24 Acta Orthopaedica 2012; 83 (1): 22–25 Area of low uptake, cm2 30 25 20 15 10 5 0 0 1 2 3 4 5 6 Years after operation Figure 2. Time scale of development of areas of low tracer uptake in the femoral heads for the 11 cases. methods, enables us to visualize the metabolism and viabil- ity of bone in the remaining part of the head. The patients in the present study featured in one or other of 2 previous F-PET studies. In the prospective study (Ullmark et al. 2009), the volume of bone in the femoral heads that had reduced uptake of the F-tracer (indicating lack of new bone formation) appeared in the late postoperative time period, 4–12 months after surgery. This finding excludes the possibility of low- uptake bone being spongious bone mixed with bone cement from the time of surgery. In our retrospective F-PET study of 35 resurfacing THAs analyzed 2–5 years after surgery (Ull- mark et al. 2011), 3 were clinical failures. In addition, 7 of the 32 F-PET-scanned arthroplasties had areas of low uptake in the femoral heads bone, which were assessed as being loss of bone metabolism. In an F-PET study of 10 resurfacing THAs that were analyzed 10–33 months after surgery (Forrest et al. 2006), the metabolism in part of the head in 1 case had declined to half of the normal level. There have been reports of late failures with femoral neck fractures after resurfacing THA. Morlock et al. (2008) studied a large number of resurfacing revisions by means of histol- ogy and morphology. They found that fractures involving the rim of the implant occurred during the first months, whereas fractures within the femoral head often took place closer to 1 year after surgery. Campbell et al. (2006) also analyzed failed surface arthroplasties. They found that neck fractures occurred a few months after surgery, while failures from loosening of the femoral component often took place 17–100 months after surgery. Steffen et al. (2010) analyzed 2 groups of failed resur- facing arthroplasties (failures from neck fractures and failures from other causes). The group with failure from neck fracture was found (to a higher degree) to be associated with osteo- necrosis in the retrieved heads, compared to the group with failures for other reasons. The 11 cases of resurfacing THA in this study, which all had areas of reduced bone metabolism, were all asymptom- atic 3–6.5 years after surgery. There were still no signs of fail- 1 2 3 4 5 6 7 8 9 10 11 ure on plain radiographs. Surprisingly, in 5 of the cases the region of low uptake had become further enlarged, indicat- ing progression. On 3 occasions, the area of low uptake had decreased from one PET scan to the next (Table), when a swift increase in uptake then ensued. This finding raises the interest- ing question of whether the bone tissue without any detectable metabolism was actually in a state of necrosis, or whether the bone may have been in a state of chronic but reversible isch- emia, indicating that healing of metabolically damaged bone may occur without any clinical consequences. The present study shows that relatively large metabolic changes can occur in the residual femoral head a long time after surgery in resurfacing THA. Apparently, the biology underly- ing the F-PET-derived findings of absence of bone metabolism is diverse and includes the possibility of spontaneous heal- ing. This is encouraging, and might lead to novel therapeu- tic approaches if the mechanisms could be more thoroughly understood. Given the difficulties associated with obtaining histopathological material from asymptomatic patients, the use of PET with 18F-fluoride and other relevant tracers of bone activity in longitudinal studies should be explored further. The long-term clinical implications of our findings are still unclear. However, the presence of regions without any metabolism in the femoral head adjoining the femoral compo- nent, in combination with the observation that late failure in resurfacing arthroplasty can be due to femoral neck fracture, indicate that implant loosening in THA may be attributable to metabolic changes in bone. This leads us to the conclusion that resurfacing THA should be used with caution until the long-term outcomes of the procedure have been determined. JS: design and data collection, KS and JM: performed surgery and data col- lection, ON: design, analysis and interpretation of data, GU: design, data collection, analysis and interpretation of data. All the authors prepared the manuscript. This work was supported financially by the Center for Research and Develop- ment, Uppsala University/County Council of Gävleborg, Sweden. No competing interests declared. 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