Hinged Knee Replacement
During a knee replacement surgery, the surgeon removed the diseased ends of the lower end of the thigh bone and the upper part of the shin bone. The prosthetic replacement of the removed parts recreates the natural knee joint without any disease.
The prosthetic parts are made of metal and high-grade plastic. The femoral and the tibial component are made up of metal while the insert or the spacer is made up of high-grade plastic. The plastic spacer helps in the smooth gliding of the opposing metal surfaces.
A natural knee has motion in the forward and the backward direction as well as a small amount of rotation and side-to-side motion. The thigh bone also slides back on the upper part of the shin bone during bending of the knee which is known as femoral rollback.
Constrain is the ability of the implant to provide stability in forward-backward and side-to-side motion. The constrained implants are needed in the face of bone loss or labor of the ligaments. Implants may be non-constrained, semi-constrained, or fully constrained depending upon the amount of stability they provide.
During a primary knee replacement for arthritis, only the bony ends are diseased and may be associated with mild to moderate deformity. The surgeon removes only the diseased ends and may stretch or release ligaments to correct the deformity.
The bone quantity and the soft tissue support is however adequate to provide stability to the prosthetic joint. Therefore the commonly used knee replacement implants in primary surgery are non-constrained.
All prosthetic implants are limited by their self-life as they wear down over the years. The life of a usual knee implant may be well over more than 20 years and for patients over 60 years of age, they may last their lifetime.
Owing to the success of the surgery, knee replacement is increasingly being performed in the younger age groups. In younger age groups after many years, the implants may need revision surgery. Additionally, revision surgery may be needed in patients with complications of primary surgery such as infection, aseptic loosening, fracture, instability, etc.
During revision surgery, at times there may be an excessive bone loss that may warrant a modular constrained implant. Similarly, in patients with excessive ligament laxity, a constrained implant may be necessary to provide stability. Patients suffering from some kinds of tumors of the bone may require removal of the stabilizing structures of the joint along with the bone tumor. These patients may also require constrained implants.
The constrained implants come in different designs. The originally used hinged constrained implant has been largely superseded by a rotating constrained implant. In a hinged knee replacement implant, there is a connecting rod in between the femoral and tibial components.
The connecting rod has a hinge mechanism to prevent overextension of the joint. This design only allows forward and backward motion and prevents any side to side and rotating motion.
The loss of side to side and rotating motion puts additional stress at the bone-implant interface and may lead to implant loosening or periprosthetic fracture. The rotating constrained implant additionally allows rotating motion about the prosthetic hinge. The hinge is located near the femoral component and a rotating platform is attached to the tibial plate.
The rotating action helps to distribute the stresses more evenly during motion while maintaining stability in the face of bone loss or ligament laxity. The hinged implants are primarily used in revision surgeries and in complex primary knee replacement surgery involving severe deformity or tumors, or salvage surgeries, etc.
The hinged knee replacement implants are also modular that allows the addition of metal wedges or triangles to compensate for bone loss. The stem modularity also allows the addition of long stems to provide stability.
The constrained implants wear out earlier compared to non-constrained but provide excellent stability in difficult cases. The results of constrained hinged knee replacement are better when performed by a surgeon with expertise in performing complex knee replacement surgeries.