ACCURACY OF OUR NEW TENSOR IN JOINT GAP MEASUREMENT OF POSTERIOR-STABILISED TOTAL KNEE ARTHROPLASTY
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BackgroundFlexion-extension gap balancing is recognized as an essential part of total knee arthroplasty (TKA). The gap is often evaluated using spacer blocks, laminar spreader, or tensor device. The evaluation of gap balancing with the patella in the reduced position is more physiological and reproducible than with patellofemoral (PF) joint everted. However, in the knee with a reduced PF joint, it is difficult to comprehend the anteroposterior position of the tibia to the femur. So, we developed a new tensor to lift up the tibia ahead and fix the anteroposterior position of the tibia to the femur with the PF joint reduced [Fig.1].PurposeTo investigate how accurate the extension and flexion gaps would be measured by comparing our new tensor with the conventional tensor which could not fix the position of the tibia to the femur.MethodsThis study includes 60 knees in 48 patients underwent TKA using the Posterior Stabilized (PS) Prosthesis (Striker), for varus osteoarthritis. The mean age of patients was 78.2...Keywords:
Patellofemoral joint
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Introduction: Currently, minimal attention has been paid to thorough preoperative planning in primary total knee arthroplasty. The aim of this study was to evaluate the results and the effectiveness of a previously reported x-ray view as a simple way of preoperative planning in total knee arthroplasty. Materials & Methods: The rotational alignment of the distal end of the femur is usually evaluated by measuring the angle (posterior condylar angle, PCA) between the surgical transepicondylar axis (TEA) and the posterior condylar line (PC line), which connects the posterior aspects of the femoral condyles. A simple and convenient technique for assessing the TEA and PC line using plain radiography is the kneeling view. The kneeling view has been described as a posteroanterior projection at right angles to the tibial shaft with the knee in approximately 80° of flexion and with the hip joint in neutral rotation. Preoperative planning is possible using the kneeling view in measuring the rotational alignment of the distal femur using the posterior condylar angle. Additionally, information about the varus inclination of the upper part of the tibia may be obtained using the same x-ray view. Kneeling views were obtained in fifty patients with advanced osteoarthritis (classified as 4 on the Kellgren–Lawrence scale) that were admitted in our department for primary TKA. The varus inclination of the upper part of the tibia and condylar twist angle were measured using digital protractors. Results: There was no statistically significant correlation between the posterior condylar angle and the varus inclination of the upper part of the tibia. Bivariate linear regression analysis did not reveal any prediction equation or relation between the two computed variables in advanced osteoarthritic knees. Conclusions: Using this method of preoperative planning, the current practice of cutting the tibia perpendicular to its mechanical axis and applying a predefined external rotation of the femoral component is strongly disputed, especially in advanced osteoarthritic knees. The results of this study show that preoperative planning may be very helpful in assessing the rotational deformity of the distal femur. The final amount of external rotation of the femoral component must be approached on an individual basis based on a thorough preoperative planning.
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One strategy for aligning the limb and positioning components in total knee arthroplasty (TKA) in the coronal plane is mechanical alignment, which has the goal of positioning the center of the hip, knee, and ankle on a straight-line by establishing a femoral and tibial joint line at the knee that is perpendicular to the mechanical axis of the femur and tibia respectively. Another strategy is gap balancing, which has the goal of creating equal gaps between the medial and lateral compartments at 0° of extension and 90° of flexion.
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During total knee arthroplasty, balancing is necessary for long-term stability and longevity of implants as improper balancing leads to abnormal surface strain. A routine practice among surgeons is to add more posterior slope to the proximal tibia to provide an increase in the flexion gap to balance the knee throughout the entire range of motion, particularly when doing cruciate-retaining knees. The aim of this technique guide is to provide a simple estimate of the posterior slope added or subtracted when cutting the proximal tibia using a standard extramedullary guide. It can also be applied to predict the amount of coronal change instituted using a standard extramedullary drop guide. Using a few basic calculations with a sine equation, a surgeon can accurately predict the amount of change in the slope applied when cutting the proximal tibia. This can be done to control the degree of slope added to the anterior-posterior direction and can be used to predict coronal alignment changes as well. This technique can be applied to any length extramedullary guide and applied across all companies to provide surgeons with an exact degree change in the tibial slope and coronal alignment with simple calculations.
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Background: Successful total knee arthroplasty requires component alignment according to the mechanical axes and restoration of ideal knee kinematics. This requires adequate ligament balancing, stable tibia-femoral and patello-femoral joints, and a non-restricted range of motion. We developed a computer assisted total knee arthroplasty system to help the surgeon achieving more intra-operative accuracy. Material and methods: An OPTOTRAK camera is used to track relative motions between femur, tibia, and instruments. In contrast to other systems we avoid fixation of reference bases onto acetabulum and foot. The surgeon generates a representation of the patient’s anatomy using the technique of “surgeon defined anatomy”. Based on recorded landmarks the system calculates the femoral and tibial mechanical axes, the position of the knee joint line, the level of the defects on femoral and tibial side, the anatomically best fitting femoral component size, the femoral ventral level, and the natural tibial rotation. These values enable an initial planning situation, which features alignment of the tibial and femoral distal resection planes according to the mechanical axes as well as the definition of the anterior and posterior femoral resection planes with respect to the ventral cortex and the prosthesis design. To consider soft-tissue behaviour the surgeon loads both collateral ligaments in extension and flexion, a Results: During a clinical study we performed thirteen total knee arthroplasties. Postoperatively passive extension was 0.8-4.2° (mean 1.9°) in the coronal plane and 0.2-3.9° (mean 1.8°) in the sagittal plane. Varus-valgus instability was 7.2°. The results of the subsequent patients of this ongoing study will be available during the conference.
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Introduction Neutral mechanical alignment in TKA has been shown to be an important consideration for survivorship, wear, and aseptic loosening. However, native knee anatomy is described by a joint line in 3° of varus, 2–3° of mechanical distal femoral valgus, and 2–3° of proximal tibia varus. Described kinematic planning methods replicate native joint alignment in extension without changing tibiofemoral alignment, but do not account for native alignment through a range of motion. An asymmetric TKA femoral component with a thicker medial femoral condyle and posterior condylar internal rotation paired with an asymmetric polyethylene insert aligns the joint line in 3° of varus while maintaining distal femoral and proximal tibial cuts perpendicular to mechanical axis. The asymmetric components recreate an anatomic varus joint line while avoiding tibiofemoral malalignment or femoral component internal rotation, a risk factor for patellofemoral maltracking. The study seeks to determine how many patients would be candidates for a kinematically planned knee without violating the principle of a neutral mechanical axis (0° ± 3°). Methods A cohort comprised of 55 consecutive preoperative THA patients with asymptomatic knees and 55 consecutive preoperative primary unilateral TKA patients underwent simultaneous biplanar radiographic imaging. Full length coronal images from the thoracolumbar junction to the ankles were measured by two independent observers for the following: mechanical tibiofemoral angle (mTFA), mechanical lateral distal femoral angle (mLDFA), and mechanical medial proximal tibial angle (mMPTA). Patients who met the following conditions: mTFA 0°±3°; mLDFA 87°±3°; and mMPTA 87°±3°, were considered candidates for TKA with an asymmetric implant that would achieve a kinematic joint line and neutral mechanical axis. Similarly, patients with the following conditions: mTFA 0°±3°; mLDFA 90°±3°; and mMPTA 90°±3°, were considered candidates for TKA with a symmetric implant that would achieve a kinematic joint line and neutral mechanical axis. Results In this cohort of 110 patients, the mean mTFA was 1° varus ± 5°, the mean mLDFA was 87° ± 3°, mMPTA 87°± 2°. The comparison of patients meeting each of the three conditions required for a TKA with a neutral mechanical axis and a kinematic joint line are outlined in Table 1. Conclusion A TKA with kinematic 3° varus joint line and neutral mechanical axis was possible in 52% of patients using an asymmetric implant and 23% of patients using a symmetric implant. Previous descriptions of kinematic planning using standard TKA components required compromise of neutral mechanical axis alignment with detrimental effects on overall survivorship. Knee arthroplasty using an asymmetric implant may achieve the best of both worlds, neutral mechanical axis and a kinematic joint line, in a large percentage of patients.
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The aim of total knee arthroplasty (TKA) is to align both the femoral and tibial components perpendicular to the mechanical axis of the leg. Most instrument systems cut the femur and tibia independently. Accurate alignment of the femoral component is hampered by our inability to define precisely the centre of the hip in three-dimensional space. Femoral resection is therefore based on a number of assumptions, which unfortunately do not hold true all of the time. First, it assumes that an intramedullary rod follows a predictable path in the femur; secondly, that there is a fixed relationship between the rod and the mechanical axis of the leg, and thirdly that the shape of the distal femur is constant. Fourthly, even if the resection is correct, it assumes that the femoral component sits perfectly on cut surfaces. Further, there are inherent inaccuracies in the assessment of femoral component position, in that rotation of the limb with a 10° fixed-flexion deformity greatly affects apparent component position. The exact entry point into the femur also influences alignment in that an intramedullary rod placed through an entry point 10 mm anterior to the intercondylar notch of the femur gives a mean valgus angle of 8°. When the tibia is cut perpendicular to its long axis in the coronal plane, assuming 3° of tibial varus, the femur needs to be cut with the corresponding degree of valgus, i.e., 5°. Even this argument is based on a small number of cadavers and does not take account of variations in the anatomy of the distal femur. In particular, a valgus bow can result in valgus malposition of the component. Extramedullary alignment carries the problem of using only a surface representation of the centre of the hip in a single plane, which becomes inaccurate as the femoral jig is rotated. Malalignment of the tibial component increases the stress on the ultra-high molecular weight polyethylene insert, predisposing it to increased wear and subsidence. Studies comparing intramedullary and extramedullary guidance systems for cutting the proximal tibia have shown that 71% to 94% of prostheses inserted with an intramedullary guide, and 82% to 88% inserted with an extramedullary guide, are within 2° of being perpendicular to the long axis of the tibia. To set a benchmark for comparison with computer assisted and robotic techniques currently being developed, we felt that it was important to assess the accuracy of placement of both the tibial base plate and femoral component in the coronal plane using current guidance systems. We developed a series of radiographs allowing accurate independent assessment of femoral and tibial components. A long anteroposterior view of the distal femur with the patient prone was used to assess femoral placement. Coned views of the proximal and distal femur on the same plate were used to assess tibial placement. Correct rotational alignment of the radiograph was confirmed by the profile of the components. Using this technique, we radiologically assessed the varus/valgus alignment of the tibial components of 350 TKAs. All the tibial components were implanted using an extramedullary guide with no posterior slope. We implanted 96.3% of components within 2° of the perpendicular to the longitudinal axis of the tibia. In order to validate our radiological assessment, a subgroup of 40 knees was re-assessed on CT scan. Analysis of this subgroup showed a close correlation between the results using the two different methods (mean difference 0.88°, SD 0.75). We also assessed the position of the femoral component in 362 TKAs. A subgroup of 32 knees, 18 with perfect alignment and 14 with imperfect alignment, underwent CT scout scan of the femur from which the mechanical axis of the femur could be measured. Radiologically, 92% of all components were implanted within 3° of the target value and 83% were within 2° of target. There was close correlation between the CT and radiological measurements in the subgroup. Deviation from the mechanical axis was 1.16° (− 2.5° to +2°) in the perfectly aligned knees, validating both surgical technique and radiological assessment. Although the findings for the femoral components compared favourably with other studies, there was still room for improvement. We set out to achieve this through direct measurement of the mechanical axis of the femur. In a series of 80 TKAs, patients were subjected to a preoperative CT scout scan of the femur. We took care to eliminate rotational error. The angle between the slope of the distal femur and the mechanical axis of the femur was calculated. During surgery the distal cutting block (Wright Medical Medial Pivot Arthroplasty System) was applied directly to the distal femur without use of an intramedullary alignment rod and the angle corrected so as to be perpendicular to the mechanical axis. A right-angled jig resting on the anterior femoral cortex was used to assess the flexion/extension of the cut. Patients were scanned again postoperatively. In 76 knees (95%) the femoral component was within 2° of the mechanical axis. The remaining three were within 3°. We continue to evaluate the technique with the use of a new jig, which allows incremental 1°-correction of the distal femoral cut. In conclusion, accurate cutting of the tibia during knee arthroplasty is possible with careful use of extra-medullary instrumentation. The use of a simple pre-operative CT scan eliminates the errors inherent in intramedullary femoral systems and takes into account the femoral anatomy of each individual patient. Robotic-assisted surgery may offer the opportunity of accurate placement of components. It is, however, likely to be both time consuming and expensive. We should not yet abandon thoughts of improving the use of our current mechanical instruments. Robots have yet to prove their superiority.
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The use of navigation for total knee arthroplasty (TKA) improves limb alignment in the coronal and sagittal planes. However, similar improvements in femoral and tibial component rotation have not yet been realized using currently available systems.We developed a modified navigated TKA technique in which femoral rotation was set using the resected tibial plane as the reference with the aim of achieving a rectangular flexion gap. Limb alignment was assessed in a cohort of 30 knees using the navigation system. Post-operative limb alignment was measured using long-leg standing radiographs. Computed tomography was used to determine post-operative component orientation.Sagittal alignment data improved from a mean of 7.8° varus (pre-operative) to 0.0° (post-operative), assessed by intra-operative navigation. Post-operative hip-knee-ankle axis alignment was 0.9° valgus (mean; standard deviation [SD] 1.7°). Mean femoral component rotation was 0.5° internally rotated (SD 2.6°), relative to the surgical transepicondylar axis. Mean tibial component rotation was 0.9° externally rotated (SD 5.5°). No soft tissue releases were performed.These results confirm that the desired femoral rotation, set using a tibia-first approach with the resected tibial plane as the reference, can be achieved without compromising overall limb alignment. Femoral component rotation was within a narrow range, with a moderate improvement in achieving more consistent tibial component rotation compared with other techniques. This technique may prove to be useful for surgeons wishing to employ a tibia-first philosophy for TKA while maximizing the benefits associated with computer-assisted navigation.
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