Vitreous collagen fragment dimensions after vitrectomy: a pilot study addressing the cut-rate enigma

to measure vitreous fragments generated at 5,000 and 20,000 cuts-per-minute (CPM) by means of Scanning Electronic Microscopy (SEM; fragments > 550 nm), Optical Profilometry and Dynamic Light Scattering (DLS; fragments 10-550 nm) and assess if cur-rate affects vitreous fragment size.

Vitreous samples at 5k and 20 K CPM were collected from 3 patients undergoing 25G pars plana vitrectomy for macular pucker. SEM and Profilometry measured fragment size in 6 drops from each sample, DLS analysed the remainder. Each patient contributed 2 samples at 5 K and 20 K CPM taken at the beginning of surgery.

The median volume was 1,153.6 µm3 (IQR 2,322.3 µm3) at 5k and 72,4 µm3 (IQR 923.4 µm3) at 20k, respectively (Mann-Whitney p = 0.074); the variability of vitreous fragment volume was higher for 5 K CPM group (p = 0.036). DSL median fragment dimension in the 10-550 nm range was 50.7 nm (IQR 87.8 nm) for the 5 K CPM group and 43.8 nm (IQR 95.9 nm) for the 20 K CPM group (p < 0.001).

A fourfold increase in cut-rate did not produce significantly smaller vitreous fragments but the 20 K CPM samples showed less variability. Fragments size spanned 5 orders of magnitude (from nanometers to hundreds of microns), challenging the assumption that blade action produces uniform fragments. Fragment size variability minimally affected by cut-rate, questions the blade action as the main or only factor shearing vitreous. Elongation stress and shear stress most likely play a prominent role.