Introduction. Unicompartmental Knee Replacement Arthroplasty (UKA) is a treatment option for early knee OA that appears under-utilised, partly because of a lack of clear guidance on how to best restore lasting knee function using such devices. Computational tools can help consider inherent uncertainty in patient anatomy, implant positioning and loading when predicting the performance of any implant. In the present research an approach for creating patient-specific finite element models (FEM) incorporating joint and muscle loads was developed to assess the response of the underlying bone to UKA implantation. Methods. As a basis for future uncertainty modelling of UKA performance, the geometriesof 173 lower limbs weregenerated from clinical CT scans. These were segmented (ScanIP, Simpleware Ltd, UK) to reconstruct the 3D surfaces of the femur, tibia, patella and fibula. The appropriate UKA prosthesis (DePuy, U.S.) size was automatically selected according to tibial plateau size and virtually positioned (Figure 1). Boolean operations and mesh generation were accomplished with ScanIP. A patient-specific musculoskeletal model was generated in open-source software OpenSim (Delp et al. 2007) based on the Gait2392 model. The model was scaled to a specific size and muscle insertion points were modified to corresponding points on lower limb of patient. Hip joint load, muscle forces and lower limb posture during gait cycle were calculated from the musculoskeletal model. The FE meshes of lower limb bones were transformed to the corresponding posture at each time point of a gait cycle and FE analyses were performed (Ansys, Inc. U.S) to evaluate the strain distribution on the tibial plateau in the implanted condition. Results. With the tibial component positioned above, along or below the joint line, the lower limb alignment was more varus, remained unaltered or more valgus respectively (Figure 2). With the tibial component positioned 3mm above the joint line, the peak strain in the underlying bone was 670 µstrain on medial (UKA) side and 6780 µstrain on the intact side. With the tibial component positioned 3mm below the joint line, the peak strain was 3010 µstrain on the medial side and 5330 µstrain on the intact side. Here, the strains on the medial side increased by 2640 µstrain whilst they were reduced by 1450 µstrain on the intact side compared to the unimplanted case. Conclusion. The present research has delivered a framework which can be exploited in future uncertainty modelling of UKA performance predictions. The patient-specific model incorporates loading, anatomical and material property variability, and can be applied to evaluate the performance of UKA prostheses for metrics such as stress/strain/micromotions in larger patient populations. For figures/tables, please contact authors directly.

UKA with mobile bearing is a one of the treatment of medial osteoarthritis. However, some reports refer to the risk of dislocation of the mobile bearing. Past reports pointed out that medial gap might be enlarged in deep flexion position (over 120 degrees), and says that it will lead to instability of the mobile bearing. The purpose of this study is to research the risk factors of enlargement of medial gap in deep flexion position.

We performed 81 UKAs with mobile bearing system from November 2013 to December 2015, and could evaluate 41 knees. This study of 41 knees included 9 males and 32 females, with average operation age of 75.4years(63–89years). The diagnosis was osteoarthritis in 39 knees and osteonecrosis in 2 knees. The UKA(Oxford partial knee microplasty, Biomet, Warsaw, IN) was used in all cases. We performed distal femur and proximal tibia osteotomy using CT-Free navigation system(Stryker Navigation System II/precision Knee Navigation ver4.0). And we inserted femoral and tibial trial component, then we placed an UKA tensioner on the medial component of the knee. Using tensioner under 30 lbs, we measured joint medial gap at 0,20,45,90,130(deep flexion) degrees. When we compared medial gap at 90 degrees position with at 130 degrees, we defined it as ‘instability group’ if there was gap enlargement more than 1mm, and defined it as ‘stability group’ if there wasn't.

We compared this two groups with regard to age, BMI, femoro-tibial angle (FTA), the diameter of anterior cruciate ligament (ACL), tibial angle and tibial posterior slope angle of the implant. We evaluated preoperative and postoperative FTA by weightbearing long leg antero-posterior alignment view X-rays. We measured ACL diameter at its condyle level in coronal view of MRI. Also we evaluated tibial component implantation angle by postoperative CT using 3D template system. These measurement were analyzed statistically using t test.

The stability group contained 26 knees, and the instability group contained 15 knees. Compared with the stability group, the instability group indicated higher FTA (p=0.001). Between 20 and 90 degrees flexion position, there was no change of medial gap.

Dislocation of the mobile bearing is one of the complications of UKA and it will need re-operation. It is said to be caused by impingement of the bearing and osteophyte of femur. However, some reports said that dislocation was happened when the knee was flexed deeply or twisted, and there was no impingement. We think it may means that dislocation could be caused by medial gap enlargement.

This study indicates that higher FTA could be risk factor of dislocation of mobile bearing. It is important to evaluate preoperatively FTA by X-ray.

The purpose of this study is to evaluate accuracy of tibia cutting and tibia implantation in UKA which used navigation system for tibia cutting and tibia component implantation, and to evaluate clinical results.

We performed 72 UKAs using navigation system from November, 2012. This study of 72 knees included 56 females and 16 males with an average operation age of 74.2 years and an average body mass index (BMI) of 24.8 kg/m2. The diagnosis was osteoarthritis (OA) in 67 knees and osteonecrosis (ON) in 5 knees. The UKA (Oxford partial knee microplasty, Biomet, Warsaw, IN) was used all cases. We evaluated patients clinically using the Japanese orthopaedic association (JOA) score, range of motion (ROM), operation time, the amount of bleeding and complications. Patients were evaluated clinically at preoperation and final follow up in JOA score and ROM. As an radiologic examination, we evaluated preoperative and postoperative lower limb alignment in FTA (femoro-tibial angle) by weightbearing long leg antero-posterior alignment view X-rays. Also we evaluated a tibial component implantation angle by postoperative CT, and tibia cutting angle by intraoperative navigation system. We defined the tibial angle which a tibia functional axis and the tibia component made in coronal plane, also tibial posterior slope angle which a tibia axis and tibia component made in sagittal plane by CT. We measured tibial angle and tibial posterior slope angle by 3D template system.

We performed UKA in all cases mini-midvastus approach. At first we performed osteotomy of the proximal medial tibia using CT-Free navigation. At this procedure we performed osteotomy to do re-cut if check did cutting surface in navigation, and there was cutting error (>3°), and then to do check again in navigation. Next we did not use navigation and went the osteotomy of the distal femur with an IM rod and drill guide of microplasty system. And then we performed a trial and decided bearing gap and moved to cementing. At first we went cementing of the tibia component. At this procedure we went to drive implant again if check did implant surface in navigation, and there was implantation error(>3°), and to do check. We checked did tibia cutting, tibia implantation carefully in navigation. In addition, We sterilize a clips and use it came to be in this way possible for the check of the first osteotomy side exactly.

ROM was an average of 122.7° of preoperation became an average of 128.2° at final follow up, and JOA score was an average of 50.5 points of preoperation improved an average of 86.6 points at final follow up after UKA. An average of the operation time was 94 minutes, an average of the amount of bleeding was 137.7ml, and complications were one proximal type deep venous thrombosis (DVT) and one pin splinter joining pain by navigation, .Asetic loosening(tibial component) was one case, and this conversed the TKA.

In the radiologic evaluation, FTA was an average of 182.1° of preoperation corrected an average of 175.9°after UKA. In other words, an average of 6.2° were corrected by UKA. The tibia component implantation angle was an average of 90.18° in a measurement by the CT after UKA, intoraoperative tibia component implantation angle was an average of 90.32° in a measurement by the navigation system. These two differences did not accept the significant difference at an average of 1.33°.(P=0.5581). Similarly, the posterior slope angle were as follow; average of 5.65°by CT and average of 5.75°by navigation. These two differences did not accept the significant difference at an average of 1.33°. (P=0.6475)

Discussion: We performed UKA using navigation and evaluated the implantation accuracy for tibia osteotomy, tibia implantation. They were good alignment with an average of 90.18°, and outliers more than 3° were two cases(2.8%). It will be necessary to examine long-term progress including clinical results complications in future. We are performed UKA now in femur side using PSI(patient specific instruments) and tbia side using Navigation.

Objectives: The results of knee arthroplasty are commonly assessed by survival analysis using revision as the endpoint. We have used the assessment of pain by a patient based questionnaire as an alternative after the Oxford Phase 3 UKA implanted by a minimally invasive technique.

Materials and Methods: Between January 1999 and May 2007, 223 consecutive Oxford arthroplasties were implanted by a single surgeon in a county hospital. Mean followup period was 35 months. Patients were assessed prospectively pre-operatively and after UKA in each year subsequently by a questionnaire. Survival analysis was undertaken.

Results: Preoperatively 85.8% had moderate or severe pain. Postoperatively, of five patients(2.6%) with persisting pain due to failure of using proper patient selection three were revised to TKA and two are still being followed. Three patients(1.6%) with moderate pain after using proper indication criteria accepted their complaints. Ten other patients (5.2%) experiencing moderate pain some time during the eight year period were successfully treated by arthroscopy. If after surgery patients experienced pain which had spontaneous improved by the second year, the initial pain was ignored. Totally 9.6% of patients experienced moderate or severe pain at some stage, and the failure rate was 4.2% in this period of 8 years’ experience.

Conclusion: When strict indications are followed the failure rate of the procedure can be minimised till 1.6% when moderate pain is considered the endpoint. As relief of pain is the primary reason for joint replacement, this is likely to be the most important factor in determining the long-term outcome for the patient.

Introduction: The introduction of Oxford UKA byminimally invasive techniques has signiþcantly changed the treatment of anteromedial osteoarthritis of the knee joint. It is therefore necessary to evaluate this treatment option both clinically and radiologically. Aims: To study the radiographic and clinical results of the Oxford medial UKA in patients with a minimum follow up of 2 years in a single centre. Methods: A prospective independent study in which 67 consecutive UKAs were implanted by a single surgeon, using a minimally invasive technique. All patients with a minimum follow up of 2 years were pre- and postoperatively clinically evaluated by the AKSS and radiologically according to the Oxford Centre criteria, including ßuoroscopy. Results: Clinical: 28 patients with 30 prostheses (mean FU: 2.54 yr; mean age: 71.4 yrs) were included. The Knee Score improved signiþcantly from 58.7 (pre-op) to 95.0 (FU). The Function Score improved from 54.5 to 88.8. Mean ROM was 125û preoperatively and 121û at FU.Radiological: preop: varus deformity (n=18; mean 3.4û; range 2–10û), and valgus (n=12; 5.4, 2–12û). Postop: valgus alignment in all (n=30; 6.3û, 4–12û). Fifteen cases (50%) showed signs of patellofemoral arthritis (PFA); 11 cases with grade 2–3 PFA had a maximum pain score of 50. Full congruency of the tibial and femoral components was obtained in 18 cases, 10 cases were within and 2 out of margin according to the Oxford Centre criteria. Radiolucency below the tibial component was seen in 2 cases. Conclusions: This independent study has conþrmed preliminary þndings that using a minimally invasive approach good radiological and clinical results can be obtained. Presence of PFA had no inßuence on good clinical outcome.

Aims. Day-case arthroplasty is gaining popularity in Europe. We report outcomes from the first 12 months following implementation of a day-case pathway for unicompartmental knee arthroplasty (UKA) and total hip arthroplasty (THA) in an NHS hospital. Methods. A total of 47 total hip arthroplasty (THA) and 24 unicompartmental knee arthroplasty (UKA) patients were selected for the day-case arthroplasty pathway, based on preoperative fitness and agreement to participate. Data were likewise collected for a matched control group (n = 58) who followed the standard pathway three months prior to the implementation of the day-case pathway. We report same-day discharge (SDD) success, reasons for delayed discharge, and patient-reported outcomes. Overall length of stay (LOS) for all lower limb arthroplasty was recorded to determine the wider impact of implementing a day-case pathway. Results. Patients on the day-case pathway achieved SDD in 47% (22/47) of THAs and 67% (16/24) of UKAs. The most common reasons for failed SDD were nausea, hypotension, and pain, which were strongly associated with the use of fentanyl in the spinal anaesthetic. Complications and patient-reported outcomes were not significantly different between groups. Following the introduction of the day-case pathway, the mean LOS reduced significantly by 0.7, 0.6, and 0.5 days respectively in THA, UKA, and total knee arthroplasty cases (p < 0.001). Conclusion. Day-case pathways are feasible in an NHS set-up with only small changes required. We do not recommend fentanyl in the spinal anaesthetic for day-case patients. An important benefit seen in our unit is the so-called ‘day-case effect’, with a significant reduction in mean LOS seen across all lower limb arthroplasty. Cite this article: Bone Jt Open 2021;2(11):900–908

Aims. A functional anterior cruciate ligament (ACL) or posterior cruciate ligament (PCL) has been assumed to be required for patients undergoing unicompartmental knee arthroplasty (UKA). However, this assumption has not been thoroughly tested. Therefore, this study aimed to assess the biomechanical effects exerted by cruciate ligament-deficient knees with medial UKAs regarding different posterior tibial slopes. Methods. ACL- or PCL-deficient models with posterior tibial slopes of 1°, 3°, 5°, 7°, and 9° were developed and compared to intact models. The kinematics and contact stresses on the tibiofemoral joint were evaluated under gait cycle loading conditions. Results. Anterior translation increased in ACL-deficient UKA cases compared with intact models. In contrast, posterior translation increased in PCL-deficient UKA cases compared with intact models. As the posterior tibial slope increased, anterior translation of ACL-deficient UKA increased significantly in the stance phase, and posterior translation of PCL-deficient UKA increased significantly in the swing phase. Furthermore, as the posterior tibial slope increased, contact stress on the other compartment increased in cruciate ligament-deficient UKAs compared with intact UKAs. Conclusion. Fixed-bearing medial UKA is a viable treatment option for patients with cruciate ligament deficiency, providing a less invasive procedure and allowing patient-specific kinematics to adjust posterior tibial slope. Patient selection is important, and while AP kinematics can be compensated for by posterior tibial slope adjustment, rotational stability is a prerequisite for this approach. ACL- or PCL-deficient UKA that adjusts the posterior tibial slope might be an alternative treatment option for a skilled surgeon. Cite this article: Bone Joint Res 2022;11(7):494–502

Aims. Unicompartmental knee arthroplasty (UKA) is the preferred treatment for anterior medial knee osteoarthritis (OA) owing to the rapid postoperative recovery. However, the risk factors for UKA failure remain controversial. Methods. The clinical data of Oxford mobile-bearing UKAs performed between 2011 and 2017 with a minimum follow-up of five years were retrospectively analyzed. Demographic, surgical, and follow-up data were collected. The Cox proportional hazards model was used to identify the risk factors that contribute to UKA failure. Kaplan-Meier survival was used to compare the effect of the prosthesis position on UKA survival. Results. A total of 407 patients who underwent UKA were included in the study. The mean age of patients was 61.8 years, and the mean follow-up period of the patients was 91.7 months. The mean Knee Society Score (KSS) preoperatively and at the last follow-up were 64.2 and 89.7, respectively (p = 0.001). Overall, 28 patients (6.9%) with UKA underwent revision due to prosthesis loosening (16 patients), dislocation (eight patients), and persistent pain (four patients). Cox proportional hazards model analysis identified malposition of the prostheses as a high-risk factor for UKA failure (p = 0.007). Kaplan-Meier analysis revealed that the five-year survival rate of the group with malposition was 85.1%, which was significantly lower than that of the group with normal position (96.2%; p < 0.001). Conclusion. UKA constitutes an effective method for treating anteromedial knee OA, with an excellent five-year survival rate. Aseptic loosening caused by prosthesis malposition was identified as the main cause of UKA failure. Surgeons should pay close attention to prevent the potential occurrence of this problem. Cite this article: Bone Jt Open 2023;4(12):914–922

Aims. The aim of this study was to describe the pattern of revision indications for unicompartmental knee arthroplasty (UKA) and total knee arthroplasty (TKA) and any change to this pattern for UKA patients over the last 20 years, and to investigate potential associations to changes in surgical practice over time. Methods. All primary knee arthroplasty surgeries performed due to primary osteoarthritis and their revisions reported to the Danish Knee Arthroplasty Register from 1997 to 2017 were included. Complex surgeries were excluded. The data was linked to the National Patient Register and the Civil Registration System for comorbidity, mortality, and emigration status. TKAs were propensity score matched 4:1 to UKAs. Revision risks were compared using competing risk Cox proportional hazard regression with a shared γ frailty component. Results. Aseptic loosening (loosening) was the most common revision indication for both UKA (26.7%) and TKA (29.5%). Pain and disease progression accounted for 54.6% of the remaining UKA revisions. Infections and instability accounted for 56.1% of the remaining TKA revision. The incidence of revision due to loosening or pain decreased over the last decade, being the second and third least common indications in 2017. There was a decrease associated with fixation method for pain (hazard ratio (HR) 0.40; 95% confidence interval (CI) 0.17 to 0.94) and loosening (HR 0.29; 95% CI 0.10 to 0.81) for cementless compared to cemented, and units UKA usage for pain (HR 0.67, 95% CI 0.50 to 0.91), and loosening (HR 0.51; 95% CI 0.37 to 0.70) for high usage. Conclusion. The overall revision patterns for UKA and TKA for the last 20 years are comparable to previous published patterns. We found large changes to UKA revision patterns in the last decade, and with the current surgical practice, revision due to pain or loosening are significantly less likely. Cite this article: Bone Jt Open 2023;4(12):923–931

Aims. The COVID-19 pandemic has caused unprecedented disruption to elective orthopaedic services. The primary objective of this study was to examine changes in functional scores in patients awaiting total hip arthroplasty (THA), total knee arthroplasty (TKA), and unicompartmental knee arthroplasty (UKA). Secondary objectives were to investigate differences between these groups and identify those in a health state ‘worse than death’ (WTD). Methods. In this prospective cohort study, preoperative Oxford hip and knee scores (OHS/OKS) were recorded for patients added to a waiting list for THA, TKA, or UKA, during the initial eight months of the COVID-19 pandemic, and repeated at 14 months into the pandemic (mean interval nine months (SD 2.84)). EuroQoL five-dimension five-level health questionnaire (EQ-5D-5L) index scores were also calculated at this point in time, with a negative score representing a state WTD. OHS/OKS were analyzed over time and in relation to the EQ-5D-5L. Results. A total of 174 patients (58 THA, 74 TKA, 42 UKA) were eligible, after 27 were excluded (one died, seven underwent surgery, 19 non-responders). The overall mean OHS/OKS deteriorated from 15.43 (SD 6.92), when patients were added to the waiting list, to 11.77 (SD 6.45) during the pandemic (p < 0.001). There were significantly worse EQ-5D-5L index scores in the THA group (p = 0.005), with 22 of these patients (38%) in a health state WTD, than either the TKA group (20 patients; 27% WTD), or the UKA group (nine patients; 21% WTD). A strong positive correlation between the EQ-5D-5L index score and OHS/OKS was observed (r = 0.818; p < 0.001). Receiver operating characteristic analysis revealed that an OHS/OKS lower than nine predicted a health state WTD (88% sensitivity and 73% specificity). Conclusion. OHS/OKS deteriorated significantly among patients awaiting lower limb arthroplasty during the COVID-19 pandemic. Overall, 51 patients were in a health state WTD, representing 29% of our entire cohort, which is considerably worse than existing pre-pandemic data. Cite this article: Bone Jt Open 2023;4(3):138–145

Aims. Return to sport following undergoing total (TKA) and unicompartmental knee arthroplasty (UKA) has been researched with meta-analyses and systematic reviews of varying quality. The aim of this study is to create an umbrella review to consolidate the data into consensus guidelines for returning to sports following TKA and UKA. Methods. Systematic reviews and meta-analyses written between 2010 and 2020 were systematically searched. Studies were independently screened by two reviewers and methodology quality was assessed. Variables for analysis included objective classification of which sports are safe to participate in postoperatively, time to return to sport, prognostic indicators of returning, and reasons patients do not. Results. A total of 410 articles were found, including 58 duplicates. Seven articles meeting inclusion criteria reported that 34% to 100% of patients who underwent TKA or UKA were able to return to sports at 13 weeks and 12 weeks respectively, with UKA patients more likely to do so. Prior experience with the sport was the most significant prognostic indicator for return. These patients were likely to participate in low-impact sports, particularly walking, cycling, golf, and swimming. Moderate-impact sport participation, such as doubles tennis and skiing, may be considered on a case-by-case basis considering the patient’s prior experience. There is insufficient long-term data on the risks to return to high-impact sport, such as decreased implant survivorship. Conclusion. There is a consensus that patients can return to low-impact sports following TKA or UKA. Return to moderate-impact sport was dependent on a case-by-case basis, with emphasis on the patient’s prior experience in the sport. Return to high-impact sports was not supported. Patients undergoing UKA return to sport one week sooner and with more success than TKA. Future studies are needed to assess long-term outcomes following return to high-impact sports to establish evidence-based recommendations. This review summarizes all available data for the most up-to-date and evidence-based guidelines for returning to sport following TKA and UKA to replace guidelines based on subjective physician survey data. Cite this article: Bone Jt Open 2022;3(3):245–251

Aims

Unicompartmental and total knee arthroplasty (UKA and TKA) are successful treatments for osteoarthritis, but the solid metal implants disrupt the natural distribution of stress and strain which can lead to bone loss over time. This generates problems if the implant needs to be revised. This study investigates whether titanium lattice UKA and TKA implants can maintain natural load transfer in the proximal tibia.

Aims

In-hospital length of stay (LOS) and discharge dispositions following arthroplasty could act as surrogate measures for improvement in patient pathways, and have major cost saving implications for healthcare providers. With the ever-growing adoption of robotic technology in arthroplasty, it is imperative to evaluate its impact on LOS. The objectives of this study were to compare LOS and discharge dispositions following robotic arm-assisted total knee arthroplasty (RO TKA) and unicompartmental arthroplasty (RO UKA) versus conventional technique (CO TKA and UKA).

Aims

The primary objective of this registry-based study was to compare patient-reported outcomes of cementless and cemented medial unicompartmental knee arthroplasty (UKA) during the first postoperative year. The secondary objective was to assess one- and three-year implant survival of both fixation techniques.

Aims

Bi-unicondylar arthroplasty (Bi-UKA) is a bone and anterior cruciate ligament (ACL)-preserving alternative to total knee arthroplasty (TKA) when the patellofemoral joint is preserved. The aim of this study is to investigate the clinical outcomes and biomechanics of Bi-UKA.

Aims. Mobile-bearing unicompartmental knee arthroplasty (UKA) with a flat tibial plateau has not performed well in the lateral compartment, leading to a high rate of dislocation. For this reason, the Domed Lateral UKA with a biconcave bearing was developed. However, medial and lateral tibial plateaus have asymmetric anatomical geometries, with a slightly dished medial and a convex lateral plateau. Therefore, the aim of this study was to evaluate the extent at which the normal knee kinematics were restored with different tibial insert designs using computational simulation. Methods. We developed three different tibial inserts having flat, conforming, and anatomy-mimetic superior surfaces, whereas the inferior surface in all was designed to be concave to prevent dislocation. Kinematics from four male subjects and one female subject were compared under deep knee bend activity. Results. The conforming design showed significantly different kinematics in femoral rollback and internal rotation compared to that of the intact knee. The flat design showed significantly different kinematics in femoral rotation during high flexion. The anatomy-mimetic design preserved normal knee kinematics in femoral rollback and internal rotation. Conclusion. The anatomy-mimetic design in lateral mobile UKA demonstrated restoration of normal knee kinematics. Such design may allow achievement of the long sought normal knee characteristics post-lateral mobile UKA. However, further in vivo and clinical studies are required to determine whether this design can truly achieve a more normal feeling of the knee and improved patient satisfaction. Cite this article: Bone Joint Res 2020;9(7):421–428

Aims. Unicompartmental knee arthroplasty (UKA) has become a popular method of treating knee localized osteoarthritis (OA). Additionally, the posterior cruciate ligament (PCL) is essential to maintaining the physiological kinematics and functions of the knee joint. Considering these factors, the purpose of this study was to investigate the biomechanical effects on PCL-deficient knees in medial UKA. Methods. Computational simulations of five subject-specific models were performed for intact and PCL-deficient UKA with tibial slopes. Anteroposterior (AP) kinematics and contact stresses of the patellofemoral (PF) joint and the articular cartilage were evaluated under the deep-knee-bend condition. Results. As compared to intact UKA, there was no significant difference in AP translation in PCL-deficient UKA with a low flexion angle, but AP translation significantly increased in the PCL-deficient UKA with high flexion angles. Additionally, the increased AP translation became decreased as the posterior tibial slope increased. The contact stress in the PF joint and the articular cartilage significantly increased in the PCL-deficient UKA, as compared to the intact UKA. Additionally, the increased posterior tibial slope resulted in a significant decrease in the contact stress on PF joint but significantly increased the contact stresses on the articular cartilage. Conclusion. Our results showed that the posterior stability for low flexion activities in PCL-deficient UKA remained unaffected; however, the posterior stability for high flexion activities was affected. This indicates that a functional PCL is required to ensure normal stability in UKA. Additionally, posterior stability and PF joint may reduce the overall risk of progressive OA by increasing the posterior tibial slope. However, the excessive posterior tibial slope must be avoided. Cite this article: Bone Joint Res 2020;9(9):593–600

Aims. Commonly performed unicompartmental knee arthroplasty (UKA) is not designed for the lateral compartment. Additionally, the anatomical medial and lateral tibial plateaus have asymmetrical geometries, with a slightly dished medial plateau and a convex lateral plateau. Therefore, this study aims to investigate the native knee kinematics with respect to the tibial insert design corresponding to the lateral femoral component. Methods. Subject-specific finite element models were developed with tibiofemoral (TF) and patellofemoral joints for one female and four male subjects. Three different TF conformity designs were applied. Flat, convex, and conforming tibial insert designs were applied to the identical femoral component. A deep knee bend was considered as the loading condition, and the kinematic preservation in the native knee was investigated. Results. The convex design, the femoral rollback, and internal rotation were similar to those of the native knee. However, the conforming design showed a significantly decreased femoral rollback and internal rotation compared with that of the native knee (p < 0.05). The flat design showed a significant difference in the femoral rollback; however, there was no difference in the tibial internal rotation compared with that of the native knee. Conclusion. The geometry of the surface of the lateral tibial plateau determined the ability to restore the rotational kinematics of the native knee. Surgeons and implant designers should consider the geometry of the anatomical lateral tibial plateau as an important factor in the restoration of native knee kinematics after lateral UKA. Cite this article: Bone Joint Res 2019;8:593–600

Objectives. Unicompartmental knee arthroplasty (UKA) is an alternative to total knee arthroplasty with isolated medial or lateral compartment osteoarthritis. However, polyethylene wear can significantly reduce the lifespan of UKA. Different bearing designs and materials for UKA have been developed to change the rate of polyethylene wear. Therefore, the objective of this study is to investigate the effect of insert conformity and material on the predicted wear in mobile-bearing UKA using a previously developed computational wear method. Methods. Two different designs were tested with the same femoral component under identical kinematic input: anatomy mimetic design (AMD) and conforming design inserts with different conformity levels. The insert materials were standard or crosslinked ultra-high-molecular-weight polyethylene (UHMWPE). We evaluated the contact pressure, contact area, wear rate, wear depth, and volumetric wear under gait cycle loading conditions. Results. Conforming design inserts had the lower contact pressure and larger contact area. However, they also had the higher wear rate and volumetric wear. The improved wear performance was found with AMD inserts. In addition, the computationally predicted volumetric wear of crosslinked UHMWPE inserts was less than half that of standard UHMWPE inserts. Conclusion. Our results showed that increasing conformity may not be the sole predictor of wear performance; highly crosslinked mobile-bearing polyethylene inserts can also provide improvement in wear performance. These results provide improvements in design and materials to reduce wear in mobile-bearing UKA. Cite this article: Bone Joint Res 2019;8:563–569

Aims. Unicompartmental knee arthroplasty (UKA) is a bone-preserving treatment option for osteoarthritis localized to a single compartment in the knee. The success of the procedure is sensitive to patient selection and alignment errors. Robotic arm-assisted UKA provides technological assistance to intraoperative bony resection accuracy, which is thought to improve ligament balancing. This paper presents the five-year outcomes of a comparison between manual and robotically assisted UKAs. Methods. The trial design was a prospective, randomized, parallel, single-centre study comparing surgical alignment in patients undergoing UKA for the treatment of medial compartment osteoarthritis (ISRCTN77119437). Participants underwent surgery using either robotic arm-assisted surgery or conventional manual instrumentation. The primary outcome measure (surgical accuracy) has previously been reported, and, along with secondary outcomes, were collected at one-, two-, and five-year timepoints. Analysis of five-year results and longitudinal analysis for all timepoints was performed to compare the two groups. Results. Overall, 104 (80%) patients of the original 130 who received surgery were available at five years (55 robotic, 49 manual). Both procedures reported successful results over all outcomes. At five years, there were no statistical differences between the groups in any of the patient reported or clinical outcomes. There was a lower reintervention rate in the robotic arm-assisted group with 0% requiring further surgery compared with six (9%) of the manual group requiring additional surgical intervention (p < 0.001). Conclusion. This study has shown excellent clinical outcomes in both groups with no statistical or clinical differences in the patient-reported outcome measures. The notable difference was the lower reintervention rate at five years for roboticarm-assisted UKA when compared with a manual approach. Cite this article: Bone Joint J 2021;103-B(6):1088–1095