Anterior knee pain is a common symptom that poses difficult diagnostic and therapeutic problems.13 The clinical presentation of anterior knee pain usually is diagnosed as patellofemoral pain syndrome.3,39,41,43 Relatively rare etiologies of anterior knee pain include peripatellar tendinitis or bursitis, plica syndromes, Sinding Larsen's disease, Os-good Schlatter disease, and neuromas.39 Previously, patients with patellofemoral pain often were diagnosed with chondromalacia patellae, suggesting symptoms were caused by pathologic changes in the retropatellar cartilage with no definite assessment of the underlying abnormality.39 However, two studies showed that numerous patients with symptoms consistent with patellofemoral pain syndrome had no arthroscopic evidence of articular cartilage damage.12,25
A better understanding of patellofemoral biomechanics has contributed to the successful classification and treatment. Fulkerson described six major anatomic structural sources of patellofemoral pain: subchondral bone, synovial lining, retinaculum, skin, muscle, and nerve.13 He proposed that the major contributing factors to the development of patellofemoral pain syndrome are malalignment of the lower extremity, patellar muscular imbalance, and overactivity with cartilage disruption or softening being a late sequela.13 Regardless, the cause of pain remains elusive in some patients despite clinical and radiographic evaluation, suggesting a functional etiology.13
Anterior knee pain is common in the military population. It is the second most common overuse injury after stress fractures.17 Finestone et al11 reported 15% of elite Israeli infantry recruits were affected with this anterior knee pain during basic training. Jordaan and Schwellnus17 found an incidence of 0.22/1000 training hours among new military recruits.
Various physical examination tests have been used to diagnose patellofemoral pain syndrome.27,31,32,40 However, reports of their scientific validity are scarce. Malanga et al27 reviewed articles regarding the sensitivity and specificity of common orthopaedic physical examination maneuvers of the knee. They reported the common tests for patellofemoral pain syndrome lack sensitivity when correlated with pathologic operative findings.27Only one recently published study has assessed the diagnostic value of five physical examination tests by prospectively comparing patients with patellofemoral pain syndrome with control subjects.31The authors reported the vastus medialis coordination test, patellar apprehension test, and eccentric step test had a positive ratio, indicating positive test results increased the likelihood of patellofemoral pain syndrome.31
Plain radiography is the standard for evaluating patients with patellofemoral pain.39 Various radiographic measures have been linked to patellofemoral pain syndrome.1,18,28 However, Laprade and Culham, who compared several radiographic parameters in patients with patellofemoral pain syndrome and in healthy controls, found no significant differences between the two groups.22
In the current study, we prospectively investigated physical examination and plain radiographic findings in a population of infantry soldiers with and without patellofemoral pain syndrome. Our primary research question was: How sensitive and specific are the key physical examination tests for the diagnosis of patellofemoral pain syndrome in this population? Our secondary research questions were: Which basic physical examination findings are significantly more prevalent in infantrymen with patellofemoral pain syndrome than in asymptomatic controls, and which plain radiographic measures are more prevalent in soldiers with patellofemoral pain syndrome than in control subjects?
MATERIALS AND METHODS
We prospectively examined and followed up 61 male soldiers with anterior knee pain (mean age, 19.4 ± 1.2 years; range, 18.5-20.2 years) and 25 control subjects (mean age, 24.1 ± 6.5 years; range, 19-27 years). The cohort consisted of consecutive patients previously diagnosed with patellofemoral pain syndrome by a Medical Corps physician and referred for additional evaluation at a central military orthopaedic clinic. Other eligibility criteria included symptoms consistent with patellofemoral pain syndrome that started or were aggravated during military training. The inclusion criteria were: (1) anterior knee pain after increased physical activity aggravated by walking up and down stairs, squatting, or sitting with knees flexed, and accompanied by crepitus, giving way, and catching; (2) persistent pain despite conservative treatment for at least 3 months with rest and reduction of physical activity level, analgesics and/or nonsteroidal antiinflammatory drugs, and physical therapy; and (3) military discharge because of knee pain. Exclusion criteria were: (1) no history of knee injury or knee surgery; and (2) no medical conditions that could require discharge from the infantry unit. Pain had persisted for a mean of 19 months before soldiers were entered in the study; 87% had bilateral involvement (Table 1). Approximately ⅓ of the soldiers experienced less severe knee pain before military enlistment, but only five (8%) soldiers had a family history of knee pain. The study was approved by the Medical Corps Ethics Committee, and informed consent was obtained from all subjects.
We performed the following physical examinations to rule out other disorders that can generate anterior knee pain: McMurry's and Appley's tests for meniscal tears, anterior drawer test, Lachman's test, and Pivot's shift test for anterior cruciate ligament (ACL) injury, posterior drawer test for posterior cruciate ligament (PCL) injury, valgus and varus stress tests for medial collateral ligament (MCL) and lateral collateral ligament (LCL) injury, and supine hip internal rotation for referred pain.26 Anteroposterior (AP) radiographs of the knees were checked routinely to rule out other pathologic conditions.
The control group of volunteers consisted of male infantry soldiers without a history of knee pain, knee injury, or knee surgery who were referred during the study period to the same central military orthopaedic clinic for various orthopaedic problems not related to the knee and who had served in the army for at least 2 years. Control subjects had the same physical examination and radiographic screening as the soldiers with knee injuries.
Soldiers completed a questionnaire at the time of examination which included demographics and army service details. The questions were: (1) Did you suffer from knee pain before military service?; (2) How long have you been suffering from knee pain?; (3) Is the pain confined to one knee or is it present in both knees?; (4) Did you have a knee injury before or during army service?; (5) Is the pain exacerbated by any of the following: stair climbing, stair descent, prolonged sitting with the knees flexed (eg, as in watching a movie or riding a bus)?; (7) Does the pain resolve at rest?; (8) Have you experienced a sensation of locking of the knee?; and (9) Is there a history of knee pain in any of your family members (parents and siblings)?
The following physical examination tests were performed to determine their sensitivity and specificity in patellofemoral pain syndrome: patellar tilt test,32 patellar apprehension test,27 patella alta test, and active instability test. The patella alta test was performed with patients positioned supine. Pressure was applied over the lower pole of the patella while the knee was fully extended and then flexed; pain during flexion was suggestive of patella alta. We performed the active instability test with patients positioned supine with the lower extremity in a neutral position and the knee flexed at 15°. Patellar tracking was assessed during isometric quadriceps contraction. Any lateral patellar motion was noted and recorded in millimeters. The test was considered positive (ie, suggesting patellar instability) if the patella moved laterally more than 3 mm, as this was thought by the examiner (SD) to be the clinical sensitivity limit.
We documented patients' physical examination findings to determine the parameters most prevalent in the group of soldiers with patellofemoral pain syndromecompared with asymptomatic control subjects. Soldiers were examined for lower-limb alignment (qualitative assessment of genu varum or genu valgum deformity),26 foot posture (pes cavus and pes planus),32 quadriceps angle (Q angle) while standing,32 presence of knee effusion using the patellar ballottement test,26 patellofemoral joint crepitation,32 presence of tenderness over the medial and lateral patellar retinacula,32 patellar tilt test,32 medial and lateral patellar glide (as percentage of patellar width),32 and presence of squinting patellae. Qualitative assessment of squinting patella (inward tilt) was performed by viewing the patient from the front, standing with the medial aspects of the knees and medial malleoli as close together as possible.26
We also documented patients' plain radiographic findings. Lateral and axial patellar radiographs were taken at 30° knee flexion. The axial view was taken using a special standardizing device that maintains knee flexion at 30°. We measured the Insall-Salvati index (for patella alta),15 sulcus angle,26 Merchant angle,30 and Laurin angle.23 The presence of patellar subluxation and subchondral sclerosis were determined qualitatively from the axial radiographs.2
All clinical and radiologic evaluations were performed by the senior author (SD), who was not blinded to the group assignment.
Statistical analysis was performed using the Student's t test for continuous variables and the chi square test for categorical variables. Significance was set at p < 0.05.
We documented patients' physical examination findings to determine the parameters most prevalent in soldiers with patellofemoral pain syndrome compared with asymptomatic control subjects (Table 2).
The sensitivity of the four patellofemoral tests evaluated was low (less than 50%), whereas their specificity was moderate to high (92%, 100%, 72%, and 92% for patellar tilt, active instability, patellar apprehension, and patella alta tests, respectively) (Table 2). The positive patellar tilt test and active instability test were significantly more prevalent in subjects with patellofemoral pain (43% vs 8% in control subjects, p = 0.002; and 25% vs none of the control subjects, p = 0.004, respectively) (Table 2). However, the results of the patellar apprehension and patella alta tests did not significantly differ between the two groups (Table 2).
Among the physical examination findings that were tested, the following were significantly more prevalent in the patellofemoral pain syndrome group: increased Q angle, lateral and medial retinacular sensitivity (p < 0.001), patellofemoral crepitation (p < 0.001), squinting patella (p = 0.045), and reduced mobility of the patella (as assessed by the patellar glide test) (p = 0.018) (Table 3). An increased Q angle (> 20°) was more frequent among patients with a positive active instability test than in patients with a negative active instability test (11 of 15 patients [73%] compared with 21 of 46 patients [46%]).
The only abnormal plain radiographic finding that was significantly more frequent in patients with patellofemoral pain syndrome was patellar subluxation in the axial view [15 of 61 patients (25%) vs one of 25 control subjects, (4%), (p = 0.032)] (Table 4). There were no differences in the sulcus angle, patellar height (determined by the Insall-Salvati index), patellar tilt (determined by Laurin angle), and patellar displacement (determined by Merchant angle) between groups.
Physical examination and plain radiography play a pivotal role in the diagnosis of patellofemoral pain syndrome, yet the literature contains few reports regarding the diagnostic value of these parameters. We prospectively compared infantry soldiers diagnosed with patellofemoral pain syndrome with an appropriate group of control subjects to assess the validity of clinical findings and radiographic measures used to diagnose patellofemoral pain syndrome.
Our study has several limitations. First, there may have been a selection bias because the initial diagnosis of patellofemoral pain syndrome by the referring physician was based partially on physical examination findings. However, although the study population might be selective, inclusion criteria were based on objective factors, and all subjects were screened carefully before being included. Second, another selection bias could have resulted from administrative and psychologic factors inherent to all studies investigating a military population. Nevertheless, the abundance of patellofemoral pain syndrome in infantry soldiers and the uniformity of this population is an ideal study cohort. Third, observer bias may have occurred because the examiner was not blinded to the group assignment.
Physical examination tests are considered the cornerstone of the diagnosis and evaluation of patients with patellofemoral pain syndrome.32,33 However, published studies concerning the specificity of these tests are scarce and inconclusive. The active instability test was positive in 15 of 61 patients (sensitivity, 25%) compared with no patients from the control group (specificity, 100%) (p = 0.004). Watson et al reported that the lateral pull test also had poor interobserver reliability for PFPS.40 Although the lateral pull test was performed with full extension of the knee,40 we did the active instability test at 15° knee flexion. It is our impression that one can correctly observe lateral subluxation when the quadriceps is contracted in this position. This subluxation can be felt and even seen in slim patients. In patients with substantial instability, notable subluxation can be observed during the test with increased patellar tilt. Similarly, the patellar tilt test was found to be positive in 26 of 61 patients compared with two of 25 control subjects (p = 0.002), and therefore, had high specificity (92%), but low sensitivity (43%). In contrast to Nijs et al,31 our findings did not confirm the utility of the patellar apprehension test in diagnosing patellofemoral pain syndrome. Likewise, we found that the patella alta test was neither specific nor sensitive for patellofemoral pain syndrome.
Another important finding was the high prevalence of an increased Q angle, which contrasts with other reported results.6,10 In a literature review of traditional methods of clinical evaluation of patellofemoral pain syndrome, Post reported no scientific correlation between Q angle measurements and patellofemoral disorders.33 Other physical examination findings more prevalent in soldiers with patellofemoral pain syndrome were squinting patella, patellofemoral crepitation, patellar retinacular sensitivity, patellar tilt test, and reduced mobility of the patella (as assessed by the patellar glide test). These parameters have been suggested as indicative of patellofemoral pain syndrome,13,14,16,19,20,38,39 but their scientific validity has not been reported. We found no differences between the groups regarding foot posture and lower extremity alignment, which have been used for diagnosis of patellofem-oral pain syndrome.17,19,35,38,41 In contrast to Nijs et al,31 our findings did not confirm the specificity of the patellar apprehension test for diagnosing patellofemoral pain syndrome.
Computed tomography,5,34,35 radionuclide bone scans,9 and MRI36,37,42 have been used to assist in the diagnosis of patellofemoral pain syndrome. Lateral and axial radio-graphs at 30° knee flexion provide information regarding the morphologic characteristics of the patellofemoral joint and remain the standard for evaluating patients with patellofemoral pain.39 Patellar subluxation was the only radiographic measure different (p = 0.032) between soldiers with patellofemoral pain syndrome and control subjects. There were no differences between sulcus angle, patellar height (determined by the Insall-Salvati index), patellar tilt (determined by Laurin angle), and patellar displacement (determined by Merchant angle). These findings agree with those of previous studies that questioned the diagnostic usefulness of plain radiographic measures in diagnosing patellofemoral pain syndrome.18,21,22,24 possible explanation for the limited value of radiographic studies is that the patellofemoral joint is examined statically.29
We suggest the patellofemoral joint examination also should include active tests that may reveal findings not shown on passive tests. McNally et al29 observed active patellar maltracking on radiographs. This concept should be used routinely when the clinical examination suggests maltracking.
The etiology of patellofemoral pain syndrome seems to be multifactorial. It has been proposed that high-usage overloading of a malaligned (susceptible) extensor mechanism ultimately leads to persistent secondary degenerative changes and pain.13 Other factors reported as increasing the risk of having patellofemoral pain syndrome develop include patellar malaliganment,39 abnormal patellar tracking,4 and vasti neuromuscular imbalance (ie, delayed onset of the vastus medialis obliquus in relation to the vastus lateralis).7,8,38 Similarly, tightness of the lateral knee retinaculum, hamstring, iliotibial band, and gastrocnemius have been implicated in the etiology of patellofemoral pain syndrome.38 Witvrouw et al suggested that patellofemoral pain syndrome is a wastebasket comprised of several different pathophysiologic entities.40,43 Its heterogeneity might account for the relatively low sensitivity and high specificity of the clinically relevant physical examination tests performed in our study (43% sensitivity and 92% specificity for the patellar tilt test; 52% and 100%, respectively, for Q angle > 20°; and 25% and 100%, respectively, for positive active instability). Therefore, diagnosis and evaluation of patients with anterior knee pain should be based on a combination of several clinical findings. Similarly, a negative result should not rule out a diagnosis of patellofemoral pain syndrome.
Because all soldiers experienced knee pain during their military service and most had malalignment parameters, it is likely that the patellofemoral pain was from preconditioned knee characteristics subjected to overuse. It is possible that clinical parameters can identify individuals at increased risk for having patellofemoral pain develop before the onset of symptoms. Additional prospective studies in which patients are tested before the onset of anterior knee pain are needed to confirm this hypothesis.
We provide evidence regarding the validity of clinical and radiographic features commonly used for evaluation of patellofemoral pain syndrome. Physical examinations were more useful clinically than radiographic studies for evaluating and diagnosing patellofemoral pain syndrome. Positive active instability tests and patellar tilt tests are more frequent, but their sensitivity is relatively low. Increased Q angle, squinting patella, patellofemoral crepitation, patellar retinacular sensitivity, and reduced mobility of the patella were more prevalent in soldiers with patellofemoral pain syndrome. In contrast, all radiographic measures other than patellar subluxation had no diagnostic value.
We thank M. Perlmutter for help in the preparation of this paper.
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