Abstract
Dear Editor,
We read with interest the article titled, “Preoperative Hip Extension Strength Is an Independent Predictor of Achieving Clinically Significant Outcomes After Hip Arthroscopy for Femoroacetabular Impingement Syndrome,” written by Beck et al. 1 It is an interesting article with new insight into the role that muscle weakness plays in femoroacetabular impingement syndrome (FAIS).
It is one of only a handful of studies that have investigated muscle strength in FAIS. We commend the team on conducting the study, which showed that hip extension weakness can predict the outcome of hip arthroscopy. In addition, using a Patient Acceptable Symptomatic State is a novel end point to define success of the procedure.
Despite the many fine points of the article, there were a number of issues in the study design, which has cast some doubts on the accuracy of the findings obtained in the study. First, a priori power calculations were not conducted; this was recognized as a limitation in the article. The reason was stated as lack of previous data, but there have been at least 2 studies on muscle weakness in FAIS with strength data widely available in the published literature.2,3 In addition, the post hoc analysis shows a power of 78%, based on hip flexion strength. This is peculiar, especially as the main finding of the article was based on extension strength. Furthermore, there was a significant dropout rate of nearly 20%. This certainly raises the question of validity of the conclusions, and whether the findings are generalizable.
Second, the strength measurements were taken using a handheld dynamometer. Although using this device is more practical, it will yield less reliable figures. Thorborg et al 5 found that female assessors consistently produced lower values using a handheld dynamometer. This is due to the dynamometer’s need to exert an equal and opposite reaction force, according to Newton’s third law of motion. If this is dependent on an assessor holding the device, it can lead to low inter- and intraobserver reliability. It is reported in the article that muscle strength was measured by 2 assessors, but the reliability of their measurements is not reported. We are curious to know whether these figures were calculated for the measurements taken in this study.
Measuring hip muscle strength accurately and consistently is difficult because of a number of factors; and we would like to highlight our previous review on standardizing muscle strength measurement in FAIS. 4 In addition, we have tested our protocol on a number of healthy volunteers, showing high inter- and intraobserver reliability. These data are awaiting publication. We would also recommend a warm-up period prior to the start of muscle strength measurement. This may improve maximum strength measurements and has been shown to reduce the risk of muscle injury.
The last point was noted as a limitation in the article; however, using a motorized dynamometer would have eliminated this issue altogether. In addition, according to the printed images, the patients were not stabilized in any way on the examination couch. Utilizing the patient’s core muscles, instead of isolating the hip joint, could lead to inaccurate or inconsistent data.
Finally, our institution is in the process of conducting a very similar study. We have obtained funding and ethical approval to correlate hip muscle strength with postoperative outcomes of hip arthroscopy. Our study has been powered at 90% for hip strength in all planes, and we are using an isokinetic dynamometer to measure isometric muscle strength. According to our systematic review, we recommended that patients undergo a 5-minute warm-up on a stationary exercise bicycle and be stabilized in the machine to prevent erroneous measurements. We also suggest that all future studies on the subject follow the same protocol to generate comparable data across different centers. We look forward to sharing our findings in due course.
Footnotes
The following author declared potential conflicts of interest: V.K. reports payments received for lectures, including service on speaker’s bureaus from Smith & Nephew and Arthrex.