In this study we have analysed the magnitude of cartilage loss throughout cartilage plates of the knee in patients with advanced osteoarthritis, before TKA, using quantitative MR imaging technology. In addition, we studied the relation between tibial, femoral, and patellar cartilage loss and valgus/varus malalignment of the knee in advanced osteoarthritis.
Limitations of this study include the modest sample size and its cross sectional nature. However, to investigate cartilage loss and its relation to alignment longitudinally in advanced osteoarthritis (before knee arthroplasty), observation periods of 10 years and more are required. The strength of the study is that the accuracy of the measurements was confirmed in the same study sample by applying established invasive measurements postoperatively,6
and that, in contrast to radiography20
quantitative MR imaging allowed us to differentiate tibial, femoral, and patellar cartilage loss accurately in advanced osteoarthritis.
Previous MRI studies have focused on cartilage volume as a quantitative end point. However, cartilage volume scales strongly with bone size and this therefore coincides with a large intersubject variability, both in healthy volunteers and in patients. This variability severely limits the capacity to differentiate effectively between healthy subjects and patients in cross sectional studies. Though cartilage volume can be normalised to body weight, body height, age, and other factors,21
the correlation with anthropometric variables is relatively week,22,23
rendering this approach relatively ineffective. As the individual subchondral bone area (tAB) can be reliably determined from MR image data6,7,19
and correlates more strongly with cartilage volume than other variables, normalisation to tAB can be used to effectively enhance T scores of cartilage morphology for osteoarthritis patients in cross sectional studies.7
Note, however, that this requires a different approach to cartilage segmentation, as not only cartilage but also the denuded bone interface area of each cartilage plate needs to be traced in each slice. This tracing must exclude peripheral osteophytes which render the bone interface area larger than it is before the onset of osteoarthritis.
The current data confirm that normalisation to total subchondral bone area is more effective than normalisation to other variables such as body weight and height, when trying to discriminate between healthy subjects and patients with osteoarthritis. This applied without exception to all cartilage plates of the knee examined here.
When comparing cartilage loss in different cartilage plates of the knee, we found a substantial amount of heterogeneity and only a weak correlation between femoral and tibial cartilage loss. These findings are important as they clearly suggest that femoral and tibial cartilage plates should be measured separately and that measuring cartilage loss in just one of them is insufficient for estimating cartilage loss in the other.14
Cicuttini et al24
described correlation coefficients of around 0.75 between femoral and tibial cartilage volume in healthy subjects and patients with moderate osteoarthritis. They concluded that it may therefore be sufficient to measure only tibial cartilage in femorotibial osteoarthritis. Note that a similar correlation was observed in our current study (r
0.65 between medial tibial and femoral cartilage volume), but that the correlation became weaker when cartilage volume was normalised to tAB (r
0.51). This effect is readily explained by the fact that the intersubject variability of cartilage volume was reduced when normalising it to bone size. Our findings suggest that in advanced osteoarthritis only approximately 25% of the variability in femoral cartilage loss is explained by the cartilage loss measured in the tibia and vice versa, and the correlations are even weaker between the medial and lateral femorotibial compartments, and with the patella. This suggests that the proportion of femoral and tibial cartilage loss is highly variable between osteoarthritis patients, and that it is essential to measure both cartilage plates separately. Other studies have determined an aggregate value for cartilage volume in the tibia and femur.25,26
As the factors determining the proportion of femoral and tibial cartilage loss in femorotibial osteoarthritis are currently unknown and need to be explored further, we suggest that both cartilage plates should be determined separately.
As expected, we found a significant correlation between femorotibial cartilage loss and malalignment of the knee, and this result is in agreement with previous longitudinal studies employing radiography11
and MR imaging.12
We cannot determine retrospectively the extent to which malalignment was the cause or effect of cartilage loss, but the range in static alignment of the knee among the subjects was large (14° varus to 19° valgus). Also, a recent longitudinal study has confirmed that the degree of malalignment at baseline was associated with prospective cartilage loss in the relevant femorotibial compartments.12
An interesting finding of our study is that, in patients with severe osteoarthritis, the correlation of cartilage loss and alignment was greater for the tibia than for the femur. This observation has not been made in patients with moderate osteoarthritis.12
Given the moderate sample size, this finding should be interpreted with caution and will need to be confirmed in larger cohorts. However, the current results indicate that malalignment may represent a stronger determinant of tibial than femoral cartilage loss, and this may be a potential reason for the heterogeneity in femoral versus tibial cartilage loss in advanced osteoarthritis.
When considering alignment independent of valgus and varus, there was a weak correlation with patellar cartilage loss. Although the correlation did not reach statistical significance in this sample, it was almost as strong as that between alignment and femoral cartilage loss. These findings indicate that patellar cartilage loss may also be associated with alignment of the knee, potentially because of the higher pressure in the medial or lateral patellar facet. Future studies should thus investigate the cartilage in the medial and lateral facet separately, to determine whether valgus malalignment is specifically correlated to lateral facet cartilage loss, and varus malalignment to medial facet cartilage loss.
In summary, this study shows that quantitative MR imaging is most discriminative between healthy subjects and patients when cartilage volume is normalised to the total subchondral bone area. Cartilage loss was found to be highly variable in patients with advanced osteoarthritis, this also applying to the femur and tibia of the same compartment. Femorotibial cartilage loss was associated with alignment of the knee, but the association was found to be considerably stronger for the tibia than for the femur. The study also provides some evidence that patellar cartilage loss may be weakly associated with alignment, the correlation being of the same order of magnitude as for femoral cartilage. The current data show that, in cross sectional studies, cartilage volume must be normalised to bone interface area to provide useful T scores of cartilage loss in osteoarthritis. Patellar, tibial, and femoral T scores should be measured separately, due to the larger heterogeneity of tibial cartilage loss in advanced osteoarthritis. Our findings further suggest that the mechanism and cause–effect relation of malalignment and tibial, femoral, and patellar cartilage loss should be examined more closely in larger cohorts of patients with osteoarthritis, using quantitative MR imaging technology.