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Matrix growth and plate-advection balance relates newly supplied nail material to movement and changing geometry of the nail plate. The nail matrix contains the growth-producing region, whereas the mature nail plate is transported material.^[ref] Measured advance is consequently a material-flow observation, not a direct count of cell divisions.

For an original one-dimensional conservation of mass model, define line density $m(x,t)=\rho (x,t)A(x,t)$, where $A$ is plate cross-sectional area, $\rho$ its density and $v$ its advance speed. A local production rate $s(x,t)$ per unit length gives the continuity equationIf the chosen downstream segment has negligible production or loss, $s=0$. At steady state its mass flux $mv$ is constant. Uniform density then makes the volume flux $Q=Av$ constant, recovering the simple width-times-thickness model.

For stipulated $Q=0.30\mathrm{m}{\mathrm{m}}^3/day$, cross-sectional areas of 3 and $2\mathrm{m}{\mathrm{m}}^2$ require speeds 0.10 and 0.15 mm/day. Equal production can therefore accompany unequal advance when geometry changes. Conversely, equal speeds do not establish equal production without knowing area and density.

For a fixed segment $[a,b]$, integrating the continuity equation givesIf inflow exceeds outflow, material accumulates rather than vanishing from the accounting. Growth, hydration-related geometry change and clipping must be assigned to appropriate terms before interpreting an apparent imbalance.

The nail-marking study measured displacement over known time intervals in 22 young adults.^[ref] Extending that design with repeated width and thickness measurements would test a nail-plate storage balance, rather than assuming steady geometry. Inferring a mitotic rate would require further information about cell size and conversion into mature material.