Femoral Total Knee Arthroplasty Research Articles

Comparison of anatomic axes with a navigated functional rotation axis determined by ligament tension for rotational femoral component alignment in cadaver knee arthroplasty.

The malimplantation of the total knee arthroplasty (TKA) components is one of the main reasons for revision surgery. For determining the correct intraoperative femoral rotation several anatomic rotational axes were described in order to achieve a parallel, balanced flexion gap. In this cadaveric study prevalent used rotational femoral axes and a navigated functional rotational axis were compared to the flexion-extension axis defined as the gold standard in rotation for femoral TKA component rotation. Thirteen body donors with knee osteoarthritis (mean age: 78.85 ± 6.09; eight females and five males) were examined. Rotational computer tomography was performed on their lower extremities pre- and postoperatively. Knee joint arthroplasties were implanted and CT diagnostics were used to compare the preoperatively determined flexion-extension axis (FEA). The FEA is the axis determined by our surgical technique and serves as an internal reference. It was compared to other axes such as (i) the anatomical transepicondylar axis (aTEA), (ii) the surgical transepicondylar axis (sTEA), (iii) the posterior condylar axis (PCA) and (iv) the functional rotation axis (fRA). Examination of 26 knee joint arthroplasties revealed a significant angular deviation (p*** < 0.0001) for all axes when the individual axes and FEA were compared. aTEA show mean angular deviation of 5.2° (± 4.5), sTEA was 2.7° (± 2.2), PCA 2.9° (± 2.3) and the deviation of fRA was 4.3° (± 2.7). A tendency towards external rotation was observed for the relative and maximum axis deviations of the aTEA to the FEA, for the sTEA and the fRA. However, the rotation of the posterior condylar axis was towards inwards. All axes showed a significant angular deviation from the FEA. We conclude that the presented technique achieves comparable results in terms of FEA reconstruction when compared with the use of the known surrogate axes, with certain deviations in terms of outliers in the internal or external rotation.

Anatomic Referencing of the Distal Femur in Total Knee Arthroplasty: The Impact of Orientation on Femoral Component Size

BackgroundAnatomic referencing in total knee arthroplasty places the femoral component flush to the anterior cortex while maintaining posterior condylar offset (PCO). The intent of this study was to evaluate how component position influences the femoral component size. MethodsDigital surface models were created using 446 femora from an established computed tomography database. Virtual bone resections, component sizing, and component placement were performed assuming neutral (0°) flexion and neutral (3°) rotation relative to the posterior condyles. The appropriately sized femoral component, which had 2 mm of incremental size, was placed flush with the anterior cortex for optimal restoration of the PCO. Sizing and placement were repeated using 3 and 6° flexion and 0, 5, and 7° external rotation (ER). ResultsAt 0° flexion, decreasing ER from 3 to 0° resulted in an average decreased anterior-posterior height (APH) of 1.9 mm, corresponding to a component size decrease of 1 for 88% of patients. At 7° ER, component size increased by an average of 2.5 mm, corresponding to a size increase for 80% of patients. Flexing the femoral component to 3° with ER at 3° resulted in a decrease in APH of 2.2 mm (1 size decrease in 93% of patients). At 3° flexion and 3° ER, 86% had the same component size as at 0° flexion and 0° ER. Increasing ER at 3° flexion increased APH by 1.2 mm at 5° and 3.1 mm at 7° on average, relative to 3° ER. Increasing flexion from 3 to 6° extended this effect. ConclusionsFlexion decreases the APH when the ER is held constant. The ER of the femoral component increases the APH across all tested flexion angles, causing an increase in the ideal femoral component size to maintain PCO. With anatomic referencing, alterations in femoral component positioning and subsequent changes in component size can be accounted for.

Distal femoral torsion positioning in total knee arthroplasty shows equal reliability using a computer-assisted system or a robotic arm

IntroductionComputer-assisted or robot-assisted surgery in theory enables optimal positioning in total knee arthroplasty (TKA). The aim of the present study was to evaluate the rotational positioning of the femoral TKA implant using the ROSA Knee® robotic system. The hypothesis was that the accuracy of the rotation obtained by the ROSA Knee® robotic arm was comparable to that obtained by navigation. Material and methodsThis was a retrospective comparative consecutive study involving 140 patients receiving first-line TKA, divided into two equal groups: Navigation, and Robot. Distal epiphyseal torsion of the femur (DETF) was analyzed by comparing preoperative (native DETF) and postoperative (prosthetic DETF) CT scans between groups. ResultsThe mean prosthetic DETF was 2.3°±1.3 for the Navigation group and 2.3°±1.4 for the Robot group (p=0.96), with no significant difference in deviation from DETF target (p=0.07). Assuming±2° range of error, the Navigation group showed 88.6% and the Robot group 90% accuracy (p=0.79). ConclusionThe Rosa Knee® robotic system provided accuracy equivalent to navigation for rotational positioning of the femoral implant. Level of evidenceIV.

Comparison of Single-Radius with Multiple-Radius Femur in Total Knee Arthroplasty: A Meta-Analysis of Prospective Randomized Controlled Trials.

BackgroundWhether there was clinical superiority for the single‐radius prosthesis over the multi‐radius prothesis in total knee arthroplasty (TKA) still remains to be clarified. We updated a meta‐analysis including prospective randomized controlled trials (RCTs) to compare the clinical prognosis of patients receiving single‐radius TKA (SR‐TKA) or multi‐radius TKA (MR‐TKA).MethodsWe searched the databases of PubMed, Web of Science, EMBASE, Cochrane Library, MEDLINE for eligible RCTs. Two reviewers evaluated the study quality according to the Risk of Bias tool of the Cochrane Library and extracted the data in studies individually. The extracted data included the baseline data and clinical outcome. The baseline data include the author's name, country, and year of included studies, the name of knee prosthesis used in studies, sample size, follow‐up time, and BMI of patients. The clinical data comprised primary indicators including postoperative knee range of motion (ROM), sit‐to‐stand rest, severe postoperative scorings, such as visual analog scale (VAS), American Knee Society knee score (AKS), Oxford knee scoring (OKS), and SF‐36 Quality of Life Scale, as well as various secondary indicators of complications including anterior knee pain, postoperative infection, aseptic prosthesis loosening, and prosthesis revision. The data analysis was performed using Review Manager 5.3 software and STATA 12.0. The sensitivity analysis was performed using STATA 12.0.ResultsA total of 13 RCTs, along with 1720 patients and 1726 knees, were finally included in our present meta‐analysis. We found that patients in SR‐TKA group performed better in the sit‐to‐stand test (OR = 1.89, 95% CI: 1.05–3.41, p = 0.03) and satisfaction evaluation (OR = 3.27, 95% CI: 1.42–7.53, p = 0.005), which were only evaluated in two included RCTs. While no significant difference was found between SR‐TKA and MR‐TKA groups in terms of postoperative ROM, VAS scoring, AKS scoring, SF‐36 scoring, OKS scoring, and various complications including anterior knee pain, postoperative infection, aseptic prosthesis loosening, and prosthesis revision.ConclusionIn conclusion, our present meta‐analysis indicated that SR implants were noninferior to MR implants in TKA, and SR implants could be an alternative choice over MR implants, since patients after SR‐TKA felt more satisfied and performed better in the sit‐to‐stand test, with no significant difference in complications between SR‐TKA and MR‐TKA groups. While more relevant clinical trials with long‐term follow‐up time and specific tests evaluating the function of knee extension mechanism should be carried out to further investigate the clinical performance of SR implants.

The femoral intercondylar notch is an accurate landmark for the resection depth of the distal femur in total knee arthroplasty

IntroductionConventionally, the depth of distal femoral resection in total knee arthroplasty is referenced from the most prominent distal femoral condyle. This surgical technique does not consider pathological alterations of articular surfaces or severity of knee deformity. It has been hypothesized that the femoral intercondylar notch is a clinically reliable and more accurate alternative landmark for the resection depth of the distal femur in primary total knee arthroplasty.MethodsThe resection depths of the distal femur at the medial and lateral femoral condyles and intercondylar notch were measured using computer navigation in 406 total knee arthroplasties. Variability between the bone resection depths was analyzed by standard deviation, 95% confidence interval and variance. Clinical follow-up of outcome to a minimum of 12 months was performed to further inform and validate the analysis.ResultsMean resection depth of the medial condyle was 10.7 mm, of the lateral condyle 7.9 mm and of the femoral intercondylar notch 1.9 mm. The femoral intercondylar notch had the lowest variance in resection depth among the three landmarks assessed, with a variance of 1.7 mm2 compared to 2.8 mm2 for the medial femoral condyle and 5.1 mm2 for the lateral femoral condyle. The intercondylar notch reference had the lowest standard deviation and 95% confidence interval. The resection depth referencing the notch was not sensitive to the degree of flexion contracture pre-operatively, whereas the medial and lateral condyles were. For varus deformed knees, distal femoral resection depth at the notch averaged 2 mm, which corresponds to the femoral prosthesis thickness at the intercondylar region, while for valgus deformed knees, the resection was flush with the intercondylar notch.ConclusionsThe femoral intercondylar notch is a clinically practical and reproducible landmark for appropriate and accurate resection depth of the distal femur in primary total knee arthroplasty.Level of evidenceLevel III: Retrospective cohort study.

Preoperative planning of total knee arthroplasty: reliability of axial alignment using a three-dimensional planning approach.

Preoperative templating of total knee arthroplasty (TKA) can nowadays be performed three-dimensionally with software solutions using computed tomography (CT) datasets. Currently there is no consensus concerning the axial orientation of TKA components in three-dimensional (3D) planning. To assess intra-/inter-observer reliability of detection of different bony landmarks in planning axial component alignment using axial CT images and 3D reconstructions. Intra- and inter-observer reliability of determination of four predefined axial femoral and tibial axes was calculated using data from CT scans. Axes determination was performed on the axial slices and on the 3D reconstruction using preoperative planning software. In summary, 61 datasets were analyzed by one medical student (intra-observer reliability) and 15 datasets were analyzed by four different observers independently (inter-observer reliability). For the femur, clinical epicondylar axis and posterior condylar axis showed the best reliability with an inter-observer variability of 0.7° and 0.5°, respectively. For the tibia, posterior condylar axis provided best reliability (inter-observer variability: 1.7°). Overall variability was greater for tibial than for femoral axes. Reliability of axis determination was more accurate using axial CT slices rather than 3D reconstructions. The femoral clinical epicondylar axis is highly reliable. Landmarks for the tibia are not as easily identifiable as for the femur. The tibial posterior condylar axis presents the axis with highest reliability. Based on these results, clinical epicondylar axis for orientation of the femoral TKA component and posterior condylar axis for the tibial implant, both defined on axial slices can be recommended.

Investigation of the effect of the angle between femoral and prosthesis mechanical axes on bone remodeling of femur in total knee arthroplasty.

The bone remodeling is the process in which the bone adapts its structure to the variation of environmental loads. The joint might be broken or damaged as a result of aging or an accident. To remedy this situation, Total Knee Arthroplasty (TKA) and prosthesis implantation is recommended. The main goal of this research is to investigate the effects of femur implanting angle on the bone remodeling process after TKA in the Coronal, Sagittal and horizontal planes over seven years. First, the 3D CAD model from CT images is created then the bone behavior is simulated using a model with a USDFLD subroutine. The results show that as the implant rotates in one direction, the stress and density distribution increases in the same direction whereas the load and consequently the bone density decrease substantially in the opposite direction. Consequently, the bone density might even decrease 77 and 31 percent in the coronal and sagittal plane respectively, so in the total knee arthroplasty, the mechanical axes of prosthesis and femur should be parallel. The active bone which occurs as a result of mechanical axes of prosthesis and femur parallelism and consequently uniform load distribution, can protect the implant from prosthesis loosening and fracture.

The effect of different interference fits on the primary fixation of a cementless femoral component during experimental testing

Cementless femoral total knee arthroplasty (TKA) components use a press-fit (referred to as interference fit) to achieve initial fixation. A higher interference fit could lead to a superior fixation, but it could also introduce more damage to the bone during implantation. The purpose of the current study was to investigate the effect of interference fit on the micromotions and gap opening/closing at the bone-implant interface. Experimental tests were performed in six pairs of cadaveric femurs implanted with femoral components using a low interference fit of 350 μm and a high interference fit of 700 μm. The specimens were subjected to the peak loads of gait and squat, based on the Orthoload dataset. Digital Image Correlation (DIC) was used to measure the micromotions and opening/closing in different regions of interest (ROIs). Two linear mixed-effect statistical models were created with micromotions and gap opening/closing as dependent variables. ROIs, loading conditions, and implant designs as independent variables, and cadaver specimens as random intercepts. The results revealed no significant difference between the two interference fit implants for micromotions (p = 0.837 for gait and p = 0.065 for squat), nor for the gap opening/closing (p = 0.748 for gait and p = 0.561 for squat). In contrast, significant differences were found between loading and most of the ROIs in both dependent variables (p < 0.0001). Additionally, no difference in bone deformation was found between low and high interference fit. Changing interference between either 350 μm or 700 μm did not affect the primary stability of a femoral TKA component. There could be an interference fit threshold beyond which fixation does not further improve.

Flexible versus standard intramedullary rod in posterior stabilized primary total knee arthroplasty: protocol for a randomized controlled trial

BackgroundIn total knee arthroplasty (TKA) a flexible intramedullary rod can be used to account for sagittal bowing of the distal femur. Although patients report better post-operative functional outcome when the flexible rod was used, it is unknown how the use of the flexible rod affects the placement of the femoral TKA component, and how this relates to activities of daily living. It is expected that the use of the flexible rod will result in a more flexed femoral component, a larger patellar tendon moment arm, and consequently in better functional outcome. The goal of this study is to compare the flexible rod to the standard intramedullary rod in primary TKA in terms of fit of the TKA, functional outcome, and sizing of the femoral component.MethodsA single-blind randomized controlled trial with two groups (flexible vs standard rod), with patients blinded for group allocation, and 2 years post-operative follow-up. The fit of the TKA is quantified by two parameters: (1) the flexion angle of the TKA in the sagittal plane and (2) the sagittal profile of the distal femur compared between the pre-operative bone and the TKA. Both parameters are calculated in 3D volume images obtained using fluoroscopy. Functional outcome will be measured using (1) the timed Get-up and Go test (2), the stair climbing test (3), knee power output, and (4) patient and clinician reported outcomes. Different parameters will be measured during the TKA procedure to account for the invisibility of cartilage in the 3D volumes and to study if the amount of bone removed during the procedure is affected by group allocation.DiscussionThe sagittal fit of TKA is not a standardized outcome measure. We discuss our choice of parameters to define the sagittal fit (i.e., flexion angle and sagittal profile), our choice for the parameters we measure during the TKA procedure to account for the lack of cartilage thickness in fluoroscopy, and our choice for the parameters to study if the amount of bone removed during the procedure is affected by group allocation. Lastly, we discuss the merits of this planned trial.Trial registrationNetherlands Trial Register, 4888, registered 30 March 2015. https://www.trialregister.nl/trial/4888

Cortical Perforation Misidentified with Medial Condylar Fracture of Femur in Total Knee Arthroplasty: Case Report

Cortical Perforation Misidentified with Medial Condylar Fracture of Femur in Total Knee Arthroplasty: Case Report

Different References for Valgus Cut Angle in Total Knee Arthroplasty.

The valgus cut angle (VCA) of the distal femur in Total Knee Arthroplasty (TKA) is measured preoperatively on three-joint alignment radiographs. The anatomical axis of the femur can be described as the anatomical axis of the full length of the femur or as the anatomical axis of the distal half of the femur, which may result in different angles in some cases. During TKA, the anatomical axis of the femur is determined by intramedullary femoral guides, which may follow the distal half or near full anatomical axis, based on the length of the femoral guide. The aim of this study was to compare using the anatomical axis of the full length of the femur versus the anatomical axis of the distal half of the femur for measuring VCA, in normal and varus aligned femurs. We hypothesized that the VCA would be different based upon these two definitions of the anatomical axis of the femur. Full-length weight bearing radiographs were used to determine three-joint alignment in normal aligned (Lateral Distal Femoral Angle; LDFA = 87º ± 2º) and varus aligned (LDFA >89º) femurs. Full-length anatomical axis-mechanical axis angle (angle 1) and distal half anatomical axis-mechanical axis angle (angle 2) were measured in all subjects by two independent orthopedic surgeons using a DICOM viewer software (PACS). Angles 1 and 2 were compared in normal and varus aligned subjects to determine whether there was a significant difference. Ninety-seven consecutive subjects with normally aligned femurs and 97 consecutive subjects with varus aligned femurs were included in this study. In normally aligned femurs, the mean value of angle 1 was 5.05° ± 0.76° and for angle 2 was 3.62° ± 1.19°, which were statistically different (P= 0.0001). In varus aligned femurs, the mean value of angle 1 was 5.42° ± 0.85° and for angle 2 was 4.23° ± 1.27°, which were also statistically different (P= 0.0047). The two different methods of outlining the anatomical axis of the femur lead to different results in both normal and varus-aligned femurs. This should be considered in determination of the valgus cut angle on pre-operative radiographs and be adjusted according to the length of the intramedullary guide.

Fixation strength of a polyetheretherketone femoral component in total knee arthroplasty

IntroductionIntroducing polyetheretherketone (PEEK) polymer as a material for femoral components in total knee arthroplasty (TKA) could potentially lead to a reduction of the cemented fixation strength. A PEEK implant is more likely to deform under high loads, rendering geometrical locking features less effective. Fixation strength may be enhanced by adding more undercuts or specific surface treatments. The aim of this study is to measure the initial fixation strength and investigate the associated failure patterns of three different iterations of PEEK-OPTIMA® implants compared with a Cobalt–Chromium (CoCr) component. MethodsFemoral components were cemented onto trabecular bone analogue foam blocks and preconditioned with 86,400 cycles of compressive loading (2600 N–260 N at 1 Hz). They were then extracted while the force was measured and the initial failure mechanism was recorded. Four groups were compared: CoCr, regular PEEK, PEEK with an enhanced cement-bonding surface and the latter with additional surface primer. ResultsThe mean pull-off forces for the four groups were 3814 N, 688 N, 2525 N and 2552 N, respectively. The initial failure patterns for groups 1, 3 and 4 were the same; posterior condylar foam fracture and cement–bone debonding. Implants from group 2 failed at the cement–implant interface. ConclusionsThis study has shown that a PEEK-OPTIMA® femoral TKA component with enhanced macro- and microtexture is able to replicate the main failure mechanism of a conventional CoCr femoral implant. The fixation strength is lower than for a CoCr implant, but substantially higher than loads occurring under in-vivo conditions.

Relationship between medial flexion gap and bone resection of medial posterior condyle of femur in total knee arthroplasty

Relationship between medial flexion gap and bone resection of medial posterior condyle of femur in total knee arthroplasty

Morphometric analysis of the distal femur in total knee arthroplasty and native knees.

Analysis of the morphology of the distal femur, and by extension of the femoral components in total knee arthroplasty (TKA), has largely been related to the aspect ratio, which represents the width of the femur. Little is known about variations in trapezoidicity (i.e. whether the femur is more rectangular or more trapezoidal). This study aimed to quantify additional morphological characteristics of the distal femur and identify anatomical features associated with higher risks of over- or under-sizing of components in TKA. We analysed the shape of 114 arthritic knees at the time of primary TKA using the pre-operative CT scans. The aspect ratio and trapezoidicity ratio were quantified, and the post-operative prosthetic overhang was calculated. We compared the morphological characteristics with those of 12 TKA models. There was significant variation in both the aspect ratio and trapezoidicity ratio between individuals. Femoral trapezoidicity was mostly due to an inward curve of the medial cortex. Overhang was correlated with the aspect ratio (with a greater chance of overhang in narrow femurs), trapezoidicity ratio (with a greater chance in trapezoidal femurs), and the tibio-femoral angle (with a greater chance in valgus knees). This study shows that rectangular/trapezoidal variability of the distal femur cannot be ignored. Most of the femoral components which were tested appeared to be excessively rectangular when compared with the bony contours of the distal femur. These findings suggest that the design of TKA should be more concerned with matching the trapezoidal/rectangular shape of the native femur. The distal femur is considerably more trapezoidal than most femoral components, and therefore, care must be taken to avoid anterior prosthetic overhang in TKA

Clinical value of SPECT/CT in the painful total knee arthroplasty (TKA): a prospective study in a consecutive series of 100 TKA.

Bone single photon emission computed tomography (SPECT)/CT is considered as beneficial in unhappy patients with pain, stiffness or swelling after total knee arthroplasty (TKA). The purpose of this study was to identify typical patterns of bone tracer uptake (BTU), distribution and intensity values in patients after TKA. The above findings were correlated with the type and fixation of TKA, the time from TKA and intraoperative findings at revision surgery. A total of 100 knees of 84 consecutive patients (mean age ± SD 70 ± 11 years) after TKA with persistent knee pain were prospectively included. All patients underwent clinical examination, standardized radiographs and (99m)Tc-hydroxymethane diphosphonate (HDP) SPECT/CT as part of a routine diagnostic algorithm. The diagnosis before and after SPECT/CT and final treatment were recorded. TKA component position was determined on 3-D reconstructed images. Intensity and anatomical distribution of BTU was determined. Maximum intensity values were recorded as well as ratios in relation to the proximal midshaft of the femur. Univariate analyses (chi-square test, Pearson's correlation and t test for independent samples) were performed (p < 0.05). SPECT/CT changed the clinical diagnosis and final treatment in 85/100 (85 %) knees. Intraoperative findings confirmed the preoperative SPECT/CT diagnosis in 32/33 knees (97 %). TKA loosening as well as progression of patellofemoral osteoarthritis (OA) was correctly diagnosed in 100 % of knees. Typical patterns of BTU for specific pathologies were identified. Loose femoral TKA components significantly correlated with increased BTU at the lateral femoral regions (p < 0.05). Loose tibial TKA components significantly correlated with increased BTU at all tibial regions (p < 0.05) and around the tibial peg (p > 0.01). The diagnostic benefits of SPECT/CT in patients after TKA have been proven. Typical pathology-related BTU patterns were identified, which will improve reporting quality. Due to the benefits in establishing the correct diagnosis, SPECT/CT should be part of the routine diagnostic algorithm for patients with pain after TKA.

Cutting Error of the Distal Femur in Total Knee Arthroplasty by Use of a Navigation System

The aim of this study was to evaluate the alignment of the distal femoral cutting surface using a navigation system to determine the accuracy of bone cutting.We evaluated 20 knees in 20 patients. After cutting the distal femur, the cutting surface was validated using the navigated cutting block adapter, and the angular difference between the cutting surface and that preoperatively planned in the sagittal and coronal planes was recorded. The average error of all knees was 1.6° ± 2.2° in extension, and 14 (70%) of 20 knees were cut in an extended position. Our tendency is to cut the distal femur in an extended position with the first femoral cut in the sagittal plane.

Measurement of rotational alignment of the distal femur in total knee arthroplasty

To measure and compare the included angle between the surgical transepicondylar axis (STEA) and the posterior condylar line (PCL) and the included angle between the femoral anteroposterior line (APL) and PCL, and to discuss the value of STEA, APL, and PCL as rotational alignment landmarks of the distal femur in total knee arthroplasty (TKA). Seventy-five normal femoral specimens from Chinese adult cadavers were randomly selected. An axial photograph of every femoral specimen was taken with a digital camera and put into a personal computer. Using Photoshop 7.0.1 software, the included angle between the perpendicular line of APL and the PCL, noted as APA, together with the posterior condylar angle (PCA) between STEA and PCL were measured and compared using a paired-samples t-test. The value for PCA was 3.67°± 1.62° (range, 0.75°-5.90°) and for APA 3.50°± 1.40° (range, 1.34°-5.65°). There was no significant difference between these two angles (t= 0.949, P= 0.359). Considering their relatively small means, these two angles showed wide variations. The rotational alignment of the femoral component can not accurately be determined by using PCL as a landmark. In order to get a proper rotational alignment of the femoral component in most cases of TKA, APL and STEA should be used as a double check.

A New Radiographical Technique for Assessment of the Transepicondylar Axis of the Femur in Total Knee Arthroplasty

A New Radiographical Technique for Assessment of the Transepicondylar Axis of the Femur in Total Knee Arthroplasty

5° to 6° of distal femoral cut for uncomplicated primary total knee arthroplasty : Is it safe?

The angle formed between the anatomical axis and mechanical axis of the femur determines the angle of resection of distal femur in total knee arthroplasty. The objective of this study was to analyse the safety of the routine practice of selecting 5–6° of valgus for distal femoral cut during uncomplicated primary total knee arthroplasty. A retrospective study was undertaken involving a review of pre-operative CT scout films of osteoarthritic knees in 83 consecutive patients (44 males and 39 females; mean age 70 years) who presented for total knee arthroplasty. The anatomical and mechanical axis of femur were plotted on CT scout films and the angle between the two femoral axes were measured. Mean femoral axes angle was 5.4° (range 3.3–7.6 °, SD 0.9°). Estimated 95% confidence interval of the mean for the population was 5.2 – 5.6°. There were no significant differences for age, gender or laterality of the limb. The routine practice of selecting 5° to 6° of the distal femoral cut for an uncomplicated primary total knee arthroplasty is safe.

An articulated antibiotic spacer used for infected total knee arthroplasty: a comparative in vitro elution study of Simplex ® and Palacos ® bone cements

For the staged management of infected total knee arthroplasty (TKA), antibiotic laden polymethylmethacrylate (PMMA) spacers have been recommended. Antibiotic-impregnated PMMA spacers target drug delivery, achieving high local levels while limiting the potential for host toxicity associated with parenteral antimicrobial therapy. This study examined the elution characteristics of an articulating PMMA TKA spacer that has been useful clinically. Tobramycin and vancomycin are both active against many organisms leading to joint infections. We used various combined antibiotic concentrations (maintaining a relative ratio of 55% tobramycin to 45% vancomycin w/w), and then assayed the elution profile of the TKA spacer in vitro. Additionally, the elution qualities of two brands of bone cement, Simplex ® and Palacos ®, were compared. Briefly, three groups of PMMA spacers, impregnated with different antibiotic loads, were fashioned from a mold replicating a femoral TKA component. The entire spacer surface area was immersed in sterile phosphate buffered saline (PBS) in a 1:6 ratio of grams of cement to milliliters of PBS and incubated at 37 °C for 24 h. After 24 h, aliquot eluates were taken, the PBS discarded, and replaced with fresh, sterile PBS. PBS was changed daily and an aliquot was taken at least weekly for nine weeks. Eluate samples were stored at −70 °C until assayed. Each spacer eluate sample's antibiotic concentration was determined by disc diffusion bioassay against Bacillus subtilis. Mean zone inhibition diameters were extrapolated from the standard curve to yield micrograms per milliliter of antibiotic in PBS. In all groups the Palacos ® spacers demonstrated higher elution levels, above the MIC for the organism used, for a longer period of time than those made with Simplex ®. Based on the observed elution profiles, antibiotic-impregnated Palacos ® bone cement may offer a more effective vehicle for local drug delivery during staged treatment of infected TKA.