A two-dimensional morphometry-based model of interstitial and transcapillary flow in rabbit synovium

Area of Science:

• Biophysics
• Physiology
• Biomechanical Engineering

Background:

• The synovial lining regulates joint fluid volume and composition via specialized capillaries.
• High intra-articular pressures (IAP) increase synovial hydraulic conductance and alter interstitial pathways.

Purpose of the Study:

• To model trans-synovial flow and assess how altered pathway geometry and interstitial matrix conductivity (Ki) contribute to conductance changes at varying IAP.
• To evaluate local flow patterns and the impact of capillary wall permeability assumptions.

Main Methods:

• Developed a 2D trans-synovial flow model using morphometric data at low (5 cmH2O) and high (25 cmH2O) IAP.
• Applied Darcy's law and the Starling principle to finite elements, iterating Ki and capillary wall conductance to match experimental data.
• Compared models with localized versus uniformly distributed capillary permeability.

Main Results:

• Matched experimental data with a Ki of 1.4-2.1 x 10⁻¹⁵ m⁴ s⁻¹ N⁻¹ at low IAP, comparable to other fluid-confining tissues.
• Uniform permeability assumptions significantly underestimate pericapillary pressure gradients.
• Synovial deformation accounted for 24-50% of conductance increase at high IAP; a rise in Ki was necessary to explain the remainder.

Conclusions:

• Synovial deformation and increased interstitial matrix conductivity contribute to elevated hydraulic conductance at high IAP.
• Model highlights limitations of uniform permeability assumptions and the need for quantitative biochemical analysis of synovial tissue under pressure.