A total knee replacement (TKR) is a common orthopedic procedure that replaces the damaged surfaces of the knee joint with metal and plastic components. Standard implants resurface the ends of the thigh bone (femur) and shin bone (tibia), relying on the patient’s ligaments for stability and motion control. A hinged knee replacement (HKR) represents the highest degree of mechanical constraint available in joint arthroplasty. This specialized implant is reserved for the most complex joint destruction, providing stability impossible to achieve when the natural structures of the knee are severely compromised.
The Specialized Design and Function
The unique mechanical structure of a hinged implant involves a direct physical connection between the femoral and tibial components. Unlike standard TKR, the hinged components are linked by a central axle or pin. This pin creates a fixed mechanical axis that dictates the knee’s movement. This structural link provides maximum constraint, allowing the implant itself to manage stability when the patient’s ligaments are damaged.
Modern hinged implants are typically “rotating hinges,” incorporating a polyethylene bearing that allows for some axial rotation. This rotation, usually up to 25 degrees, is an evolution from older fixed-hinge designs that only allowed simple flexion and extension. The rotating mechanism reduces high stresses at the bone-implant interface. Components are predominantly made from cobalt-chrome or titanium alloy, with articulating surfaces separated by ultra-high molecular weight polyethylene. The overall function mimics a simple door hinge, providing a stable arc of motion while sacrificing the complex rolling and gliding movements of a natural knee.
Specific Medical Indications for Use
A hinged knee replacement is generally considered a salvage procedure, used when less constrained implant types cannot provide adequate stability or fixation. A primary indication is severe ligamentous instability, particularly when both the medial and lateral collateral ligaments are compromised or destroyed. In these cases, the knee joint is globally unstable, and the implant must assume the entire role of the supporting soft tissues. This often occurs in patients with severe inflammatory arthritis or following significant trauma.
Another reason for using a hinged implant is massive bone loss in the femur or tibia, where standard components cannot be securely anchored. These defects often result from failed previous knee replacements, tumors, or severe fractures. Furthermore, a hinged implant may be necessary to correct severe angular deformities, such as varus or valgus alignment exceeding 20 degrees. This is especially true if soft tissue releases required to correct the angle would leave the knee completely unstable. The decision to use a hinged design acknowledges that only a mechanically linked device can restore a functioning limb when structural integrity is poor.
Surgical Considerations and Fixation
Implanting a hinged knee replacement is a technically complex and often lengthy surgical procedure due to challenging patient anatomy and the high forces the device must withstand. Because the implant is highly constrained, it transmits significant forces to the bone-implant interface, necessitating robust fixation. This fixation is achieved through the mandatory use of intramedullary (IM) stems, which are long metal rods extending deep into the shafts of the femur and the tibia. These stems anchor the implant securely, distributing the high loads generated by the linked mechanism.
The surgeon must meticulously prepare the bone canals by reaming them to the correct diameter and depth. The choice of stem length and whether to cement the stems is critical; long cemented stems are often preferred as they minimize micromotion at the interface. Due to underlying bone deficiency, surgeons frequently use metal augments or bone grafts to fill large defects before seating the implant. Achieving proper soft tissue balancing is also a challenge, as the hinge’s mechanical constraint must be perfectly aligned to the limb’s overall alignment to prevent undue stress on the device.
Expected Outcomes and Device Durability
Patients receiving a hinged knee replacement should have realistic expectations regarding post-operative function; the primary goal is a stable, pain-free limb rather than high-performance function. The mechanical link inherently limits the range of motion (ROM) compared to a standard TKR. However, modern rotating hinges still allow for a mean ROM in the functional range of approximately 110 to 115 degrees. Functionally, the implant reliably provides stability for walking and daily activities, which is a major improvement for patients who previously had an unstable joint.
The high-constraint design affects the device’s long-term durability. Constant high mechanical stress on the components and the bone-implant interface leads to an increased risk of failure modes specific to this design. The 10-year implant survivorship for hinged knees used in complex revision scenarios is typically 80% to 90%, which is lower than the expected lifespan of a standard primary TKR. The most common reasons for long-term failure include deep infection and aseptic loosening, which occurs when the implant separates from the bone due to high forces acting on the intramedullary stems and cement mantle.