Fixed-bearing Total Knee Replacement Research Articles

Cross-sectional Analysis Study Between SIGMA Knee System and ATTUNE in a Cruciate-Retaining Fixed-Bearing Total Knee Replacement, with no Patellar Resurfacing.

The purpose of our study was to compare the knee implant SIGMA to the ATTUNE knee implant in a cruciate-retaining (CR) fixed-bearing (FB) total knee replacement (TKR) surgery, with no patellar resurfacing. We examined 40 patients (19 SIGMA; 21 ATTUNE) who underwent a TKR FB CR surgery, without patellar resurfacing, due to osteoarthritis, between August 2013 and July 2017. All surgeries were performed by a single surgeon. We performed a cross-sectional analytical study based on preoperative patient data and data collected in follow-up sessions. All patients were asked to fill a quality-of-life (QOL) questionnaire, SF-36 translated to the Hebrew language. All patients were clinically evaluated using the Knee Society Score (KSS) and the Knee Function Score (KFS). Patients from the SIGMA cohort reported fewer postoperative physical function limitations in the SF-36 (p = 0.01) and the KFS (p = 0.04). Patients who underwent surgery using the SIGMA implant reported increased vitality when compared to the ATTUNE (p = 0.02). No significant differences were found between the groups in other measures of quality of life, ranges of motion, valgus, and postoperative knee stability. The average follow-up period was significantly longer for the SIGMA (p < 0.00001). Significant differences were found in postoperative physical function and vitality scores between SIGMA and ATTUNE knee implants. SIGMA cohort presented superior results. Presumably, these findings are due to the differences in follow-up times.

Biomechanical Assessment of Mobile-Bearing Total Knee Endoprostheses Using Musculoskeletal Simulation

The purpose of this computational study was to analyze the effects of different mobile-bearing (MB) total knee replacement (TKR) designs on knee joint biomechanics. A validated musculoskeletal model of the lower right extremity implanted with a cruciate-retaining fixed-bearing TKR undergoing a squat motion was adapted for three different MB TKR design variants: (I) a commercially available TKR design allowing for tibial insert rotation about the tibial tray with end stops to limit the range of rotation, (II) the same design without end stops, and (III) a multidirectional design with an additional translational degree-of-freedom (DoF) and end stops. When modeling the MB interface, two modeling strategies of different joint topologies were deployed: (1) a six DoF joint as a baseline and (2) a combined revolute-prismatic joint (two DoF joint) with end stops in both DoF. Altered knee joint kinematics for the three MB design variants were observed. The commercially available TKR design variant I yielded a deviation in internal-external rotation of the tibial insert relative to the tray up to 5° during knee flexion. Compared to the multidirectional design variant III, the other two variants revealed less femoral anterior-posterior translation by as much as 5 mm. Concerning the modeling strategies, the two DoF joint showed less computation time by 68%, 80%, and 82% for design variants I, II, and III, respectively. However, only slight differences in the knee joint kinematics of the two modeling strategies were recorded. In conclusion, knee joint biomechanics during a squat motion differed for each of the simulated MB design variants. Specific implant design elements, such as the presence of end stops, can impact the postoperative range of knee motion with regard to modeling strategy, and the two DoF joint option tested accurately replicated the results for the simulated designs with a considerably lower computation time than the six DoF joint. The proposed musculoskeletal multibody simulation framework is capable of virtually characterizing the knee joint dynamics for different TKR designs.

Ten-year outcomes of a nitrided Ti-6Al-4V titanium alloy fixed-bearing total knee replacement with a highly crosslinked polyethylene-bearing in patients with metal allergy

BackgroundMetal hypersensitivity in patients with a total knee arthroplasty (TKA) is a rare condition and a very controversial topic. Despite the lack of data concerning the real effective impact of allergy on TKA failures, most of the manufactures offer the choice of ‘non-allergenic’ implants both for primary and revision TKA, some of which provide the same designs and surgical techniques as the conventional ones. Only a few studies are available on outcomes on these ‘hypersensitivity-friendly’ implants and even fewer specifically on allergic patients with a mid- to long-term follow-up. MethodsBetween 2007 and 2015, we enrolled 72 patients (57 females, 15 males), who underwent TKA treated with a non-allergenic posterior-stabilized (PS) total knee implant for a declared and proven metal allergy.Patients were followed clinically and radiographically for a mean 10 years of follow-up. ResultsWith revision as an endpoint the Kaplan–Meier survival estimate showed a survival rate of 97.2% at five years and 95.1% at 10 years. Significant improvements in range of motion (ROM), Knee Society Scoring (KSS) and Hospital for Special Surgery (HSS) knee scores were registered at final follow-up (P < 0.0001). At final follow-up validated Patient-Reported Outcome Measures (PROMs) showed the following scores: Oxford Knee Score (OKS) 42.1, EQ5D 0.80, EQ VAS 80.1, Forgotten Joint Score 71.2. ConclusionsThis nitrided Ti-6Al-4V titanium alloy fixed-bearing total knee replacement with a highly crosslinked polyethylene-bearing showed interesting results and survival rates in patients with metal allergy at mid- to long-term follow-up.

Effects of interference assembly of a tibial insert on the tibiofemoral contact mechanics in total knee replacement.

Tibial locking mechanism design is adopted to limit the backside micromotion in fixed-bearing total knee replacement. However, the effect of the interference assembly of a tibial insert on the tibiofemoral contact mechanics was usually ignored. Finite element model of a fixed-bearing total knee replacement with full peripheral locking mechanism was established to simulate the interference assembly of the tibial insert, and the corresponding effects on the tibiofemoral contact mechanics were predicted. Due to the interference assembly of the tibial insert, a maximum Mises stress of 3.24 MPa was found for the tibial insert before loading. Furthermore, the contact stress was increased by 8.77%, and the contact area was decreased by 5.43% under peak load. The interference assembly of the tibial insert in a fixed-bearing total knee replacement changed the tibiofemoral contact mechanics. This study indicated that the level of interference fit should be cautiously designed for the tibial locking mechanism in fixed-bearing total knee replacement for balancing the articular surface wear and the backside wear of the modular tibial insert.

Increased risk of aseptic loosening for 43,525 rotating-platform vs. fixed-bearing total knee replacements

Background and purpose — Given similar functional outcomes with mobile and fixed bearings, a difference in survivorship may favor either. This study investigated the risk of aseptic loosening for the most used subtypes of mobile-bearing rotating-platform knees, in Norway and Australia.Patients and methods — Primary TKRs reported to the Norwegian and Australian joint registries, between 2003 and 2014, were analyzed with aseptic loosening as primary end-point and all revisions as secondary end-point. We hypothesized that no difference would be found in the rate of revision between rotating-platform and the most used fixed-bearing TKRs, or between keeled and non-keeled tibia. Kaplan–Meier estimates and curves, and Cox regression relative risk estimates adjusted for age, sex, and diagnosis were used for comparison.Results — The rotating-platform TKRs had an increased risk of revision for aseptic loosening compared with the most used fixed-bearing knees, in Norway (RR =6, 95% CI 4–8) and Australia (RR =2.1, 95% CI 1.8–2.5). The risk of aseptic loosening as a reason for revision was highest in Norway compared with Australia (RR =1.7, 95% CI 1.4–2.0). The keeled tibial component had the same risk of aseptic loosening as the non-keeled tibia (Australia). Fixation method and subtypes of the tibial components had no impact on the risk of aseptic loosening in these mobile-bearing knees.Interpretation — The rotating-platform TKRs in this study appeared to have a higher risk of revision for aseptic loosening than the most used fixed-bearing TKRs.

The Effect on Long-Term Survivorship of Surgeon Preference for Posterior-Stabilized or Minimally Stabilized Total Knee Replacement: An Analysis of 63,416 Prostheses from the Australian Orthopaedic Association National Joint Replacement Registry.

Controversy still exists as to the optimum management of the posterior cruciate ligament (PCL) in total knee arthroplasty. Surgeons can choose to kinematically substitute the PCL with a posterior-stabilized total knee replacement or alternatively to utilize a cruciate-retaining, also known as minimally stabilized, total knee replacement. Proponents of posterior-stabilized total knee replacement propose that the reported lower survivorship in registries when directly compared with minimally stabilized total knee replacement is due to confounders such as selection bias because of the preferential usage of posterior-stabilized total knee replacement in more complex or severe cases. In this study, we aimed to eliminate these possible confounders by performing an instrumental variable analysis based on surgeon preference to choose either posterior-stabilized or minimally stabilized total knee replacement, rather than the actual prosthesis received. Cumulative percent revision, hazard ratio (HR), and revision diagnosis data were obtained from the Australian Orthopaedic Association National Joint Replacement Registry from September 1, 1999, to December 31, 2014, for 2 cohorts of patients, those treated by high-volume surgeons who preferred minimally stabilized replacements and those treated by high-volume surgeons who preferred posterior-stabilized replacements. All patients had a diagnosis of osteoarthritis and underwent fixed-bearing total knee replacement with patellar resurfacing. At 13 years, the cumulative percent revision was 5.0% (95% confidence interval [CI], 4.0% to 6.2%) for the surgeons who preferred the minimally stabilized replacements compared with 6.0% (95% CI, 4.2% to 8.5%) for the surgeons who preferred the posterior-stabilized replacements. The revision risk for the surgeons who preferred posterior-stabilized replacements was significantly higher for all causes (HR = 1.45 [95% CI, 1.30 to 1.63]; p < 0.001), for loosening or lysis (HR = 1.93 [95% CI, 1.58 to 2.37]; p < 0.001), and for infection (HR = 1.51 [95% CI, 1.25 to 1.82]; p < 0.001). This finding was irrespective of patient age and was evident with cemented fixation and with both cross-linked polyethylene and non-cross-linked polyethylene. However, the higher revision risk was only evident in male patients. There was a 45% higher risk of revision for the patients of surgeons who preferred a posterior-stabilized total knee replacement compared with the patients of surgeons who preferred a minimally stabilized total knee replacement. Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.

The influence of simulator input conditions on the wear of total knee replacements: An experimental and computational study.

Advancements in knee replacement design, material and sterilisation processes have provided improved clinical results. However, surface wear of the polyethylene leading to osteolysis is still considered the longer-term risk factor. Experimental wear simulation is an established method for evaluating the wear performance of total joint replacements. The aim of this study was to investigate the influence of simulation input conditions, specifically input kinematic magnitudes, waveforms and directions of motion and position of the femoral centre of rotation, on the wear performance of a fixed-bearing total knee replacement through a combined experimental and computational approach. Studies were completed using conventional and moderately cross-linked polyethylene to determine whether the influence of these simulation input conditions varied with material. The position of the femoral centre of rotation and the input kinematics were shown to have a significant influence on the wear rates. Similar trends were shown for both the conventional and moderately cross-linked polyethylene materials, although lower wear rates were found for the moderately cross-linked polyethylene due to the higher level of cross-linking. The most important factor influencing the wear was the position of the relative contact point at the femoral component and tibial insert interface. This was dependent on the combination of input displacement magnitudes, waveforms, direction of motion and femoral centre of rotation. This study provides further evidence that in order to study variables such as design and material in total knee replacement, it is important to carefully control knee simulation conditions. This can be more effectively achieved through the use of displacement control simulation.

Reduced patellofemoral and walking pain with mobile-bearing vs. fixed-bearing total knee replacements: a mid-term prospective analytic study

Total knee replacement (TKR) is the standard treatment for advanced stage knee osteoarthritis. The introduction of the mobile-bearing (MB) design has given rise to a series of theoretical advantages compared to fixed-bearing (FB) implants, although current literature does not reveal significant differences between the designs. The aim of this study was to estimate the clinical results of 2 cemented total knee prosthetic designs: an MB and an FB design. A series of patients with similar clinical and radiographic characteristics were treated consecutively with 100 FB followed by 94 MB implants. Patients were evaluated radiographically and clinically. Statistically significant differences were found in terms of pain at 5 years in favor of MB prostheses (p=0.006). The "pain on ascending/descending stairs" category on the KSS score showed improvement at 5 years for the MB design (p=0.003). MB implants showed better results in terms of ability to ascend/descend stairs at five years (p=0.002). With regards to the patients' ability to walk, there were differences at 1 year (p=0.020) and at 5 years (p=0.021) in favor of MB implants. At a mean follow-up of 5 years, significant differences were observed in the MB prosthesis in terms of postoperative pain, ability to ascend/descend stairs, and patellofemoral pain.

Evaluation of fixed-bearing arthroplasty in primary total knee replacement

Background Fixed-bearing primary total knee replacement is a well-established method of treatment for advanced arthritic knee joint. Aim of the study This study evaluated the clinical, functional, and radiological outcomes after the use of fixed-bearing total knee arthroplasty in cases of primary total knee replacement. Patients and method This prospective study was conducted on 14 patients (six male and eight female) who attended Suez Canal University Hospitals between January 2012 and January 2013, and the mean age was 60 years (range 50-72 years) at the time of surgery. The patients included in the study had advanced unilateral knee osteoarthritis with severe pain and functional disability with failure of conservative measures. Preoperative clinical and radiological evaluation was carried out and the knee society clinical and functional rating system (KSS) was used to evaluate the patients preoperatively and postoperatively at 1-year follow-up. The scoring system combines a relatively objective knee score that is based on the clinical parameters and a functional score based on how patients perceive their knee function in relation to specific activities. Radiological evaluation was carried out to determine the alignment of the knee and the femoral and tibial component positions. Results There was significant difference between preoperative and postoperative clinical and functional scores. The mean postoperative clinical assessment score for pain was 44.9, for range of motion it was 115.9, and for stability it was 23.5. Postoperative mean knee society clinical assessment score was 90.7 and the mean functional assessment score for knee function was 87.5. According to the knee society clinical and functional score, 85% of patients had excellent results and 15% of patients had good results. As regards radiological results, all knees were well-aligned at 1 year postoperatively and no periprosthetic loosening or radiolucent line around either component was found. Conclusion Primary total knee arthroplasty using fixed-bearing prosthesis is an effective procedure with good clinical and functional results and early mobilization.

Mobile- vs. fixed-bearing total knee replacement

Background and purpose — It is unclear whether mobile-bearing (MB) total knee arthroplasties reduce the risk of tibial component loosening compared to fixed-bearing (FB) designs. This randomized study investigated implant migration, periprosthetic bone mineral density (BMD), and patient-reported outcomes (Oxford knee score)—all at 2 years—for the P.F.C. Sigma Cruciate Retaining total knee arthroplasty.Patients and methods — 50 osteoarthritis patients were allocated to either FB or MB tibial articulation.Resultsand interpretation — At 2 years, the mean total translation (implant migration) was higher for the FB implant (0.30 mm, SD 0.22) than for the MB implant (0.17 mm, SD 0.09) (p = 0.04). BMD decreased between baseline and 1-year follow-up. At 2-year follow-up, BMD was close to the baseline level. The knee scores of both groups improved equally well. The FB tibial implant migrated more than the MB, but this was not clinically significant. The mobile polyethylene presumably partly absorbs the force transmitted to the metal tibial tray, thereby reducing micromotion.

A systematic review and meta-regression of mobile-bearingversusfixed-bearing total knee replacement in 41 studies

Mobile-bearing (MB) total knee replacement (TKR) was introduced to reduce the risk of aseptic loosening and wear of polyethylene inserts. However, no consistent clinical advantages of mobile- over fixed-bearing (FB) TKR have been found. In this study we evaluated whether mobile bearings have an advantage over fixed bearings with regard to revision rates and clinical outcome scores. Furthermore, we determined which modifying variables affected the outcome. A systematic search of the literature was conducted to collect clinical trials comparing MB and FB in primary TKR. The primary outcomes were revision rates for any reason, aseptic loosening and wear. Secondary outcomes included range of movement, Knee Society score (KSS), Oxford knee score (OKS), Short-Form 12 (SF-12) score and radiological parameters. Meta-regression techniques were used to explore factors modifying the observed effect. Our search yielded 1827 publications, of which 41 studies met our inclusion criteria, comprising over 6000 TKRs. Meta-analyses showed no clinically relevant differences in terms of revision rates, clinical outcome scores or patient-reported outcome measures between MB and FB TKRs. It appears that theoretical assumptions of superiority of MB over FB TKR are not borne out in clinical practice.

Risk of Revision for Fixed Versus Mobile-Bearing Primary Total Knee Replacements

Mobile-bearing total knee arthroplasty prostheses were developed to reduce wear and revision rates; however, these benefits remain unproven. The purposes of this study were to compare the short-term survivorship and to determine risk factors for revision of mobile-bearing and fixed-bearing total knee replacements. A prospective cohort study of primary total knee arthroplasties performed from 2001 to 2009 was conducted with use of a community total joint replacement registry. Patient characteristics and procedure details were identified. Cox regression models were used. Bearing type was investigated as a risk factor for revision while adjusted for other risk factors such as age, American Society of Anesthesiologists (ASA) score, body mass index, sex, race, diagnosis, bilateral procedures, cruciate-retaining versus posterior-stabilized components, surgical approach, fixation, patellar resurfacing, hospital and surgeon volumes, and fellowship training. The study cohort consisted of 47,339 total knee arthroplasties, with 62.6% of the procedures in women. Fixed bearings were used in 41,908 knees (88.5%) and mobile bearings in 4830 (10.2%). Rotating-platform designs were used in all mobile-bearing total knee arthroplasties (3112 had a Rotating-Platform Press-Fit Condylar posterior-stabilized design; 1053, a Low Contact Stress [LCS] design; and 665, a Rotating-Platform Press-Fit Condylar cruciate-retaining design). Patients who received fixed-bearing total knee arthroplasty systems were older (mean age, 68.1 years) than those who received mobile-bearing total knee arthroplasty systems (mean age, 62.2 years); the difference was significant (p < 0.001). Overall, 515 knees (1.1%) were revised for reasons other than infection. The survival rate was 97.8% (95% confidence interval [CI], 97.4% to 98.0%) at 6.7 years. The adjusted risk of aseptic revision for the LCS total knee replacements was 2.01 times (95% CI, 1.41 to 2.86) higher than that for fixed-bearing total knee replacements (p < 0.001).There was no significant revision risk for the other mobile-bearing total knee arthroplasty systems. There was no association with surgeon and hospital case volumes and the risk of revision total knee arthroplasty. Our study suggests the benefit of potential long-term wear reduction with the LCS implant may not be realized in a community-based setting, where a variety of surgical skills, surgical experience, and diverse patient demographic factors may affect early outcomes. Therapeutic Level II.

Single-radius, multidirectional total knee replacement

The present study was designed to evaluate the mid-term (5- to 8-year) survival rate of a new total knee replacement (TKR) with a single-radius femoral component and a multidirectional, highly congruent tibial component, in comparison with an historical group (TKR with multi-radius design and fixed bearing). Four hundred and thirty patients were included, of which 369 patients (86 %) completed the 5-year follow-up with Knee Society Score evaluation and radiograph examination (study group = 387, control group = 83). There was a significant improvement for all analysed items between pre-operative status and late follow-up. Mean knee score was 93 ± 9 points in the study group and 88 ± 16 points in the control group (p < 0.001). Mean flexion angle was not different (118 ± 11 vs. 114 ± 13 degrees). Mean functional score was 87 ± 16 points in the study group and 71 ± 24 points in the control group (p < 0.001). Survival rate at 5-year follow-up was 98.8 % (vs. 98.0 %) for mechanical revisions only, and 96.4 % (vs. 98.1 %) for all revisions. The new TKR allowed obtaining significantly better results than the fixed-bearing TKR. These results are in line with the best series published in the current literature, but there was no evidence of any superiority of this TKR against already published standards.

Corrigendum

Wear of moderately cross-linked polyethylene in fixed-bearing total knee replacements Claire L Brockett, Louise M Jennings, Catherine Hardaker and John Fisher The authors would like to bring attention to the error published in ‘Wear of moderately cross-linked polyethylene in fixed-bearing total knee replacements’ Proceedings of the IMechE Part H: J. Engineering in Medicine, 2012, Vol. 226(7), pp. 529–535, doi 10.1177/0954411912445265

Long-Term Comparison of Fixed-Bearing and Mobile-Bearing Total Knee Replacements in Patients Younger Than Fifty-one Years of Age with Osteoarthritis

There is limited information comparing the results of fixed-bearing total knee replacement and mobile-bearing total knee replacement in patients with osteoarthritis who are younger than fifty-one years and who have a fixed-bearing implant in one knee and a mobile-bearing implant in the other. The purpose of this study was to compare our long-term clinical and radiographic results of fixed-bearing total knee replacement and mobile-bearing total knee replacement in a group of patients from this population. We prospectively compared the results of 108 patients with osteoarthritis who were younger than fifty-one years (mean age, forty-five years) who had received a fixed-bearing prosthesis in one knee and a rotating platform mobile-bearing prosthesis in the other. The mean follow-up was 16.8 years (range, fifteen to eighteen years). The patients were assessed clinically and radiographically. Knee motion and function were assessed as a primary outcome. Patients were assessed with questionnaires, and each knee was assessed separately. Although there was significant improvement in both groups of knees, there was no significant difference between the groups (i.e., fixed-bearing and mobile-bearing knees) with regard to the mean postoperative knee motion (126° and 128°, respectively; p = 0.79), the mean Knee Society knee clinical score (95 and 94 points, respectively; p = 0.79), or the Knee Society knee functional score (84 and 85 points, respectively; p = 0.19) at the latest follow-up. In the fixed-bearing group, one knee was revised because of infection, two for aseptic loosening of the tibial component, and two because of wear of the tibial polyethylene insert. In the rotating platform mobile-bearing group, two knees were revised because of instability and one because of infection. The Kaplan-Meier survivorship for revision at 16.8 years of follow-up was 95% (95% confidence interval, 91 to 100) for the fixed-bearing prosthesis and 97% (95% confidence interval, 93 to 100) for the rotating platform mobile-bearing prosthesis. Long-term results of both fixed and mobile-bearing total knee arthroplasties were encouraging in patients who were younger than fifty-one years of age with osteoarthritis. However, we found no superiority of the mobile-bearing total knee prosthesis over the fixed-bearing total knee prosthesis.

Wear of moderately cross-linked polyethylene in fixed-bearing total knee replacements

Cross-linked polyethylene has been introduced into total joint replacement to improve wear resistance. Although the performance of highly cross-linked polyethylene is well documented clinically and experimentally for total hip replacements, the reduction in mechanical properties with increasing irradiation is of concern for application to total knee replacement. The aim of this study was to investigate the wear performance of a moderately cross-linked polyethylene material in a fixed-bearing total knee replacement. The study was conducted using two femoral geometries, a conventional cruciate-retaining femoral and a high-flexion femoral geometry. The femoral geometry appeared to have no effect on the wear of the knee replacement under standard gait conditions. A significant reduction in wear volume was measured with the moderately cross-linked polyethylene compared with the conventional polyethylene over a six-million-cycle wear study. This study indicates the use of a moderately cross-linked polyethylene in a fixed-bearing total knee replacement may provide a low wearing option for total knee replacement.

The Role of Registry Data in the Evaluation of Mobile-Bearing Total Knee Arthroplasty

The conventional wisdom that a mobile-bearing design may offer a benefit compared with a fixed-bearing design in total knee arthroplasty has not been supported by evidence. We reviewed the published literature and annual registry reports of all national and regional registries to determine the differences in clinical outcomes between mobile and fixed-bearing designs. We found only single-center reports and studies with small sample sizes in the published literature. These studies did not demonstrate any advantages of mobile bearings over fixed bearings. Moreover, major national joint registries reported higher failure rates associated with mobile-bearing total knee replacement compared with fixed-bearing total knee replacement. Similar findings from a U.S. national study in a community setting suggest that mobile-bearing knees have an increased risk of revision. After harmonization of methodologies, international collaborations of registries may provide the best insight into the performance of mobile-bearing total knee arthroplasty in real-world settings.

The wear of fixed and mobile bearing unicompartmental knee replacements

Unicompartmental knee replacements (UKR) are an option for surgical intervention for the treatment of single-compartment osteoarthritis. The aim of this study was to compare the wear of a low-conformity fixed-bearing UKR with a conforming mobile bearing UKR under two kinematic conditions, to investigate the effect of implant design and kinematics on wear performance in a physiological knee wear simulator. Under both sets of kinematic conditions, the relatively low-conforming fixed UKR showed lower wear, compared with the more conforming anterior-posterior sliding mobile bearing. However, it should be noted that differences in materials between the two designs also contribute to the relative wear performance of the bearings. The combined wear of the medial and lateral bearings of the fixed-bearing UKR as a 'total knee' were significantly reduced compared with a fixed-bearing total knee replacement studied under the same kinematic conditions.

Internal rotational error of the tibial component is a major cause of pain after total knee replacement

This study used CT analysis to determine the rotational alignment of 39 painful and 26 painless fixed-bearing total knee replacements (TKRs) from a cohort of 740 NexGen Legacy posterior-stabilised and cruciate-retaining prostheses implanted between May 1996 and August 2003. The mean rotation of the tibial component was 4.3 degrees of internal rotation (25.4 degrees internal to 13.9 degrees external rotation) in the painful group and 2.2 degrees of external rotation (8.5 degrees internal to 18.2 degrees external rotation) in the painfree group (p = 0.024). In the painful group 17 tibial components were internally rotated more than 9 degrees compared with none in the painfree group (p < 0.001). Additionally, six femoral components in the painful group were internally rotated more than 6 degrees compared with none in the painfree group (p = 0.017). External rotational errors were not found to be associated with pain. Overall, 22 (56.4%) of the painful TKRs had internal rotational errors involving the femoral, the tibial or both components. It is estimated that at least 4.6% of all our TKRs have been implanted with significant internal rotational errors.

Fixed versus mobile bearing total knee replacement: methodological considerations in systematic reviewing

Dear Editors, We thank Dr. Jacobs for the interest in our article ‘‘Clinical and radiological outcomes of fixedversus mobile-bearing total knee replacement: a meta-analysis’’ [26] published in this journal. We thank him for the comments regarding the approach and presentation which we adopted in order to appropriately answer this controversial research question. Regarding the points that were raised: Firstly, he rightly acknowledged that the data from retrospective studies was included in this paper’s analysis. It is acknowledged that retrospective data can be subject to methodological limitations such as selection and allocation bias [5]. However, by not including all pertinent studies relating to the research question, such a meta-analysis would have exhibited selection bias [11]. By excluding studies based on quality and methodological rigour, it is true that the authors may be able to make more confident inferences on the results [5]. Nonetheless, by undertaking such a method, the conclusions drawn would not represent those findings of the evidence-base as a whole. This later factor was the overwhelming reason for undertaking a ‘best-evidence synthesis’ [24, 25]. The primary objective of this study was to compare the clinical and radiological differences of fixedand mobilebearing total knee replacement (TKR). Whilst previous studies have reported a difference in outcome between fixed-bearing TKR such as posterior cruciate sacrificing and retaining prostheses [20], an assessment of this was not the primary aim of this study. However, with this in mind, we detailed which specific implant used, using all the information provided from each paper. As Dr. Jacobs correctly points out, in a small number of papers (three out of 33), a non-posterior stabilising mobile-bearing prosthesis was compared to a posterior-stabilising fixed comparator [1, 6, 32]. Whilst this may provide some heterogeneity in the type of implant used, and therefore may underestimate a treatment effect between the two groups, the papers reviewed largely provided limited information as to the specific implants used. In response to this, it would have been inappropriate, with such insufficient information, to perform a formal subgroup analysis based on whether prostheses were cruciate retaining or sacrificing or whether the posterior cruciate-sacrificing implants were the ‘postand-cam’ variety. This, nonetheless, may be an area for further evidence synthesis to determine the potential importance of this specific type of cruciate-sacrificing implant. Dr. Jacobs rightly advocates the inclusion of unpublished or ‘grey’ literature used in our study. It is commonly considered important to include unpublished results in order to create an unbiased sampling frame for meta-analysis [22]. By not including all published and unpublished literature, the scope of a literature search is thereby reduced. Consequentially, the conclusions drawn from such reviews would be biased [11]. Dr. Jacobs should not be mislead over the simplicity of the search strategy used to assess the published or ‘white’ literature. T. O. Smith (&) School of Allied Health Professions, University of East Anglia, Norwich NR4 7TJ, UK e-mail: toby.smith@uea.ac.uk