Abstract
Summary
The BMP-2 content and bone forming potential of 2 leading allograft products (OsteoAMP® and Osteocel® Plus) was tested across 3 commercially available lots. Surprisingly, there was no BMP-2 content associated with the cells contained within Osteocel® Plus. OsteoAMP® contained greater than 1000 times the overall BMP-2 content than Osteocel® Plus. Correspondingly, Osteocel® Plus did not form new bone at any timepoint in the 12 week in vivo study while OsteoAMP® had increasing new bone formation at each sequential timepoint. Interestingly, the highest cellularity of Osteocel® Plus was just prior to implant at t0, decreasing at each timepoint, decreasing further at the terminal endpoint of the study at 12 weeks (82% of cells had died or migrated). Conversely, the cellularity of OsteoAMP® increased at each timepoint.
Introduction
Implants containing living cells are often characterised by the orthobiologics industry as ‘osteogenic’. The positive function and ultimate fate of these cells has been assumed with little to no proof of efficacy. In this study we compare the bone forming ability of the market leading stem cell product claiming osteoinductivity as well as osteogenicity, Osteocel® Plus, against the market leading allograft derived growth factor product, OsteoAMP® which claims osteoinductivity but contains no viable cells. The goal of the study is to determine if a cellular product will form new bone or produce a false positive when evaluated histomorphometrically using an osteoinductive control over time in vivo. Additionally, the osteoinductive potential from each product will be quantified by in vitro by measurement of BMP-2 content via ELISA.
Methods
Three different lots of each OsteoAMP® and Osteocel® Plus were implanted by an independent lab into muscle pouches of athymic rats. Implants were assessed for bone formation using H&E histology at time of implant, 1 day, 2w, 4w, 6w, 8w, and 12w. Histomorphometric measurements were done using Image J (NIH) using four sections per implant per timepoint (n=12). Each lot of both products was also digested in water, hydrochloric acid, and guanidine-hydrochloride (Gua-HCl) to facilitate BMP-2 content quantification via ELISA (R&D Systems). The extracts were intended to independently measure cellular, mineral, and collagen phase content of BMP-2. All extracts were analyzed by ELISA independently to characterise the source of osteoinductivity and the resulting release profile in vivo.
Results
With each subsequent timepoint, all lots of Osteocel® Plus exhibited increasing osteonecrosis with increasing time. By the terminal 12 week timepoint used in this study, 82% of living bone that was implanted for Osteocel® Plus had died or migrated away from the implant site. In contrast, new bone formation and cellularity increased with OsteoAMP® at each timepoint, still increasing at the terminal 12 week endpoint of this study. The BMP-2 results support the bone formation in vivo. Neither of the 3 Osteocel® Plus lots registered any BMP-2 from the cells on ELISA, nor was there any free BMP-2 that leached out of the product. The mineral phase of Osteocel® Plus exhibited traces of BMP-2 for only one of the 3 donors with an average of 0.04 ng/g for all three donors. The protein phase of Osteocel® Plus had the highest concentration of BMP-2 at 0.63 ng/g BMP-2. For all three lots, OsteoAMP® showed no leaching of BMP-2 after exposure to water and a BMP-2 content in the mineral phase of 50.85 ng/g. The collagen phase of OsteoAMP® had the highest concentration of BMP-2 in all groups tested at 915.54 ng/g.
Discussion
The results in this study would suggest that orthobiologic implants that contain living cells and claim osteogenic properties do not maintain viable cells over time and did not form bone. Similarly, the results would indicate the living cells do not contribute to osteoinductivity. In contrast, the higher BMP-2 content and osteoinductive property exhibited by OsteoAMP® was a stronger producer of new bone.
