DEXA STUDY ON MINERALIZATION REDISTRIBUTION UNDER THE TIBIAL BASE PLATE IN TOTAL KNEE ARTHROPLASTY: 38 KNEES WITH MINIMUM FIVE YEARS FOLLOW-UP

Abstract

Purpose: After implantation of a total knee arthroplasty (TKA) for osteoarthritis, early demineralisation of the superior tibial epiphysis occurs with modification of bone mineral density (BMD) in the two compartments. The long-term trend is not known to date. We report the results of 38 prostheses followed prospectively with densitometric measurements at minimum five years.

Material and methods: This prospective analysis included 38 TKA implanted for primary degenerative disease in patients with a mean age of 70±4 years at implantation, 60% women. Clinical assessment (IKS) and radiography (HKA) as well as osteodensitometry were recorded before surgery, at six months, one year, two years and five years. Bone mineral status was assessed using the densitometry of the femoral neck before surgery and at five years for all patients. The DEXA method was used for each knee on the anteroposterior film. Seven zones were defined around the tibial implant, in particular two under the medial and lateral plateaus, and under the stem. We studied changes in each zone over time. The alpha risk was set at 5%. Inter- and intraobserver reproducibility was 2.9% and 2.8% respectively.

Results: At five years follow-up, the mean IKS function score was 85±19 and the knee score was 918. The mean mechanical axis was 180±2° with symmetrical distribution. There were no progressive lucent lines. The BMD of the homolateral femoral neck did not change significantly (0.763 g/cm2 at inclusion and 0.750 g/cm2 at five years), unlike the natural evolution in a reference population (women -4.5%, men -2.4%). The mean BMD in the seven zones decreased significantly (11.6%, p< 0.0001. Mean BMD was 0.936 g/cm2 at inclusion and 0.863 g/cm2 at six months), 0.823 g/cm2 at five years. BMD decreased very strongly from 0 to 6 months (−6.51%, p< 0.0001) then more slowly to the end of the first year (−3%) and finally declining regularly, but non-significantly, at a slower rate from 1 to 5 years. Study of the seven zones showed a difference in changes in the BMD between the medial, lateral and stem zones. The two medial zones decreased significantly from 6.33% to 6.18% especially during the first year (−2.06% and −2.09%) and more moderately from 1 to 5 years (−1.6%, −2.65%). The lateral zones showed a greater average decline in BMD (−10.5%, −8.92%) between 0 and 5 years: −8.57% and −6.75% during the first year then at a slower rate. The greatest loss in BMD was found under the stem; −14.3% at five years. Here again, between 0 and 6 months the decline was rapid: −8.09%. It reached −12.74% at one year then varied little, −1% and −2% between 1 and 5 years.

Conclusion: 1) Bone remodelling under the tibial base plate occurs early after implantation of a TKA. It occurs during the first year (especially the first six months). 2) Remodelling is more pronounced laterally than medially (good realignment but persistent varus stress with greater stress on the medial than lateral side). 3) The greatest loss in BMD occurs under the stem. 4) BMD of the femoral neck remains stable, unlike the evolution observed in a reference population.