In this cross-sectional study, we found that central osteophytes protruding from the normal curvature of the femoral head into the articular cartilage were a common feature of patients with OA. Healthy subjects also had central osteophytes, but they were not as common and were smaller than in patients with OA. Marginal osteophytes are a hallmark of osteoarthritis. This was clearly apparent in the present study as the patients with OA had larger and more frequent marginal osteophytes than the healthy subjects.
To our knowledge, central osteophytes have not previously been quantified using histology. McCauley et al. investigated central osteophytes from 200 patients who were referred for MR imaging of the knee. In that study, central osteophytes were defined as focal excrescences that extended from the cortical surface and were surrounded by articular cartilage on all sides. This definition is similar to the definition of central osteophytes used in the present study. McCauley et al. found that central osteophytes were associated with more articular cartilage defects and meniscal tears. Moreover, patients with central osteophytes were significantly older and had a higher BMI than patients without central osteophytes. Increased weight and age are known to predispose to osteoarthritis. Interestingly, in the present study, the central osteophytes were slightly more common than marginal osteophytes in healthy subjects. It has been shown that bone growth, in the form of marginal osteophytes, occurs before a loss of articular cartilage could be detected. Therefore, it is not unreasonable to suggest that central osteophytes could also be an early event of OA, especially as the size and frequency of central osteophytes were greater in patients with OA than in healthy subjects. Furthermore, 17% of the central osteophytes had denuded bone in patients with OA compared to 4% of the healthy subjects. However, this difference was not statistically significant, but indicate that the central osteophytes invade the articular cartilage from the bone-cartilage unit. Currently, the formation of marginal osteophytes, joint space narrowing, and subchondral bone sclerosis are the only features determined in a radiographical OA assessment. However, these features are, nevertheless, not sensitive enough to detect OA at an early stage. Therefore, we suggest that the central osteophytes investigated in the present study may be an early marker for diagnostic imaging of OA.
It is well known that marginal osteophytes grow by a form of endochondral ossification with progressive changes of cellular proliferation, differentiation, and elaboration of intercellular matrix5. However, it is currently unknown how central osteophytes form. A possible mechanism for the progression of central osteophytes could be that they form as a consequence of microcracks in the calcified cartilage and subchondral bone. This increases bone turnover and may result in HDMP, supposing that HDMP initiates reactivation of the secondary ossification centre. The articular cartilage may be lost due to endochondral ossification as illustrated in Fig. 3, and not as a consequence of fragmentation leading to a process of wear and tear. However, longitudinal studies are needed to investigate this hypothesis.
A weakness of the current study is that it is a cross-sectional study, and we can, therefore, only speculate on how the central osteophytes develop. Another weakness is that some areas of the femoral head from patients with OA were devoid of articular cartilage. Hence, in some areas, it was not possible to define whether central osteophytes were present or not. Thus, our estimates for the number of central osteophytes in patients with OA were likely underestimated, while this was not the case for the healthy subjects. Lastly, the healthy subjects were slightly younger than the OA patients, although this difference was not statistically significant.