Preoperative radiographs of case example demonstrating bone-on-bone medial compartment arthritis on the AP, maintained posteromedial tibial plateau on the lateral, and maintained joint space between the lateral facet and trochlea
Unicompartmental knee arthroplasty (UKA) was introduced in the 1970s by Marmor and was initially met with great enthusiasm, promising an alternative to osteotomy . Early excitement disappeared with poor results reported by several authors. Laskin reported on 37 patients in 1978, finding contralateral compartment degeneration at 4–7 years and results inferior to bicompartmental and tricompartmental arthroplasty of the knee . In 1980, Insall and Aglietti also reported poor results, with early deterioration at an average follow-up of 6 years . These early failures were likely due to the degree of constraint of the initial implants, poor implant design, imprecise instrumentation, and unclear indications. A unicompartmental knee arthroplasty could not be treated as “half” a total knee arthroplasty (TKA) and in that early form, with excellent results being reported with TKA, most surgeons abandoned UKA.
Learning from the initial mistakes, indications were proposed, instrumentation improved, and implant designs were refined. Proposed indications addressed age, weight, level of activity, pain, range of motion, and angular deformity, and limitations in the degeneration of the remaining compartments . Several authors then began to publish promising results. In 1999, Berger et al. found 98% survival at 10 years in 62 knees , and more recently 90% survival at 20 years . Pandit also published results from 1000 cases, showing 94% survival at 10 years and 91% survival at 15 years . These promising results suggest that solutions had been found to the early challenges and partial knee replacements were once again a viable treatment option for select patients with knee arthritis.
With the development of surgical pathways, multimodal pain strategies, improvement in surgical technique and the advent of same-day surgery protocols, patient’s expectations about unicompartmental knee replacement surgery have changed. The promise of simpler, quicker surgery, and faster recovery has led to a significant increase in the frequency of UKA , particularly for isolated medial compartment arthritis. Additionally, indications have expanded with continued excellent results [9–11]. Long-term follow-up has shown encouraging longevity and function from the implants currently available and published data suggests increasing utilization of ambulatory surgery centers (ASC) is safe and effective for same-day procedures [12–14]. To that end, Centers for Medicare and Medicaid Services has designated partial knee replacements as an outpatient procedure and indications for hospital stays greater than 23 hours must now be well indicated.
The primary indications for medial UKA are isolated anteromedial arthritis and spontaneous osteonecrosis of the knee [15–17]. Approximately three decades ago, Kozinn & Scott described their ideal candidate as less the 60 years of age, weighing less than 180 pounds, and low demand with minimal pain at rest, less than 5° flexion contracture, a preoperative arc of flexion of 90°, and a passively correctable angular deformity of less than 10° of varus. Additionally, patellofemoral joint arthritis, inflammatory arthropathy, chondrocalcinosis, and cruciate ligament deficiency were suggested as contraindications .
Initial failures suggest that constraint is best limited in implant design [2, 3, 39]. Learning from early design limitations has led to reliable survivability in multiple designs. Options for the medial compartment of the knee include fixed- and mobile-bearing designs, monoblock and modular tibial options, and cemented and cementless implants. Functional outcomes and longevity appear similar between fixed- and mobile-bearing designs [40–43], although progression of lateral compartment arthritis is more common in the mobile-bearing group and polyethylene insert dislocation is a complication unique to mobile-bearing implants . Retrospective analysis has also shown that fixed-bearing implants better tolerate suboptimal rotation of the tibial component . While reasonable midterm results have been reported in an all-polyethylene tibial designs [45, 46] and there is valid concern regarding bone loss in revision of metal-backed modular tibial components , the literature more consistently shows superior survivability with metal-backed designs and the risk of early failure with monoblock all-polyethylene tibial implants [48–51]. Biomechanical data help support and explain these findings, with significantly greater strain on the cancellous bone of the proximal tibia with all-polyethylene tibial components [52, 53]. Cementless implants offer the potential for faster surgery, avoidance of cementation errors, and diminished aseptic loosening. The limited available evidence is promising, with survival, reoperation rate, failure, and clinical outcomes similar to cemented implants [54, 55]. However, the majority of results are restricted to a single, mobile-bearing implant design , and include only midterm follow-up of 5 years. Long-term follow-up is necessary to validate these findings.
Interest in leveraging technological advances in surgical technique continues to grow among surgeons, researchers, manufacturers, and patients. The goal of robotics and patient-specific instrumentation (PSI) is to minimize limb malalignment and component malposition in an effort to improve implant durability and outcomes. However, these technologies are expensive and have failed to show significant clinical benefit as of yet. Two Level 1 studies have been conducted using PSI guides for medial unicompartmental knee arthroplasty [56, 57], and neither showed an advantage when compared to conventional instrumentation. The majority of studies investigating robotic-assisted UKA have reported on accuracy of component placement and shown a statistical advantage when compared to conventional techniques, but there are few reports documenting clinical outcomes and long-term follow-up results are lacking [58, 59]. It remains unknown if more accurate component position leads to improved clinical outcomes or enhances long-term survival of implants.
Outcomes and Survival
Although there is some debate regarding improvement in patient-reported outcomes when compared to TKA [60–63], it is generally accepted that appropriately selected patients have high satisfaction rates and improved function following UKA. Nevertheless, UKA is associated with a lower occurrence of complications, readmission, and mortality [64, 65]. If 100 patients receiving TKA received UKA instead, the result would average one fewer death and three more reoperations in the first 4 years following surgery . It is important to point out that the threshold for revision of UKA is much lower, and UKA still compares favorably in economic evaluations of estimated cost and health outcome even when considering slightly higher rates of revision .
The results of UKA have significantly improved in the past few decades, with greater than 94% survival at 10 years for metal-backed, fixed-bearing medial UKA in multiple cohort studies [6, 67, 68] and an average of 91% for all medial UKA in a systematic review . Of note, registry data consistently shows worse outcomes, with an average 10-year survival of only 84.1% in the aforementioned systematic review . A plausible explanation for this trend is that registry data includes multiple implants performed by multiple surgeons with varying levels of experience. The revision rate is significantly lower for surgeons performing at least 30 UKAs per year . Cohort studies may allow better understanding of how specific implants perform at single center institutions by high volume surgeons .
A recent systematic review found the most common modes of failure for fixed-bearing medial UKA are progression of adjacent compartment arthritis (36%) and aseptic loosening (28%). Instability (12%), polyethylene wear (12%), tibial subsidence (4%), unexplained pain (2%), and infection (2%) are less common. When looking at all medial UKAs, that is, mobile- and fixed-bearing, early failures (<5 years) were most commonly caused by aseptic loosening (25%), progression of osteoarthritis (20%), and bearing dislocation (17%) .
Newer designs and better instrumentation have significantly reduced the incidence of aseptic loosening. Varus deformity , younger age, and weight have been advocated as possible risk factors for mechanical failure . Mechanical loosening is also likely influenced by undercorrection of constitutional deformity, component malalignment, excessive tibial slope, and anterior cruciate ligament deficiency. In addition, all these factors may contribute to wear-induced periprosthetic osteolysis , with a further increase in component subsidence and/or loosening . Progression of adjacent compartment arthritis was responsible for 38% and 40% of midterm (5–10 years) and late failures (>10 years), respectively . Overcorrection of the leg mechanical axis may cause degenerative changes in the contralateral compartment . Degeneration of the patellofemoral joint may occur in the presence of an oversized femoral component . Disease progression and component failure are discussed further in Chap. 17.