Subchondral bone cysts regress after correction of malalignment in knee osteoarthritis: comply with Wolff’s law

Purpose

The purpose of this study is to discuss the mechanical function of subchondral bone cysts and its relationship with Wolff’s law.

Methods

One hundred forty symptomatic knees (120 patients) with osteoarthritis were subjected to MRI before high tibial osteotomy (HTO). Subchondral bone cysts (SBCs) were detected on 72 knees of these 140 knees. SBCs, bone marrow lesion (BML), and hip-knee-ankle (HKA) axis were measured by using validated methods. After HTO, the evolution of cysts was evaluated on MRI performed with a five year follow-up on the 72 knees with pre-operative cysts.

Results

At baseline, 70 (97%) of these 72 knees had a BML surrounding the SBCs; the maximum cyst volume was 874 mm³ and the average cyst volume 9. 6 ± 4.1 mm³. In the subregions where cysts were present, adjacent cartilage was still present without full thickness defects. The mean pre-operative hip-knee-ankle angle was 7.3 ± 3 degrees (0° to 14°) of varus and differed significantly (p = 0.01) between the 68 knees without cysts (average 3 ± 2 degrees) and the 72 knees with cysts (average 9.2 ± 4 degrees). Five years after HTO, the number of cysts had decreased, the maximum cyst volume was 532 mm³, and the average cyst volume was 6.3 ± 2.8 mm³.

Conclusion

Regression of subchondral bone cysts may be related to restoration of an appropriate load at the subchondral bone. With applying poroelasticity to bone mechaincs, this finding may suggest that SBCs and BMLs may be a physiological adaptation to mechanic overload. More basic research is needed to prove this matter.