Capsule thickness correlates with vascular density and blood flow within foreign-body capsules surrounding surgically implanted subcutaneous devices.

Medical devices implanted in subcutaneous tissue elicit wound-healing events that encapsulate the device. To investigate the foreign-body response to devices of clinically relevant size, we studied the relationship between capsule thickness, vascular density, and blood flow within the capsule. In nine canines, we implanted 40 subcutaneous devices (polysulfone n=14, titanium n=17, silicone-coated n=9). Devices and surrounding tissue structures were harvested at 208 ± 142 (mean ± standard deviation) days after implantation. Tissues were processed for histological analysis with light microscopy. Foreign-body capsule (FBC) thickness and vascular density were determined. A subset of five animals was instrumented via thoracotomy for microsphere injections and reference blood sampling. In these animals, regional blood flow was determined in mL/min/g of tissue in the FBC to examine the relationship between capsule thickness and blood flow. One-way analysis of variance and linear regression analyses were performed. The FBC thickness was 0.92 ± 0.67 mm for all devices studied and was not influenced by device material. Capsule thickness correlated with vascular density for all devices (R(2)=0.85, P<0.0001) and for each device material (polysulfone R(2)=0.85, P<0.0001; silicone R(2)=0.95, P<0.0001; titanium R(2)=0.87, P<0.0001). Capsule thickness correlated modestly with blood flow for all devices studied (R(2)=0.59, P<0.0001) and for each device material (polysulfone R(2)=0.77, P<0.001; silicone R(2)=0.65, P=0.10; titanium R(2)=0.52, P=0.03). Capsule thickness, vascular density, and blood flow were similar within individual animals but variable across animals. Material type did not influence the thickness of FBCs surrounding devices of clinically relevant size. Capsule thickness correlated strongly with vascular density and modestly with blood flow within the capsule. Individual animals exhibited variable foreign-body responses.