Alignment Analyses in the Varus Osteoarthritic Knee Using Computer Navigation
Osteoarthritic (OA) knees with severe extension varus deformity seem to have correspondingly more severe flexion varus, especially beyond a certain tibiofemoral angle. Clinical measurement of flexion varus and fixed flexion deformity (FFD), which had been difficult to perform because of the spatial alignment of the knee in flexion, was recently made possible with computer navigation.
We conducted a study to evaluate the relationship of extension and flexion varus in OA knees and to determine whether severity of FFD in the sagittal plane correlates with severity of coronal plane varus deformity. The study included 317 consecutive cases of computer-navigated total knee arthroplasty performed on OA knees with varus deformities. Three sets of values were extracted from the navigation data: varus angle at maximal knee extension, 90° knee flexion, and maximal knee extension. Correlation analyses were performed for extension and flexion varus, FFD, and coronal plane deformity.
OA knees with extension varus of more than 10° had an incremental likelihood of more severe flexion varus. When the extension varus angle exceeded 20°, probability became almost certainty. There was no correlation between FFD and coronal plane varus deformity.
The strengths of this study are that it was a single-surgeon series with knee data from consecutive patients who had computer-navigated TKA. Patient data were prospectively generated from the navigation software and retrospectively analyzed. All navigation alignment was performed by a single surgeon, thereby eliminating examination bias during the time knee alignment data were being obtained. The study was adequately powered and had a large number of patients for data analysis. The authors believe that this is the first study to analyze alignment in both the coronal and sagittal plane in varus OA knees.
We acknowledge a few limitations in our study. Although several investigators have found that navigation can be used to achieve accurate postoperative alignment,10,15,16 subtle errors may be inadvertently introduced at different points of alignment measurement. These error points include identification of visually selected anatomical landmarks; kinematic registration of hip, knee, and ankle; and intraoperative changes in the navigation environment (eg, inadvertent movement of pins or rigid bodies). In addition, different surgeons have different techniques for kinematic registration. However, the surgeries in our study were performed by the same surgeon, so this confounding factor was effectively removed. Another limitation was that navigation alignment was obtained during surgery, when patients were under anesthesia and in a supine, non-weight-bearing position, whereas routine clinical weight-bearing radiographs are taken with nonanesthetized patients and this might overestimate the deformities intraoperatively. However, all parameters were measured in the same patient under the same anesthetic effects, so this should not have affected the analyses. Most surgeons would make an intraoperative assessment of the severity of any deformity before the surgery proper anyway. Nevertheless, some authors have found that knee alignment obtained with intraoperative navigation correlated well with alignment obtained with weight-bearing radiographs.17,18
Conclusion
Our study results showed that, in OA varus knees, extension varus highly correlated with flexion varus. However, there was no correlation between FFD and coronal plane varus deformity.
