Unicompartmental arthroplasty (UKA) is a partial knee replacement, it resurfaces either the medial (90%) or lateral(10%) compartment. UKA is becoming an increasingly popular procedure. In the 1990s, the high tibial osteotomy (HTO) was the gold standard procedure for treatment of unicompartmental arthritis, particularly in younger patients, while the UKA was avoided largely due to the stigma of high failure rate associated with the first generation of UKA implants. At the Mayo clinic for example there were over 8,000 total knees performed in the 1990s and only 3 UKA, yet that number increased 50x by 2003, while HTO procedures declined. In 2006, the medial UKA was the procedure of choice in 11% and 29% of surgeons for a 45 year old male or female (respectively) with unicompartmental arthritis. The overall number nationally continues to rise in the 21st century, whereby roughly 10-15% of patients are indicated for this procedure. However indications remain controversial.
The initial indications were delineated by Kozin & Scott[1, 2]: isolated medial compartmental DJD (no lateral compartment arthritis, only mild patellofemoral arthritis on merchant view), no lateral joint line tenderness, intact ACL (look at the wear pattern on lateral x-ray of the knee: posterior tibial bone loss indicates disrupted ACL because the tibia has shifted anteriorly without the ACL restraint, exposing the posterior tibia to contact stresses from the distal femoral condyles); noninflammatory arthroplasty; weight under 82 kg; correctable varus deformity (< 5° deformity); over 60 years old; flex contracture <5°; range of motion > 90°.
This criteria served as the foundation for patient selection for decades even though there isn’t good data to support many of the specifics (and the criteria is surprisingly specific with regards to degrees, pounds, years, etc.) Recent studies have questioned many of the established thresholds. Scott et al. reevaluated his guidelines in a series of papers in CORR [3, 4], amending the age restriction by suggesting that the UKA is an alternative to the HTO in the younger population, increasing the weight restriction to 90 kg, and increasing the flexion contracture to <15 °, and < 10° varus or 5° valgus deformity.
The Nuffield Criteria similarly encourages modifications to the initial exclusion criteria, and suggests that patellofemoral arthritis, weight, age, and activity level should not be contraindications.
Expanded indications for UKA has been further supported by Berend, Lombardi et al . They eliminated age, BMI, and activity level from the selection criteria without observing increased failure rates . They report age (+/- 60) had no effect on outcomes; BMI >40 no effect on survivorship and patellofemoral arthrosis (as determined by Altman scoring system) similarly did not effect revision rates.
The necessity of ACL function is also controversial . The ACL appears less important in fixed-bearing UKA compared to the Oxford mobile-bearing UKA, which requires more knee stability to prevent the mobile poly from dislocating (although studies show success with the Oxford UKA in the ACL-deficient knee as well). It is controversial whether the ACL is reliably intact in the arthritic knee. Studies suggest that 61% are intact in a knee with DJD , however, an aging knee becomes increasingly arthritic and the surrounding soft tissue envelop stiffens, and thus offers increased stability to the knee, reducing the role of the ACL. Furthermore, ACL function is probably not binary but rather its function diminishes with age as it becomes attenuated.
The exclusion of inflammatory arthritis is an important theoretic distinction in the progression of knee degeneration. Inflammatory arthritis creates a toxic environment to chondrocytes throughout the knee joint and therefore it is expected that all compartments of the knee will eventually develop arthritis. However, the natural history of primary osteoarthritis of the knee is less clear. If medial compartment arthritis is observed, will that arthritis progress to involve the patellofemoral joint and the lateral compartment? Is the medial compartment simply the first domino to fall, or can the progression be contained by addressing the medial compartment. Many argue that osteoarthritis can be contained. The conversion rate to TKA after a medial compartment UKA, due to arthritis progression, is less than 5% after 20 years. Why is this true, when tricompartmental arthritis is so common? One model for knee arthritis is a stepwise deterioration that begins with anterior-medial tibial wear (the focal contact point of the Medial Femoral Condyle, which moves very little during knee kinematics). The wear creates a varus deformity of the knee, which leads to increased stress on the ACL and meniscus, which leads to attenuation of these soft tissues and progression of arthritis to the posterior-medial tibia (the tibia subluxes anteriorly as the soft tissue restrains attenuate). As wear progresses, knee kinematics become deranged, patellofemoral wear worsens with abnormal tracking, and lateral compartment wear develops leading to tri-compartmental arthritis. Proponents of this theory believe the UKA is a great procedure because it stops disease progression, nips the arthritis at the bud, and preserves most of the native knee architecture.
We discussed the history of TKA above and highlighted two theories behind TKA design: Anatomic and Mechanical. Anatomic Designs preserve knee ligaments and allow this soft tissue to guide knee kinematics. The more popular TKA design, the Mechanical Design, sacrifices the cruciate ligaments and adjusts the collateral ligament tension to allow the implant’s surface geometry to guide knee motion.
The UKA is an Anatomic design. The cruciate ligaments are preserved, the collateral ligament tension is not manipulated, and the implant is placed to “go with the flow”, it doesn’t want to get in the way of the knees natural motion. More specifically, the polyethylene bearing surface cannot guide knee motion or it risks causing a “kinematic mismatch” - the lateral compartment and surrounding soft tissue envelop is guiding the knee along a very specific track and if the UKA had a conforming poly, this would try guiding the knee along a slightly different track, and would effectively prevent smooth knee motion.
There are two design types for the UKA – a fixed-bear and a mobile-bearing poly. Both have pros and cons. In both cases the femoral component is relatively the same.
1) Fix-bearing design has the poly "fixed" to the tibial baseplate. This is what you see in all TKA designs. However, the poly in a TKA is usually concave, and this surface geometry guides some of the knee motion. This is avoided however in a UKA by making the poly flat, so the articular geometry offers no guided motion.
One problem is that the femoral component is curved, and by sitting on a flat poly, all of the force gets focused on a very small contact point. If pressure = force/area, and the contact area is small, then there is a lot of pressure on the poly and this can accelerate wear. The concern about focused contact area is addressed in the mobile-bearing design.
2) Mobile-bearing design has a curved tibial poly that matches perfectly with the curve of the femoral component . This conformity means that there is a large contact area and this diminishes the pressure on the poly and reduces wear. The problem is that we still don’t want conformity in the implant to direct knee kinematics. This problem is avoided by allowing the poly to move freely on the tibial base plate. The rest of the knee (native kinematics) will move the poly as needed (while it remains conformed to the femoral component). This design is a great idea, and works great in clinical practice, as long as the procedure is performed properly. The concern is that the poly may be too mobile and risks dislocating if there were any errors in the procedure (this is a unique mode of failure for the mobile bearing design).
There is debate about which design is superior. There is evidence to support both sides.
The Mobile-bearing design was developed by the Oxford Knee Group in England, and studies from that group demonstrate excellent survivorship – 98% at 10 years, 92% at 15 years [11, 12]), while some studies based on registry data in other countries, Australia and Sweden, show increased complications compared to the Fixed-bearing design. Specifically the mobile UKA showed higher failure in first 3 years, then a low steady state failure, which may be attributed to surgical technique. Together the results suggest that the Mobile-Bearing design is more technically challenging and theres greater risk of failure for surgeons inexperienced with the procedure. However, if done properly, the outcomes are excellent.
The Fixed-Bearing design demonstrates excellent survivorship, although it may not match that of the best Mobile-bearing studies. It appears to be more utilitarian, shorter learning curve, and good overall results.
The main advantage of the Mobile-bearing design was the increased contact surface on the poly that reduces poly wear. However, over the past decade, one could argue that a major paradigm shift has occurred in the material science of joint arthroplasty – the ultra-high molecular weight polyethylene (UHMWP). Increased cross-linking within the polyethylene has decreased wear. Its possible that wear will become so minor that the issue of poly wear will no long be a major limitation in the longevity of implants. This means that Fixed-bearing implants may offer equivalent survivorship to Mobile-bearing implants. It also means that a UKA is an increasingly good procedure for young patients who want to be active with their implant.
The outcomes for UKA are separated into 2 important topics: 1) functional outcome measures and 2) durability.
Spoiler alert: While still debated, the current concensus among surgeons is that UKA offers improved outcome scores with inferior longevity.
Therefore, surgeons are divided over values - do they prefer faster recovery, better function, decreased complications, or superior long-term longevity?
1) Functional outcome measures
The UKA has notable advantages over the TKA[13-15]:
faster patient recovery
lower incidence of complications
higher patient satisfaction - better return to activities, improved ROM, and feels like a “normal knee”  [18, 19]
The surgery removes less joint surface and less soft tissue. Articular congruity and soft tissue tension play a central role in regulating knee kinematics. Studies demonstrate that a UKA better reapproximates normal knee motion, and recreation of knee kinematics is associated with improved patient satisfaction, as is lower pain scores and return to function. UKA appears to achieve higher patient satisfaction as compared to TKA (although these are patients with better knees at baseline so its comparing apples and oranges). While both TKA and UKA are excellent at relieving pain, the UKA scores higher in “forgotten implant” score, indicating patients more forget about their implants.
The UKA can be durable if done well and done in the properly selected patient, yet UKA is shown to be less durable than TKA (albeit differences are not dramatic). Metanalysis demonstrates UKA have 1.53%/year failure rates compared with 1.26%/year of TKA. Fix-bearing UKA have 10 year survival rates of 98%, 13 year survival of 95% (with revision due to progression of patellofemoral arthritis) . Mobile-bearing UKA have reported 99% 10 year survival.
Age has a significant impact on survivorship. In patients under 55, that rate increases to 2.55%/year, 55-64 years old its 1.8%/year and over 65 its 1.3%/year. Although one study on UKA in patients under 60 demonstrate 90% survivorship at average of 11 years, with 90% satisfaction and return to sport .
Interestingly there is a higher number of UKA with excellent scores that still get revised. There is possibly an independent factor for the higher revision rate, because the TKA is always an option for postop pain, while a TKA does not have a good secondary option .
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