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Iowa Orthop J. 2010; 30: 104–108.
PMCID: PMC2958280




Dynamic intraoperative assessment of patella tracking utilizes femoral nerve stimulation to contract the quadriceps muscles in assessing the proper distance to transfer the tibial tubercle during distal realignment procedures for patellofemoral instability.


We describe the effects of tourniquet inflation and catheter placement on intraoperative femoral nerve stimulation for assessment of patel-lar tracking.


Percutaneous electromyographic (EMG) needles were placed into the quadriceps and sartorius muscles to monitor muscle activity and changes in amplitude threshold (mA) required for femoral nerve stimulation with increasing tourniquet inflation times. Eleven patients used ultrasound for catheter placement and ten were manually placed based upon body landmarks.


Tourniquet application time correlated positively with the change in amplitude threshold required to generate muscle contraction. Patients had an average four-fold increase in required stimulus amplitude from the baseline thresholds (pre-tourniquet inflation) to final thresholds (tourniquet inflated) with a two-hour tourniquet inflation time. The use of ultrasound for catheter placement significantly decreased the baseline amplitude required in comparison with catheters placed without ultrasound, (p = 0.0330).


Increased tourniquet inflation times require greater stimulus amplitude to generate quadriceps muscle contraction. Ultrasound guidance for catheter placement can provide femoral nerve stimulation at low amplitudes.


The senior author has utilized peripheral nerve stimulation in an attempt to provide a dynamic assessment during patellar realignment procedures.1,2 The purpose of this study was to evaluate the influence of the use of a tourniquet on intraoperative femoral nerve stimulation and the resultant muscle contraction/activity. The study objective is to evaluate the threshold required to stimulate the femoral nerve with increased tourniquet time. A second objective is to compare the accuracy of catheter placement for nerve stimulation with and without ultrasound-guided assistance.

Brief History of Realignment and Rationale

There have been over a hundred procedures described to treat PFI.3,4 A key question in all procedures, is just how far does one need to transfer the tendon? The senior author has focused on the use of quadriceps muscle contraction intraoperatively to assess the kinematic tracking of the patella for determining the proper transfer distance of the tendon for proper patellofemoral alignment.

Peripheral nerve stimulation is being used to add further dynamic evaluation of the patellar realignment procedures.1 This current study is to evaluate effects of the tourniquet use on this dynamic assessment and provide guidelines for its use. This dynamic evaluation technique allows the surgeon to assess how the patella tracks within the trochlea when the quadriceps mechanism is stimulated in the absence of other soft tissue constraints. Thus, we are hypothesizing that this dynamic tracking provides a more realistic picture of what can be expected post-operatively.

Lavery et al. described an innovative approach known as dynamic intraoperative assessment utilizing a “stimulating” femoral catheter to directly activate the quadriceps muscle to assess patellar tracking and accomplish proper placement for the tibial tubercle osteotomy.2 Ebinger et al. also published preliminary results of the procedure.1 These two publications made it evident that the tourniquet plays a pivotal role in femoral nerve stimulation, not only regarding the duration of tourniquet inflation but also in the accurate placement of the catheter for initial stimulation. In this study, we explore the effects of the pneumatic tourniquet on direct femoral nerve stimulation used to perform dynamic in-traoperative assessment for patella tracking. Tourniquet application is hypothesized to increase the amplitude required to generate muscle contraction during EMG intraoperative assessment.

In this study, EMG activity was used to evaluate the intraoperative tourniquet effects by measuring the changes in amplitude threshold (mA) required to produce a palpable muscle contraction of the extensor mechanism via femoral nerve stimulation. A second focus of the study was to determine whether there was a difference in the baseline threshold required with femoral catheters placed with the aid of ultrasound guidance in comparison to catheter placement by free-hand method, utilizing only anatomical body landmarks.


Twenty-one patients participated in the study (11 males and 10 females). Patients were included in the study if they were to have a lower extremity surgery and receive a femoral block to relieve post-operative knee pain. The procedures performed were not restricted to FulkersonVmedial patellofemoral ligament (MPFL) reconstructions; the majority of patients were being treated for anterior cruciate ligament (ACL) reconstructions. Patients receiving operations that did not allow for sufficient use of the tourniquet to observe its effects on femoral nerve stimulation were excluded from participation in the study. With the patient lying in the supine position, a “stimulating” femoral catheter was placed anteriorly in the patient's operative leg by an anesthesiologist. The “stimulating” catheter was connected to an EMG monitoring system, NIM-Eclipse Spinal System (Medtronic, Minneapolis, MN) that provided direct stimulation to the femoral catheter in a pulsating frequency of 50 Hz. A more detailed description of catheter placement can be found in the primary paper describing the procedure.6 Because of an observed variation in the placement of the catheters early in the study, ultrasound was also utilized for proper placement of the femoral catheters in one group (eleven patients) and free-hand placement was used in the other group (ten patients). A standard sized cuff pneumatic tourniquet was set at lOOmmHg above the patient's systolic blood pressure during its application. The tourniquet was applied in a standard manner for a time period of at least one hour, but not exceeding two hours. Percutaneous EMG needles were placed distal to the tourniquet in four muscle bellies receiving their innervations from the femoral nerve: the vastus lateralis, rectus femoris, vastus medialis, and the sartorius muscle (Figure 1). The needles were spaced approximately 4.0 cm apart and covered with Tegaderm. Muscle activity was monitored by the EMG monitoring system and by palpation of the muscle contractions in the sterile field. This provided confirmation that the EMG activity being recorded was produced by the muscle contractions generated because of the femoral nerve stimulation.

Figure 1
Percutaneous EMG electrode placement in quadriceps.

The protocol for evaluating the effects of the tourniquet on femoral nerve stimulation were as follows: 1) An initial stimulation was performed before tourniquet inflation to establish the patient's baseline amplitude threshold (mA) required to produce contraction of the quadriceps and sartorius muscles; 2) At 30-minute intervals, while the tourniquet was inflated, the femoral nerve was stimulated to observe the threshold of stimulus required to produce muscle contraction; and 3) When the tourniquet was deflated, femoral nerve stimulation was performed every 1 to 2 minutes to observe the length of time required for the threshold of required stimulus to return to that of the patient's initial baseline. For patients having a distal realignment procedure to treat PFI, the EMG evaluation included the aforementioned protocol and stimulations with the NIM-Eclipse Spinal System (Medtronic, Minneapolis, MN) before and after tetanus stimulations for dynamic intraoperative assessment. This was used to assess whether muscle fatigue occurred over the course of dynamic intraoperative assessment for patellar tracking. The research study was given full approval by the Institutional Review Board.


Thirty patients were recruited to the study and 21 participants completed the study (Table 1). There were 14 ACL reconstructive operations, four FulkersonVMPFL reconstructions, two isolated MPFL reconstructions, and one posterior cruciate ligament (PCL) reconstruction. Nine patients were lost to the study for the following reasons; the catheter accidentally being dosed with local anesthetic (three patients), intubation of patient with long-acting muscle relaxant (one patient), patient opting out of the study prior to surgery (four patients), and equipment malfunction (one patient). The participants in the study had a positive correlation between the tourniquet application time and the magnitude of stimulus threshold (mA) required to generate contraction of the quadriceps and sartorius muscles. The patients demonstrated an average of a four-fold increase in the magnitude of required stimulus amplitude (mA) change from their baseline response (tourniquet not inflated) as compared to that of the final stimulation (tourniquet inflated) after a period of 90 to 120 minutes duration (Figures 2A, ,2B).2B). Baseline stimulations ranged from as low as 0.5 mA to as high as 11.0 mA. Final stimulations with the tourniquet applied ranged from 1.3 mA to 30.0 mA. There was a wide range of baseline stimulus thresholds in the patients. We hypothesized that the precision of the catheter placement nearest the nerve might play a role in achieving muscle contraction at lower thresholds via femoral nerve stimulation. To evaluate whether a more precise placement of the catheter in closer proximity to the femoral nerve would result in lower baseline thresholds, ultrasound was utilized to guide the placement of the catheters in eleven of the twenty-one patients. An acceptable baseline threshold was set at 5 mA or lower. Only three of the twenty-one patients (14%) had baseline thresholds (6 mA, 8 mA, and 11 mA) above the set point for a desirable placement of the femoral catheter. These three individuals used free-hand placement of the catheters. The group of patients with ultrasound-guided placement of the catheters (eleven patients) had an average baseline threshold of 1.782 mA (SD = 1.266; SEM = 0.382; Range = 0.5 mA to 4.5 mA; Mdn = 1.0mA; Mode = 1.0mA). In the free-hand catheter placement group (ten patients) there was an average baseline threshold of 4.240 mA (SD = 3.295; SEM = 1.042 Range = 0.5 mA to 11.0 mA; Mdn = 4.0 mA; Mode = 4.0 mA). The difference between the average baseline stimulation of both groups was statistically significant (p = 0.0330) (Figure 3). The final stimulation (tourniquet inflated) threshold for the group using ultrasound had an average of 8.909 mA (SD = 7.569; SEM = 2.282; Range = 3.0 mA to 30 mA; Mdn = 6.0mA; Mode = 5.0mA) and the average final stimulation for the free-hand placement group was 10.50 mA (SD = 8.807; SEM = 2.785; Range=1.3 mA to 30 mA; Mdn = 10.5mA; Mode = 15.0mA) and not statistically significant (p = 0.6612). During the distal realignment procedures, the tetanus stimulations did not appear to have a significant influence upon the magnitude of change in thresholds observed throughout the application of the tourniquet. Thresholds appeared to follow a similar increase that was observed in patients who were not receiving multiple tetanic stimulations. Follow-ing tourniquet deflation, an average of six minutes was required for each patient's stimulus threshold to return to baseline (Figure 4).

Patient data
Figure 2A
Bar graph of average threshold at different stimulation times.
Figure 2B
Linear graph of averaged thresholds versus tourniquet application time.
Figure 3
Ultrasound versus free-hand catheter placement.
Figure 4
Recovery post-tourniquet release.


Tourniquet application has an appreciable effect on femoral nerve stimulation and the performance of dynamic intraoperative assessment. Ischemic conditions and compression of the soft tissues that occur during tourniquet application have been shown to decrease the amount of muscle contraction force generated post tourniquet application.6,7 This study evaluated the electrophysiological changes of nerve conduction during tourniquet application. The longer the tourniquet application time, the greater the threshold of stimulus required to produce an unequivocal muscle contraction. Understanding the effects of the tourniquet on femoral nerve stimulation is essential for physicians who will implement dynamic intraoperative assessment into their treatment of PFI with distal realignment procedures. Precise placement of the femoral catheter in close proximity of the femoral nerve can allow the physician to establish lower baseline thresholds to produce quadriceps muscle contraction and observe the tracking of the patella. There was a significant difference in the baseline thresholds established in patients with the ultrasound-guided catheter placement in comparison to those with catheters placed with free-hand technique. There was not a significant difference in the final thresholds between the two groups; however, through a close examination of the data it is evident that patients who had higher baseline thresholds were more likely to have higher final thresholds. In the ultrasound group (eleven patients) there were only three cases (11 mA, 12 mA, 30 mA) resulting in a final threshold stimulation of 10 mA or greater, whereas the free-hand group (ten patients) had six cases of greater than 10 mA (10 mA, 11 mA, 14 mA, 15 mA, 15 mA, 30 mA).

Patients were not randomly assigned to one group or the other for the purposes of this study, nor were the anesthesiologists placing the catheters. Therefore, it is unknown whether a novice placing the ultrasound catheter would equate to, or be superior to an expert performing free-hand catheter placement. The use of ultrasound guidance is not essential to carry out dynamic intraoperative assessment, it merely serves as an aid to provide closer placement of the catheter to the femoral nerve. We believe that ultrasound may allow for lower amplitudes of stimulus for dynamic intraoperative assessment. When higher amplitudes are required to contract the extensor mechanism as a result of the catheter being placed further away from the femoral nerve, it could be the result of direct stimulation of muscle in conjunction with the stimulation of the femoral nerve. In the three cases where we observed baseline thresholds above 5 mA, we hypothesize that there was a field effect in which the stimulation was not isolated to the femoral nerve and may have resulted in the direct stimulation of the muscles as well. Lower baseline thresholds of 3 mA or less were achieved in both groups; however, these low thresholds were more frequently established in the ultrasound group with ten cases (0.5 mA, 1 mA, 1 mA, 1 mA, 1 mA, 1 mA, 1.1 mA, 2.5 mA, 3 mA, 3 mA) in comparison to only four cases in the free-hand group (0.5 mA, 1.4 mA, 1.5 mA, 2 mA). For this reason, ultrasound placement can be helpful for those who are familiar with ultrasound imaging.

Implementing dynamic intraoperative assessment into the operating room will not require any major changes in the manner in which a surgeon operates and assesses the transfer distance of the tibial tubercle. Consequently, the senior author has not experienced any significant difference in the length of surgery time in comparison to his performing distal realignment procedures in patients with PFI prior to his use of dynamic intraoperative assessment. In conducting this study, collaboration between the surgeon and anesthesiologist was important. At our institution, regional blocks for surgery on the lower extremities are very commonly employed to provide localized post-operative pain relief. If a surgeon wants to perform dynamic intraoperative assessment, it is imperative that the femoral catheter is not dosed until after dynamic intraoperative assessment is completed. Some patients who were consented to the study were unable to participate because their catheters had been dosed prior to entering the operating room. In this event, one would have to resort to passive assessment for patellar tracking. We have found this situation to be easily avoided by notifying the anesthesia department of our protocol and adding a clinical note in the patient's records for the treating anesthesiologist. If the surgeon plans to perform dynamic intraoperative assessment early on in an operation, endotracheal intubations of the patient for general anesthesia should also not include any long-acting muscle relaxants that can impede contraction of the extensor mechanism. We recommend using succinylcholine when the surgeon requires earlier assessment as in isometry testing of a semitendinosis graft in an isolated MPFL reconstruction. Through our experimentation we have noted that should an operation require extended use of the tourniquet over two hours, or in the event of a patient's threshold increases to such a point beyond the amplitude that is producible by the nerve stimulator, dynamic intraoperative assessment can continue to be performed by releasing the tourniquet and stimulating the femoral nerve at the patient's baseline threshold within approximately six minutes after deflation of the tourniquet cuff.


The use of a pneumatic tourniquet has an effect on femoral nerve stimulation that should be taken into consideration when performing dynamic intraoperative assessment. The longer the tourniquet is applied, the greater the threshold of required stimulus to generate sufficient contraction of the extensor mechanism to observe patellar tracking. The use of ultrasound for placement of the femoral catheter can provide proficient localization of the femoral nerve and allow for use of lower amplitudes of stimulus to perform dynamic intraoperative assessment for patellar tracking. The use of ultrasound is not required for performing dynamic intraoperative assessment. Lower baseline thresholds of 3mA or less were also achieved with free-hand placement of the catheter; however, they were more frequently observed in the ultrasound group.


We gratefully acknowledge Medtronic's loan of EMG equipment.

The authors express their appreciation and thanks to Diana Johannes for her assistance in the preparation of this article.


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