Another Defining Number in Knee Pathology Assessment

In clinical orthopaedics, as in medicine generally, we like to have numbers that we can reliably quote and reference when determining the presence or absence of a particular pathology. From blood pressure to mechanical alignment, we rely on numbers. Around the knee we use many numbers measuring parameters, such as alignment, patellar height, and anteroposterior laxity, among others. These numbers have been determined by studies on normal anatomy and extended to imply an association with pathology when outside of certain predetermined ranges. This study by Tanaka et al. has given us another discriminatory number, in this case to quantify patellar instability. There are many clinical tests and measurements to assess for patellar instability, including observational features such as the Q angle and J tracking as well as lateral patellar shift and tilt, all of which are subjectively determined by the examining clinician and not accurately quantifiable. Tanaka et al. took information from a cadaveric study that indicated medial patellofemoral complex (MPFC) insufficiency could be accurately assessed by ultrasound1. They then used this technique on patients with a history of patellar instability. They measured the distance from the medial edge of the patella to the trochlea with and without slight lateral loading of the patella, and showed that a difference of ≥2 mm between the 2 measurements identified patellar instability and was highly discriminatory for MPFC insufficiency. To the authors’ credit, they studied a large number of knees divided into 4 cohorts: symptomatic knees, asymptomatic contralateral knees, knees that had undergone stabilization, and normal knees. They also assessed recognized clinical and morphological risk factors in each patient. Reliability of measurements is critical, and appropriate inter- and intra-rater reliability testing was done. The statistical analysis was robust. There are, however, some limitations to regular use by the average practicing physician, along with some cautions in considering the use of this test in day-to-day practice. Very few surgeons have an ultrasound scanner available in their consulting room, let alone the time to perform such testing. The authors state that a handheld dynamometer was used to apply “approximately” 20 Newtons (N) of lateral force, but it was not used in all cases, as the assessor developed a “feel” for that force. However, 20 N is not much, about the force of gravity on 2 kg, and I believe that is key to understanding this paper. When performing a lateral patellar glide test, most surgeons would use maximal force and most normal patellae will shift laterally by more than 2 mm, indeed to around 1 quarter of the patellar diameter. However, the authors have shown that with a force of only 20 N, only those patellae with a deficient MPFC will shift by ≥2 mm. Another concern is the positioning of the knee. The authors state that the patient was supine with the knee in full extension. In this position, the patella is proximal to the medial aspect of the medial femoral condyle, and more so in a patient with patella alta. In the cadaveric study referenced by the authors, the testing with a displacing force of 20 N was performed at 20° of flexion1. This seems a more reliable position for testing. If possible, the authors could report patient testing at this angle in a future study for comparison. The most accurate test that we have in clinical examination of the knee is the Lachman test for anterior cruciate ligament laxity2. However, very few people would use any instrument for quantifying the Lachman test unless for research purposes. Some other tests, such as the pivot-shift test, are not yet reliably quantifiable, but are highly sensitive and used regularly in clinical practice2,3. The authors have shown that lateral patellar laxity due to MPFC insufficiency is indeed quantifiable by ultrasound examination. The technique is reproducible, accurate, and noninvasive. As such, it is a valuable tool that needs to find its place in clinical practice. The test may be helpful in doubtful cases, in postoperative assessment, and in a research setting.