Revisiting the Indications for Unicondylar Knee Replacement

Unicompartmental knee arthroplasty (UKA) is an effective treatment for unicompartmental arthritis (OA), which can significantly relieve knee pain and improve function. In 1989, Kozinn and Scott proposed the selection criteria for UKA. Since then, the failure rate of UKA surgery has been significantly reduced, and this indication has become a classic. However, this standard is too strict, and only 5% of patients with knee OA meet it.


In recent years, with the advancement of prosthetic design, improvement of surgical techniques, and expansion of indications, the usage rate of UKA has gradually increased worldwide. From 2000 to 2009, the number of UKA surgeries in the United States increased 6.2 times, accounting for 4.5% of knee replacements. The proportion of UKA in primary knee replacement surgeries increased steadily from 2014 to 2019, reaching 11.5% in 2019. According to incomplete statistics, the number of UKA surgeries in my country in 2020 is about 23,000, and the proportion of primary knee replacement surgeries is only 5.4%. This creates a scissor gap with the huge population of knee OA patients in my country, and may be inconsistent with orthopedic surgeons’ over-compliance. It is related to the classic indications, so it is of practical significance to reconsider the indications for UKA surgery. Based on the first-hand experience of more than 4,000 UKA surgeries performed by our team in clinical practice, and combined with the latest research progress, this article discusses the indications of UKA and provides a reference for clinical practice.

1. Obesity

In the past few decades, the impact of obesity on UKA has remained controversial. Obesity is a clear risk factor for conversion to total knee arthroplasty (TKA) after UKA, a 10-year study shows. However, recent research shows that obesity does not affect the mid-term results of UKA. Plate et al. followed 212 UKA patients for an average of 12 years and found that obesity did not affect the long-term survival rate and clinical outcomes after UKA with removable spacers. Wang Fangxing et al. found that patients with moderate to severe obesity and non-obese groups were more satisfied 2 years after UKA surgery with removable spacers, and there was no statistically significant difference in functional scores and complication rates. In the study on fixed-platform UKA, Xu et al. conducted a follow-up study of at least 10 years and found that the clinical outcomes of obese patients were poorer and the revision rate increased significantly. Woo et al. followed 673 UKA patients with fixed spacers for 8.5 years and found that preoperative body mass index (BMI) did not affect the early prognosis of UKA patients. But when the BMI exceeds 30kg/m2, the revision rate is higher. Foo et al also found that severely obese patients with a BMI ≥35kg/m2 who used fixed spacer prostheses had lower American Knee Society knee and function scores (KSKS scores) and lower satisfaction. Therefore, it is questionable that UKA strictly follows the selection criteria based only on weight. For general obese patients, the use of removable or fixed spacer UKA prostheses will not affect the mid- and long-term efficacy and survival rate. However, it is not recommended to use fixed spacer prostheses for severely obese patients.

2. Young Patients

Young patients are usually defined as ≤60 years of age, and UKA treatment for patients in this age group remains controversial. Kennedy et al. conducted a prospective study of 1,000 cases of UKA and found that there was no statistically significant difference in the 10-year prosthesis survival rate between the younger age group and the older age group. The European Knee Association found that UKA can significantly improve the knee joint function of patients under the age of 60, with a 10-year prosthesis survival rate of 86% to 96.5% and a 15-year implant survival rate of 94.8%. Wang et al also found that the knee joint function of young patients (<60 years old) improved significantly after UKA, and the 3-year follow-up prosthesis survival rate reached 99.1%. Salman et al followed 6130 UKA patients for 7.5 years and found that younger age was not associated with higher revision rates or lower functional scores. Based on the development of modern UKA technology and materials science, both removable and fixed spacer prostheses are suitable for young patients. However, young patients are active a lot, and the risk of dislocation and wear of the movable spacer still exists. Therefore, if you need to squat or kneel frequently during daily activities or work, it is recommended to use a UKA prosthesis with a fixed spacer.

3. Elderly Patients

Elderly patients are usually defined as ≥75 years old, elderly patients are defined as ≥80 years old, and very elderly patients are defined as ≥85 years old. Elderly patients have relatively low functional requirements for the knee joint, and the lifespan of the prosthesis may be longer than expected, making them theoretically more suitable for UKA treatment. Fabre Aubrespy et al. compared 101 patients over 75 years old who underwent UKA or TKA. The knee joint function score of the UKA group was higher, and the 16-year survival rate was similar to that of the TKA group (91.8% vs. 94.6%). Liow et al also found that the 8.3-year survival rate of patients aged ≥75 years was as high as 98.9%. Ode et al found that the clinical outcomes of UKA and TKA in super-elderly patients were similar, but there were fewer early complications. Tu Yihui’s team conducted early and mid-term follow-up of patients >70 years old or ≥85 years old and found that their functional recovery after UKA was satisfactory, with low complications and mortality. Therefore, if physical conditions permit, elderly patients are also suitable for UKA surgery.

4.Patellofemoral Joint OA

Whether patellofemoral joint degeneration is suitable for UKA surgery has always been controversial. Beard et al believed that except for groove-like changes in the lateral patellar articular surface or patellar subluxation, patellofemoral joint OA should not be considered a contraindication to UKA. Hamilton et al also found that medial patellofemoral joint OA does not affect knee joint function and prosthesis survival rate after UKA. Adams et al showed that the functional recovery of UKA with fixed spacers was not adversely affected by degeneration of the medial patella or central pulley. Ma Tong et al. followed 95 patients (100 knees) for 50 months and found that medial patellofemoral joint OA did not affect the efficacy of the movable spacer UKA, but lateral patellofemoral joint OA increased the risk of surgery. UKA can correct the alignment of the lower limbs and restore normal patellar movement trajectory, allowing knee joint function to recover faster. Therefore, medial patellofemoral joint OA, central trochlear degeneration, and mild to moderate lateral patellofemoral OA are suitable for UKA, while changes in the patellofemoral joint groove and subluxation are not suitable for UKA.


5. Anterior Cruciate Ligament Loss (ACLD)

The traditional view is that ACLD will cause knee joint instability, leading to a significant increase in complications such as prosthetic loosening and pad wear. Ma Tong et al. used reverse engineering technology to reconstruct the three-dimensional finite element model of the movable spacer UKA, and found that ACLD did not increase the maximum contact stress of the knee joint, but it increased the displacement of the knee in straight position. ACLD has different pathogenesis. In anteromedial OA, anterior cruciate ligament (ACL) injury is secondary, which is completely different from primary ACL injury leading to OA. Plancher et al found that if the knee joint is stable, UKA can still achieve good results even if the ACL is completely ruptured. After an average follow-up of 6.9 years, Du et al found that the daily living score of the ACLD group was higher than that of the control group, but the revision rate did not increase significantly. Another study on 114 UKA patients showed that there was no statistically significant difference in functional scores and 10-year survival rates between the ACLD group and the intact ACL group. Wen Tao et al. followed up 60 patients with ACLD for an average of 30.5 months and found that knee varus deformity, mobility and functional scores were significantly improved after UKA surgery, and no prosthesis-related complications occurred. For young patients with OA secondary to primary ACL injury, Jaber et al. used UKA combined with one-stage ACL reconstruction in 23 cases. After 10 years of follow-up, the patients’ pain and functional scores were significantly improved, and the 14.5-year graft survival rate was as high as 91.4%. After research by Tang Hai et al., it is recommended that young patients with medial knee compartment OA secondary to ACL defect undergo UKA combined with ACL reconstruction surgery, which can achieve good mid-term effects. Therefore, ACLD should be treated with caution. Patients with primary ACL injury and secondary OA are generally not suitable for UKA surgery, but young patients can try one-stage UKA combined with ACL reconstruction treatment. For patients with ACLD secondary to OA, if there is no instability in the knee joint, experienced surgeons can proceed with caution, but it must not be used as a routine choice for UKA treatment.


6. Severe Knee Varum Deformity

Classic UKA indications limit knee varus deformity to less than 15°. Excessive varus deformity is often difficult to correct. At the same time, excessive residual varus can easily cause overload and increase the risk of gasket wear and prosthesis loosening. In the past 30 years, although UKA technology has continued to mature and prosthetic design has continued to advance, there is still a lack of consensus on whether UKA surgery can be performed in patients with >15° knee varum. The focus of the debate is how much varus residue is allowed after UKA surgery. Kleeblad et al found that only 1 of 8 patients with knee varum between 15° and 18° was undercorrected after UKA. Gulati et al evaluated 160 UKA patients and found that 8% of patients had varus residual exceeding 10° after surgery, but this did not affect the knee joint function score and imaging results. The author believes that no matter how the lower limb force line changes after UKA, the contact stress center is near the prosthesis, so UKA has greater tolerance for residual knee varum. In addition, the ultra-high wear-resistant polyethylene spacer helps improve the long-term survival rate of the prosthesis. Seng et al. analyzed 13 patients with 15° to 20° knee varum. There was no significant difference in knee joint function and quality of life scores 2 years after UKA compared with the mild varus group. No case required revision during the 5-year follow-up. Research by Ge Juncheng and others found that UKA gap pressure is related to the degree of postoperative lower limb alignment correction. In patients with good postoperative lower limb alignment correction, the intraoperative knee joint gap pressure at 0° and 20° is greater than in patients with residual varus after surgery.

Nonetheless, existing research results are still very limited. For patients with severe genu varum (15°~20°) whose lesions are limited to the medial compartment and can be corrected, UKA may be cautiously considered, especially in patients who are unable to tolerate TKA due to advanced age. However, there is a lack of literature for patients with genu varum >20°. Support, UKA surgery is not recommended at this stage.


7. Flexion contracture deformity (FCD)

The typical indications for UKA limit FCD to less than 5°. Most scholars believe that UKA is different from TKA in that it can only partially solve the pathological changes of knee joint contracture. In recent years, the indications for UKA have been broadened, and relevant literature has reported the clinical results of UKA for FCD >5°. Purcell et al. treated 53 patients with FCD of 11° to 20° using UKA with a fixed spacer, and matched them with 53 patients with posterior cruciate ligament-preserving TKA. After 8 to 10 years of follow-up, the knee joint mobility and American Knee Society score in the UKA group were (KSS score) was significantly higher than that in the TKA group, but the residual FCD was greater in the UKA group (4.1° vs. 2.1°). Goh et al found that 87 patients with severe FCD (average 18°) had poor KSS and quality of life scores at 6 months, and at least 1/3 still had moderate to severe FCD. However, the functional scores, quality of life, and surgical satisfaction of patients in the FCD group and the control group after 2 years were similar, and the long-term prosthetic survival rates at 11.5 years were similar. Yeh et al. grouped 172 patients according to residual FCD after surgery. A 10-year long-term follow-up showed that knee joints with residual FCD >10° had poorer function, and the larger the residual FCD, the worse the functional recovery. Chen et al found that there was no difference in functional recovery and quality of life between patients with FCD >10° and <10° before UKA surgery, but residual FCD >10° after surgery was associated with poorer function. The authors suggest that UKA can be used with caution in the treatment of patients with moderate to severe FCD (<20°) at this stage. FCD should be corrected as much as possible during surgery to avoid excessive residual deformity after surgery that affects functional recovery. TKA is recommended for patients with FCD >20°.


8. Failure After High Tibial Osteotomy (HTO)

In recent years, with the popularization of the concept of knee preservation, HTO is more used to treat knee varum OA, but its efficacy is still controversial. Insall et al reported that as many as 23% of patients required revision during an average follow-up of 8.9 years after HTO surgery. The main reason was that the medial space of the distraction significantly narrowed or even disappeared. TKA revision is a classic surgical option, but UKA revision has inconsistent opinions on its efficacy. Rees et al earlier reported the effect of Oxford UKA revision in a group of 18 patients with failed HTO. After an average follow-up of 5.4 years, 5 cases (27.8%) required TKA revision again. The revision rate was 9 times that of the initial UKA, and the 10-year survival rate was only 66%. The reason for failure was postoperative ectropion leading to aggravated lateral compartment degeneration. On the contrary, in 2021, Schlumberger et al reported 30 cases of failed HTO revision using medial movable spacer UKA. After an average follow-up of 4.3 years, good results were obtained, with a prosthesis survival rate of 93%. The author believes that HTO failure is not an absolute contraindication for medial UKA. Based on this, Valenzuela et al. used fixed spacer UKA to treat 22 cases of HTO failure. The efficacy was equivalent to the initial UKA and TKA revision after HTO failure, and the results were satisfactory. Despite this, there is still a lack of clinical evidence on which type of prosthesis to choose after HTO failure. Recently, Morales Avalos et al. compared the results of fixed or mobile spacer UKA in the treatment of HTO failure. The functional scores of the two groups were similar, but the revision rate was lower and the prosthesis survival time was longer in the fixed group. Therefore, based on the limited clinical data, taking into account the inherent risk of dislocation of the removable spacer prosthesis and the potential loosening of the medial structures caused by initial HTO release of the medial collateral ligament, the authors recommend that UKA revision with fixed spacers is the first choice for patients who fail after HTO surgery. Avoid eversion of the lower limbs. Patients with preoperative lower extremity valgus are not suitable for UKA.


9. Post-traumatic Arthritis (PTA)

The incidence of PTA ranges from 21% to 83%, and patients are usually younger than those with primary OA and are more likely to have adverse outcomes due to previous surgery or trauma. Some clinical cases only involve unilateral compartment, and the optimal treatment plan is still controversial. In recent years, some scholars have reported the effect of UKA in single-compartment PTA. Edmiston et al evaluated 65 cases of lateral UKA. After an average follow-up of 82 months, the authors pointed out that PTA is a risk factor for surgical failure. Argenson et al. treated 12 patients with PTA secondary to tibial plateau fractures using lateral UKA. After an average of 12.6 years of follow-up, knee joint function was significantly improved, and the 10-year prosthesis survival rate reached 92%. Lustig et al. conducted an average follow-up of 10.2 years on 13 cases of PTA treated with lateral UKA surgery. As a result, the International Knee Society (IKS) score was significantly improved, and the 5-year and 10-year prosthesis survival rates were 100%. Romagnoli et al also found that lateral UKA can significantly improve the function of PTA patients and effectively relieve pain, with a 9-year survival rate of 92%. Xue Huaming et al. retrospectively analyzed 30 PTA patients treated with UKA. The postoperative knee joint function was significantly improved, with a satisfaction rate of 86.7% and a 6-year prosthesis survival rate of 100%. Therefore, the author suggests that if PTA only involves the unilateral femoral-tibial space, the ligament function is intact, and the deformity is reversible, it can be treated with UKA surgery, but it is still necessary to expand the sample and extend the follow-up time to further observe the efficacy.



In summary, UKA is an effective method for the treatment of unicompartmental knee osteoarthropathy. With the continuous development of treatment concepts, prosthesis design, supporting tools and surgical techniques, more and more clinical studies have provided theoretical basis for the expansion of classic indications more than 30 years ago. On the premise of strictly following the four basic elements of UKA (severe degree of degeneration, reversible deformity, ligament function and intact contralateral compartment cartilage), the author believes that: obese (BMI≥30kg/m2), young (<60 Years old) or elderly (≥75 years old) patients, patellofemoral OA without groove degeneration or lateral patellar subluxation, severe varus and FCD (<20°), ACLD without knee instability, failed HTO surgery, and single room Indoor PTA is not an absolute contraindication to UKA. Looking to the next stage, in terms of indication expansion, it is necessary to carry out multi-center high-quality evidence-based medical research, expand samples, and conduct mid- and long-term follow-up to ensure the effectiveness and safety of UKA surgery.


Cite this article: Xue Huaming, Ma Tong, Wen Tao, et al. Revisiting the indications of unicondylar knee replacement [J]. Chinese Medical Journal, 2024, 104 (5): 319-324. DOI: 10.3760/cma.j .cn 112137-20230518-00820.


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Post time: May-17-2024