Aims. Micromotion of the polyethylene (PE) inlay may contribute to backside PE wear in addition to articulate wear of total knee arthroplasty (TKA). Using radiostereometric analysis (RSA) with tantalum beads in the PE inlay, we evaluated PE micromotion and its relationship to PE wear. Methods. A total of 23 patients with a mean age of 83 years (77 to 91), were available from a RSA study on cemented TKA with Maxim tibial components (Zimmer Biomet). PE inlay migration, PE wear, tibial component migration, and the anatomical knee axis were evaluated on weightbearing stereoradiographs. PE inlay wear was measured as the deepest penetration of the femoral component into the PE inlay. Results. At mean six years’ follow-up, the PE wear rate was 0.08 mm/year (95% confidence interval 0.06 to 0.09 mm/year). PE inlay external rotation was below the precision limit and did not influence PE wear. Varus knee alignment did not influence PE wear (p = 0.874), but increased tibial component total translation (p = 0.041). Conclusion. The PE inlay was well fixed and there was no relationship between PE stability and PE wear. The PE wear rate was low and similar in the medial and lateral compartments. Varus knee alignment did not influence PE wear. Cite this article: Bone Joint Res 2024;13(5):226–236

Aims. The primary objective of this study was to compare the five-year tibial component migration and wear between highly crosslinked polyethylene (HXLPE) inserts and conventional polyethylene (PE) inserts of the uncemented Triathlon fixed insert cruciate-retaining total knee arthroplasty (TKA). Secondary objectives included clinical outcomes and patient-reported outcome measures (PROMs). Methods. A double-blinded, randomized study was conducted including 96 TKAs. Tibial component migration and insert wear were measured with radiostereometric analysis (RSA) at three, six, 12, 24, and 60 months postoperatively. PROMS were collected preoperatively and at all follow-up timepoints. Results. There was no clinically relevant difference in terms of tibial component migration, insert wear, and PROMs between the HXLPE and PE groups. The mean difference in tibial component migration (maximal total point migration (MTPM)) was 0.02 mm (95% confidence interval (CI) -0.07 to 0.11), which is below the value of 0.2 mm considered to be clinically relevant. Wear after five years for HXLPE was 0.16 mm (95% CI 0.05 to 0.27), and for PE was 0.23 mm (95% CI 0.12 to 0.35). The mean difference in wear rate was 0.01 mm/year (95% CI -0.02 to 0.05) in favour of the HXLPE group. Wear is mainly present on the medial side of the insert. Conclusion. There is no clinically relevant difference in tibial component migration and insert wear for up to five years between the HXLPE conventional PE inserts. For the implant studied, the potential advantages of a HXLPE insert remain to be proven under clinical conditions at longer-term follow-up. Cite this article: Bone Joint J 2023;105-B(5):518–525

Aims. The use of vitamin E-infused highly crosslinked polyethylene (HXLPE) in total knee prostheses is controversial. In this paper we have compared the clinical and radiological results between conventional polyethylene and vitamin E-infused HXLPE inserts in total knee arthroplasty (TKA). Patients and Methods. The study included 200 knees (175 patients) that underwent TKA using the same total knee prostheses. In all, 100 knees (77 patients) had a vitamin E-infused HXLPE insert (study group) and 100 knees (98 patients) had a conventional polyethylene insert (control group). There were no significant differences in age, sex, diagnosis, preoperative knee range of movement (ROM), and preoperative Knee Society Score (KSS) between the two groups. Clinical and radiological results were evaluated at two years postoperatively. Results. Differences in postoperative ROM and KSS were not statistically significant between the study and control groups. No knee exhibited osteolysis, aseptic loosening, or polyethylene failure. Additionally, there was no significant difference in the incidence of a radiolucent line between the two groups. One patient from the study group required irrigation and debridement, due to deep infection, at six months postoperatively. Conclusion. Clinical results were comparable between vitamin E-infused HXLPE inserts and conventional polyethylene inserts at two years after TKA, without any significant clinical failure. Cite this article: Bone Joint J 2019;101-B:559–564

Aims. Vitamin E-infused highly cross-linked polyethylene (E1) has recently been introduced in total knee arthroplasty (TKA). An in vitro wear simulator study showed that E1 reduced polyethylene wear. However there is no published information regarding in vivo wear. Previous reports suggest that newly introduced materials which reduce in vitro polyethylene wear do not necessarily reduce in vivo polyethylene wear. To assist in the evaluation of the newly introduced material before widespread use, we established an in vivo polyethylene wear particle analysis for TKA. The aim of this study was to compare in vivo polyethylene wear particle generation between E1 and conventional polyethylene (ArCom) in TKA. Methods. A total of 34 knees undergoing TKA (17 each with ArCom or E1) were investigated. Except for the polyethylene insert material, the prostheses used for both groups were identical. Synovial fluid was obtained at a mean of 3.4 years (SD 1.3) postoperatively. The in vivo polyethylene wear particles were isolated from the synovial fluid using a previously validated method and examined by scanning electron microscopy. Results. The total number of polyethylene wear particles obtained from the knees with E1 (mean 6.9, SD 4.0 × 10. 7. counts/knee) was greater than that obtained from those with ArCom (mean 2.2, SD 2.6 × 10. 7. counts/knee) (p = 0.001). The particle size (equivalent circle of diameter) from the knees with E1 was smaller (mean 0.5 μm, SD 0.1) than that of knees with ArCom (mean 1.5, SD 0.3 μm) (p = 0.001). The aspect ratio of particles from the knees with E1 (mean 1.3, SD 0.1) was smaller than that with ArCom (mean 1.4, SD 0.1) (p < 0.001 ). Conclusion. This is the first report of in vivo wear particle analysis of E1. E1 polyethylene did not reduce the number of in vivo polyethylene wear particles compared with ArCom in early clinical stage. Further careful follow-up of newly introduced E1 for TKA should be carried out. Cite this article: Bone Joint J 2020;102-B(11):1527–1534

Aims. The aim of this study was to compare the migration of the femoral component, five years postoperatively, between patients with a highly cross-linked polyethylene (HXLPE) insert and those with a conventional polyethylene (PE) insert in an uncemented Triathlon fixed insert cruciate-retaining total knee arthroplasty (TKA). Secondary aims included clinical outcomes and patient-reported outcome measures (PROMs). We have previously reported the migration and outcome of the tibial components in these patients. Methods. A double-blinded randomized controlled trial was conducted including 96 TKAs. The migration of the femoral component was measured with radiostereometry (RSA) at three and six months and one, two, and five years postoperatively. PROMs were collected preoperatively and at all periods of follow-up. Results. There was no clinically relevant difference in terms of migration of the femoral component or PROMs between the HXLPE and PE groups. The mean difference in migration (maximum total point motion), five years postopeatively, was 0.04 mm (95% CI -0.06 to 0.16) in favour of the PE group. Conclusion. There was no clinically relevant difference in migration of the femoral component, for up to five years between the two groups. These findings will help to establish a benchmark for future studies on migration of femoral components in TKA. Cite this article: Bone Joint J 2024;106-B(8):826–833

Aims. Wear of the polyethylene (PE) tibial insert of total knee arthroplasty (TKA) increases the risk of revision surgery with a significant cost burden on the healthcare system. This study quantifies wear performance of tibial inserts in a large and diverse series of retrieved TKAs to evaluate the effect of factors related to the patient, knee design, and bearing material on tibial insert wear performance. Methods. An institutional review board-approved retrieval archive was surveyed for modular PE tibial inserts over a range of in vivo duration (mean 58 months (0 to 290)). Five knee designs, totalling 1,585 devices, were studied. Insert wear was estimated from measured thickness change using a previously published method. Linear regression statistical analyses were used to test association of 12 patient and implant design variables with calculated wear rate. Results. Five patient-specific variables and seven implant-specific variables were evaluated for significant association with lower insert wear rate. Six were significant when controlling for other factors: greater patient age, female sex, shorter duration in vivo, polished tray, highly cross-linked PE (HXLPE), and constrained knee design. Conclusion. This study confirmed that knee wear rate increased with duration in vivo. Older patients and females had significantly lower wear rates. Polished modular tibial tray surfaces, HXLPE, and constrained TKA designs were device design factors associated with significantly reduced wear rate. Cite this article: Bone Joint J 2021;103-B(11):1695–1701

Abstract. Introduction. The Oxford Unicompartmental Knee Replacement's (OUKR's) fully-congruent design minimises polyethylene wear. Consequently, wear is a rare failure mechanism. Phase-3 OUKR linear wear at 5 years was higher than previous OUKR phases, but very low compared to fixed-bearing UKRs. This study aimed to measure OUKR bearing wear at 10 years and investigate factors that may affect wear. Methodology. Bearing thickness for 39 OUKRs from a randomised study was calculated using radiostereometric analysis at regular intervals up to 10 years. Data for 39 and 29 OUKRs was available at 5 and 10 years, respectively. As creep occurs early, wear rate was calculated using linear regression between 6 months and 10 years. Relationships between wear and patient factors, fixation method, Oxford Knee Score (OKS), bearing position, and component position were analysed. Results. The mean wear rate was 0.06mm/year. Fixation method, age, OKS, component size, and bearing size had no correlation with wear. A higher BMI was associated with lower wear (p=0.01). Bearings more than 4mm from the wall had significantly more wear (p=0.04) than those less than 4mm from the wall. There was a linear correlation between the femoral component contact area on the bearing and wear (p=0.04). Conclusions. Phase-3 bearing wear rate is constant, significantly higher than previous OUKR phases at 10 years, and may increase the risk of long-term bearing failure. To minimise complications associated with wear, size 4 bearings should be used in young patients and manufacturing bearings from more durable, highly crosslinked polyethylene should be studied

Aims. The aim of this study was to establish the results of isolated exchange of the tibial polyethylene insert in revision total knee arthroplasty (RTKA) in patients with well-fixed femoral or tibial components. We report on a series of RTKAs where only the polyethylene was replaced, and the patients were followed for a mean of 13.2 years (10.0 to 19.1). Patients and Methods. Our study group consisted of 64 non-infected, grossly stable TKA patients revised over an eight-year period (1998 to 2006). The mean age of the patients at time of revision was 72.2 years (48 to 88). There were 36 females (56%) and 28 males (44%) in the cohort. All patients had received the same cemented, cruciate-retaining patella resurfaced primary TKA. All subsequently underwent an isolated polyethylene insert exchange. The mean time from the primary TKA to RTKA was 9.1 years (2.2 to 16.1). Results. At final follow-up, 13 patients had died, leaving 51 patients for study. Only seven of these patients had required re-operation. Knee Society scores (KSS) prior to RTKA were a mean of 78.4 (24 to 100). By six weeks post-revision, the mean total KSS was 93.5 (38 to 100) and at final follow-up, they had a mean of 91.6 (36 to 100). Conclusion. In appropriate circumstances, where the femoral and tibial components are satisfactorily aligned and well fixed, and where the soft tissues can be balanced, a polyethylene exchange alone can provide a durable solution for these RTKA patients. Cite this article: Bone Joint J 2019;101-B(7 Supple C):104–107

Aims. The aim of this study was to evaluate the surface damage, the density of crosslinking, and oxidation in retrieved antioxidant-stabilized highly crosslinked polyethylene (A-XLPE) tibial inserts from total knee arthroplasty (TKA), and to compare the results with a matched cohort of standard remelted highly crosslinked polyethylene (XLPE) inserts. Materials and Methods. A total of 19 A-XLPE tibial inserts were retrieved during revision TKA and matched to 18 retrieved XLPE inserts according to the demographics of the patients, with a mean length of implantation of 15 months (1 to 42). The percentage areas of PE damage on the articular surfaces and the modes of damage were measured. The density of crosslinking of the PE and oxidation were measured at loaded and unloaded regions on these surfaces. Results. A-XLPE inserts had higher rates of burnishing and lower rates of pitting and scratching compared with XLPE. There were no differences in the density of crosslinking at loaded and unloaded regions. A-XLPE showed higher oxidation indices in the unloaded surface region compared with XLPE. There were no differences in the levels of oxidation in the loaded regions. Conclusion. Retrieval analysis of A-XLPE did not reflect a clinically relevant difference in surface damage, density of crosslinking, or oxidation compared with XLPE tibial inserts at short-term evaluation. Cite this article: Bone Joint J 2018;100-B:1330–5

Aims. There is insufficient evidence to recommend the use of alternative polyethylene bearings in modular, fixed-bearing total knee arthroplasty (TKA). The purpose of this study was to compare standard polyethylene (SP) and highly crosslinked polyethylene (XLP) tibial liners in posterior-stabilized TKA, with osteolysis as the primary outcome and clinical results and the rate of re-operation as the secondary outcomes. Patients and Methods. This is a single-surgeon, prospective randomized study involving one design of modular posterior-stabilized TKA. An analysis of 122 TKAs with an SP compression moulded liner and 123 with an XLP liner was performed, with a mean follow-up of six years (2 to 11). Patients were evaluated clinically using the Knee Society score, Lower Extremity Activity Score (LEAS), and the presence of an effusion, and standard radiographs were assessed for radiolucent lines and osteolytic lesions. Results. Osteolysis was present in four TKAs (3.3%) in the SP group, and no knees in the XLP group (p = 0.06). There were no significant differences between the Knee Society total score, change in total score, knee function score, change in function score, LEAS, and change in LEAS in the two groups. There was a significant difference in the presence of an effusion (10/122 with SP liners, 1/123 with XLP liners; p = 0.02). There was no significant difference in the rate of re-operation between the two groups (p = 0.36). There were no complications related to the XLP liner. Conclusion. At this length of follow-up, there were no advantages and no complications related to the use of this XLP tibial liner. The presence of effusion and small osteolytic lesions was more frequent with SP than XLP liners, but of unknown clinical significance. Cite this article: Bone Joint J 2019;101-B(7 Supple C):33–39

Abstract. Introduction. At our national explant retrieval unit, we identified an unusual pattern of backside-deformation on polyethylene (PE) inserts of contemporary total-knee-replacements (TKRs). The PE backside's margins were inferiorly deformed in TKRs with central-locking trays. We reported that this backside-deformation appeared to be linked to tray debonding. Moreover, recent studies have shown high-rate of tray debonding in PS NexGen TKRs. Therefore, we hypothesised that backside deformation on PS inserts may be more than on CR inserts. Methodology. We used peer-reviewed techniques to analyse changes in the bearing (wear rate) and backside surfaces (deformation) of PE inserts using coordinate measuring machines [N=61 NexGen (CR-39 and PS-22) TKRs with non-augmented-trays]. Multiple regression was used to determine which variable had the greatest influence on backside-deformation. The amount of cement cover on trays was quantified as a %of the total surface using Image-J software. Results. There was no statistically significant difference (p=0.238) in median (IQR) wear rate of the CR PEs 18 (12–28) mm. 3. /year and PS PEs 14 (8–20) mm. 3. /year. The PE backside-deformation median (IQR) of PS [297(242–333) µm] was significantly higher (p=0.011), when compared with CR [241(161–259) µm]. Multiple regression modelling showed that duration in-vivo (p=0.037), central-clearance between insert and tray (p<0.001) and constraint (p=0.003) were significantly associated with PE backside-deformation. 27(69%) of CR and 20(91%) PS exhibited ≤10% of cement cover on tray. Conclusion. This explant study showed backside-deformation on PS inserts was more than on CR inserts. Therefore, indicating a high-rate of tibial tray debonding in PS compared to CR NexGen TKRs

Aims. The aim of this study was to investigate whether wear and backside deformation of polyethylene (PE) tibial inserts may influence the cement cover of tibial trays of explanted total knee arthroplasties (TKAs). Methods. At our retrieval centre, we measured changes in the wear and deformation of PE inserts using coordinate measuring machines and light microscopy. The amount of cement cover on the backside of tibial trays was quantified as a percentage of the total surface. The study involved data from the explanted fixed-bearing components of four widely used contemporary designs of TKA (Attune, NexGen, Press Fit Condylar (PFC), and Triathlon), revised for any indication, and we compared them with components that used previous generations of PE. Regression modelling was used to identify variables related to the amount of cement cover on the retrieved trays. Results. A total of 114 explanted fixed-bearing TKAs were examined. This included 76 used with contemporary PE inserts which were compared with 15 used with older generation PEs. The Attune and NexGen (central locking) trays were found to have significantly less cement cover than Triathlon and PFC trays (peripheral locking group) (p = 0.001). The median planicity values of the PE inserts used with central locking trays were significantly greater than of those with peripheral locking inserts (205 vs 85 microns; p < 0.001). Attune and NexGen inserts had a characteristic pattern of backside deformation, with the outer edges of the PE deviating inferiorly, leaving the PE margins as the primary areas of articulation. Conclusion. Explanted TKAs with central locking mechanisms were significantly more likely to debond from the cement mantle. The PE inserts of these designs showed characteristic patterns of deformation, which appeared to relate to the manufacturing process and may be exacerbated in vivo. This pattern of deformation was associated with PE wear occurring at the outer edges of the articulation, potentially increasing the frictional torque generated at this interface. Cite this article: Bone Joint J 2021;103-B(12):1791–1801

The Oxford Unicompartmental Knee replacement (UKR) was introduced as a design to reduce polyethylene wear. There has been one previous retrieval study involving this implant, which reported very low rates of wear in some specimens but abnormal patterns of wear in others. There has been no further investigation of these abnormal patterns. The bearings were retrieved from 47 patients who had received a medial Oxford UKR for anteromedial osteoarthritis of the knee. None had been studied previously. The mean time to revision was 8.4 years (. sd. 4.1), with 20 having been implanted for over ten years. The macroscopic pattern of polyethylene wear and the linear penetration were recorded for each bearing. The mean rate of linear penetration was 0.07 mm/year. The patterns of wear fell into three categories, each with a different rate of linear penetration; 1) no abnormal macroscopic wear and a normal articular surface, n = 16 (linear penetration rate = 0.01 mm/year); 2) abnormal macroscopic wear and normal articular surfaces with extra-articular impingement, n = 16 (linear penetration rate = 0.05 mm/year); 3) abnormal macroscopic wear and abnormal articular surfaces with intra-articular impingement +/− signs of non-congruous articulation, n = 15 (linear penetration rate = 0.12 mm/year). The differences in linear penetration rate were statistically significant (p < 0.001). These results show that very low rates of polyethylene wear are possible if the device functions normally. However, if the bearing displays suboptimal function (extra-articular, intra-articular impingement or incongruous articulation) the rates of wear increase significantly

Introduction. Instability following total knee arthroplasty is a leading cause of failure and is often treated with component revision. The goal of this study was to determine if isolated tibial polyethylene insert exchange (ITPIE) to a higher-level constraint would afford similar outcomes to component revision in the properly selected patient. Methods. We retrospectively evaluated 176 consecutive patients between 2016–2017 who were revised for symptomatic instability at a single institution. Demographic information and level of constraint preoperatively and postoperatively were documented. Radiographic parameters were also recorded for patients undergoing ITPIE. Outcome measures included all cause re-revision rates as well as patient reported outcome measures (PROMs) obtained preoperatively and at minimum 1-yr follow up. Descriptive analysis including sample t-test and chi square test were performed with statistical significance set at p <0.05. Results. 100 patients underwent component revision. 76 patients underwent ITPIE. Patients undergoing ITPIE were not found to have gross malalignment, malrotation, compromised fixation or insufficient collateral ligaments. No statistically significant differences were noted in terms of demographic characteristics between the groups. Similar increases in constraint as well as the thickness of the polyethylene were noted at the time of revision surgery in both groups. Further, patients underwent similar rates of re-revision (15.9% for component revision and 11.8% for ITPIE), p=0.8. In analyzing PROMs, no statistically significant differences were noted. Radiographic parameters of those who underwent ITPIE showed majority of patients to have well aligned components. The average follow-up was 2.3(±1.0)yrs for the component revision group and 2.2(±1.1)yrs for the ITPIE group. Conclusion. In the appropriately selected patient, ITPIE offers similar rates of success to component revision including similar re-revision rates and similar improvement in PROMs. Degree of constraint is generally increased and recommended when patients are treated for instability regardless of component revision or ITPIE. Abbreviations. ITPIE - isolated tibial polyethylene insert exchange. PROMs - patient reported outcome measures

Objectives. Bone void fillers are increasingly being used for dead space management in arthroplasty revision surgery. The aim of this study was to investigate the influence of calcium sulphate bone void filler (CS-BVF) on the damage and wear of total knee arthroplasty using experimental wear simulation. Methods. A total of 18 fixed-bearing U2 total knee arthroplasty system implants (United Orthopedic Corp., Hsinchu, Taiwan) were used. Implants challenged with CS-BVF were compared with new implants (negative controls) and those intentionally scratched with a diamond stylus (positive controls) representative of severe surface damage (n = 6 for each experimental group). Three million cycles (MC) of experimental simulation were carried out to simulate a walking gait cycle. Wear of the ultra-high-molecular-weight polyethylene (UHMWPE) tibial inserts was measured gravimetrically, and damage to articulating surfaces was assessed using profilometry. Results. There was no significant difference (p  >  0.05) between the wear rate of implants challenged with CS-BVF (3.3 mm. 3. /MC (95% confidence interval (CI) 1.8 to 4.8)) and the wear rate of those not challenged (2.8 mm. 3. /MC (95% CI 1.3 to 4.3)). However, scratching the cobalt-chrome (CoCr) significantly (p < 0.001) increased the wear rate (20.6 mm. 3. /MC (95% CI 15.5 to 25.7)). The mean surface roughness of implants challenged with CS-BVF was equivalent to negative controls both after damage simulation (p = 0.98) and at the conclusion of the study (p = 0.28). Conclusion. When used close to articulating surfaces, a low-hardness, high-purity CS-BVF had no influence on wear. When trapped between the articulating surfaces of a total knee arthroplasty, CS-BVF did not scratch the surface of CoCr femoral components, nor did it increase the wear of UHMWPE tibial inserts compared with undamaged negative controls. Cite this article: R. M. Cowie, S. S. Aiken, J. J. Cooper, L. M. Jennings. The influence of a calcium sulphate bone void filler on the third-body damage and polyethylene wear of total knee arthroplasty. Bone Joint Res 2019;8:65–72. DOI: 10.1302/2046-3758.82.BJR-2018-0146.R1

Aims

Mid-level constraint designs for total knee arthroplasty (TKA) are intended to reduce coronal plane laxity. Our aims were to compare kinematics and ligament forces of the Zimmer Biomet Persona posterior-stabilized (PS) and mid-level designs in the coronal, sagittal, and axial planes under loads simulating clinical exams of the knee in a cadaver model.

The Oxford unicompartmental knee replacement (UKR) was designed to minimise wear utilising a fully-congruent, mobile, polyethylene bearing. Wear of polyethylene is a significant cause of revision surgery in UKR in the first decade, and the incidence increases in the second decade. Our study used model-based radiostereometric analysis to measure the combined wear of the upper and lower bearing surfaces in 13 medial-compartment Oxford UKRs at a mean of 20.9 years (17.2 to 25.9) post-operatively. The mean linear penetration of the polyethylene bearing was 1.04 mm (0.307 to 2.15), with a mean annual wear rate of 0.045 mm/year (0.016 to 0.099). The annual wear rate of the phase-2 bearings (mean 0.022 mm/year) was significantly less (p = 0.01) than that of phase-1 bearings (mean 0.07 mm/year). The linear wear rate of the Oxford UKR remains very low into the third decade. We believe that phase-2 bearings had lower wear rates than phase-1 implants because of the improved bearing design and surgical technique which decreased the incidence of impingement. We conclude that the design of the Oxford UKR gives low rates of wear in the long term

Introduction. Polyethylene wear and osteolysis remain a concern with the use of modular, fixed bearing total knee arthroplasty (TKA). A variety of highly cross-linked polyethylenes (XLPs) have been introduced to address this problem, but there are few data on the results and complications of this polyethylene in posterior-stabilized knee prosthesis. We have previously reported an interim analysis of a study comparing polyethylene tibial liners. Methods. This is a prospective randomized study of one modular posterior-stabilized total knee arthroplasty by a single surgeon. 265 patients (329 knees) were randomized to receive a standard compression molded liner (SP) or a highly cross-linked (6.5 CGy electron-beam irradiated and remelted) polyethylene liner (XLP). Patients were evaluated clinically using the classic Knee Society scores, LEAS score, presence of a knee effusion, and by standard radiographs for radiolucent lines and osteolytic lesions. The analysis was performed at a mean of 6 years (range, 2–11 years). Results. There were no clinical differences between 122 knees with SP and 123 knees with XLP in Knee Society total score; change in total score; knee function score; change in function score; LEAS score; and change in LEAS score. There was a difference in the presence of effusion (one of 123 XLP, and 10 of 122 SP; p=0.02). There was no difference in the frequency of radiolucent lines (21 knees with SP and 22 with XLP). Osteolysis was present in 4 knees (3.3%) with SP, and no knees with XLP (p=0.06). There was no difference in frequency of reoperation between the two groups (3 infection in 123 knees allocated to XLP group and six (3 infection, 1 femoral loosening, 1 instability, 1 fracture plating) in 122 knees allocated to SP group. There were no complications related to the XLP liner. Conclusion. At this length of follow-up time, with the numbers available, there were no complications, but no advantages, related to the use of this XLP tibial liner. The presence of effusion and small osteolytic lesions are more frequent with SP than XLP, but of unknown clinical importance

The purpose of this study was to undertake a meta-analysis to determine whether there is lower polyethylene wear and longer survival when using mobile-bearing implants in total knee replacement when compared with fixed-bearing implants. Of 975 papers identified, 34 trials were eligible for data extraction and meta-analysis comprising 4754 patients (6861 knees). We found no statistically significant differences between the two designs in terms of the incidence of radiolucent lines, osteolysis, aseptic loosening or survival. There is thus currently no evidence to suggest that the use of mobile-bearing designs reduce polyethylene wear and prolong survival after total knee replacement. Cite this article: Bone Joint J 2013;95-B:1057–63

Introduction. Clinical observations suggest mid-flexion instability may occur more commonly with rotating platform (RP) total knee arthroplasty (TKA), including increased revision rates and patient-reported instability and pain. We propose that increased gap laxity leads to liftoff of the lateral femoral condyle with decreased conformity between the femoral component and polyethylene (PE) insert surface leading to PE subluxation or dislocation. The objectives of this study were to define “at risk” loading conditions that predispose patients to PE insert subluxation or spinout, and to quantify the margin of error for flexion/extension gap laxity in preventing these adverse events under physiologic loading conditions. Methods. Biomechanical testing was performed on six fresh frozen cadaveric knees implanted with a posterior stabilized RP TKA using a gap balancing technique. Rotational displacement and torque were measured over time, while stiffness, yield torque, max torque and displacement were calculated using a post-processing, custom MatLab code. Revision with varying size femoral components (size 3–6) and PE insert thicknesses (10–15mm), by downsizing one step, were used to create a spectrum of flexion/extension gap mismatch. Each configuration was subjected to three loaded testing conditions (0°, 30° and 60° flexion) in balanced and eccentric varus loading, known to represent daily clinical function and “at risk” circumstances. Results. PE insert rotational instability was primarily determined by conformity and contact area between the femoral condyle and the upper surface of the PE insert. In this RP design, contact area is known to decrease with flexion greater than 35°, which predisposed to abnormal motion of the femur on PE insert (Figure 1). Under all flexion/extension gap testing conditions, PE insert rotational displacement significantly decreased with increasing knee flexion (differences ranged from 0.42 to 1.01cm, p<0.05), confirming that decreased conformity allows unintended motion to occur on the upper rather than the lower insert surface, as kinematically designed. This decrease in insert rotation was further exacerbated with eccentric medial-sided loading (differences ranged from 0.77 to 1.18cm, p<0.05). Yield torque (19.66±6.79N-m, p=0.033) and max torque (19.76±5.93N-m, p=0.014) significantly increased with increasing flexion from 0° to 60° under gap balanced conditions. Yield torque significantly decreased with greater flexion gap laxity at 60° of flexion (−24.82±5.96N-m, p=0.004). The depth of the lateral PE insert concavity (1.7–3.6mm) varied with insert size and thickness and determined femoral condylar capture. The lateral insert concavity defines a narrow margin of error in flexion/extension gap asymmetry leading to rotational insert instability, especially in smaller sized knees (size 3) where the jump height (1.7mm) is less than the insert sizing increment of 2.5mm. Conclusions. Contact area is known to decrease with flexion greater than 35° in this TKA-RP design. Flexion gap laxity further increased the risk of unintended top-side rotation of the femur on the insert, especially with increasing flexion and smaller components. In RP-TKA, in addition to medial-lateral gap symmetry and flexion-extension balance, a snug flexion gap with less than 2mm lateral laxity is critical to avoid insert instability and condylar escape with insert subluxation. For any figures or tables, please contact authors directly