A compliant intramedullary stem to increase longevity of revision total knee arthroplasty

The National Institute of Arthritis and Musculoskeletal and Skin Diseases awarded $613,742 to Flex Orthopaedics, Inc. on September 19, 2025, under the Arthritis, Musculoskeletal and Skin Diseases Research program (CFDA 93.846) to optimize and conduct pre-clinical studies of a compliant intramedullary stem for revision total knee arthroplasty implants.

Revision total knee arthroplasty implants frequently fail within ten years due to overconstraint that increases bearing wear and bone-implant interfacial stress. The recipient is developing a compliant tibial stem that accommodates rotational motion about the tibia's long axis through flexible element bending rather than additional articulating surfaces, thereby reducing constraint-related wear and stress without introducing new wear mechanisms or instability. The device originated in an engineering laboratory at UCLA and has demonstrated feasibility through simulation, benchtop testing, and large animal models. Under this award, Flex Orthopaedics will optimize the stem design for revision implants and execute initial pre-clinical studies required for regulatory clearance prior to clinical testing.

Work is performed in Agoura Hills, California through August 31, 2027.

Abstract

Total knee arthroplasty is a common and extremely successful procedure that treats knee osteoarthritis. In some cases, knee arthroplasty implants do not last the remaining lifetime of the patient. When primary total knee arthroplasty (PTKA) implants fail, they are replaced by larger revision total knee arthroplasty (RTKA) implants, which typically have more inherent constraint to provide stability in the face of ligamentous incompetence. This constraint can cause more wear in the implant and higher stresses at the bone-implant interface, leading to ten- year failure rates as high as 30%. The ability of the implant to accommodate rotational motion about the tibia's long axis is crucial to alleviating overconstraint in RTKA implants, thereby reducing both bearing wear and interfacial stress. This is typically accomplished through the rotational bearing element of a rotating hinge knee, which allows the hinge bearing to rotate freely with respect to the tibial stem. However, this rotational component necessarily introduces an additional articulating surface that serves as a new source of wear. The objective of the proposed research is to extend the lifespan of RTKA implants through a compliant RTKA stem that alleviates internal-external rotational constraint without creating additional wear surfaces or introducing potential instability. Our product provides compliance about the tibia's long axis via bending of flexible elements, rather than rubbing or sliding of independent components. The device originated in one of the applicants' engineering lab at ucla. Feasibility has already been demonstrated in the applicant's lab in simulation, on the benchtop, and in a large animal model. In this work, we will optimize the stem for RTKA implants and conduct the initial pre-clinical studies that are the next step toward regulatory clearance, prior to clinical testing. Specifically, we intend to show that I) acute failure of the implant happens in a way that does not endanger the surrounding tissues and only under super-physiologic loads, and II) the implant has the durability necessary to support human gait for decades inside the body. We will conduct this research over 12 months, and the data we obtain will be move US toward an FDA investigational device exemption application to support an early feasibility study, as well as in the NIH sbir phase II application. Major milestones to be achieved with this grant include direct experimental validation of short- and long-term mechanical survivability of the compliant RTKA stem as a crucial step toward clinical testing.