|Abstract / Summary
Total knee arthroplasty (TKA) is a popular and well-established treatment of pain-full knee osteoarthritis. Cemented tibia components are the considered to be the “golden standard”, however, aseptic loosening of the tibial component is still a major problem especially in younger patients. Theoretically, uncemented fixation of the tibia components by bone ingrowth offers a possible solution to this problem. The NexGen® Trabecular Metal™ (Zimmer®) tibia components are designed with a highly porous trabecular tantalum surface to enhance bone-ingrowth.
We compared a monoblock versus a modular polyethylene design version of this tibial component in a randomized study, and 67 patients below the age of 70 years scheduled for a TKA because of knee osteoarthritis were randomized. All patients received the same uncemented cruciate retaining Titanium Nexgen Flex® femur component and a cemented all-poly patella component. Marker based radiostereometric analysis (RSA) with two years of follow-up was used for evaluating the migration of the tibial implants (study I), and we also investigated the periprosthetic adaptive bone remodelling of the proximal tibia using dual energy X-ray absorptiometry (DEXA) (study II). Using DEXA we also examined changes in bone mineral density (BMD) of the distal femur around the femoral component used in both groups (study III).
In the second part of the thesis we investigated a possible relation between tibia BMD measured by DEXA preoperatively at the proximal tibia and migration of uncemented tibia components (study IV). We combined the subjects (n=92) of the above mentioned randomized RSA-study, and patients from another prospective randomized RSA study evaluating Vanguard PPS® (Biomet) versus Vanguard Regenerex® (Biomet).
In study I, we found the expected pattern of migration for uncemented tibia implants with the highest average migration (expressed as Maximum Total Point Motion (MTPM)) initially within the first three months in both the modular group and the monoblock group reaching 0.85 mm and 0.58 mm respectively. Hereafter the MTPM migration curves flattens to reach 1.01 mm and 0.65 mm after 12 months, indicating stabilization of the implants. However, there was a tendency towards higher migration in the modular group. The difference became statistically significant after 12 months (p=0.02), and at 24 months (p=0.02), where the average MTPM was 1.15 mm and 0.72 mm in the modular and monoblock groups respectively.
In study II, we found significant changes in BMD, and after 24 months BMD had decreased by 15.0% (p=0.02) and 13.3% (p=0.02) in the medial and lateral tibial condyles of knees with monoblock implants. In patients with modular implants, BMD only showed minor changes. The differences in BMD changes between groups were statistically significant in both the medial (p=0.03) and lateral (p=0.02) region of interest (ROI).
In study III, we found significant changes in BMD of the distal femur and after 24 months of follow-up, BMD had decreased by 23.6% in the anterior ROI behind the anterior flange of the prosthesis (p<0.001), 10.1% in the posterior ROI (p<0.001) and 5.5% in the most proximal ROI (p<0.001).
In study IV we found that linear regression analysis performed to predict MTPM from the preoperative BMD of the proximal tibial showed statistically significant relations at all follow-ups
with R2-values of 20-37%. Continuous migration (MTPM from 12 to 24 months) was also statistically significant related to preoperative BMD, however, with a much lower R2-value of 11%.