Increased Osseous 99mTc-DPD Uptake in End-Stage Ankle Osteoarthritis

Background: We analyzed the histopathologic findings in end-stage osteoarthritic ankle joint tissue that display increased uptake of bone-seeking radiotracer in single-photon emission computed tomography–computed tomography (SPECT-CT) imaging. Methods: Six consecutive patients with end-stage osteoarthritis undergoing total ankle replacement received preoperative SPECT-CT imaging using 99mTechnetium dicarboxypropane diphosphonate (99mTc-DPD). Using imaging data for stratification, osteochondral tissue sections were prepared from SPECT-positive (+) and -negative (–) areas of tibial and talar resection specimens. Histomorphometric analyses of osteoblast numbers, collagen deposition, and cartilage degeneration were performed on hematoxylin and eosin, van Gieson’s and Safranin- O stained tissue sections. Osteoclast activity was visualized using tartrate-resistant acid phosphatase (TRAP) staining. Results: Increased 99mTc-DPD uptake was observed exclusively subjacent to the subchondral bone plate of tibial and talar joint compartments. SPECT(–) tissues displayed typical fatty marrow morphology containing mainly collagen-positive blood vessels and few marrow and bone-lining cells. SPECT(+) tissues were characterized by increased numbers of active bone-lining osteoblasts depositing collagen fibers. Collagen area fraction of subchondral bone marrow was significantly increased in SPECT(+) (0.52 ± 0.21) compared with SPECT(–) (0.29 ± 0.13) tissues ( P = .30). Multinucleated TRAP(+) osteoclasts were absent from bone formation sites, but associated with vascular structures invading articular cartilage through the subchondral bone plate. Increased 99mTc-DPD uptake was specifically and strongly correlated with increased osteoblast numbers ( P = .011), and with collagen area fraction ( P = .030) but not with Mankin score ( P = .202), or with osteoclast number ( P = .576). Conclusion: Subchondral bone tissues in SPECT(+) areas of end-stage ankle osteoarthritis were histologically characterized by increased osteoblast-mediated bone formation in the absence of functional osteoclasts, and increased cellularity and collagen deposition in marrow tissues. Clinical Significance: Our findings suggest a pathologic bone-remodeling process in end-stage ankle OA areas with increased 99mTc-DPD uptake. Level of Evidence: Level IV, case series.