Measurement of lower limb alignment: there are within-person differences between weight-bearing and non-weight-bearing measurement modalities

Previous studies have compared weight-bearing mechanical leg axis (MLA) measurements to non-weight-bearing measurement modalities. Most of these studies compared mean or median values and did not analyse within-person differences between measurements. This study evaluates the within-person agreement of MLA measurements between weight-bearing full-length radiographs (FLR) and non-weight-bearing measurement modalities (computer-assisted surgery (CAS) navigation or MRI).

Two independent observers measured the MLA on pre- and postoperative weight-bearing FLR in 168 patients. These measurements were compared to non-weight-bearing measurements obtained by CAS navigation or MRI. Absolute differences in individual subjects were calculated to determine the agreement between measurement modalities. Linear regression was used to evaluate the possibility that other independent variables impact the differences in measurements.

A difference was found in preoperative measurements between FLR and CAS navigation (mean of 2.5A degrees with limit of agreement (1.96 SD) of 6.4A degrees), as well as between FLR and MRI measurements (mean of 2.4A degrees with limit of agreement (1.96 SD) of 6.9A degrees). Postoperatively, the mean difference between MLA measured on FLR compared to CAS navigation was 1.5A degrees (limit of agreement (1.96 SD) of 4.6A degrees). Linear regression analysis showed that weight-bearing MLA measurements vary significantly from non-weight-bearing MLA measurements. Differences were more severe in patients with mediolateral instability (p = 0.010), age (p = 0.049) and ae3A degrees varus or valgus alignment (p = 0.008).

The clinical importance of this study lies in the finding that there are within-person differences between weight-bearing and non-weight-bearing measurement modalities. This has implications for preoperative planning, performing total knee arthroplasty (TKA), and clinical follow-up after TKA surgery using CAS navigation or patient-specific instrumentation.

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