Abstract
Background
Polymethylmethacrylate (PMMA) has been used for total knee arthroplasty (TKA) as a method of fixation; however, its durability has been questionable for the long-term use because of the loosening after the cement deterioration, its vulnerability toward infectious resistance, and a smaller amount of healthy bone left for the knee revision surgery. Especially, a decrease of bone density on the proximal tibia has been believed to be triggered as a result of stress shielding. When compared with a cemented TKA, a cementless TKA reduces the amount of bone loss after surgery. In 1999, the Trabecular Metal (TM), with its main composition being the porous tantalum metal, became available as a choice of the porous cementless knee joint prosthesis. The characteristics of porous tantalum metal are its great affinity to the bone as well as its similarity to cancellous bone. The porous tantalum metal starts to bond with osteoblasts, and fills up 80% of porous structure in one year; therefore, it has been characterized by its higher initial fixation strength. However, it is questionable if strong fixation strength due to bone ingrowth between the tibial tray mainly made up with the porous tantalum metal and a cancellous bone will continually be kept. Bobyn, JD, Dunbar et al. have acknowledged the existence of bone ingrowth based on the radiographic evaluation; however, their data had not been quantified in their report. In this study, the bone ingrowth density have periodically quantified using 3D bone morphometric software (TRI/3D-BON64.RATOC) after taking CT of the knee joint prosthesis.
Material and Methods
From October 2011, we have reviewed 45 medial osteoarthritis knees that underwent MIS-TKA using Trabecular Metal Modular Tibia CR-type (Zimmer, Inc, Warsaw, Indiana). Ages range from 61–89 years (mean, 74.5 years), and 5 males (7 knees), and 32 females (38 knees) participated in this study. After taking CT picture with the Phantom under lower extremities, the bone ingrowth density are quantified utilizing 3D bone morphometric software (TRI/3D-BON63.RATOX). Measured areas are divided into 6 zones that are right under the pegs of TM femoral component, and the bone ingrowth density (BMC/TC) between TM and cancellous bone were periodically measured on 3, 6, 9, 12,15,18,21,24.27 months after the surgery. Also, intra-zone comparison were implemented by each period among Medial (Zone 1), Lateral (Zone 2), Medial Anterior (Zone 3), Medial Posterior (Zone 4), Lateral Anterior (Zone 5), and Lateral Posterior (Zone 6). Mann-Whitney U test and Student's t-test were used for statistical analysis. All cases of tibial component alignment was within 3 degree varus-valgus to neutral alignment.
Results
Bone ingrowth and formation was increased to nine months from six months after surgery and was reduced to 12 months postoperatively. But bone resorption was aboloished 18 months after surgery without influence stress shieldings. In detail, the result was significant higher bone ingrowth and formation in medial than lateral region. I recognized that lateral lesion was affected by stress shieldings. The results was not significant difference of bone ingrowth between medial anterior and posterior region but significant difference of bone ingrowth in lateral posterior than lateral anterior.
