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Lai W-H, Shih Y-F, Lin P-L, Chen W-Y, Ma H-L. Specificity of the femoral slump test for the assessment of experimentally induced anterior knee pain.
To assess the specificity of the femoral slump test (FST) when assessing experimentally induced anterior knee pain.
Cross-sectional, exploratory study.
Asymptomatic subjects (N=12; 6 men; 6 women) for the study. An experimental pain model was used to simulate anterior knee pain by injecting .25mL of hypertonic saline solution (5% NaCl) into the medial infrapatellar fat pad.
Main Outcome Measure
The changes in pain intensity and diameter after applying the structure differential maneuver (neck flexion/extension) during the FST were recorded and analyzed.
Results revealed that the structure differential maneuver of the FST did not alter the pain intensity or diameter in 9 (neck extension) and 10 (neck flexion) out of 12 subjects, which meant that the FST provided appropriate testing responses in 75% to 83% cases when the anterior knee pain did not originate in neural tissues.
The FST had a specificity of more than .75 when detecting nerve mechanosensitivity problems of anterior knee pain.
ANTERIOR KNEE PAIN describes pain or discomfort around or under the patella. Despite various management methods to correct or improve the underlying problems, some patients with anterior knee pain suffer long-lasting pain.
suggested that one possible cause leading to a lack of recovery arose from problems with neural structures around the patella. Sensory innervation of the patellofemoral joint and its surrounding tissues comes mainly from 2 branches of the femoral nerve, the medial and lateral patellar nerves.
hypothesized that excessive local pressure over the patella might cause periodic short episodes of ischemia, which would trigger neural proliferation and cause pain.
The femoral slump test (FST) comprises the neck, trunk, and knee flexion and hip extension. The test is used to assess the femoral component of the nervous system including the midlumbar nerve root and the femoral nerve.
During the FST, neck movement in the direction of flexion and extension is commonly used as the structure differential maneuver to elicit the responses produced by changes of mechanosensitivity of the femoral nerve.
the FST had a sensitivity of 100% and specificity of 85% in diagnosing radicular leg pain, and the likelihood ratios for diagnosing L4 root impingement increased when combined with other neurodynamic tests. A recent publication by Lai et al
examined the neurodynamic responses of the FST in young asymptomatic subjects and revealed that the measurement of hip range of motion and pain on application of the FST could indicate a change in femoral nerve mechanosensitivity in individuals free of lower extremity problems, and that this measurement was not influenced by muscle flexibility or sex.
Based on the neurodynamic theory, researchers suggested that the FST could be used to assess whether anterior knee pain relates to neural tissue damage.
In patients with anterior knee pain, the test is considered positive if the specific clinical symptoms can be reproduced and if the intensity of these symptoms can be influenced by the structure differential maneuver.
there have been no published data examining the use of the FST in patients with anterior knee pain.
However, a fundamental question concerning the test validity arises when using the FST to determine whether or not a mechanosensitivity problem of the femoral nerve contributes to the symptoms of anterior knee pain. In an attempt to assess the specificity of the neurodynamic test, Coppieters et al
used a hypertonic saline induced muscle pain model to demonstrate that application of the structural differential maneuver of the slump test, straight leg raise test, and upper limb tension test had no additional effect on muscle pain perception (both pain intensity and diameter). Their findings suggested that a negative neurodynamic test resulted when pain was not from neural tissues. However, no study has been done to examine the specificity of the FST in detecting nerve involvement in anterior knee pain.
To establish the testing specificity of the FST for the assessment of mechanosensitivity problems of anterior knee pain, we designed this exploratory study to examine responses (increase, decrease, or unchanged) to the FST when assessing anterior knee pain experimentally induced by injection of hypertonic saline solution. If the experimentally induced anterior knee pain symptoms were altered by the FST, then the specificity of the FST for the assessment of pain of nonneural origin was compromised. The null hypothesis of this study was that the application of the structure differential maneuver would have no impact on the intensity and diameter of the experimentally induced anterior knee pain.
Twelve asymptomatic subjects (6 men and 6 women) were recruited from the campus area of National Yang-Ming University, Taiwan. Participants had to be free of back, neck, hip, groin, and knee pain in the year prior to the start of the experiment, had no spine or lower limb surgery before, and no range of motion limitation in all peripheral joints. Subjects with a history of neurogenic disorders or scoliosis were excluded. All participants were given a full explanation of the procedure and signed a written consent prior to the commencement of the study. The study protocol was approved by the Institutional Review Board of National Yang Ming University, Taipei, Taiwan (serial no. 990030).
Two examiners performed the test. The principle examiner carried out all the tests while the second examiner recorded the testing responses. Subjects were first examined to rule out problems of the lumbar spine and lower extremity using tests for problems of articulations, passive and active restraints of the joints, and special tests such as the forward bending test, slump test, Patrick's test, and Clarke's test.
After physical examination, subjects performed 10 minutes of stationary cycling and muscle stretching exercises (back extensors, hip extensors and flexors, and knee extensors) as warm-up, followed by the FST before and after the injection of hypertonic saline to the patellar fat pad.
Femoral Slump Test
The FST was performed based on the recommendations of Butler et al
and the testing sequences are illustrated in figure 1. First, the subject was positioned lying on their side, with the testing (right) side on the top, the trunk and neck in the slump position, and the nontesting leg flexed fully. Several straps were used to stabilize the testing position, and a Terapi Master suspension systema was used to keep the testing leg in the horizontal plane. The testing knee was stabilized in full flexion throughout the test to avoid interferences of the knee flexion movement on the level of experimentally induced anterior knee pain. Afterward, we pushed the subject's thigh into extension until the onset of pain and then brought it back to the point where the pain disappeared. This hip position was defined as the submaximum hip extension, and it was used so that we could avoid confusing subjects between the anterior thigh pain produced by the FST and the anterior knee pain induced by the hypertonic saline injection. When the hip was in position, the subject was given a knee chart to mark the pain location, and a diagram of 10 circles from 1 to 10cm in diameter were used to indicate the size of the pain area. The intensity of the pain/discomfort was measured using a 100-mm visual analog scale (VAS). Lastly, neck flexion and extension were randomly performed 3 times as the structure differential maneuvers in order to challenge the mechanosensitivity of the femoral nerve, and the pain location, diameter, and intensity were recorded again. The change of pain/discomfort on the application of neck movement was also categorized as an increase, decrease, or unchanged for each trial and for each subject. If an individual reported pain decrease in 2 or more trials out of 3 tests, the subject was in the pain decrease group. The same principle applied to the pain increase and unchanged groups.
Experimental Anterior Knee Pain Model
We injected a single bolus of .25mL hypertonic saline solution (5% NaCl) into the infrapatellar fat pad to produce symptoms similar to anterior knee pain.
The injection site, the medial part of the infrapatellar fat pad, was manually palpated and sterilized by alcohol swabs and iodine pads, and a 27G needle was used to minimize cutaneous pain. The needle was inserted to a depth of about 10mm. To understand the characteristics of the anterior knee pain induced by the hypertonic saline injection, this protocol was tested prior to the conduction of the main study. The injection resulted in a rapid increase in pain over the knee cap, and the pain resolved slowly within 20 minutes in all subjects. The FST after the injection was therefore performed between minutes 2 and 5 during which both pain intensity and size reached the steady state.
The percentage of pain intensity change in each category (increase, decrease, or unchanged) was described, and the specificity of the FST was calculated. In addition, we used the paired t test to assess the differences in the pain VAS score and the diameter before and after application of the structure differential maneuver. The significance level was set at .05. All statistic analyses were performed using SPSS version 12.0.b
Table 1 summarizes the demographic data of the subjects. All participants were naïve to the concept of neurodynamic testing. Before injection, no subject reported any anterior knee pain when the FST was performed, and neck movements did not have any influence on the sensation over the thigh region. The hypertonic saline injection was successfully given and no subject reported any infection after the experiment. The pain induced by the hypertonic saline injection was reported over the anteriomedial side of the anterior knee region (fig 2). The averaged initial pain was 49.17 for the VAS score and 3.64cm in diameter, which reached its peak (VAS=57.2, diameter=5.5cm) at approximately 150 seconds postinjection. This pain remained at the same level for about 180 seconds (see fig 2), during which the postinjection FST was completed.
Of all the testing trials, 75% and 70% showed no change in pain after neck flexion and neck extension movement, respectively. In addition, 83% of the subjects showed no change in pain with neck flexion, and 75% showed no change in pain with neck extension (fig 3). These data indicated that the FST had a specificity of more than .75 for diagnosing the femoral nerve involvement in anterior knee pain.
Table 2 lists the VAS score and diameter of the injection induced anterior knee pain of all individual subjects before and after the application of the structure differential maneuvers. Despite a small sample size, our data of pain intensity and diameter were normally distributed. The results of the paired t test showed that neither neck flexion nor extension significantly changed the intensity and diameter of the induced anterior knee pain (P>.05) (table 3).
Table 2Intensity and Diameter of the Experimentally Induced Anterior Knee Pain Before and After the Application of the Structure Differential Maneuvers (neck flexion and neck extension) During the FST
To our knowledge, this is the first study investigating the specificity of the FST in the differential diagnosis of a nonneural anterior knee pain. We used an experimental pain model to ensure that the simulated anterior knee pain symptoms did not originate from problems with nerve tissues. Findings of this study indicated that application of the structure differential maneuver during the FST did not change the intensity or diameter of the experimentally induced anterior knee pain, and the FST had a specificity between .75 and .83 when assessing femoral nerve involvement in individuals with anterior knee pain.
injected hypertonic saline intramuscularly and observed pain changes during different stages of straight leg raise and slump test. Their results showed no significant interaction of position × test for pain intensity and diameter, which indicated that the structure differentiation procedure did not impact pain perception when the origin of the pain was predominantly related to muscle nociception. A similar study was conducted to evaluate the specificity of the median nerve neurodynamic test in diagnosing hand symptoms, and the results of this study also proved that the perception of experimentally induced muscle pain did not vary due to mechanical nerve provocation.
Because factors contributing to anterior knee pain rarely occur in isolation, the validity of the FST in differentiating nerve tissue involvement in anterior knee pain was difficult to determine. The injection induced experimental pain model used in this study had the advantage that the exact origin of the symptoms was known. Therefore this nonneural knee pain model was chosen to assess the specificity of the FST. After the hypertonic saline injection, no change in the level of experimental knee pain was reported in 83% of the subjects after neck flexion was performed, and in 75% of the subjects after neck extension was performed. Our data showed that use of the FST provided appropriate responses in more than 75% of cases when assessing anterior knee pain with no nerve mechanosensitivity problem involved, that is a specificity of more than .75 when diagnosing femoral nerve involvement in individuals with anterior knee pain. The disadvantage of such a method was that the other facet of the validity, the sensitivity, of the FST could not be assessed because using an injection to produce a neural originated pain is more risky with possible permanent damages to the nerve tissues.
In this study, the FST was modified in which the hip joint was moved to the submaximum extension without causing any sensory responses. Previous studies used a similar method to avoid confusing subjects between pain produced by the neurodynamic testing and by the injection.
In our previous study, we found that the FST caused symptoms mainly over the anterior thigh region in asymptomatic young individuals. Although the submaximum hip extension was used in order to prevent any symptoms that might distract the subject from the experimentally induced anterior knee pain, we felt that this level of tension was likely to expose the neural structures to considerable challenge because the tension was near the threshold of pain production.
Concerns regarding the possible influences from the flexibility of the surrounding tissues on the neurodynamic testing results have been discussed in the literature.
Although we did not control the flexibility issue in this study, our previous data showed that general flexibility and the flexibility of hip flexors and knee extensors did not impact on the results of the FST in young asymptomatic subjects.
Because participants in this study were of a similar age and in similar physical condition, we therefore ruled out the influence of flexibility on our study results.
According to the neurodynamic theory, pain that does not originate in neural tissues should not change with the application of the structure differential maneuver. However, 2 out of 12 subjects showed decreased pain with neck flexion and neck extension. One possible explanation for this unexpected pain decrease was the dispersion of hypertonic saline over time, and thus less pain was experienced in the later stage of the experiment. Although most of the subjects had a window of 3 to 5 minutes during which the induced pain was in a steady state (see fig 2), some individuals showed a downward slope of the pain earlier than the others after saline injection, which might account for the pain decrease responses after the application of the structure differential maneuver.
In this study, an experimental pain model was used to determine the specificity of the FST in differential diagnosis of anterior knee pain. Although the hypertonic saline injection produced a painful condition similar to the symptoms reported by patients with anterior knee pain, these 2 conditions were quite different in nature, because the experimental pain produced in this study was an acute pain, whereas the anterior knee pain is a chronic pain, which might involve certain chronic adaptations in the surrounding tissues. Concerns regarding statistical power arose because of the small sample size (12 subjects) in this study. Owing to the invasive procedures involved in this investigation, we were unable to recruit more subjects of diverse background. This limitation should be considered if the results of our study were to be generalized. In addition, the hip extension used in this study was a submaximum extension, which might not be sufficient to elicit responses in some subjects. Although our data indicated the specificity of the FST was between .75 and .83, we felt more evidence is needed to support the use of the FST in clinical assessment of anterior knee pain.
To the best of our knowledge, this is the first study describing the specificity of the FST for the assessment of anterior knee pain. Our findings indicated that the FST had a specificity of more than .75 when detecting nerve mechanosensitivity problems of anterior knee pain. Future research should carry out examining the neurodynamic responses in patients with clinical anterior knee pain in order to determine whether or not the FST could add a new facet in the assessment and treatment of anterior knee pain.
Supported by the National Science Council, Taiwan (grant no. NSC 100-2314-B-010-004 ).
No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated.