ENDO-Modell rotating knee prosthesis: A functional analysis

Abstract

Material damage in the joint mechanism of the ENDO-Modell Rotating Knee prosthesis is most frequently found in prostheses retrieved during exchange operations from patients with severe varus deformity and hyperextension. The bending stresses resulting from these conditions apparently lead to material damage if the functions of stabilising structures such as the capsule, ligaments and muscles are deficient. Malposition during rotation, in contrast, does not hinder the axial force flow and by itself does not cause any significant incidence of material damage. Judging from the distribution of the different types of damage to the joint bush, the damage appears to arise in a particular sequence beginning with migration of the polyethylene bush and buckling around the rim, followed by the formation of cracks which eventually lead to the bush being completely torn out of its socket. Premature material failure is accelerated by high body weight which was observed mainly in the male patients in the group studied here. Careful handling of joint structures with economic notching in the lateral ligaments during surgical exposure of the joint are, in addition to correct axial implantation of the prosthesis, indispensable pre-conditions for long service life of the implant. In view of the premature material failure described here, implantation of the ENDO-Modell Rotating Knee prothesis has to be considered critically in cases with severe axial deviation, particularly when this is coupled with high body weight and activity level, severe fixed flexion contracture, loss of bone substance and irreparable deficiency of muscle and soft tissue. Such conditions are often found in patients with advanced rheumatoid arthritis or neurological disease, and in the elderly. For these patients, the Hinge mechanism is the better alternative, in spite of the disadvantage of immediate transmission of rotational forces into the tibial component.