Unicompartmental knee arthroplasties: Tips and tricks
The number of unicompartmental knee arthroplasties (UKAs) being performed is increasing. This is due in part to the prevalence of osteoarthritis and expanded indications for the procedure. UKA poses technical demands that requires experience and surgical frequency for best outcomes. Surgeons should keep proper patient selection, biomechanics, and alignment front of mind when undertaking an UKA.
Robert Hube, Professor of Orthopedic Surgery Charité—University Medicine, Berlin, Germany, and Past President of the German Knee Society, acknowledges the challenges but also asserts that it is possible to achieve excellent UKA outcomes. “The surgical priorities for UKAs are achieving the correct alignment based on preoperative planning, correctly positioning the components, and securing sufficient fixation of the implants.”
Patient selection is key
As discussed in Part 1 and Part 2 of this article series, proper patient selection is critical for UKA success. Over time, indications and contraindications for the procedure have evolved from patient characteristics such as body mass index (BMI) and age, among others  to the consideration of more pathoanatomical indications.
To see if published contraindications to UKA actually generated poorer outcomes, Hamilton et al looked at 1000 mobile-bearing UKAs (818 patients), comparing the outcomes at 10- and 15-years follow-up of 322 traditionally indicated knees and 678 contraindicated knees. They concluded that for their study group, there was “evidence that patients with the previously reported contraindications do as well as, or even better than, those without contraindications” .
In 2015, six surgeons with combined experience of 8,000 UKAs (representing between 10% to 50% of their primary knee practices) published a consensus statement in the Journal of Surgical Orthopedic Advances on the indications and contraindications for medial UKA (see Table 1).
All the same, consensus is no substitute for a surgeon’s experience and does not take into account the need for patients to be involved in the decision-making to a certain degree. When choosing between options, for example UKA or total knee arthroplasty (TKA), lifestyle, activity levels, scarring, and other factors may be weighted differently by a patient than by their surgeon .
Does anterior cruciate ligament stability make a difference?
On the matter of the stability of the anterior cruciate ligament (ACL), Robert Hube says, “A successful UKA requires a stable knee. If the muscles have compensated for the compromised ACL, and the knee is about functionally stable, in my opinion the patient is still a candidate for a UKA.”
A cadaveric study of 15 knees looked how fixed-bearing UKA in ACL-intact, partial ACL, and ACL-deficient knees would affect the posterior tibial slope’s restoration of knee stability and flexion. The researchers determined that a slope of just 1° in ACL-deficient knees almost doubled the degree of translation compared to ACL-intact knees, leading them to conclude that UKA in ACL-deficient knees was “challenging” .
Figure 1. The ACL should be stable for a successful UKA. However, there are cases where ACL compromised patients may still be good candidates for UKA and/or simultaneous ACL reconstruction. Image in public domain. Source. By Kevin Tanenbaum.
Of course, UKA is not limited to the medial compartment; lateral UKA accounts for roughly 10% of cases . In this situation, Robert Hube says that there is insufficient data to indicate whether medial or lateral UKA is a better candidate if there is also a compromised ACL. “If the knee is functionally stable, I would go for both, medial and lateral UKA. The problem is that most of the knees with a deficient ACL are not stable. In these cases, we would go for a TKA. However, this is in contrast to data for high tibia osteotomies. Here, we can achieve good results even with insufficient ACL.”
Even if the “right” patients are selected for UKA, another important factor that has been shown to impact outcomes: surgeon experience.
Surgeon experience makes a difference
UKA revision rates and implant survivorship embodies the complex relationship between patient indications, the type of implant used, and a surgeon’s annual UKA volume . Several studies have determined that if UKA is not part of a surgeon’s routine practice, the outcomes are not as favorable as for high volume surgeons [7–10]. Surgeons performing lower volumes of UKAs have higher revision rates and survivorship over time decreases faster than high volume surgeons. Baker et al suggest a minimum of 13 annual UKAs per center/surgeon to achieve results comparable to high volume centers/surgeons .
Figure 2. Graph showing the relationship between revision rate (revisions per 100 component years) and usage of UKA (the percentage of a surgeon’s knee arthroplasties that are UKA), based on data from the UK National Joint Registry for Oxford (mobile-bearing) unicompartmental knee arthroplasty (UKA). Used under CC BY-NC 2.0 license. Source: Murray DW, Liddle AD, Dodd CA, et al. Unicompartmental knee arthroplasty: is the glass half full or half empty? Bone Joint J. 2015 Oct;97-b(10 Suppl A):3–8.
Liddle et al suggest that ideally, 40% to 60% of surgical volume should be UKAs for “optimal” results—20% of surgical volume produces “acceptable” results and lower revision rates . Hamilton et al recommend that “≥ 20%, or ideally > 30% of [high- or low-volume surgeons’] knee arthroplasties are UKA for optimal results” . Further reinforcing the importance of surgeon familiarity and training, Robertsson et al concluded that higher revision rates were associated with the more technically demanding mobile-bearing (Oxford) versus a fixed-bearing (St Georg) .
For surgeons working in low-volume centers it can be difficult to access an adequate number of UKA cases each year to maintain their skills and comfort levels with the procedure. One option for these surgeons that has been proposed to improve clinical outcomes is to implement “very strict indications” . In lieu of access to actual patients on which to practice, the use of patient-specific instrumentation (PSI) was shown, in a sawbone model, to allow inexperienced surgeons to make saw cuts with an accuracy equivalent to that of expert surgeons, allowing better adherence to their preoperative plan .
Alternatively, computer navigated and robot-assisted UKA surgery have been proposed as methods that could help low-volume surgeons achieve better outcomes, but more studies are needed to confirm this . Improved alignment and positioning have been documented with the use of these emerging technologies [18, 19]. [See Surgical Insights issue on computer-assisted and robotic surgery.]
Preoperative planning to limit intraoperative decisions
All surgeons recognize the importance of adequate preoperative planning. Digital software programs have become the standard tool surgeons turn to when planning an orthopedic procedure, including UKA. Writing down the steps of a procedure with a pen and paper and/or using tracing paper to sketch out placements over an x-ray is no longer the norm . Digital planning can help surgeons decrease the number of decisions they must make intraoperatively.
Generally, these digital tools have short learning curves and intuitive, user-friendly features, such as being touch screen enabled, that assist surgeons with selecting appropriate implant sizes and determining positioning. The ability to plan in this manner helps to lower implant stocks by more accurately predicting required device sizes; this can offer a financial benefit . It also eliminates costs associated with the time and supplies needed to print hard copies of radiographs, which, depending on the size of the institution, could be significant annual savings.
A short video demonstrating the user interface and selected features of a touch screen UKA planning software.
When it comes to sharing images within an institution, Steinberg et al note that, “A considerable amount of time will be saved by using PACS [picture archiving communication systems] compatible software: the various built-in tools have been constructed according to common orthopedic consensus and in collaboration with software developers, the PACS producers and the orthopedic community at large.” 
Figure 3. The workflow of a radiology department after the implementation of Picture Archiving Communication Systems (PACS). PACS offer centralized storage for different imaging modalities (eg, x-rays, computed tomography [CT], magnetic resonance images [MRIs]) and gives surgeons the ability to import these images into orthopedic planning software in a variety of devices. Image used under CC BY-NC 3.0 license. Source: Khaleel H, Rahmat R, Zamrin D. Components and implementation of a picture archiving and communication system in a prototype application. Reports in Medical Imaging. 2019 12:1–8.
Mobile or fixed bearing?
One thing a surgeon decides preoperatively is whether to use a mobile or fixed bearing. This can be a personal decision, as Robert Hube emphasizes, “The literature does not indicate an advantage of one design over the other—with both philosophies we can achieve excellent results. It seems to be that fixed bearing UKAs are more robust and forgiving of surgical mistakes, so I prefer a fixed bearing in my UKAs.”
Aside from surgeon preference, there are other considerations for surgeons to weigh. In Table 2, Robert Hube briefly introduces each bearing type. Ultimately, for high-volume surgeons, he recommends a mobile bearing for young patients with a stable knee. Cao et al put it this way as they reminded surgeons that each patient is a unique case: Mobile-bearing UKAs tend “to fail in early postoperative years whereas fixed-bearing UKA in later postoperative years. Therefore, treatment options should be carefully considered for each patient, and surgeons should still use their personal experience when deciding between these options.” 
Table 2. Comparison of fixed and mobile bearings.
Abbreviation: Unicompartmental knee arthroplasty, UKA.
Based on slides from 2016 European Knee Society presentation “Principles in UKA” and “Surgical Technical Challenges in UKA” slides, courtesy of Robert Hube
“In UKA we just replace the joint surface without changing the biomechanics of the knee. This means that accuracy of implant positioning is even more important than in TKA,” says Robert Hube. He reminds us of four biomechanical basics of UKA:
- All four ligaments are intact (there is anteromedial arthritis present)
- Axis correction is addressed with insert height
- The bone cuts change joint lines
- Any alteration of the slope changes biomechanics of the joint
Figure 4. Illustration of how the UKA insert thickness changes the alignment axis in the knee.
Proper sizing and positioning are important
Over half of UKA failures are traced to polyethylene wear . The length of time a device is in situ certainly plays a role in wear, but high localized stress when the implant surfaces are incongruent and a too conservative resection on the bone also contributes . Correct sizing and positioning are central to reducing wear and extending the life of the UKA . “Different companies supply similar alignment guides, spacers, or ligament tensioners to increase the accuracy of implant positioning,” says Robert Hube.
When Horsager et al investigated the wear rates of the polyethylene bearing for cemented and cementless hydroxyapatite-coated Oxford medial UKAs, they found wear to increase by 0.014 mm/year for every millimeter increment of bearing overhang—half of the 80 patients in the study had medial bearing overhang, with a mean of 2.5 mm .
Figure 5. How polyethylene wear can be evaluated: the minimal joint space width (mJSW) is an indirect measure of the bearing thickness. The frontal view to the left outlines the measurement of the minimal joint space width (mJSW). The dotted line represents the sagittal cross-sectional view presented to the right. The mJSW reflects the bearing thickness and the projected dotted line on the tibial component represents the femorotibial contact point. This allows the estimation of polyethylene wear and the position of the bearing. Overhang is seen when the bearing exceeds the outline of the tibial plateau. Impingement can be identified if the bearing slides against the vertical wall (see frontal view). Used under CC BY 4.0 license. Source: Horsager K, Madsen F, Odgaard A, et al. Similar polyethylene wear between cemented and cementless Oxford medial UKA: a 5-year follow-up randomized controlled trial on 79 patients using radiostereometry. Acta Orthop. 2019 Feb;90(1):67–73.
Preoperative planning software should help surgeons gauge the approximate size of implant components they will need and where they should be positioned. However, in the operating room, surgeons must be prepared to revise their plan based on what they encounter. Malpositioning has risks: if the implant is too small the risk is stress on the bone and edge loading; if there is medial overhang, the collateral is at risk of being irritated and impingement can result from anterior overhanging of the femur.
Robert Hube encourages surgeons to place the tibial side of the implant in neutral position, selecting a size as big as possible without discriminating the attachment of the cruciate ligaments. In addition, to eliminate impingements, any overhanging of the components should be avoided. He notes that the anteroposterior diameter is smaller on the lateral side so posterior overhanging is a risk, especially for lateral UKAs.
Robert Hube’s tips for placing the femoral UKA component:
- Place the femoral component in neutral position.
- For the lateral UKA, the femoral component should be placed as lateral as possible to ensure a central track on the tibial implant.
- In contrast, for the medial UKA, the medial aspect of the component should be placed about 2 mm lateral to the medial edge of the condyle.
Undercorrect is correct
“When performing a UKA we try to slightly undercorrect the deformity to avoid transferring the load to the opposite side. This contrasts with TKA where we’ve tried to achieve a neutral alignment for decades. But we have learned lessons about this over the last few years in TKA and practices are changing. Based on good data, more and more surgeons undercorrect their TKA’s and aim for a kinematic alignment, especially in varus arthritis,” notes Robert Hube.
Kinematic alignment (KA), personalized alignment based on each patient’s unique anatomy, has been widely adopted for TKA over the last decade as it has contributed to improved outcomes compared to the more conventional mechanical alignment technique . Anatomical medial articular geometry is important as well as this surface predicts the surgeon’s likelihood of restoring rotational kinematics . Rivière et al explain the principles behind KA, as it relates to UKA, and point out that UKA fixed bearing implantation employed a kinematic approach to placement for many years before the idea took hold for TKA :
“The principles are to anatomically position (true resurfacing) and kinematically align (cylindrical femoral and anteroposterior [AP] tibial axes) the components, in order to restore the highly inter-individually variable native knee’s articular surface and improve prosthetic interaction (or biomechanics). Interestingly, for decades the principles for implanting fixed bearing UKA components were consistent with those promoted by the KA technique, but differently formulated.”
Hube also emphasizes the need to “control alignment with an insert” and to appropriately use your instruments when making cuts. The carpenters’ saying “measure twice, cut once” is not necessarily the golden rule for the UKA surgeon. He says to always check your measurements before cutting but recut if necessary.
A word on osteophytes
“Take care of osteophytes!”, urges Robert Hube. However, it is not always possible to remove all of them during UKA if they are large. If an osteophyte extends to the inferior part of the medial tibial plateau there is a risk of releasing the medial collateral ligament during their removal. If these protrusions are encountered, it may be helpful to know that Pongcharoen et al reported that mobile bearing UKA patients with residual osteophytes smaller than 9 mm had acceptable and similar clinical outcomes to those without residual osteophytes .
Figure 6. Osteophytes cannot always be removed during UKA. The size of a residual osteophyte was measured from the medial cortex of the tibial plateau to the outer margin of osteophyte. Used under CC BY 4.0 license. Source: Pongcharoen B, Chanalithichai N. Clinical outcomes of patients with residual medial osteophytes following mobile bearing unicompartmental knee arthroplasty. PLoS One. 2018 13(10):e0205469.
Although their description of a notchplasty technique to address osteophytes was not done in tandem with a UKA, Ferrari et al’s “pearls and pitfalls” are helpful pointers to keep in mind when tackling these growths (Table 3 ) .
Tibial slope considerations
The posterior tibial slope (PTS) influences knee biomechanics and is thought to be crucial for good clinical function . If the PTS is increased, dislocation of a mobile bearing becomes a greater risk . Posterior tibial slope should be maintained at an angle (< 7°) to protect the ACL [44, 45] and “the down slope adapted to the natural one both in the medial and lateral UKA,” says Robert Hube.
However, Weber et al caution that, in mobile-bearing UKAs, as the tibial slope increases, more external rotation occurs in the tibia which alters kinematics and strains ligaments (Figure 7). They encourage surgeons to approach each patient individually and think about pre-and desired postoperative kinematics, ligament integrity, and where any retropateller chondral damage may be when determining optimal slope .
Figure 7. A tibiofemoral kinematic analysis showed an anterior translation of the tibia during flexion. Position of the inlay on the left side with a 0° slope and on the right side with a 10° slope. With an increasing slope, the mobile bearing inlay shifted ventrally because of the congruent spherical form of the femur. Used under CC BY 4.0 license. Source: Weber P, Woiczinski M, Steinbrück A, et al. Increase in the Tibial Slope in Unicondylar Knee Replacement: Analysis of the Effect on the Kinematics and Ligaments in a Weight-Bearing Finite Element Model. Biomed Res Int. 2018 2018:8743604.
When it comes to fixation, the problem is “always the tibia”, declares Robert Hube, who also reminds us to make sure cement remains are removed before closing the site. Aseptic loosening of this component is a significant source of failure . Cemented is the “gold standard”, particularly considering that cementless (interference/press fit) decreases the contact area and carries risks such as compromised stability of the implant, implant migration, loosening, and failure .
Cement penetration can be used as an indication of fixation strength and implant stability. A group of researchers have validated a computed tomography (CT) based noninvasive method to measure cement penetration depth in a clinical setting to help evaluate this important interface .
Figure 8. Cement penetration can indicate the fixation strength and stability of the UKA tibial implant. Frontal cut through the implant–cement–bone interface with cement mantle (yellow) and prosthesis (blue). Used under CC BY 4.0 license. Source: Scheele CB, Müller PE, Schröder C, et al. Accuracy of a non-invasive CT-based measuring technique for cement penetration depth in human tibial UKA. BMC Med Imaging. 2019 Jan 21;19(1):9.
Robert Hube offer this final piece of advice:
“Help manage your patient’s expectations for the UKA procedure,” he says. “We may be able to achieve good outcomes with a UKA but ultimately, what we surgeons define as a clinical success may not always align with what patients were wishing for, such as a complete return to their activities or even the ability to take on new high-impact sports.”
There are many details in a UKA that require surgeons’ full attention, careful consideration, and accumulated experience to manage satisfactorily. Patient selection, implant sizing and placement, alignment, kinematics, and fixation are all important factors in achieving a successful 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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