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On the applicability of two different stimulation techniques for intra-operative peroneal nerve conduction testing
ELSEVIER
Journal of Orthopaedic Research Journal of Orthopaedic Research 19 (2001) 160-165
www.elsevier.nl/locate/orthres
On the applicability of two different stimulation techniques for intra-operative peroneal nerve conduction testing W. Nebelung *, H. Wissel, F. Awiszus Neurornutcular Re yearch Group, Otto-von Gueritke-Uriivervity Magdehurg, Clinic f o r Orthopedic Surgery, Leipziger Str 44, 39120 Magdehurg, Germany Received 14 May 1999, accepted 23 March 2000
Abstract
Dysfunction of the peroneal nerve is an important complication of knee surgery. We compared two monitoring procedures of peroneal nerve function during a standardized operation, a closing wedge high tibial osteotomy. For two types of stimulation the evoked compound motor unit action potentials (CMAPs) were recorded on the tibialis anterior muscle. We used direct perineural electrical Stimulation of the common peroneal nerve distal to the cuff (dCMAPs) after nerve identification in the surgical field. Additionally, magnetic stimulation of the sacral plexus proximal to the cuff (pCMAPs) was performed. It was found that dCMAPs were recorded during almost one hour of tourniquet time whereas the pCMAPs were blocked after 25-30 min in 9 out of 11 cases. On the other hand, the CMAP obtained after proximal stimulation exhibited a latency shift with tourniquet yielding an indicator of ischaemic changes present beneath and distal to the tourniquet cuff. In conclusion, different applicabilities of both stimulation techniques under tourniquet conditions were demonstrated. 0 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.
Introduction It is generally accepted that the peroneal nerve is at risk during knee surgery [ 11,13,15-171. Peroneal nerve palsies may occur following in particular high tibial osteotomies (HTO) and may compromise the functional result. For HTO these palsies belong to the most important complications and occur up to a frequency of 16Yo [6,12,18,19,25]. There is some doubt as to what extent iatrogenic surgical injury or ischaemic changes due to the tourniquet cuff are responsible for the observed postoperative palsies. One reason might be the iatrogenic damage due to the lateral approach and fibular osteotomy [19]. On the other hand, the clinical use of a tourniquet although very common in the operative setting of orthopedic surgery, has been shown to be responsible for nerve damage [9,23]. Even if this functional loss appears to be subclinical in most of the cases [6], such neuromuscular
*Corresponding author. Tel.: +49-391-6714032; fax: +49-39167190143. E-mail addrrsx [email protected] (W. Nebelung).
deficiencies might influence optimal postoperative muscular rehabilitation [S]. Several techniques for testing the neural function during surgical procedures are available. Some reports about monitoring neurological deficiencies using somatosensory evoked potentials (SSEPs) in orthopedic surgery were published [2,14,22,28]. As the functional disability of the lower extremity depends primarily on motor function testing this appears to be of impact for the evaluation of available techniques. Testing motor conduction distal to the lumbosacral plexus using a magnetic stimulator was first performed during total knee replacements [27]. In addition, intraoperative orthodrome stimulation of the common peroneal nerve during a tibial osteotomy was described in a former study [21]. Both procedures test action potential conduction in motor fibres. Strength and weakness of both procedures are currently unknown. In order to evaluate the advantages and disadvantages we performed a simultaneous application of two different motor pathway stimulation techniques during a relatively standardized surgical procedure, a high tibia1 osteotomy for medial osteoarthritis of the knee joint.
0736-0266/01/$ - see front matter 0 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved PIr: s o 7 ~ 6 - o 2 6 6 ( o o ) o o o6i - 4
W Nebelung et ul. I Journal of' Orthopuedic Research I 9 (2001) 160-165
Materials and methods Put ients We monitored the function of common peroneal nerve fibers by stimulation above and below the tourniquet in 11 patients undergoing a closing wedge high tibial osteotomy. The age of the patients ranged between 44 and 68 (mean: 54 years, 8 female, 5 male). All patients were suffering from degenerative osteoarthritis of the medial compartment of the knee. Prior to the monitored operation in all patients an arthroscopy was performed. If accompanying meniscal lesions were present they were treated by resection. The lateral compartment was confirmed to have no degenerative cartilage damage. Patients with neurological disorders were excluded from the study. Since the peroneal nerve needs to be exposed for the proximal fibular osteotomy the monitoring did not alter the surgical course. All experiments procedures were approved by the local ethical committee. Operative technique A standard spinal anesthesia was performed with intradural application of 2.5-3.5 ml 0.5% Bupivacain. Muscle relaxants were not given throughout the entire procedure. We performed a lateral longitudinal approach to the knee joint. Posterior to the distal insertion of the biceps muscle the common peroneal nerve was identified. In all cases we performed a high fibular osteotomy directly through the fibular head. Following this a closing wedge osteotomy superior to the tibial tubercle was performed. We fixed the fragments using an AO-plate and a cancellous bone screw in the technique described by Weber [30]. Postoperatively, neither a cast nor limitation of range of motion was ordered. We allowed increasing weightbearing after 6 weeks in all cases and 10 weeks after operation full weightbearing was admitted.
161
maintained a pressure of 300 mm Hg until fixing the osteotomy. When the fragments were fixed the tourniquet was released and recordings were continued until skin closure. Dutu unulysis Recorded CMAPs were analyzed off-line. A purpose-written software was used to display and analyze the recorded data. A cursor was used to evaluate the latencies of all recorded CMAPs. Latency-shift time courses were fitted with a single or double exponential using a nonlinear least-square algorithm.
Results Fig. 1 shows typical recordings of pCMAPs (Fig. l(a)-(e)) and dCMAPs (Fig. l(Q-(j)) during an HTO in a 51-year-old male patient. Prior to tourniquet inflation the strength of the magnetic stimulator was increased up to 940/0 for the case of Fig. 1 until a reliable pCMAP exhibiting a latency of 18.6 ms and an amplitude of 115 pV was obtained (Fig. l(a)). After 20 min of tourniquet (Fig. 1(b)) pCMAPs were still obtainable with the same stimulus strength, whereas latency increased to about 25 ms and amplitude was reduced to 80 pV. Thirty minutes after tourniquet inflation no pCMAPs could be evoked, even with an increase of
Recording set-up Tibialis-anterior-EMG recordings were taken with conventional silver/silverchloride surface electrodes in a belly-tendon fashion. The surface EMG was amplified with a conventional EMG-apparatus (Dantec Counterpoint, Denmark) with filter settings between 1 and 5 kHz. Signals were sampled by a laboratory computer with an AID conversion card at a sampling rate of 14 kHz.
f
a
Stirnuluting setup Proximally, a magnetic stimulator Magstim 200 (Magstim Company, UK) with a conventional Magstim 9 cm coil was placed beneath the lumbosacral junction during the positioning of the patient prior to surgery. The coil was positioned beneath the surgical table in such a way that the placement of the patient for surgery was not affected 1271. Distally, electrical stimulation was performed with commercially available stimulation electrodes for direct nerve stimulation (Xomed Company, Jacksonville, USA) by a constant current source (Dantec Counterpoint, Denmark). Single square-wave stimuli with a duration of 50 ps were used to stimulate the common peroneal nerve following nerve identification posterior to the fibular head. Optimal stimulationelectrode placement was performed by the surgeon directly on the perineurium. Magnetic stimuli were given with an interstimulus interval of 3 s. According to a technique described by Awiszus and Feistner [l], electrical stimuli were given between two magnetic stimuli. The strength of the magnetic stimulator was increased until a proximally evoked compound motor unit action potential (henceforth denoted pCMAP) with an amplitude of more than 50 pV was recorded in response to each stimulus. The electrical stimulus strength was adjusted to evoke a distal compound motor unit action potential (henceforth called dCMAP) with an amplitude comparable to the corresponding pCMAP (stimulus strengths ranging from 200 pA to 4 mA). Each assessment involved the recording of 5-8 responses to both types of stimuli and was repeated at 5-min intervals throughout surgery. Each measurement halted the surgical procedure for less than 30 s. The first assessment was taken before application of the tourniquet. Following the first readings we applied the tourniquet cuff and
C /
20
ms
e
Fig. 1. (a)-(e): pCMAPs of the tibialis anterior muscle evoked by magnetic stimulation of the sacral plexus (stimulus strength 94% of maximal stimulator output) during a HTO. (0-6): dCMAPs evoked by peripheral electrical stirnulation at the fibular head. First row: CMAPS obtained directly before tourniquet application: second row: 20 min after tourniquet inflation; third row: 30 min after tourniquet onset; fourth row: seventy-two seconds after tourniquet release: fifth row: recordings obtained eight minutes after tourniquet deflation.
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W. Nehelung et al. I Journal of Orthopedic Research 19 (2001) 160-165
stimulus strength to 100% of the stimulator output (Fig. 1(c)). Seventy-two seconds after tourniquet opening pCMAPs reappeared with a stimulus strength of 94% of stimulator output (Fig. l(d)) exhibiting an amplitude of 112 pV and a latency of 33 ms. The pCMAP latency recovered rapidly to 25 ms within the next 5 min (Fig. l(e)). The dCMAPs recorded (almost) simultaneously with the pCMAPs are shown in the right column of Fig. 1. It can be seen that throughout the entire surgical procedure dCMAPs were evoked exhibiting only small latency variations (range 7-10 ms). Especially, during the time interval after pCMAP-conduction block dCMAPs were readily obtained (Fig. l(h)). Whereas, pCMAPs appeared with a rather constant shape as long as they were recordable, dCMAPs revealed some shape variability. The CMAP latency changes for the single, representative patient shown in Fig. 1 are summarized in Fig. 2(a). The pCMAP latencies (filled circles) increased with time after tourniquet inflation until the complete conduction block occurred after 30 min, whereas dCMAP latencies (open circles) showed only a slight latency increase during the entire ischaemic period of 55 min. After tourniquet opening (indicated by the dashed line) pCMAPs reappeared with a latency that was considerably longer than the latencies immediately before the conduction block. Both latency increase and recovery appeared in a nonlinear fashion. The pCMAP latency shift compared to the pre-tourniquet latency is shown in Fig. 2(b) on a logarithmic scale. It can be seen that the increase followed a single exponential time course with a time constant of 9.3 min. The recovery appeared to be more complex and could be fitted with two exponentials with time constants of 2.3 and 13.1 min. Apparently, a residual latency shift of 3 ms was present at skin closure appearing 8 min after tourniquet opening in this patient. Similar results were obtained in all 11 the subjects monitored during HTO and the results are summarized in Table 1. Reliable pCMAPs were obtainable in all cases, however, stimulus strengths close to the maximal stimulator output were required. Nine out of the eleven patients exhibited a complete pCMAP conduction block after a tourniquet time of 25-30 min. The two patients without pCMAP block had tourniquet-inflation periods of only 20 and 35 min, respectively. Regardless of conduction-block occurrence, all patients exhibited a pCMAP latency shift following a single exponential time course (Table 1). After tourniquet deflation, pCMAPs reappeared in all cases with conduction block, and the latency shift decayed rather rapidly in all cases. In eight patients the latency-recovery time course could be well described with a double exponential, whereas a single exponential was sufficient for the remaining three cases (Table 1). In all but one patient the latency did not recover completely until skin closure, the patient with
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8
18
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0.24
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I
0 20 40 time after tourniquet inflation
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Fig. 2. CMAP latency changes during the operative procedure (monitoring of Fig. 1; filled circles: pCMAP latencies; open circles dCMAP latencies; dashed line: tourniquet deflation time), (a) absolute CMAP latencies on a linear scale, (b) pCMAP latency difference with respect to pre-tourniquet latency on a logarithmic scale.
complete recovery being the one with the shortest tourniquet-inflation time of only 20 min. In contrast to the pCMAPs, dCMAPs could be obtained throughout the entire surgical procedure in all but two cases. These two patients developed an apparent inexcitability of the peroneal nerve after a tourniquet time of 55 min. Nonetheless, even in these cases the time interval during which dCMAPs were available for peroneal nerve function monitoring exceeded that of the pCMAP availability.
Discussion
The results given show that the motor peroneal nerve function monitoring may be performed by both procedures tested.
W. Nebelung et al. I Journal of Orthopaedic Research 19 (2001) 160-165
m
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As shown in another report [21], dCMAP monitoring during HTO allows testing of the peroneal nerve conduction through the surgical field for almost one hour of tourniquet inflation. However, the size of the sample and the descriptive nature of the results limit the statistical value of the results. As our data did not show iatrogenic neurological deficiencies, clinical efficiency of dCMAP-monitoring remains to be investigated. The dCMAP recording is available almost twice as long for functional monitoring when compared to pCMAP recording. On the other hand, the shape of the dCMAPs was critically dependent on the stimulus electrode position and thus different nerve fibers were persumably tested during different assessments. This should not present a problem when mechanical damages to the entire nerve due to the surgical procedure are to be detected. On the other hand, nerve damage only to some portion of the peroneal nerve fibers may be left undetected. The liberation of the nerve is only justified in certain cases as for example during an HTO combined with a high osteotomy through the fibular head or in tumor resections. Additionally, the dCMAP monitoring requires positioning of a stimulus electrode for each assessment thus halting the surgical procedure. Moreover, recording equipment including an EMGamplifier, an electrical stimulator and stimulus electrodes are necessary to perform the method. The pCMAP recording, on the other hand, is much easier to perform with respect to the surgical procedure [27]. The original procedure reported by Unwin and Thomas [27], however, was performed during the administration of muscle relaxants. Consequently, it was necessary to take pCMAPs from the non-operated leg as a control for the mere drug effects. Moreover, muscle relaxants block neuromuscular transmission in axons in quite an unpredictable fashion and thus it cannot be expected that pCMAP recordings during muscle relaxation monitor a constant subset of motor axons. Our results, however, show that in the absence of neuromuscular-transmission block (although this is not always avoidable during knee surgery) pCMAP shapes are quite reliable indicating that the subset of motor axons monitored by the procedure remained identical throughout the entire recording period. Muscle relaxants have a severe impact on the latency of the recorded pCMAPs [27] making it impossible to evaluate pCMAP-latency shifts as a parameter for axonal conduction time. Our results show, that avoiding these drugs during pCMAP monitoring, provides information on axonal conduction velocity as well as on action-potential conduction. It is well known that ischaemia of a peripheral human nerve leads to a slowing of axonal conduction velocity [3] indicated by a latency shift of the compound muscle action potential. The apparent conduction slowing
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W Nebelung el al. I Journul of Orthopaedic Research 19 (2001) 160-165
appears to be attributable to the complex physiologicalcondition changes accompanying ischaemia [4]. Dysfunction of the electrogenic sodium-potassium pump inevitably leads to extracellular potassium and intracellular sodium accumulation. These changes in turn yield an axonal depolarization that inactivates sodium channels responsible for action potential generation. As a first consequence conduction velocity is slowed and with further deterioration of the ionic gradients conduction is biocked [3]. It has been described that the resistance of the axonal membrane to ischaemic changes increases in a peripheral direction [20]. Consequently, it is not surprising that dCMAPs were available during proximal conduction block. Additionally, the greatest changes in excitability both during and after ischaemia are known to occur directly beneath the cuff [3]. This increased sensitivity appeared to depend on exclusion of blood supply rather than on mechanical deformation of the nerve. Probably reduction of extrafascicular volume by pressure reduced the nervous oxygen capacity. Thus the pCMAP recording appears to be ideally suited to monitor ischaemic changes as those axonal sections with the most prominent alternations are included in the functional testing. Both excitability changes and conduction slowing were found to be fully reversible as long as the ischaemic period was kept below 10 min [4]. Thus, for each short tourniquet applications the intraaxonal sodium accumulation was found to have no permanent inadvertent effect on the axonal function. On the other hand, during longer tourniquet periods, commonly used in orthopedic surgery, subclinical irreversible loss of motor function was reported [9,3 11. Prospective studies of meniscectomies with 45 min tourniquet application demonstrate electromyographic abnormalities consistent with denervation of motor units in comparison to a group without tourniquet. Additionally, in a prospective study regarding revisions of carpal tunnel syndromes significant denervations by using a tourniquet were described [23]. A possible route to such a permanent functional loss has been described in the rat optic nerve fibers [26]. The intraaxonal sodium accumulation leads to activation of the sodium-calcium exchanger in a reverse direction thus allowing calcium entry with subsequent irreversible axonal damage [29]. A disadvantage in performing pCMAP-monitoring is the limited time period of 25 min tourniquet time. Especially, performing technically demanding procedures with a high risk of neural injury takes longer than 25 min, furthermore they are preferably performed in a bloodless operation field. Our measurements were performed under spinal anesthesia, for which muscle relaxants can completely be avoided. Spinal anesthesia is well established in elective knee surgery [7,10]. Moreover, even under
general anesthesia pCMAP recording is feasible, even if additional monitoring of muscle-relaxant effects is necessary [27]. Another important aspect is the need for an additional magnetic stimulation unit. Such units are widely used for the assessment of central motor function [24] or epidural monitoring in scoliosis correction [5]. The stimulation of the sacral plexus can be performed with the standard equipment, recording can be performed by a conventional EMG-amplifier. In summary, our results show that a pCMAP monitoring provides, in addition to mere conduction testing, at least some information on the ischaemic axonal changes due to tourniquet application. To what extent the different parameters of the observed latency shift (e.g. time constants of recovery) are useful indicators for permanent axonal damage remains to be determined.
Acknowledgements
This work was supported in part by Deutsche Forschungsgemeinschaft AW5l2- 1, AW5l2-2, and AW5l2-4. References [I] Awiszus F, Feistner H. Rapid on-line estimation of responses to transcranial magnetic and peripheral nerve electrical simulation in single human motoneurons. J Neurosci Methods 1995;61:91--7. [2] Baumgaertner MR, Wegner D, Booke J. SSEP monitoring during pelvic and acetabular fracture surgery. J Orthop Trauma 1994;8: 127-33. [3] Bostock H, Baker M, Grafe P, Reid C . Changes in excitability and accommodation of human motor axons following brief periods of ischaemia. J Physiol Lond 1991;441. [4] Bostock H, Cikurel K, Burke D. Threshold tracking techniques in the study of human peripheral nerve. Muscle Nerve 1998;21: 137-58. [S] Burke D, Hicks RG. Surgical monitoring of motor pathways [see comments]. J Clin Neurophysiol 1998;1S:19&205. [6] Curley P, Eyres K, Brezinova V, Allen M, Chan R, Barnes M . Common peroneal nerve dysfunction after high tibial osteotomy. J Bone Joint Surg [Br] 1990;72:405-8. [7] Dahl JB, Schultz P, Anker-Moller E, Christensen EF, Staunstrup HG, Carlsson P. Spinal anaesthesia in young patients using a guage needle: technical considerations and an evaluation of postoperative complaints compared with general anaesthesia. Br J Anaesth 1990;64:178-82. IS] Daniel DM, Lumkong G , Stone ML, Pedowitz RA. Effects of tourniquet use in anterior cruciate ligament reconstruction. Arthroscopy 1995;11:307-11. [9] Dobner JJ, Nitz AJ. Postmeniscectomy tourniquet palsy and functional sequelae. Am J Sports Med 1982;10:2114. [lo] Drakeford MK, Pettine KA, Brookshire L, Ebert F. Spinal narcotics for postoperative analgesia in total joint arthroplasty. A prospective study. J Bone Joint Surg [Am] 1991;73:424-8. [ll] Ejsted R, Hindso K, Mouritzen V. The total condylar knee prosthesis in osteoarthritis. A 5- to 10-year follow-up. Arch Orthop Trauma Surg 1994;113:61-5. [I21 Flier1 S, Sabo D, Hornig K, Perlick L. Open wedge high tibial osteotomy using fractioned drill osteotomy: a surgical modifica-
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tion that lowers the complication rate. Knee Surg Sprots Traumatol Arthrosc 1996;4:149-53. [I 31 Guanche CA. Tourniquet-induced tibial nerve palsy complicating anterior curciate ligament reconstruction. Arthroscopy 1995;ll: 620-2. [14] Helfet DL, Koval KJ, Hissa EA, Patterson S, DiPasquale T, Sanders R. Intraoperative somatosensory evoked potential monitoring during acute pelvic fracture surgery. J Orthop Trauma 1995;9:28-34. [I51 Huo MH, Sculco TP. Complications in primary total knee arthroplasty. Orthop Rev 1990;19:781-8. [16] Idusuyi OB, Morrey BF. Peroneal nerve palsy after total knee arthroplasty. Assessment of predisposing and prognostic factors. J Bone Joint Surg [Am] 1996;78:177-84. [17] Jurist KA, Greene PW, Shirkhoda A. Peroneal nerve dysfunction as a complication of lateral meniscus repair: a case report and anatomic dissection. Arthroscopy 1989;5:141-7. [I81 Kettelkamp DB, Leach RE, Nasca R. Pitfalls of proximal tibial osteotomy. Clin Orthop 1975:23241. [19] Kirgis A, Albrecht S. Palsy of the deep peroneal nerve after proximal tibial osteotomy. An anatomical study [see comments]. J Bone Joint Surg [Am] 1992;74:1180-5. [20] Kugelberg E. Activation of human nerves by ischemia. Arch Neurol Psychiatr 1948;60:140-52. [21] Nebelung W, Wissel H, Awiszus F. Intraoperative threshold hunting for monitoring of common peroneal nerve function during high tibial osteotomies. Zentralbl Chir 1998;123:1267--70. [22] Nercessian OA, Gonzalez EG, Stinchfield FE. The use of somatosensory evoked potential during revision or reoperation for total hip arthroplasty. Clin Orthop 1989:13842.
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[23] Nitz AJ, Dobner JJ. Upper extremity tourniquet effects in carpal tunnel release. J Hand Surg [Am] 1989;14:499-504. [24] Rothwell JC. Techniques and mechanisms of action of transcranial stimulation of the human motor cortex. J Neurosci Methods 1997;74:113-22. [25] Slawski DP, Schoenecker PL, Rich MM. Peroneal nerve injury as a complication of pediatric tibial osteotomies: a review of 255 osteotomies. J Pediatr Orthop 1994;14:166-72. [26] Stys PK, Waxman SG, Ransom BR. Ionic mechanisms of anoxic injury in mammalian CNS white matter: role of Na' channels and Na(+)-Ca2+ exchanger. J Neurosci 1992;12:430-9. [27] Unwin AJ, Thomas M. Intra-operative monitoring of the common peroneal nerve during total knee replacement. J R SOCMed 1994;87:70 1-3. [28] Vrahas M, Gordon RG, Mears DC, Krieger D, Sclabassi RJ. Intraoperative somatosensory evoked potential monitoring of pelvic and acetabular fractures. J Orthop Trauma 1992;6: 50-8. [29] Waxman SG, Black JA, Ransom BR, Stys PK. Anoxic injury of rat optic nerve: ultrastructural evidence for coupling between Na' influx and Ca(2+)-mediated injury in myelinated CNS axons. Brain Res 1994;644:197--204. [30] Weber BG, Worsdorfer 0. Zuggurtungsosteosynthese bei Tibiakopfosteotomie. 2 Orthop 1980;124:637. [31] Weingarden SI, Louis DL, Waylonis GW. Electromyographic changes in postmeniscectomy patients. Role of the pneumatic tourniquet. JAMA 1979;24I :1248-50.
Journal of Orthopaedic Research Journal of Orthopaedic Research 19 (2001) 160-165
www.elsevier.nl/locate/orthres
On the applicability of two different stimulation techniques for intra-operative peroneal nerve conduction testing W. Nebelung *, H. Wissel, F. Awiszus Neurornutcular Re yearch Group, Otto-von Gueritke-Uriivervity Magdehurg, Clinic f o r Orthopedic Surgery, Leipziger Str 44, 39120 Magdehurg, Germany Received 14 May 1999, accepted 23 March 2000
Abstract
Dysfunction of the peroneal nerve is an important complication of knee surgery. We compared two monitoring procedures of peroneal nerve function during a standardized operation, a closing wedge high tibial osteotomy. For two types of stimulation the evoked compound motor unit action potentials (CMAPs) were recorded on the tibialis anterior muscle. We used direct perineural electrical Stimulation of the common peroneal nerve distal to the cuff (dCMAPs) after nerve identification in the surgical field. Additionally, magnetic stimulation of the sacral plexus proximal to the cuff (pCMAPs) was performed. It was found that dCMAPs were recorded during almost one hour of tourniquet time whereas the pCMAPs were blocked after 25-30 min in 9 out of 11 cases. On the other hand, the CMAP obtained after proximal stimulation exhibited a latency shift with tourniquet yielding an indicator of ischaemic changes present beneath and distal to the tourniquet cuff. In conclusion, different applicabilities of both stimulation techniques under tourniquet conditions were demonstrated. 0 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved.
Introduction It is generally accepted that the peroneal nerve is at risk during knee surgery [ 11,13,15-171. Peroneal nerve palsies may occur following in particular high tibial osteotomies (HTO) and may compromise the functional result. For HTO these palsies belong to the most important complications and occur up to a frequency of 16Yo [6,12,18,19,25]. There is some doubt as to what extent iatrogenic surgical injury or ischaemic changes due to the tourniquet cuff are responsible for the observed postoperative palsies. One reason might be the iatrogenic damage due to the lateral approach and fibular osteotomy [19]. On the other hand, the clinical use of a tourniquet although very common in the operative setting of orthopedic surgery, has been shown to be responsible for nerve damage [9,23]. Even if this functional loss appears to be subclinical in most of the cases [6], such neuromuscular
*Corresponding author. Tel.: +49-391-6714032; fax: +49-39167190143. E-mail addrrsx [email protected] (W. Nebelung).
deficiencies might influence optimal postoperative muscular rehabilitation [S]. Several techniques for testing the neural function during surgical procedures are available. Some reports about monitoring neurological deficiencies using somatosensory evoked potentials (SSEPs) in orthopedic surgery were published [2,14,22,28]. As the functional disability of the lower extremity depends primarily on motor function testing this appears to be of impact for the evaluation of available techniques. Testing motor conduction distal to the lumbosacral plexus using a magnetic stimulator was first performed during total knee replacements [27]. In addition, intraoperative orthodrome stimulation of the common peroneal nerve during a tibial osteotomy was described in a former study [21]. Both procedures test action potential conduction in motor fibres. Strength and weakness of both procedures are currently unknown. In order to evaluate the advantages and disadvantages we performed a simultaneous application of two different motor pathway stimulation techniques during a relatively standardized surgical procedure, a high tibia1 osteotomy for medial osteoarthritis of the knee joint.
0736-0266/01/$ - see front matter 0 2001 Orthopaedic Research Society. Published by Elsevier Science Ltd. All rights reserved PIr: s o 7 ~ 6 - o 2 6 6 ( o o ) o o o6i - 4
W Nebelung et ul. I Journal of' Orthopuedic Research I 9 (2001) 160-165
Materials and methods Put ients We monitored the function of common peroneal nerve fibers by stimulation above and below the tourniquet in 11 patients undergoing a closing wedge high tibial osteotomy. The age of the patients ranged between 44 and 68 (mean: 54 years, 8 female, 5 male). All patients were suffering from degenerative osteoarthritis of the medial compartment of the knee. Prior to the monitored operation in all patients an arthroscopy was performed. If accompanying meniscal lesions were present they were treated by resection. The lateral compartment was confirmed to have no degenerative cartilage damage. Patients with neurological disorders were excluded from the study. Since the peroneal nerve needs to be exposed for the proximal fibular osteotomy the monitoring did not alter the surgical course. All experiments procedures were approved by the local ethical committee. Operative technique A standard spinal anesthesia was performed with intradural application of 2.5-3.5 ml 0.5% Bupivacain. Muscle relaxants were not given throughout the entire procedure. We performed a lateral longitudinal approach to the knee joint. Posterior to the distal insertion of the biceps muscle the common peroneal nerve was identified. In all cases we performed a high fibular osteotomy directly through the fibular head. Following this a closing wedge osteotomy superior to the tibial tubercle was performed. We fixed the fragments using an AO-plate and a cancellous bone screw in the technique described by Weber [30]. Postoperatively, neither a cast nor limitation of range of motion was ordered. We allowed increasing weightbearing after 6 weeks in all cases and 10 weeks after operation full weightbearing was admitted.
161
maintained a pressure of 300 mm Hg until fixing the osteotomy. When the fragments were fixed the tourniquet was released and recordings were continued until skin closure. Dutu unulysis Recorded CMAPs were analyzed off-line. A purpose-written software was used to display and analyze the recorded data. A cursor was used to evaluate the latencies of all recorded CMAPs. Latency-shift time courses were fitted with a single or double exponential using a nonlinear least-square algorithm.
Results Fig. 1 shows typical recordings of pCMAPs (Fig. l(a)-(e)) and dCMAPs (Fig. l(Q-(j)) during an HTO in a 51-year-old male patient. Prior to tourniquet inflation the strength of the magnetic stimulator was increased up to 940/0 for the case of Fig. 1 until a reliable pCMAP exhibiting a latency of 18.6 ms and an amplitude of 115 pV was obtained (Fig. l(a)). After 20 min of tourniquet (Fig. 1(b)) pCMAPs were still obtainable with the same stimulus strength, whereas latency increased to about 25 ms and amplitude was reduced to 80 pV. Thirty minutes after tourniquet inflation no pCMAPs could be evoked, even with an increase of
Recording set-up Tibialis-anterior-EMG recordings were taken with conventional silver/silverchloride surface electrodes in a belly-tendon fashion. The surface EMG was amplified with a conventional EMG-apparatus (Dantec Counterpoint, Denmark) with filter settings between 1 and 5 kHz. Signals were sampled by a laboratory computer with an AID conversion card at a sampling rate of 14 kHz.
f
a
Stirnuluting setup Proximally, a magnetic stimulator Magstim 200 (Magstim Company, UK) with a conventional Magstim 9 cm coil was placed beneath the lumbosacral junction during the positioning of the patient prior to surgery. The coil was positioned beneath the surgical table in such a way that the placement of the patient for surgery was not affected 1271. Distally, electrical stimulation was performed with commercially available stimulation electrodes for direct nerve stimulation (Xomed Company, Jacksonville, USA) by a constant current source (Dantec Counterpoint, Denmark). Single square-wave stimuli with a duration of 50 ps were used to stimulate the common peroneal nerve following nerve identification posterior to the fibular head. Optimal stimulationelectrode placement was performed by the surgeon directly on the perineurium. Magnetic stimuli were given with an interstimulus interval of 3 s. According to a technique described by Awiszus and Feistner [l], electrical stimuli were given between two magnetic stimuli. The strength of the magnetic stimulator was increased until a proximally evoked compound motor unit action potential (henceforth denoted pCMAP) with an amplitude of more than 50 pV was recorded in response to each stimulus. The electrical stimulus strength was adjusted to evoke a distal compound motor unit action potential (henceforth called dCMAP) with an amplitude comparable to the corresponding pCMAP (stimulus strengths ranging from 200 pA to 4 mA). Each assessment involved the recording of 5-8 responses to both types of stimuli and was repeated at 5-min intervals throughout surgery. Each measurement halted the surgical procedure for less than 30 s. The first assessment was taken before application of the tourniquet. Following the first readings we applied the tourniquet cuff and
C /
20
ms
e
Fig. 1. (a)-(e): pCMAPs of the tibialis anterior muscle evoked by magnetic stimulation of the sacral plexus (stimulus strength 94% of maximal stimulator output) during a HTO. (0-6): dCMAPs evoked by peripheral electrical stirnulation at the fibular head. First row: CMAPS obtained directly before tourniquet application: second row: 20 min after tourniquet inflation; third row: 30 min after tourniquet onset; fourth row: seventy-two seconds after tourniquet release: fifth row: recordings obtained eight minutes after tourniquet deflation.
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stimulus strength to 100% of the stimulator output (Fig. 1(c)). Seventy-two seconds after tourniquet opening pCMAPs reappeared with a stimulus strength of 94% of stimulator output (Fig. l(d)) exhibiting an amplitude of 112 pV and a latency of 33 ms. The pCMAP latency recovered rapidly to 25 ms within the next 5 min (Fig. l(e)). The dCMAPs recorded (almost) simultaneously with the pCMAPs are shown in the right column of Fig. 1. It can be seen that throughout the entire surgical procedure dCMAPs were evoked exhibiting only small latency variations (range 7-10 ms). Especially, during the time interval after pCMAP-conduction block dCMAPs were readily obtained (Fig. l(h)). Whereas, pCMAPs appeared with a rather constant shape as long as they were recordable, dCMAPs revealed some shape variability. The CMAP latency changes for the single, representative patient shown in Fig. 1 are summarized in Fig. 2(a). The pCMAP latencies (filled circles) increased with time after tourniquet inflation until the complete conduction block occurred after 30 min, whereas dCMAP latencies (open circles) showed only a slight latency increase during the entire ischaemic period of 55 min. After tourniquet opening (indicated by the dashed line) pCMAPs reappeared with a latency that was considerably longer than the latencies immediately before the conduction block. Both latency increase and recovery appeared in a nonlinear fashion. The pCMAP latency shift compared to the pre-tourniquet latency is shown in Fig. 2(b) on a logarithmic scale. It can be seen that the increase followed a single exponential time course with a time constant of 9.3 min. The recovery appeared to be more complex and could be fitted with two exponentials with time constants of 2.3 and 13.1 min. Apparently, a residual latency shift of 3 ms was present at skin closure appearing 8 min after tourniquet opening in this patient. Similar results were obtained in all 11 the subjects monitored during HTO and the results are summarized in Table 1. Reliable pCMAPs were obtainable in all cases, however, stimulus strengths close to the maximal stimulator output were required. Nine out of the eleven patients exhibited a complete pCMAP conduction block after a tourniquet time of 25-30 min. The two patients without pCMAP block had tourniquet-inflation periods of only 20 and 35 min, respectively. Regardless of conduction-block occurrence, all patients exhibited a pCMAP latency shift following a single exponential time course (Table 1). After tourniquet deflation, pCMAPs reappeared in all cases with conduction block, and the latency shift decayed rather rapidly in all cases. In eight patients the latency-recovery time course could be well described with a double exponential, whereas a single exponential was sufficient for the remaining three cases (Table 1). In all but one patient the latency did not recover completely until skin closure, the patient with
I
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OJ
0.24
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0 20 40 time after tourniquet inflation
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Fig. 2. CMAP latency changes during the operative procedure (monitoring of Fig. 1; filled circles: pCMAP latencies; open circles dCMAP latencies; dashed line: tourniquet deflation time), (a) absolute CMAP latencies on a linear scale, (b) pCMAP latency difference with respect to pre-tourniquet latency on a logarithmic scale.
complete recovery being the one with the shortest tourniquet-inflation time of only 20 min. In contrast to the pCMAPs, dCMAPs could be obtained throughout the entire surgical procedure in all but two cases. These two patients developed an apparent inexcitability of the peroneal nerve after a tourniquet time of 55 min. Nonetheless, even in these cases the time interval during which dCMAPs were available for peroneal nerve function monitoring exceeded that of the pCMAP availability.
Discussion
The results given show that the motor peroneal nerve function monitoring may be performed by both procedures tested.
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m
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As shown in another report [21], dCMAP monitoring during HTO allows testing of the peroneal nerve conduction through the surgical field for almost one hour of tourniquet inflation. However, the size of the sample and the descriptive nature of the results limit the statistical value of the results. As our data did not show iatrogenic neurological deficiencies, clinical efficiency of dCMAP-monitoring remains to be investigated. The dCMAP recording is available almost twice as long for functional monitoring when compared to pCMAP recording. On the other hand, the shape of the dCMAPs was critically dependent on the stimulus electrode position and thus different nerve fibers were persumably tested during different assessments. This should not present a problem when mechanical damages to the entire nerve due to the surgical procedure are to be detected. On the other hand, nerve damage only to some portion of the peroneal nerve fibers may be left undetected. The liberation of the nerve is only justified in certain cases as for example during an HTO combined with a high osteotomy through the fibular head or in tumor resections. Additionally, the dCMAP monitoring requires positioning of a stimulus electrode for each assessment thus halting the surgical procedure. Moreover, recording equipment including an EMGamplifier, an electrical stimulator and stimulus electrodes are necessary to perform the method. The pCMAP recording, on the other hand, is much easier to perform with respect to the surgical procedure [27]. The original procedure reported by Unwin and Thomas [27], however, was performed during the administration of muscle relaxants. Consequently, it was necessary to take pCMAPs from the non-operated leg as a control for the mere drug effects. Moreover, muscle relaxants block neuromuscular transmission in axons in quite an unpredictable fashion and thus it cannot be expected that pCMAP recordings during muscle relaxation monitor a constant subset of motor axons. Our results, however, show that in the absence of neuromuscular-transmission block (although this is not always avoidable during knee surgery) pCMAP shapes are quite reliable indicating that the subset of motor axons monitored by the procedure remained identical throughout the entire recording period. Muscle relaxants have a severe impact on the latency of the recorded pCMAPs [27] making it impossible to evaluate pCMAP-latency shifts as a parameter for axonal conduction time. Our results show, that avoiding these drugs during pCMAP monitoring, provides information on axonal conduction velocity as well as on action-potential conduction. It is well known that ischaemia of a peripheral human nerve leads to a slowing of axonal conduction velocity [3] indicated by a latency shift of the compound muscle action potential. The apparent conduction slowing
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appears to be attributable to the complex physiologicalcondition changes accompanying ischaemia [4]. Dysfunction of the electrogenic sodium-potassium pump inevitably leads to extracellular potassium and intracellular sodium accumulation. These changes in turn yield an axonal depolarization that inactivates sodium channels responsible for action potential generation. As a first consequence conduction velocity is slowed and with further deterioration of the ionic gradients conduction is biocked [3]. It has been described that the resistance of the axonal membrane to ischaemic changes increases in a peripheral direction [20]. Consequently, it is not surprising that dCMAPs were available during proximal conduction block. Additionally, the greatest changes in excitability both during and after ischaemia are known to occur directly beneath the cuff [3]. This increased sensitivity appeared to depend on exclusion of blood supply rather than on mechanical deformation of the nerve. Probably reduction of extrafascicular volume by pressure reduced the nervous oxygen capacity. Thus the pCMAP recording appears to be ideally suited to monitor ischaemic changes as those axonal sections with the most prominent alternations are included in the functional testing. Both excitability changes and conduction slowing were found to be fully reversible as long as the ischaemic period was kept below 10 min [4]. Thus, for each short tourniquet applications the intraaxonal sodium accumulation was found to have no permanent inadvertent effect on the axonal function. On the other hand, during longer tourniquet periods, commonly used in orthopedic surgery, subclinical irreversible loss of motor function was reported [9,3 11. Prospective studies of meniscectomies with 45 min tourniquet application demonstrate electromyographic abnormalities consistent with denervation of motor units in comparison to a group without tourniquet. Additionally, in a prospective study regarding revisions of carpal tunnel syndromes significant denervations by using a tourniquet were described [23]. A possible route to such a permanent functional loss has been described in the rat optic nerve fibers [26]. The intraaxonal sodium accumulation leads to activation of the sodium-calcium exchanger in a reverse direction thus allowing calcium entry with subsequent irreversible axonal damage [29]. A disadvantage in performing pCMAP-monitoring is the limited time period of 25 min tourniquet time. Especially, performing technically demanding procedures with a high risk of neural injury takes longer than 25 min, furthermore they are preferably performed in a bloodless operation field. Our measurements were performed under spinal anesthesia, for which muscle relaxants can completely be avoided. Spinal anesthesia is well established in elective knee surgery [7,10]. Moreover, even under
general anesthesia pCMAP recording is feasible, even if additional monitoring of muscle-relaxant effects is necessary [27]. Another important aspect is the need for an additional magnetic stimulation unit. Such units are widely used for the assessment of central motor function [24] or epidural monitoring in scoliosis correction [5]. The stimulation of the sacral plexus can be performed with the standard equipment, recording can be performed by a conventional EMG-amplifier. In summary, our results show that a pCMAP monitoring provides, in addition to mere conduction testing, at least some information on the ischaemic axonal changes due to tourniquet application. To what extent the different parameters of the observed latency shift (e.g. time constants of recovery) are useful indicators for permanent axonal damage remains to be determined.
Acknowledgements
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