Role of Modern Sonography in the Assessment of Knee Pathologies: Review of a Large Series of Patients

Meniscal injuries of the knee are common. Acute meniscal tears occur most often from twisting injuries; chronic degenerative tears occur in older patients and can occur with minimal twisting or stress. Meniscal cysts present with symptoms of pain often associated with a palpable mass at or adjacent to the knee joint and are frequently accompanied by horizontal or complex meniscal tears. Bedside ultrasound (US) is being used more frequently to assess patients with acute knee injuries. Ultrasound allows the skilled practitioner to examine the knee dynamically and to compare injured and uninjured joints. Different previous studies showed inconsistent results regarding the sensitivity and specificity of US examination: some showed that sonography is not accurate enough to be used as the only imaging modality for diagnosing tears of the knee menisci, and another few studies showed a sensitivity of US matching that of MRI. In the daily practice we still get many US referrals with a question of meniscal tear. Objectives: The main purpose of this study is to determine the role of sonography in detecting knee pathologies, and if it can be used as the main single exam for diagnosing meniscal injuries. Materials and Methods: A retrospective review of US exam results done by musculoskeletal radiologist in Ziv Medical Center, for 358 patients/370 knees between the years 2013-2015. The prevalence of different knee pathologies shown on US exam will be calculated, and the relationship between these pathologies will lateral tear, the Sensitivity was 50% and the Specificity was 94.4%, PPV was 75% and NPV was 85%. There was no major difference between genders who complaints of knee pain, 56.7% females and 43.3% males. The median age is 45.9 years; no prevalence of knee pain is seen in one age group. In 92.2% of the patients who were tested by US for a knee pain there was a pathology seen, so we can conclude that, with a skilled radiologist, US is a reliable first line modality for knee pain complaints. The most common pathology found in knee US was Baker’s cyst in 59.2% of patients. On the contrary, the least common pathology found was lateral sub-luxation in 1.1% of patients. When we compared the results of the US of knee with the MRI, we found a Sensitivity of 78.5% and a Specificity of 70%, PPV of 78.57% and NPV of 70% for medial meniscal tears. For lateral meniscal tears the Sensitivity was 50% and the Specificity was 94.4%, PPV was 75% and NPV was 85%. These results are consistent with those published in the literature. is a high correlation between medial meniscal tear and osteoarthritic changes in the knee joint, likewise between lateral meniscal tear and lateral subluxation and parameniscal cyst. Therefore, US exam is a good first line modality in patients with knee complaints, and patients with positive results for meniscal tear, meniscal subluxation, degenerative medial changes or parameniscal cyst, should be directed toward MRI exam and further investigation and treatment. However, patients with negative US exam and high suspicion of internal knee derangements (meniscal tear, cruciate ligament tear) should be directed for MRI examination, due NPV around 70-85%.


Introduction
The knee is the largest joint in human body considering its intraarticular cartilage volume and area. Among the other body joints, it is mostly prone to traumatic injuries, wear and septic or inflammatory arthritis. The most common complaint is sensitivity at its medial side, and least common in lateral side. Meniscal tears are common injuries with many aetiologies, traumatic injuries in young patients and wear injuries in older patients are the most common ones. Para-meniscal cyst can present as a painful palpable mass inside or near the knee, and it's commonly a result of longitudinal or complex meniscal tear. Several studies have shown that the accuracy of physical examination in detecting meniscal tear depends on the type and location of the tear. The ultrasound modality is used in diagnosis of a painful knee. MRI can detect presence and type of meniscal tear, it is the most sensitive noninvasive method in detecting small tears. In one meta-analysis, MRI has sensitivity and specificity of 91.4% and 81.1% respectively in detecting medial tears, as for the lateral meniscus tear, sensitivity 76% and specificity of 93.3% [1][2][3][4][5].
Some of the advantages of MRI are non-invasive, high sensitivity and specificity, no ionizing radiation and is not operator dependent.
Disadvantages include not readily available, expensive, has some contraindications and is time consuming. As a result, in 1989, US started to be used for detecting pathologies in painful knee. Its advantages are cheap, widely available, fast, dynamic examination, the ability to simultaneously compare both knees, safe and almost no contraindications. On the other hand, it is operator dependent, highly influenced by the experience of the performer and the resolution depends on the machine frequency. Studies to test the sensitivity and specificity of the US in detecting meniscal tears have shown different results: a) Some showed that the US is not sensitive enough (sensitivity of 60% and specificity of 21%), others showed sensitivity ranging between 83-86% and specificity of 69% in comparison to MRI scan which was used as the definitive diagnostic method. b) Because of theses contradicting results, we decided to conduct this study to test the role of US in detecting different pathologies in the knee joint, such as: Baker's cyst, parameniscal cyst, meniscal extrusion, meniscal tears etc. and discover if there are any relations between those pathologies.
The sensitivity and specificity of the US will be calculated in patients participating in the study who also underwent MRI or Arthroscopy of the knee [6][7][8][9][10].

Study Assumption
US is a good first line modality for knee pathology, it can detect a wide range of pathologies and sign for meniscal tears, although it is not sensitive nor specific enough for direct detection of meniscal tears, hence it can be used as a screening modality, and when it rises the suspicion of meniscal tear, it is recommended to proceed to MRI.

Study Expectations
Diagnosing a wide range of different pathologies in the knee joint using US. There are causative relations between different pathologies. US is a good screening test for a painful knee, but it is not sensitive enough in detecting meniscal tears.

Meanings and Implications
The results of our study have practical clinical impact; they would help clinicians to forward their patients to the right examination. Also, they have a financial impact, which could save money and time by referring the patients to the right examination and avoiding unnecessary ones.

Methods and Materials
We conducted a retrospective analytical study. Results of the US examination of the knee done in Ziv Medical Centre by the same senior musculoskeletal radiologist were collected from the patient's medical files. The distribution of the pathological findings was calculated. Pathologies that were tested are -lateral meniscal tear (LMT), medial meniscal tear (MMT), medial and lateral subluxation, Baker's cyst, parameniscal cyst, degenerative changes, tendon pathology, ligament pathology and other pathologies. The relation between different pathologies and meniscal tears was also considered. Finally, we compared US results with MRI and arthroscopy results to determine the specificity, sensitivity, PPV and NPV of US in diagnosing meniscal tears [11][12][13][14].

Devices
Two US devices were used, PHILIPS IU 22 and PHILIPS EPIQ 5G, using musculoskeletal intended transducer 5-12 MHZ.

Statistical Analysis
CHI square test was used to determine the relations between different pathologies. Logistic regression test, while the dependent variable is meniscal tear, and the explanatory variables are the other pathologies. This regression was applied to all pathologies against all other pathologies. Statistical significance needed is 5% and test strength is 80%. Sample size was all participants who underwent knee US by the same radiology musculoskeletal senior physician, in the radiology department in Ziv Medical Centre, this factor will eliminate the interindividual variability.   was a trend toward relation between para-meniscal cyst and LMT (P=0.08) (Figures 2-12).     Representative examples of different knee pathologies as seen in US: Figure 6: Baker's cyst -located in the popliteal fossa between the tendon of the semimembranosus and the medial gastrocnemius. It's size ranges from few millimeters to few centimeters, and it is a common reason for knee complaints, it can cause pain, or it might be asymptomatic. It also can be in relation to intraarticular effusion from mechanical or inflammatory aetiologies.

Discussion
Knee pain is a common complaint, more pronounced at the medial compartment; as a result, many patients are referred to diagnose the aetiology. Nowadays, US exam is considered a first line modality for patients with knee pain. The ability of the US in revealing the above-mentioned pathologies is manifested by that 92.2% of the referred patients in our study were diagnosed with at least one knee pathology. Reasonable explanations for the inability to find pathology in US examination are referred pain to the knee area or pathologies that cannot be diagnosed with US. In our study, the most common finding was Baker's cyst with incidence of 59.2% (212 patients), higher than showed in previous studies (6.6 -25%).
The least common pathology was lateral meniscal subluxation (1.0%). There was no gender difference in the referred patients, 56.7% females and 43.7% males. The median age is 45.9 years which shows that there is no predilection for a certain age group The lateral and medial menisci are important structures in the knee joint. They function as shock absorbers and transfer the weight from the axial skeleton to the calves. Meniscal tear may result from degenerative changes or from acute trauma [15][16][17].
Risk factors for meniscal tears include age above 60 years, was in patients with LMT injured in a in rotatory mechanism during a physical activity also in new injuries (less than one month since the injury). The PPV was above 90% in recent injuries only (less than a month), on the other hand, NPV less than 90% in males with sport injury with LMT [18][19][20].