Three-dimensional analysis of subchondral cysts in hip osteoarthritis: an ex vivo HR-pQCT study

Abstract

Introduction: Subchondral cysts are deeply related to the pathogenesis of osteoarthritis (OA), but the factors contributing to cyst formation are not well known. A three-dimensional analysis of subchondral cysts at the micro-structural level was conducted using a high-resolution peripheral quantitative CT (HR-pQCT), and their relationships with cartilage attrition and subchondral bone microstructure were investigated.

Methods: Femoral heads extracted from ten female patients with hip OA were scanned using an HR-pQCT at a voxel size of 41μm. The volume fractions, numbers, and sizes of the cysts were measured in the subchondral bone region under the area of cartilage loss. Furthermore, the areas of cartilage loss, as well as the microstructure of the subchondral bones, were also measured, and their correlations with the cysts were analyzed.

Results: The volume fractions of cysts within subchondral bone regions varied from 2% to 33%, the numbers of cysts varied from 6 to 87, and the sizes varied from 1mm(3) to 657mm(3). There was a positive correlation between the number of cysts and bone volume (r>0.8, p<0.01).

Conclusion: The degree of cyst formation showed a wide distribution in number and volume, and there was a close relationship between multiple cyst formation and bone sclerosis, which might be caused by reactive bone formation that occurred around each cyst.

Keywords: HR-pQCT; Microstructure; Osteoarthritis; Subchondral bone; Subchondral cyst.

Fig. 1

(A-C): A cartilage loss region is defined as the femoral head outline uncovered by soft tissues, and its surface area is measured. (D): A subchondral bone region is defined as a region 5 mm under the cartilage loss region. (E-G): Subchondral cysts are defined as spaces >1 mm in diameter and connected with the cartilage loss regions, and their volume and number are measured. By eroding bone marrow and cyst spaces 0.5 mm from their surfaces, spaces <1 mm in diameter vanish. In this step, most of the normal bone marrow spaces could be deleted. By dilating the remaining spaces 0.5 mm from their surfaces and extracting only the spaces connected to cartilage loss regions, subchondral cysts could be obtained. (H-I): Subchondral trabecular structure is measured in the subchondral bone region, excluding cyst regions.

Fig. 2

3D CT images of (A) femoral head with OA, (B) subchondral cysts, (C) cartilage loss region, and (D) subchondral trabecular bone. The subchondral bone region is shown as green and soft tissue as yellow. The different colors of subchondral cysts mean that there is no connectivity between them. The following cyst related parameters are measured: Cyst.V: Total volume of all cysts in the subchondral bone region. Cyst.V/TV: Cyst volume fraction. Cyst.N: Total number of cysts in the subchondral bone region. Cyst.N/TV: Number of cysts per unit volume. CystV.Ave: Average volume of each cyst. CystV.SD, CystV.Min, CystV.Max: SD, minimum, and maximum volume of each cyst.

Fig. 3

Distribution of subchondral cysts in the subchondral bone region (yellow line). The volume and number of cysts are extremely different among individuals. Cyst volume fraction (Cyst.V/TV) ranges from 2.2% to 32.7%.

Fig. 4

Cyst.N/TV and BV/TV have a positive correlation (P<0.004, R=0.82), suggesting that multiple cyst formation is related to bone sclerosis. (A-B): 2D images of OA femoral heads show that the trabecular bone sclerosis occurs particularly around the cysts. These areas of bone sclerosis might be caused by the reactive bone formation against bone absorption or the change of mechanical load distribution in the subchondral bone.