Unicompartmental knee arthroplasty (UKA) is a surgical technique used for the treatment of osteoarthritis in one compartment of the knee, most commonly in the medial compartment. In contrast, a total knee arthroplasty (TKA) is used for the treatment of osteoarthritis in all three compartments of the knee. UKA was first introduced in the 1970s. Proponents for UKA argued that the procedure more closely mimics normal knee kinematics, leads to lower perioperative morbidity and intraoperative blood loss, and allows for earlier mobilization and rehabilitation compared to conventional bicondylar knee arthroplasty. Early results, however, demonstrated high rates of failures, with 28% rate of conversion to TKA at an average follow-up of six years. Over time, modifications in the implant design, surgical technique and the expansion of surgical indications have led to renewed interest in UKA. Furthermore, the increased demand for minimally invasive approaches has also increased the popularity of UKA, which requires a smaller incision compared to traditional TKA. The development of robotic-assisted techniques have improved surgical precision and component alignment in an effort to increase survivorship.
The knee has three compartments: medial, lateral, and patellofemoral. The medial compartment is the articulation between the medial condyle of the femur and the medial aspect of the tibial plateau. The lateral compartment is the articulation between the lateral femoral condyle and the lateral aspect of the tibial plateau. The patellofemoral compartment is the articulation between the patella and the trochlear groove of the femur. A patient can develop arthritis in one, two, or all three of these compartments. The alignment of the limb is one factor that can determine the location of osteoarthritis development. Patients with varus (or bow-legged) malalignment will have greater contact stresses in the medial compartment of the knee and be more likely to develop medial compartment arthritis. Patients with valgus (or knock-kneed) malalignment will be more likely to develop arthritis in the lateral compartment.
As with other elective procedures, prior to considering surgery, the patient should have attempted treatment with conservative measures, including activity modification, physical therapy, anti-inflammatory medications, bracing, and/or injections. Surgical intervention can be considered only if these conservative measures fail to provide relief. The indications for UKA are controversial and have evolved since its introduction. Kozinn and Scott established a set of indications in 1989. They included: low-demand patients older than 60 years of age, weight less than 82 kg (181 lbs.), osteoarthritis or osteonecrosis confined to one compartment, low pain at rest, greater than 90 degrees of preoperative arc of motion, less than 5 degrees of flexion contracture, and less than 15 degrees of angular deformity that is passively correctable to neutral. Strict adherence to these criteria is difficult, with one analysis of over 4,000 TKAs demonstrating that only 6.1% of the cases met the anatomic indications and that only 4.3% met the clinical indications for UKA as outlined above.
More recent evidence has shown improved outcomes and survivorship in younger (age less than 60 years) and obese patients. The improvements in implant design have, in part, contributed to the expansion of traditional indications for UKA. Conventional teaching suggested that high BMI does lead to a risk of aseptic loosening from the excess load on the components. Similarly, younger patients typically have a more active lifestyle and higher functional expectations, which may also predispose to early loosening from excess wear. A meta-analysis that included six registry studies demonstrated no increased likelihood of revisions or poor outcomes in obese patients compared to control. The same study found a higher risk of revisions in younger patients and females.
The contraindications to UKA as initially described by Kozinn and Scott  continue to be challenged, as strict conformity to their selection criteria severely limits the number of eligible patients. In addition to the patient demographics and anatomic factors listed above, contraindications also include arthritis of the patellofemoral joint (PFJ) and the opposite compartment, inflammatory arthritis, and insufficiency of the anterior cruciate ligament (ACL).
Recent studies have demonstrated no difference in function and revision rates in patients with and without PFJ arthritis. Traditionally, UKA in ACL-deficient knees were associated with high failure rates. Cadaveric and in vivo studies have demonstrated biomechanical instability in ACL-deficient knees that may predispose to polyethylene wear and worsening degeneration in the lateral and patellofemoral compartments. Recent studies have shown acceptable outcomes in patients with deficient ACL when UKA is compared to ACL-intact patients. Simultaneous reconstruction of the ACL at time of UKA has also been described and has demonstrated greater implant survival rate compared to ACL-deficient knees without reconstruction.
Preoperative evaluation should include a detailed history and physical examination. Evaluation of the knee should encompass testing for ligamentous stability and range of motion limitations based on criteria established by Kozinn and Scott. Weight-bearing radiographs should be scrutinized for varus or valgus deformity. A merchant view is helpful to evaluate the PFJ and lateral patella subluxation. Advanced imaging, such as CT, may be necessary for preoperative mapping for robotic-assisted UKA to guide positioning of the implants and soft-tissue balancing. MRI is often unnecessary and may overestimate the severity of articular pathology.
The surgical approach to UKA should allow adequate exposure of the medial compartment while minimizing the release of the soft tissues. The tibial resection should match the native tibial slope. The sagittal resection should be as close to the tibial spine as possible to maximize the area for positioning of the component without causing damage to the ACL. In the coronal plane, the part should be placed perpendicular to the long axis of the tibia. Undersizing of the tibial component should be avoided to prevent implant subsidence. Over-correction of varus deformity may result in excess stress on the medial soft tissues and increased lateral compartment degeneration. This may be prevented by not formally releasing the medial collateral ligament and sizing the polyethylene insert that allows for 2 mm of joint laxity in both full extension and flexion.
The ideal position of the femoral component is central or slightly lateral on the femoral condyle to optimize tracking with the tibial component. For lateral UKA, femoral component impingement on the patella has been described. These were noted to be associated with anterior placement and oversizing of the femoral component. The sulcus terminalis of the lateral femoral condyle has been described as a landmark beyond which the femoral component should not be sized or implanted.
Other considerations in UKA include the use of mobile- (MB) versus fixed-bearing (FB) implants. MB implants were introduced in the 1980s and were designed to distribute loads over a large surface area to decrease polyethylene wear. Modern FB implants are characterized by low conformity between the femoral and tibial components that permit greater ROM and decreased backside wear. Long-term outcomes in the literature have failed to demonstrate the superiority of one type of bearing over the other concerning implant survival and patient outcomes.
Another consideration is whether to use cement during component implantation. Cementing in UKA can prove challenging given the limited exposure and surface area. Furthermore, cementing prolongs the operative duration and may result in cementation errors. Another disadvantage of cementing is the greater rate of radiographic subsidence; however, most of those cases do not correlate with aseptic loosening. Short- and mid-term follow-up has shown similar survival rates between cemented and uncemented UKA.
Since the introduction of UKA several decades ago, the primary mechanism of failure has remained consistent in the literature, most commonly stemming from aseptic loosening, followed by progressive osteoarthritis. A systematic review of UKA demonstrated that aseptic loosening (25%) and osteoarthritis progression (20%) accounted for more than half of all revisions in the first five years, while infection (5%) and polyethylene wear (4%) were less frequent. Approximately 40% of mid- and late-term revisions were attributed to osteoarthritis progression. Despite technological advances in UKA implants, the revision-free survival rate has remained constant, unlike improvements seen in TKA survival rate trends. Some authors attribute this to the lower threshold by the surgeon to convert a UKA to a TKA, whereas a TKA revision is viewed as more technically demanding, with higher morbidity. Recent literature has refuted these claims by demonstrating similar complication and revision rates after UKA conversion and after primary TKA.
Since its initial introduction, indications for UKA has continued to expand. Data from large case series, registries, and meta-analyses have consistently demonstrated the long-term efficacy and survivorship of UKA in the appropriate patient populations. When performed for the correct indications and in the appropriate settings, UKA is a minimally invasive surgery that can return the patient to their previous level of recreational activity. Heightened patient expectations especially in younger and more active patients, however, can lead to early wear of the implant and need for revision. Nevertheless, conversion of a failed UKA to total knee arthroplasty is associated with lower morbidity compared to revision of a TKA.
Despite the excellent long-term survival rates after UKA in recent literature, there are substantial differences in outcomes demonstrated by cohort and registry data. At 15 years, the average survival rate was 87% in cohort studies, as opposed to 67% in registry studies. This discrepancy may be explained by variability in outcomes reporting and the inclusion of multiple surgeons with varying degrees of clinical volume. Registry studies offer the benefits of demonstrating trends in UKA over time and capturing a higher number of cases compared to cohort studies. For example, online data provided by the National Joint Registry for England, Wales, and Northern Ireland demonstrated better outcomes in UKA performed by surgeons who perform them 40-60% of the time compared to surgeons who perform them less than 5% of their total practice. This data highlights that in addition to careful patient selection, surgeon experience plays a substantial role in optimizing outcomes after UKA. High-volume centers that employ dedicated orthopedic operating room staff, as well as nursing and therapy staff familiar with the procedure, will ensure both operational efficiency and postoperative care to optimize the patients’ outcomes. Preoperatively, a thorough assessment of the patient’s co-morbid medical conditions helps minimize postoperative complications and length of stay. [Level V]
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