Abstract
Introduction
Current techniques in total knee arthroplasty aim to restore the coronal mechanical axis to neutral. Preoperative planning has historically been based on long-leg standing films (LLSF) which allow surgeons to plan bony resection and soft tissue releases. However, LSSF can be prone to error if malrotated. Recently, patient-specific guides (PSG) utilizing supine magnetic resonance imaging (sMRI) have become an accepted technique for preoperative planning. In this study we sought to compare the degree of coronal deformity using LLSF and sMRI.
Methods
Two hundred thirty knees underwent planning for total knee arthroplasty with sMRI and LLSF. Coronal plane deformity was determined based on the femoral-tibial angle (FTA) as defined by the angle formed between a line from the center of the femoral head to the intercondylar notch and a line from the middle of the tibial spines to the middle of the ankle joint. Mechanical axis values from the sMRI were compared with values obtained from LLSF
Results
There were 172 varus knees and 58 valgus knees. There was significant correlation (r=0.9215) between LLSF and sMRI for the measurement of coronal plane deformity for all knees. sMRI underestimated the severity of deformity by 2.19 degrees of varus (p<0.001). Additionally, as the severity of the deformity increased, there was also an increase in the discrepancy between sMRI and LLSF. There was a smaller discrepancy for valgus knees (−0.66 degrees) than varus knees (3.15 degrees, p<0.001). The discrepancy between the two modalities was not affected by gender (p=0.386).
Conclusion
sMRI based imaging can help approximate coronal plane deformity in the preoperative planning of TKA but it has limitations. This MRI-based technique tended to underestimate deformity in varus knees and patients with extreme deformity. Surgeons may use sMRI for pre-operative planning but must understand that they tend to underestimate the severity of deformity.
