Abstract
INTRODUCTION:
Clinical densitometry studies indicate that following TKR implantation there is loss of bone mineral density in regions around the implant. Bone density below the tibial tray has been reported to decrease 36% at eight years after TKR. This bone loss (∼5%/year) is substantially greater than osteoporosis patients in the same age group (∼1–2%/year) and could contribute the loss of mechanical support provided by the peri-implant leading to loosening of components in the long term. High patient mass and body mass index have also been implicated in increased loosening rates, and was thought to be due to high stress or strain on the tibial constructs. These findings suggest that peri-implant bone strain may be affected by time in service and patient factors such as body mass.
The goal of this project was to assess the proximal tibial bone strain with biomechanical loading using en bloc retrieved TKR tibial components. Note that the implants were not obtained from revision surgery for a loose implant, but rather after death; thus the implants can be considered to be successful for the lifetime of the patient. We asked two research questions, guided by the clinical and laboratory observations: (1) are the peri-implant bone strain magnitudes for cemented tibial components greater for implants with more time in service and from older donors?, (2) is tibial bone strain greater for constructs from donors with high body weight and lower peri-implant BMD?
METHODS:
Twenty-one human knees with cemented total knee replacements were obtained from the SUNY Upstate Medical University Anatomical Gift Program. Clinical bone density scans were obtained of the proximal tibia in the anterior-posterior direction. Axial loads (1 body weight, 60/40% medial to lateral) were applied to the tibia through the contact patches identified on the polyethylene inserts. Strain measures were made using a non-contacting 3-D digital image correlation (DIC) system. Strain was measured over six regions of the bone surface (anterior (A), posterior (P), medial (M), lateral (L), postero-medial (PM), postero-lateral (PL)) (Figure 1).
RESULTS:
For a donor population of 54 to 90 years (78 ave) with 0 to 22 years in service (ave 9 years), the peri-implant bone strains ranged from 119 to 791 ue. Maximum strains exceeded 3000 ue. Peri-implant bone strains were greater for implants with more time in service (p = 0.044), but not age of the donor (p = 0.333) (Figure 2). Peri-implant bone strains were greater for donors with greater mass (p = 0.028) and lower bone density (p = 0.0039) (Figure 3).
DISCUSSION:
To the authors knowledge, these results show for the first time (using cemented tibial components) that bone remodeling after in-vivo service does not result in constant bone strain as would be expected for ‘homeostatic’ strain conditions. Even though loading was applied based on body weight, heavier donors had higher bone strains. Donors with more time in service also had higher bone strains. Combined, these results suggest that the supporting bone stock could diminish in some patients to the point at which bone failure occurs resulting in component migration.
