Introduction. Robotics have been applied to total knee arthroplasty (TKA) to improve surgical precision in component placement and joint function restoration. The purpose of this study was to evaluate prosthetic component alignment in robotic arm-assisted (RA)-TKA performed with functional alignment and intraoperative fine-tuning, aiming for symmetric medial and lateral gaps in flexion/extension. It was hypothesized that functionally aligned RA-TKA the femoral and tibial cuts would be performed in line with the preoperative joint line orientation. Methods. Between September 2018 and January 2020, 81 RA cruciate retaining (CR) and posterior stabilized (PS) TKAs were performed at a single center. Preoperative radiographs were obtained, and measures were performed according to Paley's. Preoperatively, cuts were planned based on radiographic epiphyseal anatomies and respecting ±3° boundaries from neutral coronal alignment. Intraoperatively, the tibial and femoral cuts were modified based on the individual soft tissue-guided fine-tuning, aiming for symmetric medial and lateral gaps in flexion/extension. Robotic data were recorded. Results. A total of 56 RA-TKAs performed on varus knees were taken into account. On average, the tibial component was placed at 1.9° varus (SD 0.7) and 3.3° (SD 1.0) in the coronal and sagittal planes, respectively. The average femoral component alignment, based on the soft tissue tensioning with spoons, resulted as follows: 0.7° varus (SD 1.7) in the coronal plane and 1.8° (SD 2.1) of external rotation relative to surgical transepicondylar axis in the transverse plane. A statistically significant linear direct relationship was demonstrated between radiographic epiphyseal femoral and tibial coronal alignment and femoral (r=0.3, p<0.05) and tibial (r=0.3, p<0.01) coronal cuts, resepctively. Conclusion. Functionally aligned RA-TKA performed in varus knees, aiming for ligaments’ preservation and balanced flexion/extension gaps, provided joint line respecting femoral and tibial cuts on the coronal plane

Introduction

The posterior condylar axis of the distal femur is the common reference used to describe femoral anteversion. In the context of Total Hip Arthroplasty (THA), this reference can be used to define the native femoral anteversion, as well as the anteversion of the stem. However, these measurements are fixed to a femoral reference. The authors propose that the functional position of the proximal femur must be considered, as well as the functional relationship between stem and cup (combined anteversion) when considering the clinical implications of stem anteversion. This study investigates the post-operative differences between anatomically-referenced and functionally-referenced stem and combined anteversion in the supine and standing positions.

Dislocation is one of the most common complications in total hip arthroplasty (THA) and is primarily driven by bony or prosthetic impingement. The aim of this study was two-fold. First, to develop a simulation that incorporates the functional position of the femur and pelvis and instantaneously determines range of motion (ROM) limits. Second, to assess the number of patients for whom their functional bony alignment escalates impingement risk. 468 patients underwent a preoperative THA planning protocol that included functional x-rays and a lower limb CT scan. The CT scan was segmented and landmarked, and the x-rays were measured for pelvic tilt, femoral rotation, and preoperative leg length discrepancy (LLD). All patients received 3D templating with the same implant combination (Depuy; Corail/Pinnacle). Implants were positioned according to standardised criteria. Each patient was simulated in a novel ROM simulation that instantaneously calculates bony and prosthetic impingement limits in functional movements. Simulated motions included flexion and standing-external rotation (ER). Each patient's ROM was simulated with their bones oriented in both functional and neutral positions. 13% patients suffered a ROM impingement for functional but not neutral extension-ER. As a result, 48% patients who failed the functional-ER simulation would not be detected without consideration of the functional bony alignment. 16% patients suffered a ROM impingement for functional but not neutral flexion. As a result, 65% patients who failed the flexion simulation would not be detected without consideration of the functional bony alignment. We have developed a ROM simulation for use with preoperative planning for THA surgery that can solve bony and prosthetic impingement limits instantaneously. The advantage of our ROM simulation over previous simulations is instantaneous impingement detection, not requiring implant geometries to be analysed prior to use, and addressing the functional position of both the femur and pelvis

It has been proposed that higher knee adduction moments and associated malalignment in subjects with severe medial knee joint osteoarthritis (OA) is due to anatomical deformities as a result of OA [1, 2]. The emergence of patient-matched implants should allow for correction of any existing malalignment. Currently the plans for such surgeries are often based on three dimensional supine computed tomography (CT) scans or magnetic resonance imaging (MRI), which may not be representative of malalignment during functional loading. We investigated differences in frontal plane alignment in control subjects and subjects with severe knee joint OA who had undergone both supine imaging and gait analysis. Fifteen subjects with severe knee OA, affecting either the medial or lateral compartment, and 18 control subjects were selected from a database established as part of a larger study. All subjects had undergone gait analysis using the Vicon motion capture system. OA subjects had undergone routine CT scans and were scheduled for knee joint replacement surgery. Control subjects had no known musculoskeletal conditions and had undergone MRI imaging of hip, knee and ankle joints. Frontal plane knee joint angles were measured from supine imaging (supine) and from motion capture during standing (static) and during gait at the first peak ground reaction force (gait). OA subjects had a significantly higher BMI (p < 0.01) and different gender composition (13 males and 2 females vs 4 males and 5 females; p = 0.03) compared with controls. Multiple linear regression analysis indicated no significant confounding effect of these differences on frontal plane angles measured in supine, static or gait conditions. For both OA and healthy subjects, frontal plane knee angles were significantly higher during gait compared with supine (p = 0.03 and 0.02, respectively). There were also significant differences in knee alignment between OA and healthy subjects for supine and static (p < 0.05) but not for gait, although this was approaching significance (p = 0.052). Overall there seemed to be higher variation in alignment in the OA subjects (Fig. 1). The significantly higher frontal plane knee joint angles measured in both control and OA subjects during gait compared with supine imaging indicate that functional alignment should be taken into consideration when planning patient-specific surgeries. Higher variation in OA patients may be due to alterations in gait patterns due to pain or degree of wear in their osteoarthritic joints, and requires further investigation. In addition, methodological considerations should be taken when comparing alignment from measurements taken with imaging and motion capture to avoid systematic errors in the data. In conclusion, we believe that both supine and loadbearing imaging are insufficient to gain a full representation of functional alignment, and analysis of functional alignment should be routinely performed for optimal surgical planning

There is a difference between “functional instability” of a total knee arthroplasty (TKA) and a case of “TKA instability”. For example a TKA with a peri-prosthetic fracture is unstable, but would not be considered a “case of instability”. The concept of “stability” for a TKA means that the reconstructed joint can maintain its structure and permit normal motion and activities under physiologic loads. The relationship between stability and alignment is that stability maintains alignment. Instability means that there are numerous alignments and almost always the worst one for the loading condition. In the native knee, “instability” is synonymous with ligament injury. If this were true in TKA, then it would be reasonable to treat every “unstable TKA” with a constrained implant. But that is NOT the case. If the key to successful revision of a problem TKA is understanding (and correcting) the specific cause of the problem, then deep understanding of why the TKA is unstable is essential. A case of true “instability” then, is the loss of structural integrity under load as the result of problems with soft tissue stabilizing structures and/or the size or position of components. It is rare that ligament injury alone is the sole cause of instability (valgus instability invariably involves valgus alignment; varus instability usually means some varus alignment and compromised lateral soft tissues). There will be forces (structures) that create instability and forces (structures) that stabilise. There are three categories of instability: Varus-valgus or coronal: Assuming that the skeleton, implant and fixation are intact. These are usually cases that involve ligament compromise, but the usual cause is CORONAL ALIGNMENT, and this must be corrected. The ligament problem is best solved with mechanical constraint. Gait disturbances that increase the functional alignment problems (hip abductor lurch causing a valgus moment at the knee, scoliosis) may require attention of additional compensation with re-alignment. Plane of motion: Both fixed flexion contractures and recurvatum may result in buckling. The first by exhaustion of the quadriceps (consider doing quadriceps “lunges” with every step) and the second because recurvatum is usually a compensation for extensor insufficiency. The prototype for understanding recurvatum has always been polio. This is perhaps one of the most difficult types of instability to treat. The glib answer has been a hinged prosthesis with an extensor stop but there are profound mechanical reasons why this is flawed thinking. The patient with recurvatum instability due to neurologic compromise of the extensor should be offered an arthrodesis, which they will likely decline. The simpler problem of recurvatum secondary to a patellectomy will benefit from an allograft reconstruction of the patella using a modified technique. A common occurrence is obesity with patellofemoral pain, that the patient has managed with a “patellar avoidance” or “hyperextension gait”. Plane of motion instability is a problem of the EXTENSOR MECHANISM DEFICIENCY. Flexion instability. This results from a flexion gap that is larger than the extension gap, where a polyethylene insert has been selected that permits full extension but leaves the flexion gap unstable. These patients achieve remarkable flexion easily and early, but have difficulties with pain and instability on stairs, with recurrent (non-bloody) effusions and peri-articular tenderness. Revision surgery is necessary. Flexion instability may also occur with posterior stabilised prostheses. So-called “mid-flexion” instability is a contentious concept, poorly understood and as yet, not a reported cause for revision surgery distinct from “FLEXION INSTABILITY”. Flexion instability is a problem of GAP BALANCE

Introduction. Total Knee Replacement (TKR) alignment measured intra-operatively with Navigation has been shown to differ from that observed in long leg radiographs (Deep 2011). Potential explanations for this discrepancy may be the effect of weight bearing or the dynamic contributions of soft tissue loads. Method. A validated, 3D, dynamic patient specific musculoskeletal model was used to analyse 85 post-operative CT scans using a common implant design. Differences in coronal and axial plane tibio-femoral alignment in three separate scenarios were measured:. Unloaded as measured in a post-op CT. Unloaded, with femoral and tibial components set aligned to each other. Weight bearing with the extensor mechanism engaged. Scenario number two illustrates the tibio-femoral alignment when the femoral component sits congruently on the tibia with no soft tissue acting whereas scenario three is progression of scenario number two with weight applied and all ligaments are active. Two tailed paired students t-test were used to determine significant differences in the means of absolute difference of axial and coronal alignments. Results. The mean coronal alignment were 1.7° ± 2.1° varus (range, −3.0° to 7.0°), 0.8° ± 2.0° varus (range, −3.7° to 4.8°), 0.4° ± 2.0° varus (range, −3.9° to 5.1°) for unloaded, unloaded with implants set aligned and weight bearing scenarios respectively. The mean of absolute difference in coronal alignment between the unloaded and weight bearing scenario was 1.8° ± 1.5° (range 0.0° to 5.9°). The mean axial alignment were 6.8° ± 5.5° external rotation (ER) (range, 20.0° ER to 11.0° internal rotation (IR)), 5.2° ± 6.1° ER (range, 24.8° ER to 12.6° IR), 7.1° ± 5.5° ER (range, 20.7° ER to 6.8° IR) for unloaded, unloaded with implants set to congruency and weight bearing scenarios respectively. The mean of absolute difference in axial alignment between the unloaded and weight bearing scenario was 2.8° ± 2.0° (range 0.1° to 8.8°). Statistically significant absolute differences in coronal and axial alignments were found. Conclusions. ‘Correct’ alignment has long been considered and important predictor of longevity and function following TKR surgery (Sikorski 2008). However, recent reports have challenged these long held beliefs. One possible reason is that these alignments are measured in static condition, not in a functional position where soft tissue is active. This study showed that knee joint alignment changes significantly between unloaded and loaded scenarios. This suggest that static, unloaded measurements do not represent functional alignment. Thus, tibio-femoral alignment measured from unloaded condition may not describe a ‘correct’ alignment for a particular patient. Further work should focus on dynamic and functional descriptions of component and/or limb alignment

Computer assisted total knee arthroplasty may have advantages over conventional surgery with respect to component positioning. Femoral component mal-rotation has been shown to be associated with poor outcomes, and may be related to posterior referencing jigs. We aimed to determine the variation between the transepicondylar axis (TEA) and posterior condylar axis (PCA) in a series of knees undergoing navigated total knee arthroplasty, and to determine the correlation between final intra-operative coronal alignment and post-operative radiographic functional alignment. A review of 170 consecutive patients undergoing primary total knee arthroplasty between June 2007 and August 2010, using Precision navigation and Triathlon implants (Stryker). The difference between the TEA and PCA was measured as was the initial coronal alignment. Referencing of the TEA had been previously validated against computerised tomography in a previous study. During arthroplasty, neutral alignment was aimed for, and the final alignment after implant insertion was recorded. Pre- and 1 year post-operative flexion was measured. A standing four foot alignment radiograph was obtained 6 weeks after surgery to determine the weight-bearing mechanical axis. The mean difference between the TEA and PCA was 3.94 degrees (−2.80 to 11.59) and median difference was 3.6 degrees. (A positive value implies the PCA is internally rotated with respect to the TEA). The median pre-operative flexion was 120 degrees (80–130) and the median post-operative flexion was 125 (85–145). The mean change in flexion was −2.5 degrees (−40 to 40; p=0.001). The mean intra-operative alignment was 0.75 degrees (−3 to 6, SD 1.9) and the mean radiographic alignment was 1.24 degrees (−6.5 to 6.5, SD 1.6). Taking −3 to +3 to be neutral, the outlier rate intra-operatively was 6.5% and radiographically was 16.5%. The intra-operative and radiographic alignment showed correlation (coefficient 0.289). There was poor correlation between pre-operative deformity and degree of difference between intra-operative and radiographic alignment (coefficient −0.1). Conclusion: There is a wide variation in the difference between the TEA and PCA, and there is not a good relationship with coronal alignment. Although most valgus knees had a bigger difference, such a difference was also seen in many varus knees. This should alert the surgeon when using posterior referencing jigs when determining the femoral component size and rotation. Although these patients achieved good post-operative flexion, this was determined by the pre-operative range. There was reasonable correlation between the final intra-operative mechanical alignment and the weight-bearing alignment as determined by a standing radiograph

Knee alignment is a fundamental measurement in the assessment, monitoring and surgical management of patients with osteoarthritis [OA]. In spite of extensive research into the consequences of malalignment, our understanding of static tibiofemoral alignment remains poor with discrepancies in the reported weight-bearing characteristics of the knee joint and there is a lack of data regarding the potential variation between supine and standing (functional) conditions. In total knee arthroplasty [TKA] the lower limb alignment is usually measured in a supine condition and decisions on prosthesis placement made on this. An improved understanding of the relationship between supine and weight-bearing conditions may lead to a reassessment of current surgical goals. The purpose of this study was to explore the relationship between supine and standing lower limb alignment in asymptomatic, osteoarthritic and prosthetic knees. Our hypothesis was that the change in alignment of these three groups would be different. A non-invasive infrared position capture system (accuracy ±1° in both coronal and sagittal plane) was used to assess the knee alignment for 30 asymptomatic controls and 31 patients with OA, both before and after TKA. Coronal and sagittal mechanical femorotibial (MFT) angles in extension (negative values indicating varus in the coronal plane and hyperextension in the sagittal plane) were measured with each subject supine and in bi-pedal stance. For the supine test, the lower limb was supported at the heel and the subject told to relax. For the standing position subjects were asked to assume their normal stance. The change in alignment between these two conditions was analysed using a paired t-test for both coronal and sagittal planes. To quantify the change in 3D, vector plots of ankle centre displacement relative to the knee centre from the supine to standing condition were produced. Alignment in both planes changed significantly from supine to standing for all three groups. For the coronal plane the supine and standing measurements (in degrees, mean(SD)) were 0.1(2.5) and −1.1(3.7) in the asymptomatic group, −2.5(5.7) and −3.6(6) in the OA group and −0.7(1.4) and −2.5(2) in the TKA group. For the sagittal plane the numbers were −1.7(3.3) and −5.5(4.9); 7.7(7.1) and 1.8(7.7); 6.8(5.1) and 1.4((7.6) respectively. This change was most frequently towards relative varus and extension. Vector plots showed that the trend of relative varus and extension in stance was similar in overall magnitude and direction between the three groups. Knee alignment can change from supine to standing for asymptomatic and osteoarthritic knees, most frequently towards relative varus and hyperextension. The similarities between each group did not support our hypothesis. The consistent kinematic pattern for different knee types suggests that soft tissue restraints rather than underlying joint deformity may be more influential in dynamic control of alignment from lying to standing. In spite of some evidence suggesting a difference between supine and standing knee alignment a mechanical femorotibial (MFT) angle of 0° is a common intra-operative target as well as the desired post-operative weight-bearing alignment. These results indicated that arthroplasties positioned in varus intra-operatively could potentially become ‘outliers’ (>3° varus) when measured weight-bearing. Mild flexion contractures may correct when standing, reducing the need for intra-operative posterior release. These potential changes should be considered when positioning TKA components on supine limbs as post-operative functional alignment may be different