The effects of valgus medial opening wedge high tibial osteotomy on articular cartilage pressure of the knee: a biomechanical study.

PURPOSE: The objective of this study was to quantify the effect of different loading axes and of a valgus opening wedge high tibial osteotomy (HTO) on tibiofemoral cartilage pressure.

METHODS: Six human knee specimens were tested with a load of 1000N in extension in a materials testing machine using a specially designed fixture. Pressure in the medial and lateral joint compartment was recorded using pressure-sensitive films. Different loading alignments (varus, straight, and valgus) were simulated. A medial opening wedge HTO was performed adjusting the loading axis to slight valgus. The first measurement was performed with intact medial collateral ligament (MCL). Then the MCL was dissected gradually and the cartilage pressure again analyzed.

RESULTS: There was a significant correlation of the load distribution with the position of the loading axis. The medial compartment was predominantly loaded in the varus setting. The more lateral the loading line intersected the knee, the more pressure was redistributed laterally. The opening wedge HTO without the MCL release resulted in a significant increase of the pressure medially (P = .002). Only after a complete release of the MCL was a significant decrease of pressure medially observed after opening wedge HTO (P = .003).

CONCLUSIONS: The position of the loading axis in the frontal plane has a strong effect on the tibiofemoral cartilage pressure distribution of the knee. The medial compartment is predominantly loaded in a varus knee; a neutral mechanical axis slightly loads the lateral more than the medial compartment. In valgus alignment, the main load runs through the lateral compartment.

CLINICAL RELEVANCE: A medial opening wedge HTO maintains high medial compartment pressure despite the fact that the loading axis has been shifted into valgus. Only after complete release of the distal fibers of the MCL does the opening wedge HTO produce a decompression of the medial joint compartment.