Varus alignment increases medial meniscus extrusion and peak contact pressure: a biomechanical study

Purpose: Assessment of medial meniscus extrusion (MME) has become increasingly popular in clinical practice to evaluate the dynamic meniscus function and diagnose meniscus pathologies. The purpose of this biomechanical study was to investigate the correlation between MME and the changes in joint contact pressure in varus and valgus alignment. It was hypothesized that varus alignment would result in significantly higher MME along with a higher joint contact pressure in the medial compartment. Methods: Eight fresh-frozen human cadaveric knees were axially loaded, with a 750 N compressive load, in full extension with the mechanical axis shifted to intersect the tibial plateau at 30% and 40% (varus), 50% (neutral), 60% and 70% (valgus) of its width (TPW). Tibiofemoral peak contact pressure (PCP), mean contact pressure (MCP) and contact area (CA) were determined using pressure-sensitive films. MME was obtained via ultrasound at maximum load. Results: MME was significantly increased from valgus (1.32 ± 0.22 mm) to varus alignment (3.16 ± 0.24 mm; p < 0.001). Peak contact pressure at 30% TPW varus alignment was significantly higher compared to 60% TPW valgus (p = 0.018) and 70% TPW valgus (p < 0.01). MME significantly correlated with PCP (r = 0.56; p < 0.001) and MCP (r = 0.47, p < 0.01) but not with CA (r = 0.23; n.s.). Conclusion: MME was significantly increased in varus alignment, compared to neutral or valgus alignment, with an intact medial meniscus. It was also significantly correlated with PCP and MCP within the medial compartment. However, valgus malalignment and neutral axis resulted in reduced MME and contact pressure. Lower limb alignment must be taken into account while assessing MME in clinical practice. Level of evidence: Controlled laboratory study.