Interpreting Periprosthetic Metabolism After Total Ankle Arthroplasty—Why Time Matters

One of the ongoing challenges in the evaluation of total ankle arthroplasty (TAA) is understanding what “normal” postoperative bone metabolism actually looks like. Although most of the work in nuclear imaging has taken place around hip and knee arthroplasty—fields with 2 orders of magnitude greater volume—the ankle has remained something of a blank canvas. Clinicians have often borrowed assumptions from larger joints, treating any elevated periprosthetic tracer uptake as a warning sign. The work by Cody et al ¹ brings a perspective to ankles that has been a long time coming.

The authors prospectively examined 32 asymptomatic TAA patients at 6, 12, 18 and 24 months using ¹⁸F-NaF positron emission tomography / computed tomography (PET/CT). Their message is clear: uptake peaks at around 12 months and remains elevated at 24 months, especially in the talus. In other words, a “hot” scan in the first 2 years is not necessarily a failing ankle—and might reflect normal findings.

This is an important correction to a widespread misconception. Persistent postoperative metabolic activity has been well documented in other joints. Schweizer et al ² demonstrated increased periprosthetic uptake in asymptomatic hip arthroplasties, often overlapping with patterns seen in symptomatic cases. Son et al’s ³ serial PET/CT work similarly showed prolonged metabolic activity in the hip and knee for up to 2 years. Jin et al ⁴ reported that even well-functioning hip replacements maintain elevated osteoblastic activity long after implantation. These studies highlight a simple truth: uptake alone is not a diagnosis.

Yet until now, the ankle has lacked comparable data. Dyke et al ⁵ offered insight into the very early postoperative period, showing increased bone blood flow and turnover at 3 months, but the metabolic story beyond that point remained untold. Cody et al now fill that gap with a thoughtful, structured evaluation across the first 2 postoperative years.

Several findings deserve particular attention. First, uptake—measured by standardized uptake values (SUVs), bone blood flow and bone turnover—consistently peaked at 12 months and declined thereafter without normalizing by 24 months. ¹ If clinicians are ordering single-photon emission computed tomography (SPECT)/CT or PET/CT during this period and interpreting persistent uptake as loosening, they risk mislabelling normal physiology as failure. These results align seamlessly with the hip and knee literature, reinforcing that TAA should be evaluated along a similarly extended metabolic timeline, especially given the high costs of the tests.

Second, the talus behaves differently from the tibia. Across all later time points, the talus showed higher metabolic activity. ¹ The reasons for this remain an enigma and promote the need for further research. The higher anterior uptake patterns described by the authors echo the regional stress-driven variations seen in hip arthroplasty imaging. ²

Third—and importantly—metabolic recovery lags far behind symptomatic recovery. Prior work by Agarwalla et al ⁶ demonstrated that patient-reported outcomes after TAA plateau at approximately 1 year. Cody et al show that bone metabolism remains elevated long after patients feel “fully recovered.”

Finally, elevated uptake alone should not be taken as evidence of loosening. PET/CT is an exceptionally sensitive tool, and sensitivity without context can mislead. Adesanya et al ⁷ warned that both septic and aseptic loosening can present with similar PET/CT patterns. Sterner et al ⁸ showed that PET findings may even precede symptoms. With regards the ankle, Mason et al ⁹ and Gurbani et al ¹⁰ found SPECT/CT useful in evaluating painful TAA, but emphasized clinical correlation above all. Cody et al now warn us to exercise caution, in the use of nuclear imaging tests in evaluating post-surgical loosening or osteolysis as a diagnosis. This would include soft tissue pathology and referred pains and points to the importance of clinical findings in particular.

This study advances the field in several meaningful ways. It establishes the first postoperative metabolic timeline for modern fixed-bearing TAA implants, supplies normative values for SUVs and kinetic parameters and clarifies that elevated activity at 12-24 months is not in itself diagnostic of loosening.

Of course, important questions remain. We still do not know when uptake fully normalizes as they stopped at 2 years. We do not have comparative scan data of symptomatic and asymptomatic patients as has been shown in hip arthroplasty. ² Whether implant design or implant materials affects metabolic behaviour is another question deserving future evaluation. And the anterior-dominant uptake patterns observed in both bones may benefit from correlation with gait mechanics or finite element modelling.

Still, the takeaway is unmistakable: nuclear imaging within the first 2 years after TAA must be interpreted with caution. A bright scan is not a verdict. The ankle remodels slowly—especially the talus—and its metabolic timeline does not mirror a patient’s subjective recovery. Cody et al remind us that clinical findings and clinical suspicion must remain paramount. Expensive nuclear tests should be used sparingly and are unlikely to offer meaningful value, and might simply reflect biology rather than pathology.

Supplemental Material

sj-pdf-1-fai-10.1177_10711007251410157 – Supplemental material for Interpreting Periprosthetic Metabolism After Total Ankle Arthroplasty—Why Time Matters

Supplemental material, sj-pdf-1-fai-10.1177_10711007251410157 for Interpreting Periprosthetic Metabolism After Total Ankle Arthroplasty—Why Time Matters by Andrew J. Goldberg in Foot & Ankle International

Andrew J. Goldberg, MD, MBBS, FRCS(Tr&Orth) The Wellington Hospital, London, United KingdomImperial College London, United KingdomUCL Division of Surgery, London, United Kingdom