Unicompartmental knee arthroplasty (UKA) preserves the articular cartilage, bone, and menisci in the unaffected compartments, as well as the cruciate ligaments, thus preserving proprioception and more normal kinematics in the knee than total knee arthroplasty (TKA). For some, it is a bridging procedure before TKA becomes necessary; therefore, it is important to preserve bone during implantation of the UKA. For other patients, it is the definitive procedure that will last their lifetimes.

The classic indications and contraindications for UKA (1) are equally appropriate when considering use of robotic-arm technology, although expanding indications for UKA, in general, continue to be evaluated (2, 3, 4, 5). The classic recommendations are attributed to Kozinn and Scott who advocated restricting UKA to low-demand patients older than age 60 with unicompartmental osteoarthritis or focal osteonecrosis. Additionally, they recommended that patients weigh less than 82 kg (181 lbs), have a minimum 90-degree flexion arc and flexion contracture of less than 5 degrees, an angular deformity not exceeding 10 degrees of varus or 15 degrees of valgus (both of which should be correctable to neutral passively after removal of osteophytes), an intact anterior cruciate ligament (ACL), and no pain or exposed bone in the patellofemoral or opposite tibiofemoral compartment (1).

More recently, the indications for UKA have expanded to include younger and more active patients, without substantial compromise in outcomes or implant survivorship (2, 3, 4, 5), making it a legitimate alternative to periarticular osteotomy or TKA in younger patients. Obese patients have been shown to have compromised outcomes, although UKA is a reasonable option for patients who are only mildly obese (6). I would not advocate the procedure in morbidly obese patients. Incompetence of the ACL may cause abnormal knee kinematics, and anterior tibial subluxation will typically result in posterior tibial wear. However, although ACL insufficiency had historically been considered an absolute contraindication to UKA, it is now considered a reasonable option if there is limited functional instability, if the area of femoral contact on the tibia in extension and the tibiofemoral arthritis is anterior (3). Minimizing the tibial slope in the ACL-deficient knee is critical, however, to ensure durability (7).

If subchondral bone loss is significant—caused, for instance, by a large cyst or extensive focal osteonecrosis with structural compromise—then I advise against UKA because this may predispose to component subsidence. Additionally, this procedure should be restricted to patients without inflammatory arthritis and crystalline arthropathy (e.g., gout and chondrocalcinosis), as these ailments can
increase the risk of pain and accelerated degeneration of the remaining compartments of the knee. If there are areas with grade IV chondromalacia in the other compartments of the knee, I would not recommend UKA. Lesser stages of chondromalacia are not contraindications to UKA, however, unless the patient complains of pain in those compartments.

There are no specific contraindications to the use of robotic assistance for UKA, although the added duration of surgery early in the learning curve may make robotic assistance undesirable for some patients in whom a briefer anesthetic time is desirable. Anesthetic times are typically approximately 2 hours during the initial few cases with robotic assistance but quickly decrease soon thereafter to approximately 1 hour, as the surgical team becomes more proficient with the setup, preparation, and nuances of the surgical procedure.

The results of UKA are affected by a variety of factors including the underlying diagnosis, patient selection, prosthesis design, polyethylene quality, and implant alignment and fixation. If we assume that patients are appropriately selected and an implant of sound design with good polyethylene is used, then the accuracy of implantation is likely the most important variable having an impact on whether an implant will perform well and survive as long as expected.

An excessive posterior tibial slope; or tibial component or mechanical axis varus malalignment predisposes the prosthesis to early failure (7, 8, 9). Studies have shown that using conventional approaches and instrumentation, it is difficult to consistently accurately align the tibial component in UKA (8,10,11). Outliers beyond 2 degrees of the preoperatively planned alignment may occur in as many as 40% to 60% of cases using conventional methods (11,12). Additionally, the range of component alignment varies considerably, even in the hands of skilled knee surgeons (8). The problem is compounded when using minimally invasive surgical approaches, which is how most contemporary UKAs are likely performed (10,13). One study analyzing the results of 221 consecutive UKAs performed through a minimally invasive surgical approach found a large range of tibial component alignment, with a mean of 6 degrees (SD ± 4) and a range from 18 degrees varus to 6 degrees valgus (10).

Computer navigation was introduced in an effort to reduce the number of outliers and improve the accuracy of UKA. Even with computer navigation, however, the number of outliers (beyond 2 degrees of the preoperatively planned implant position) may approach 15% (11). Robotic guidance was therefore introduced to capitalize on the improvements seen with computer navigation but also to further refine and enhance the accuracy of bone preparation, even with minimally invasive techniques (12).

Finally, the complexity of revision of the failed UKA to TKA, and the results of the revision, partially depend on the extent of bone compromise. When greater bone resection has been performed during the UKA, revision is more challenging, and the need for augments to fill bone defects is greater. With robotic assistance, the intention is not only to make component position more consistent but also to limit bone resection and reduce the thickness of the tibial polyethylene inserts needed to balance the knee.

The Tactile Guidance System (TGS) Robotic Arm (MAKO Surgical Inc., Ft. Lauderdale, FL) is a surgeon-interactive robotic arm that uses preoperative images of the patient’s lower extremity to allow accurate preoperative planning, intraoperative navigation, and robotic assistance to prepare bone for implantation of UKA components (Fig. 25-1). The system provides a stereotactic interface that constrains the surgeon in the preparation of the femur and the tibia. Stereotactic boundaries are virtual walls created by the software and implemented through the robotic arm hardware to restrict the cutting tip to within a predefined resection volume.

The TGS technology is an alternative to standard UKA instrumentation with their intra- and extramedullary guides, pinned cutting blocks and jigs, and saws. In their place are burrs of varying sizes, which have reduced inventory and further optimized the application of minimally invasive approaches to UKA without having to squeeze miniaturized, but still bulky, instruments into a small wound (Fig. 25-2). The TGS robotic arm is enabling technology that has improved the accuracy and the performance of UKA through a minimally invasive approach.