Unicompartmental Knee Arthroplasty: An Overview

Introduction:

Unicompartmental knee arthroplasty (UKA), or partial knee replacement, is a surgical innovation targeting the specific area of knee deterioration, preserving healthy portions while addressing osteoarthritis—a degenerative joint condition. Unlike total knee arthroplasty (TKA), which replaces all three knee compartments, UKA precisely targets and replaces only the damaged segment.

Proponents highlight UKA's post-operative advantages, including reduced complications, minimized blood loss during surgery, and accelerated recovery compared to TKA. However, historical data reveals higher failure rates, with approximately 28 percent of cases requiring conversion to TKA within six years post-UKA.

Despite these challenges, advancements in UKA techniques, notably its minimally invasive approach, have garnered favor among surgeons and patients. Furthermore, the integration of robotic assistance shows promising potential to refine the precision and efficacy of UKA procedures, heralding a new era in knee joint restoration.

What Is Unicompartmental Knee Arthroplasty?

Unicompartmental knee arthroplasty stands out by selectively targeting the afflicted compartment of the knee, replacing it with precision-engineered metal or plastic components. The efficacy of this treatment hinges on meticulous patient selection and the adeptness of the surgical team. Given its technical intricacies, early failure rates can be elevated.

However, its minimally invasive nature translates to shorter hospital stays and quicker resumption of daily routines for patients, minimizing both time and financial strain. Noteworthy advantages encompass rapid recovery, diminished intraoperative blood loss, reduced postoperative discomfort, and a lowered susceptibility to infections. Yet, the procedure does not have drawbacks, with some patients eventually requiring supplementary surgeries.

What Are the Compartments of the Knee?

Within the intricate anatomy of the knee, three distinct compartments delineate its structure:

• Medial: This compartment entails the juncture where the medial condyle of the femur meets the medial aspect of the tibial plateau. In bow-leggedness, characterized by varus malalignment, heightened pressure is exerted on the medial compartment, precipitating the onset of medial compartment arthritis (inflammation of the joints).

• Lateral: This compartment encompasses the interface between the lateral femoral condyle and the lateral aspect of the tibial plateau. With knock-kneed conditions characterized by valgus malalignment, increased strain is directed toward the lateral compartment, culminating in the development of lateral compartment arthritis.

• Patellofemoral: This compartment pertains to the connection between the trochlear groove of the femur and the kneecap.

What Are the Indications of Unicompartmental Knee Arthroplasty?

Unicompartmental knee arthroplasty typically emerges as a consideration when conventional approaches such as physical therapy, activity adjustment, bracing, anti-inflammatory medications, or injections prove ineffective in managing knee osteoarthritis.

Notably, the indications for UKA were delineated by Kozinn and Scott in 1989, which include:

• Individuals aged 60 years or older, weighing less than 181 pounds.

• Osteonecrosis (a condition where there is a loss of blood supply to a bone, resulting in its death or necrosis) or osteoarthritis (a degenerative joint disease) localized within a single compartment of the knee joint.

• Candidates should experience low levels of pain at rest.

• Preoperative assessment should reveal a range of motion greater than 90 degrees.

• Candidates should exhibit less than 5 degrees of flexion contracture.

• Correctable angular deformities should be less than 15 degrees.

• Candidates should not exhibit symptoms of inflammatory arthritis.

Navigating these criteria proves challenging; however, recent advancements in implant design pave the way for favorable outcomes among younger individuals and those with an elevated body mass index (BMI). Contrary to common belief, obese patients display no heightened probability of revisions. Yet, heightened activity levels in younger patients may predispose them to increased implant loosening. Both cohorts necessitate vigilant monitoring and customized interventions.

What Are the Contraindications of Unicompartmental Knee Arthroplasty?

Contraindications for unicompartmental knee arthroplasty (UKA) encompass several factors:

• Affliction of the patellofemoral joint (PFJ) with arthritis.

• Prevalence of conditions such as rheumatoid arthritis.

• Arthritis in the knee compartment opposing UKA's intended site could compromise surgical function and outcome.

• Insufficiency or damage to the anterior cruciate ligament (ACL), renders this technique unsuitable.

What Is the Procedure Involved in Unicompartmental Knee Arthroplasty?

Thorough physical assessments and comprehensive medical histories are imperative. Evaluations should encompass assessing the knee for limitations in range of motion and ligamentous stability, adhering to criteria established by Kozinn and Scott. Utilization of the specialized Merchant View X-ray aids in enhanced visualization of the patellofemoral joint (PFJ). Advanced imaging modalities such as CT (computed tomography) scans are recommended, particularly for planning robotic-assisted UKA procedures.

The steps include:

Medical Compartment:

• After giving anesthesia, ensuring optimal visibility of the knee's medial compartment while minimizing interference with soft tissues is essential.

• The implant's alignment with the angle of tibial resection should harmonize with the natural slope of the patient's tibia, thus promoting stability.

• Resecting bone proximate to the tibia while precisely positioning the implant is crucial for preserving the anterior cruciate ligament (ACL, located in the center of the knee).

• The implant component must be oriented perpendicular to the longitudinal axis of the tibia.

• To prevent implant subsidence into the bone, it is crucial to avoid undersizing during implantation.

• Steer clear of overcorrecting varus deformities to maintain optimal alignment.

Lateral Compartment:

• For optimal placement, aim to position the femoral component of the implant centrally or with a subtle lateral inclination on the femoral condyle.

• Avoiding anterior placement and oversizing during implantation is crucial to preventing patellar impingement.

Following implantation, a meticulous alignment assessment precedes the incision closure. Consistent monitoring through follow-up visits is paramount to tracking the healing trajectory. The efficacy of this procedure hinges on factors such as ligament equilibrium, limb orientation, and secure implant anchorage.

What Are the Complications of Unicompartmental Knee Arthroplasty?

Complications arising from knee arthroplasty encompass a spectrum of challenges:

• Aseptic loosening, wherein the implant gradually separates from the bone without infection.

• Progressive osteoarthritis (advancement of osteoarthritis over time).

• Infections.

• Polyethylene wear (deterioration of the plastic component within the joint replacement).

• Pre-prosthetic fracture (fracture occurring around the area of the artificial joint before or during surgery).

• Bearing dislocation, where the implant components dislocate from their intended position.

• Ankylosis (abnormal stiffening of a joint due to the fusion of bones) of the knee.

• Persistent pain.

Conclusion:

Executing unicompartmental knee arthroplasty (UKA) under precise criteria presents a minimally invasive approach with the potential to reinstate patients to their prior recreational engagements. However, accelerated implant wear may ensue within younger cohorts boasting heightened physical activity levels, prompting the need for revision surgery. Turning a failed UKA into a total knee arthroplasty (TKA) boasts fewer complications than revising a primary TKA.