Unicondylar Knee Replacement DOES NOT Provide a Superior Functional Outcome to Total Knee Arthroplasty in the Young and Active Adult

Michel Malo, M.D., FRCSC
Montreal, QC

Arthritic disease in people less than 55 years old is increasingly common. Younger, active individuals often present to orthopaedic surgeons requiring treatment for knee osteoarthritis. Knee arthroplasty has been offered as a viable option to provide pain relief and improve function in the middle-aged patient1. According to the Canadian Joint Replacement Registry2, when age-gender-specific rates for joint replacements are examined, the most notable increases for knee replacement in the last decade occurred in the 45-to-54 age group (these have more than doubled for both males and females).

 

Unicompartmental knee arthroplasty (UKA) has been performed for more than 30 years with variable results. Over the last decade, there has been a renewed and increased interest in UKA generated by reports of improved results. Although UKA was originally reserved for more elderly and sedentary patients, with the evolution of the technique and implant design, and the growing confidence of better results, the inclusion criteria for UKA have progressively broadened to include younger and more active individuals. This has been a source of debate in recent years3,4, and there are many reasons to be concerned.

Long-term Successful Results of UKA are Closely Related to Specific Patient Selection
First of all, it is very important to point out that better results of UKA have been obtained using strict patient selection criteria. The classic criteria for UKA have been defined by Kozinn and Scott5, including specifically elderly and sedentary patients with isolated tibiofemoral unicompartmental osteoarthritis, correctible deformity, stable joint, and no eburnated bone in the patello-femoral articulation. Essentially all of the available survivorship studies for UKA showing long-term results comparable to total knee arthroplasty (TKA) adhered to a conventional group of aged patients 6-10.

There are only a limited number of studies reporting results of UKA in younger patients. The first reports emerged in the late 90's. Engh and McAuley11 have suggested UKA as an acceptable alternative for the young and high-demand patient. However, at that time, almost no substantial clinical results specifically addressed this issue. Their analysis was primarily an estimation based on information extracted from long-term outcome studies of UKA in a general patient population. They estimated the success rate of UKA in younger, active patients to be about 80% at ten years. Schai et al.12 reported the early results at the Brigham Hospital with the PFC system (Johnson & Johnson) in a small cohort of 28 patients with a mean age of 52 years (range, 37-60 years). Activity level analysis using the Tegner scale revealed only a slight increase from a preoperative mean of 2.3 to 2.7 points. Two (7%) femoral components were revised for loosening. They concluded that "UKA in middle-aged patients yields 2- to 6-year results competitive with osteotomy but inferior to TKA in terms of early reoperation rate." Tabor and Tabor13 reported data of 67 knees (58 patients) with minimum 5-year follow-up (mean, 9.7 years; range, 5-20 years) using a Marmor-style non-metal-backed cemented tibial component. The average age was 61 years (range, 41-80), and 31 knees were in patients younger than 60 years old at surgery. Overall survivorship was 91% at five years, 84% at ten years, and 79% at 15 years. Pennington et al.14 reviewed retrospectively 46 Miller-Galante (Zimmer) UKA in 41 patients of 60 years of age or younger and physically active. Survivorship was 92% at 11 years. The University of California (UCLA) activity-level average score was 6.6 ± 1.4 for the knees in which the original prosthesis had been retained and 7.3 ± 1.5 for those revised (3). More recently, a few other reports15,16 have shown favourable results of another system (Oxford phase III-Biomet) at short-term follow-up. One of these studies15 assessed, specifically, the sporting and physical activity level of patients following a UKA with a mean age of 66 years at review and at a mean follow-up time of 18 months (range, 4-46). They recorded a significant improvement in UCLA activity-level scores from 4.2 to 6.5, with 93% of patients successfully returning to their regular sporting and physical activities following surgery. Nevertheless the authors acknowledged the relatively short-term follow-up of their review and the necessity for "a more detailed study to evaluate the long-term effects of sporting activity on the Oxford UKA."

So, UKA might offer good, early performance benefits with near normal kinematics, quicker recovery and regained mobility, and accelerated return to physical activity. All of these make UKA an attractive treatment option. But, although the short-term results look promising in a younger group of patients, consistent long-term data are lacking. It is very questionable whether the long-term encouraging results of UKA in the conventionally selected patients could be reproduced in the younger, active patients. This has not yet been proven. As new indications are introduced, it is imperative to compare the outcomes with established standards. As indications for UKA expand to include younger and more active patients, there is reasonable concern that this could lead to more complications and failures, and poorer survival outcomes17 (Figure1).

malo1a malo_1b malo1c

Figure 1: A 51-year-old active female with a five-year postop painful cemented UKA. Anteroposterior, lateral and merchant views showing loosening of the tibial component and appearance of osteoarthritis in the patellofemoral and lateral compartment.

TKA Has Proved Its Durability in the Young Population
On the other hand, TKA has an excellent history of long-term successful functional outcome and survival in the young population. Gill et al18 published results on 68 knees performed by a single surgeon in 50 patients younger than 55 years old with osteoarthritis and rheumatoid arthritis at an average of 9.92 years follow-up. The function score in both subgroups improved significantly at the latest follow-up. Two knees were revised for aseptic loosening. Based on the cumulative survivorship rate of 96.5% at ten years, the authors concluded that cemented TKA in younger and more active patients can achieve outcomes comparable to those in the older population. The group of Insall19 reported the long-term results of 108 TKA using one implant in 84 patients younger than 55 years old (average, 51 years) who had either osteoarthritis or posttraumatic arthritis. The anticipated overall rate of survival was 94% at 18 years with revision of the femoral or tibial component as the end point, and 87% when patella revision or spacer exchange were included in the failures. The Tegner activity average score improved from 1.3 to 3.5 points. Duffy et al20 from the Mayo Clinic reviewed 74 consecutive TKA in 54 patients who were 55 years of age or younger (average age, 43 years) at a minimum follow-up of ten years (range, 10-17 years). The majority of patients had rheumatoid arthritis (64% of the total knee arthroplasties). Implant survivorship was estimated to be 99% at ten years and 95% at 15 years.

Evidence for UKA or TKA in treating the middle-aged patient is based on individual, non-comparative survivorship series for both types of prostheses. There are very few direct comparative studies between UKA and TKA in comparable patients21,22,23. The existing reports all provide only short-term clinical results and none of them are specific to a group of patients younger than 55 years. Some investigation has been done on the activity level following knee arthroplasty.24 Similarly, the current literature on this issue is limited, and there is no good-quality, objective, evidence-based information available.

National Joint Registries Are Valuable Sources of Data
The continuing contribution of data to national joint registries is a valuable aid in validating the current trends, particularly in knee survival after UKA or TKA. Register-based studies, despite some inherent limitations, provide valuable insights into the use and performance of a surgical procedure in a certain patient group.

Furnes et al25 reported the rates of failures of cemented UKA and TKA based on the Norwegian Arthroplasty Register. The ten-year survival probability of UKA (80.1%) was inferior to that of TKA (92%), with a rate of revision following UKA twice as high as that following TKA. The increased risk of revision following UKA was seen in all age-categories. UKA was associated with more revisions because of pain, aseptic loosening of the tibial and of the femoral component, and periprosthetic fracture. In a study just recently published26, relevant information is available specifically on patients younger than 55 years of age from the Swedish Knee Arthroplasty Register for the period 1998-2007. During the 10 years, the use of TKA in these patients increased fivefold and UKA increased threefold, but its use decreased in the last two years. The risk of revision increased in these patients and it was lower for TKA (9%) than for UKA (24%). The authors conclude that today, TKA is the preferred method for young osteoarthritic patients in Sweden. Similar powerful conclusions are observed in the Australian Registry27 concerning the outcome of UKA. There is a significant difference in the risk of revision depending on age: the risk of revision decreases with increasing age. The highest revision rate occurs in the under 55-age group with an 8-year cumulative revision rate of 18.0%. Another study28 reported data also of patients less than 55 years, but from a community joint registry over a 14-year period. Cemented TKA performed best, with a cumulative revision rate of 15.5%, compared to 32.3% in UKA patients.

UKA Is a Technically More Demanding Procedure
The current enthusiasm for minimally invasive surgery has been a contributing factor in the recent explosion of interest for UKA. Undoubtedly, UKA is a technically more demanding procedure than TKA, particularly when done with a minimally invasive approach, even though the evolution of instrument systems has allowed more precision in performing UKA. In fact, there are reports on higher rates of early complications and failures after a minimally invasive technique29,30. The learning curve is steep. Many pitfalls may occur, which could compromise the outcome, and attention to details is essential. Accurate component placement and limb alignment are critical to the long-term su ccess of the prosthesis (Figure 2).

malo_fig2

Figure 2: Malalignment of UKA femoral and tibial components.

A major issue with UKA is the limited experience most surgeons have with this procedure compared with TKA. The ideal criteria for UKA are encountered in only a small number of subjects. The usual percent of suitable candidates for UKA among all knee arthroplasties is between six and ten percent4,31. Infrequent performance of an arthroplasty may lead to poorer results, especially since UKA requires increased technical experience. It has been showed by Robertsson et al.32, based on the Swedish Arthroplasty Register, that the revision rate of UKA is affected by operating volume. Also, the long-term success and reliability of the procedure has been mainly reported by high-volume and experienced surgeons at specialized total joint centres. Thus, the widespread use of UKA is definitely an area of concern, especially in the general orthopaedic community where only a minimum number of cases are performed per year.

UKA procedure has been suggested by Scott and others as a time-buying operation before TKA. Ease of conversion of UKA to TKA has been debated, and controversy still exists. Advances in implant design and emphasis on preservation of bone stock during UKA may make revision to TKA less complicated, although this may be offset by potential severe UKA failures with greater damage in the younger, more active patient.

Conclusion
The painful osteoarthritic knee in the middle-aged population presents a therapeutic challenge. The younger and more active patient, often referred to as the 'new' patient, is difficult to treat. Needs are somewhat different and expectations are high: not only are these patients looking for an end to pain but also for a return to normal function. The care of this group age of patients should be individualized.

Undoubtedly, UKA offers early potential benefits, but concern exists regarding the durability and survival of uni-compartmental knee replacement in the younger and more active adult. Patient selection and surgical technique issues play a major role in the long-term functional outcome and longevity of UKA. The indications for UKA are specific, and strict adherence to them is predictive of successful results. UKA is a technically more challenging procedure and expertise and surgical experience is required. Until further well-designed research provides more information, caution is advised.

To date, TKA has shown to be an effective, more reliable and established option with consistent long-term results in the active, younger patients. Despite long-term survival of TKA in the middle-aged population, wear, aseptic loosening and osteolysis remain potential concerns, the same as those after UKA. The emergence of so-called modern high-performance implants and the recent development of improved biomaterials, notably, highly crossed-linked polyethylene and highly porous metals (trabecular metal33), provide hope for even better results in treating this specific group of patients.

References

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