This protocol describes the tibial neuroma transposition model, which entails a lesion of the tibial nerve with subsequent transposition of the proximal nerve end toward a subcutaneous pretibial or lateral position. Behavioral testing of neuroma pain and plantar hyperalgesia is quantified using Von Frey monofilaments.
The tibial neuroma transposition (TNT) is a rat model in which allodynia at the neuroma site (tibial nerve) can be independently evaluated from allodynia at the plantar surface of the hind paw innervated by the intact sural nerve. This TNT model is suitable to test therapies for neuroma pain, such as the potential superiority of certain surgical therapies that are already used in the clinic, or to evaluate new drugs and their effect on both pain modalities in the same animal. In this model, a distal lesion (neurotmesis) is made in the tibial nerve, and the proximal nerve end is transposed and fixed subcutaneously and pretibially to enable assessments of the neuroma site with a 15 g Von Frey monofilament. To assess allodynia over the sural nerve, Von Frey monofilaments can be used via the up-down method on the plantar lateral region of the hind paw. After cutting the tibial nerve, mechanical hypersensitivity develops at the neuroma site within 1 week after surgery and persists at least until 12 weeks after surgery. Allodynia at the sural innervated plantar surface develops within 3 weeks after surgery compared to the contralateral limb. At 12 weeks, a neuroma forms on the proximal end of the severed tibial nerve, indicated by dispersion and swirling of axons. For the TNT model surgery, multiple critical (micro)surgical steps need to be followed, and some surgery practice under terminal anesthesia is advised. Compared to other neuropathic pain models, such as the spared nerve injury model, allodynia over the neuroma site can be independently tested from sural nerve hypersensitivity in the TNT model. However, the neuroma site can be tested only in rats, not in mice. The tips and directions provided in this protocol can help research groups working on pain successfully implement the TNT model in their facility.
Every wound, varying from simple lacerations to whole limb amputation, is accompanied by varying degrees of peripheral nerve injury. Such nerve injury can result in the formation of a neuroma, a disorganized entanglement of sprouting nerve fibers. Neuromas become painful in 8%-30% of patients, severely impacting their quality of life1,2,3,4,5. After limb amputation, neuroma pain develops in 50% of patients6,7,8. Reported symptoms include tenderness, spontaneous pain, allodynia, hyperalgesia, and mechanical or thermal hypersensitivity in the innervated area9. When not treated adequately within 1 year, neuroma pain can advance to a chronic pain state, resulting in high societal burden and associated medical costs10,11,12,13,14. Due to the poor efficacy of current pharmacological interventions, neuroma pain is preferably treated by surgical removal of the painful neuroma, and the nerve treated by various surgical techniques, as described in the literature15. It is important to note that complete pain relief is rare, pain often worsens over time, and 40% of patients do not benefit from the surgery, indicating that new treatments are needed1,16.
A standardized rat model of neuroma pain aids in understanding the mechanisms that drive neuroma pain, and may help identify new treatments or evaluate existing ones used in the clinic. The tibial neuroma transposition (TNT) model was first described by Dorsi et al. in 200817 and has been used by different research groups18,19,20. The overall goal of this method is to be able to test different treatment techniques for neuroma pain. The advantage of the model over, for example, the spared nerve injury (SNI) model21, is that it allows to test allodynia at the neuroma site. This is because the model involves transposing the proximal nerve ending of the tibial nerve to a subcutaneous pretibial position, where it can be probed with von Frey monofilaments. Moreover, allodynia develops at the plantar surface of the hind paw innervated by the intact sural nerve, which can be assessed independently from the neuroma pain in the same animal. This is similar to symptoms of neuroma pain in patients, where persistent neuropathic pain after removal of a painful neuroma is sometimes caused by the neighboring nerves22. Moreover, allodynia over a severed nerve with a neuroma is a different pain modality than allodynia over the intact neighboring nerve. Thus, this model facilitates assessment of the effect of new therapies on both allodynia present at the neuroma site and more widespread neuropathic pain tested in the plantar surface of the hind paw. As the surgery performed to create the TNT model can be challenging, this paper elaborates on the procedure to support researchers implementing the model in their facility.
This research was performed in accordance with the IVD (Instantie voor Dierenwelzijn Utrecht) and the guidelines for animal research, project number AVD1150020198824.
1. Von Frey baseline measurements
2. Anesthesia and preparation
NOTE: This study was conducted on 15 male Sprague Dawley rats that were 12 weeks old.
3. Surgery
4. Post-surgical treatment
5. Von Frey testing of the plantar side of the hind paws
NOTE: Von Frey testing (step 5 and 6) is performed prior to surgery (for baseline measurement), and from 3 days after the surgery.
6. Von Frey testing of the neuroma site
7. Specimen recovery for histology and preparation
NOTE: Histological examination is performed 12 weeks after initial surgery.
Assessment at the neuroma site showed increased sensitivity to the application of the 15 g von Frey monofilament. At baseline, rats typically responded to 10%-15% (± 13%) of the 25 applications of a 15 g monofilament. The response rate increased to 45%-50% (± 24%) 1 week after TNT surgery. On the contralateral side, the number of responses after surgery was similar to those at baseline (Figure 2A). Around 20% of the rats did not develop a painful neuroma; the response rate did not increase compared to the baseline (Figure 2B). This is comparable to the human situation, where not all patients (50% after amputation) develop pain after the formation of a neuroma. All rats developed a neuroma at the end of the transected and transposed tibial nerve stump 12 weeks after surgery (Figure 3). This neuroma was characterized by swirling axons and microfascicles within collagen depositions.
Transection of the tibial nerve reduced mechanical sensitivity at the middle of the plantar side of the hind paw, innervated by the tibial nerve (Figure 1). The hyposensitivity was present at 1 week after surgery, was significantly different from the contralateral side and baseline from 3 weeks after surgery, and remained until at least 12 weeks after surgery (Figure 4). At the lateral part of the plantar side of the hind paw innervated by the intact sural nerve, the rats developed mechanical hypersensitivity that was significantly different from the contralateral side and baseline from 1 week after surgery (Figure 4). This hypersensitivity persisted to at least 12 weeks after surgery. At the contralateral paw, mechanical sensitivity was not affected compared to baseline in the areas either innervated by the sural or tibial nerve (Figure 4).
Figure 1: Nerve distribution on the plantar side of the hind paw. Red = sural nerve distribution (lateral); purple = tibial nerve distribution (middle); green = saphenous nerve distribution (medial). Please click here to view a larger version of this figure.
Figure 2: Von Frey of the neuroma site. The tibial nerve was transected and the course of mechanical sensitivity over the neuroma site was assessed with a 15 g monofilament applied in five clusters of five applications each, with a total of 25 applications. A response is scored as one point. (A) The neuroma site showed a significantly higher response 1 week after surgery compared to baseline and the contralateral side. N = 15; error bars: standard error of the mean (SEM); mixed model analysis with multiple comparisons and Tukey test. * = p < 0.05, ** = p < 0.01, *** = p < 0.001. (B) Individual values of the ipsilateral site show diversity in reaction. Three rats (20%) had a relatively high baseline score, and three rats (20%) did not show any changes in pretibial sensitivity. Please click here to view a larger version of this figure.
Figure 3: Morphology of the neuroma. Histological images of a 12 week old neuroma. (A) Hematoxylin-Eosin staining, (B) Masson's trichrome staining, and (C) Neurofilament staining. Green arrow = tibial nerve just proximal to the neuroma. Orange arrow = neuroma, identified by swirling of axons and diffusion of the fascicles. Scale bar = 500 µm. Please click here to view a larger version of this figure.
Figure 4: Von Frey of the plantar hind paw (tibial and sural nerve). The tibial nerve was transected and the course of mechanical sensitivity was assessed by Von Frey testing at the plantar surface of the hind paw. The middle part of the ipsilateral operated hind paw innervated by the tibial nerve showed hyposensitivity. The lateral part of the ipsilateral operated hind paw innervated by the sural nerve showed hypersensitivity. The middle and lateral parts of the plantar contralateral hind paw showed no changes in sensitivity compared to the baseline. N = 15; error bars: standard error of the mean (SEM); mixed model analysis with multiple comparisons and Tukey test. * = p < 0.05, ** = p < 0.01, *** = p < 0.001, **** = p < 0.0001. Please click here to view a larger version of this figure.
Critical steps in the protocol
The TNT model involves cutting the tibial nerve and transposing it laterally and subcutaneously to a pretibial location to enable sensitivity testing of the neuroma, in addition to plantar hyperalgesia over the sural nerve. In the TNT model, it is key that the place of the neuroma is visible to the researchers. Therefore, an albino rat strain is preferred because subcutaneous sutures are easily visible through the skin and the color of the suture should preferably be dark blue or black.
When the surgery is conducted and the tibial nerve is exposed, there is variation in the place (e.g., proximal or distal) of the bifurcation of the tibial nerve. If rats have a proximal bifurcation, it is possible that two nerves can be found (the medial and lateral plantar nerve) proximal to the ankle (Figure 5A), instead of just one tibial nerve (Figure 5B). It is important that both branches are cut and transposed in order to induce plantar hyperalgesia over the sural nerve. One could choose to only transpose one plantar nerve; however, a distinction between the lateral and medial plantar nerve is not easily made on this level and could influence the results. Therefore, it is advised to transpose both nerves. Moreover, some rats might have a more distal bifurcation of the tibial nerve, and transposing only one plantar nerve might be impossible.
Figure 5: Proximal and distal bifurcation of the tibial nerve. Anatomical variation in the level of bifurcation (*) of the tibial nerve. (A) Proximal bifurcation of the tibial nerve; (B) distal bifurcation of the tibial nerve. Abbreviations: MPN = medial plantar nerve, LPN = lateral plantar nerve. Please click here to view a larger version of this figure.
In the original paper by Dorsi et al.17, a ligature is placed around the proximal tibial nerve end and the nerve is transposed and fixed via this ligature suture. As the ligature around the nerve can induce constriction pain24, an alternative described in this method is to fix the nerve using epineurinal sutures. If the tibial nerve is cut and transposed, it is important that the suture to fix the nerve subcutaneously is placed through the epineurium and not the nerve fascicles themselves, as this could influence pain measures as well. In addition, one should avoid placing the suture through the skin, as rats tend to gnaw any visible sutures, resulting in displacement of the neuroma and thus non-reliable outcomes of pain measures.
When the skin is closed, it is also important to use intra-epidermal sutures to avoid gnawing that will result in an open wound. Moreover, after transposing the tibial nerve, it will be situated in a more superficial layer just under the skin. An open wound in combination with a superficially placed nerve is undesirable.
Von Frey measurements can be performed to test neuroma pain on the neuroma site and to test plantar hyperalgesia over the sural nerve, on the lateral side of the hind paw. The neuroma site is visible after surgery due to the dark color of the suture. For testing the sural nerve hypersensitivity, one location should be chosen where the Von Frey monofilament is applied. This can be proximal or distal to the lateral food pad, but should be approximately the same place during baseline measurements and measurements in the weeks after surgery.
Troubleshooting of the method
If the ratsreact to all stimuli applied to the pretibial area already during baseline measurements, make sure that they are calm and relaxed, and that they are properly acclimatized to the testing area. Repeat baseline measurement until the rats react less to the stimuli. In addition, it is preferable to not wear any perfume while performing measurements. Ideally, rats do not react to the pretibial stimulus when a 15 g monofilament is applied before surgery. However, if the rats are calm, and 50%-100% of the rats still react to the 15 g monofilament, change it to a monofilament that has a 10%-20% response rate during baseline. However, if the monofilament is changed, it is advised to first perform a pilot experiment to test whether the TNT rats react to this lower strength of monofilament. In the initial paper of the TNT model, the neuroma site was measured by applying the monofilament through an opening at the bottom of a Perspex box17. In pilot experiments, rats were found to react to each stimulus when applied via the bottom of the cage and tended to attack the monofilament. When closely held by the researcher, the rats were in a calm state, resulting in a lower response rate on the monofilament during baseline measurement.
If the proximal tibial nerve end cannot reach far enough in the subcutaneous tunnel, follow the course of the tibial nerve more proximal and remove any collagenous and fatty tissue around the nerve. Cut any minor nerve branches or vessels that keep the nerve fixed to its surrounding. This will give the nerve a wider range of motion to be transposed more laterally. Note that in the original paper by Dorsi et al.17, the nerve was transposed more laterally. In pilot experiments, it was found to be impossible to reach the lateral position. Therefore, this method describes a pretibial position of the neuroma site.
Limitations of the method
A limitation of the TNT model is that the surgery involves multiple (micro)surgical steps to be followed. Another limitation is that the TNT model is not easily translated to mice. From experience, mice tend to be rather sensitive to stimuli applied at a pretibial area, even with the application of a 0.008 g monofilament.
Significance of the method with respect to existing/alternative methods
In the TNT model, neuroma pain can be tested independently from plantar hyperalgesia over the sural nerve. The latter is also induced in other neuropathic pain models such as the SNI model, but here, neuroma pain cannot be independently tested21. In addition, both the tibial and peroneal nerve are cut in the SNI model, resulting in more loss of motor function which results in paralyzed intrinsic muscles of the paw21. Because only the tibial nerve is cut at a distal level in the TNT model, the intrinsic muscles of the feet show only a negligible loss of motor function.
Potential applications of the method
Previous research has already shown that the TNT model can be used to test various pain medication, nerve caps, or other surgical tools for neuroma treatment18,19,20. However, all research groups that are interested in pain could have potential benefit from using the TNT model, as two different pain modalities can be tested in the same animal.
The authors have nothing to disclose.
We would like to thank Sabine Versteeg for assisting during microsurgery and Anja van der Sar and Trudy Oosterveld-Romijn from the Common Animal Laboratory (Gemeenschappelijk Dieren Laboratorium) for their help in preparing the microscope and surgical room and taking care of the animals.
This research was funded by Axogen.
Aesthesio | Linton Instrumentation | 514007 until 514015 | 0.6 g until 15 g monofilaments |
Carprofen | Local Veterinary Pharmacy | n/a | The local veterinary pharmacy makes caprofen dilution |
Cotton swabs | Nobamed | 974255 | |
Electrocautery | Fine Science Tools | 18010-00 | |
Ethanol 70% | Interchema BV | 400406 | |
Ethilon 4.0 | Johnson & Johnson | 1854G | IMPORTANT: the color should be blue or black |
Ethilon 8.0 | Johnson & Johnson | BV130-5 | |
Isoflo, isoflurane Zoetis | Dechra Veterinary Products | B506 | |
Mesh bottom cages | StoeltingCo | 57816 and 57824 | |
Micro forceps | Fine Science Tools | 11251-35 | |
Micro needle holder | Fine Science Tools | 12076-12 | |
Micro scissors | Fine Science Tools | 15019-10 | |
Micro tweezers | Fine Science Tools | 11254-20 | |
NaCl 0.9% | Trademed | H7 1000-FRE | |
Needle holder | Fine Science Tools | 12004-16 | |
Ophthalmic ointment | Local Veterinary Pharmacy | n/a | The local veterinary pharmacy makes the ophthalmic ointment |
Scalpel | Fine Science Tools | 10003-12 | |
Scissors | Fine Science Tools | 14001-12 | |
Stereo surgical microscope | Leica | A60 F | |
Sterile sheet with hole | Evercare OneMed | 1555-01 | |
Surgical blade nr.15 | Fine Science Tools | 10015-00 | |
Tweezers | Fine Science Tools | 11617-12 |