Alignment Options and Assessment after Total Knee Arthroplasty

Myles R. J. Coolican, FRACS, AUSTRALIA Giacomo Dal Fabbro, MD, AUSTRALIA

ISAKOS eNewsletters   Current Perspective 2025   Not yet rated

Following many years of adherence to the traditional concept of mechanical alignment, the past two decades have seen the description and adoption of several other alignment philosophies for total knee arthroplasty (TKA). The aim has been to improve patient outcomes and provide the most personalised treatment possible.

Malpositioning of implants in the coronal, sagittal, and axial planes can cause unsatisfactory outcomes following knee replacement. Accordingly, the imaging assessment of component alignment remains a cardinal aspect of both routine postoperative care and scientific research analysis.

This article aims to describe the rationales behind the available alignment strategies and to outline a consistent and standardised methodology to assess implant alignment and orientation after TKA on the basis of the evidence in the literature.

Alignment Strategies and Their Rationale

Mechanical alignment (MA), which was the mainstay of knee arthroplasty from the 1970s through the early 1980s, aims to create identical rectangular spaces between the distal femur and proximal tibia both in full extension and at 90°. The plane of bone cuts results in the centres of the hip, knee, and ankle being in a straight line; this line is vertical when the feet are placed as far apart as the hips, with the goals being equal distribution of loads across the joint and greater implant longevity. As this is the natural alignment in 15.4 % of the community, ligament releases are frequently required to balance the knee. To create a more normal knee, a variation of MA, called anatomic alignment (AA), was proposed by Hungerford in the early 1980s to respect the natural medial tibial proximal angle; however, that strategy was later abandoned. Two major factors that led surgeons away from MA were (1) the 15%-20% rate of patient dissatisfaction after knee arthroplasty and (2) the introduction of computer navigation, and now robotic surgery, with the ability to accurately remove bone and achieve the desired alignment.

Kinematic alignment (KA) aims to restore native alignment by resecting the correct amount of bone and cartilage that is to be replaced by the implant, with adjustment for any tissue that is lost through disease, meaning that ligament releases are rarely required but that adjustment of the tibial cut occasionally is necessary for balance. This strategy carries the risk of extreme alignment, and, accordingly, restricted KA with boundaries has evolved. Inverse kinematic alignment (iKA) is a tibia-first procedure in which the tibia is cut identical to the existing joint line to recreate the same joint-line obliquity and then the position of the femoral component is adjusted by altering the femoral cutting guide in a standard gap-balancing fashion. As with KA, boundaries can be used to minimise extreme alignment.

Functional alignment (FA) requires assessment of the gaps between the medial and lateral compartments at or near full extension and at 90°, with removal of the correct amount of bone and cartilage to allow the implant to fill the space. Soft-tissue releases are uncommon, but, as with restricted KA, boundaries limit extreme alignment.

Imaging Tools to Assess Component Alignment

In 1989, The (American) Knee Society Total Knee Arthroplasty Roentgenographic Evaluation and Scoring System was published, with the goal of standardising the radiographic rating of TKA on the basis of the anatomical axis¹. Subsequently, conventional weight-bearing radiographs became the gold standard for the assessment of implant position. Standard anteroposterior, lateral, and axial views detect the angles between the anatomical and mechanical axes and the various components having been reported in the literature, with moderate to good inter-observer reliability (intraclass correlation coefficient [ICC], 0.65-0.83)². More recently, in 2015, an update of the scoring system was proposed, with the technique requiring three essential views for an accurate TKA evaluation: (1) a weight-bearing anteroposterior (AP) radiograph made with the patella facing toward the x-ray beam, which is targeted parallel to and in line with the slope of the tibial baseplate; (2) a lateral radiograph made with the knee flexed at 30°, with an emphasis on the superimposition of the implant’s posterior femoral condyles; and (3) a patellofemoral radiograph made with the knee flexed to 45° and the beam directed at 30° from the horizontal (known as a Merchant view)³. In addition to these three views, a standing full-length radiograph of both legs must be included in the analysis in order to evaluate coronal component alignment relative to mechanical alignment. Full-length imaging remains an important part of evaluation in the setting of non-mechanical alignment approaches. Additionally, the literature has shown that short-leg radiographs may result in misinterpretation of component positioning. Given the various alternative approaches that aim to totally or partially restore native alignment, the postoperative analysis also needs to include ipsilateral preoperative radiographs or radiographs of the contralateral, normal knee as a reference for comparison.

The limitations of standard radiographs include difficulty in the assessment of implant rotational alignment as well as subtle variations in assessment resulting from changes in limb position and magnification. The use of computed tomography (CT) has become the gold standard for the assessment of implant positioning in the axial plane as it allows for both direct assessment on two-dimensional (2D) CT slices and three-dimensional (3D) reconstruction⁴. The supplementation of standard radiographs with CT scanning of the knee is mandatory in order to perform a complete assessment of component alignment after TKA. While 2D-CT scanning is an accurate method for assessing rotation, the identification of anatomical landmarks may be difficult. 3D-reconstructed CT imaging has been reported to have significantly higher intra- and inter-observer reliability in comparison with both radiographs and 2D-CT scans for sagittal and rotational alignment and is recommended for the assessment of a poorly functioning TKA when there are concerns related to component positioning².

The acquisition of a CT scan with the patient in the supine position represents a limitation in the assessment of coronal and sagittal alignment where its value is to assess component rotation. For this reason, as reported in a review from the University College London Hospital, there has been increasing research on the use of weight-bearing CT scans made with use of a cone-beam scanner, which allows for the evaluation of the knee under loading conditions and would result in introducing the use of standing CT scans in clinical practice. Another flaw in the CT assessment of alignment and rotation is the high variability associated with the 3D analysis of knee alignment, making it impossible to define 3D reference values for alignment parameters. However, the presence of underlying principles to the methods reported in the literature is allowing to reach an agreement for the 3D alignment analysis which should be considered even in the analysis of the prosthesis components⁵.

Metal artifacts in CT scans represent a major limitation in the use of this tool for the assessment of TKA component position, and, in recent years, considerable effort has been devoted to developing metal artifact reduction techniques in CT.

Assessment of Component Alignment

Assessment of component position after TKA is most accurately achieved with use of radiographic methods, which allow for the identification of relevant landmarks and are the gold standard. Other assessment tools, such as the use of a goniometer either clinically or during surgery, do not allow for the accurate identification of landmarks and are of little use.

Femoral Component

Coronal Alignment. The coronal anatomic alignment of the femoral component is evaluated on an AP radiograph as the angle between the anatomical axis of the bone and a line tangential to the articulating surface of the femoral component (Figure 1, A)^1,3, with moderate interobserver reliability having been reported in the literature (ICC, 0.65)². The assessment of the coronal varus-valgus angle on 3D-CT scans has been reported to have a significantly higher interobserver reliability than radiographs (ICC, 0.89)². The coronal mechanical alignment of the femoral component is assessed on a standing long-leg view as the angle between the femoral mechanical axis and the tangent to the femoral implant distal surface (also known as the lateral distal femoral angle or femoral joint line angle).

Sagittal alignment. The sagittal alignment of the femoral component is evaluated on a lateral radiograph as the angle between the most distal femoral fixation surface and the axis of the femoral shaft (Figure 1, B)^1,3, with a good interobserver reliability having been reported in the literature (ICC, 0.82)². Sagittal alignment also may be assessed with 3D-CT scanning, which has been reported to have higher reliability as compared with radiographs and 2D-CT scanning².

Axial alignment. The axial rotation of the femoral component is assessed according to the method of Berger et al. on 2D-CT scans, with the posterior condylar angle being defined as the angle between the surgical epicondylar axis (consisting of the line between the lateral epicondyle and the sulcus of the medial epicondyle) and the posterior condylar line (the tangent to the posterior condylar surfaces) (Figure 2). Assessment of component rotation with 3D-CT scanning has been reported to have significantly higher interobserver agreement as compared with 2D-CT scanning².

Tibial Component

Coronal alignment. The coronal alignment of the tibial component is assessed on an AP radiograph as the angle between the tangent to the long axis of the baseplate and the mechanical or anatomical axis of the tibia (Figure 3, A). ^1,3, with good interobserver reliability having been reported (ICC, 0.70). A more accurate assessment of the mechanical tibial axis and the medial proximal tibial angle can be assessed on a standing long-leg radiograph⁴. AP radiographs also allow for the assessment of joint-line obliquity and joint-line height. The joint-line height in the coronal plane can be calculated as the average of lateral femoral condyle and medial femoral condyle height, defined as the distance between a line perpendicular to the intramedullary axis of the tibia located at the level of the most proximal fibula and the most distal lateral and medial point of the femoral condyles, respectively.

Sagittal alignment. The tibial slope is assessed on a lateral radiograph as the angle between the anatomical tibial axis and the line tangent to the baseplate^1,2, with good interobserver reliability having been reported (ICC, 0.79) The joint-line height also may be assessed on a lateral radiograph according to the method of Figgie et al. by measuring the perpendicular distance between the superior margin of the tibial tubercle and the weight-bearing parallel surface of the tibial component (Figure 3, B).

Axial alignment. The universal intraoperative anatomical landmark and the optimal target of tibial component rotation are still a matter of debate. The Akagi line, which is among the most used intraoperative landmarks, connects the medial border of the patellar tendon and the middle of the posterior cruciate ligament tibial insertion (Figure 4). The postoperative axial rotation of the tibial component is assessed with 2D-CT scanning. The Berger protocol is to measure the angle subtended by the tibial tubercle axis (a line connecting the geometric centre of the tibia and the tip of tibial tubercle) and the tibial component axis (a line perpendicular to the transverse axis of the tibial component) (Figure 5). Bedard et al. proposed a modification of this technique for assessing the rotation of asymmetric tibial components; with this modified technique, the tibial component axis is considered to be a line perpendicular to the flexion axis (defined as the line connecting the centres of each hemi-plateau of the tibial component). The Kim protocol is to measure the angle between the posterior margin of the tibial bearing and the tangent to the posterior margin of the tibial plate, with the aim to consider the true rotational alignment in case of mobile bearing rotation platform implant. Furthermore, a 3D-CT protocol was proposed for the assessment of tibial rotation as the angle between the posterior tibial condylar axis and the posterior tibial prosthesis implant. Since excellent intra- and intertester reliability have been reported for this method (ICC, 0.99 for both), it should be taken into account when assessing tibial component rotation².

Patella

Patellofemoral alignment is determined on the Merchant or skyline view with the assessment of patellar tilt and displacement^1,3. Patellar tilt is defined according to the method of Gomes et al. as the angle between the anterior limits of the femoral component condyles and the prosthesis-bone interface line of the patellar component, and patellar displacement is defined as the distance between the line intersecting the centre of the patella and the line through the deepest part of the trochlear groove.

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Figure 5

Summary

Malpositioning of implants is a major cause of unsatisfactory outcomes following TKA, and standardised definitions for implant alignment terminology are still lacking. The assessment of standard weight-bearing AP, lateral, and Merchant views and long-leg radiographs according to the updated Knee Society Total Knee Arthroplasty Roentgenographic Evaluation and Scoring System represents the gold standard for the evaluation of component alignment and orientation in the coronal and sagittal planes. The coronal and sagittal assessment includes the angles between the anatomical and mechanical axes of the femur and the tibia and the lines tangential to the distal and proximal part of the components. 2D-CT analysis must be performed to accurately measure axial femoral and implant rotation, which is assessed as the angle between the epicondylar axis and the posterior condylar line for the femur and the angle between the tibial tubercle orientation and the tibial component axis for the tibia. Assessment of preoperative or contralateral knee radiographs must be included in order to assess joint height and to achieve a complete and reliable assessment of implant orientation. The analysis of 3D-reconstructed CT imaging to assess coronal femoral alignment and axial femoral and tibial rotation is recommended when confronted with a poorly functioning TKA associated with concerns about component positioning or for research purposes.

Future Directions

As various alignment philosophies for TKA have recently been introduced, there is a need to outline a consistent and standardised methodology to assess implant alignment and orientation after TKA. Standardisation will allow surgeons to assess accuracy and alignment outcomes after TKA and to investigate the association between component positioning and patient function and satisfaction. The application of advanced imaging techniques such as 3D-CT imaging should be implemented and integrated with dynamic functional examination such as the gait analysis in order to make postoperative care more effective and to provide powerful tools for research purposes.

References

1. Ewald FC. The Knee Society total knee arthroplasty roentgenographic evaluation and scoring system. Clin Orthop. 1989;(248):9-12.
2. Hirschmann MT, Konala P, Amsler F, Iranpour F, Friederich NF, Cobb JP. The position and orientation of total knee replacement components: a comparison of conventional radiographs, transverse 2D-CT slices and 3D-CT reconstruction. J Bone Joint Surg Br. 2011;93(5):629-633. doi:10.1302/0301-620X.93B5.25893
3. Meneghini RM, Mont MA, Backstein DB, Bourne RB, Dennis DA, Scuderi GR. Development of a Modern Knee Society Radiographic Evaluation System and Methodology for Total Knee Arthroplasty. J Arthroplasty. 2015;30(12):2311-2314. doi:10.1016/j.arth.2015.05.049
4. Cyteval C. Imaging of knee implants and related complications. Diagn Interv Imaging. 2016;97(7):809-821. doi:10.1016/j.diii.2016.02.015
5. Veerman QWT, Ten Heggeler RM, Tuijthof GJM, de Graaff F, Fluit R, Hoogeslag RAG. High variability exists in 3D leg alignment analysis, but underlying principles that might lead to agreement on a universal framework could be identified: A systematic review. Knee Surg Sports Traumatol Arthrosc. Published online October 26, 2024. doi:10.1002/ksa.12512

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