Lower limb alignment becomes more varus and hyperextended from supine to bipedal stance in asymptomatic, osteoarthritic and prosthetic neutral or varus knees

PURPOSE: Knee alignment is a fundamental measurement in the assessment, monitoring and surgical management of patients with osteoarthritis. There is a lack of data regarding how static tibiofemoral alignment varies between supine and standing conditions. This study aimed to quantify the relationship between supine and standing lower limb alignment in asymptomatic, osteoarthritic (OA) and prosthetic (TKA) knees. METHODS: A non-invasive position capture system was used to assess knee alignment for 30 asymptomatic controls and 31 patients with OA both before and after TKA. Coronal and sagittal mechanical femorotibial angles were measured supine with the lower limb in extension and in bipedal stance. Changes between conditions were analysed using paired ttests. Vector plots of ankle centre displacement relative to the knee centre from supine to standing were produced to allow three-dimensional visualisation. RESULTS: All groups showed a trend towards varus and extension when going from supine to standing. Mean change for asymptomatic knees was 1.2° more varus (p = 0.001) and 3.8° more extended (p < 0.001). For OA knees this was 1.1° more varus (p = 0.009) and 5.9° more extended (p < 0.001) and TKA knees 1.9° more varus (p < 0.001) and 5.6° more extended (p < 0.001). CONCLUSION: The observed consistent changes in lower limb alignment between supine and standing positions across knee types suggests the soft tissue envelope restraining the knee may have a greater influence on dynamic alignment changes than the underlying bony deformity. This highlights the importance of quantifying soft tissue behaviour when planning, performing and evaluating alignment dependent surgical interventions of the knee. When routinely assessing any type of knee, clinicians should be aware that subtle consistent alignment changes occur under weightbearing conditions and tailor their treatments accordingly. LEVEL OF EVIDENCE: II.