An in vitro model to explore subchondral perfusion and intraosseous pressure

Abstract

Background: Little is known about subchondral perfusion physiology. We developed a 3Rs (Replace, Reduce, Refine) compliant in vitro calf foot model to explore perfusion and intraosseous pressure (IOP).

Methods: Calf feet were catheterised and perfused with serum. IOP was measured at three sites, the metacarpal diaphysis (MCD), metacarpal subchondral epiphysis (MCS) and proximal phalanx diaphysis (PPD) using intraosseous needles with pressure transducers and digital recorders. Fresh (< 4 h post mortem) and old feet (> 4 h post mortem) were perfused at different pressures, with and without a proximal tourniquet.

Results: There was a wide range in basal IOP with a mean IOP of 30.0 mmHg, SD 14.4, range 7.6 mmHg to 52.7 mmHg (n = 40 records) in 15 subjects. There was no significant difference between the three sites tested (p = 0.54, 0.12 and 0.051). At each individual site IOP correlated with perfusion pressure (r = 0.993). With a proximal venous tourniquet, IOP increased from 15.1 mmHg (SD 11.3 mmHg) to 44.9 mmHg (SD 24 mmHg), p < 0.0001, n = 9. Filling and emptying curves during perfusion and with using a tourniquet were similar, indicating that the model behaves in an elastic hydrodynamic manner. In fresh feet IOP peaked after about 1 min irrespective of perfusion pressure, possibly due to auto regulation. Older feet showed a continuously rising IOP and became oedematous. There was no significant difference in IOP between fresh and old feet perfused with serum at 150 cms pressure for 1 min.

Conclusion: Though basal intraosseous pressure varies, IOP behaves predictably. IOP measurements reflect the perfusion microclimate at the individual needle tip. This 3Rs compliant model will be used for further exploration of subchondral perfusion physiology with loading.

Keywords: In vitro model; Intraosseous pressure; Perfusion; Physiology; Subchondral; Tourniquet.

Fig. 1

Difference in IOP at 60 s perfusion at the different sites tested, Blue - MCD - metacarpal diaphysis, Red - MCS - metacarpal subchondral, Green - PPD - proximal phalanx diaphysis n = 40 sites in 15 subjects. For each individual site IOP was correlated with perfusion pressure, r = 0.993, error bars SD

Fig. 2

Rise in IOP with time among 15 subjects at different perfusion pressures, Blue - perfusion at 150 cms, Red – perfusion at 100 cms and Green – perfusion at 50 cms pressure, error bars SD

Fig. 3

Additional rise in IOP with a proximal tourniquet for 60 s and effect of removal of the tourniquet after a further 60 s with continued perfusion of serum at 150 cms pressure, error bars SD

Fig. 4

Relative rates of rise and fall with perfusion at 150 cm serum, n = 15, error bars SD

Fig. 5

For fresh feet IOP peaked at about 1 min with no further increase up to 100 seconds (t-test p = 0.4). Blue = 150 cm, Red = 100 cm, Green = 50 cm perfusion pressure, error bars SD.

Fig. 6

Older feet had a different filling curve profile with continued perfusion. They become increasingly oedematous with no obvious IOP end point. Blue = 150 cm, Red = 100 cm, Green = 50 cm perfusion pressure, error bars SD