Abstract
Research Type:
Level 5 - Case report, Expert opinion, Personal observation
Introduction/Purpose:
Ankle Fusion is a common treatment for end stage osteoarthritis [1]. Current clinical adjuvants include Bone Morphogenic Protein or Platelet Derived Growth Factor. However, these factors still have significant nonunion rates, motivating alternative bioactive strategies [2]. One molecule of interest is FK506, a known approved immunosuppressant. It functions in upregulating osteogenesis by binding to FKBP12, an intracellular BMP inhibitor, and enhances BMP-2 activity by blocking its repressor. Previous research demonstrated FK506 as capable of initiating osteogenesis and bone formation in vitro, while also displaying successful fusion in an in vivo rabbit spine model [3,4]. We hypothesize in a representative in vivo model, FK506 will enhance osteogenesis.
Methods:
To evaluate the effects of FK506 in an in vivo model, nine New Zealand White rabbits underwent ankle fusion. A 2.3 mm diameter, 18 mm length screw was inserted through the medial malleolus into the talus and calcaneus following thorough debridement of the articular cartilage and preparation of the joint space (Fig. 1A/B). Approximately 50 µl of fibrin gel, preloaded with either PBS (Control) or FK506 (0.4 mg total), was mixed and immediately injected into the joint space. After four weeks, the rabbit ankles were harvested and analyzed using mCT for bone volume and tissue mineral. Additionally, paraffin-embedded tissue blocks were sectioned for histological staining (H&E, Safranin O/Fast Green, Goldner’s Trichrome, and Gomori’s Trichrome).
Results:
At the early 4-week time point, there was notable joint motion by manual manipulation. While osteogenesis between the tibia and talus was limited, the FK506 treated animals showed significantly greater signs of bone formation anterior to the joint (Fig. 1C), including increased bone volume and tissue mineral deposition (Fig. 1D/G). Moreover, FK506-treated animals exhibited increased hypertrophic cartilage and mineralized tissue in this anterior pocket of the fusion site (Fig. 1F/E).
Conclusion:
The results of this study demonstrate that FK506 enhances bone formation in a rabbit ankle environment, indicating its potential as a fusion adjuvant. Despite lack of obvious fusion (possibly due to less than optimal fixation or evaluation of ankle at too early a timepoint of only 4 weeks) this work demonstrates promising increased osteogenesis in the ankle environment. Further studies will aim to optimize biomechanical stabilization and biochemical and physical properties of the microenvironment to enhance FK506-mediated potency and maximize its therapeutic efficacy.
