A Pressure Bound for Wheelchair Cushions and a Low-Cost Cushion Specification That Respects It

Long-term wheelchair users who sit for many hours on thin or worn cushions develop pressure ulcers over the ischial tuberosities, and pressure ulcers can become deep, infected, and fatal. This paper computes, with a stated first-order load-spreading model, the peak interface pressure and the longest safe sitting interval for six cushion options for user masses of 40-110 kg. The seat carries 75% of body weight, the contact patch grows with immersion depth, and peak pressure is the mean pressure multiplied by a concentration factor for the ischial tuberosities. Peak pressures are compared with a capillary closing pressure of 32 mmHg, used as an input, and converted to safe intervals with a pressure-time relation approximating the published Reswick and Rogers curve. At 70 kg the computed peak pressure is 97.0 mmHg on a 50 mm flat foam slab (safe interval 1.52 h), 77.3 mmHg on a 100 mm flat slab (2.13 h), 59.4 mmHg on a commercial contoured foam cushion (3.17 h), and 43.8 mmHg on an air-cell cushion (4.99 h). A specified low-cost layered contoured cushion - a 50 mm reconstituted chip-foam base under a 50 mm medium-foam top with soft ischial-well inserts, in a washable cover - computes to 57.3 mmHg and 3.34 h at 70 kg and 68.1 mmHg and 2.57 h at 90 kg, and keeps the safe interval at or above the stated 2.0 h threshold across the stated mass range of 40-100 kg, with a minimum of 2.32 h at 100 kg. Its parts cost is $22.00. The model is first-order only: it contains no tissue mechanics and no shear, and it does not protect a user who cannot reposition and has no repositioning schedule or alarm. SIMULATED, NOT MEASURED: nothing described has been built, sat on, or tested on people.