Role of Sonographic Examination in Traumatic Knee Internal Derangement

Methods 

Sonography 

Sonographic examinations were performed by a member of the author group, an expert in sonography (5 years of musculoskeletal sonographic experience). A 10-MHz linear array transducer in a portable ultrasounda was used to scan knee joints. Subjects were maintained in a supine position with knee full extension, and the transducer was aligned along the longitudinal axis of the knee joint, with the lower end of transducer at the upper margin of patella. In this position, the suprapatellar recess was located between the quadriceps tendon and prefemoral fat. Then the transducer was swept to the medial and lateral site of quadriceps tendon to fully scan the suprapatellar recess.4 Effusions appeared as an anechoic area between the quadriceps tendon and prefemoral fat (fig 1). In patients with mild effusion, it could be only detected in the lateral or medial compartment due to effect of gravity. The effusion was recorded as the widest anechoic width on sonogram without compressing the transducer or quadriceps contraction. Sonography criteria for the presence of knee effusion was fluid accumulation exceeding 2mm, because the physiologic fluid accumulation is usually less than 2mm.4 To be certain that anechoic area was effusion rather than synovium or other soft tissue mass, Doppler examination and compressing the area were performed. Knee joint fluid was highly compressible, and did not exhibit vascularity.

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Fig 1. (A) Effusion in the suprapatellar recess of the knee (↓). (B) Compression of suprapatellar recess by transducer shows near complete disappearance of the anechoic area (effusion). Abbreviations: F, femur; P, prefemoral fat; Q, quadriceps tendon.

Results 

Table 1 depicts the results of sonographic examination with respect to MRI diagnosis for knee effusion. MRI showed knee effusion in 24 patients. Nineteen of these patients were similarly diagnosed by sonography. The sensitivity of sonographic examination in detecting effusion was found to be 79.1%, and the specificity 50%. The positive-predictive value (PPV) was 86.3% and negative-predictive value (NPV) was 37.5%.

Table 1. Results of Sonographic Examination With Respect to MRI Diagnosis (N=30)

	Sonographic Diagnosis	MRI Diagnosis
		Effusion (positive)	Effusion (negative)
	Effusion (positive)	19	3
	Effusion (negative)	5	3

NOTE. Sensitivity, 79.1%; specificity, 50%; positive-predictive value, 86.3%; and negative-predictive value, 37.5%.

Table 2 shows the predictive accuracy of knee effusion by ultrasound to knee internal derangement. Twenty of 22 subjects diagnosed as knee effusion by ultrasound were found to show knee internal derangement. Among the 8 subjects without knee effusion on sonogram, 5 had knee internal derangement on MRI. The sensitivity of knee effusion to internal derangement was 80.0% and the specificity was 60.0%. The PPV and NPV were 90.9% and 37.5%, respectively.

Table 2. The Relationship Between Knee Sonographic Effusion and Internal Derangement in MRI (N=30)

	Sonographic Effusion	Internal Derangement in MRI
		Positive	Negative
	Positive	20	2
	Negative	5	3

NOTE. Sensitivity, 80%; specificity, 60%; PPV, 90.9%; and NPV, 37.5%.

Of the 25 patients with knee internal derangement, 10 had multiple injuries. Fifteen had ACL tears, 5 PCL tears, and 15 meniscus tears in these 25 patients. There were 5 patients who had internal derangement but no evident knee effusion on sonogram, whereas 2 patients had sonographic knee effusion in the absence of internal derangement. Of the 5 patients exhibiting internal derangement but no sonographic effusion, 3 had no effusion in MRI either, and the interval between their injuries was almost 3 months. The other 2 subjects had only a little effusion in MRI and the interval between their injuries was 5 days and 1 month. These 5 subjects were individually diagnosed with the following: one had ACL and meniscal injury, one ACL injury alone, and 3 meniscal injury. Of the 2 patients with sonographic effusion but no internal derangement in MRI, one had a mild synovitis, and one had a severe inflammatory arthritis with synovitis.

Discussion 

Findings from this study indicate that sonographic examination is a useful imaging tool for detecting knee effusion, with an accuracy of around 73%. There were 5 patients in whom effusion was detected by MRI, but not by sonographic analysis. The extent of effusion detected in all of these particular patients by MRI was minimum amount, suggesting that sonographic examination may be sufficiently accurate to detect knee effusion in all but the most minimal cases. In the study by Schweitzer et al,19 MRI could detect as little as 4mL of fluid at mid-sagittal line. However, Delaunoy et al7 stated that the lowest amount of effusion detected by sonography in saline or blood was 10mL. These findings may imply that sonographic examination is less sensitive than MRI in detecting knee effusion. Whether such amounts of knee effusion have clinical significance still needs to be determined.

Knee effusion can also be detected by physical examination and can be used to predict the knee internal derangement. Good validity of physical examination in detecting a large amount of knee effusion was reported by Hauzeur et al8 but poor reliability was noted when the effusion was moderate to small. Obesity, joint deformities in osteoarthritis, and edema due to venous diseases might further increase the clinical evaluation errors.8 Differentiating joint effusion from hypertrophy of the synovium is sometimes difficult, either by physical examination or ultrasound. However, the compression maneuver on sonographic examination made it easy to differentiate synovium from effusion, and the compression maneuver was performed in this work.8 Hauzeur8 classified the amount of knee effusion into 3 grades on sonogram. Large effusion was defined as fluid collection evident in the suprapatellar pouch, moderate effusion as fluid detected only when the lateral pouches were being compressed, no effusion as no fluid collection even by compression. We did not classify the amount of effusion in our subjects due to the small number of cases. It might be interesting to classify the effusion amount in the further study.

The other important finding of this study is that subjects with knee effusion detected by ultrasound had a 90.9% PPV of knee internal derangement. This suggests that when knee effusion is detected by sonographic examination, it is highly likely that knee internal derangement also exists, and that further diagnostic evaluation (by MRI or arthroscopy) is warranted. Sonographic examination could hence be used as a valuable screening tool for patients with traumatic knee injury, preventing the need for the more costly and invasive techniques of MRI and arthroscopy, respectively.

In 1 retrospective study,9 59% of 115 patients with knee effusion (as diagnosed by MRI) had internal derangement. The correlation between knee effusion and internal derangement was significantly lower than the present study. However, in another study,20 90% of acute knee injury patients were found to have both effusion and internal derangement as determined by MRI. Several factors may influence the correlation between the knee effusion and internal derangement. First, the duration between injury and evaluation should be considered, because effusions absorb over time. This was shown by Boks et al,21 who noted that effusion was related to recent, but not old, trauma. In our study, most of the patients visited the clinic several days or even weeks after the knee injury occurred. Second, the amount or extent of effusion is a possible determining factor. Small amounts of effusion may result from inflammation of the knee joint, rather than injury of the internal knee structure. For instance, in a study conducted by Kolman et al,13 it was determined that only 14% of patients with 10mm or less of effusion in the lateral aspect of the suprapatellar pouch had concurrent derangement. It was concluded that if there is no significant effusion evident in the suprapatellar pouch there is unlikely to be associated derangement. Last, the differing definitions of internal derangement may influence the results. Some researchers have included bony fracture or dislocation as internal derangement, but we did not. Internal derangement was broadly defined in the study conducted by Kolman,13 with anterior, posterior, median, and lateral cruciate ligament tears, meniscal tears, moderate to severe osteoarthritis, osteochodrosis dissecans lesions, patellar microfracture, bone bruising, tibial plateau fracture, tibial epiphyseal fracture, patellar retinacular tears, and popliteal tears all included. In keeping with most of the studies,3, 11 we defined internal derangement as by the presence of either ACL, PCL, or meniscal injury.

Conclusions 

Sonographic examination can accurately detect knee effusion. When knee effusion is evident in patients of traumatic knee injury, it is strongly indicative of knee internal derangement. We suggest that sonography is an ideal screening tool to determine whether more detailed knee examination (ie, to diagnose internal derangement) is warranted.