeling is also evident on Tc 99m bone scans and should not be confused with malignant recurrence (figure 7A; figures 7B, 7C, 7D). This can last for many years and is not pathologic unless the patient complains of new pain in which case one should consider loosening, infection, or local recurrence of tumor. In some cases, reparative remodeling is a bony reaction to unusual stress placed on the bone (figure 8A; figure 8B). In all cases, it is best to correlate the patient's nuclear medicine imaging with his or her clinical picture, laboratory results, and other imaging studies.

Infection; Hardware Loosening; Reparative Remodeling; Synovitis

Nuclear medicine imaging along with MRI and CT is often a sensitive method for diagnosing osteomyelitis (figure 9). With three-phase bone scans, blood flow and blood pool images show increased radiotracer in the soft tissues around the infected bone and increased radiotracer in the infected bone on delayed imaging. Indium-111 tagged WBC scans give good results for identifying a prosthetic joint infection in the right clinical setting (figure 10). Hardware loosening may be secondary to infection, but it may also occur without an infection being present. Its radiographic appearance can be unremarkable, and its appearance on technetium 99m MDP studies may resemble those seen with a prosthetic infection (figure 11; figure 12).

The distinction between an infected versus non-infected prosthesis is often made based on the clinical picture. In those patients lacking clinical signs of an infection (fever, redness, localized swelling, elevated WBC count, and so forth) a diagnosis of hardware loosening or a reparative bony remodeling process is made. Frequently, these patients will have to undergo surgery to address the loosening and to ascertain for sure there is no soft tissue or bone infection.

Degenerative disease with osteophyte formation and sclerosis also is a common cause for increased radiotracer uptake (figure 13). Reparative bony remodeling changes may be evident in patients with paraplegia or quadriplegia (figure 14). This in part may be from heterotopic bone formation, and any nuclear medicine studies in these patients should be closely correlated with the patient's radiographic images and clinical picture (figure 14E).

Synovitis can be demonstrated on nuclear medicine imaging (figure 15; figure 16; figure 17A-B; figure 17C-F). Synovitis is a common inflammatory reaction with pannus formation around joints from degenerative and inflammatory arthritis. It is sometimes a reaction to tiny particulate matter associated with joint prostheses (figure 16). Or, it may have an infectious origin. Its exact cause cannot usually be determined by imaging alone as it has a non-specific appearance. It is not a normally expected finding and requires clinical evaluation for its specific etiology.

Small-particle disease results from abnormal or increased implant wear, leading to particle shedding and provocation of a histiocytic response, which manifests as bone destruction. The most common cause of small-particle disease is polyethylene wear, and most patients remain asymptomatic until extensive bone loss is present (see orthopedic medical devices and cross-sectional imaging: protocols and artifacts for further discussion of small particle disease and metallosis associated with metal-on-polyethylene and metal-on-metal implants).

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Figure 9A	Figure 9B	Figure 9C	Figure 9D
30 year-old man with history of bimalleolar right ankle fracture status post fixation (A, B) presented with fever and sepsis. Technetium 99m MDP bone scan shows increased radiotracer around the right ankle on blood flow (C), blood pool (D) and delayed MIP fused images (E) consistent with osteomyelitis.
Figure 9E	Figure 10A	Figure 10B
30 year-old man with history of bimalleolar right ankle fracture status post fixation. Delayed MIP fused image is consistent with osteomyelitis.	55 year-old man status post total right knee arthroplasty (A) presented with right knee pain and fever. Indium-111 tagged WBC scan planar images show increased localization of radiotracer around the prosthesis (B) consistent with prosthetic joint infection in this clinical setting.
Figure 11A	Figure 11B	Figure 11C	Figure 11D
65 year-old woman with right knee arthroplasty with persistent knee pain. Increased radiotracer uptake on the blood flow (A), blood pool and delayed images (B), delayed MIP SPECT (C), and fused (D) images around the prosthesis is consistent with hardware loosening (given the patient's lack of infectious signs and symptoms).
Figure 12A	Figure 12B	Figure 12C	Figure 12D
50 year-old man with history of partial right medial knee hemiarthroplasty with persistent knee pain. Increased radiotracer uptake on the blood flow (A), blood pool (B), delayed MIP SPECT (C) and fused (D) images in the medial compartment are consistent with hardware loosening (lack of infection symptoms). Focal radiotracer uptake in the lateral right femoral condyle is due to degenerative changes.
Figure 13A	Figure 13B	Figure 13C	Figure 13D
54 year-old man with history of right knee arthroplasty presented for chronic knee pain. Technetium 99m MDP bone scan shows no abnormal increased radiotracer activity on blood flow (A) and blood pool (B) images. Multiple foci of radiotracer uptake around the right knee prosthesis on delayed MIP SPECT (C) and fused (D) images are consistent with degenerative/reparative/remodeling changes.
Figure 14A	Figure 14B	Figure 14C	Figure 14D
24 year-old paraplegic man presented for fever of unknown origin. Technetium 99m MDP bone scan shows no abnormal increased radiotracer activity on blood flow (A) or blood pool (B) images. Multiple foci of radiotracer uptake around the both hip joints on delayed MIP SPECT (D) and fused (D) images are consistent with remodeling changes as seen on MIP bone image (E). Increased radiotracer activity in the lower lumbar spine is also related to remodeling.
Figure 14E	Figure 15A	Figure 15B	Figure 16A
24 year-old paraplegic man presented for fever of unknown origin. remodeling heterotopic bone changes are visible on an MIP bone image.	60 year-old woman with lymphoma presented for staging. FDG-PET and PET/CT fusion MIP images demonstrate bilateral total knee arthroplasty with ill-defined periprosthetic metabolic activity around the left knee most compatible with synovitis. FDG avid nodal conglomerates seen in right inguinal and right external iliac regions are consistent with known lymphoma.		57 year-old woman with bilateral total hip replacements. Coronal reconstruction CT (A), MIP FDG PET (B), and coronal PET CT fusion (C) images demonstrate increased FDG uptake overlying the left hip prosthesis with osteolysis of the left acetabulum and high riding femoral head prosthetic component, consistent with polyethylene wear.
Figure 16B	Figure 16C	Figure 17A	Figure17B
57 year-old woman with bilateral total hip replacements. Coronal reconstruction CT (A), MIP FDG PET (B), and coronal PET CT fusion (C) images demonstrate increased FDG uptake overlying the left hip prosthesis with osteolysis of the left acetabulum and high riding femoral head prosthetic component, consistent with polyethylene wear.		65 year-old woman with lymphoma and right hip arthroplasty. Axial CT, PET, PET-CT fusion axial and coronal (A), and MIP (B) images demonstrate intense FDG uptake around the right hip consistent with inflammatory changes. Note: FDG avid nodal conglomerate in the left inguinal region is related to known lymphoma.
Figure 17C	Figure 17D	Figure 17E	Figure 17F
57 year-old man with lung cancer presented for follow up scan. FDG MIP (C) and axial PET-CT fused (D, E, F) images demonstrate increased FDG activity within the soft tissue around multiple large joints, consistent with inflammatory arthritis.

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Other Prostheses and Physiologic States