The study of tissue reaction to wear particles generated from metal on plastic articulation was revived by Willert and Semlitsch [1]. They examined 123 explanted endoprostheses of metal, almost exclusively chrome cobalt, and plastic: polyethylene or polyethylene terephthalate articulations. Their conclusion was stated clearly: “We believe that this was responsible for the loosening of endoprostheses with polyester components because an extensive foreign body reaction was found in the narrow spaces close to the prosthesis as well as in the joint cavity.

Our histological findings are supported by clinical and radiographic observations” [1].

It is probably this very detailed report, supported by easy to understand line drawings of wear particle pathways, that revived the issue of tissue reaction to wear particles in general and component loosening in particular. Questions were not asked, and explanations not given, how did wear particles manage to migrate to “marrow spaces close to the prosthesis.”?

With an increasing number of publications on the subject the pattern of “cause and effect” became accepted as the inevitable sequence of events. Radiographic appearances of localised endosteal bone erosion gave rise to the term: “osteolysis.” With time, demarcation of bone-cement interface of the cup was labelled: “linear osteolysis.” It is not clear who was the first to use the term “osteolysis” what is clear is that the term “osteolysis” has become synonymous with tissue reaction to wear particles – initially UHMWPE and the cause of component loosening. Harris summarised his views by stating that a number of observations, which appear to be unrelated, “can be drawn together to support this thesis that osteolysis is the dominant problem in total hip arthroplasty” [2].

(It should be pointed out that OSTEOLYSIS is a term defining “dissolution of bone, applied especially to the removal or loss of the calcium of bone.” Dorland’s Illustrated Medical Dictionary 39th Edition: 2000 p. 1334).

How, why or when do these UHMWPE wear particles enter “the marrow spaces close to the prosthesis?”

Wear particles are generated at the articulating surfaces due to their relative motion under load. It is unlikely that macrophages would be attracted to the metal-plastic articulation; wear particles are more likely to follow the pathways generated by fluid movement within what functionally is a foreign body bursa housing a neuropathic spacer. For the particles to reach “marrow spaces close to the prosthesis” [1] a free pathway must be available. Combined with changes in volume and pressure, erosion, cavitation and “osteolysis” will result – with UHMWPE wear particles – as passengers. It is only at this stage that macrophage reaction may become a part of the mechanism leading to bone resorption. Ingress of UHMWPE particles to narrow spaces must precede any macrophage reaction.

It is essential to appreciate a controversy does not concern facts but the explanation of the facts. It is the explanation of the facts that becomes the reason for action.

The facts are: wear, wear products, tissue reaction to wear products, bone resorption, endosteal cavitation and component loosening. The explanation of the facts attempts to correlate wear – as volume of UHMWPE particles shed into the tissues – and the incidence of component loosening,

For the purpose of the discussion it is assumed that neither the method nor the clinical application of the method of component fixation, is at fault.

Wear and loosening of the UHMWPE cup in the Charnley LFA was considered to be the one factor limiting the life of the arthroplasty [3]. This was the conclusion drawn from a review of 104 LFAs in 71 patients under the age of 40 at the time of the operation at a mean follow-up of 9.3 years (range 4–17). The number of publications on the subject confirmed the pattern [4–7].

Increasing depth of cup penetration releases an increasing volume of UHMWPE wear particles into the tissues which is reflected in the increasing incidence of cup loosening. This would suggest a biological cause of cup loosening.

The correlation between the depth of cup penetration and the incidence of cup loosening does not hold for the incidence of stem loosening (Table 37.1, Fig. 37.1). The incidence of stem loosening does not follow the pattern observed with the cup.