Introduction: The Donjoy OA Orthesis has been developed to treat unicompartimental osteoarthrosis of the knee joint. The idea is to reduce the load in the affected joint compartment. Sofar only clinical evaluations and gait analysis have been published. The aim of this study is to use intraarticular pressure sensors in an anatomical model to work out the existing intraarticular pressures.

Method: 5 fresh non fixated anatomical knee models were used. The models were prepared leaving the 2/3 of the femur as well as the tibia and fibula. We used a right and left-sided version of the Donjoy OA Orthesis. A defined pressure was applied via the pressure application screw.

The dorsal capsule of the knee joint was opened and the meniscus were mobilised in a way that the pressure foils (Fuji typ super low) could be placed between the undersurface of the meniscus and the tibia plateau in each joint compartment. The measurements were recorded in four different joint positions (0°, 30°, 60° flexion and 0° with removed meniscus).

Each anatomical model was well fixed in a universal testing machine” (Instron, System ID: 5565 H1703). The used axial pressure in all set ups used was 1500N.

We used 4 four different test series:

no pressure, no orthesis

no pressure, with orthesis

with pressure, no orthesis

with pressure, with orthesis

After removing the pressure foils the actual pressure could be estimated by the diameter of the coloured foil. Using the programm Mortphomet it was possible to calculate these pressure areas and give procentual figures.

Results: The Antivarus Orthesis could reduce the intraarticular joint pressure between 55–70%

Without pressure:

Knee → 64,37 %

Knee → 55,30 %

Knee → 54,43 %

Knee → 58,75 %

Knee → 44,80 %

Mean value 55%.

With pressure:

Knee → 74,59 %

Knee → 74,00 %

Knee → 67,91 %

Knee → 86,34 %

Knee → 49,69 %

mean value 70%.

The Antivalgus Orthesis could reduce the intraarticular joint pressure between 46–74%

Without pressure:

Knee → 79,78 %

Knee → 76,22 %

Knee → 75,20 %

Knee → 62,55 %

Knee → 76,49 %

Mean value 74%.

With pressure:

Knee → 42,68 %

Knee → 46,24 %

Knee → 64,61 %

Knee → 40,08 %

Knee → 37,20 %

Mean value 46%.

Conclusion: This anatomical investigation has proved for the first time via intraarticular pressure measurements that it is possible to considerable reduce the intraarticular pressure using the Donjoy antivarus and antivalgus Orthesis.