Long-term changes in bone mineral density following total knee replacement.

Bone density was measured using dual photon absorptiometry and dual xray absorptiometry below the tibial component under the medial and lateral plateaus, under the central peg, and distal to the central peg. Measurements of bone density were taken at 1 week, 6 weeks, 6 months, and 1 year after surgery in 31 patients. Seven of these patients also had measurements taken 8 years after surgery. The average age of the patients at surgery was 69 years old. One week after surgery, the mean bone density from all regions was 0.77 g/cm2 in women compared with 1.14 g/cm2 in men. Between 6 weeks and 6 months, the bone density below the tibial component declined inconsistently in most patients, but by 1 year the bone mineral density was statistically equal to the initial bone density at 1 week after surgery. Eight years after total knee arthroplasty, there was a 36.4% decrease in bone mineral density between men and women was not statistically significant. Medial plateau, lateral plateau, under the peg, and distal to the peg bone density changes tended to be the same. Maximum rates of bone resorption in patients with osteoporosis of the same age group have been reported to be between 1% and 2% per year. In this study, after an initial stable period, the bone density of the proximal tibia beneath the tibial component consistently decreased at a rate of 5% per year. Surgeons must consider this relatively large decrease in bone density if they are relying on bone for long-term stable fixation of the prosthesis.(ABSTRACT TRUNCATED AT 250 WORDS)