Abstract
Objectives:
The quadriceps angle (Q-angle) measures the force vector of pull of the extensor mechanism of the knee. A larger Q-angle is thought to be a predisposing factor for patellar dislocation, but previous literature has reported poor reliability and validity of clinically measured Q-Angle.
In response to poor reliability, a new “Standard Q-Angle” (SQA) was developed by Merchant et al. (Merchant 2020) in an attempt to create a standardized and repeatable method of clinically measuring the Q-Angle. The SQA may be a practical, inexpensive, and valuable part of physical examinations to help clinicians decide if concomitant medializing osteotomy is indicated in patellar stabilization procedures. However, SQA has not yet been validated in patients with anatomic risk factors for recurrent patellar instability such as patella alta and trochlear dysplasia. Furthermore, while SQA has previously been demonstrated to have strong interrater reliability, it has never been shown to correlate with standardized measurements for a lateralized tibial tuberosity, such as the Tibial Tuberosity – Trochlear Grove (TT-TG) distance. Therefore, the purpose of our study was to determine the correlation between SQA and TT-TG distance in patients with recurrent patellar instability. We hypothesized that there would be a positive correlation between SQA and TT-TG distance.
1. Merchant AC, Fraiser R, Dragoo J, Fredericson M. A reliable Q angle measurement using a standardized protocol. Knee. 2020;27(3):934-939.
Methods:
Patients at a single institution treated by a single surgeon with recurrent patellar instability who had previously obtained a dynamic 4D CT scan were included in this study. Patients who had previous patellar stabilization procedures were excluded. The Standard Q-angle was measured as previously described by Merchant et al. (Merchant 2020). The patient was placed in the supine position with the knee extended. A long-arm goniometer was centered over the patella with the distal arm over the tibial tuberosity and the proximal arm extended up to the anterior superior iliac spine (ASIS). The examiner then centered the patella within the trochlear groove and the SQA was measured. A second examiner stood at foot of the patient’s bed to confirm positioning. Measurements were taken in the office, as well as in the operating room after the induction of general anesthesia. All measurements were performed by the senior author (***). Utilizing 4D CT scans, the TT-TG, Caton-Deschamps index (CDI), and lateral trochlear inclination (LTI) were measured as previously described.
Patients whose CDIs were between 0.8 and 1.2 were considered to have normal patellar height. Patients whose LTIs were >11° were considered to have normal LTI, which has been shown to have a sensitivity of 93% and a specificity 87% for detecting trochlear dysplasia.
Mean and standard deviation values were calculated for patient demographic information, and correlation coefficients were calculated to compare SQA with TT-TG distance in both awake and anesthetized patients. The statistical software Stata was utilized for all analyses (StataCorp. 2021. Stata Statistical Software: Release 17. College Station, TX).
Results:
Fifty-seven patient knees were included in the study. The mean age was 22 years-old (±6 years). For awake patients, the correlation coefficient (R2) between SQA and TT-TG was -0.006 (Figure 1), and in anesthetized patients R2 = 0.093 (Figure 2). In the “normal patellar height” group, the R2 value was higher in the anesthetized group (R2 = 0.334) than the awake group (R2 = 0.239). In the normal lateral trochlear inclination group, there was also a greater R2 value in the anesthetized group (R2 = 0.403) than the awake group (R2 =0.093). The correlation between SQA and TT-TG was the strongest in anesthetized patients with both a CDI < 1.2 and LTI >11° (R2 = 0.635).
Conclusions:
There was a strong positive association between SQA and TT-TG in anesthetized patients with normal patellar height and a normal lateral trochlear inclination. However, in awake patients and patients with anatomic risk factors for recurrent patellar instability there was only a weak association between SQA and TT-TG. The SQA may be able to provide useful information, but only in selected cases. Therefore, it remains important to have objective radiographic measurements to characterize the position of the tibial tuberosity when planning patellar stabilization surgery.
