The Engage is a constrained Total Knee Replacement that can be selectively ‘locked’ in extension by patients with a deficient knee extensor mechanism or soft tissue instability, temporarily simulating arthrodesis in order to provide the full constraint and stability needed to ambulate. Unlike arthrodesis, the Engage can be ‘unlocked,’ allowing a patient to freely flex the knee and engage in activities that may otherwise be difficult or impossible. Patients may lock the knee in extension by briefly passing a specially designed two-sided magnetic key across the knee while it is extended. This handheld key contains specifically oriented magnets, and its magnetic field serves to actuate the internal locking mechanism of the implant system, which contains several fully sealed magnets of its own. The implant will remain locked in extension until the handheld key is reapplied in its reversed polarity orientation (performed by simply turning the key over).

The growth of total knee replacement has led to a concomitant rise in revision procedures. Patients who undergo revision are 5.7x more likely to require re-revision, and the compounded effects of each surgical intervention include loss of anatomy, muscle impairment and atrophy, and instability. Knee extensor weakness is common, and can severely limit patient function. Irreversible extensor damage is also commonly present in patients with neurologic disorders or traumatic injuries. No existing TKR design compensates for a weakened or damaged knee extensor mechanism. Knee arthrodesis is sometimes indicated, and may also be performed in cases of arthrofibrosis, instability, bone/tissue loss, or osteosarcoma. By fusing the joint in extension, arthrodesis provides stability and eliminates the need for muscular control during ambulation. Unfortunately, many activities of daily living become difficult or impossible without the ability to bend the knee, including driving and bathing. Permanent limb extension also induces fatigue and back pain.