Progression of Arthritis

Progression of osteoarthritis in the remaining compartments is one of the most common modes of failure. Multiple studies cite the rate of UKA failure due to progression of arthritis between 1% and 9% [3, 36]. However, among UKA failures, progression of arthritis accounts for 15–50% of failures and is the most common mid- to late-term mode of failure [3, 5–7]. Patient selection and failure to follow specific indications may contribute, as patients with inflammatory arthritis, higher American Society of Anesthesiologists (ASA) score, and obesity are at a higher risk of developing adjacent compartment disease [37–39].

Technical aspects that contribute to progression of arthritis include overcorrection of the mechanical axis [13, 40] (Fig. 17.2). Hernigou et al. reported in their series of 58 medial UKAs that a hip-knee-ankle angle over 180° was associated with a higher and more rapidly occurring degeneration of the lateral compartment. Putting in a tight knee to avoid mobile-bearing dislocation can increase the contact stress in the adjacent compartment and contribute to degenerative wear [29, 40]. In contrast to total knee arthroplasty, component placement also affects femorotibial contact independent of the mechanical axis of the limb. Implant placement can reduce contact area and thereby increase local stress [41].

The clinical significance of patellofemoral joint degeneration is controversial. While it was initially thought that patellofemoral arthritis could contribute to anterior knee pain and patient dissatisfaction, recent literature has shown no difference in functional outcomes or reoperation rates in patients with patellofemoral arthritis documented at the time of surgery [11]. An oversized femoral component can also lead to patellofemoral impingement, which may be more symptomatic than patellofemoral arthritis [3]. This is more common with lateral UKA [42]. Interestingly, recent studies have demonstrated improved patellofemoral joint congruence following UKA [43]. The recommended management of symptomatic adjacent compartment degeneration is revision to total knee arthroplasty [37].

Aseptic Loosening

Another major cause of UKA failure is aseptic mechanical loosening of the components. Overall rates of aseptic loosening in UKA have been cited between 0.5% and 18% [3]. Of UKA failures, aseptic loosening accounts for 31–45% of failures and is the major mode of failure in the first several years following UKA [5–7]. Patients at a higher risk for aseptic loosening include young, overweight patients with significant varus deformity [37, 44]. Aseptic loosening is more common with the tibial component than in the femoral component [6] (Fig. 17.3). Mechanical factors that increase stress on the tibial component and can contribute to loosening include malalignment, overcorrection of deformity, excessive tibial slope, and anterior cruciate ligament (ACL) deficiency [45]. Mechanically, lowering the joint line corresponds to increased stress on the UKA components and aseptic loosening, while raising the joint line leads to early polyethylene wear and progressive degenerative changes in the other compartment [12]. Initial data suggested that fixed-bearing implants have a higher rate of aseptic loosening due to lower conformity than mobile-bearing components, but recent data have shown that mobile-bearing components may paradoxically demonstrate higher rates of aseptic loosening [27, 28, 46]. All-polyethylene tibial components have a higher rate of loosening than metal-backed components [7, 39, 44]. Aseptic loosening is best addressed by conversion to total knee arthroplasty, although revision UKA may be considered for select patients.

Polyethylene Wear

Polyethylene wear is a less common mode of UKA failure but still accounts for 12–25% of UKA failures [7]. It usually presents as a late mode of failure, after 8 years, but early cases of catastrophic failure have also been reported [47, 48]. Technical factors that contribute to polyethylene wear include component malposition and under-correction of deformity [13, 47, 49]. Implant-specific factors associated with polyethylene wear include thickness less than 6 mm, lack of design conformity, and flaws in the manufacturing and sterilization of PE [47, 49]. Debris from polyethylene causes an osteolytic reaction at the bone-implant interface and can affect alignment and stability of the implant. Uneven load distribution from component malalignment/instability can accelerate aseptic loosening [47]. Increased ligamentous laxity and subluxation of the tibiofemoral implant surface can also contribute to increased polyethylene wear [48]. In addition, progression of osteoarthritis may also cause attenuation of the anterior cruciate ligament, leading to increased subluxation as well [48]. Subluxation of the implants concentrates force over the peripheral aspect of the tibial component, which is also where the polyethylene layer is the thinnest [48]. Improvements in wear-characteristics of polyethylene may decrease the prevalence of UKA failure from polyethylene wear. Management options of polyethylene wear include polyethylene exchange or revision to total knee arthroplasty.

Bearing Dislocation

Mobile-bearing dislocation accounts for 1.5–4.6% of UKA failures [5]. These dislocations more commonly affect the lateral compartment secondary to the increased laxity of the lateral collateral ligament (LCL), the convexity of the lateral tibial condyle, and the medial femoral rollback during flexion [5, 7, 19–22, 49]. Medial mobile-bearing dislocations can occur in the setting of unbalanced flexion/extension gaps, instability due to medial collateral ligament (MCL) injury, or component malposition with impingement of the insert on the adjacent bone [46, 50]. In contrast to total knee arthroplasty, soft tissue releases are not recommended in UKA due to risk of instability and bearing dislocation, as the goal of UKA is to restore ligament tension to normal, thereby restoring knee kinematics [40]. Joint instability following UKA can also contribute to early polyethylene wear and aseptic loosening, which can, in turn, increase the risk of mobile-bearing dislocation [47]. Management for bearing dislocation includes revision UKA with a fixed bearing or conversion to total knee arthroplasty [37].

Tibial Collapse

Tibial subsidence is the most common cause of periprosthetic fracture following UKA and accounts for 3.6–10.4% of UKA failures [6, 51]. Tibial collapse commonly presents as a late complication and is more common in elderly patients, suggesting osteoporosis as a contributing factor [51, 52]. It most commonly affects the medial tibial plateau due to increased pressure and load [37]. Technical factors, which may contribute to tibial subsidence , include excessive posterior tibial slope [52, 53]. Surface area of the tibial component and depth of the tibial resection may also play a role, but this has not been demonstrated in the literature [52]. Tibial collapse is also more commonly associated with fixed-bearing all-polyethylene implants. These implants have higher contact stresses at localized points in the anterior and medial tibia, leading to localized edge loading and tibial collapse [54]. Management options for tibial collapse include percutaneous screw fixation and revision to TKA and may require cement, augments, cones, and stems depending on the degree of bone loss, status of the other knee compartments, and degree of deformity [31–34, 46, 52, 55].

Infection

Infection following UKA occurs at a rate of 0.2–1% [56]. While this rate is slightly lower than that of total knee arthroplasty, UKA infections are unique in that they involve both the prosthesis and native cartilage [56]. The diagnostic workup of infection in UKA is based on that of total knee arthroplasty with preoperative laboratory studies used to guide potential aspiration [57, 58]. Given the involvement of both native cartilage and prosthesis, slightly different thresholds for the diagnosis of infection have been proposed. In a study of 259 patients undergoing revision UKA, Schwartz et al. [59] found a 10.8% infection rate and proposed cutoff values of the following: ESR: 27 mm/h, CRP: 14 mg/L, synovial fluid WBC count of 6200 cells/μL, and 60% neutrophils . The proposed cutoff for aspirate leukocyte count is higher than that of total knee arthroplasty. This may be attributable to the involvement of more of the native cartilage but requires further investigation and validation [59]. The causative organisms in UKA infections are similar to those in total knee arthroplasty infection with coagulase-negative Staphylococcus, S. aureus, group B Streptococcus, E. coli, and P. acnes among the most common organisms [56, 59]. Management of UKA infection is similar to that of total knee arthroplasty. Acute infections can be managed with irrigation, debridement , polyethylene liner exchange, and antibiotics, while chronic infections require irrigation and debridement with antibiotic spacer, antibiotics, and conversion to total knee arthroplasty [37]. Labruyere et al. proposed synovectomy with one-stage conversion to total knee arthroplasty and 3 months of antibiotics as a reasonable alternative to two-stage conversion to total knee arthroplasty, with no infection recurrence and good functional outcomes at early follow-up [56].

Unexplained Pain

Unexplained pain is an important cause for revision after UKA. While etiology is unknown, it accounts for up to 23% of revision surgery according to registry data from England and Wales [60]. This is significantly higher than the rates of revision for total knee arthroplasty for unexplained pain, which is estimated to be about 9% [60]. Overall, the rates of the unexplained pain following UKA vary from 5% to 15% [5–7]. While the etiologies may differ on a patient-by-patient basis, proposed etiologies include loose bodies, cement extrusion, meniscal tears in the native compartment, and chronic regional pain syndrome or reflex sympathetic dystrophy [37]. Due to the limited exposure and single-piece implants used in some systems, it can be technically challenging to remove extruded cement from the posterior aspect of a UKA. Given the hybrid of native cartilage and prosthesis, failure to restore normal joint alignment and mechanics can contribute to pain generators such as meniscal tears and loose bodies. Furthermore, all-polyethylene tibial components have been associated with higher rates of unexplained pain [61]. This may be related to the higher rates of tibial collapse seen with fixed-bearing all-polyethylene tibial component due to higher load transfer to the tibia resulting in persistent bone remodeling [61]. The threshold for revision due to unexplained pain may also differ based on surgeon experience and familiarity with UKA [62]. In an examination of the New Zealand Joints Registry, Goodfellow et al. showed that in knees that had very poor outcome (Oxford Knee Score <20), 63% of UKAs went onto revision surgery, while only 12% of TKAs were revised [62]. Management of the unexplained pain following UKA is surgeon-specific, but the conversion of a UKA to a total knee arthroplasty is less technically demanding than revision of a total knee arthroplasty. Thus, surgeons may have a lower threshold to offer revision to total knee arthroplasty as an option for patients with unclear pain generators following UKA [60, 62].