In this study we found that physical activity level was an important factor affecting periprosthesis bone density after TKA and moderate activity was associated with less BMD loss than either low activity and high activity. This research showed that periprosthesis bone density loss was the lowest at activity levels of 6 and 7, while it increased at both low and high activity levels.
A nonlinear (parabolic) relationship between the UCLA activity rating after TKA and rBMD% in the proximal tibia suggested that moderate physical activity was beneficial to maintain bone mass, while insufficient or excessive functional exercise aggravated bone loss. This phenomenon is consistent with Wolff's law[20], which indicates that mechanical stress stimulates bone formation, while disuse leads to bone loss. Petersen et al[9] revealed that a decreased load led to rapid bone loss, while an increased load led to a small increase in BMD in the tibial condyles. This might suggest that a low activity level loads the tibia at a smaller or lower frequency than does a moderate activity level. Gallo, J et al. [21] reported that for man, high levels of physical activity, and a high historical level of physical activity were associated with a higher risk of reoperation; for women, the early reoperation was associated with lower physical activity. It seemed that both low and high levels of activity were detrimental to TKA.
The aim of the present study was to evaluate the effect of activity level, determined by the UCLA activity rating, on BMD in periprosthetic bone after TKA and suggest the best activity level to guide clinical rehabilitation postoperatively. Although the rBMD measured on standard X-ray images is not the true BMD, Hernandez-Vaquero et al[10] have proven that the relationship between the rBMD measured on standard X-ray images and the true BMD measured by DXA is linear (Cronbach’s correlation of 0.72 to 0.92), namely, rBMD could serve as an alternative to BMD.
After TKA, the bone density around the prosthesis decreased, and the medial side decreased more than the lateral side. In fact, the decrease in BMD following TKA had been widely reported and was generally believed to be the result of a biomechanical response to stress shielding of the tibial component[8, 22-26]. Surgery altered the intra-articular environment and osteotomy exposed cancellous bone. The implant is in direct contact with cancellous bone, and the pressure load directly affected cancellous bone, contributing to bone loss. Due to the stress shielding effect, the stress stimulation on the bone around the prosthesis was reduced, and bone tissue absorption occurred, which was manifested as bone loss. Postoperative limb immobilization also contributed to disuse osteoporosis.
We also found that the changes of BMD in the medial, lateral, and distal regions were inconsistent. After correction of malalignment of lower limbs, the load distribution in the medial side was larger than that in the lateral side, so medial bone density decreased even more.
The significant influence of activity level on rBMD after 1 and 3 years could be attributed to adaptive bone remodeling and changes in load, as Jaroma, A et al. reported that tibial metaphyseal periprosthetic bone was remodeled after TKA due to mechanical axis correction [8]. However, the postoperative activity level had no significant effect on BMD after 5 years. Seitz et al[27] found that bone density loss demonstrated a reparation phase and a stabilization phase and that no significant BMD change was observed during the stabilization phase. Therefore, bone might be in the stabilization phase at 5 years and not be affected by activity level. An increase in BMD was seen at a moderate activity level at both 1 and 3 years. As postoperative bone loss is general and increases the risk of prosthetic loosening and revision, it makes sense to reduce bone loss in patients after TKA. Therefore, we suggest that patients should undergo rehabilitation training with a moderate activity level within 3 years after TKA. This means that within 3 years of surgery, patients should be engaged in functional exercise at a moderate intensity level (neither too much nor not enough), so that bone density loss is minimized.
When considering the influence of load on BMD, it was unexpected that the activity level had a significant effect on the BMD in the lateral and medial metaphysis. The greater reduction in the BMD in the medial metaphysis might be the result of the typically higher medial load distribution in a varus knee preoperatively, adaptive bone remodeling and changes in the load following correction of the mechanical axis. However, it was unexpected that the activity level had no significant effect on the BMD in the distal metaphysis; the reason for this finding is unclear and merits further investigation, especially regarding load transfer and distribution. A significant difference in rBMD% between lateral and medial regions was observed in the regions closest to the base plate, and the greatest difference in BMD between proximal medial and lateral regions was found in the regions closest to the base plate and further from the stem of prosthesis. However, differences in rBMD% between the lateral and medial regions closest to the stem of the prosthesis were not significant. These results might suggest that the load is evenly distributed on both sides of the stem of the prosthesis.
There are some limitations to this study. Firstly, the sex distribution was atypical with 84% females. However, according to relevant statistics, the ratio of male to female knee osteoarthritis is about 1:7, so female subjects are the majority. Secondly, in this study, the number of follow-up subjects decreased over time. We revealed a interesting relationship between activity level and periprosthetic bone density, but a further study that included more patients and complete data is needed. Thirdly the BMD measured on standard X-ray images is a relative value, and while it can reflect the trend of the true BMD, there may be quantitative error with respect to the true BMD.