The Monk hip arthroplasty. Preliminary report on the uncemented standard Monk prosthesis

The Monk hip prosthesis is a one-piece block prosthesis consisting of a metal femoral component with the head enclosed in a polyethylene cup. The standard type is fixed without cement. Most of the hip movements take place between the metal and the polyethylene cup. 104 Monk arthroplasties were performed. 95 hips in 84 patients have been available for regular examination and follow-up. The follow-up period was from 12 to 30 months, an average of 19 months. Out of the 95 cases, 72 were classified as excellent, 10 as good and 13 as unchanged. The results obtained during the first year were comparable to those obtained with the Ring prosthesis, but during the following 1 1/2 years eight revisions had to be performed. The main reason for the deterioration was loosening of the femoral component, probably because in the standard type it is too short and the neck has too great a varus inclination, resulting in a rather strong tilting force. Because no cement has been used the revisions have been quite easy to perform. Another reason for failure has been wear of the polyethylene cup, in one case with a massive foreign body reaction. In spite of a short follow-up period this report illustrates the failures due to the design of the prosthesis, and therefore an improved construction is now in use.