Unicompartmental Knee Arthroplasty


Unicompartmental knee arthroplasty (UKA) was first introduced as an alternative to tibial osteotomy for unicompartmental osteoarthritis in the 1970s. , Inconsistent early outcomes diminished enthusiasm at a time when total knee arthroplasty (TKA) was also evolving and promising results were being published. , Consequently, many surgeons abandoned UKA in favor of TKA. Early failures were likely related to unclear indications and primitive equipment. Improved implant design, instrumentation, indications, and surgical techniques have produced impressive results. Additionally, UKA is associated with a lower occurrence of complications, readmission, and mortality. , The possibility of equal or better function from a simpler surgery that is easier to recover from has led to a renewed interest and considerable increase in the frequency of UKA.

Options for the medial compartment of the knee include mobile- and fixed-bearing designs, modular and monoblock tibial options, and cementless and cemented implants. Survival and functional outcomes are similar between mobile and fixed-bearing designs for medial compartment arthroplasty. In the lateral compartment, a mobile-bearing implant is prone to dislocation; thus, a fixed-bearing design is preferred. , Monoblock all-polyethylene tibial designs have produced reasonable midterm results, , but there is a risk of early failure and a preponderance of literature supporting superior survivability with metal-backed implants. Cementless designs have the potential advantage of diminished aseptic loosening and avoidance of cementation errors. The limited available evidence is encouraging, with clinical outcomes, survival, reoperation rates, and failure similar to cemented implants. However, the majority of results are limited by midterm follow-up of 5 years and are restricted to a single mobile-bearing implant design.

UKA is indicated for isolated tibiofemoral compartment (medial or lateral) arthritis or osteonecrosis ( Fig. 12.1 ). Surgical goals of UKA are to relieve pain and restore function by resurfacing the affected compartment, recreating predisease knee alignment, and reestablishing native ligament tension. In a classic article published in 1989, Kozinn and Scott described the perfect candidate as low demand with minimal pain at rest, younger than 60 years of age, weighing less than 180 pounds, a preoperative arc of flexion of 90 degrees, less than a 5-degree flexion contracture, and a passively correctable angular deformity of less than 10 degrees of varus. Inflammatory arthropathy, chondrocalcinosis, patellofemoral joint arthritis, and cruciate ligament deficiency were suggested as contraindications. Modern indications do not strictly exclude patients based on activity level, age, weight, presence of chondrocalcinosis, the existence of arthritis within the patellofemoral joint, or anterior cruciate ligament (ACL) deficiency. , ,

Fig. 12.1

A, Preoperative anteroposterior (AP) view of isolated medial compartment arthritis. B, Postoperative AP view following medial unicompartmental knee arthroplasty with appropriate component alignment.

It is generally accepted that appropriately selected patients have improved function and high satisfaction rates following UKA. Despite multiple cohort studies with greater than 93% survival at 10 years, , , , , cynics reference registry data that invariably show an increased revision rate when compared with TKA. , , However, registry data include multiple surgeons with varying levels of experience utilizing a myriad of implants at facilities with different arthroplasty volumes. Revision rates are significantly lower for surgeons who use UKA for at least 20% of their knee arthroplasties , and for those performing at least 30 UKAs per year. There is also a higher risk of revision when UKA is performed at a low-volume hospital. , Cohort studies may allow better understanding of how specific implants perform at single-center institutions by high-volume surgeons. It was recently estimated that the lifetime revision risk of UKA when performed on a patient at age 55 years is only 15% and drops to 7% in a patient who is 75 years old at time of index procedure. Furthermore, a blinded review of radiographs in a sample of 107 revisions from multiple centers enrolled in the National Joint Registry between 2006 and 2010 found that only 20% were implanted for recommended indications, using appropriate surgical technique, and had a mechanical problem. Despite a threshold for revision that is much lower than for TKA, UKA still compares favorably in economic evaluations of health outcome and estimated cost. ,

The purpose of this chapter is to review potential complications of unicompartmental arthroplasty, including preoperative patient factors that can predispose to revision, intraoperative pitfalls to avoid, and common postoperative issues.

Preoperative Factors

Medical comorbidities that increase the risk of TKA complication outcomes likely influence UKA outcomes as well. Although there is less published on UKA perioperative medical optimization, there are data to suggest increased complications in smokers and patients with hypoalbuminemia. , Obesity is a growing problem worldwide, and cutoffs based on body mass index are controversial. While high failure rates in the obese population have been reported, , satisfactory survival and results have also been demonstrated. UKA has been shown to be a viable option in patients over 75 years , , as well as in younger, active patients. , Chondrocalcinosis does not influence functional outcome or survival following UKA. , Mild patellofemoral arthritis has not been shown to adversely affect outcomes. Moreover, the presence of lateral osteophytes does not preclude outstanding results following medial UKA at 15-year follow-up. Adequate outcomes have been achieved in ACL-deficient patients without subjective instability by reducing the posterior slope of the tibial component , and also in medial UKA with concomitant ACL reconstruction. , Strict contraindications are currently limited to active infection, inflammatory arthropathy, and ligamentous instability. , Additional preoperative factors that can negatively impact patient outcomes include partial thickness cartilage loss, large coronal deformity, significant flexion contracture, and previous high tibial osteotomy.

Ligamentous Instability

Improved satisfaction with UKA when compared with TKA is presumably related to near-normal knee kinematics. Functional instability secondary to a lack of cruciate and/or collateral ligaments predictably results in poor outcomes and early failure. This has been verified using validated fine-element models and intuitively makes sense. In select cases, the slope of the tibial component can be adjusted to compensate for a deficient ACL or an ACL reconstruction can be performed ( Fig. 12.2 ). However, there are no published data on staged or simultaneous collateral ligament reconstruction. A coronal plane deformity that is overcorrectable past neutral is either ligamentously deficient or has contralateral compartment arthritis, causing pseudolaxity. Both circumstances predispose to early failure and are contraindications to UKA.

Fig. 12.2

Anterior cruciate ligament (ACL) tibial tunnel is slightly lateral and more vertical than typical to accommodate the tibial baseplate during concomitant reconstruction. In the stable knee without an ACL, the tibial slope is reduced, minimizing anterior translation of the tibia.

Partial-Thickness Cartilage Loss

Patients with partial-thickness cartilage loss can have a positive clinical result but are subject to significantly higher revision rates and prone to worse functional outcomes. The presence of full-thickness cartilage loss and/or subchondral edema on magnetic resonance imaging (MRI) do not provide additional prognostic information. Abnormal pathology in adjacent compartments is overestimated by MRI and does not influence outcomes when clinical and radiographic criteria are met. UKA appears to be reserved for patients with bone-on-bone arthritis identified by plain radiographs ( Fig. 12.3 ).

Fig. 12.3

Varus stress radiographs demonstrating partial-thickness cartilage loss in the right knee and bone-on-bone arthritis in the left knee.

Coronal Deformity

With the goal in mind of recreating predisease knee alignment, the optimal mechanical alignment for a medial UKA is likely slightly varus. , Postoperative valgus alignment induces lateral compartment disease progression and excessive varus predisposes to component failure. The majority of knees with 10 degrees or less of mechanical varus on standing hip-to-ankle radiographs are correctable to less than 3 degrees of varus, and the correctability of alignment is improved with the removal of osteophytes intraoperatively. Improved function and quality of life with UKA have been demonstrated in patients with deformities >15 degrees, but there are limited data on this population. By limiting UKA to patients with <15 degrees of coronal deformity, you are more likely to achieve satisfactory results without risking early failure due to undercorrection.

Flexion Contracture

Recent evidence indicates that a flexion contracture of up to 20 degrees may not be a strict contraindication to UKA. However, a fixed flexion deformity after UKA of greater than 10 degrees is associated with worse functional outcomes , and the ability to correct flexion contracture during the resurfacing of a single compartment is limited. For this reason, UKA should not routinely be undertaken in patients with >10 degrees flexion contracture preoperatively.

Previous High Tibial Osteotomy

UKA in a patient with previous high tibial osteotomy (HTO) is not strictly prohibited but should be undertaken with caution. , HTO is predominantly done to correct varus deformities in young active patients. The goal of HTO is to shift the distribution of force away from the affected medial compartment. When done properly, the weight-bearing axis is overcorrected into gentle valgus. As mentioned previously, optimal alignment for a medial UKA is neutral or slightly varus. Therefore, a patient with persistent neutral or varus alignment following HTO with isolated medial compartment disease could be considered a candidate, but valgus mechanical alignment should be avoided.

Intraoperative Complications

One of the appealing features of UKA is the simple, reproducible nature of the operation. With less exposure and fewer bony cuts required, accurate performance of each step of the operation becomes crucial to success. Appropriate femoral resection is contingent on an accurate tibial cut. Maintained integrity of the unaffected compartments and ligamentous structures is of vital importance. The goal is to restore ligament tension and normal kinematics. Therefore, soft-tissue releases are not recommended. Careful measured resection and precise component positioning optimize balance, minimize early failure, and reduce the need for revision.

Cartilage Injury

Unlike during TKA, when sharp dissection or electrocautery can be utilized with impunity, care should be taken during exposure to avoid damage to the intact cartilage at the superior aspect of the arthrotomy. All compartments of the knee should be inspected to confirm that moving forward with UKA is suitable. Full-thickness cartilage loss on the lateral side of the patellofemoral joint may compromise ability to matriculate stairs, but moderate medial and central patellofemoral arthrosis can be overlooked. , ,

Iatrogenic Collateral Ligament Injury

Appropriate collateral ligament tension is critical for natural kinematics and stability. In addition to being the primary stabilizer to valgus stress at 30 degrees of knee flexion, the medial collateral ligament (MCL) is a secondary restraint to tibial external rotation and translation. Similarly, the lateral collateral ligament (LCL) is the primary restraint to varus stress at 30 degrees of knee flexion and a secondary restraint to posterolateral rotation. Too much tension on the MCL or LCL overloads the contralateral compartment and can lead to early failure. Too little tension creates laxity, allowing for translation in a fixed-bearing construct and putting a mobile-bearing implant at risk for dislocation. Accordingly, the surgeon must be mindful during exposure, meniscus removal, and bony resection to protect the collateral ligaments. Iatrogenic injury to a collateral ligament during UKA resulting in laxity warrants conversion to TKA with attempted repair and consideration of constrained condylar knee prosthesis.

Aberrant Tibial Resection

The tibial cut is the first and most important bony resection, affecting both the flexion and extension gaps. An extramedullary tibial cutting guide is placed parallel to the long axis of the tibia in the coronal plane and matching the tibia’s native slope in the sagittal plane (up to 7 degrees). Extreme varus or valgus resection may compromise mechanical limb alignment, overloading the replaced or unaffected compartment and resulting in early polyethylene wear, subsidence, fracture, or disease progression with increased revision rates. Too little slope will produce an asymmetrically tight flexion gap. Excessive slope can lead to loose flexion gap, subjective instability, posterior collapse, and premature failure.

A conservative tibial resection is preferred, typically 1 to 2 mm below the arthritic surface, to ensure that the implant is resting on maximal metaphyseal bone with a large cortical rim. Lateral compartment degeneration is often associated with bone loss and aggressive bony resection may necessitate a larger polyethylene insert than manufactured in a given implant system. This potential issue is more pronounced in the lateral compartment secondary to added intrinsic laxity. An inadvertently thick resection may necessitate conversion to TKA for appropriate ligament tension and balance.

The tibial resection is completed with a vertical cut. On the medial side, the vertical limb is made immediately adjacent to the ACL, at the apex of the medial tibial eminence and parallel to the medial wall of the intercondylar notch. This maximizes the size of the tibial component that can be utilized without overhang. In contrast, the lateral tibial plateau is wider medial to lateral, and the implant is more susceptible to overhang in the anteroposterior direction. Consequently, it is not necessary to border the cruciate ligaments, compartment and it can be helpful to remove osteophytes from the medial aspect of lateral femoral condyle prior to making the sagittal cut. Additionally, the vertical cut in the lateral compartment should be approximately 10 to 15 degrees internally rotated from the midsagittal plane of the tibia to allow for a screw home mechanism.

Care should be taken to avoid injury to the cruciate ligaments centrally, the collateral ligaments peripherally, and breaching the posterior tibial cortex distally. Cruciate and collateral ligament integrity is imperative for a normally functioning UKA, and past-pointing with the sagittal saw cut distally can predispose to medial tibial plateau fracture postoperatively ( Fig. 12.4 ). The ACL can be directly injured during the sagittal cut or undermined by excursion of the saw blade during horizontal resection. Many systems offer a pin hole and vertical slot in the extramedullary guide to minimize risk to the ACL and posterior tibial cortex. The slot guides trajectory of the reciprocating saw blade, and the pin acts as a saw stop to prevent undermining of the ACL and violation of the posterior tibial cortex. As previously indicated, retractors should be maintained between the collateral ligaments and saw blades at all times.

Fig. 12.4

An extended sagittal cut increases the risk of periprosthetic fracture following unicompartmental knee arthroplasty.

Component Malposition

Component malposition is likely an underlying factor contributing to increased revision rates of low-usage and low-volume surgeons. Proper component position begins with appropriate sizing. Tibial coverage should be maximized without overhang to minimize the risk of subsidence and stress reaction or fracture postoperatively. An optimally implanted tibial component would be flush with all edges of the cut tibial surface, but this is not always possible. Overhang gives rise to impingement on peripheral soft tissues, and underhang puts increased stress on the underlying bony surface , ( Fig. 12.5 ). When forced to compromise, cortical coverage to prevent subsidence is recommended posteriorly on the medial side and anteriorly on the lateral side. Conversely, impingement of the MCL medially and popliteus posterolaterally should be avoided.

Fig. 12.5

A, Lateral film showing excessive posteromedial tibial overhang. B, Merchant radiograph demonstrating patellar impingement from oversized lateral unicompartmental knee arthroplasty femoral component.

Axial alignment is also important, specifically in mobile bearing designs. If the tibial component is placed in significant external rotation (or too far medially), the mobile bearing can impinge on the lateral wall of the tibial component during flexion. , Retrospective analysis has shown that fixed-bearing implants better tolerate suboptimal rotation of the tibial component.

A perfectly positioned femoral component balances the flexion gap and maximizes bony coverage without patellar impingement. A fixed-bearing femoral component should be sized such that 1 to 2 mm of exposed bone remains between the anterior medial edge of the component and the cartilage tidemark in order to avoid patellar impingement. The mobile-bearing design is spherical, and the anterior border is positioned below the anterior femoral bone to mitigate the risk of patellar impingement. In fixed-bearing designs, the component should be biased laterally in an effort to articulate centrally on the tibia and avoid edge loading. In mobile-bearing designs, the component is placed in the central one-third of the femoral condyle. The spherical design of mobile-bearing femoral components and round-on-flat interface of fixed-bearing implants permits some latitude with respect to femoral component rotational alignment.

Postoperative Complications

When compared with TKA, UKA is associated with a lower occurrence of readmission, complications, and mortality. , Nevertheless, complications are inevitable. The following are common complications specific to UKA, likely origins, and suggestions for how to deal with them.

Bearing Dislocation

Mobile-bearing dislocation is a complication unique to mobile-bearing designs ( Fig. 12.6 ), with an incidence between 1.5% and 3.6%. , The causes of bearing dislocation include component malposition, flexion-extension gap imbalance, MCL injury/laxity, and bearing impingement. Most dislocations occur within 2 years of the index operation, and the vast majority are related to component malposition and gap imbalance. A meta-analysis comparing Asian and Western patients showed equivalent reoperation rates, but reoperation for bearing dislocation more commonly in Asian patients. Bearing dislocation can be treated with bearing exchange, revision UKA, or conversion to TKA based on the cause for dislocation. In a series that included 52 bearing exchanges, 25% redislocated at an average of 15 months following exchange.

Fig. 12.6

Example of posteriorly dislocated mobile bearing following medial unicompartmental knee arthroplasty.

Aseptic Loosening

Aseptic loosening accounts for 23% to 44% of failures and is the most common early failure mode. , Poor cement technique is difficult to quantify, but undoubtedly plays a role. Removing potential for cementation errors is a proposed advantage of cementless implant designs. Younger age, overweight, and varus deformity have been suggested as possible risk factors for mechanical failure. , Aseptic loosening is less common in the femoral than the tibial component. Overcorrection of deformity, malalignment, ACL deficiency, and excessive tibial slope can contribute to loosening. Altering the joint line also corresponds to decreased prosthesis survival. Lowering the joint puts increased stress on the UKA components and can lead to mechanical failure. Monoblock all-polyethylene tibial implants have been associated with a risk of early failure; metal-backed components are typically preferred. Recent systematic reviews indicate that mobile-bearing components may demonstrate slightly higher rates of aseptic loosening. , Revision UKA may be considered in select situations, but mechanical loosening is customarily addressed by conversion to TKA.

Progression of Arthritis

Progression of arthritis accounts for 14% to 29% of UKA failures. It is the most common failure mode of lateral UKA , and second only to aseptic loosening in medial UKA. Symptomatic adjacent compartment arthritis was responsible for 38% and 40% of midterm (5–10 years) and late failures (>10 years) according to a medial UKA meta-analysis. Overcorrection of the mechanical axis may cause degenerative changes in the contralateral compartment , , ( Fig. 12.7 ). Additionally, patient factors presumably contribute. Younger, obese, and more active patients may be at higher risk of developing adjacent compartment disease.

Fig. 12.7

A, Postoperative films showing neutral alignment. B, Subsequent rapid progression of arthritis in the contralateral compartment 3 years following surgery.

The clinical significance of patellofemoral joint degeneration is controversial. Recent literature has shown no difference in reoperation rates or functional outcomes in patients with documented patellofemoral arthritis without bone loss and grooving of the lateral facet. , , Degeneration of the patellofemoral joint can emerge in the presence of an oversized femoral component. This complication is more common with lateral UKA. Remarkably, patellofemoral congruence has been shown to improve following fixed-bearing medial UKA. Progression of arthritis should be treated with UKA of the newly symptomatic compartment or conversion to TKA.

Polyethylene Wear

Polyethylene wear accounts for 4% to 12% of UKA failures. , , , Early cases of catastrophic failure have been reported, but wear usually presents as a late mode of failure. , Polyethylene wear is more common in fixed-bearing designs. , Technical factors that contribute to wear include undercorrection of deformity and component malposition. , Design-related factors associated with polyethylene wear include lack of design conformity, polyethylene thickness less than 6 mm, prolonged shelf life, and flaws in the manufacturing/sterilization. Osteolysis of polyethylene debris can cause an osteolytic reaction at the bone-implant interface affecting implant alignment and stability. Progression of arthritis may cause attenuation of the ACL, leading to increased translation and concentration of force over the thinnest portion of the polyethylene liner at the peripheral aspect of the tibial component. Changes in manufacturing and sterilization of polyethylene may decrease the prevalence of this complication in the future. Isolated liner exchange has been shown to be a useful treatment option in a well-fixed, metal-backed, fixed-bearing UKA ( Fig. 12.8 ). Depending on progression of arthritis in other compartments and stability of implants, revision to TKA is routinely justified.

Fig. 12.8

A, Preoperative anteroposterior (AP) radiograph showing a significant decrease in polyethylene (PE) thickness with well-positioned and well-fixed components and without osteolysis. B, AP radiograph 6 years after isolated PE liner exchange are showing that the components are still well fixed and well aligned, without evidence of loosening or osteolysis.


Infection following UKA is rare, occurring at a rate of <1.0%. While this rate is slightly lower than that of TKA, UKA infections are peculiar in that they involve native cartilage and a prosthesis. The diagnostic workup for UKA infection parallels that of TKA with preoperative laboratory studies used to guide potential aspiration. Alternative thresholds for diagnosis of infection have been proposed based on a study of 259 revision UKAs with 28 infections. The optimal cutoff values were 27 mm/h for erythrocyte sedimentation rate, 14 mg/L for the C-reactive protein, 6200 cells/ μ L for the synovial fluid white blood cell (WBC) count, and 60% for the differential. Inflammatory marker limits are comparable to TKA thresholds, but modified synovial WBC count is presumptively linked to unresurfaced compartments. Staphylococci are most often the responsible organism. Group B Streptococcus , E. coli , P. acnes , and Pseudomonas have also been reported. Patients typically present with an acute infection (<4 weeks) and are treated with debridement, antibiotics, and implant retention. Outcomes are not as positive as one might expect, with survivorship free from all-cause revision at 5 years of only 49% to 55%. , Synovectomy with one-stage conversion to TKA and 3 months of antibiotics has been proposed as an alternative to two-stage conversion to TKA with good functional outcomes and no infection recurrence at early follow-up. The prevalence of poor outcomes should be addressed with patients, and surgeons should follow infections closely for aseptic causes of failure in addition to considering early and aggressive treatment with two-stage exchange.

Periprosthetic Fracture

Periprosthetic fracture following UKA is an uncommon but important complication that orthopaedic surgeons may be able to prevent and should be prepared to treat. With a reported incidence of 0.3% to 2.2%, there is a paucity of literature analyzing risk factors and the majority of reported fractures involve the medial tibial plateau. , , , Patients with poor bone quality are at risk for periprosthetic fracture in general. Fractures have also been associated with use of multiple pin holes to affix the tibial cutting guide, the vertical tibial cut, the resection depth, and tibial component size and position. , A myriad of pin holes placed in the proximal tibia to secure the extramedullary guide may result in a stress riser. The sagittal tibial cut can breach the posterior cortex and increase the risk of periprosthetic fracture. Similarly, preparation of the tibia with a gouge, used in some systems, may also create a stress riser if the posterior tibial cortex is breached. A biomechanical model found that medial tibial resection greater than 6 mm may increase the risk of periprosthetic fracture after medial UKA. An oversized component with significant posterior overhang results in force on the nonsupported portion of the tibial tray in weight-bearing flexion. Alternatively, an undersized and medialized tibial component results in increased stress on a smaller and eccentric region of the tibial plateau.

Periprosthetic femoral fractures may occur but happen with even less frequency than tibial plateau fractures. The trajectory of impaction during femoral component placement should be slightly anterior, in line with the femoral condyle. A posterior directed impaction places a shear force on the condyle and can generate a periprosthetic fracture.

Management of a periprosthetic fracture is based on the patient, fracture location, degree of displacement, and stability of the components. Nonoperative treatment can be successful in lower-demand, elderly patients with nondisplaced fractures and stable components. Surgical intervention is typically recommended in displaced fractures, but it is paramount to determine the stability of the UKA prosthesis. Open reduction internal fixation with a buttress plate is recommended for a displaced fracture with stable components , ( Fig. 12.9 ). Patients with a fracture around a UKA with a loose prosthesis must be revised. Conversion to TKA can be a technically complex operation requiring stems, bone graft, augments, and revision components.

Jun 18, 2022 | Posted by in ORTHOPEDIC | Comments Off on Unicompartmental Knee Arthroplasty

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