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Iowa Orthop J. 2010; 30: 131–140.
PMCID: PMC2958284

HIGH TIBIAL OSTEOTOMY VERSUS UNICOMPARTMENTAL KNEE ARTHROPLASTY FOR MEDIAL COMPARTMENT ARTHROSIS OF THE KNEE: A REVIEW OF THE LITERATURE

Abstract

This review examined the literature regarding high tibial osteotomy (HTO) and imicompartmental knee arthroplasty (UKA), focusing on indications, survivorship and functional outcomes of the two procedures, as well as revision to total knee arthroplasty (TKA) after failed HTO or UKA. HTO and UKA share the same indications in selected cases of medial unicompartmental knee arthrosis. These indications include patients who are: 1) 55 to 65 years old; 2) moderately active; 3) non-obese; 4) have mild varus malalignment; 5) no joint instability; 6) good range of motion; and 7) moderate unicompartmental arthrosis. Few studies are available in the literature comparing the outcomes of HTO and UKA. Those few studies show slightly better results for UKA in terms of survivorship and functional outcome. Nevertheless, the differences are not remarkable, the study methods are not homogeneous and most of the papers report on closing wedge HTOs. For these reasons, no definitive conclusions can be drawn. TKA represents the revision option for both treatments and yields satisfactory functional outcomes and survivorship. Whether revision HTO and UKA-to-TKA perform any worse than primary TKA is still controversial. With the correct indications, both treatments produce durable and predictable outcomes in the treatment of medial unicompartmental arthrosis of the knee. There is no evidence of superior results of one treatment over the other.

INTRODUCTION

High tibial osteotomy and unicompartmental knee arthroplasty represent a“strange couple”in the treatment of medial compartment arthrosis of the knee. Even though they are very different procedures with different philosophies, in some cases they share the same indications. Therefore, some authors describe them as alternative options, while others deny any overlaps of indication.

HTO has long been considered a successful and widely performed procedure to address malalignment and subsequent unicompartmental arthrosis of the knee. UKA has gained popularity in the management of unicompartmental arthrosis, when total knee arthroplasty and HTO are the only alternative treatments available.

The aim of this review is to identify the correct indications for HTO and UKA, analyze the results from both treatments, and report on the comparison studies in the literature.

INDICATIONS FOR HTO AND UKA Indications for HTO

The original intent of HTO is correction of a knee angular deformity or metaphyseal tibial malalignment, which determines a medial symptomatic overload or initial arthrosis. The ideal candidate for an HTO is a young (less than 60 years old), active patient affected by symptomatic mild-to-moderate varus knee (5 to 15 degrees) with mild medial compartment involvement (less than grade III, Ahlback classification), intact lateral and patellofemoral compartments, good knee range of motion (knee flexion >120 degrees), and no joint laxity or instability. However, the indications for HTO have been recently expanded to include posterolateral laxity and varus hyperextension thrust, anterior cruciate ligament (ACL) deficiency and varus thrust or alignment, and combined ligamentous laxity with varus or posterolateral thrust.1 A better understanding regarding the importance of tibial slope in knee stability and modern surgical techniques, which allow multiplanar correction of alignment, led to this expansion in terms of indication.2 The body mass index (BMI) of the patient is a controversial factor. In fact, whether obesity significantly affects the outcome of HTO has not yet been proven.3

In the last few decades, HTO has been associated with other procedures in order to improve the outcomes in younger patients, and these mainly include: 1) cartilage resurfacing procedures; 2) meniscal transplantation; and 3) ligament reconstruction.3,4

The degree of osteoarthritis affecting the medial compartment is a key factor in determining the outcome of HTO. Even though cartilage repair procedures have been associated with HTO for the treatment of focal chondral defects in malaligned knees, severe degenerative involvement of the medial compartment still remains a contraindication for HTO.5

Arthritic involvement of the lateral and patellofemoral compartments is another contraindication to HTO. It has been shown that when HTO for medial compartment arthrosis is performed in an otherwise healthy knee, no degenerative changes occur in the lateral or patellofemoral compartments.6 On the other hand, HTO does not manage symptoms related to bi- or tri-compartmental arthrosis of the knee.

Another key factor in the selection of patients amenable to HTO is age: the risk of failure increases 7.6% per year of age, and in patients over 65 years the relative risk is 1.5 times that of younger patients. Therefore, HTO is not recommended in patients older than 65 years.7

Indications for UKA

UKA is the partial surface replacement of the knee joint. Its increasing popularity is due to: 1) the possibility of replacing a severely damaged compartment; 2) the preservation of bone stock; together with 3) a faster recovery time and minimal invasiveness compared to TKA. With recent technical improvements, UKA is consistently less invasive, and newer designs with arthroscopic techniques will soon be introduced into the marketplace.8

The ideal indications for UKA include: 1) unicompartmental osteoarthritis or femoral condyle avascular necrosis, with intact lateral and patellofemoral compartments; 2) age over 60 years; 3) low demands; 4) no obesity; 5) minimal pain at rest; 6) range of motion (ROM) arc over 90 degrees with less than 5 degrees flexion contracture; and 7) within 10 degrees of axial malalignment, which can be passively corrected almost to neutral.9,10

Anterior cruciate ligament (ACL) deficiency has been considered a contraindication for UKA because of increased stresses across the components with subsequent polyethylene wear and early failure.11 Nevertheless, some authors have reported UKA can be done safely in ACL-deficient knees when the tibial component slope does not exceeded 7 degrees. In UKA as well as in HTO, the importance of tibial slope for anterior/posterior stability of the knee has been proven, but there is no evidence yet about the reliability of UKA in ACL-deficient knees.12,13

Indications for both HTO and UKA

Despite the differences in indications between the two procedures, a small population of patients can be considered amenable to either HTO or UKA. The current indications for both treatments are summarized in Table 1.

TABLE 1
Ideal indications for UKA, HTO and overlaps between the two treatments

OUTCOMES OF HTO AND UKA

Many papers in the literature described the outcomes of HTO and UKA. These mainly focused on survivorship analyses, technical features (such as closed versus open HTO, or all polyethylene versus metal-backed tibial UKA), complications and adverse effects of the procedures, as well as outcomes of revisions to TKA. Only a few papers reported a direct comparison of the two procedures.

Results of HTO

Survivorship analyses have reported increasing survival rates for HTO, with correct patient selection and precise surgical technique. As the authors gained knowledge and experience over a period of years, the 10-year survivorship increased from around 50% to 80% (Naudie, 1999;u Papachristou, 2006;15 Gstottner, 200816), then to 90% (Sprenger, 200317), 95.1% (Koshino, 200418), and up to 97.6% (Akizuki, 200819). The same authors reported a survivorship at 15 years increasing from 65.5%,16 66%,15 86.9%,18 to 90.4%.19 Flecher et al. in 200620 reported an 85% survivorship at 20 years of follow-up. Patient satisfaction and clinical results were reported to be good as well, with 50 to 80% achieving good to excellent results at five-to-seven years follow-up, and 30 to 60% good to excellent results at 10-to-15 years follow-up.21,22 Advanced age, over- or under-correction, instability and severe arthrosis were reported as unfavorable factors. The goal to be achieved in alignment assessment is a slight valgus overcorrection (2 to 5 degrees).3

Controversies still exist regarding three topics and these include: 1) the most reliable HTO technique (closing, opening, or dome), 2) the clinical implications of patellar height changes, and 3) the technical difficulties related to surgical technique/means of synthesis. No conclusions can be drawn on which techniques are to be preferred when comparing between closing wedge, opening wedge, or dome-like, as none has shown significantly better outcome over the others. Interestingly, all papers reporting long-term outcomes and all studies comparing HTO and UKA considered only closing wedge osteotomies, and this lack of the literature regarding the newer opening wedge technique requires further investigation.

Before the introduction of internal fixation and early motion in HTO, when cast immobilization was part of the postoperative treatment, authors recorded a risk between 7.6% and 8.8% of patients having patella baja following a lateral closing wedge osteotomy.23 This complication was probably due to contracture of the patellar tendon during cast immobilization.4 A more recent study24 showed that closing wedge osteotomy increases patellar height, whereas opening wedge osteotomy lowers patellar height. The clinical implications of patellar height changes on outcomes and following TKA are still controversial.

Lateral closing wedge HTO has for many years been considered the gold standard in treating medial knee osteoarthritis. However, this technique entails fibular osteotomy or proximal tibiofibular joint disruption, pe-roneal nerve dissection, more demanding subsequent TKA, loss of bone stock, and more difficulty controlling tibial slope (with a tendency to decrease it). For all these reasons, the opening wedge HTO gained popularity and became a widely used alternative. This technique however is not itself free from drawbacks including the necessity for bone grafting and possible collapse or loss of correction.3

Results of UKA

UKA was introduced in the 1970s but did not gain wide acceptance due to poor early results, high failure rates, and high technical demands as reported by Insall et al. in 1980.25 In the last few decades, opinion has completely changed and the latest reports in the literature showed highly satisfactory survival rates and considerable patient satisfaction.

In a study on the Finnish Arthroplasty Register, Koskinen26 reported a 10-year survival rate ranging from 53% to 81%, depending on the prosthetic model implanted. In single-center studies, 10-year survivorship was reported to be up to 93%,27 94%,28 and 98%.29 Berger et al. reported a 15-year survivorship of 95.7%,30 while O'Rourke et al. reported an 85% survival rate at 15 years and 72% at 25 years.31 Patient satisfaction mirrors these excellent survivorship results, particularly in activities requiring complete knee ROM, such as going down stairs and kneeling.32

Aspects which most likely affect the outcome of UKA are prosthetic design, alignment, and stability.33

Koskinen et al.,26 reported a remarkable difference in terms of 10-year survivorship for four different UKA designs (81%, 79%, 78% and 53%). Furthermore, Borus et al. in their review of the literature, reported overall inconclusive results regarding the use of fixed versus mobile-bearing components and metal-backed versus all polyethylene tibial components.10

Robertsson et al.34 stated that surgeons’confidence with this technically demanding procedure markedly influenced the outcome. They showed that hospitals performing less than 23 UKAs per year reported a revision rate 1.6 times higher compared to that of departments performing a higher volume of UKAs. These authors also concluded that implant design influenced the outcome of UKA.

The importance of tibial slope for the antero-posterior stability of the knee has been proven in UKA, but there is no evidence yet about the reliability of the procedure in ACL deficient knees.12,13

Comparisons between HTO and UKA

Only a few studies have compared groups of patients treated with HTO and UKA (Table 2), and only two of these studies were randomized controlled trials (RCT).

TABLE 2
Summary of studies that compared groups of patients treated with HTO and UKA

Karpman and Volz35 performed a retrospective study, reviewing the records of patients treated with HTO and UKA with 20- to 40-month follow-up. HTO obtained a 100% survival rate at two years follow-up, while UKA showed a 91% survival rate at three years. Good-to-excellent results were achieved in 57% of the HTO group, and in 91% of the UKA group. The authors concluded that UKA is a valid alternative to HTO, and performed better.

Broughton et al.36 reported a retrospective study comparing 49 HTO and 42 UKA procedures at a mean follow-up of 7.8 years and 5.8 years respectively. The matching of groups was made post-hoc, by comparing the pre-operative findings. HTO was performed with lateral closing-wedge technique and either excision of the fibular head or release of the tibiofibular joint. No internal fixation was performed. Postoperatively, plaster casts were used for six weeks. The UKA implanted was a St. Georg Sledge (Waldemar Link), cemented in all cases. Both medial and lateral compartment procedures were included. Patients were assessed by interview, examination and radiographs when possible, using the Baily knee score, an adaptation of the Hospital for Special Surgery score. UKA showed significantly better results compared to HTO in terms of Baily score (76% and 43% good results, respectively), revision rates (7% and 20% respectively), pain (87% and 23% with mild or no pain), and early complications (including wound problems, deep venous thrombosis and infection). The main cause of unsatisfactory results in the HTO group was inadequate correction of the deformity. Although the authors concluded that UKA performed better than HTO, concerns were raised about the differences in follow-up time, accuracy of the osteotomies, patient selection criteria, and the inclusion of lateral compartment procedures.

Weale and Newman37 reassessed the same cohort at 12 to 17 years follow-up in 1991. Only 21 osteotomies and 15 UKAs were available at this end-of-study follow-up. Good results were found in seven HTOs (21%) and eight UKAs (42%). The revision rate was 34% in the osteotomy group, and 12% in the UKA group. The surviving HTOs and UKAs showed a similar Baily knee score of 31 and 34 respectively. Interestingly, the same patients had significantly different Baily knee scores six years before: 35 and 42 respectively. A total of 43% of the HTOs had mild or no pain, compared to 80% of the UKAs. The authors concluded that the superior results of UKA compared with HTO were maintained even at long-term follow-up.

Ivarsson and Gillquist38 prospectively evaluated the rehabilitation programs of ten patients undergoing a closing wedge HTO (12-month rehabilitation program) matched to ten patients undergoing a UKA (4 Oxford-Biomet, 6 PCA-Howmedica, 6-month rehabilitation program). The mean age of the patients was 63 years (range 53-72 years). Lysholm knee function score, a six-step activity grading scale, the torque of the thigh muscle, VAS pain score, and gait analysis were performed. Muscle strength at six months was better in UKA than in HTO, but it was comparable in the two groups at 12 months. The UKA group tended to reach a higher Lysholm postoperative score, but the difference was significant only with a statistical p of 10%. Pre- and post-operative gait analysis showed an increase in the maximal gait velocity and duration of single-leg support in the UKA group, while no differences were observed in the HTO group. Based on their data, the authors recommended UKA in older patients.

Borjesson et al.39 performed a similar study on 18 HTO and 22 UKA patients, evaluating British Orthopaedic Association (BOA) score, time-distance variables of gait, range of motion, patient satisfaction, and physical activity level achieved. The design of this study was prospective and randomized: Patients (from 55 to 70 years of age) with moderate medial knee arthrosis (Ahl-back's grades I—III) were randomly assigned to one of the two surgical procedures by drawing lots, and were examined before surgery as well as at three months, 1 year and 5 years after surgery. HTO was performed as first described by Coventry (closing wedge osteotomy, postoperatively immobilized in a whole-leg plaster cast for six weeks) and the UKA used was Brigham (Depuy). No differences were detected between the two groups regarding BOA score, range of motion, and patient satisfaction. Time-distance variables of gait showed clinically significant differences in favor of the UKA group at three-months after surgery, but these became insignificant at one-year and five-year follow-up.

Stukenborg-Colsman et al.40 published a prospective randomized trial, comparing the clinical outcomes of 32 patients treated with HTO and 28 patients treated with UKA. Patients were computer randomized. The osteotomies were performed according to the technique described by Coventry: closing wedge osteotomy, fixation with a two-thirds tubular plate and a cortical screw. The UKA used was the Tubingen unicondylar knee sliding prosthesis (Aesculap). Postoperatively, full weight bearing in the UKA and partial weight bearing for six weeks in the HTO was allowed. Patients were assessed at an average of 2.5, 4.5, and 7.5 years after surgery using the Knee Society scoring system. At the latest follow-up, the surviving HTOs obtained an average knee score of 76, while UKAs scored 74. The mean functional score was 71 for HTO and 59 for UKA. Average range of motion was 117 degrees in the HTO group and 103 degrees in the UKA group. HTOs showed more postoperative complications. Kaplan-Meier analysis predicted a survivorship of 82% at five years and 77% at 10 years for UKA, compared to 78% and 60% for HTO. None of the differences reported in this study were statistically significant. The authors concluded that both operative techniques are reliable and have their own indications for the treatment of unicompartmental osteoarthritis. The authors stated that HTO is indicated in younger active patients with mild arthritis (Ahlback I or II), a malalign-ment less than 10 degrees, and no ligament instability, while older patients (>60 years) are amenable to UKA.

At the 2008 AAOS Annual meeting, Dettoni et al.41 presented the only study, to our knowledge, comparing medial opening wedge HTO to UKA. Fifty-four patients treated with HTO and internal fixation with a Puddu plate (Arthrex) were compared to 56 patients treated with an Accuris unicompartmental knee prosthesis (Smith&Nephew). The Knee Society score and the WOMAC were evaluated at two to four years of follow-up. The two groups were comparable in terms of preoperative assessment and gender, but differed regarding age (mean age 55 years for HTO and 65 for UKA). Both groups obtained satisfactory results: 93% good-to-excellent results in the HTO group and 95% in the UKA group. Although differences were not significant, the UKA group showed a slightly better knee score (93 points compared to 76 in the HTO group), while HTO showed a better function score (91 points compared to 84 from the UKA group).

W-Dahl et al.42 reported the outcomes of all UKAs, TKAs and HTOs performed in Sweden from 1998 to 2007. In this study, 8793 UKA and 65661 TKA were included. For HTOs, a single institutional series of 450 patients, age 30-64 years, was used to calculate the revision rate and to compare to UKAs (n = 4,799; age 30-64 years). The 10-year cumulative revision rate (CRR) for HTO was 17%. Comparable CRR was observed in UKAs, but the risk of revision for the HTOs appeared to be somewhat lower for the first years of follow-up.

Some papers performed a review of literature regarding HTO and UKA, with inconclusive results due to difficulties in pulling data from nonhomogeneous studies.

Griffin et al.43 reported that, despite the poor-to-average quality of studies in the literature, HTO seemed to have a higher complication rate, lower survivorship and similar functional results compared to UKA.

Chang et al.44 determined that UKA provided improved range of motion, decreased rehabilitation time, and immediate weight bearing, while the advantages of HTO over UKA were the ability to maintain a higher level of activity without potential wear of arthroplasty components. The authors concluded that there was insufficient evidence to prefer one method over another with regard to results and longevity.

Brower et al.45 concluded in their meta-analysis (Co-chrane Database Systematic Review, 2007) that there is silver-level evidence that HTO causes more complications than UKA, and that there is silver-level evidence for no significant difference in pain, function and gait analysis after HTO compared to UKA.

Richmond46 reported that for younger, more active patients who have abnormal alignment, HTO seems to be a better choice than UKA. For the older, more sedentary patient, UKA is supported by the literature as a more reliable operation.

REVISION TO TKA

Whether revision HTO or UKA to TKA performs worse than primary TKA is also still controversial. Given that TKA represents the endpoint of every failed HTO or UKA, this aspect is particularly important, and can drive a surgeon's preference for one treatment over another.

Revision HTO to TKA

Windsor et al.47 analyzed the outcomes of 45 TKAs implanted after failed HTO, and compared them to data from other TKAs retrieved from other studies at the same institution. The incidence of poor results (by HSS score) was higher in the group of TKA after HTO than in other TKAs, implanted as primary or as revision after failed TKA. Fixation of the tibial component did not appear to be compromised, but an alteration of patel-lofemoral mechanics (patella infera) was observed. The authors concluded that patients treated with HTO should be informed that the results of TKA after a failed HTO may not be as good as TKA without previous osteotomy.

Karabatsos et al.48 performed a retrospective matched cohort study at a mean follow-up of five years, comparing 20 patients who underwent TKA after HTO and 20 matched patients who received a primary TKA. Surgical difficulties were more frequently encountered in the TKA-after-HTO group, with longer operative times, more difficult exposure, and an increased number of lateral releases. There were differences in pain and function according to WO MAC and SF-36 scores, but these were not statistically significant. The authors concluded that TKA after HTO is a more challenging procedure, technically, than primary TKA and that functional outcomes after TKA after previous HTO tended to be inferior, even though differences were not significant.

Van Raaij et al.49 performed a systematic review of the literature on TKA after prior HTO. The authors reported prolonged surgical time, extra operative procedures and less postoperative knee ROM, but no increase in revision surgeries for patients receiving TKA after prior HTO compared to patients receiving primary TKA. Improvements in pain and physical function scores were detected within the first three-to-six months after TKA implants in patients receiving primary TKA compared to patients receiving TKA after prior HTO. But at mid-term and long-term follow-up, no significant differences in knee clinical scores were found between the two populations. In patients receiving TKA after prior HTO, tibial component fixation was reported to be at risk due to the loss of metaphyseal bone stock and modification of proximal tibial anatomy. A revision tibial component with a long stem can increase primary stability, but may prevent accurate placement of the component due to an asymmetric medullary canal subsequent to HTO. Nevertheless, no differences in revision rates at minimum five years of follow-up were found between patients receiving TKA after previous osteotomy compared to primary TKA. The authors concluded that osteotomy does not compromise subsequent TKA.

Similar conclusions were drawn by Amendola et al.50 and Kazakos et al.51

Revision UKA to TKA

Revision of a failed UKA to TKA is thought to raise problems related to bone stock management and inferior clinical outcomes.

Barrett and Scott52 reported on 29 failed first-generation UKAs that were converted to TKA. The mechanism of failure was loosening in 55% of the cases and progression of degenerative disease in the opposite compartment in 31% of cases. The average time to revision was 47 months (range 4-113 months). In 66% of the failures, the authors identified technical errors at the time of initial UKA or poor patient selection.

McAuley et al.53 reported on a series of 39 consecutive UKA revisions to TKA, and observed that bone deficiencies were not challenging. The failed UKAs were easily managed using standard primary modular components and local autograft to treat bony defects. Good-to-excellent results were obtained in all patients at 24-120 months of follow-up, with an average range of moti8on (ROM) of 111 degrees and a mean knee society score and function score of 89 and 81 points respectively.

Becker et al.54 performed a matched-pair comparative study on 28 patients treated with TKA following failed UKA and 28 patients with primary TKA at a mean follow-up of 40-70 months. The authors found that revision of a UKA to a TKA showed inferior functional results in comparison to primary TKA, with significant differences in knee society scores, WO MAC and ROM. Patients were satisfied after revision UKA to TKA, but not as much as patients who received a primary TKA. In addition, the authors reported technical difficulties associated with UKA revisions to TKA. mainly about bone loss on the tibial side. TKA after UKA required significantly thicker polyethylene inlays in comparison with primary TKA.

Oduwole et al.55 reported on the mode of failure of a series of 106 Oxford phase III (Biomet) UKAs. The authors observed a 13.2% failure rate at one-to-four years of follow-up and concluded that long-term results of TKA after failed UKA were inferior compared to primary TKA in terms of pain and functional outcome (WOMAC, SF-36, ROM) and that revision could be complex.

Robertsson et al.56 evaluated the satisfaction of 27,372 patients operated upon between 1981 and 1995 in Sweden (Swedish Knee Arthroplasty Register-SKAR), with a simple four-point questionnaire. Patients who received a TKA after a failed UKA were less satisfied than patients who received a primary TKA.

Springer et al.57 reported the outcomes of 22 consecutive failed UKAs that underwent conversion to TKA. In 27% of the patients, defects requiring bone graft were found on the femoral condyle. No femoral stems or metal augmentations were required. On the tibial side, 45% of the patients had defects requiring bone graft. Metal wedge augmentation was required in 23% of the knees and stems were used in 9% of the cases. According to the knee society clinical rating system, the results of TKA after failed UKA were similar to both primary TKA and revision TKA. The authors concluded that patients should be informed that surgical results may not be as successful as a primary TKA, and surgeons should be aware that not all failed UKAs are easily converted to TKA.

Comparison between Revision HTO and Revision UKA

Gill et al.58 compared a group of 30 revision HTOs-to-TKAs with a group of 30 revision UKAs-to-TKAs, matched by age, gender, type of TKA, primary disease and length of follow-up. The study investigated technical difficulties at revision, complications, and outcomes using the knee society rating system. The most frequent technical difficulties in the revision HTO group were: 1) obtaining an acceptable exposure, and 2) achieving correct tibial component positioning. In the revision UKA group, the major technical difficulty was managing bony defects, on both the tibial and femoral sides. Significantly more osseous reconstructions were required in the UKA group (77%) compared to the HTO group (20%). The HTO group required more procedures to improve exposure: seven V-Y quadricepsplasties, 11 lateral retinacular releases, 3 PCL releases and 1 PCL sacrifices were performed. Nevertheless, these associated procedures did not affect the outcome of TKA at final follow-up. In the UKA group, two quadricepsplasties, 11 medial or lateral compartment releases, 1 PCL resection and 1 tibial tubercle osteotomy were performed. Post-revision ROM did not differ in the two groups. The HTO group scored significantly better in both Knee Society Knee score (87.3 points compared to 78.3 in the UKA group) and function score (88.6 compared to 67.7 in the UKA group). The authors concluded that the results of revision UKA-to-TKA approached but did not equal those obtained with revision HTO-to-TKA, which remains superior.

CONCLUSIONS

Although both HTO and UKA are effective for managing medial compartment knee arthrosis, they should not be considered equivalent treatment options. The literature review showed that correct patient selection is fundamental to obtaining durable and predictable results with both techniques. In addition, the indications seem to be very different for the two procedures. Nevertheless, a small population can be amenable to either HTO or UKA, and this includes patients: 1) from 60 to 65 years old; 2) who are moderately active; 3) who are non-obese; 4) with mild varus malalignment (from 5 to 10 degrees); 5) without joint instability; 6) with a good range of motion; and 7) with moderate unicompartmental arthritis.

Both HTO and UKA showed satisfactory results and survival rates at mid- and long-term follow-up. A few papers attempted to make comparisons between the two procedures and generally showed slightly better results for UKA in terms of survivorship and functional outcome. However, the differences were not remarkable and the quality of these studies is insufficient to draw any definitive conclusion. Furthermore, as shown in the present review, UKA and HTO are different procedures with different indications and a comparison between them is meaningful only in the small population of patients amenable to both treatments.

Whether revision HTO or UKA to TKA performs worse than primary TKA is a debatable issue. Given that TKA represents the endpoint of every failed HTO or UKA, this is particularly important, and was considered in the present paper. Both revision HTO and revision UKA to TKA are technically more challenging than primary TKA: 1) HTO in terms of surgical exposure and tibial component positioning, and 2) UKA in terms of bone stock loss and the need for bone grafting both on the femoral and the tibial sides. While HTO does not seem to affect the results of subsequent TKA, revision UKA to TKA apparently performs worse than primary TKA. It must be mentioned here that all the studies in the English literature reported the results of TKA after closing wedge HTO and no data are available about TKA after opening wedge HTO. This is an important issue because, theoretically, TKA is easier after opening wedge HTO than after closing wedge HTO. Indeed, with opening wedge HTO, there is no risk of patella alta, bone stock is maintained and the risk of impingement between the tibial stem and the anterior tibial cortex is decreased.

In conclusion, further investigation is required to determine the most reliable management choice for those patients that can currently be treated with either HTO or UKA.

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