Correlation Between Merchant’s Standard Q Angle and TT-TG Distance

Abstract

Background:

In response to the poor reliability of clinically measured quadriceps angle (Q angle), the standard Q angle (SQA) was developed as a standardized, repeatable measurement method.

Purpose:

To determine the correlation between SQA and tibial tubercle-trochlear groove (TT-TG) distance in awake and anesthetized patients with recurrent patellar instability.

Study Design:

Cross-sectional study; Level of evidence, 3.

Methods:

We included 47 patients (94 knees; mean patient age, 23 ± 9 years) treated by 1 surgeon for recurrent patellar instability who had undergone computed tomography (CT) scans as part of their preoperative workup. SQA measurements were taken in the clinic and in the operating room after the induction of general anesthesia. Using these CT scans, we measured the TT-TG distance, Caton-Deschamps index (CDI), and lateral trochlear inclination (LTI).

Results:

The correlation coefficient (R²) between the SQA and the TT-TG distance was 0.03 (β = 0.18) for awake patients, and 0.15 (β = 0.39) for anesthetized patients. When the CDI was <1.2, the correlation was stronger in anesthetized patients (R² = 0.32; β = 0.56) than in awake patients (R² = 0.06; β = 0.24). Similarly, when the LTI was >11°, the correlation was greater in anesthetized patients (R² = 0.27; β = 0.52) than in awake patients (R² = 0.02; β = 0.12). The correlation between the SQA and the TT-TG distance was strongest in anesthetized patients with both a CDI of <1.2 and an LTI of >11° (R² = 0.57; β = 0.76).

Conclusion:

We observed a strong positive correlation between the SQA and the TT-TG distance in anesthetized patients with normal patellar height and a normal LTI. However, in awake patients and those with anatomic risk factors for recurrent patellar instability, correlations between the SQA and the TT-TG distance were weakly positive or nonsignificant. The SQA may provide useful information, but not in patients with anatomic findings that put them at increased risk for patellofemoral instability.

Keywords

patella alta patellar instability patellar tracking Q angle tibial tuberosity trochlear dysplasia tibial tubercle–trochlear groove distance

Patellofemoral instability is a common condition, representing approximately 3% of all knee injuries, and can be associated with debilitating pain, limitations in basic function, and long-term osteoarthritis.^11,16,24 Although first-time dislocations are often managed nonoperatively, reported recurrence rates are as high as 44% after initial dislocation, and even higher among patients with repeat dislocations.^11,14 Numerous surgical options are available to stabilize the patella, including medial patellofemoral ligament reconstruction, tibial tubercle osteotomy, and trochleoplasty.⁵ The choice of stabilization procedure depends on multiple factors, including objective measurements of patients’ underlying pathoanatomy.¹⁰

The quadriceps angle (Q angle) is measured as a representation of the lateral force vector applied to the patella by the quadriceps and patellar tendon. It is calculated by drawing a line from the anterior superior iliac spine to the center of the patella, and another line from the tibial tubercle through the center of the patella.¹² A large Q angle may be a risk factor for patellar dislocation. Studies have reported poor reliability and validity of clinically measured Q angle.^6,18

In response to poor reliability, a new standard Q angle (SQA) was developed by Merchant et al¹⁸ to provide a standardized and repeatable method of Q angle measurement. The SQA was offered as a practical, inexpensive, and valuable part of physical examinations to help clinicians decide whether concomitant medializing osteotomy is indicated in patellar stabilization procedures. However, the SQA has not yet been validated in patients with anatomic risk factors for recurrent patellar instability, such as patella alta and trochlear dysplasia, nor has it been measured in anesthetized patients. Significant differences in other clinical measurements, such as the Lachman and pivot-shift tests, have been reported between awake and anesthetized patients.¹⁷ Measuring the SQA before and during surgery may be highly valuable to help prevent both undercorrection and overcorrection during tibial tubercle osteotomies. Furthermore, although the SQA has been demonstrated to have strong interrater reliability,¹⁸ it has not been shown to correlate with standardized measurements for a lateralized tibial tuberosity, such as the tibial tuberosity-trochlear groove (TT-TG) distance.

Therefore, this study aimed to determine the correlation between the SQA and the TT-TG distance in awake and anesthetized patients with recurrent patellar instability. We hypothesized that there would be a strong positive correlation between the SQA and the TT-TG distance; however, we expected the correlation to be lower in patients with anatomic risk factors for patellar instability—including patella alta and trochlear dysplasia.

Methods

Patient Characteristics

Institutional review board approval was granted to conduct a retrospective review of patient data and imaging. We included patients treated by a single surgeon (A.J.C.) from 2020 to 2024 who underwent a computed tomography (CT) scan for lateral patellar instability. Patients were excluded if they did not have symptomatic patellar instability or if SQA measurements had not been performed. As part of their routine evaluation, we used a kinematic CT scanner (Aquilion ONE; Canon Medical Systems Corp), which had been previously validated for assessing patellar tracking.^22,23 Patients who had previously undergone patellar stabilization procedures were excluded.

Data Collection

The SQA was measured as previously described¹⁸ (Figure 1). Patients lie supine with the knees and hips extended and relaxed with the patella pointing directly upward (anteriorly). The center of the tibial tubercle was marked, and a 21-cm plastic goniometer with an extendable long arm was placed over the thigh (GoPro Goniometer; PFJ Solutions). First, the long arm was positioned over the anterior superior iliac spine, which was identified via direct palpation. Next, the examiner centered the goniometer pivot over the patella, with the patella manually placed in the trochlear groove. Finally, the distal arm of the goniometer was centered over the tibial tubercle, and the Q angle was measured. Measurements were taken in the clinic during the preoperative appointment and in the operating room after induction of general anesthesia. Not all patients who underwent clinic measurements also had surgery and anesthetized measurements. All measurements were performed by the senior author (A.J.C.).

Figure 1.

Photograph showing measurement of the standard Q angle in an anesthetized patient. The long arm of the goniometer is extended to the ASIS, and the patella is held within the trochlear groove by the surgeon’s right hand. The surgeon’s left hand centers the distal arm of the goniometer over the tibial tubercle. ASIS, anterior superior iliac spine.

Radiographic Measurements

Data collection, including radiographic analysis, was performed by a senior orthopaedic surgery resident (J.D.M.). Using CT scans with the knee relaxed in extension, we measured the TT-TG (Figure 2),⁷ the Caton-Deschamps index (CDI),⁸ and the lateral trochlear inclination (LTI),⁴ as previously described. The CDI was calculated as the ratio of the distance from the anterior tibial plateau to the inferior patellar articular surface divided by the length of the patellar articular surface. Measurements were obtained on sagittal CT slices where the patella appeared largest, as previously described.⁸ The LTI was measured, on the axial slice where the intercondylar notch appeared as a Roman arch, as the angle between the lateral trochlear ridge and a line perpendicular to the posterior condylar axis, with the vertex positioned at the deepest point of the trochlear groove. ⁴ These radiographic measurement techniques have been shown to have high interobserver reliability.^22,26 Patients whose CDI values were between 0.8 and 1.2 were considered to have normal patellar height. An LTI of > 11° was considered normal.

Figure 2.

Measurement of the tibial tuberosity-trochlear groove distance on a CT scan. CT, computed tomography.

Statistical Analysis

We reported mean (± SD) values for demographic information and SQA measurements. Linear regression analysis was performed to evaluate the relationship between the SQA and the TT-TG distance in both awake and anesthetized patients using the coefficient of determination (R²) and standardized β coefficients calculated to determine the strength of the regressions by giving the proportion of the variation in SQA that is predictable from the TT-TG distance. Subgroup analyses were also performed for patients with and without obesity (body mass index [BMI] ≥30 kg/m²), patella alta (CDI ≥1.2), trochlear dysplasia (LTI ≤11°), and long (≥20 mm) TT-TG distance. To ensure the validity of our linear regression analyses, we verified that 4 key assumptions were satisfied: linearity, independence of observations, homoscedasticity (constant variance of residuals), and normality of residuals. We assessed homoscedasticity by performing the Breusch-Pagan test. Normality of residuals was evaluated using the Shapiro-Wilk test. For all comparisons, regressions were considered significant at P < .05. An a priori power analysis, based on the hypothesized correlation between SQA and TT-TG distance (R² = 0.5; α = .05; power = 80%), determined that a minimum of 29 knees would be required.¹⁵ All analyses were conducted using the statistical programming language Python version 3.11.2 (Python Software Foundation; docs.python.org/3.11/) and Statistics Kingdom Linear Regression Calculator (Statistics Kingdom; https://www.statskingdom.com/linear-regression-calculator.html).

Results

A total of 94 knees were included in this study (Figure 3).

Figure 3.

Flowchart showing patient selection and subgroup stratification. BMI, body mass index; CDI, Caton-Deschamps index; LTI, lateral trochlear inclination; OR, operating room; TT-TG, tibial tuberosity-trochlear groove distance.

The mean (± SD) age was 23 ± 9 years (range 14-52 years). The mean (± SD) value for the SQA was 20.2 ± 4.1. In the full cohort (90 awake, 56 anesthetized), we found no correlation between the SQA and the TT-TG distance in awake patients (R² = 0.03; β = 0.18; P = .10) and a weak correlation in patients under anesthesia (R² = 0.15; β = 0.39; P = .003) (Table 1).

Table 1

Correlation Between Standard Q Angle and TT-TG Distance in Awake and Anesthetized Patients ^a

Group	Awake				Anesthetized
Group	N	R²	β	P	N	R²	β	P
All patients	90	0.03	0.18	.10	56	0.15	0.39	.003
Patellar height
Normal, CDI < 1.2	69	0.06	0.24	.048	41	0.32	0.56	<.001
Patella alta, CDI ≥1.2	21	0.04	−0.20	.38	15	0.02	−0.13	.65
LTI
Normal, >11°	55	0.02	0.12	.37	35	0.27	0.52	.001
Trochlear dysplasia, ≤11°	35	0.07	0.26	.13	21	0.01	−0.08	.72
TT-TG
Normal, <20 mm	42	0.38	0.62	<.001	24	0.27	0.52	.01
Elevated, ≥20 mm	48	0.17	−0.41	.004	32	0.06	0.25	.17
Body mass
Non-obese, BMI <30	70	0.05	0.23	.06	44	0.22	0.47	.001
Obese, BMI ≥30	20	0.003	−0.05	.82	12	0.03	0.18	.58
Normal patellar height and normal LTI	44	0.02	0.14	.38	27	0.57	0.76	<.001
Normal patellar height, normal LTI, and non-obese	35	0.001	0.04	.83	22	0.48	0.69	<.001
Normal patellar height, normal LTI, and normal TT-TG	21	0.24	0.49	.02	12	0.23	0.48	.11
Normal patellar height, normal LTI, normal TT-TG, and non-obese	15	0.13	0.36	.19	8	0.08	0.28	.50

Standard Q angle measured as described in Merchant AC, Fraiser R, Dragoo J, Fredericson M. A reliable Q angle measurement using a standardized protocol. Knee. 2020;27(3):934-939.

Bold values indicate statistical significance. BMI, body mass index; CDI, Caton-Deschamps index; LTI, lateral trochlear inclination; N, number of knees; TT-TG, tibial tubercle-trochlear groove.

When patients were stratified by key modifiers, in those with a CDI of <1.2, the awake SQA to TT-TG distance correlation was statistically significant, although relatively weak (R² = 0.06; β = 0.24; P = .048); nonetheless, in anesthetized patients, it was stronger (R² = 0.32; β = 0.56; P < .001). In contrast, those with a CDI of ≥1.2 showed no correlation in either state (awake, R² = 0.04; β = −0.20; P = .38; anesthetized, R² = 0.02; β = −0.13; P = .65).

A similar pattern was seen in patients with LTI >11°, for whom awake measurements yielded R² = 0.02 (β = 0.12; P = .37) and anesthetized measurements yielded R² = 0.27 (β = 0.52; P = .001). In patients with an LTI of ≤11°, we found no correlation between the SQA and the TT-TG in either state (awake, R² = 0.07; β = 0.26; P = .13; anesthetized; R² = 0.01; β = −0.08; P = .72). Patients with a TT-TG of <20 mm showed a strong awake correlation (R² = 0.38; β = 0.62; P < .001) that remained so under anesthesia (R² = 0.27; β = 0.52; P = .009). Finally, the correlation in non-obese patients improved from awake (R² = 0.05; β = 0.23; P = .057) to anesthetized (R² = 0.22; β = 0.47; P = .001).

The greatest effect was observed in all cohorts in patients who had both a CDI of <1.2 and an LTI of >11°, for whom the anesthetized R² was 0.57 (β = 0.76; P < .001) (Table 1). In the awake state, this group did not have a significant correlation between the SQA and the TT-TG distance (R² = 0.02; β = 0.14; P = .38).

Discussion

The results of this study demonstrated a positive correlation between the SQA and the TT-TG distance in anesthetized patients with normal patellar height and a normal LTI. However, in awake patients and patients with anatomic risk factors for recurrent patellar instability, we found only a weak or no correlation between the SQA and the TT-TG distance. These findings suggest that the SQA can provide useful information, but only in select patients, for whom measurement of SQA under anesthesia may guide medialization decisions during tibial tubercle osteotomy.

Q-angle measurements reflect the lateral force vector of the knee extensor mechanism and have historically been used for clinical evaluation and surgical planning in patients with recurrent patellar instability.¹⁸ However, with the widespread availability of advanced imaging, radiographic measurements such as TT-TG distance have become the standard for measuring knee extensor mechanism malalignment in the frontal plane.⁶ Furthermore, advanced imaging such as magnetic resonance imaging (MRI) or CT can provide critical information regarding intra-articular structures, cartilage condition, soft-tissue integrity, and osseous abnormalities. However, there are distinct potential advantages to using SQA measurements in clinical practice. Because the TT-TG distance cannot be reassessed intraoperatively, measuring the SQA before and during surgery may be valuable in helping to prevent both undercorrection and overcorrection during tibial tubercle osteotomies. Brown et al³ reported that in patients undergoing medializing tibial tubercle osteotomies for patellar instability, postoperative Q-angle measurements correlated with clinical outcomes: all patients with good or excellent results had postoperative Q angles of ≤10°, whereas those with fair or poor outcomes had Q angles of ≥15°. Paulos et al¹⁹ also reported excellent outcomes when achieving a postoperative Q-angle equivalent of ≤5°, indicating that there may be clinical relevance to Q-angle values. Intraoperative measurement of the SQA may also help avoid excessive medialization of the tubercle, which has been shown to increase the risk of iatrogenic medial patellar subluxation.²⁰ Although intraoperative measurement of the SQA has been advocated,¹⁸ we have not found intraoperative SQA measurements to be useful.

Smith et al²¹ highlighted the historical lack of standardized methodology for Q-angle measurement. For example, an area of ongoing debate is whether the Q angle should be measured in full extension or at various degrees of knee flexion. Whereas Smith et al²¹ argued that measuring in some degree of flexion provides more clinically meaningful information, others recommended measuring the Q angle with the knee fully extended, which is the position in which the Q angle is maximized due to the screw-home mechanism shifting the tibial tuberosity laterally.⁵ Merchant et al¹⁸ ultimately decided that extension was the most repeatable method of measuring a standard Q angle, by also manually aligning the patella in the trochlear groove. The SQA addresses both the issue of the lateralized patella in extension and the screw-home mechanism, allowing for accurate and repeatable measurements of the Q angle. However, when they described this modified method for measuring the Q angle, they included only patients without a history of knee problems or a family history of patellar instability in the study. In contrast, the present study included patients with patellar instability, and this is the patient population in whom SQA would most commonly be measured clinically. In summary, our study applies the SQA methodology to a clinically relevant cohort with patellar instability, thereby extending its application beyond the asymptomatic populations evaluated by Merchant et al.¹⁸

The TT-TG distance is commonly used to assess 1 component of patellofemoral malalignment, namely, lateralization of the tibial tuberosity. While the TT-TG distance quantifies the spatial relationship between the tibial tuberosity and the trochlear groove, it does not directly account for the position of the patella relative to the trochlea. Therefore, it is more commonly considered a surrogate measure when evaluating patellar instability⁶ and may help estimate the risk of instability. In their original study, Dejour et al⁷ assessed the TT-TG distance with patients lying supine and their knees fully extended, reporting a mean TT-TG distance of 12.7 ± 3.4 mm in asymptomatic patients. In contrast, patients with objective patellar instability had a significantly greater mean TT-TG distance of 19.8 ± 1.6 mm. Based on these findings, they proposed a threshold of 20 mm as indicating patellar instability.

A major advantage of the Q-angle measurement over the TT-TG distance is its lower cost, making it feasible in a wide range of clinical settings worldwide. Unlike advanced imaging techniques such as CT or MRI, it requires only simple tools, making it highly accessible. It also avoids radiation exposure, which is especially important for younger patients.

Many previous studies have reported that both a greater Q angle and a greater TT-TG distance are associated with patellar instability.^1,2,21,27 Cooney et al⁶ investigated the relationship between the Q angle and the TT-TG distance measured on CT scans. They found a negative correlation between the Q angle measured with the quadriceps relaxed and the TT-TG distance, which is the opposite of the positive correlation found in some of the subgroups in the present study. However, their sample size was much smaller (n = 17 patients), and they did not use the SQA measurement of Merchant et al,¹⁸ which may explain some of the difference between their results and our results. This difference between our results and those of Cooney et al⁶ underscores the importance of standardizing Q-angle measurement. Cooney et al also did not perform subgroup analysis controlling for anesthetized measurements, patellar height, trochlear morphology, or BMI value.

Together, the data from our study indicate that SQA measurements taken under anesthesia—and in patients with a normal CDI value, LTI >11°, a normal TT-TG distance, and a BMI of <30 kg/m²—tend to produce stronger linear correlations between the SQA and the TT-TG distance. The greatest correlation was observed in patients who were anesthetized and had both a CDI of <1.2 and an LTI of >11°. Although the exact mechanisms underlying these observations are unclear, we postulate that patients in the clinic may not be fully relaxed during measurement; apprehension or pain caused by manipulation of the patella could provoke involuntary quadriceps contraction, thereby altering limb positioning and SQA measurement. Furthermore, patients with patellar dislocation most often have disruption of the medial patellofemoral ligament, which is likely to result in a lateralized resting position of the patella, making relocation of the patella into the trochlear groove more difficult. A high CDI value, indicative of patella alta, may make it difficult to center the patella within the trochlear groove during measurement because the patella sits proximal to the trochlear groove, again altering the SQA. Similarly, a low LTI, suggestive of trochlear dysplasia, could impair stable patellar engagement and thus affect SQA accuracy. Finally, the overlying soft tissue in patients with high body mass may impede the identification of bony landmarks, further diminishing measurement precision. These explanations are speculative, and further investigation is necessary to determine the cause of variability in the strength of correlations between the SQA and the TT-TG distance.

This study has some limitations. First, some subgroup analyses had small sample sizes, which may have limited our ability to detect significant correlations. For example, the “normal patellar height, normal LTI, and normal TT-TG” cohort had an R² = 0.235 and β = 0.484 but included only 12 knees (fewer than the result of our power analysis, which determined that a minimum of 29 knees would be required) and had a nonsignificant P value (P = .11). Second, although the SQA provides a standardized method of measuring the Q angle, not all factors could be perfectly controlled. For example, small differences may have been present in the pelvis and leg rotation during clinical SQA measurements. Third, differences between subgroups may have existed that we were unable to control for. For example, Merchant et al¹⁸ reported an inverse relationship between SQA and patient height. Fourth, we did not assess intra- and interobserver reliability. This decision was intentional because 3 previous studies using Q-angle measurements with a long-arm goniometer demonstrated excellent intraobserver reliability and very good interobserver reliability using the same measurement technique, albeit with different positioning protocols.^9,13,18,25 Across these studies, the mean intraobserver intraclass correlation coefficient was 0.90, indicating near-perfect agreement, and the mean interobserver intraclass correlation coefficient was 0.76, reflecting very good agreement.¹⁸ Despite these findings, intra- and interobserver reliability for Q-angle measurements has not been thoroughly studied in patients with anatomic risk factors for patellar instability, and this is an important area for future study. Fifth, although the TT-TG distance is used extensively for measuring frontal malalignment, the measurement has limitations. The TT-TG distance is highly sensitive to limb positioning (particularly knee flexion and rotation) during imaging, which can reduce measurement accuracy. In cases of trochlear dysplasia, where the groove itself may be shallow or dysplastic, accurately measuring the TT-TG distance can be challenging. Investigating alternatives to the TT-TG, as measured on CT scans, including TT-posterior cruciate ligament and TT-TG measured on MRI, may provide an area for future study. Sixth, the commonly used threshold of 20 mm to define abnormal values may not be universally applicable, as it does not account for individual anatomical variation. Seventh, there was no control group that included patients without symptomatic patellar instability. Finally, measurements in the present study were performed by a single surgeon. Single-surgeon measurements may have introduced bias, and it remains unclear whether the results are generalizable to all surgeons. Nevertheless, our study is the largest to date comparing the SQA with validated TT-TG distance measurements.

Conclusion

We observed a strong positive correlation between SQA and TT-TG distance in anesthetized patients with normal patellar height and normal LTI. However, in awake patients and those with anatomic risk factors for recurrent patellar instability, the correlation between SQA and TT-TG distance was only weakly positive. The SQA may provide useful supplemental information, but only in select patients. Therefore, it remains important to have objective radiographic measurements to characterize frontal alignment when planning patellar stabilization surgery, particularly in patients with patella alta, trochlear dysplasia, or a BMI of >30 kg/m².

Footnotes

Acknowledgements

For editorial assistance, the authors thank Denise Di Salvo, MS, in the Editorial Services group of the Johns Hopkins Department of Orthopaedic Surgery.

Final revision submitted October 8, 2025; accepted October 9, 2025.

The authors have declared that there are no conflicts of interest in the authorship and publication of this contribution. AOSSM checks author disclosures against the Open Payments Database (OPD). AOSSM has not conducted an independent investigation on the OPD and disclaims any liability or responsibility relating thereto.

ORCID iDs

Jacob D. Mikula

John J. Elias

Andrew J. Cosgarea

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