This study is a prospectively and retrospectively enrolled, post-market, open-label, non-randomized, single institution evaluation of clinical outcomes of primary or revision knee surgery patients who meet the eligibility criteria and received devices from the Triathlon Hinge Knee System according to its indications for use (IFU).

Clinical evaluation for all cases will include data collection of patient demographics, surgical details, early postoperative status, functional and clinical outcomes, and survivorship at 1, 2, 6, and 10-years.

The purpose of this study is to evaluate the safety and performance of the Triathlon Hinge Knee System for both primary and revision cases in accordance with the Indications for Use.

The main objective of this study is to determine (1) mid- (5 year) and long-term (10 year) survivorship, and (2) drivers of failure for the Triathlon Hinge Knee (THK) hinge. Data from this study will be used to develop a predictive model to identify the most important factors in achieving fixation, including the role of augments, cones, thresholds for cement mantle thickness, and number of zones of fixation required for survivorship.

Survivorship is defined as the absence of aseptic revision to the implanted device.

The Triathlon Hinge Knee System consists of Triathlon Hinge femoral components, the Triathlon Revision baseplate, Triathlon Revision Tibial Augments, Triathlon Hinge Femoral Distal Augments, Triathlon Hinge Insert, Triathlon Revision Insert X3, and Triathlon Hinge Bumpers, Axle, Bushings, Tibial Sleeve, and Bearing component.

It is designed to be a single platform for revision, difficult primary, and limb salvage surgeries.

Inclusion Criteria:

• The subject undergoes a primary or revision procedure implanting at least the Revision Baseplate portion of the Triathlon Hinge Knee System in accordance with the Indications for Use.
• The subject has signed an Institutional Review Board (IRB)/Ethical Committee (EC)-approved, study-specific Informed Consent Form (ICF).
• The subject is a male or non-pregnant female at the time of enrollment.
• The subject agrees to comply with the protocol-mandated clinical evaluations.

Exclusion Criteria:

• Any active or suspected latent infection in or about the knee joint; overt infection;
• Distant foci of infection which may cause hematogenous spread to the implant site; rapid disease progression as manifested by joint destruction or bone resorption apparent on roentgenogram;
• skeletally immature patients;
• Severe instability of the knee joint secondary to the absence of collateral ligament integrity and function.
• Bone stock compromised by disease, infection or prior implantation which cannot provide adequate support and/or fixation to the prosthesis.
• Known or suspected sensitivity and/or allergy to any material in the device.
• Conditions presenting an increased risk of failure include:
• uncooperative patient or patient with neurologic disorder, incapable of following instructions;
• osteoporosis;
• metabolic disorders which may impair bone formation or cause bone loss;
• osteomalacia; and,
• previous arthrodesis.
• A higher incidence of implant failure has also occurred in paraplegics, cerebral palsy and patients with Parkinson's disease.
• Any mental or neuromuscular disorder which would create an unacceptable risk of prosthesis instability, prosthesis fixation failure, or complications in postoperative care.
• The subject is a prisoner

Participants who meet the eligibility criteria and receive/received devices from the Triathlon Hinge Knee System according to its indications for use (IFU).