Abstract
Traditional procedures for orthopedic total joint replacements have relied upon bone cement to achieve long-term implant fixation. This remains the gold standard in number of procedures including TKR and PKR. In many cases however, implants fixed with cement have proven susceptible to aseptic loosening and 3rd body wear concerns. These issues have led to a shift away from cement fixation and towards devices that rely on the natural osteoconductive properties of bone and the ability of porous-coated implants to initiate on-growth and in-growth at the bone interface, leading to more reliable fixation.
To facilitate long-term fixation through osseointegration, several mechanical means have been utilized as supplemental mechanism to aid in stabilizing the prostheses. These methods have included integrated keels and bone screws. The intent of these components is to limit implant movement and provide a stable environment for bone ingrowth to occur. Both methods have demonstrated limitations on safety and performance including bone fracture due keel induced stresses, loosening due to inconsistent pressfit of the keel, screw-thread stripping in cancellous bone, head-stripping, screw fracture, screw loosening, and screw pullout. An alternative method of fixation utilizing blade-based anchoring has been developed to overcome these limitations.
The bladed-based fixation concept consists of a titanium alloy anchor with a “T-shaped” cross-section and sharped-leading end that can be impacted directly into bone. The profile is configured to have a bladed region on the horizontal crossbar of the “T” for engagement into bone and a solid rail at the other end to mates with a conforming slot on the primary body of the prosthesis. A biased chisel tip is added to the surface of the leading blade edge to draw the bone between the anchor's horizontal surface and surface of the implant, thus generating a compressive force at the bone-to-prothesis interface. The anchoring mechanism has been successfully been integrated into the tibial tray component of a partial knee replacement; an implant component that has a clinical history of revision due to loosening.
A detailed investigation into the pulloff strength, wear debris generation, compressive-force properties, and susceptibility to tibial bone fracture was carried out on the anchor technology when integrated in a standard tibial tray of a partial knee replacement. When tested in rigid polyurethane bone foam (Sawbones, Grade 15) the pulloff strength of the construct increased by 360% when utilizing the anchor. The tibial tray and anchor construct were cycled under compressive loading and demonstrated no evidence of interface corrosion or wear debris generation after 1 million cycles. In addition, the anchor mechanism was shown to generate 340N of compressive force at the tibial tray-to-bone interface when evaluated with pressure sensitive film (Fuji Prescale, Medium Grade). Finally, the ultimate compressive load to induce tibial fracture was shown to increase by 17% for the anchored tray as compared to a traditional keeled tray when tested in an anatomic tibial sawbones model; and by 19% when evaluated in human cadaveric tibias.
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