One remaining concern is whether a specific joint implant will accelerate and/or start tumor development over a longer period of time.
As mentioned, it has been clearly shown that it is possible to have an indolent local malignancy in the bone marrow adjacent to an osteoarthritic joint. In cases of resection of the joint, i.e. on total joint replacement and if the tumor is only located adjacent to the joint, this might prevent tumor development. Whether or not this will have any influence on the MOM SR concept is unclear, and it is unlikely considering the generalized nature of MDS and lymphoma. All the published long-term studies on small MOM bearings and hematological malignancies have been on total joint replacements where the proximal and distal parts of the joint have been resected, i.e. McKee-Farrar THA. Due to the vast amount of large MOM bearings inserted in the USA and the UK, this should be analyzed continuously and needs to be followed in the long term. Pooling and aggregation of the available ongoing MOM registry data from several countries is possible, but complicated because of data-protection laws. However, cross-checking of joint registries with the national cancer registries is warranted.
As an extension of our and others’ earlier published studies (Lewold et al. 1996
, Lidgren 2008
, Mackie et al. 2011
, Wagner et al. 2011
), we compared the incidence of hematological malignancies taken from the national cancer database in Sweden, bone biopsy tumor findings from the large Australian Hip OA cohort, and data from OA patients in the prospective national knee prosthetic register cohort in Sweden (going back to 1975 and covering all clinics). The aim was therefore to determine whether there has been—in addition to the baseline level of malignancies seen in bone marrow at primary prosthetic replacement—a long-term increase in blood malignancies related to joint implants releasing low levels of metal particles. This could give some indication of the follow-up necessary for MOM bearings releasing several orders of magnitude higher levels of metal ions.
Assuming that the disease duration of hematological malignancies was equal in the Australian and Swedish knee cohorts, an approximate annual incidence of 31/105 could be calculated using the prevalence of indolent blood malignancies in femoral heads taken from the large Australian study (De Steiger et al. 2011
). This is almost the same incidence as was observed using data from the Swedish study (Wagner et al. 2011
) in the 10 years prior to knee replacement: 34/105. After surgery, the Swedish incidence of blood malignancies was 137/105 (unpublished data). The corresponding incidence in the general population of Sweden, had it had the same sex-, age-, and calendar-year distribution, would have been 81/105 before the operation and 122/105 afterwards. Part of this increase in incidence may have been due to changes in the cohort’s distribution of age, sex, and calendar year.
Furthermore, the low preoperative incidence level may have been affected by the fact that the preoperation cohort was a selected population, as it is likely that it included healthier individuals than in the general population, with respect to hematological malignancies. Consequently, the most severe cases were automatically excluded. In addition, patients with multiple diseases and in poor condition because of comorbidities may have been prevented from having surgery, thereby excluding them from the preoperative knee cohort.
However, irrespective of whether the increase was caused by other confounding factors such as frequent radiation exposure and virus infection, or indeed metal exposure, these data can be seen as a baseline reference for an extended follow-up of large MOM bearings.
The blood malignancies diagnosed in the bone marrow at prosthetic replacement could not fully explain the low but clear increase found after long-term knee replacement. We therefore propose that the pre-existing condition of lymphoid aggregates in OA may be further activated by metal ions after implantation. Depending on the stage of the condition, the combination of OA pathology in bone marrow and the reaction to metal ions after implantation can contribute to the development of the blood malignancies.