The Importance of Different Modularity of the Polyethylene Insert for Tibial Component Migration and Adaptive Bone Remodeling After Uncemented Total Knee Arthroplasty Using Trabecular Metal Technology (TMT) Zimmer NexGen®, (Monoblock vs. Modular Design)

Introduction In general a primary TKA is a durable and well-functioning solution with a 10-year implant survival of more than 90 % (Robertsson et al. 2001). Both implants to be tested in the present study have a trabecular metal coating of the tibial component. However, a different manufacturing technique of the baseplate with one having a monoblock polyethylene insert and the other a modular polyethylene insert have been used. Failure secondary to polyethylene wear is ranked as a frequent cause for revision. Many issues have been identified as contributory to polyethylene wear; the quality of the polyethylene, manufacturing process, nature of the tibial-femoral or patellofemoral articulation, area of contact, sterilization process, and issues pertaining to modularity (backside wear) (Lombardi et al. 2004, Lombardi et al. 2008). Long-term clinical studies have documented the success of direct compression-molded monoblock tibial components (Weber et al. 2002). However, the benefits of modularity are widely recognized and include the ability to fine tune soft-tissue balance after implantation of metallic devices.

New techniques are developed and the patients´ demands for increased durability of the implants are increasing. Failure because of aseptic loosening of the tibia component remains one of the leading courses of revision surgery in TKA (Sierra et al. 2004, Danish Knee Arthroplasty Register, Annual Report 2010). It is believed that both the implants, planned to be tested in the present study, with the trabecular surface coating will facilitate bone ingrowth and secure a better fixation to the host bone, thus increasing the implant survival and improving the function of the prosthesis.

One of the implants NexGen® , monoblock(TMT) has been used for routine clinical use abroad, but also in Denmark for some years, and so far the clinical results have been good (Bobyn et al. 2004). There have been made two randomized controlled studies using RSA mesurement of the implant showing promising results with good stable fixation at follow up between twelve and twenty-four month (Dunbar et al. 2009, Henricson et al. 2008).

The other type of prosthesis is identical but has a modular polyethylene design of the tibia component. Modular designs are used frequently in most TKA designs and have also shown good long-term results. However, there are potential problems concerning back-side wear and aseptic loosening because of polyethylene foreign body reaction (Lombardi et al. 2004, Lombardi et al. 2008, Weber et al. 2002).

There have been made previous randomized RSA studies evaluating uncemented modular TKA designs with good results showing stable fixation at twelve and twenty-four month (Dunbar et al. 2008, Hansson et al. 2005, Hansson 2008).