Unicompartmental knee arthroplasty versus total knee arthroplasty
When should a surgeon opt for unicompartmental knee arthroplasty (UKA) versus total knee arthroplasty (TKA) to treat knee osteoarthritis (OA)? UKA may be associated with higher revision rates but there are situations when UKA offers benefits over TKA. Should orthopedic surgeons make efforts to increase the number of UKAs they perform?
Knee OA is a common problem. There is a 50% lifetime risk of developing symptomatic arthritis in the knee . In the US alone, it was estimated in 2016 that close to 14 million people were afflicted with symptomatic knee OA, and there is a growing trend for this condition to develop in younger persons (< 65 years of age) .
The associated knee pain, aching, loss of range of motion (ROM) and function, as well as stiffness are all symptoms driving the increasing arthroplasty rates, with predictions of “a four-fold demand for knee arthroplasty in OECD countries by 2030.”  Coupled with risk factors such as obesity, gender, injury, and genetics, [4, 5] knee arthritis has been on orthopedic surgeons’ radar for many years…and will continue to be so.
Knee arthroplasty is an accepted surgical treatment to address knee OA. Ideally, patients, together with their surgeons, consider treatment options and prioritized patient-specific considerations, to make fully informed decisions. When asked, patients take into account how fast they could recover and return to work, the potential for revision and complications, functional outcomes, and mortality likelihood [6–8]. It has also been suggested that patients also consider length of the surgical scar when making treatment modality decisions .
Typically, depending on the location and progression of the OA, patient characteristics such as age, comorbidities, activity level, and the condition of the anterior cruciate ligament (ACL), a surgeon is presented with several treatment options after conservative management has been exhausted . Simply put, the presence of single compartment (unicondylar) OA is the primary indicator that UKA should at least be considered,  together with other factors; this is a special subset of OA patients . Approximately a third of knee OA patients will have the disease confined to a single compartment .
This article compares UKA and TKA and asks, “In which situations would you opt for these procedures?” Caution: This decision may not be as straightforward as it appears; both UKA and TKA are accepted and established treatment options for OA of the knee [9, 14, 15].
Jean-Noël Argenson, Professor and Chairman of the University Hospital of Marseille’s Orthopaedics Department, France, explains that, “For many years, TKA has been considered the standard of surgical treatment for knee OA due to demonstrated reproducibility and effectiveness for relieving pain and restoring function; however UKA has increased in popularity over the last twenty years as shown in national orthopedic registries . The indications for UKA are relatively strict with fewer indications compared to TKA since the OA should be limited to one femorotibial compartment of the knee with intact ACL.”
Lateral or medial compartment: Which is more common in UKA?
The lateral compartment is usually involved in 10% of unicompartmental femorotibial arthroplasty cases due to the natural history of OA preferentially loading the medial femorotibial compartment and the higher proportion of “knees in varus” among the general population . Both the anatomical and the biomechanical characteristics are different in each of the knee femorotibial compartments, and similar surgical treatment may not give reproducible results when applied to a different compartment [18, 19]. Jean-Noël Argenson notes that, “UKA in the lateral compartment has been described as technically more challenging and is performed ten times less frequently than medial UKA,” for the reasons explained above.
Table 1. A systematic review of 19 cohort studies and seven registry-based studies that reported combined medial and lateral survivorship indicated no difference in the survivorship of medial and lateral UKA. Based on information in Source: van der List JP, McDonald LS, Pearle AD. Systematic review of medial versus lateral survivorship in unicompartmental knee arthroplasty. Knee. 2015 Dec;22(6):454–460.
“However, published studies have suggested that lateral UKA is a reasonable alternative to TKA for isolated lateral femorotibial compartment disease. The mid- and long-term results of lateral UKA have shown satisfying clinical and radiological results. The specific anatomical and biomechanical characteristics of the lateral compartment should be accommodated at the time of surgery when performing lateral UKA. Improvements in patient selection and surgical ancillaries over time have also decreased the failure rate, allowing similar survival for medial and lateral UKA,” says Argenson.
Treatment options: TKA or UKA?
UKA offers an option to TKA that potentially allows for a more natural feeling knee as more of the native anatomy and kinematics are retained  and this may prolong the time until a full knee replacement is needed . However, these decisions are rarely simple and there are many considerations a surgeon and their patient must weigh when deciding whether UKA is an acceptable option. Danford et al found patient-reported outcomes (PROs) between UKA and TKA to be similar .
Figure 2. Total knee arthroplasty (TKA) replaces the whole knee joint with prosthetic components (left image), while unicompartmental knee arthroplasty (UKA) is confined to a single compartment of the knee—either the medial (shown) or lateral compartment (right image).
Jean-Noël Argenson states that, “In the case of symptomatic OA with documented bone-on-bone disease severe enough to warrant consideration for TKA radiographically and clinically, and after failure of local treatment (such as injections), UKA should be considered over TKA. This will preserve the cruciate ligaments and the intact cartilage and offers the patient a quick rehabilitation protocol. Additionally, if revision is necessary, a UKA revision surgery will be less difficult than a TKA revision in terms of operative time and blood loss, and use of a standard postero-stabilized implant is usually possible .”
A 2009 study published in the Lancet discovered that despite candidacy for UKA sitting around 47.6% of knee OA patients, only 8–15% of these patients received them, the rest received TKA . Why is this the case? There is controversy around patient selection for UKA,  conflicting contraindications against performing UKA, and surgeon UKA procedure volume also plays a role.
Certainly, when OA has infiltrated more than one compartment of the knee , or if there is chondrocalcinosis present , TKA could be the better choice. Jean-Noël Argenson also reminds us that in cases of rheumatoid arthritis, TKA is required. But what motivates surgeons to select TKA when only one compartment is involved?
Table 2. Pros and cons of UKA and TKA. Used under CC BY 4.0 license. Source: Mortensen JF, Rasmussen LE, Østgaard SE, et al. Randomized clinical trial of medial unicompartmentel versus total knee arthroplasty for anteromedial tibio-femoral osteoarthritis. The study-protocol. BMC Musculoskelet Disord. 2019 Mar 20;20(1):119.
Kozinn and Scott contraindications
According to Kozinn and Scott’s 1989 publication on UKA, a narrow range of patients are appropriate for the procedure . They suggested that patients under 60 years of age, with a weight over 82 kg, having exposed bone in the patellofemoral compartment, or are physically active/heavy laborers should not receive UKA . This criteria excludes a lot of knees. “Despite several studies confirming the value of such limited indications both for young patients  and for older patients , there is still existing controversy in the orthopedic community regarding the respective use of UKA and TKA [30, 31]”, says Jean-Noël Argenson.
However, the Kozinn and Scott contraindications were based on fixed-bearing implants and for these, damage to the patellofemoral joint, obesity, age, and high activity levels are still suggested as good contraindications . Indeed, “fixed bearings have many advantages. The surgical technique is easier and you can use it on the lateral side of the knee where it is much more dangerous to use a mobile bearing, and therefore use the same device or system on any patient,” noted Emmanuel Thienpont, MD, MBA, in an Orthopedics Today (2015) article .
In the same article, David Murray, MD, FRCS, from the Nuffield Orthopaedic Centre in Oxford, said, “If surgeons adhere to the Kozinn and Scott contraindications, then only 5% to 10% of patients are appropriate, whereas if you ignore them, then 20% to 50% are appropriate.”
David Murray stated that his center ignores these contraindications: “We have always had the philosophy that if a patient has anteromedial osteoarthritis, we will do a mobile-bearing UKA. We have therefore operated on numerous cases with the Kozinn and Scott contraindications and when we have reviewed our long-term data we have found that not one of these contraindications compromises the results ."
Table 3. Indications for UKA remain controversial. ACL: anterior cruciate ligament; BMI: body mass index; UKA: unicompartmental knee arthroplasty. Source: hipandkneebook.com; by Bert Parcells.
This is echoed by Jean-Noël Argenson who says that, “Thirty years after Kozinn and Scott’s 1989 publication, a consensus was made to redefine the concept of UKA with a new and updated list of indications and contraindications , supporting the current use of UKA over TKA in these revised indications.”
Hamilton et al evaluated the outcomes at a mean follow-up of ten years in contraindicated patients who received mobile-bearing UKA and found them to be doing the same or better than the patients with no contraindications—including the young, heavy, highly active males. In fact, the contraindicated knees had fewer poorer outcomes and significantly higher American Knee Society (AKS) scores. At 15 years follow-up, implant survival was the same between the groups. The authors concluded that mobile-bearing UKA implants should not be subject to the Kozinn and Scott contraindications and that the Oxford UKA indications, which are based on pathoanatomy, were preferred patient selection criteria for this type of implant .
Patient selection: Oxford indications
When Goodfellow et al suggested patient selection criteria for UKA,  they were working solely with the Oxford mobile bearing implant, however, the indications put forward represented a “paradigm shift” when compared to what had traditionally been used to select appropriate patients .
Pathoanatomical indications for UKA in patients with anteromedial osteoarthritis and spontaneous osteonecrosis of the knee are:[24, 25, 36–38]
- Bone-on-bone OA in the medial compartment
- Retained full thickness of lateral cartilage (viewed on valgus stress x-ray)
- Functionally normal medial collateral ligament (MCL) and ACL*
- Note: patellofemoral joint status is not considered (except in cases of bone loss and lateral grooving)
- Frontal deformity < 15°
- Flexion contracture < 15°
- Absence of inflammation-caused OA
* Note: Mancuso et al suggest a treatment algorithm for when to consider UKA vs TKA in medial OA and ACL-deficient knees. They identify the combination of UKA with ACL repair as “technically demanding” but feasible. This is supported by Suter et al’s findings that ACL deficiency may not always be a contraindication.
Should a patient’s age, body mass index, or activity level preclude them from a UKA?
In short, the answer is no. Studies are finding that age, body mass index (BMI), and activity levels are not necessarily indicative of the success of a UKA, especially when the above list of anatomical indications are weighted more heavily in decision-making . To reiterate, UKA is not an appropriate procedure for single compartment OA from knee degeneration related to rheumatoid (inflammatory) arthritis, cases with significant knee stiffness, or in some cases with ligament damage .
A patient’s age does not seem to influence UKA outcomes . Indeed, patients around age 50 are “considered ideal candidates” and perform better . On the other end of the age spectrum, a retrospective study of 276 patients over the age of 75 who received TKA (170 patients) and UKA (106 patients) for medial compartment arthritis determined that the UKA group had faster initial recovery and comparable complications and survivorship . The authors encouraged surgeons to consider this option in older patients that meet the anatomical criteria as it is less invasive.
Obesity has been linked to a higher likelihood of revision surgery in TKA . However, a rationale for UKA vs TKA based on BMI is not necessarily justified, as some studies have found. Molloy et al found no significant difference in 10-year UKA Oxford mobile-bearing implant survival between different BMI groups . In fact, they found the BMI 35+ group to benefit most from UKA.
Figure 3. Oxford Knee Score by time of follow-up and BMI group for patients who received a medial Oxford UKA. Used under CC BY 4.0 license. Source: Molloy J, Kennedy J, Jenkins C, et al. Obesity should not be considered a contraindication to medial Oxford UKA: long-term patient-reported outcomes and implant survival in 1000 knees. Knee Surg Sports Traumatol Arthrosc. 2019 Jul;27(7):2259–2265.
This finding supports the 2018 published study comparing TKA (1'300 patients) and medial UKA (650 patients) outcomes in severely obese patients. Those who received medial UKA had fewer reoperations, deep infections, and perioperative complications. Their range of motion (ROM) and Knee Society Functional (KFS) scores were higher than the TKA group .
In UKA patients under age 65, improved functional outcomes, ROM, and return to activity were found to be higher than after TKA . Compared to TKA for medial OA, UKA patients are more likely to return to both low and high impact sports  with hiking, biking, and swimming most practiced after UKA surgery . In addition, Waldstein et al reported that although some UKA patients (2–9%) had decreased participation in sports after their procedure, 87–98% returned to activity. They also noted a significant increase in low-impact activities after UKA .
Figure 4. X-rays of a fixed bearing unicompartmental knee implant. Image in public domain. Source: MBq.
Revision rates influence surgeon procedure selection
Numerous studies [8, 45, 50–52] have identified that UKA has higher revision rates than TKA, especially in younger patients . When surgeons look to the collected evidence to inform their choice of procedure, revision rates are heavily weighted, particularly because this information is collected within National Registries and other registries, such in Sweden and Australia, notify surgeons with high UKA revision rates and “encourage them to […] rectify the situation” .
Murray et al mention the “vicious cycle” that develops for surgeons to self-limit UKA . “…surgeons, who are anxious about revision of UKA, have narrow indications and do small numbers, and then have a high revision rate. They will further narrow their indications and may eventually stop UKA completely.”
New Zealand Joint Registry (NZJR) data reveals a five times higher UKA revision rate compared to TKA six months postoperatively. For patients with an Oxford Knee Score (OKS) under 20, the TKA group had a 10% chance of revision while the UKA group’s was 60% . This very large difference could indicate a low threshold for revision, considering that UKA revision is straightforward when compared to a much more complex TKA revision .
Classifying combined PKAs
PKA is an umbrella term for several procedures—as a term it can refer to a UKA alone or a UKA in combination with another compartment. A standardized classification system would be handy and reduce confusion. Garner et al surveyed 230 orthopedic specialty registrars, surgeons, and biomechanical engineers to see what sub-terminology they favored .
|CPKA:Combined partial knee arthroplasty as the umbrella term|
Bi-UKA: Bi-unicondylar arthroplasty
BCA-M: Medial bicompartmental arthroplasty
BCA-L: Lateral bicompartmental arthroplasty
TCA: Tricompartmental arthroplasty
Generally, revision data is associated with implant failure, but this perception carries a bias because the reasons for revision vary and carry different magnitudes—this nuance is not necessarily captured in registry data. The Danish Joint Register has made efforts to avoid using revision rates solely as the basis of comparison of UKA and TKA by collecting information on a wide range of variables associated with UKA and other orthopedic knee procedures .
Why are UKA revision rates higher than in TKA?
“Higher reported frequencies of revision and reoperations with UKA may contribute to this debate of which procedure to select,” says Jean-Noël Argenson. “The issue has been identified using observational, registry-based studies , in one such example, the study showed that equivalent low clinical score UKA has a 60% chance to be revised compared to 10% for a TKA .”
Table 4. The revision rate sensitivity is different for the two different implants. Based on information in Source: Goodfellow JW, O'Connor JJ, Murray DW. A critique of revision rate as an outcome measure: re-interpretation of knee joint registry data. J Bone Joint Surg Br. 2010 Dec;92(12):1628–1631.
Perhaps the lower threshold reasoning mentioned above offers a solid cause. The most common reasons for UKA revisions are conversion to primary TKA, lateral UKA due to OA progression, and bearing replacement. TKA revisions are oftentimes more complex procedures in that they require “stems, wedges and stabilized components” . However, if failure is re-defined to include all re-operations and not just revisions, then the failure rate of UKA was found to be less than that of TKA .
OA patients’ knee(s) will require further attention as the disease advances over time. Seen through this lens, a revision for this reason is not necessarily a failure of a UKA but an inevitable progression in treatment . But there is another element alluded to above—a surgeon’s experience with the procedure that influences revision rates.
Increase the practice proportion of UKA to 20%?
Surgical caseload plays a big role in UKA revision rates. Surgeons performing small numbers of UKA have higher revision rates and tend to operate on younger healthier patients. When Liddle et al looked at data from the England and Wales National Joint Registry they concluded that high-volume UKA surgeons (> 10 cases per year, with revision rate flatlining at 30 cases) demonstrated revision rates equal to that of surgeons performing TKA . In another study, optimal usage of Oxford UKA was identified as 40–60% of arthroplasties and the number for fixed bearing implants was around 20% .
Figure 5. LOWESS (Locally Weighted Scatterplot Smoothing) curve demonstrating the effect of increasing caseload on revision rates following UKR and TKA (up to fifty cases). Source: Liddle AD, Pandit H, Judge A, et al. Effect of Surgical Caseload on Revision Rate Following Total and Unicompartmental Knee Replacement. J Bone Joint Surg Am. 2016 Jan 6;98(1):1–8. Used with permission.
According to Jean-Noël Argenson, there is a discussion whether a surgeon should expand their UKA indications in order to attain the 15% to 20% regular use to achieve success or failure rates comparable to the ones they can expect when using TKA.
“The main debate is centered on the state of the patellofemoral joint with apparently less restrictions when using mobile bearing than for fixed bearing UKA. But on the other hand, longer learning curve and less surgical tolerance is associated with mobile bearing UKA. When UKA indication was recently revisited by a group of knee society surgeons using either fixed or mobile bearing UKA, it was accepted that the only remaining contraindication for UKA was in cases of total cartilage loss on the lateral aspect of the patellofemoral joint ,” says Argenson.
Broadening the appropriate indications for UKA would allow surgeons to gain more experience in UKA. It should be noted that fixed and mobile bearings place different demands on a surgeon’s technical experience – mobile devices are more challenging to place correctly. but even with this under advisement it has been recommended that surgeons should aim for UKAs to be 20% or more of their practice [32, 62] or a minimum of 13 procedures per year .
Benefits of UKA
Critics of UKA argue that TKA should be the preference due to improved implant survivorship and decreased revision rates [8, 64]. Proponents of UKA claim improved functional outcomes, decreased complications, and improved cost-effectiveness compared to TKA in part due to lower postoperative use of healthcare services [8, 65].
When indicated, “UKA offers surgeons the potential to reduce patient length of stay, use a so-called minimally invasive technique, routinely perform the surgery in an ambulatory setting, and perform bilateral simultaneous UKA in an easier way compared to bilateral simultaneous TKA without increasing the complications rate, [66, 67]” says Jean-Noël Argenson.
“UKA has lower morbidity and mortality compared to TKA and in systematic review and metaanalysis evaluation it was clear that more patients did return to physical activities and sports after UKA than after TKA . There are better pain scores and function with UKA compared to TKA . Finally, the patient perception and satisfaction of the surgery at 5 years of follow-up is clearly in favor of UKA . The long-term survival reported for UKA compares equally to that usually reported for TKA .”
The benefits of UKA when compared to TKA should be discussed by patients and their surgeons when deciding between the two procedures [8, 11, 32, 56, 69]:
- Shorter hospital stays
- No difference in patient reported pain
- Higher functional patient reported outcome measures (PROMs)
- Lower risk of mortality
- Fewer venous thromboembolic and cardiac events
- Lower overall opioid usage
- Lower likelihood of readmission
- Shorter recovery time
Patient selection, based on pathoanatomical characteristics, but customized to the type of implant (fixed versus mobile bearing) is critical for partial knee arthroplasty success. UKA may be associated with higher revision rates than TKA but there are suggested mitigative measures for this. Using England and Wales National Joint Registry information, Liddle et al calculated that for every 100 fewer patients receiving TKA and instead having UKA, there would be one fewer death and three more reoperations within the first four years postsurgery . Jean-Noël Argenson reaffirms that, “For all types of surgery, training is a key aspect and regular use of a reproducible surgical technique is a factor of success for the procedure.”
Part 2 of this article series compares HTO to UKA. Are there indications that point to the use of HTO or UKA?
This series of articles was created with the support of the following specialists (in alphabetical order):
This issue was created by Word+Vision Media Productions, Switzerland
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