Effects of hypertonic dextrose injections in the rabbit carpal tunnel

Abstract

Objective: This study investigated the effects of different doses of hypertonic dextrose injection on the carpal tunnel subsynovial connective tissue (SSCT) and median nerve in a rabbit model.

Methods: Thirty-eight New Zealand white rabbits weighing 4.0-4.5 kg were used. One forepaw carpal tunnel was randomly injected with one of five different treatments: saline-single injection; saline-two injections 1 week apart; 10% dextrose-single injection; 20% dextrose-single injection; or 10% dextrose-two injections 1 week apart. Animals were sacrificed at 12 weeks after the initial injection and were evaluated by electrophysiology (EP), SSCT mechanical testing and histology.

Results: There were significant increases in the energy absorption of the SSCT in the 10% dextrose-double injection group compared to the saline injection groups. SSCT stiffness was also significantly increased in the 10% dextrose-double injection group compared to the other groups. There was a significant increase in the thickness of the SSCT in the 10% dextrose-double injection group compared to the saline-single injection group and a significant decrease in the nerve short-long diameter ratio in the 10% dextrose-double injection group compared to the saline-single injection group. There were no changes in EP among the groups.

Conclusions: SSCT fibrosis is present for up to 12 weeks after dextrose injection; multiple injections have bigger effects, including what appears to be a secondary change in nerve flattening. This model may be useful to study the effects of external fibrosis on nerve morphology and physiology, such as occurs clinically in carpal tunnel syndrome.

Figure 1. Histological Measurements

a. Subsynovial Connective Tissue Thickness 1: FDS-FDP Thickness, 2: FDS-FDS Thickness The shortest distance between middle finger FDS tendon and flexor digitorum profundus (FDP) tendon, and the average of the shortest distance between index and middle, middle and ring finger FDS tendons were measured at the mid-carpal tunnel level. b. Nerve Short-Long Diameter Ratio 1: Short Diameter, 2: Long Diameter The short and long cross sectional diameter ratio of the median nerve was measured. To measure the short-long diameter ratio, the long diameter (axis) was defined as the longest diameter of the median nerve, while the short diameter was defined as the longest diameter which was perpendicular to the long axis. The short diameter was divided by the long diameter to generate the ratio.

Figure 1. Histological Measurements

a. Subsynovial Connective Tissue Thickness 1: FDS-FDP Thickness, 2: FDS-FDS Thickness The shortest distance between middle finger FDS tendon and flexor digitorum profundus (FDP) tendon, and the average of the shortest distance between index and middle, middle and ring finger FDS tendons were measured at the mid-carpal tunnel level. b. Nerve Short-Long Diameter Ratio 1: Short Diameter, 2: Long Diameter The short and long cross sectional diameter ratio of the median nerve was measured. To measure the short-long diameter ratio, the long diameter (axis) was defined as the longest diameter of the median nerve, while the short diameter was defined as the longest diameter which was perpendicular to the long axis. The short diameter was divided by the long diameter to generate the ratio.

Figure 2. Mechanical Testing

a. The results of ultimate load The ultimate load was greater in the dextrose injection groups, but the difference did not reach statistical significance (P=0.08). b. The results of energy absorption Same mark shows significant difference between each combination (P<0.05). There were significant increases in the 10% dextrose-double injection group than in the saline injection groups (P<0.05).

Figure 2. Mechanical Testing

a. The results of ultimate load The ultimate load was greater in the dextrose injection groups, but the difference did not reach statistical significance (P=0.08). b. The results of energy absorption Same mark shows significant difference between each combination (P<0.05). There were significant increases in the 10% dextrose-double injection group than in the saline injection groups (P<0.05).

Figure 3. Mechanical Testing (Stiffness)

Significant difference between 1: Saline-single and 10% dextrose-double 2: Saline-double and 10% dextrose-double, 3: 10% dextrose-single and 10% dextrose-double, 4: 20% dextrose-single and 10% dextrose-double (P<0.05). There were significant increases in the 10% dextrose-double injection group compared to the saline injection controls (P<0.05).

Figure 4. Histology of Subsynovial Connective Tissue (x200)

The SSCT was irregular and thickened in the dextrose injection groups compared to the saline injection groups.

Figure 5. Subsynovial Connective Tissue Thickness

Same mark shows significant difference between each combination (P<0.05). The thickness of the SSCT was significantly larger in the 10% dextrose-double injection group compared to the 10% saline-single injection group.

Figure 6. Nerve Short-Long Diameter Ratio

Identical marks indicate a significant differences (P<0.05). The nerve short-long diameter ratio was significantly lower in the dextrose injection groups than in the saline-single injection group (P<0.05).