Is Mobile-Bearing Medial Unicompartmental Knee Arthroplasty Appropriate for Asian Patients With the Risk of Bearing Dislocation?

Unicompartmental knee arthroplasty for lateral compartment knee osteoarthritis: a credible surgical approach and short-term clinical outcomes

PURPOSE

We aimed to investigate the technical advantages and clinical outcomes of lateral unicompartmental knee arthroplasty (LUKA) through the medial parapatellar approach.

METHODS

From August 2022 to June 2024, 104 patients who underwent LUKA via the medial parapatellar approach were enrolled. The operation time, hospital stays, surgical complications and follow-up period were collected. Pre- and postoperative Range of movement (ROM), Oxford knee score (OKS) and Hospital for special surgery knee score (HSS) were recorded and further investigated by paired-samples t-test analysis.

RESULTS

Primary demographic data of the retrospective case series collected include gender (30 males and 74 females), age (63.1 ± 8.37 years, range 49-84 years), operation time (86 ± 17 min, range 52-135 min), hospital stays (4.0 ± 1.1 days, range 3-8 days) and follow-up period (9.9 ± 5.7 months, range 1-23 months). There was a significant improvement in ROM (P < 0.001), OKS (P < 0.001) and HSS (P < 0.001) compared to preoperative values. Patient satisfaction at 1-month postoperative follow-up was up to 95%. The incidence of deep venous thrombosis was 16.3%. There was no incidence of postoperative infection.

CONCLUSION

The medial parapatellar approach is a understandable, easy to master and credible surgical approach for LUKA, showing a striking improvement in clinical outcomes without adverse events in the short-term follow-up.

Outcomes of fixed versus mobile-bearing medial unicompartmental knee arthroplasty

Aims

The optimal bearing surface design for medial unicompartmental knee arthroplasty (UKA) remains controversial. The aim of this study was to compare outcomes of fixed-bearing (FB) and mobile-bearing (MB) UKAs from a single high-volume institution.

Methods

Prospectively collected data were reviewed for all primary cemented medial UKAs performed by seven surgeons from January 2006 to December 2022. A total of 2,999 UKAs were identified, including 2,315 FB and 684 MB cases. The primary outcome measure was implant survival. Secondary outcomes included 90-day and cumulative complications, reoperations, component revisions, conversion arthroplasties, range of motion, and patient-reported outcome measures. Overall mean age at surgery was 65.7 years (32.9 to 94.3), 53.1% (1,593/2,999) of UKAs were implanted in female patients, and demographics between groups were similar (p > 0.05). The mean follow-up for all UKAs was 3.7 years (0.0 to 15.6).

Results

Using revision for any reason as an endpoint, five-year survival for FB UKAs was 97.2% (95% CI 96.4 to 98.1) compared to 96.0% for MB (95% CI 94.1 to 97.9; p = 0.008). The FB group experienced fewer component revisions (14/2,315, 0.6% vs 12/684, 1.8%; p < 0.001) and conversion arthroplasties (38/2315, 1.6% vs 24/684, 3.5%; p < 0.001). A greater number of MB UKAs underwent revision due to osteoarthritis progression (FB = 21/2,315, 0.9% vs MB = 16/684, 2.3%; p = 0.003). In the MB group, 12 (1.8%) subjects experienced bearing dislocations which required revision surgery. There were 15 early periprosthetic tibia fractures (0.6%) in the FB group compared to 0 for MB (p = 0.035).

Conclusion

In similar patient populations, FB UKAs demonstrated slightly higher survival than a commonly used MB design. Adverse event profiles differed by bearing type, with an increased risk of bearing dislocation and OA progression with MB designs, and early periprosthetic tibia fractures for FB designs.

Advantages and limitations of mobile-bearing unicompartmental knee arthroplasty: an overview of the literature

INTRODUCTION

Interest in unicompartmental knee arthroplasty (UKA) has recently grown. Mobile bearing UKA, in which the bearing is not fixed but rather perfectly conforms with femoral and tibial components and moves completely passively between the femoral and tibial implant, has now been used for approximately half a century.

AREAS COVERED

Alongside the recognized advantages of UKA, the mobile-bearing variant benefits from an extremely low rate of polyethylene wear and tolerable minor malalignment. Revision rates for UKA have been reported to exceed those of total knee arthroplasty, but long-term survival rates and outcomes from mobile-bearing UKA have been found to be satisfactory. In addition to the lateral osteoarthritis and loosening, which are main complications of UKA, bearing dislocation is a specific complication of mobile bearing UKA. Fractures and valgus subsidence are more prevalent than in the cementless UKA. While these continue to be features to be addressed, they have been partially solved.

EXPERT OPINION

Given the manifold benefits of UKA, its application could be extended to a larger patient population. Successful outcomes rely on careful patient selection and the surgeon's extensive familiarity with the procedure. Looking ahead, the incorporation of robotic surgery, already a feature of some fixed-bearing UKAs, might shape the future trajectory of mobile-bearing UKA.

Propensity of the tight mid-flexion and loose flexion gap despite the adjustment of extension and flexion gaps in Oxford unicompartmental knee arthroplasty: retrospective study

PURPOSE

A well-balanced joint gap is necessary in Oxford unicompartmental knee arthroplasty (OUKA) to prevent mobile-bearing dislocation. While the gaps between 20° (extension) and 100° (flexion) are precisely adjusted using the incremental mill system, there has been insufficient evaluation of gaps in other angles. We hypothesized that the gap is not always the same in other angles. This retrospective study aimed to evaluate the gap in full-extension (0°), mid-flexion (60°) and deep flexion (130°) for comparison with those in extension and flexion gaps.

METHODS

We evaluated 119 knees in 83 patients (51 females, 31 males, aged 71.9 years). The full-extension and mid-flexion gaps were compared with the extension gap, and the deep flexion gap was contrasted with the flexion gap. Each gap was classified into isometric, tight or loose, for evaluation of contributing factors.

RESULTS

Although the full-extension gap tended to be isometric (45%), the mid-flexion tended to be tight (48%), whereas the deep-flexion was loose in most knees (84%) (P = 0.002). The tight mid-flexion and loose deep flexion gap pattern accounted for 44% of the total knees, especially so with smaller femoral components (P = 0.004).

CONCLUSION

Our results highlight the propensity of tight mid-flexion and loose flexion gap despite the adjustment of extension and flexion gaps in OUKA. Although the effect of such a minor gap imbalance is still unknown, the pattern was more prevalent in patients with smaller-sized femoral components. Use of a larger femoral component may equalize the gap throughout the motion arc.

Bearings can dislocate with smaller femoral components and thicker bearings in Oxford™ medial unicompartmental knee arthroplasty

BACKGROUND

A mobile bearing can dislocate when joint laxity is larger than jumping height, the height difference between the bottom and the peak of the bearing (the highest point of the upper bearing surface on each side). Significant laxity due to improper gap balancing should therefore be avoided. However, once the bearing rotates vertically on the tibial component, the bearing can dislocate with smaller laxity than the jumping height. We mathematically calculated the required laxity for dislocation (RLD) and the required rotation of the bearing for dislocation (RRD). The current study addressed the question: 1) could the femoral component size and the bearing thickness affect the RLD and RRD?

HYPOTHESIS

The femoral component size and the bearing thickness could affect the MLD and MRD.

METHODS

The RLD and RRD were calculated using the bearing dimensions provided by the manufacturer with femoral component size, bearing thickness, and directions (anterior, posterior, and medial/lateral) as the variables on a two-dimensional basis.

RESULTS

The RLD was 3.4 to 5.5mm in the anterior, 2.3 to 3.8mm in the posterior, and 1.4 to 2.4mm in the medial or lateral directions. The RLD decreased with a smaller femoral size or a thicker bearing. Similarly, the RRD decreased with a smaller femoral size or a thicker bearing thickness in all directions.

CONCLUSIONS

Increased bearing thickness and decreased femoral component size deceased the RLD and RRD, which would relate to an increased risk of dislocation. Selecting the femoral component as large as possible and the bearing as thin as possible would therefore be helpful in the prevention of dislocation.

LEVEL OF EVIDENCE

III; comparative computer simulation study.

Pre-operative predictive factors of residual varus on the mechanical axis after Oxford unicompartmental knee arthroplasty

Background

Residual varus after Oxford unicompartmental knee arthroplasty (UKA) happens frequently. This study aims to evaluate the pre-operative contributing factors of residual varus.

Methods

A total of 1,002 knees (880 patients, 201 patients were male, and 679 were female) underwent Oxford UKA in the Orthopedic Surgery Department of the Beijing Jishuitan Hospital from March 2018 to April 2021. The mean age of the patient was 64.7 ± 7.7 years. To assess residual varus, the full-length lower extremity is placed upright for EOS imaging, with the knee fully extended. The angle of post-operative residual varus was measured as described by Noyes et al. Of the knees studied, they were either categorized into an under-corrected group (post-operative Noyes angle >5°) or a corrected group (post-operative Noyes angle ≤5°). Age, gender, body mass index (BMI), range of motion (ROM), Clinical American Knee Society Score (Clinical AKSS), and Function American Knee Society Score (Function AKSS) were compared. The following additional parameters were measured: pre-operative Noyes angle, lateral distal femoral angle (LDFA), medial proximal tibial angle (MPTA), the posterior slope of the proximal tibia angle (PPTA), joint line converge angle (JLCA), and fixed flexion deformity (FFD).

Results

There was no statistically significant difference between the two groups in regards to gender (p = 0.428), surgical leg (p = 0.937), age (p = 0.851), BMI (p = 0.064), pre-operative Clinical AKSS (p = 0.206) and Function AKSS (p = 0.100). However, pre-operative ROM statistically differed between the two groups (p < 0.001). The contributing factors of post-operative residual varus were determined to be the following parameters: pre-operative MPTA (p < 0.001, OR = 4.522, 95% CI: 2.927-6.984), pre-operative Noyes (p < 0.001, OR = 3.262, 95% CI: 1.802-5.907) and pre-operative FFD (p = 0.007, OR = 1.862, 95% CI: 1.182-2.934). The effects of pre-operative LDFA (p = 0.146), JLCA (p = 0.942), and pre-operative PPTA (p = 0.899) on the post-operative mechanical axis did not show statistical significance.

Conclusions

Patients with severe pre-operative varus, particularly varus deformity mainly from the tibial side or pre-operative FFD, are more prone to get extremity mechanical axis residual varus after UKA with Oxford.

Approximately 41% of knees have a looser gap in full extension than in 20° flexion after Oxford unicompartmental arthroplasty

INTRODUCTION

In Oxford unicompartmental knee arthroplasty (OUKA), the flexion and extension gaps should be adjusted to prevent mobile-bearing dislocation. The extension gap is recommended to be evaluated in the 20° flexion position to avoid underestimation due to tension of the posterior capsule. However, we have become aware of a looser gap in full extension than in 20° flexion in some instances.

MATERIALS AND METHODS

We retrospectively investigated 83 knees in 60 patients who underwent OUKA between January and June 2020. During surgery, the extension gaps were measured in both full extension and 20° flexion. The knees were classified into two groups: the gap was looser in full extension (0° group), and the gap was equal or looser in 20° flexion than in full extension (20° group). The hip-knee-ankle angle (HKAA), the lateral distal femoral angle (LDFA), the medial proximal tibia angle (MPTA), the posterior tibial slope angle (PTSA), and the last spigot size were also measured and compared between the groups.

RESULTS

There was looseness in approximately 41% of knees (34 out of 83 knees) in full extension. In the knees in the 0° group, the last spigot size was significantly smaller (median 1 and 2, P < 0.01). However, there were no significant differences in the HKAA, MPTA, LDFA or PTSA between the groups.

CONCLUSIONS

Approximately 41% of knees have a looser gap in full extension than in 20° flexion after OUKA. Further investigation is needed to better understand which extension gap should be used in such cases, and to find the contributing factors in loose full extension gap other than the size of the last spigot.

Effect of tibial component overhang on survivorship in medial mobile-bearing unicompartmental knee arthroplasty

BACKGROUND

Some studies have shown that the position of the tibial component in Oxford unicompartmental knee arthroplasty with a mobile bearing will affect the clinical outcome of patients. Hence, our study aimed to investigate the relationship between the overhang distance of the tibial component and the survival of the implant.

METHODS

A retrospective analysis of patients who underwent unicompartmental knee arthroplasty at the same institution from 2014 to 2018 was presented. The study was divided into three groups: minor underhang group (underhang between -3 and 0 mm); minor overhang group (overhang 0-3 mm); and major overhang group (overhang ≥ 3 mm). Demographic and clinical profile characteristics of each group were compared, and survival curves of each group were also compared using Kaplan-Meier and modeled using multivariate Cox regression.

RESULTS

A total of 351 knees were included in this study with a minimum follow up of three years and a mean follow up of 4.8 ± 1.5 years. The revision rates in each group were 3.6% (minor underhang group), 2.7% (minor overhang group), and 20.9% (major overhang group) (P < 0.001). From the three groups' cumulative survival rates, the major overhang group was significantly lower than the other two groups (log rank P < 0.001). Multivariate Cox regression showed an association between the major overhang group and implant survival rate (hazard ratio = 7.515, 95% confidence interval = 2.500-22.593, P < 0.001) CONCLUSION: The risk of revision will increase if the tibial component overhangs more than 3 mm medially. Moreover, the reasons for revision are generally bearing dislocation and aseptic loosening.

Bearing Separation From the Lateral Wall of the Tibial Component Is a Risk of Anterior Dislocation of the Mobile Bearing in Oxford Unicompartmental Knee Arthroplasty

BACKGROUND

Bearing dislocation is a serious complication after Oxford unicompartmental knee arthroplasty. Bearing separation from the lateral wall can cause it to spin (90° horizontal rotation) and eventually dislocate because there is just a 2 mm difference in height in both the lateral and medial sides from the bottom of the bearing, compared with the anterior (5 mm) and posterior (3 mm) sides. The details of this problem have not been previously examined.

METHODS

Twenty-one dislocations in 12 patients were retrospectively analyzed. Bearing separation was defined as the bearing position being sufficiently distant from the lateral wall of the tibial component to allow spinning. We analyzed the incidence of separation, the direction and the recurrence of the dislocations, and their causes and treatments.

RESULTS

Five of the 12 patients had separation. Of the total of 21 dislocations, 11 occurred in cases of separation (52%). Seven of 11 anterior dislocations were found to have separation, whereas nine of 13 posterior dislocations occurred without separation (P = .0237). Three of 5 patients with separation had recurrence of dislocation, and eventually 2 underwent revision to fixed-bearing unicompartmental knee arthroplasty.

CONCLUSION

Bearing separation from the lateral wall of the tibial component can cause bearing dislocation, especially in an anterior direction. To prevent separation, the wall-bearing distance should be evaluated before the keel slot preparation, with manipulation as necessary. Conversely, posterior dislocation was predominant in our nonseparation cases.

Potential factors in postoperative dislocation of Oxford phase III mobile bearing UKA in Chinese patients: a single-center retrospective study

BACKGROUND

Bearing dislocation is the main complication after mobile bearing unicompartmental knee arthroplasty. The purpose of this study was to analyze the potential risk factors of bearing dislocation after Oxford phase III mobile bearing unicompartmental knee arthroplasty in Chinese patients.

METHODS

We retrospectively investigated 492 patients (578 knees) who underwent Oxford phase III mobile bearing unicompartmental knee arthroplasty in our institution between February 2009 and June 2019. The patients were divided into two groups based on surgeons' annual surgical volume. Those with/ without bearing dislocation were compared based on patient, surgeon and implant factors.

RESULTS

Among the 492 patients, 21 (4.3%, 4 men and 17 women) experienced bearing dislocation. Of these, 14 (4.0%) were in the high surgical volume group and 7 (5.1%) were in the low surgical volume group. Multivariate analysis revealed that trauma to the operated leg and daily life involving high knee flexion cumulatively predicted bearing dislocation (p < 0.05).

CONCLUSIONS

Trauma to the operated leg and daily life involving high knee flexion were risk factors for bearing dislocation after Oxford phase III mobile bearing unicompartmental knee arthroplasty.

Bearing dislocation of mobile bearing unicompartmental knee arthroplasty in East Asian countries: a systematic review with meta-analysis

Background

Bearing dislocation is a common postoperative complication of mobile-bearing unicompartmental knee arthroplasty, and East Asian patients tend to be at higher risk. However, whether this high dislocation rate is common in all East Asian populations remains unclear. This meta-analysis was performed to explore this issue and describe various features of dislocation in East Asians, including the onset time, dislocation direction, and re-dislocation rate.

Methods

The literature was searched in PubMed, Embase, Ovid, and Cochrane Library up to May 2020. Studies were scrutinized by two independent authors, and the bearing dislocation rate, onset time, direction, and re-dislocation rate were specifically analyzed. RevMan 5.3 was used for the statistical analysis.

Results

Seven case series from Korea, China, and Japan were included. The pooled analysis showed that the total dislocation rate was 2.37%, while the subgroup analysis showed that the dislocation rate in Korea and other countries was 4.50% and 0.74%, respectively (P < 0.01). Another subgroup analysis of the onset time showed a significant difference before and after the first 5 years postoperatively (P < 0.01). Anterior and posterior dislocations were more frequent than medial and lateral dislocations (P < 0.01). The average re-dislocation rate was 32.45%, which was approximately seven times higher than the primary dislocation rate (P < 0.01).

Conclusion

Our meta-analysis demonstrated that Korea had a higher bearing dislocation rate among East Asian countries, especially in the first 5 years after primary UKA. Anterior and posterior dislocations were common. The most important finding is that the re-dislocation rate can be much higher than the initial dislocation rate.

Survivorship of fixed vs mobile bearing unicompartmental knee replacement: A systematic review and meta-analysis of sixty-four studies and National Joint Registries

BACKGROUND

Unicompartmental knee replacement (UKR) prostheses can use fixed (FB) or mobile bearing (MB) constructs. We compared survivorship and failure modes of both designs.

METHODS

The inclusion criteria were studies published between 2005 and 2020 with minimum average follow-up of five years reporting the survival and/or number of revisions of specific designs in medial and lateral UKRs. Pooled rate of revision per 100 patient years (PTIR) was estimated using a random effects model.

RESULTS

Seventy cohorts of 17,405 UKRs with weighted mean follow-up of 7.3 years (0.1-29.4 years) were included. A total of 170,923 UKRs were identified in registry reports at a weighted mean implant survival time of 15.4 years. PTIR in MB UKR versus FB UKR was similar [1.45 vs 1.40, (p = 0.8)]. In cohort studies, the overall PTIR for MB was also similar to FB [1.03 vs 0.78, (p = 0.1)]. For medial UKR, the PTIR for MB was marginally greater but not significantly different to FB [0.96 vs 0.81, (p = 0.3)], whilst for lateral UKR, the PTIR for MB was significantly worse than for FB [2.20 vs 0.72, (p < 0.01)]. Polyethylene wear is more common in FB implants, whilst MB implants are revised more often for bearing dislocation.

CONCLUSIONS

Overall implant survival in mid- to long-term studies is similar for MB versus FB medial UKRs. MB have a four-fold higher risk of revision in comparison to FB when used for lateral UKR.

Does new instrument for Oxford unicompartmental knee arthroplasty improve short-term clinical outcome and component alignment? A meta-analysis

Background

The Microplasty (MP) instrumentation designed for the Phase III Oxford mobile-bearing unicompartmental knee arthroplasty (UKA) system is considered a better option to achieve more accurate component positioning and alignment. In the present study, we focused on short-term clinical and radiological outcomes to determine whether the MP instrumentation can reduce the short-term revision rate and occurrence of outliers of metallic components.

Methods

The literature in PubMed, Embase, the Cochrane Library, and Web of Science was searched up to May 2020. Studies were scrutinized by two independent authors, and the revision rate, complication spectrum, and radiological assessment with outlier rates were specifically analyzed. RevMan 5.3 was used for the statistical analysis.

Results

Seven studies were included in the meta-analysis. Four studies reported both clinical and radiological outcomes, two reported only radiological outcomes, and one reported only clinical outcomes. The pooled analysis showed that the revision rate in the MP instrumentation group was 0.866 per 100 component years, while that in the control group was 1.124 (odds ratio, 0.77; p < 0.05). The subgroup analysis of the bearing dislocation rate showed a significantly greater reduction in the Korean population than in the populations of other countries (p < 0.05). The radiological assessment showed that the alignment of the femoral component was significantly improved (p < 0.05), while that of the tibial component was not (p > 0.05).

Conclusion

The newly developed MP instrumentation for Oxford UKA significantly reduced the revision rate of this treatment. The positioning of the femoral component was also proven to be better by radiological assessments.