Abstract
INTRODUCTION
Adequate osseointegration of knee resurfacing implants for the treatment of focal cartilage defects is an important prerequisite for good clinical outcomes. Inadequate initial fixation and sustained micromotion may lead to osteolysis and ultimately implant failure. PET/CT with the bone seeking tracer 18F-sodium fluoride (18F-NaF) allows for localisation and quantification of abnormalities in bone metabolism. 18F-NaF PET/CT has been shown to correlate with loosening of implants in the hip and spine. Here, we asses osseointegration of the knee resurfacing implants using micro-computed tomography (µCT) and correlate µCT parameters to 18F-NaF uptake on PET/CT scans taken 3 and 12 weeks after surgery. We hypothesize that 18F-NaF uptake at 12 weeks and its relative decrease between 3 and 12 weeks correlates with osseointegration at 12 weeks postoperatively. Polymer implants with Young”s moduli approximately equal to- and below the Young's modulus of bone, with- and without surface modification were used in this study next to a control metal implant.
METHODS
Five different osteochondral implants were implanted bilaterally in critically-sized osteochondral defects in 16 goats. At 3 and 12 weeks postoperatively, a 10-minute static PET/CT-scan (Philips, Gemini TF PET/CT) was made 60 minutes after intravenous injection of 18F-NaF. Image processing resulted in an overall bone metabolism parameter, i.e. standardized uptake value (SUV). A cylindrical region of interest was drawn around each implant to obtain the maximum SUV (SUVmax). Bone quality parameters were quantified in a cylinder surrounding the implant using µCT after sacrifice as a measure for osseointegration. The in vivo 18F-NaF PET/CT uptake parameters were correlated to the bone quality parameters.
RESULTS
Implant osseointegration strongly varied for the different implants. Some implant groups exhibited very poor osseointegration with clear signs of osteolysis, while titanium implants exhibited good osseointegration. A strong correlation was observed between bone quality parameters as determined using µCT and SUVmax at 12 weeks. The SUVmax of the implants with poor osseointegration remained high, while implants with good osseointegration showed a relative decrease in SUVmax between 3 and 12 weeks.
CONCLUSION
This study suggests that the SUVmax of PET/CT 12 weeks after surgery correlates well for the quality of osseointegration assessed on µCT 12 weeks after surgery. De relative decrease of SUVmax between the given time points had a strong correlation with the degree of osseointegration. In this study, large differences in the quality of osseointegration were observed. The role of surface modification, elasticity and micromotion still remain to be determined as well as if 18F-NaF is sensitive enough to discriminate between smaller differences and what the optimum time point would be to predict the ultimate osseointegration.
