Neuroma 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 group, 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 formation of neuroma while 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 the possible dilution effects of distal nerve stump factors that may contribute to the extent of neuroma formation. To completely exclude distal nerve stump influences, burying the proximal stump into the pectoral muscle in the muscle-covered group was used in combination with a long defect of 2 cm.

Nine months post operation, the 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 increasing with increasing distance and muscular covering of the proximal stump.

The results of this study demonstrated a strong correlation between proximal stump neuroma formation and distal stump distance, indicating the 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 applications, the authors recommended not only burying the proximal stump into a skeletal muscle but, in addition, surgical augmentation of the gap between the proximal and distal portions.