Abstract
Up until this point in time, total knee replacement implants have relied on standardised sizes and shapes. The design process for the ‘off-the-shelf’ implants has typically involved designing a standard size implant and then scaling the design up and down to provide a series of standard sizes. More recently, some suppliers have paid more attention to providing sizes that meet the particular needs for either women or men, but these implants are largely standard designs with adjustments to the medial to lateral width or the anterior to posterior depth.
To design an implant that not only provides the correct size for every patient's knee, but more importantly to provide an implant that duplicated the patient's exact geometry is the goal. A CT scan is obtained of the patient's lower limb. The CT data is converted into a surface model of the knee joint with proprietary software. The surface model is then utilised to create a near exact match of the articular surface in a knee femoral component. The sagittal geometry is preserved for the medial, trochlear and lateral ‘J’ curves with correction for disease as required. The coronal trochlear and condyle geometries are engineered surfaces that respect the laws of knee design for low contact stress. The bone cuts are individualised for each femoral component based on maximising bone preservation and utilising design rules that are based on finite elemental analysis and fatigue testing. The tibial articular surface geometry is derived from the femoral component. Separate medial and lateral inserts are supplied in varying thicknesses that allow precise balancing of the joint.
Patient specific instrumentation is supplied with the implant that allows either femur first or tibial first techniques.
