The diagnosis of a meniscal tear may require MRI, which is costly. Ultrasonography has been used to image the meniscus, but there are no reliable data on its accuracy. We performed a prospective study investigating the sensitivity and specificity of ultrasonography in comparison with MRI; the final outcome was determined at arthroscopy. The study included 35 patients with a mean age of 47 years (14 to 73).

There was a sensitivity of 86.4% (95% confidence interval (CI) 75 to 97.7), a specificity of 69.2% (95% CI 53.7 to 84.7), a positive predictive value of 82.6% (95% CI 70 to 95.2) and a negative predictive value of 75% (95% CI 60.7 to 81.1) for ultrasonography. This compared favourably with a sensitivity of 86.4% (95% CI 75 to 97.7), a specificity of 100.0%, a positive predictive value of 100.0% and a negative predictive value of 81.3% (95% CI 74.7 to 87.9) for MRI.

Given that the sensitivity matched that of MRI we feel that ultrasonography can reasonably be applied to confirm the clinical diagnosis before undertaking arthroscopy. However, the lower specificity suggests that there is still a need to improve the technique to reduce the number of false-positive diagnoses and thus to avoid unnecessary arthroscopy.

Total knee replacements and high tibial osteotomies are commonly performed orthopaedic operations with low complication rates. Both of these procedures involve surgery in close proximity to the popliteal artery with the use of power tools and sharp instruments. The behaviour of the popliteal artery during knee flexion, in particular the change in distance between itself and the posterior tibial cortex, is poorly understood. Many previous studies have been on stiff embalmed knees or with the patient lying supine, so as to subject the popliteal artery to an anterior pull from gravity.

We used duplex ultrasonography on 100 healthy knees to determine the distance of the popliteal artery from the posterior tibial surface at 0 and 90 degrees of flexion. One observer was used throughout. At 1–1.5cm below the joint line, we found the artery was closer to the posterior tibial surface in 24% of knees when the knee was flexion. This was also the case for 15% of knees at 1.5–2cm below the joint line. These two levels were chosen as they represent the usual positions for the tibial cuts performed in total knee replacement and tibial osteotomy. We provide an anatomical account to help explain our findings using cadaveric dissections, arteriography and static MRI studies. The first of our explanations for this posterior movement of the artery is the increase in the antero-posterior thickness of the popliteus muscle during knee flexion. We also observed a posterior pull on the popliteal artery from the sural vessels.

6% of the knees had a high branching anterior tibial branch. We highlight this anatomical variant as an example of an extremely vulnerable vessel. We review the existing literature regarding the popliteal artery dynamics, and conclude that 90 degrees of knee flexion is the safer position for tibial procedures, but repeat the warning that the surgeon must still take great care.