Evaluation of the Safety of a Novel Knee Load-Bypassing Device in a Sheep Model

Background: Unloading treatments for osteoarthritis of the knee, such as valgus braces, have been shown to reduce medial joint-loading and pain. The purpose of this preclinical study was to investigate the tissue response to an extracapsular implantable device (KineSpring System; Moximed, Hayward, California) that is designed to reduce loading across the medial compartment of the knee. Methods: Eleven sheep were implanted with the KineSpring System, consisting of bases secured to the medial femoral and tibial cortices with bone screws and also consisting of a joint-spanning load-absorber composed of a piston and spring assembly inside a polymeric sheath. In nine sheep, titanium alloy washers with either an uncoated or a plasma-sprayed titanium-hydroxyapatite (TPS-HA) coated undersurface were fixed with screws to the contralateral femur and tibia to investigate the effects of surface treatment on extracortical bone apposition. The functionality of the KineSpring System was assessed by fluoroscopy, tissue response was determined by gross and microscopic histology, and implant fixation was determined by radiography, microradiography, and histomorphometry at four, twelve, twenty-six, and fifty-two weeks. Results: The function of the KineSpring System was confirmed in all sheep. Seventeen of eighteen TPS-HA coated bases and thirty-six of thirty-six washers had stable bone fixation; one coated base was loose with evidence of infection. At four weeks, there was an early, expected inflammatory response, but this response resolved by twelve weeks and a pseudosynovial membrane formed around the sheath. Bone apposition increased over time for the TPS-HA coated bases and for uncoated washers (p < 0.05). There was good apposition of the TPS-HA coated washers even at early time points, with no subsequent increase in apposition over time. Conclusions: The KineSpring System demonstrated safety in a simulated use model for periods of up to fifty-two weeks. Screws result in stable implant fixation and TPS-HA coating allows for early and long-lasting cortical bone apposition and integration.