Formation in a Rat Median Nerve Model – Influence of Distal Stump and Muscular Coating

The purpose of this study was to investigate neuroma formation and the impact of distal stump factors on this process in a rat median nerve model. In three different groups, the median nerve was exposed in the axilla and a gap was created. In the first group, a short gap of 1 cm (short gap group) was created while, in the second, a long gap of 2 cm (long gap group) was made between the proximal and distal nerve stump. A further group of animals was used to analyze the development of neuroma formation under circumstances of burying the proximal stump into adjacent muscle with the presence of a long gap of 2 cm (muscle-covered group). The use of different gap lengths between the proximal and distal stump should allow one to gain more information about possible dilution effects of distal nerve stump factors that may contribute to the size of neuroma formation. In the muscle-covered group, burying the proximal stump into the pectoral muscle was used in combination with a long defect of 2 cm. Nine months post procedure, animals were sacrificed and histologic analysis was performed. The area of the neuromas in cross section was measured and the neural-to-connective-tissue ratio was estimated.

The results of the investigation of cross-sectional area demonstrated that neuroma formation was significantly higher in the short-gap group (5.33 mm², 95% confidence interval (CI) 4.77 to 5.88 mm²) than in the long-gap group (4.05 mm², CI 3.5 to 4.6 mm²), with the smallest neuroma formation found in the muscle-covered group (2.53 mm², CI 1.97 to 3.08 mm²). The percentage of neural tissue was highest in the muscle-covered group (55.6%, CI 46.99 to 64.27%) and the long-gap group (42.3%, CI 33.62 to 50.9%) and lowest in the short-gap group (34.1%, CI 25.5 to 42.78%), indicating neural tissue to increase with increasing distance and muscular covering of the proximal stumps.

The results of this study demonstrated a strong correlation between proximal stump neuroma formation and distal stump distance, indicating direct impact of distal stump factors on the proximal stump. These factors coming from the distal nerve stump were best blocked when the proximal nerve stump was completely isolated when buried into the adjacent muscle. For clinical application, the authors recommended not only burying of the proximal stump into a skeletal muscle, but also surgical augmentation of the gap between the proximal and distal parts.