deep and plastic deformation, both of \\hich induce permanenl deformation, also contribute Collie scaroivihc articular surfaces and on the backside of modular polvethyicr.-c components (72).

a-Dclaminatlon:

Repeated cyclic 'oading may cause subsurface fatigue fractures in one material and cause eventually detachment of relatively large subsurface-fragments (fatigue wear) (7).

During in vivo use, the tibial insert is subjected to compressive stresses immediately under the contact area, with a peak stress at a depth of 1-2mm below the surface. The stresses outside the contact area are reversed and are tensile in nature (72).

As a result, during the rolling-sliding motion of the femoral component, the subsurface region of the polyethylene is subjected to completely reversing cycles of stress (compression and tension). -This fluctuating environme"*- is the precursor to fatigue, leading to the initiation and propagation of cracks in the material (72).

Delamination occurs due to the initiation and propagation of subsurface cracks and can result in the removal of large (>0.5 mm) flakes of polyethylene wear debris (72).

Pitting is another fatigue-related phenomenon.  It results from                     the

coalescence of shallow subsurface cracks    or cracks initiated on                     the

surface (72).                                             '                                                                '

Delamination can result in the loss of geometric conformity, can alter the pattern of load distribution, and can eventually result in accelerated implant failure by disengagement or fracture of the tibial insert or wear-through (72).