The Evolution of Unicompartmental Knee Arthroplasty

In the present day, with the continuous deepening of the concept of stepwise knee joint treatment, the concept of unicompartmental knee replacement (UKR) is becoming increasingly popular. Looking back at the development history of unicompartmental knee prostheses and replacement techniques, improvement can be considered the main theme of its development.

In the 1950s, Duncan C. McKeever proposed the theory of single-compartment knee joint unilateral replacement. Subsequently, McKeever and MacIntosh proposed the precursor of unicondylar knee replacement by introducing a metal tibial component that replaced only the tibial plateau.

Gunston developed a knee replacement technique, which Charnley further developed for the unicompartmental knee prosthesis. Thackeray introduced and distributed it as a load-bearing angled insert. Its feature was a prominent femoral component connected to a flat plateau. The femoral component experienced loosening, deformation, and wear, with a relatively low survival rate, while the tibial component performed well.

From 1970 to 1972, Dr. Leonard Marmor (in collaboration with Richards) developed a prosthesis known as the Marmor modular knee prosthesis. In 1973, a design error led to inconsistencies between the prosthesis components and the molds, delaying the development of unicompartmental joint replacement for a decade. However, Marmor still innovated in the design and operational techniques of UKA components, making significant contributions of great importance.

Currently, the majority of fixed-bearing UKA designs follow Marmor's design, with a convex and multi-curvature metal femoral surface and a flat or shallow concave polyethylene surface. Initially, this high-contact stress design could lead to excessive polyethylene wear and subsequent prosthesis failure.

In recent years, new materials have been introduced to address such issues. According to the AJRR (American Joint Replacement Registry) annual report, in terms of material selection for the bearing, highly cross-linked polyethylene bearings account for the highest proportion, remaining above 50%.

In 1974, Goodfellow and O'Connor from the UK National Research Development Corporation proposed the "meniscal bearing" concept for OUKA. This design included a flat tibial plateau, a spherical femoral condyle component, and a polyethylene platform insert with high conformity to the femoral condyle surface and minimal constraint on the tibial plateau. This design concept was first used in unicompartmental knee replacement in 1982. Based on this concept, the mobile unicondylar prosthesis gained widespread application.

JUST Unicondylar Design Concept:

JUST continued this structural concept and further developed its own design philosophy and unicondylar products, releasing both fixed-bearing and mobile-bearing unicondylar systems.

Fixed-Bearing Unicondylar:

Earlier studies generally believed that the femoral condyle should have multiple radii, forming a "J"-shaped curve with the instantaneous center of rotation during flexion. [1]

The multi-radius concept remains the mainstream design. The design of fixed-bearing unicondylar prostheses also follows this philosophy, matching the multi-radius condyle with a near-flat platform component, avoiding constraint, and reducing the risk of platform-side loosening.

To address the wear issues that appeared previously, wear between the two interfaces is reduced through material selection and process improvements to ensure long-term survival.

Mobile-Bearing Unicondylar:

Natural menisci possess compliance; during knee joint flexion, the natural meniscus changes shape to adapt to the changing curvature of the femoral condyle. [2]?

Mobile-bearing unicondylar prostheses simulate the compliance of natural menisci but have rigid inserts. Therefore, the condyle is designed with a single radius and high conformity with the insert, increasing the contact area and reducing contact pressure.

To address the high constraint issue arising from this, the insert and platform adopt a non-constrained movable design, reducing shear forces transmitted to the platform and avoiding the risk of loosening. This leads to the concept of mobile-bearing unicondylar.

Regarding the design of the mobile platform-insert interface, it aims to avoid dislocation risks arising from inner OA adaptation, optimized surgical instruments, and widespread surgical technique adoption.