Biocompatibility of a novel expandable threaded screw using an ovine bilateral femoral model: a preliminary study

Screwtype titanium alloy implants are routinely used in orthopedics to stabilize fractured bone and allow natural healing. New demands now exist for screw fixation systems demonstrating the capacity to sustain loads while allowing motion. A new titanium screw type was therefore developed, offering a novel dynamic thread design, which utilizes an expandable crest component instead of a fixed one. Following insertion, the resulting thread has the ability to expand into areas of low bone density and exert a radial force with an improved initial fixation for osseointegration. The intent of this study was to evaluate the preliminary safety and efficacy of this design using a subchronic ovine model. Materials and Methods: Biomaterials were composed a highstrength Ti15Mo screw shank (Ø4.5mm; Intelligent Implant Systems) with an expandable thread crest made of either TiAlV alloy (TG1, n=4), commercially pure titanium (TG2, n=4) or crestless (Ctrl, n=2). Following an IACUCapproved protocol as well as AAALAC and CCAC regulations, a female Dorset Rideau Arcott hybrid sheep (>12mo.; 40kg) was placed in lateral recumbency to receive a total of 10 bilateral femoral diaphyseal and bicortical screw insertions leaving a 2mm gap between the screw heads and the femoral surface. At 6 weeks, bone ingrowth was reconstructed by MicroCT (SkyScan 1172) and imaged by highresolution radiography (Faxitron MX20). Explanted screws were sectioned longitudinally, processed for undecalcified histology, infiltrated with PMMA, microground (Exakt 400 CS) and stained with Toluidine Blue. The resulting slides were examined histologically and then scanned using a NanoZoomer Digital Pathology 2.0HT for histomorphometry. Using Aperio ImageScope, new bone formation was measured in cortical, periosteal, and endosteal areas. Results: Both TiAlV (TG1) and cpTi (TG2) expandable threaded screw types exhibited variable amounts of new bone formation (Figure 1) and prominent osteoblasts without evidence of cartilage formation. New bone formation was graded as mild to moderate at the periosteal surface and minimal to mild at the endosteal surface and cortex. The new bone adjacent to the expandable threaded implants was predominantly lamellar with a minimal woven bone component, consistent with normal bone formation and remodeling. Good osseointegration, characterized by moderate to marked bone apposition to the implant surface with minimal fibrous tissue at the boneimplant interface was seen in all groups. Histopathology results indicated an excellent safety and biocompatibility in all test and control groups as evidenced by a mild tissue reaction. Histomorphometry also revealed increased new bone formation surrounding the expandable threaded implants when compared to controls.