Unicompartmental knee arthroplasty versus high tibial osteotomy
Each step you take sends forces that amount to three to eight times your body weight between your femur and tibia, through your knee . Considering this, it is no wonder that the most common type of arthritis is osteoarthritis (OA) of the knee . It is associated with “wear and tear”, injury, and genetic predisposition, and is more likely to occur in people over 50 years old (although younger individuals can also have OA) . Surgical treatment options are available that aim to support improved mobility, reduce pain, and facilitate active lives, including sports—which is a priority, especially for many younger people with knee OA.
Three procedures that seek to surgically manage degenerative OA are HTO, UKA, and total knee arthroplasty (TKA). Successful HTO and UKA can delay a TKA, allowing patients to retain more aspects of their native knee(s) for longer [4, 5].
As Georg Matziolis, Chief Physician at the German Centre for Orthopedics in Eisenberg, Germany, points out, “TKA has the best long-term survival of all the options but the worst functional outcomes and patient reported outcomes measures (PROMs). TKA replaces all compartments (medial, lateral, and patellofemoral) and is therefore indicated for end-stage arthritis in a minimum of two compartments. If arthritis is secondary to systemic diseases such as rheumatoid arthritis, gout, and pseudogout, TKA is recommended even if only one knee compartment seems to be affected.”
Part 1 explored UKA in relation to TKA. This article focuses on similar questions applied to HTO and UKA, comparing these two procedures and examining the indications that would prompt surgeons to select one over the other.
Matziolis reminds us that, “HTO, UKA, and TKA are all viable options to treat degenerative arthritis of the knee but each of these procedures is appropriate for different kinds and degrees of OA.”
The full joint is replaced with prosthetic components.
A wedge of tibial bone is removed from under the healthy side of the knee to allow more even weight distribution and less pain. It is then fixed with a plate (not shown).
Either the medial (shown) or lateral compartment is replaced with a prothesis.
Figure 1. TKA, HTO, and UKA are all viable options to address osteoarthritis of the knee. However, there are situations that indicate the selection of one over the others.
Since 1958, when the procedure was first performed by Jackson , and popularized by Coventry  starting in the 1960s , HTO has been used to treat unicompartmental OA in knees. Whereas UKA and TKA replace elements of the knee joint with prostheses, either in part or in full, respectively, HTO takes a biomechanical approach and is viewed as a realignment procedure. “HTO does not replace the knee joint but optimizes the joint biomechanics by reducing the pressure on one compartment,” says Georg Matziolis. Patient-specific alignment is widely recognized as “the foundation step when planning any surgical intervention” .
It is not only a useful treatment for certain cases of single compartment knee OA, but also knee instability. It is sometimes paired with ligament reconstruction (anterior cruciate ligament [ACL]) to address stability issues . Generally, ACL reconstruction accompanies an HTO if there is a posterior slope of more than 12° or severe varus malalignment .
A wedge-shaped cut (see Figure 1) is made in the top of the tibia to change the alignment of the knee by shifting the weight/loading of the arthritic part of the joint to the healthier part (see Figure 2) . It is mostly performed for varus deformities, less commonly for valgus deformities . Preoperative planning using imaging helps surgeons to calculate the size of the wedge/osteotomy needed to correct the weight-bearing axis (see Figure 3). For best results, it is critical that correct alignment is achieved . There is controversy around what constitutes the ideal mechanical alignment but generally the recommended alignment range is between 2° and 6° of mechanical valgus .
Figure 3. Line D shows the distance of the proposed osteotomy site and the same length (line d) is drawn over one of the mechanical axes. The distance between the two mechanical axes at this point (line G) indicates what size of the gap at the posterior aspect of the osteotomy should be in order to correct the weight-bearing axis to the correction point. Used under CC BY 4.0 license. Source: Herman BV, Giffin JR. High tibial osteotomy in the ACL-deficient knee with medial compartment osteoarthritis. J Orthop Traumatol. 2016 Sep;17(3):277–285.
HTO: closed wedge or open wedge
Lateral closed wedge and medial open wedge are two different HTO techniques (see Figure 4 ) . Lateral closed wedge is the traditional approach. It is known for advantages such as its potential for greater correction, elimination of bone grafting, and faster healing [5, 17-19]. Sabzevari et al inventories the following disadvantages: “concomitant fibular osteotomy or release of the proximal tibiofibular joint, risk of peroneal nerve injury (occurs in 3.3–11.9%), the need for two bone cuts, ability of malalignment correction in only one plane (frontal), shortening of the leg, loss of bone stock, and more difficult conversion to arthroplasty with muscle detachment.” 
Medial open wedge lets the surgeon correct alignment in both the coronal and sagittal planes and provides the option to adjust the correction intraoperatively, does not require fibular osteotomy, requires just a single cut with no muscle detachment, does not shorten the limb, has no bone loss, and is converted quite easily to a TKA . As a counter to these positive attributes are the disadvantages of potential delayed union or nonunion and the need of a bone graft [21–23]. Additionally, if a correction that exceeding 12.5 mm is needed, then a concomitant tibial tuberosity osteotomy has been suggested to circumvent increased patellofemoral compartment pressure and the adverse effects of patella infra .
“Typically, a medial open-wedge HTO is performed to treat a medial arthritis of the joint. The best indication is an extraarticular deformity located in the tibial metaphysis and a non-end stage arthritis of the medial compartment,” says Georg Matziolis.
Both techniques can deliver good results. However, some research indicates that open wedge gives better early and late follow-up functional scores, fewer days to weight bearing, and a shorter time to return to activities .
Table 1. Comparison of femorotibial angle for open-wedge vs closed-wedge osteotomies. The knee joint range of motion was only better in the open wedge group at 6-weeks postoperatively, otherwise, no noteworthy difference was noted. The open-wedge group also had a quicker time to full weight bearing than the closed-wedge group.
Abbreviation: ROM, range of motion.
Used with permission. Source: Agarwala S, Sobti A, Naik S, et al. Comparison of closing-wedge and opening-wedge high tibial osteotomies for medial compartment osteoarthritis of knee in Asian population: Mid-term follow-up. J Clin Orthop Trauma. 2016 Oct–Dec;7(4):272–275.
Tabrizi et al reported 87.5% of open wedge patients were satisfied while only 75% of closed-wedge patient reported similar satisfaction . It should be noted that two metaanalyses did not find superiority of one over the other [26, 27]. Another study suggested that open wedge HTO results in fewer revisions to TKA than UKA, and closed wedge HTO is associated with higher revision to TKA .
HTO—superficial medial collateral ligament: to reattach or not reattach [29–31]?
It is well known that during HTO, the superficial medial collateral ligament (sMCL) must be released to successfully perform the procedure. However, there is disagreement on whether it should be reattached. Since the sMCL is a primary restraint on valgus stress, medial laxity could result if it is not reattached, yet some studies have not conclusively shown this laxity to be of concern to patients. Malinowski et al have described a technique for sMCL reattachment that works for any correction angle and aims to return “near native pressure on the medial compartment by matching the tension on the sMCL to the size of the osteotomy gap.”
HTO: Which patients benefits most?
HTO can reduce knee pain, delay a knee replacement , and may be more cost effective than UKA and TKA in patients between the ages of 50 and 60 . However, as in most orthopedic procedures, patient selection is critical contributor to this technique being successful. What patient characteristics are aligned with the best HTO outcomes? There are several considerations that surgeons should weigh, remembering that HTO patient selection criteria are stricter that than for UKA . Georg Matziolis summarizes the primary indications for HTO as: “non-end stage arthritis, extraarticular tibial deformity, and young aged patient with high demand for activity in sports or work.”
Younger patients, under the age of 65, are ideal candidates . Flecher et al noted that age over 50 was a significant risk factor in predicting revision and survival , a suggestion echoed by Gstöttner et al  and others, particularly for females . An increased risk of failure of 7.6% for every year of age has been reported, and this grows to 1.5 times the risk in the over-65 age group, when compared with younger patients .
Of course, higher activity levels are more likely in younger patients and younger, more active patients have higher expectations of their procedures [38, 39]. One study showed that up to 50% of patients with knee OA indicated for surgery have knee-demanding jobs that include activities such as lifting, kneeling, and walking stairs . Retaining all aspects of a native knee and adjusting alignment eliminates prothesis wear, facilitates a quicker return to sports (Table 1) , and carries the potential for normal kinematic function . One study found that 95% of HTO patients were able to return to work within six months postoperatively .
Table 2. HTO patients were found to return to sports and work activities faster when compared to a matched group of UKA patients. Based on information in Source: Jacquet C, Gulagaci F, Schmidt A, et al. Opening wedge high tibial osteotomy allows better outcomes than unicompartmental knee arthroplasty in patients expecting to return to impact sports. Knee Surg Sports Traumatol Arthrosc. 2020 Feb 1.
Body mass index
Considering that for every kilogram of weight loss, the forces through the knee decrease four-fold , it follows that lower weight or body mass index (BMI) would result in lower forces through the knee and facilitate a more successful HTO. However, BMI consideration has been identified as controversial . Lighter patients (< 25 BMI) are more active, cause more stress on the osteotomy site, and this can result in poor HTO survival—even worse than in high BMI (> 27.4) patients . Akizuki et al recommends a normal-range BMI  noting that a preoperative BMI above 27.5 is a significant risk factor for early failure. HTO patients with BMI over 30 reported lower Knee Society Scores at five years follow-up .
Alignment and instability
HTO is indicated in patients with mild to moderate varus alignment (5–15°) . It is less commonly done in patients with valgus alignment . Loia et al note that, “Combined knee instability and malalignment is a challenging problem for the surgeon” . There are primary varus, double varus, or triple varus knees; these are characterized by different degrees of deformity and levels of instability . When varus malalignment presents in combination with instability, a combined or staged procedure, including both ligament procedures and open wedge HTO, are suggested (see Table 3). Georg Matziolis finds that, “moderate instabilities of the ACL and PCL [posterior cruciate ligament] as well as flexion contracture can be addressed by change of the posterior tibial slope.”
Table 3. Depending on the type of malalignment and degree of instability, the HTO procedure can be accompanied by increasingly complex ligament reconstruction.
Abbreviations: ACL, anterior cruciate ligament; HTO, high tibial osteotomy; PCL, posterior cruciate ligament. [Table content based on information in [48–52].]
Worse preoperative range of motion (ROM) has been associated with poorer HTO outcomes [45, 46, 53] Ideally, a preoperative ROM greater than 120° is desired in potential HTO candidates [14, 44, 48].
The severity of OA is a predictor of HTO outcomes [34, 54]. The lower the degree of OA, the better the outcomes . If the OA involves more than one compartment, this is a counterindication for HTO . The Ahlbäck classification is often used to quantify the progression of knee OA and no higher than grade 2 is recommended for HTO patients, ie, mild to moderate .
Ahlbäck classification 
The Ahlbäck classification is one of many ways to grade knee osteoarthritis.
Grade 1: joint space narrowing (< 3 mm)
Grade 2: joint space obliteration
Grade 3: minor bone attrition (0–5 mm)
Grade 4: moderate bone attrition (5–10 mm)
Grade 5: severe bone attrition (> 10 mm)
HTO and cartilage regeneration: a helping hand
Cartilage has a limited ability to regenerate and once it has deteriorated, surgical intervention (HTO, TKA, UKA) is generally indicated . Since HTO decreases the pressure in the arthritic compartment of the knee joint, the procedure creates an environment that, with the right conditions, can encourage cartilage regeneration. A June 2020 publication in the World Journal of Stem Cells highlights the results of the implantation of allogenic human umbilical cord blood-derived mesenchymal stem cells (hUCB-MSCs) during HTO to regenerate cartilage.
Figure 5. Arthroscopic observation of stem cell implantation procedures during HTO. A: Medial compartment osteoarthritis (arrow) in a 61-year-old woman; B: Multiple holes, 4 mm in diameter and 4 mm in depth (arrow), were drilled using a drill bit; C: Human umbilical cord blood-derived mesenchymal stem cells were mixed with hyaluronic acid hydrogel and implanted in the holes (arrow). Used under CC BY-NC 4.0 license. Source: Song JS, Hong KT, Kong CG, et al. High tibial osteotomy with human umbilical cord blood-derived mesenchymal stem cells implantation for knee cartilage regeneration. World J Stem Cells. 2020 Jun 26;12(6):514–526.
Figure 6. After second-look arthroscopic observation. A: Cartilage lesions classified as International Cartilage Repair Society (ICRS) grade IV in the medial femoral condyle (MFC) (arrow) and tibial plateau in a 61-year-old female patient; B: Cartilage was regenerated to ICRS grade I (arrow) via second-look arthroscopy, performed 13 months after the initial operation; C: A cartilage lesion classified as ICRS grade IV in the MFC (arrow) of a 52-year-old male patient; D: Cartilage was regenerated to ICRS grade II (arrow) via second-look arthroscopy, performed 22 months after the initial operation; E: A cartilage lesion of ICRS grade IV in the MFC (arrow) of a 68-year-old female patient; F: Cartilage was regenerated to ICRS grade III (arrow) via second-look arthroscopy, performed 16 months after the initial operation. Used under CC BY-NC 4.0 license. Source: Song JS, Hong KT, Kong CG, et al. High tibial osteotomy with human umbilical cord blood-derived mesenchymal stem cells implantation for knee cartilage regeneration. World J Stem Cells. 2020 Jun 26;12(6):514–526.
HTO: What are the drawbacks?
Patient selection is critical for the success of HTO but even when the “right” patient undergoes the procedure, ideal outcomes do not always materialize. It is a complicated, demanding procedure that benefits from “careful preoperative planning and meticulous surgical technique” .
Selected complications include [13, 15, 58–62]:
- Patella baja
- Thromboembolic disease
- Compartment syndrome
- Delayed union and nonunion
- Neurovascular complications
- Under correction and recurrence of deformity
Campbell was the first to investigate arthroplasty solely within the medial compartment of the knee, reporting preliminary results of vitallium plate placement in 1940 . The modern era of UKA was ushered in in the early 1970s when Marmor introduced his modular hemiarthroplasty in 1972 [64, 65], and shortly after, in 1974, the first mobile-bearing UKA (Oxford Knee) was placed . Implants, and their associated procedures, have continued to evolve as materials and kinematic understanding has advanced. UKA replaces a single compartment of the knee, usually the medial, but sometimes the lateral  with a prosthetic.
Figure 7. Painful OA of the medial compartment of the right knee (varus deformity) in a 57-year-old man. UKA was indicated. (a) Preoperative x-ray and (b) magnetic resonance imaging (MRI) showing advanced osteoarthritis of the medial compartment of the knee; (c) final x-ray appearance of the knee with the prosthesis in position. Used under CC BY-NC 4.0 license. Source: Rodríguez-Merchán EC, Gómez-Cardero P. Unicompartmental knee arthroplasty: Current indications, technical issues and results. EFORT Open Rev. 2018 Jun;3(6):363–373.
UKA: fixed and mobile bearings
There are two kinds of UKA implants that are used: fixed bearing and mobile bearing. As their names suggest, each of these devices have different modes of action once implanted. Both are “technically demanding and require appropriate training and attention to operative detail” . However, many surgeons have a personal preference for the one they most like to use .
Both types of implants have good outcomes associated with them , but there are specific modes of failure for each of these. Parratte et al identified all reasons for revision in two groups of UKA patients and found the causes to be aseptic loosening, dislocation, and arthritic progress in 15% of their mobile-bearing patients, and wear and arthritis progression in 12% of the fixed-bearing group .
UKA: Which patients are best suited?
Patient selection indicators are broader for UKA than for HTO . When selecting patients for UKA, Georg Matziolis looks for those “with end-stage arthritis, intraarticular deformity by cartilage and perhaps bone wear as the cause of the varus leg. In addition, I make sure there is no flexion contracture (< 5°) and clinically stable anterior and posterior cruciate ligaments. It is important that the postoperative mechanical axis does not result in excessive varus.”
Part 1 took a closer look at the controversy around how patients are selected for UKA, explaining that there is a movement towards pathoanatomical characteristics being the predominant factors that surgeons consider. However, for the sake of comparing “apples with apples” we will break down indications into similar categories as we did for HTO.
When examining the UKA outcomes of 16,000 patients under the age of 65 in the Australian and Swedish knee registries, W.Dhal et al discovered that there was a significantly higher cumulative risk of revision in patients younger than 55 than that for patients between the ages of 55 and 64 years . While Dettoni et al stated that UKA patients should be older than 60 years , Panni et al did not think that age influenced outcomes , Vasso et al posited that age 50 was “ideal” , and Siman reported good UKA results in an older than age 75 population . Younger patients (< 60 years) are less likely to report satisfaction with their UKA than those over the age of 60 .
High activity levels have traditionally been viewed as a contraindication for UKA [14, 78] with low to moderate demands being preferred to maximize the lifespan of the implant. Interestingly, Koh et al found in high activity individuals, patient satisfaction was higher after UKA than HTO .
Again, there is disagreement around the weight/BMI-related indications for UKA. Generally, nonobese  with a weight under 82 kg is desired, but keep in mind that in a group of BMI > 35 patients, those who received UKA were found to benefit more than those with a TKA [79, 80]. As a more pathoanatomical approach is taken when assessing patients for their appropriateness for UKA, weight/BMI may play a lessened role in the future.
Alignment, instability, range of motion
Detonni et al indicates that UKA patients should have alignment of 0–5° and either no anteroposterior or mediolateral instability or grade 1 (0–5 mm) on the American Medical Association rating. Ideally, a UKA candidate would also have ROM of arc 90° and < 5° flexion contracture .
Severity of OA
The OA should be confined to either the lateral or medial compartment and not be inflammatory in nature (rheumatoid), as this is a contraindication. However, as long as the lateral and patellofemoral compartments are intact, any degree of OA in the compartment is compatible with UKA. According to Georg Matziolis, “Arthritis of the lateral patella compartment as well as patella maltracking is a contraindication for UKA. Arthritis of the medial patella compartment without or with only moderate symptoms can be tolerated. The typical indication for a medial UKA is the anteromedial arthritis, presenting with a cartilage wear pattern only in the central and anterior part of the tibia.”
UKA: potential complications
Certainly, periprosthetic joint infection (PJI) after UKA happens  and other complications are possible. Ji et al shared their list of intra-and postoperative complications from 245 patients who received medial UKA (see Table 4) .
Table 4. Complications of unicompartmental knee arthroplasty.
Abbreviations: MCL: medial collateral ligament, TKA: total knee arthroplasty, OR/IF: open reduction and internal fixation.
Used under CC BY-NC 3.0 license. Source: Ji JH, Park SE, Song IS, et al. Complications of medial unicompartmental knee arthroplasty. Clin Orthop Surg. 2014 Dec;6(4):365–372.
UKA or HTO: Which one to use?
In 2014, Nwachukwu et al identified an inverse correlation in the use of UKA and HTO, with UKA use increasing and HTO decreasing . If you ask Georg Matziolis, he will tell you that the functional results are aligned in a sort of continuum, with “HTO in between those of TKA and UKA. The long-term survival of HTO and UKA is comparable but an important point to consider is that rehabilitation to normal joint function is significantly longer with HTO compared to UKA and even TKA.” A fast return to activity is usually achieved with UKA .
HTO patients have been shown to achieve larger ROM than UKA patients, suggesting it is the better alternative for those who pursue high levels of activity [41, 85]. However, Kim et al counters this with their finding that “UKA had better short-term functional outcomes and return to recreational and sports activities than did HTO in patients with medial OA” . At five years follow-up, the UKA study group displayed fewer pain symptoms and a higher quality of life than the HTO group .
In terms of survivorship, long-term survival was similar up to roughly 12 years but after this time UKA had a better chance of still being in place, concluded one study. In this case, fixed-bearing UKAs were most likely to fail due to femoral component loosening, while HTOs failed due to OA progression .
Consideration of future risk
HTO and UKA generally address OA in a single compartment and are considered viable techniques to prolong the time to a TKA. Indeed, a prior HTO does not preclude a UKA as long as excessive mechanical valgus axis is avoided .
One thing that surgeons should discuss with their patient when deciding between the two procedures is the finding that those who underwent conversion to a TKA after UKA were at close to double the risk of needing revision after a TKA than those who had previously had HTO . Additionally, in terms of future surgical considerations, Lee et al reported conversion TKA after UKA needing more components and thicker polyethylene in contrast to conversion TKA after HTO sometimes requiring a stem to bypass the osteotomy gap . However, most people that have a HTO or UKA never need a TKA .
Appropriate Use Criteria
The American Academy of Orthopedic Surgeons (AAOS) developed a web-based tool called the Appropriate Use Criteria (AUC) for surgical management of osteoarthritis of the knee. It asks several questions about the patient’s condition and function, then generates recommendations for surgical procedures as well as suggestions of which procedures are not recommended.
While one center retrospectively tested the AUC against the surgical procedures in 100 actual cases over a year and found the tool’s recommendation to align in 99% of them , Riddle and Perera highlighted the AUC’s high dependence “on traditional variables that surgeons consider when evaluating patients for TKA: patient age, knee OA severity, knee OA pattern and knee motion”  [see Part 1 to learn more about the evolving indications/contraindications for UKA].
AUC to decide TKA, HTO, or UKA? Georg Matziolis says it is not that simple
Chief Physician at the German Centre for Orthopedics, Eisenberg, Germany, Georg Matziolis, tested the AAOS AUC and had this to say: “It is a nice tool but I believe that it is better to understand the procedures in a manner that allows you to explain to the patient why and how your decision is made.” He went on to note that individual considerations must be weighted uniquely for each case, with their level of importance dependent on a surgeon’s experience and a patient’s needs and characteristics. Patient details he takes under advisement include, for example:
- Personal circumstances (work, social life, sports)
- Scars/skin condition
- Life expectancy, independent from age, related to other diagnoses
- Patient expectations and tolerance to persistent symptoms
- Cosmetic aspects (resulting varus/valgus/neutral axis in combination with Bone biology (healing of the osteotomy)
- Other diseases interfering with implants (eg, chronic infections)
- ….and more!
Georg Matziolis maintains that at the end of the day, “there will still be several patients that qualify for both UKA and HTO.” In these situations, he recommends surgeons speak to their patients, and make them partners in choosing one option over the other (see above box for additional factors to consider and discuss). For all orthopedic procedures, appropriate patient selection is the foundation to build on . This motto holds true for HTO and 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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