Our results have demonstrated that older age, female gender, and lower BMI are associated with reduced vastus medialis and lateralis cross-sectional areas. Participation in vigorous physical activity was associated with increased vastus medialis, but not vastus lateralis, cross-sectional area. Similarly, only increased vastus medialis cross-sectional area was associated with increased patella cartilage and bone volumes. These findings suggest that increased vastus medialis cross-sectional area may benefit patellofemoral joint health.
Our measure of vastus medialis and lateralis cross-sectional areas demonstrated expected relationships, with age and female gender being associated with a reduction in vastus medialis and lateralis cross-sectional areas, whereas BMI was associated with greater vastus medialis and lateralis cross-sectional areas. Though not examined specifically in this study, a reduction in muscle size has been shown to be common with aging [4
], and it has been shown that females have, on average, less muscle mass relative to males [17
]. In contrast, a larger BMI may be inherently associated with increased muscle cross-sectional area due to a larger body size. We have also demonstrated that participation in vigorous physical activity was associated with increased vastus medialis, but not vastus lateralis, cross-sectional area. A primary action of the distal portion of vastus medialis is to dynamically restrain the natural tendency of the patella to track laterally [1
]. Therefore, the relationship between vastus medialis cross-sectional area and participation in physical activity may, in the setting of vigorous physical activity, reflect vastus medialis hypertrophy in an attempt to restrain excessive lateral patella displacement.
We have previously examined the relationship between vastus medialis and lateralis cross-sectional areas and patella structures [12
] in an independent and smaller (n = 175) cohort of healthy women (mean age of 52 years). In this previous study, we demonstrated a positive association between vastus medialis cross-sectional area and patella bone volume, but not cartilage volume. In the present study, we examined an entirely independent cohort and substantiated all previously significant findings while also noting that increased patella cartilage volume is significantly associated with an increased cross-sectional area of vastus medialis. The discrepancy in cartilage volume results between the two independent studies may be attributable to the smaller sample size of our previous study. Moreover, in the present study, we included males and thus expanded the generalisability of our findings. Subsequently, the present study substantiates previous findings pertaining to both men and women. In the past and present studies, we have found vastus medialis, rather than vastus lateralis, to be the significant determinant of patella structures. The relative importance of vastus medialis at the patellofemoral joint is also supported by electromyography studies, which demonstrated that delayed activation of vastus medialis relative to vastus lateralis is associated with patellofemoral pathologies, including pain [2
] and subluxation/dislocation [3
How a greater vastus medialis cross-sectional area mediates increased patella cartilage and bone volumes is unclear. Previous studies have shown that reduced cross-sectional areas of local spinal muscles are associated with instability and low back pain [9
]. As indicated by extrapolating such data to the patellofemoral joint, it may be that a greater cross-sectional area of the vastus medialis muscle helps to prevent the natural tendency of the patella to track laterally [1
] and thus reduce any shearing damage that may occur to articular surfaces. This, in turn, may produce an optimal biomechanical environment and have a beneficial effect on patella structures, including cartilage and bone. Moreover, our recent longitudinal data have suggested that increased patella bone volume may be advantageous to patellofemoral joint health, as increased baseline bone volume was associated with a reduction in the rate of patella cartilage volume loss [20
]. Therefore, it may be that increased vastus medialis cross-sectional area benefits the patellofemoral joint via biomechanical effects on both patella bone and cartilage volumes. Longitudinal studies are required to determine whether increased vastus medialis cross-sectional area reduces the risk of long-term patellofemoral pathology.
We examined a healthy population without knee pain or pathology and our results cannot be generalised to symptomatic populations or those with established knee pathology. Nevertheless, studying a healthy population allows the identification of factors that may be associated with early structural changes at the knee and reduces the confounding effect of reduction in muscle size due to pain-related disuse. Furthermore, this study may have been limited by our method for assessing the cross-sectional area of the distal vastus medialis and lateralis muscles. We measured the vastus muscles at the MR slice 37.5 mm superior to the quadriceps tendon insertion at the proximal pole of the patella. This slice was chosen as it was the largest slice visible across all subjects. Moreover, we have calculated cross-sectional area rather than the physiological cross-sectional area (PCSA) that previously has been employed to investigate muscle-force relationships [21
]. Calculating the PCSA requires imaging of the entire length of the muscle fibre, which would be a costly and timely exercise using MRI. However, our measure of cross-sectional area showed the expected relationships with age, gender, and BMI. It may be hypothesised that people with larger muscles inherently have larger joint structures, including cartilage and bone volumes, but if this were the case, vastus lateralis would have been significantly associated with patella cartilage and bone. Moreover, the relationships observed in this study were independent of BMI, height, and weight (data not shown) as a measure of body size. Although the determinants of patella structure remain unclear, there is likely to be a complex interplay between the surrounding soft tissue support (for example, vastus medialis and lateralis muscles) and the bony geometry (for example, femoral sulcus angle and patella tilt). Therefore, future studies may benefit from examining other articular supports that may also contribute to patellofemoral joint structure and function.