Evaluation of motor recovery remains the benchmark outcome measure in experimental peripheral nerve studies. The isometric tetanic force measurement of the tibialis anterior muscle in the rat is an invaluable tool to assess functional outcomes after reconstruction of sciatic nerve defects. The methods and nuances are detailed in this article.
Traumatic nerve injuries result in substantial functional loss and segmental nerve defects often necessitate the use of autologous interposition nerve grafts. Due to their limited availability and associated donor side morbidity, many studies in the field of nerve regeneration focus on alternative techniques to bridge a segmental nerve gap. In order to investigate the outcomes of surgical or pharmacological experimental treatment options, the rat sciatic nerve model is often used as a bioassay. There are a variety of outcome measurements used in rat models to determine the extent of nerve regeneration. The maximum output force of the target muscle remains the most relevant outcome for clinical translation of experimental therapies. Isometric force measurement of tetanic muscle contraction has previously been described as a reproducible and valid technique for evaluating motor recovery after nerve injury or repair in both rat and rabbit models. In this video, we will provide a step-by-step instruction of this invaluable procedure for assessment of functional recovery of the tibialis anterior muscle in a rat sciatic nerve defect model using optimized parameters. We will describe the necessary pre-surgical preparations in addition to the surgical approach and dissection of the common peroneal nerve and tibialis anterior muscle tendon. The isometric tetanic force measurement technique will be detailed. Determining the optimal muscle length and stimulus pulse frequency is explained and measuring the maximum tetanic muscle contraction is demonstrated.
Loss of motor function following traumatic peripheral nerve injury has a significant impact on the quality of life and socioeconomic status of patients1,2,3. The prognosis of this patient population remains poor due to minimal improvements in surgical techniques over the years4. Direct end-to-end tension-free epineural repair forms the gold standard surgical reconstruction. However, in cases with extended nerve gaps interposition of an autologous nerve graft has proven to be superior5,6. The associated donor site morbidity and limited availability of autologous nerve grafts have imposed the need for alternative techniques7,8.
Experimental animal models have been used to elucidate the mechanism of peripheral nerve regeneration and to evaluate outcomes of a variety of reconstructive and pharmacological treatment options8,9. The rat sciatic nerve model is the most frequently used animal model10. Their small size makes them easy to handle and house. Due to their superlative neuroregenerative potential, the diminished time between intervention and evaluation of outcomes can result in relatively lower costs11,12. Other advantages of its use include morphological similarities to human nerve fibers and the high number of comparative/historic studies13. Although the latter should be approached cautiously, as a wide variety of different outcome measures between studies makes it difficult to compare results14,15,16,17,18.
Outcome measures to assess nerve regeneration range from electrophysiology to histomorphometry, but these methods imply a correlation but do not necessarily directly measure the return of motor function14,15. Regenerating nerve fibers might not make appropriate connections which can cause an overestimation of the number of functional connections14,15,19,20. The best and clinically most relevant measurement to demonstrate correct reinnervation of end organs remains assessment of muscle function21,22,23. Creating motor function assessment tools for animal models is, however, challenging. Medinaceli et al. first described the walking track analysis, which has since been the most frequently used method to evaluate functional recovery in experimental peripheral nerve studies21,24,25,26,27,28. The walking track analysis quantifies the sciatic functional index (SFI) based on measurements of pawprints from walking rats21,29. Major limitations of the walking track analysis, such as toe contractures, automutilation, smearing of the print and poor correlation with other measures of reinnervation, have necessitated the use of other parameters for quantification of functional recovery30,31.
In previous studies in Lewis rats32 and New Zealand rabbits33, we validated the isometric tetanic force (ITF) measurement for the tibialis anterior (TA) muscle and demonstrated its effectiveness in the evaluation of muscle recovery after different types of nerve repair34,35,36,37,38,39. The TA muscle is well suited because of its relatively large size, innervation by the peroneal branch of the sciatic nerve and well elucidated biochemical properties40,41,42,43. When muscle length (preload force) and electrical parameters are optimized the ITF provides a side-to-side variability of 4.4% and 7.5% in rats32 and rabbits33, respectively.
This article provides a detailed protocol of the ITF measurement in the rat sciatic nerve model, including a thorough description of the necessary pre-surgical planning, surgical approach and dissection of the common peroneal nerve and the distal TA muscle tendon. Using predetermined values for the stimulus intensity and duration, the optimal muscle length and stimulus pulse frequency will be defined. With these four parameters, the ITF can subsequently be consistently and accurately measured.
All animal procedures were performed with approval of the Institutional Animal Care and Use Committee (IACUC A334818).
1. Calibration of the force transducer
2. Animal subjects
3. Surgical preparation
4. Surgical approach to the common peroneal nerve
5. Dissection of the distal tibialis anterior muscle tendon
6. Isometric tetanic force measurement
Five parameters are used to measure the ITF measurement. These include muscle tension (preload force), stimulus intensity (voltage), stimulus pulse frequency, stimulus duration of 0.4 ms and a delay of 2 ms. Prior to measuring the ITF, the optimal muscle tension has to be determined using two single twitch muscle contractions at an intensity of 2 V during the parameter test. These stimuli cause dorsiflexion of the paw and produce an output signal on the graph in the VI (Figure 5). These single twitch curves ideally have a rapid vertical upswing representing the contraction period directly followed by a slower vertical decrease period demonstrating the relaxation period. The program will average these two peak output forces, but the active force has to be manually calculated by subtracting the preload force from the mean output force. In the example in Figure 5, a preload of 10 g results in two peak output forces of 411.09 g (4.03 N) and 379.78 g (3.73 N), which is averaged to a mean peak output force of 395.43 g (3.88 N). When the active forces of each preload are plot in a graph, the maximum active force can be identified. These active forces usually produce a bell-shaped curve and the maximum active force for Lewis rats weighing 300-500 g should be around 30-40 g (0.29-0.39 N) (Figure 6).
For the tetanic stimulations during the frequency test, the stimulus intensity is increased to a supra-maximal voltage (10 V) to ensure maximal activation of all TA muscle motor units using increasing frequencies. The optimal tetanic curve increases and decreases sharply and has a slowly decreasing plateau phase with minimal oscillations. Figure 7 depicts an example of a tetanic curve at a stimulus frequency of 30 Hz with an isometric tetanic force of 803.25 g (7.88 N). The highest force plateau is defined as the maximum ITF.
Figure 1: Image of customized clamp fashioned from a surgical hemostat and modified with a tightening screw that allows for adjustment of the tension. Please click here to view a larger version of this figure.
Figure 2: Graphical code for virtual instrument for isometric tetanic force measurement on LabVIEW. Please click here to view a larger version of this figure.
Figure 3: Calibration of the force transducer. Successful calibration of the force transducer with five weights (0, 10, 20, 30 and 50 g) should result in a positive linear curve. Please click here to view a larger version of this figure.
Figure 4: Schematic overview of experimental setup for isometric tetanic force measurement. (Copyrighted and used with permission of the Mayo Foundation for Medical Education and Research; all rights reserved. Reprinted from: Shin, R. H. et al. Isometric tetanic force measurement method of the tibialis anterior in the rat. Microsurgery. 28 (6), 452-457 (2008)). Please click here to view a larger version of this figure.
Figure 5: Representative single twitch curves for optimization of muscle length. For each preload measurement, two single twitches are applied. These single twitch curves have a rapid vertical upswing (contraction period) followed by a vertical decrease (relaxation period). The two peak output forces will be averaged to a mean peak output force. In this example with a Lewis rat, a preload of 10 g results in two peak output forces of 411.09 g (4.03 N) and 379.78 g (3.73 N), which is averaged to a mean peak output force of 395.43 g (3.88 N). Please click here to view a larger version of this figure.
Figure 6: Optimal muscle length (preload). The active muscle force can be calculated by subtracting the preload from the mean peak output force. The active muscle force for each preload should be documented until a drop in active muscle force is visible. The preload yielding the highest active muscle force will be used to measure the isometric tetanic force. The optimal preload for Lewis rats weighing 300-500 g should be around 30-40 g (0.29-0.39 N) (N=10). Please click here to view a larger version of this figure.
Figure 7: Representative isometric tetanic force curve. The optimal tetanic curve increases sharply, then has a slowly decreasing plateau phase followed by a sharp decrease. The highest force plateau is defined as the maximum ITF. This example depicts the tetanic curve at a stimulus frequency of 30 Hz with an isometric tetanic force of 803.25 g (7.88 N). Please click here to view a larger version of this figure.
This protocol describes a previously validated method for acquiring accurate maximum ITF measurements of the TA muscle in the rat model32. The recovery of maximum strength after experimental nerve reconstruction treatments is of primary interest in the clinical setting as it proves that the nerve not only regenerated, but also made working connections with the target muscle. The ITF can be used in a small nerve gap model, such as the rat sciatic nerve model32, and with a few modifications to the protocol, it can also be used in a larger nerve gap rabbit model33.
There are several critical steps that should be considered to ensure consistent and reliable maximum isometric muscle force measurements. The importance of carefully selecting the type of anesthesia to prevent skeletal muscle side effects has previously been described32,33. The use of isoflurane has demonstrated a time dependent decrease in muscle force, which can be explained by its ability to induce sarcoplasmic reticulum stimulated release of calcium33,48. The effect of ketamine/xylazine on the muscle force has proven to be minimal based on our experience and previous study32. Secure attachment of the distal TA muscle tendon to the force transducer is also of great importance for accurate measurements. Slippage or tearing of the tendon should be prevented or directly corrected. Therefore, a custom-made clamp was created from a surgical hemostat and modified with a tightening screw. Other research groups have described a technique of drying the tendon for about 30 minutes to mechanically strengthen the interface between the tendon and a clamp49. In order to maintain endurance of the muscle it is critical to avoid desiccation of the TA muscle and tendon with warm 0.9% NaCl and implement a 5-minutes resting period between each tetanic stimulation. The resting period is based on the activity of the phosphagen system, also known as the immediate energy source, which is important for explosive muscle contractions. It consists of adenosine triphosphate (ATP) and creatine phosphate activity and provides energy for less than 10 seconds of maximal activity. It requires approximately 3-5 minutes to replenish 100% of the phosphagens50.
We recognize the limitations of the method described in this video. The non-survival nature of the procedure does not allow for serial measurements over time. Additionally, it is a detailed and time-consuming testing protocol. During the 1 to 2 hour testing time, the nerve and muscle undergo a significant number of stimulations which may result in muscle fatigue with potential decrease in ITF. This has, however, proven to be less prominent in the rat model compared to the rabbit33.
In conclusion, the ITF measurement described in this video is an invaluable tool in experimental peripheral nerve studies to quantify motor recovery. When presented with other outcome measures such as electrophysiology and histomorphometry, a global assessment of nerve function can be provided.
The authors have nothing to disclose.
Research reported in this publication was supported by the National Institute of Neurological Disorders and Stroke of the National Institutes of Health under Award Number RO1 NS 102360. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
0.9% Sodium Chloride | Baxter Healthcare Corporation, Deerfield, IL, USA | G130203 | |
1 mm Kirshner wires | Pfizer Howmedica, Rutherford, NJ | N/A | |
Adson Tissue Forceps | ASSI, Westbury, NY, USA | MTK-6801226 | |
Bipolar electrode cables | Grass Instrument, Quincy, MA | N/A | |
Bipolar stimulator device | Grass SD9, Grass Instrument, Quincy, MA | N/A | |
Cotton-tip Applicators | Cardinal Health, Waukegan, IL, USA | C15055-006 | |
Curved Mosquito forceps | ASSI, Westbury, NY, USA | MTK-1201112 | |
Force Transducer MDB-2.5 | Transducer Techniques, Temecula, CA | N/A | |
Gauze Sponges 4×4 | Covidien, Mansfield, MA, USA | 2733 | |
Ground cable | Grass Instrument, Quincy, MA | N/A | |
Isoflurane chamber | N/A | N/A | Custom-made |
Ketamine | Ketalar, Par Pharmaceutical, Chestnut, NJ | 42023-115-10 | |
LabView Software | National Instruments, Austin, TX | ||
Loop | N/A | N/A | Custom-made |
Microsurgical curved forceps | ASSI, Westbury, NY, USA | JFA-5B | |
Microsurgical scissors | ASSI, Westbury, NY, USA | SAS-15R-8-18 | |
Microsurgical straight forceps | ASSI, Westbury, NY, USA | JF-3 | |
Retractor | ASSI, Westbury, NY, USA | AG-124426 | |
Scalpel Blade No. 15 | Bard-Parker, Aspen Surgical, Caledonia, MI, USA | 371115 | |
Slim Body Skin Stapler | Covidien, Mansfield, MA, USA | 8886803512 | |
Subminiature electrode | Harvard Apparatus, Holliston, MA | N/A | |
Surgical Nerve Stimulator | Checkpoint Surgical LCC, Cleveland, OH, USA | 9094 | |
Terrell Isoflurane | Piramal Critical Care Inc., Bethlehem, PA, USA | H961J19A | |
Testing platform | N/A | N/A | Custom-made |
Tetontomy Scissors | ASSI, Westbury, NY, USA | ASIM-187 | |
Traceable Big-Digit Timer/Stopwatch | Fisher Scientific, Waltham, MA, USA | S407992 | |
USB-6009 multifunctional I/O data acquisition (DAQ) device | National Instruments, Austin, TX | 779026-01 | |
Vacuum Base Holder | Noga Engineering & Technology Ltd., Shlomi, Isreal | N/A | Attached clamp is custom-made |
Weight (10 g) | Denver Instruments, Denver, CO, USA | 820010.4 | |
Weight (20 g) | Denver Instruments, Denver, CO, USA | 820020.4 | |
Weight (50 g) | Denver Instruments, Denver, CO, USA | 820050.4 | |
Xylazine | Xylamed, Bimeda MTC Animal Health, Cambridge, Canada | 1XYL002 |