Heterotopic Ossification (HO) is a common finding in the radiographs of patients who have had total hip replacement surgery (42–57%). HO is responsible for pain and limitation of postoperative motion in 3–10% of these patients. This study was initiated to find out if pulsed lavage affected the incidence of HO.

A prospective randomised double-blind trial was initiated consisting of 115 primary total hip arthroplasties. Intra-operative irrigation was provided by a 50 ml syringe and limited to 500 ml of room-temperature normal saline or pulsed lavage with three litres of room-temperature normal saline. The grade of HO at one year was classified according to Brooker et al. Statistical analysis of the results was made using the chi-squared test and the Kruskal-Wallis test.

The incidence of HO in this group of primary total hip arthroplasties was 57. 4%. The majority of cases with HO were graded 1 or 2 (42. 6%). No statistical significance was found to exist between the two methods of irrigation in relation to HO (chi-squared p value = 0. 456). From an analysis of the known risk factors for HO, only the type of osteoarthritis was shown to statistically influence the incidence of HO.

The process of HO is thought to be as a result of the differentiation of mesenchymal cells into osteoprogenitor cells. Theories have proposed that the inducing agent and/or the mesenchymal cells arise from bone at the time of the operation, although systemic agents have also been proposed. If the inducing agent or precursor cells were released from the bone intraoperatively, thorough irrigation could be expected to have an association with a lower incidence of HO. The lack of correlation between irrigation and incidence of HO indicates this is unlikely to be the mechanism.