emented total knee arthroplasty has been considered the accepted standard with predictable and durable results [3.4.8.9.10.16. 21.22.25.28.29]. Cementless total knee arthroplasty have induced preservation of bone stock, shoter operating time, ease of revision. Porous tantalum has been introduced as metallic implant material for total knee arthroplasty. The high volumetric porosity (70 ~ 80%), low modulas of elasticity (3~4 MPa), and high friction characteristics of trabecular metal make it conducive for biological fixation [17]. Many groups have reported satisfactory outcomes with cementless total knee arthroplasty using trabecular metal monoblock tibial components that contain porous tantalum as the primary material [5.9.14.18. 24.34].All reports was used trabecular metal monoblock and radiostereometric results for stastical analysis. No manuscript were evaluated by imaging the postoperative computed tomography. We hypothesized that trabecular metal modular type (posterior stabilized type: PS type) is affected the influence of stress shielding with mid flextion instability. The aim of present study was to compare the bone ingrowth under the peg of trabecular metal modular tibial component between cruciate retaining type (cruciate retaining: CR type) and PS type.

From October 2011 to April 2013, 46 primary total knee arthroplasties were perfomed in 46 patients with porous tantalum modular tibial component (Trabecular Metal; Zimmer, Warsaw, IN). We divided CR and PS type selectively. In all cases, the TKA surgical procedure was performed by one author (T. K.) and was minimally invasive surgery, with a skin incision of 8-11 cm. Patient walking was permitted from the day following the operation. The Knee Society Score (KSS) and The Western Ontario Mcmaster Universities Osteoarthritis Index (WOMAC) were measured preoperarively and up to two years postoperatively by two authors (T.K and T.O). Some case in which postoperative simple X-ray examination and, 3D Planning reposition simulation postoperative evaluation (ZedKnee: LEXI. Co., Ltd, Tokyo. Japan) showed malalignment and, cases was excluded from the analysis. We dicided that inclusion criteria was that all cases of tibial component alignment was within 3°varusvalgus to neutral alignment. For determination of the ossification density at 3,6,9,12,15,18,21 months postoperatively, a phantom(Taisho-Toyama Pharm. Co., Ltd, Tokyo. Japan ) consisting of a cylinder composed of a material corresponding to cortical bone and filled with a material having a bone density corresponding to cancellous bone was placed under the knee (Fig. 1), and imaging was then performed by multi detector-row computed tomography (MDCT). From the obtained images, the cancellous trabecular structure was visualized three-dimensionally with 3D osteomorphometry software (TRI/3D-BON64; RATOC System Engineering Co., Ltd., Tokyo, Japan), and the structural parameters were calculated (Fig. 2). The structural parameters subjected to analysis were those recommended by the American Society for Bone and Mineral Research [19]: bone mineral content/total volume (BMC/TV) values, representing mineralized bone volume as a percentage of total volume. In the assessment of BMC/TV values by MDCT imaging of the proximal tibia, the joint prosthesis itself generated artifacts, which prevented accurate delineation. Therefore, the overall region was divided into 6 regions of interest (ROIs), consisting of two cylindrical volumes, each 16 mm in diameter and 8 mm in height, with their tops 0.6 mm below the medial or lateral peg, with each further divided into 2 half-cylinders. [ROI. 1 (Medial), ROI. 2 (Lateral), ROI. 3 (Medial-Anterior), ROI. 4 (Medial-Posterior), ROI. 5 (Lateral-Anterior), and ROI. 6 (Lateral-Posterior) (Fig. 3). Statistical analysis(SPSS version 17.0 software: SPSS, Chicago, IL, USA) was performed for relative change in ossification density ,immediately and after surgery every 3 months in each of the two groups by the Mann-Whitney U test and for comparison between the two groups by the paired t-test. P values of less than 0.05 were considered significant .This study of these patients was approved by the Institutional Review Board and they were informed of the risk of radiation exposure required

No significant difference was recognized in age and Body mass index, gender, knee society score, between two groups before the operation (Table . 1).No osteoporosis therapeutic agent was administered in the two groups .There were no significant difference in KSS and WOMAC at 1-year follow-up between the two groups (Table . 2). No prosthetic fracture and prosthetic migration and prosthetic infection were detected during the follow-up periods. At 3.6.9.12.15.18.21 months after operation, the BMC/TV values in ROI. 1 (Medial) was no significant difference in the two groups (Fig . 4). The BMC/TV values in ROI. 2 (Lateral) was significant higher in PS type than CR type at 3 and 18. 21 months after operation (p<0.01, p<0.05. Fig . 5).

The BMC/TV values in ROI. 3 (medial-anterior) and ROI.4 (Medial-Posterior) was no significant difference in both group at all periods after operation (Fig . 6.7). The BMC/TV values in ROI.5 (Lateral-Anterior), and ROI.6 (Lateral-Posterior) was significant higher in PS type than CR type at 3.18.21 months after operation (p<0.01, p<0.05. Fig. 8.9). The monoblock type consists of a porous tantalum ingrowth surface compression molded into it and two hexagonal porous tantalum pegs for initial stability. The modular type consists of a titanium alloy modular tray with a porous tantalum layer that also includes two hexagonal pegs and includes a central boss (small circular peg) in the central posterior of the tray that is used with a lock down screw [11]. Early migration for porous tantalum monoblock tibial component was not continue but soon stabilized [12]. Porous tantalum increased initial stability and accerated bone ingrowth and retented of bone stock through reduced stress shielding [20]. The flexibility of porous tantalum modular tibial component plate may produce radiolucencies at higher rate and it exhibited higher bone ingrowth than porous tantalum monoblock tibial component and implantation time was positively correlated with bone ingrowth for monoblock tibial components [13.30].We evaluated the bone mineral content/total volume (BMC/TV) values between CR and PS type of porous tantalum modular tibial component up to twenty one months. In the current study, No manuscript were evaluated the BMC/TV values of CR and PS type by imaging the postoperative computed tomography.

The relationship between CR and PS type had the same factor for postoperative activites and accuracy position of total knee arthroplasty. There were siginificantly higher BMC/TV values for PS type than CR type in ROI.2.4.6 (Lateral, Lateral-Anterior, Lateral-Posterior) at three and eighteen , twenty one months after operation. We did not find a significant difference in the relative change in BMC/TV values in ROI.1.3.5 (Materal, Materal-Anterior, Materal-Posterior) between PS and CR type postoperatively. The presents study suggests that PS type associated with the post-cam mechanism was caused reactively higher BMC/TV values than CR type, associated with bone sclerotic change in medial tibial plateau for medial knee osteoarthritis at 3 months. We discussed that trabecular metal modular tibia (PS type) with midflextion instability was caused reactive cancellous stabilized and not occurred the influence of stress shieldings in lateral site under peg of tibial component than CR type, postoperative 18 months later. The present study had several limitations that should be considered. First, this study was prospective study, but patients could not be randomized. Additional research is required to determine the long-term benefits of porous tantalum modular tibial component for CR and PS type. Second, there are a relatively small size with short term follow up. Thid, computed tomography was not perfomed before operation, furthermore BMC TV values was not measured in view of radiation exposure. In present study we recongnized that trabecular metal modular tibia (PS type) was not affected the influence of stress shieldings.in spite of than CR type, post-operative 18 months later to 21 months.

This study revealed that trabecular metal modular tibia (PS type) with midflextion instability was caused reactive cancellous stabilized and not occurred the influence of stress shielding in lateral site under peg of tibial component than CR type postoperative 18 months later.

The authors gratefully acknowledge the valuable contributions of Eriko Yamaguchi, and Norihiko Kono M, D and Nobuhito Nango Ph,D in perfoming independent radiographic analysis.