The modified surgery is a simplified method for mouse or rat spared nerve injury model that requires only one ligation and one cut to injure both common peroneal and sural nerves.
Spared nerve injury (SNI) is an animal model that mimics the cardinal symptoms of peripheral nerve injury for studying the molecular and cellular mechanism of neuropathic pain in mice and rats. Currently, there are two types of SNI model, one to cut and ligate the common peroneal and the tibial nerves with intact sural nerve, which is defined as SNIs in this study, and another to cut and ligate the common peroneal and the sural nerves with intact tibial nerve, which is defined as SNIt in this study. Because the sural nerve is purely sensory whereas the tibial nerve contains both motor and sensory fibers, the SNIt model has much less motor deficit than the SNIs model. In the traditional SNIt mouse model, the common peroneal and the sural nerves are cut and ligated separately. Here a modified SNIt surgery method is described to damage both common peroneal and sural nerves with only one ligation and one cut with a shorter procedure time, which is easier to perform and reduces the potential risk of stretching the sciatic or tibial nerves, and produces similar mechanical hypersensitivity as the traditional SNIt model.
Nerve injury-induced neuropathic pain following surgery or trauma has a significant economic burden that impairs quality of life. A host of nerve injury models, including spinal nerve ligation (SNL)1, chronic constriction injury (CCI)2 to the sciatic nerve, partial sciatic nerve ligation (pSNL)3, sciatic nerve transaction (SNT)4 and spared nerve injury (SNI)5,6,7,8, were successfully developed to mimic the cardinal symptoms of peripheral nerve injury in rats and mice for studying the molecular and cellular mechanism of neuropathic pain6,7,8,9,10. However, each surgical model has its benefits and limitations, therefore particular attention should be given to exploring and developing the surgery models10.
The rodent SNI model produces long-term hypersensitivity to mechanical stimulation. However, it is somewhat confusing because there are two different SNI models. The initial SNI model was developed in Woolf's lab, in which the common peroneal and the tibial nerves were injured, leaving the sural nerve intact5,6. The second SNI model was developed in Basbaum's lab, in which the common peroneal and the sural nerves were injured, leaving the tibial nerve intact7,8. The initial Woolf's model is defined as SNIs here because the sural nerve is left intact, and Basbaum's model is defined as SNIt here because the tibial nerve is left intact. Because the sural nerve is purely sensory whereas the tibial nerve contains both motor and sensory fibers, the SNIt model has much less motor deficit than the SNIs model. However, unlike the SNIs model, mice in the SNIt model do not develop thermal hypersensitivity, but mechanical hypersensitivity develops in both models. Although the SNIt model is a relatively easy procedure, it requires the ligation of the sural and common peroneal nerves separately with the potential risk of stretching the sciatic or tibial nerves6,7,8,9.
The common peroneal, tibial, and sural nerves are three branches of the sciatic nerve and can be clearly identified at the superior edge of the gastrocnemius muscle (Figure 1): the tibial nerve goes under the gastrocnemius muscle, and the common peroneal (cephalad side) and sural nerve (caudal side) are above the gastrocnemius muscle11. Based on its anatomical features, a modified mouse SNIt surgery procedure was developed to ligate the common peroneal and sural nerves together with only one nerve-ligation and one nerve-cutting, which results in shortened procedure-time.
Animal experiments were approved by UCSF Institutional Animal Care and Use Committee and were conducted in accordance with the NIH Guide for the Care and Use of Laboratory animals. Adult C57BL/6 mice weighing 20-30 g was used in this study. The von Frey assessment was performed between 1:00 pm and 3:00 pm.
1. Anesthesia and mouse preparation
2. Modified SNIt surgery
3. von Frey assessment for mechanical threshold
4. Statistical Analysis
The comparison of procedure time between modified and traditional methods.
The procedure time from the beginning of cutting the skin to the end of closing skin was recorded in 5 mice with the modified approach and 5 mice with the traditional approach, respectively. A minimal number of animals was used to obtain results with statistical significance. Compared with the control of the traditional approach7,14 to perform SNIt, the modified approach took almost half the amount of procedure time (236.2 ± 28.6 s vs. 422.6 ± 53.8 s, p=0.0156; Figure 4).
The comparison of mechanical hypersensitivity between the two methods.
No difference of von Frey assessment (manual) was observed in the two groups at baseline (1.05 ± 0.10 g vs. 0.96 ± 0.13 g, p=0.9405). Compared with the traditional method (n=9), the modified method (n=14) induced similar mechanical hypersensitivity on the ipsilateral side from post-operative day 1 to post-operative day 14 (0.33 ± 0.09 g vs. 0.32 ± 0.05 g, p>0.9999, at post-operative day 1; 0.19 ± 0.02 g vs. 0.27 ± 0.06 g, p=0.9485 , at post-operative day 3; 0.20 ± 0.06 g vs. 0.15 ± 0.05 g, p=0.9979, at post-operative day 5; 0.13 ± 0.02 g vs. 0.15 ± 0.03 g, p>0.9999, at post-operative day 7; and 0.14 ± 0.02 g vs. 0.19 ± 0.03 g, p=0.9937, at post-operative day 14; Figure 5A).
Compared with percentage response between traditional (n=5) and modified method (n=5), similar mechanical hypersensitivity was observed on the ipsilateral side from baseline (4.00% ± 2.45% vs. 6.00% ± 2.45%, p>0.9999) to post-operative day 1 (20.00% ± 3.16% vs. 12.00% ± 3.74%, p=0.8987), day 3 (30.00% ± 5.48% vs. 32.00% ± 4.90%, p>0.9999), day 5 (36.00% ± 9.27% vs. 30.00% ± 5.48%, p=0.9730), day 7 (32.00% ± 8.00% vs. 36.00% ± 4.00%, p=0.9968), and day 14 (38.00% ± 5.83% vs. 36.00% ± 7.48%, p>0.9999; Figure 5B).
No neural reconnection between the distal and remaining stump at post-operative day 14.
Because the axon regeneration proceeds at a rate of 1-3 mm/d15, whether there is nerve regeneration at post-operative day 14 should be taken into account. On the post-operative day 14, the modified SNI mice were perfused intracardially with 0.1 mol/L phosphate-buffered saline followed by a phosphate-buffered 10% formalin fixative, and the nerve ligation site was dissected and exposed. No neural reconnection was observed between the distal and remaining stumps (Figure 3D).
Figure 1. Illustration of the 3 branches of the sciatic nerve that are separated at the superior edge of the gastrocnemius muscle (GM) around the knee area. The tibial nerve runs under the GM muscle, and the common peroneal nerve and sural nerve run above the GM muscle. 1: sciatic nerve; 2: tibial nerve; 3: common peroneal nerve; 4: sural nerve. Abbreviations: BFM = biceps femoris muscle; GM = gastrocnemius muscle. Please click here to view a larger version of this figure.
Figure 2. Illustration of surgical procedure in modified SNI mice. (A) In a lateral position, the left leg is upside with knee flexion. The yellow horizontal line (1) indicates the horizontal line crossing the knee level, and the yellow dashed arrow (2) indicates the incision. (B) Following a skin incision, two white lines can be visualized under the biceps femoris muscle (BFM). The femur is under the cephalad one (3), and the sciatic nerve is under the caudal one (4). (C) After careful dissection of BFM, the sciatic nerve (5) and its three branches can be visualized: the tibial (6), common peroneal (7), and sural (8) nerves. (D) The common peroneal (7) and sural (8) nerves were ligated together. 1: horizontal line crossing knee; 2: cut incision; 3: femur on the underneath; 4: sciatic nerve on the underneath; 5: sciatic nerve; 6: tibial nerve; 7: common peroneal nerve; 8: sural nerve; 9: white curve line indicates knee. Please click here to view a larger version of this figure.
Figure 3. Illustration of sciatic, tibial, sural, and common peroneal nerves with the ligation of modified SNI in intracardially perfused mice. (A) Sciatic nerve (1) and its three branches: the tibial nerve (2) passing under the gastrocnemius muscle (5), and the common peroneal (3) and the sural (4) nerves running above the gastrocnemius muscle. (B) Suture needle crossing under common peroneal and sural nerves together. (C) A 6-0 nylon suture was used for the ligation of common peroneal and sural nerves together. (D) No reconnection between distal and remaining nerve stump at post-operative day 14 was observed. 1: sciatic nerve; 2: common peroneal nerve 3: tibial nerve; 4: sural nerve; 5: gastrocnemius muscle; 6: ligation. Please click here to view a larger version of this figure.
Figure 4. The modified method is faster than the traditional SNI method to perform. Compared to the traditional SNI method (n=5), the modified method (n=5) requires much less procedure time to perform. Analyses were performed using an unpaired t-test, and data are presented as mean ± SEM. Please click here to view a larger version of this figure.
Figure 5. von Frey assessment of mechanical responsiveness in traditional and modified SNI models. (A) The modified SNI (n=9) and traditional SNI (n=14) models induced similar mechanical hypersensitivity on the ipsilateral side in 14-day follow-up. (B) The modified SNI (n=5) and traditional SNI (n=5) models acquired similar percentage responses on the ipsilateral side in a 14-day follow-up. Analyses were performed using two-way ANOVA with Sidak's multiple comparisons test. Data are presented as mean ± SEM. Please click here to view a larger version of this figure.
Compared to the traditional mouse SNIt method that ligates the common peroneal nerve and the sural nerve separately6,7,8,9, the modified SNIt model has three advantages: (1) it has less risk of contracting or stretching sciatic or tibial nerves; (2) there is no need to remove the distal nerve stumps after nerve-cutting because by ligating the common peroneal nerve and the sural nerve together, the distal nerve stumps are anatomically separated from the proximal stumps. Indeed, anatomy dissection demonstrated that no nerve regeneration was observed 14 days after modified SNIt; (3) the procedure is much easier to perform with a much shorter procedure time.
It is critical to make skin incisions low around knee level, rather than high in the thigh near the hip, when performing the modified mouse SNIt. This is because the common peroneal, tibial, and sural nerves have not branched out from the sciatic nerve at the high thigh area, which makes it challenging to separate the three branches. In contrast, at the superior edge of the gastrocnemius muscle near the knee area, the three branches from the sciatic nerve traverse separately and are easily visualized and identified, as the tibial nerve passing under the gastrocnemius muscle, and the common peroneal nerve on the cranial side and sural nerve on the caudal side running above the gastrocnemius muscle11. Interestingly, although the common peroneal nerve is medial to the tibial nerve at the thigh area, it usually crosses the tibial nerve to get closer to the sural nerve at the superior edge of the gastrocnemius muscle at the knee area in the lateral position with knee flexion, which makes it easier for ligating common peroneal and sural nerves together. Importantly, the tibial nerve remains intact without contacting or stretching when the common peroneal and sural nerves are ligated above the gastrocnemius muscle. In addition, when dissecting BFM to expose underneath nerves, it is important to avoid damaging blood vessels (lateral proximal genicular artery, popliteal artery, distal caudal femoral artery, etc.) located above the gastrocnemius muscle16.
Although this modified SNIt model successfully develops neuropathic pain, some limitations are needed to be admitted. Because of damage to the common peroneal nerve, this could produce motor dysfunction with markedly extended hind leg17. Another, the mechanical hypersensitive area is innervated by intact nerve rather than injured nerve, but the neuropathic pain is often caused by the lesion or disease of the peripheral or central nervous system in clinical pain management18. Therefore, further study needs to be explored in the development of the neuropathic pain model.
In summary, this modified mouse SNIt method is a simplified SNI procedure with only one nerve-ligation and one nerve-cutting without any removal of nerve stumps. It is much easier to perform in producing nerve injury-induced mechanical hypersensitive, with reduced risk of sciatic or tibial nerves damage.
The authors have nothing to disclose.
Z.G. is supported by NINDS R01NS100801.
6-0 suture | Henry Schein | 9007482 | Nerve ligation and close the muscular layer |
Iris Scissors | Integra Miltex | 12460598 | Cut muscle and fascia |
Mayo dissecting scissors | Fisherbrand | 895120 | Cut skin incision |
Micro forcep | Fisherbrand | 16100110 | Blunt dissection biceps femoris muscle |
Micro Scissors | Excelta | 17467496 | Cut nerve |
Microdissection Forceps | Fisherbrand | 16100123 | Separate the common peroneal and the sural nerves from the neighboring tissues |
Needle Holder | Fisherbrand | 8966 | Hold 6-0 needle |
Prism software | Graph Pad | version 8.0 | Statistical analysis software |
Wound clips | Roboz Surgical | NC1878744 | Close skin incision |