# Abstract

 

## Background and purpose The trabecular metal tibial monoblock component (TM) is a relatively new option available for total knee arthroplasty. We have previously reported a large degree of early migration of the trabecular metal component in a subset of patients. These implants all appeared to stabilize at 2 years. We now present 5-year RSA results of the TM and compare them with those of the NexGen Option Stemmed cemented tibial component (Zimmer, Warsaw IN). 

## Patients and methods 70 patients with osteoarthritis were randomized to receive either the TM implant or the cemented component. RSA examination was done postoperatively and at 6 months, 1 year, 2 years, and 5 years. RSA outcomes were translations, rotations, and maximum total point motion (MTPM) of the components. MTPM values were used to classify implants as "at risk" or "stable". 

## Results At the 5-year follow-up, 45 patients were available for analysis. There were 27 in the TM group and 18 in the cemented group. MTPM values were similar in the 2 groups (p = 0.9). The TM components had significantly greater subsidence than the cemented components (p = 0.001). The proportion of "at risk" components at 5 years was 2 of 18 in the cemented group and 0 of 27 in the TM group (p = 0.2). 

## Interpretation In the previous 2-year report, we expressed our uncertainty concerning the long-term stability of the TM implant due to the high initial migration seen in some cases. Here, we report stability of this implant up to 5 years in all cases. The implant appears to achieve solid fixation despite high levels of migration initially. # Results 

## Standard outcomes and follow-up All standard outcome measures for the 45 patients in this study are given in [Table 1](#). Originally, 37 patients were randomized to receive the trabecular metal component and 33 received the cemented component. Age and BMI were similar in the 2 groups at the time of admission. There were no differences in subjective measures (WOMAC) between implant groups at any follow-up point. ::::table-wrap

::: caption
Subject demographics and WOMAC scores. Only matched WOMAC data from
earlier follow-ups are included. Values are mean (SD)
:::

```
                     Trabecular metal   Cemented   p-value
```

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```
                     (n = 27)           (n = 18)   (ANOVA)
```

Sex, M/F 10/17 8/10\
Age 60 (8) 61 (9) 0.7
Body mass index 32 (5) 34 (5) 0.2
WOMAC preoperatively 51 (20) 44 (18) 0.2
WOMAC at 6 months 22 (23) 18 (20) 0.5
WOMAC at 12 months 15 (16) 14 (22) 0.9
WOMAC at 24 months 13 (16) 15 (14) 0.6
WOMAC at 60 months 18 (18) 19 (18) 0.8:::: For the 5-year follow-up, 45 patients could be reached and were willing to participate. There were 27/28 of the trabecular metal subjects and 18/21 cemented subjects with 2-year data recruited. The reasons for loss to follow-up for the other 25 patients are documented in [Figure 1](#) and it happened mainly in the first two years of follow-up. Of the patients with a high degree of prosthesis migration in the original study, all 9 were recruited to the 5-year follow-up. All but 3 of the patients originally recruited to the study are still being followed by the operative surgeon, and there have been no revisions to date. The three patients who were exceptions had left the province. <figure> <p><img src="" /></p> <figcaption>Consolidated Standards of Reporting Trials (CONSORT) flow diagram.</figcaption> </figure> 

## RSA results The accuracy of the RSA system was 0.025 mm, 0.025 mm, 0.063 mm, and 0.026 mm for the x-, y-, and z- directions and maximum total point motion, respectively. The precision of the RSA system was 0.07 mm, 0.07 mm, and 0.11 mm for x-, y-, and z- translations, 0.16°, 0.15°, and 0.12° for x-, y-, and z- rotations, and 0.10 mm for maximum total point motion. At the 2-year follow-up, there were no subjects "at risk" in the trabecular metal group (0/28). At the 5-year follow-up, none of the trabecular metal components were found to be "at risk". At the 2-year follow-up, there were 4 subjects "at risk" in the cemented group (4/21). At the 5-year follow-up, 1 of the cemented components "at risk" was lost to follow-up, 2 stabilized (0.09 mm and 0.07 mm of movement between 2 and 5 years) and 1 continued to migrate (0.35 mm of movement between 2 and 5 years). 1 of the cemented implants found to be stable at 2 years migrated an additional 0.42 mm and was classified as being "at risk". However, retrospectively, this implant had somewhat anomalous 2-year data (0.64 mm, 0.45 mm, and 0.87 mm at 1, 2, and 5 years), where the difference between 1 year and 5 years would make the implant appear stable (0.23 mm in 4 years) but the difference between 2 years and 5 years would make the implant appear to be continuously migrating (0.42 mm in 3 years). The condition number and mean errors for this subject were within normal limits. All other implants that were found to be stable at 2 years migrated less than 0.3 mm between the 2- and 5-year follow-up, and were still classified as stable. The proportion of "at risk" components at 5 years was 2 of 18 (0.11, 95% CI: 0.03--0.33) in the cemented group and was 0 of 27 (95% CI: 0.0--0.13) in the trabecular metal group (p = 0.2). At the 5-year follow-up, there was no difference in MTPM between the cemented group and the trabecular metal group (p = 0.9) ([Table 2](#) and [Figure 2](#)). MTPM values for individual subjects are shown in [Figure 3](#). Compared to the cemented components, the trabecular metal tibial components had more subsidence (p = 0.001) ([Table 2](#)). ::::table-wrap
::: caption
Summary of radiostereometric analysis (RSA) data by group and follow-up
:::

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```
                                    6-month follow-up   12-month follow-up   24-month follow-up   60-month follow-up   p-value**[^a^](#)**                                                   
```

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```
                                    TM                  Cemented             TM                   Cemented             TM                    Cemented        TM              Cemented        

                                    (n=30)              (n=24)               (n=29)               (n=23)               (n=28)                (n=21)          (n=27)          (n=18)          
```

Mean maximum total 0.85 0.54 0.87 0.57 0.92 0.65 0.98 0.79 0.9
point motion (range) (0.07--3.11) (0.12--1.99) (0.12--3.25) (0.16--1.87) (0.06--3.40) (0.18--2.12) (0.16--3.11) (0.06--2.16)

Lateral/medial translation (SD) --0.02 (0.13) 0.01 (0.14) --0.02(0.13) --0.00(0.14) --0.01 (0.14) 0.02 (0.16) --0.02 (0.16) 0.01 (0.26) 0.7

Superior/inferior translation (SD) --0.34 (0.31) 0.00 (0.08) --0.34 (0.34) --0.01 (0.11) --0.35 (0.35) --0.03 (0.10) --0.38 (0.37) --0.04 (0.11) 0.001

Anterior/posterior translation (SD) 0.02 (0.12) 0.02 (0.29) 0.03 (0.13) 0.06 (0.18) 0.02 (0.12) 0.09 (0.18) 0.01 (0.12) 0.08 (0.29) 0.3

Anterior/posterior tilt (SD) --0.33 (0.73) 0.02 (0.43) --0.40 (0.80) --0.02 (0.30) --0.39 (0.80) --0.05 (0.40) --0.45 (0.90) --0.11 (0.26) 0.1

Internal/external rotation (SD) 0.19 (0.38) --0.15 (0.47) 0.15 (0.39) --0.14 (0.44) 0.19 (0.45) --0.08 (0.60) 0.17 (0.45) 0.04 (0.82) 0.5



## Varus/valgus tilt (SD) --0.05(0.72) 0.07 (0.32) --0.11 (0.73) 0.06 (0.32) --0.10 (0.75) 0.06 (0.33) --0.12 (0.73) --0.02 (0.20) 0.6

\*\*^a^\*\*5-year results only:::: <figure> <p><img src="" /></p> <figcaption>Maximum total point motion (MTPM) (mean and standard deviation) according to the duration of follow-up in the trabecular metal and cemented groups. Lines joining data points do not indicate continuous measurement but are included to show the pattern of migration.</figcaption> </figure> <figure> <p><img src="" /></p> <figcaption>Individual maximum total point motion (MTPM) according to the duration of follow-up in the trabecular metal group (blue) and the cemented group (red). Lines joining data points do not indicate continuous measurement but are included to show the pattern of migration.</figcaption> </figure> At the 2-year follow-up, 9 subjects in the trabecular metal group had shown very high migration in the first 6 months postoperatively (> 1.0 mm). All of these implants migrated less than 0.2 mm between 1 and 2 years and were not considered to be "at risk" At 5 years, all of these implants continued to show stability with an average change in MTPM of 0.10 mm over the 3 years between follow-ups. Using the marker configuration models, independent bead motion was detected in 5 of the high-migration cases, between the postoperative and 6-month follow-up examinations. The error of fit of the model marker was reduced dramatically (0.103 mm to 0.0466 mm on average) when the bead in question was removed from the analysis. The position of the bead with respect to that of the bead in the model was inferior, indicating that the motion was not due to loose beads, which would have traveled radially. The magnitude of independent bead motion was from 0.3 mm to 0.5 mm in all 5 cases. At the 5-year follow-up, there was no change in the position of the independently moving bead---indicating that the deformation of the base plate was permanent.


Extracted trial result:
Outcome: subsidence (continuous)
Intervention: The trabecular metal tibial monoblock component (TM)
Comparator: cemented component.
Intervention group size: 27
Intervention mean: -0.38
Intervention standard deviation: 0.37
Comparator group size: 18
Comparator mean: -0.04
Comparator standard deviation: 0.11
Notes: Used numerical values for Superior/inferior translation (SD). Numerical values are from 5-year follow up.