Elsevier

The Knee

Volume 21, Issue 6, December 2014, Pages 1120-1123
The Knee

Relationship of the posterior femoral axis of the “kinematically aligned” total knee arthroplasty to the posterior condylar, transepicondylar, and anteroposterior femoral axes

https://doi.org/10.1016/j.knee.2014.07.025Get rights and content

Highlights

  • We analyzed the posterior femoral axis of the “kinematically aligned” TKA.

  • One hundred and fourteen patients undergoing TKA with MRI images were reviewed.

  • The kinematic alignment axis was 4.0° internally rotated to the transepicondylar axis.

  • The kinematic alignment technique internally rotates the femur versus traditional targets.

Abstract

Background

A recent proposed modification in surgical technique in total knee arthroplasty (TKA) has been the introduction of the “kinematically aligned” TKA, in which the angle and level of the posterior joint line of the femoral component and joint line of the tibial component are aligned to those of the “normal,” pre-arthritic knee. The purpose of this study was to establish the relationship of the posterior femoral axis of the “kinematically aligned” total knee arthroplasty (TKA) to the traditional axes used to set femoral component rotation.

Methods

One hundred and fourteen consecutive, unselected patients with preoperative MRI images undergoing TKA were retrospectively reviewed. The transepicondylar axis (TEA), posterior condylar axis (PCA), antero-posterior axis (APA) of the trochlear groove, and posterior femoral axis of the kinematically aligned TKA (KAA) were templated on axial MRI images by two independent observers. The relationships between the KAA, TEA, APA, and PCA were determined, with a negative value indicating relative internal rotation of the axis.

Results

On average, the KAA was 0.5° externally rotated relative to the PCA (minimum of − 3.6°, maximum of 5.8°), − 4.0° internally rotated relative to the TEA (minimum of − 10.5°, maximum of 2.3°), and − 96.4° internally rotated relative to the APA (minimum of − 104.5°, maximum of − 88.5°). Each of these relationships exhibited a wide range of potential values.

Conclusions

Using a kinematically aligned surgical technique internally rotates the posterior femoral axis relative to the transepicondylar axis, which significantly differs from current alignment instrument targets.

Introduction

Total knee arthroplasty (TKA) has been highly successful in relieving pain and restoring function in patients with degenerative joint disease, with numerous studies demonstrating implant survivorship of greater than 90% at 10 years and beyond [1], [2], [3]. However, as the volume of TKAs performed in the United States continues to increase, especially in the younger population, concerns remain regarding the expectations and functional demands placed on these prostheses [4], [5], [6], [7]. Recent studies focusing on patient satisfaction as the primary outcome measure have shown the percentage of patients who remain “unsatisfied” following a TKA to be as high as 15% to 30%, with higher rates of dissatisfaction seen in younger patients (less than 65 years old) [7], [8], [9], [10]. Parvizi et al. [10] demonstrated a high prevalence of residual symptoms in young, active patients, as only 66% of patients stated their knee to feel “normal,” with persistent pain in 33%, stiffness in 41%, and grinding or other noise in 33%.

The long-held tenet in TKA is that a successful outcome is dependent on achievement of a neutral mechanical axis of the lower extremity with the tibial and femoral components aligned perpendicular to the mechanical axis in the coronal plane. However, the significance of a neutral, mechanical alignment both on overall component survivorship and on clinical function has recently been questioned [11], [12], [13]. Furthermore, the introduction of the concept of “constitutional varus” has hypothesized that restoration of a neutral, mechanical alignment may in fact be “unnatural” for a substantial proportion of the population, thus in part contributing to residual symptoms and dissatisfaction [12], [14], [15]. A recent modification in surgical technique has been to attempt to align the angle and level of the femoral component, posterior joint line of the femoral component, and joint line of the tibial component to those of the “normal,” pre-arthritic knee [14]. This modification has been introduced as the “kinematically aligned” TKA, and preliminary results have been encouraging [16], [17], [18]. Internal–external rotation of the femoral component in the axial plane is focused on restoration of the pre-arthritic, posterior femoral joint line based on the defined thickness of the femoral component to be used. This is in contrast to the classical, mechanically aligned TKA, in which axial rotation of the femoral component is set by aligning the posterior joint line relative to one of three axes: (1) perpendicular to the anteroposterior axis of the trochlear groove (Whiteside's line) [19], (2) parallel to the transepicondylar axis [20], [21], or (3) 3° externally rotated to the posterior condylar axis [22].

Femoral component malrotation continues to be a concern in TKA, and reproducible and accurate positioning in the axial plane remains elusive due to the anatomic variability of these traditional landmarks, and the associated difficulty in identifying them intraoperatively [23], [24], [25], [26]. Furthermore, whether these traditional landmarks should even be targeted in TKA has been questioned, as Eckhoff et al. has demonstrated that they do not accurately reproduce the true flexion–extension axis of the knee [25], [27]. To our knowledge, the effect of “kinematically aligning” a TKA on rotation of the femoral component compared to these traditional axes has not yet been reported and warrants investigation.

The purpose of this study was to establish the relationship of the posterior femoral axis of the kinematically aligned TKA to the traditional axes used to determine femoral component rotation. Our hypothesis is that the use of a kinematically aligned surgical technique will lead to a posterior femoral axis that is significantly different from the current alignment instrument targets.

Section snippets

Materials and methods

This study is an IRB-approved retrospective review of a prospectively collected registry of patients undergoing total knee arthroplasty at a single institution. The inclusion criteria for the study were patients over the age of 18 years old, with a primary diagnosis of osteoarthritis or inflammatory arthritis, undergoing a unilateral TKA with preoperative magnetic resonance imaging (MRI) scans as part of the preparation for the use of patient specific cutting guides (Signature™, Biomet Inc.,

Statistical analysis

All data were collected and analyzed utilizing Microsoft Excel software (Microsoft Corporation, Redmond, WA). Interobserver correlation coefficients were graded using previously described semi-quantitative criteria: excellent for 0.9  r  1.0, good for 0.7  r  0.89, fair/moderate for 0.5  r  0.69, low for 0.25  r  0.49, and poor for 0.0  r  0.24 [30].

Results

A wide range of variability was appreciated in the relationships between the PCA, TEA, and APA. On average, the TEA was 4.5° externally rotated relative to the PCA, with a standard deviation of 2.4°. The range of values was 11.7° (minimum of − 1.3°, maximum of 10.4°). Similarly, on average, the APA was 96.9° externally rotated relative to the PCA, with a standard deviation of 3.2°. The range of values was 15.6° (minimum of 90°, maximum of 105.6°). Lastly, on average, the APA was 92.4° externally

Discussion

This study demonstrates that on average, the KAA is only 0.5° externally rotated relative to the PCA, − 4.0° internally rotated relative to the TEA, and − 96.4° internally rotated relative to the APA, with each of these relationships exhibiting a wide range of potential values. Therefore, our results support our hypothesis and demonstrate the posterior femoral axis of the kinematically aligned TKA to be internally rotated relative to the traditional axes used to set femoral component rotation in

Conflict of interest statement

The authors have no financial conflicts of interested related to this work.

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  • Cited by (0)

    This study has been approved by the Institutional Review Board at Washington University School of Medicine (St. Louis, MO).

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