Unicompartmental knee arthroplasty

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The Journal of Arthroplasty Vol. 19 No. 4 Suppl. 1 2004

Unicompartmental Knee Arthroplasty New Indications, More Complications? Kelly G. Vince, MD, and Leah T. Cyran, MD

Abstract: Despite mixed results with unicompartmental knee arthroplasty (UKA) in the 1970s, the UKA was established as a reliable procedure in the low-demand, elderly patient. Dependable results up to a decade prompted the idea that UKA may work equally well in the younger patient. Expanded indications for UKA are being evaluated: might higher failure rates and difficult revisions emerge from younger, more active patients? Key words: unicompartmental knee arthroplasty, new indications, long-term follow-up. © 2004 Elsevier Inc. All rights reserved.

A severe blow was dealt to the unicompartmental knee arthroplasty (UKA) in North America when it was abandoned at the Hospital for Special Surgery in New York. Surgeons there turned to the development of total knee arthroplasties after a poor experience with the unicondylar implant. In truth, the clinical evidence for this decision was not strong. Of 32 UKAs performed between 1972 and 1974 and followed up from 5 to 7 years, 10 were lost to follow-up, and 18 had concurrent patellectomies. The group was mixed: 17 were medial and 5 lateral, and the results were poor. At final follow-up evaluation, one knee was rated as excellent, 7 knees good, 4 knees fair, and 10 knees poor. Seven knees (28%) were converted to a bicondylar prosthesis [1]. Though other surgeons, such as Marmor in Los Angeles [2,3] had more experience and superior results, the New York stance exerted a strong influence on American surgeons.

By contrast, the UKA has long been favored at the Brigham Hospital in Boston, where classic indications were respected: older patients (age 70 and older) with pathology limited to a single compartment [4]. Eventually, failure from polyethylene wear occurred even in low-demand patients and necessitated revision arthroplasty at an advanced age [5,6]. Even advocates of the UKA acknowledged that total knee arthroplasty (TKA) was likely to provide lifelong function in the elderly and the role of the UKA in North America was devolving.

Limits of High Tibial Osteotomy Concurrently, the limitations of high tibial osteotomy (HTO) came under scrutiny. Limited pain relief (sometimes unacceptable), valgus alignment, and high rates of conversion to knee arthroplasty at 5 years were seen. The extent to which previous HTO can compromise TKA [7–9] though debated, emerged as a concern, and HTO also began to fall out of favor. Though the UKA was likely to fail at a higher rate than a TKA, it provided function, and pain relief superior to the HTO, without valgus limb alignment. Eventually, UKA failure was proposed as part of the lifelong plan for a younger patient, based largely on the belief that revision would be easier [10 –15]. UKA took precedence in some pa-

From the Keck School of Medicine, University of Southern California, Los Angeles, California. No benefits or funds were received in support of this study. Reprint requests: Kelly G. Vince, MD, Keck School of Medicine, University of Southern California, DEI 5th Floor, 1450 San Pablo Street, Los Angeles, CA 90033. © 2004 Elsevier Inc. All rights reserved. 0883-5403/04/1904-1004$30.00/0 doi:10.1016/j.arth.2004.02.022


10 The Journal of Arthroplasty Vol. 19 No. 4 Suppl. 1 June 2004 tients who might previously have been considered for HTO [16]. Not surprisingly, detractors of the UKA concept argued against the ease of revision [17] and the Swedish Knee Registry cautioned that the failed UKA should be revised to a total knee arthroplasty, not another UKA (even to treat arthritic deterioration in the contralateral compartment) [18]. A small study that directly compared the results of converting HTO or UKA to TKA identified higher wound and infection problems in the former and larger bone defects in the latter, without concluding which was preferable [19]. A new picture emerged from this debate. The results of UKA are functionally superior, and the failure rates comparable to HTO. Revision of the failed UKA may be easier than either a failed HTO or TKA. The difficulty of surgical technique for the UKA versus the HTO is debated but recovery from UKA is expeditious [20]. The UKA, recently in question for the older patient, is now being considered for the younger more active individual as an “interval” procedure, enabling deferral of the TKA. In a review that articulates this viewpoint, Deshmukh and Scott [16] stated that despite a history of UKA results with higher failure rates than TKA, 2 new long-term studies yield results comparable to TKA. They posited “that enhanced second-decade survivorship, and therefore, an expansion of the indications for UKA are possibilities.” Recent, accelerated interest in UKA for the sedentary and even more active younger individual led to a more recent piece, in which Scott has called for moderation [21]. What data can we bring to bear on the trend of “Yoonies for Youngsters?”

available (34 dead and 1 lost), 6 failures were seen (5.5%) [23]. At 10 years or longer, 44 of 143 arthroplasties (31%) were evaluated. The predicted survivorship was 97.7%. Twenty patients were evaluated at 14 years. The survivorship table predicts a survivorship of 76.9%. Both of these studies, with excellent retrieval rates, had been performed in the centers from which the designs originated; neither can be described truly as a long-term study given that the minimum and mean follow-up falls well short of a decade. In addition, both studies adhered to very conventional patient selection, with mean ages exceeding those for most series of TKA. In this important respect, it can be argued that both of these studies, although clearly reporting superb results, do not differ significantly from an established and venerable literature on the subject. An independent evaluation of 124 medial Oxford UKAs in Sweden with 37 patients who died and none lost at 10.1 to 15.6 years reported 6 revisions (6.87%) [24]. Another independent study, of the Marmor UKA experience at the University of Iowa, showed outstanding results in 48 arthroplasties at a minimum of 15 years [25]. Although age, weight, and activity level were not considered in the selection of patients, the average age at surgery was older, at 70.9 years (range, 51.1–93.6), and the average weight was 175 lb (range, 115–235). All of these studies reflect results in a very conventional group of aged patients. These studies entice us to offer the UKA to younger patients as a new venture, as yet, without vindication.

Results in the Younger Patient Long-Term Results in UKA Neither of the 2 long-term studies cited by Deshmukh and Scott has a minimum 10-year follow-up time. One evaluated 62 fixed bearing Miller-Galante UKAs (Zimmer, Warsaw, IN) at an average of 7.5 years (range, 6 –10) when 10 had died and 1 was lost to follow up [22]. Two (3.9%) repeat surgeries were performed, and the authors describe stringent selection criteria. This would not include either younger or more active patients. The other study evaluated 143 mobile bearing Oxford UKA, at a mean follow-up time of 7.6 years in patients with intact anterior cruciate ligaments. The manuscript does not specify the minimum follow-up time, but it is at least 2 years, with a maximum of 13.8 years. Although survivorship can be legitimately predicted at 10 years from this data, the study is not a minimum 10-year follow up. Of the 108 knees

Current studies, then, support UKA in older patients. Engh and McAuley [26] posed the question with regards to young or high-demand patients in 1999. However, few surgeons have implanted the UKA into enough “younger” patients to generate substantial data. Schai and Suh et al. [27], elaborating on the experience at the Brigham Hospital with the PFC system (Johnson & Johnson, Raynham, MA), considered UKA an alternative to osteotomy (Table 1). Tegner activity levels increased from a preoperative mean of 2.3 to 2.7, indicating something slightly better than a capability of walking on uneven ground but impossible to back pack or hike [28] (Table 2). Two (7.1%) femoral components came loose, and the investigators concluded that “UKA results (were) competitive with osteotomy but inferior to TKA with respect to revision (rates). In our enthusiasm for solutions to the ar-

Unicompartmental Arthroplasty • Vince and Cyran


Table 1. Unicompartmental Studies in Younger Patients Study Schai et al. [27] Tabor and Tabor [29] Pennington et al. [30]

Follow-Up Interval (y)

Knees With Follow-Up (n)

Mean Age (y)


Average Weight/BMI


Survivorship at 10 y

3.3 (2–6) 9.7 (5–20) 11 (5.6–13.8)

28 63 (31 ⱕ 60 y) 45

52 (37–60) 61 (41–80) 54 (35–60)


180 N/A 198/32

2 (7%) 11 (16%) 3 (6%)

N/A 84% 83% (ⱕ 60 y) 92%

Abbreviation: BMI, body mass index.

thritic knee in the younger active patient, we must remember these results and Tegner scores.” Tabor and Tabor [29] reported overall survivorship to be 91% at 5 years, 84% at 10 years, and 79% at 15 years (Table 1). The average age was 61 years (range, 41– 80), with 31 younger than 60 years old at surgery. Patients younger than 60 had slightly worse 5- and 10-year survival rates and slightly better 15-year survival and functional and knee scores than patients over 60. Obese patients had similar survival rates and knee scores compared with thinner patients. This study suggested that UKA can be safely performed in younger or obese patients and that good long-term results can be expected. No subgroup analysis was given regarding activity level or follow-up interval with respect to

Table 2. Tegner Activity Level Scale Level



Competitive sports: soccer, football, rugby (national elite) Competitive sports: soccer, football, rugby (lower divisions), ice hockey, wrestling, gymnastics, basketball Competitive sports: racquetball or bandy, squash or badminton, track and field athletics (jumping, etc.), down-hill skiing Competitive sports: tennis, running, motorcars speedway, handball Recreational sports: soccer, football, rugby, bandy, ice hockey, basketball, squash, racquetball, running Recreational sports: tennis and badminton, handball, racquetball, down-hill skiing, jogging at least 5 times per week Work: heavy labor (construction, etc.) Competitive sports: cycling, cross-country skiing, Recreational sports: jogging on uneven ground at least twice weekly Work: moderately heavy labor (eg, truck driving) Work: light labor (nursing) Work: light labor Walking on uneven ground possible, but impossible to back pack or hike Work: sedentary (secretarial) Sick leave or disability pension because of knee problems

9 8 7

6 5

4 3 2 1 0

NOTE. Data from Tegner and Lysolm [28].

the younger or older groups or the obese or thinner groups. Recently, Pennington and Swienckowski et al. [30] reported excellent results (Table 1) with activity levels assessed by the University of California (UCLA) scale [31] (Table 3). Patients achieved an average score of 6.5 ⫾ 1.4 (range, 4.0 –10.0) for knees in which the original prosthesis had been retained and 7.3 ⫾ 1.5 (6.0, 7.0, and 9.0) for those revised. The most recent literature shows promising results in somewhat younger patients with modest athletic levels. Studies do not yet support UKA for very athletic individuals.

Advantages and Appeal of the UKA The older literature on the UKA has been controversial, with good and less good results. The standard for comparison has always been the TKA. Recent studies have confirmed excellent results in conventionally selected patients at mid-term follow-up times. Will the promise of these results pertain to younger patients?

Table 3. UCLA Activity Assessment Class



Regularly participate in impact sports such as jogging, tennis, skiing, acrobatics, ballet, heavy labor, or backpacking Sometimes participate in impact sports Regularly participate in very active events, such as bowling or golf Regularly participate in active events, such as bicycling Regularly participate in moderate activities, such as swimming and unlimited housework or shopping Sometimes participate in moderate activities Regularly participate in mild activities, such as walking, limited housework, and limited shopping Sometimes participate in mild activities Mostly inactive, restricted to minimal activities of daily living Wholly inactive, dependent on others, cannot leave residence

9 8 7 6 5 4 3 2 1

NOTE. Data from Zahiri et al. [31].

12 The Journal of Arthroplasty Vol. 19 No. 4 Suppl. 1 June 2004 Researchers do not doubt that the UKA can provide excellent functional results and good durability. With the innovation of “minimally invasive surgery,” shorter hospital stays and perhaps lower infection rates may also be realized [32–34]. Will the rigors of a more demanding surgical technique and treatment of younger patients yield equally good results? Will, in short, more indications lead to more complications? The dearth of early complications in recent studies attests to the technical proficiency of the surgeons. Not all surgeons may be able to expect such results, even in the lower demand patient. Not only do we lack results in series of young patients, there is unresolved controversy regarding indications and surgical technique: the significance of patellofemoral arthropathy, the importance of the ACL and whether medial and lateral compartment UKA yield similar results. The debate on component position has not yet reached the degree of sophistication that characterizes the TKA literature. These variables may bear strongly on complication rates.

Complications and Limitations of UKA: Can We Still Expect Good Results? The classic indications have been strict [35]. Galante followed them, performing UKA only in patients with osteoarthritis or osteonecrosis, in whom the contralateral compartment was normal and the patellofemoral joint was involved only moderately (Outerbridge stage 2) [36]. All but one of the patients had an intact anterior cruciate ligament (ACL). If the indications become more liberal, can we expect the same degree of patient satisfaction and success? To what extent can involvement of the patellofemoral compartment be regarded as a (relative) contraindication to UKA? Some ardent proponents of UKA discount the importance of patellofemoral disease [23]. Will the results in 2 patients, one with and one without patellofemoral disease, be comparable? Do these surgeons deny that damage to hyaline cartilage in the patellofemoral joint can be a source of pain? Perhaps the improvement in tibial femoral pain is so significant that patients easily tolerate residual PF symptoms. To what extent can irregularities in tracking be tolerated? In a small study, it seemed that more (but not all) patients preferred their UKA to a contralateral TKA. This seems credible intuitively but has not been definitely substantiated [37]. Maintenance of maximal normal anatomy, though enhancing the “normal feel” of the knee does not

Fig. 1. (A) A 63-year-old woman with medial compartment osteoarthritis who initially had limited deformity. Within 3 months of surgery, medial tibial plateau fracture has displaced. Considerable amounts of bone are missing and there may be extensive avascular necrosis. Revision arthroplasty would be difficult. (B) Open reduction and internal fixation has been performed. This procedure is extensive, surgical, and involves wide exposure.

guarantee that the arthroplasty will necessarily relieve pain as expected.

Potential Problems Fracture Tibial fractures may occur during surgery, and 3 of 62 (4.8%) were reported by Berger et al. [22]. None led to failure, and one was treated with screw fixation intraoperatively. Some may require more extensive open reduction and internal fixation and others revision arthroplasty. These fractures might be avoidable by limiting the number and location of pin holes that are created in securing cutting instruments to the proximal tibia (Figs. 1 and 2) [38,39]. Lateral Compartment UKA and Bearing Dislocation The Oxford mobile bearing UKA has a 10% bearing dislocation rate as reported by the center where

Unicompartmental Arthroplasty • Vince and Cyran


Fig. 2. (A) Preoperative anteroposterior radiograph of an athletically active 66-year-old man, weighing 230 lb with a height of 6 feet, 1 inch. The deformity is correctible clinically. (B): One month postoperative radiograph with solid fixation and anatomic style tibial alignment. (C) Nine months after surgery with subsidence of tibial component. Aspirations were negative for infection.

it was developed. The problem was most prevalent in the lateral compartment: 6 of 46 cases (13%) [40]. This highlights the physiology of the knee, in which femoral rollback occurs to a greater extent in the lateral compartment [41]. The in-

Fig. 3. (A) Three month postoperative anteroposterior radiograph with a fixed bearing, all-polyethylene lateral, unicompartmental arthroplasty. The patient is a 44-year-old security guard with lateral compartment osteoarthritis. (B) Three-month postoperative lateral radiograph shows cemented, all-polyethylene tibial component in place. Note metal marker in the tibial component. (arrow). (C) Seven months after surgery, the tibial component has extruded into the infra patellar region.

creased excursion, the contribution of an intact meniscal complex to normal stability and the different articular geometry raise the question as to whether a prosthesis can be expected to perform equally well in both compartments (Fig. 3).

14 The Journal of Arthroplasty Vol. 19 No. 4 Suppl. 1 June 2004 uncemented fixation. The challenge of achieving bone ingrowth in TKA is compounded by the biomechanics of the UKA. Until significant improvements in uncemented technology are applied to the UKA, cemented fixation remains more predictable [45].


Fig. 4. A 55-year-old male baggage handler underwent unicompartmental arthroplasty 8 years previously. Note the extensive osteolysis as indicated by the arrows. This large defect was all contained and accordingly could be revised with particulate bone graft and stemmed components.

Breakage and Wear The problem of polyethylene wear has directed development of TKA materials, implants and surgical technique for over three decades. Much of this work is of benefit to UKA, however we have not necessarily applied much of it appropriately. Unacceptable wear rates have been reported for Porous Coated Anatomic (PCA), Microloc and Fibermesh UKA prostheses in one study, attributed to lack of conformity and resultant high concentration of loads in the polyethylene [42]. In a 4- to 8-year study of PCA UKA, Lindstrand et al. [43] expressed concern over the extent of wear in retrieved specimens and on radiographs. Osteolysis, though generally associated with total knee and hip arthroplasty, can result from polyethylene wear in the UKA [44]. The femoral component, smaller and sometimes thinner can break, especially if poorly supported by femoral bone (Figs. 4 and 5).

Fixation The appeal of UKA as a conservative bone sparing reconstruction in the younger active patient has led some surgeons in the past to implant them with

The current discussion on surgical technique for the UKA has focused on the exciting challenge of the limited surgical approach. The debate on details of component position is only developing. For example, some surgeons with extensive experience have long implanted the UKA in an anatomic position, meaning slight varus on the tibia. Excellent results published recently [22] emanate from a very mechanical position of the tibial component (at right angles to the long axis of the tibia). Though not anatomic, this facilitates congruence between the components in flexion, by making the choice of femoral component rotation easier. Although clinical [46] and finite element analysis [47] support the idea that tibial components should be oriented at right angles to the tibia as a means of decreasing force on the bone, there is no final opinion on the question. There is consensus however, that the surgical technique is more difficult than total knee arthroplasty and involves a significant “learning curve” (Fig. 6) [43].

Conclusions The recent recommendation of UKA for the younger patient is a promising strategy, but must be

Fig. 5. Failure of a unicondylar arthroplasty as a result of breakage of the femoral component (fracture indicated by the forceps). Evidence was also seen of arthritis having developed in the lateral and patellofemoral compartments.

Unicompartmental Arthroplasty • Vince and Cyran


Fig. 6. (A) A 79-yearold man with a satisfactory contralateral TKA. The alignment is near neutral and the lateral compartment appeared intact at the time of UKA. (B) Immediate posoperative medial UKA. Alignment has not been over corrected and lateral compartment is intact. (C) Just prior to revision, this radiograph shows bone on bone in the lateral compartment. At 9 months after the UKA, pain is unremitting, deformity is unacceptable, and dramatic cartilage space loss is seen in the lateral compartment.

regarded as a sound hypothesis rather than an “established” treatment. The UKA provides better pain relief than an HTO, without valgus deformity. Recovery and knee function may be superior to the TKA, but perhaps in part because of patient selection. Although a higher failure rate may be expected with the UKA, there may be more anatomy intact than in a failed TKA. It is not clear that this represents an “easy revision” or that the conversion will be less complicated than that of an HTO. Current clinical results reflect conservative indications for UKA. We may rightfully expect higher failure rates, perhaps with greater damage in the younger, more active patient. New, expanded indications for UKA may lead to more complications and perhaps more severe failures. It should however, also lead ultimately to greater understanding and superior results as we study our failures and devise new answers. Such is the path of progress.

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