An animal study on repair of long distance nerve defect by nerve lengthening

Abstract

AIMS: Segmental loss of a peripheral nerve can be a challenging reconstructive problem because direct repair of such injuries is often not possible. Autografting is currently used to repair the segmental defect of peripheral nerve, but has its inherent problems including the need to sacrifice a healthy nerve, sensory loss associated with the sacrificed nerve, the availability of donor nerves, and the need to cross two sutured sites. Another repair method is bio-artificial nerve conduits grafting, whose efficacy has been proved for short nerve defect repair. But the nerve regeneration is often limited and functional recovery is poor for repair the long distance defect. An alternative strategy without nerve grafting is nerve lengthening, which has provided satisfactory results for short distance nerve defect. However, the efficacy of nerve lengthening for long distance nerve defect is unclear. Low intensity pulsed ultrasound (LIPU) has been shown to accelerate healing of the bone fracture and bone non-union. Conceivably, LIPU could also promote nerve regeneration. The combined effect of nerve lengthening and LIPU is still unknown. The purposes of this study is to investigate 1) the effect of primary nerve lengthening for repair a long distance nerve defect in rabbit sciatic nerve; 2) the effect of LIPU on enhancing regeneration of the lengthened nerve; 3) the effect of nerve lengthening on its associated muscle mass and bone mineral density; 4) the expression of the nerve growth factor (NGF) and S-100 protein in rabbit sciatic nerve after lengthening. METHODS: Sciatic nerve in rabbit was selected as nerve lengthening model in this study. 66 adult New Zealand white rabbits were assigned into 5 groups. Group 1, left legs of 30 rabbits, single nerve cut and direct end-to-end coaptation; Group 2, right legs of 30 rabbits, 30mm nerve segment resection and primary nerve lengthening; Group 3, left legs of another 30 rabbits, 30 mm nerve segment resection and nerve autografting; Group 4, right legs of another 30 rabbits, 30mm nerve segment resection and nerve lengthening combined with LIPU treatment. Group 5, both legs of the rest 6 rabbits, which was normal control group. Utilizing the customized nerve lengthening device, the proximal nerve stumps of Group 2 and 4 were lengthened by 1mm/day for 30 days. Meanwhile LIPU (30mW/cm2) treatment was applied to Group 4 for 20 min daily for 30 days. For Group 1-4, 6 rabbits were sacrificed at each of the five post-surgery time points at week 4, week 6, week 8, week 12 and week 16. The motor nerve conduction velocity (MNCV) was assessed by electrophysiology, the nerve diameter and the gastrocnemius muscle wet weight were measured , the bone mineral density (BMD) of proximal tibia trabecular bone was evaluated by peripheral quantitative computed tomography (pQCT) , the nerve fiber density and the expression of nerve growth factor and S-100 protein were analyzed by immunohistochemistry. All the data for each group and time point were analyzed by one-way analysis of variance (ANOVA), followed by LSD as post hoc test. RESULTS: MNCV was calculated from the compound muscle action potential (CMAP), which was recorded firstly in Group 1 at week 6 and at week 8 in other surgery groups. At week 16, compared to the normal control group, the MNCV value of 4 surgery groups were still significantly lower. Group 1 had recovered to 63.11% of normal MNCV value, significantly better than Groups 2-4 (p<0.001). The MNCV result in Group 4 (45.08%) was greater than Group 2 (38.54%, p=0.045) and Group 3 (38.46%, p=0.039). Histomorphometry of myelinated fiber density showed that all 4 surgery groups had lower nerve fiber density (P<0.05) compared to the normal control. At week 16, the nerve fiber density in Group 4 was significantly greater than in Group 3 (p=0.005) while Group 2 had no significant difference with Group 3 (p=0.093). Group 1 had better recovery than other three groups (P<0.05). The results of the gastrocnemius muscle wet mass showed denervated muscle mass decreased more than 50% at week 4, and went on decreasing to around 35% of the original mass at week 16 for all Groups 1-4. At week 4, BMD of all 4 surgery groups decreased to 85%-90%. After that, the BMD was further decreasing for Group 1-4. At week 16, the BMD of Group 1 (82.1%) was greater than the other 3 surgery groups (Group 2: 65.2%, Group 3: 63.7% and Group 4: 72.3%)(p<0.05). Immunohistochemical analysis on sciatic nerve revealed that the expression of NGF and S-100 in Group 2 and 4 were significantly higher than Group 1 and 3 at Week 4 and 6. At Week 8, the positive staining for both NGF and S-100 was still observable in Group 4, but not in Groups 1-3. The expression of S-100 protein in Group 4 came back to normal at week 12. Merged positive expression and nuclei could be observed in merged pictures of immunostaining for either NGF or S-100. CONCLUSIONS: The present study revealed that primary nerve lengthening technique was effective to repair long distance nerve defect in rabbit model, and the effect was comparable with autografting methods. Moreover, according to the results of MNCV, myelinated fiber density, and expression of S-100 protein, the combination of nerve lengthening and LIPU treatment could promote nerve fiber regeneration. Nerve lengthening could stimulate the NGF and S-100 protein expression, which may be a potential mechanism of the nerve repair and regeneration.