Real-Time Biplane Versus Single-Plane US-guided Hip Joint Injection: A Single-Center Prospective Study

Abstract

Background:

Ultrasound (US)-guided hip joint injection is a cornerstone of musculoskeletal care, but conventional single-plane US presents a steep learning curve for novice operators. Real-time biplane US, which provides orthogonal imaging planes, may improve procedural accuracy and efficiency, yet its utility in hip injection remains understudied.

Purpose:

To compare procedural performance and learning curves between conventional single-plane US and real-time biplane US for hip joint injection, particularly in novice operators.

Study Design:

Cohort study; Level of evidence, 2.

Methods:

This prospective single-center study enrolled 70 consecutive patients (July 2024-December 2024) scheduled for US-guided hip injection, nonrandomly assigned to single-plane US (n = 35) or biplane US (n = 35) groups. All procedures were performed by a radiologist with 7 years of diagnostic US experience, limited previous US-guided interventional exposure, and no experience in hip joint injection. Primary outcomes were first-attempt success (successful intra-articular access and injection on the initial insertion without needle withdrawal or redirection) and final success (successful completion within no more than 2 insertion attempts). Secondary outcomes included puncture time, needle adjustments, operator confidence score, and complication. Learning curves were analyzed using cumulative sum (CUSUM) modeling for puncture time (seconds).

Results:

Compared with the single-plane group, the biplane group had significantly higher first-attempt success rate (48.6% vs 25.7%; relative risk [RR], 1.889 [95% CI, 1.103-3.648]; P = .048) and final success rate (91.4% vs 71.4%; RR, 1.280 [95% CI, 1.014-1.616]; P = .031). The biplane group also had shorter puncture time (median, 120 [interquartile range, IQR, 64-182] vs 153 [90- 234] seconds; P = .020), fewer needle adjustments (median, 1 [IQR, 0-2] vs 2 [0-3]; P = .008), and higher operator confidence score (median, 2 [IQR, 1-3] vs 1 [1-2]; P = .043). One minor complication occurred in the single-plane group. CUSUM analysis demonstrated cubic learning curves for both techniques (single-plane: R² = 0.893; biplane: R² = 0.979; both P < .05). Phase-specific analysis further showed that the advantage of biplane guidance was most pronounced during the learning phase, while shorter puncture time persisted after proficiency was achieved.

Conclusion:

In novice operators, real-time biplane US guidance was associated with higher procedural success, fewer needle adjustments, shorter puncture time, and faster early skill acquisition than conventional single-plane US for hip joint injection. Its greatest benefit was observed during the learning phase, although multicenter studies are needed to confirm generalizability.

Hip joint disorders represent a prevalent musculoskeletal complaint in adult populations, with intra-articular pathologies (eg, osteoarthritis, femoroacetabular impingement) frequently leading to chronic pain and functional disability. ⁵ The deep-seated anatomical location of the hip and its complex periarticular soft-tissue/osseous structures pose inherent challenges to accurate diagnosis, necessitating a multimodal assessment integrating detailed medical history, targeted physical examination, and advanced imaging modalities. ¹⁶ In this context, intra-articular hip injection serves as a cornerstone intervention in musculoskeletal diagnosis and treatment: it confirms intra-articular pain sources via anesthetic relief (diagnostic role), delivers therapeutic agents for symptom management (therapeutic role), and predicts outcomes of subsequent procedures such as hip arthroscopy.^7,15

Traditionally, hip joint injections relied on anatomical landmarks (eg, anterior inferior iliac spine, greater trochanter) for guidance.^6,13 However, the joint's depth and proximity to dense neurovascular bundles render blind injection (without imaging) technically demanding and high-risk. Image-guided techniques have since emerged to address these limitations, yet fluoroscopy (real-time radiography), computed tomography (CT), and magnetic resonance imaging (MRI) each have critical drawbacks: fluoroscopy (real-time radiography) provides limited soft-tissue contrast and involves ionizing radiation exposure, ¹³ CT involves ionizing radiation exposure, ⁴ and MRI, while valuable for diagnostic evaluation, is not routinely used for real-time needle guidance in hip joint injection. ¹ In contrast, ultrasound (US) offers real-time dynamic imaging with no radiation and low cost; it enables direct visualization of vessels, nerves, and real-time joint capsule distention during injection, thereby enhancing accuracy, safety, and patient comfort. ² A key limitation of conventional US, however, is its single-plane visualization, which demands substantial operator proficiency and presents a steep learning curve for novice practitioners. ¹⁸ For experienced clinicians, single-plane US can suffice for hip injection procedures, but novice operators often struggle with insufficient spatial positioning ability, leading to frequent needle tip “loss of visualization” or trajectory deviation under single-plane guidance, which prolongs procedure time and increases the risk of repeated adjustments.

Real-time biplane US is an emerging technology that utilizes a matrix array probe to simultaneously display orthogonal (longitudinal and transverse) imaging planes on a single screen, eliminating the need for probe rotation. This capability provides continuous visualization of the needle trajectory and clarifies the spatial relationship between the needle tip, target site, and adjacent structures during guided puncture. Its value in improving procedural success rates and reducing complications has been demonstrated in interventions such as intrahepatic bile duct drainage and central venous catheterization.^9,11,20 Nonetheless, its application and efficacy in hip joint injection—a common musculoskeletal intervention—remain inadequately investigated, especially regarding its potential to address the technical bottlenecks faced by novice operators in this procedure.

This study aimed to compare procedural performance and learning curves between conventional single-plane US guidance and real-time biplane US guidance for hip joint injection in a novice operator setting. We hypothesized that biplane guidance would improve first-attempt and final success rates, reduce puncture time and needle adjustments, and facilitate earlier proficiency acquisition.

Methods

Study Design and Patient Enrollment

This prospective, observational, single-center study compared the safety and accuracy of real-time biplane versus conventional single-plane US-guided hip joint injection. Cases in both groups were accrued during the same study period rather than in 2 completely separate procedural phases. The 2 techniques were performed in an interleaved manner over time. During this period, the operator's US-guided interventional practice was limited to the hip joint injections evaluated in this study. As a comparative observational study rather than a randomized controlled trial, a formal sample size calculation was not performed. Consecutive patients scheduled for hip joint injection between July 2024 and December 2024 were enrolled. The study protocol received approval from the Institutional Review Board (IRB) of the First Medical Center of the PLA General Hospital (IRB No. S2021-091-01), and written informed consent was obtained from all participants after a detailed explanation of the procedure and its potential risks. The primary inclusion and exclusion criteria are outlined in Table 1.

Table 1.

Inclusion and Exclusion Criteria

Inclusion Criteria	Exclusion Criteria
• Clinical manifestations of hip pain and/or limited mobility	• History of hip joint surgery
• Hip joint pathology confirmed by physical examination and imaging studies	• Concurrent lumbar spine disease, ankylosing spondylitis, or sacroiliac joint pathology
	• Femoral head necrosis
	• Documented allergy to medications used in the study
	• Inability to cooperate with the procedure
	• Contraindications to intra-articular puncture (eg, local soft tissue infection, coagulopathy)

Procedure Protocol

All hip joint injections were performed under ultrasonic guidance using a Philips EPIQ Elite color Doppler system (Philips Healthcare). A standardized imaging protocol was implemented: conventional single-plane procedures utilized a curvilinear array transducer (C5-1, 1.0-5.0 MHz), while real-time biplane procedures employed a matrix array transducer (X6-1, 1.0-6.0 MHz) with inherent biplane imaging capability.

A single radiologist (G.Z.) performed all procedures; this radiologist had 7 years of diagnostic US experience and approximately 1 year of limited previous US-guided interventional exposure, consisting of only about 10 fluid drainage procedures, and no experience in hip joint injection before study initiation. The selection of an operator with this specific background was intended to standardize baseline proficiency, thereby facilitating a comparable assessment of the learning curves associated with both techniques. However, this design does not permit assessment of reproducibility across operators with different levels of experience. A 21-gauge, 150-mm puncture needle (Becton Dickinson) was utilized. This needle length ensured adequate reach to the anterior synovial recess, while its fine gauge contributed to reduced patient discomfort during puncture.

Patients were positioned supine with the target hip in mild abduction and external rotation to optimize the acoustic window for a standardized anterior approach targeting the anterior synovial recess at the femoral head-neck junction, based on our previously reported technique. ¹⁶ This route differs from the more commonly described lateral-to-medial trajectory and was the specific standardized approach evaluated in the present study. Following a standardized aseptic protocol (skin disinfection with povidone-iodine and application of sterile drapes), the transducer was covered with a sterile single-use sheath. Only this single standardized approach was used throughout the study to ensure procedural consistency between the 2 US-guidance techniques.

Single-Plane US-Guided Technique

Under conventional B-mode imaging, a longitudinal view along the femoral neck axis was obtained and maintained. The needle was introduced in-plane from the anterolateral aspect, with the entire shaft and tip visualized in the single longitudinal plane throughout advancement toward the anterior synovial recess (Figure 1). Needle guidance relied exclusively on this single-plane visualization.

Figure 1.

Single-plane ultrasound imaging and guided puncture for hip joint injection. (A) Two-dimensional (2D) longitudinal view of the hip joint. The anterior synovial recess is located anterior to the head-neck junction, bordered by the hyperechoic femoral head (solid yellow line) and femoral neck (dashed yellow line). (B) Puncture procedure under 2D single-plane ultrasound guidance: the puncture needle (yellow arrows) is advanced toward the anterior synovial recess, with the needle tip (red circle) accurately positioned in the target area. The illustrated needle trajectory represents the standardized anterior approach used in this study.

Real-Time Biplane US-Guided Technique

The matrix transducer was used to acquire and simultaneously display orthogonal imaging planes: a longitudinal view along the femoral neck axis and a real-time transverse view at the femoral head-neck junction. The needle was advanced in-plane in the longitudinal view. During needle advancement, the operator held the probe with one hand and the needle with the other, while an assistant adjusted the trackball on the US console to keep the transverse-plane indicator aligned with the advancing needle tip. Concurrently, the transverse plane provided continuous cross-sectional visualization of the needle tip, enabling real-time spatial verification of its position relative to the articular recess. Specifically, the long-axis plane was used to monitor the needle shaft and planned trajectory. In contrast, the short-axis plane allowed direct identification of mediolateral deviation of the needle tip relative to the target. If the tip deviated to either side, the direction of offset could be inferred directly from the short-axis image and corrected accordingly, without repeated probe rotation or tilting to reidentify the needle tip. The spatial relationship between the 2 orthogonal planes and the mechanism of deviation detection are illustrated schematically in Figure 3. This dual-view feedback allowed for immediate vector correction during needle advancement without the need to rescan or tilt the probe (Figures 2 and 3).

Figure 3.

Schematic illustration of real-time biplane ultrasound guidance and directional identification of needle-tip deviation during hip joint injection. The upper inset shows the matrix-array transducer generating 2 orthogonal imaging planes: plane A corresponds to the long-axis view and plane B corresponds to the short-axis view. In the left panel, successful puncture is illustrated: the red needle advances into the anterior synovial recess along the planned trajectory in the long-axis view, and the red dot representing the needle tip is simultaneously confirmed within the recess in the short-axis view. In the right panel, needle deviation during puncture is illustrated: the dashed red line in the long-axis view indicates deviation of the needle trajectory toward the patient's left side, and the red dot in the short-axis view is displaced to the left side of the image. Thus, the short-axis view directly indicates the direction of deviation and facilitates immediate trajectory correction without repeated probe rotation or tilting. If the trajectory deviates toward the patient's right side, the needle tip would conversely appear on the right side of the short-axis image.

Figure 2.

Real-time biplane ultrasound imaging and guided puncture for hip joint injection. (A) The left panel shows the longitudinal section of the hip joint, outlining the femoral head (solid yellow line), femoral neck (dashed yellow line), and the anterior synovial recess at the head-neck junction. The right panel displays the corresponding transverse section at the head-neck junction (dashed yellow line), with the anterior synovial recess situated anteriorly. (B) Guided puncture in the real-time biplane view. The puncture needle (yellow arrow) is visualized entering the anterior synovial recess in the longitudinal plane, while the needle tip (red arrow) is simultaneously confirmed in the transverse plane. The red arrow indicates the transverse-plane indicator used to maintain alignment with the advancing needle tip during real-time biplane guidance.

Confirmation of Intra-articular Placement

Correct intra-articular needle position was confirmed using a 2-step objective criterion: (1) absence of significant resistance during the injection of a 1-2 mL test dose of normal saline, and (2) direct sonographic observation of fluid dissemination within the anterior synovial recess. Upon confirmation, a therapeutic cocktail (4 mL of 1% ropivacaine + 4 mL of 2% lidocaine) for pain relief was injected. Failure to meet either criterion prompted immediate needle repositioning under guidance, followed by repeat confirmation.

Patient pain intensity was assessed via a 10-point visual analog scale (VAS) at baseline and 20 minutes after injection. Hemodynamic parameters and patient status were monitored throughout the procedure, with immediate complications (eg, local bleeding, nerve irritation) documented in detail.

Outcome Measures

Primary Outcomes Included

• First-attempt success rate: Defined as the proportion of patients in whom the needle successfully entered the anterior synovial recess and the medication was fully injected on the initial needle insertion, without needle withdrawal or redirection of the needle tip

• Final success rate: Defined as the proportion of patients in whom successful medication injection was achieved within no more than 2 separate needle insertion attempts. Procedures requiring ≥3 or requiring takeover by a senior attending physician because of technical difficulty were classified as failures.

Secondary Outcomes Included

• Puncture time: Measured in seconds, from the moment the needle penetrated the skin until the absence of significant resistance confirmed the successful intra-articular injection

• Number of needle adjustments: The total count of instances where the needle was partially withdrawn, and its trajectory was altered due to an observed deviation from the planned path

• Operator confidence score: A subjective assessment by the operator using a 3-point scale (1 = low confidence in first-attempt success; 2 = moderate confidence; 3 = high confidence)

• Complications: The incidence of any adverse events occurring during or immediately after the procedure. These included vascular injury (eg, hematoma formation), nerve injury (eg, new-onset pain or numbness), systemic toxic reaction to local anesthetics, and postprocedural infection.

Statistical Analysis

All statistical analyses were performed using SPSS 29.0 software (IBM Corp). Normally distributed continuous data are presented as the mean ± standard deviation and compared between groups using the independent samples t test. Non-normally distributed continuous data are presented as the median (interquartile range [IQR], P₂₅-P₇₅) and analyzed using the Mann-Whitney U test. Categorical data are presented as counts (percentages) and compared using the chi-square test or the Fisher exact test, as appropriate. Learning curves for both US-guided techniques were constructed using CUSUM analysis. Curve fitting was performed with linear, quadratic, and cubic models; the model with the highest coefficient of determination (R²) and a statistically significant fit (P < .05) was selected as the optimal model. The significance level was set at P < .05.

Results

Baseline Characteristics

Baseline demographic and clinical characteristics of the 70 enrolled patients are summarized in Table 2. No statistically significant differences were observed between the conventional single-plane US group and real-time biplane US group (all P > .05), indicating adequate group balance at study initiation.

Table 2.

Baseline Characteristics of the Study Population ^a

	Single-Plane Group N = 35	Real-Time Biplane Group N = 35	P
Sex
Male	15 (42.9)	14 (40)	.808
Female	20 (57.1)	21 (60)
Age, years	35 (29-45)	36 (30-47)	.625
Height, cm	166 (163-171)	167 (163-173)	.737
Weight, kg	62.5 ± 10.2	66.1 ± 15.4	.262
BMI, kg/m2	22 ± 2.6	22.9 ± 3.7	.222
Puncture site
Left hip	17 (48.6)	16 (45.7)	.994
Right hip	18 (51.4)	19 (54.3)
Puncture depth, cm	5.09 ± 0.62	5.24 ± 0.52	.277
VAS score
Intrapuncture VAS	4.5 ± 2.1	4.9 ± 2.2	.328
Prepuncture examination VAS	14 (10-20)	17 (11-23)	.219
Pre-/postpuncture examination VAS delta	11 (6-15)	12 (6-19)	.525
Joint effusion	4 (11.4)	6 (17.1)	.495

^aData are presented as n (%), median (IQR), or mean ± SD. BMI, body mass index; IQR, interquartile range; VAS, visual analog scale.

Primary and Secondary Outcomes

Key outcomes of hip joint injection in both groups are presented in Table 3. For the primary outcomes, the real-time biplane US group achieved a significantly higher first-attempt success rate than the single-plane US group (48.6% vs 25.7%; relative risk [RR], 1.889 [95% CI, 1.103-3.648]; P = .048). A similar advantage was observed for final success rate (91.4% vs 71.4%; RR, 1.280 [95% CI, 1.014-1.616]; P = .031). For the secondary outcomes, the biplane US group showed statistically shorter puncture time (median, 120 [IQR, 64-182] vs 153 [90-234] seconds; P = .020), fewer needle adjustments (median, 1 [IQR, 0-2] vs 2 [0-3]; P = .008), and higher operator confidence scores (median, 2 [IQR, 1-3] vs 1 [1-2]; P = .043). Only 1 immediate minor complication (transient lower-limb skin numbness) occurred in the single-plane US group, whereas no complications were observed in the biplane US group.

Table 3.

US-Guided Hip Joint Injection Outcomes Comparison: Single-plane Versus Real-Time Biplane Groups ^a

Variable	Single-Plane Group N = 35	Real-Time Biplane Group N = 35	P	RR	95% CI
First-attempt success rate	9 (25.7)	17 (48.6)	.048	1.889	1.103-3.648
Final success rate	25 (71.4)	32 (91.4)	.031	1.280	1.014-1.616
Number of needle adjustments	2 (0-3)	1 (0-2)	.008
Puncture time, s	153 (90-234)	120 (64-182)	.020
Operator confidence score	1 (1-2)	2 (1-3)	.043

^aData are presented as n (%) or median (IQR). IQR, interquartile range; RR, relative risk.

Learning Curve Results

CUSUM learning curves for both US-guided techniques are illustrated in Figure 4, with sequential case number on the horizontal axis and cumulative puncture time (seconds) on the vertical axis. Cubic models provided the best fit for both curves (all P < .05):

Figure 4.

Learning curve of hip joint injection under 2D US guidance (y = −278.374 + 246.056x – 11.676x²+ 0.141x³, goodness of fit R² = 0.893; P < .05). Learning curve of hip joint injection under real-time biplane imaging guidance (y = 40.530 + 122.836x – 5.327x²+ 0.052x³, goodness of fit R² = 0.979; P < .05). 2D US, 2-dimensional ultrasound.

• Single-plane US-guidance: y = −278.374 + 246.056x – 11.676x²+ 0.141x³; R² = 0.893

• Real-time biplane US-guidance: y = 40.530 + 122.836x – 5.327x²+ 0.052x³; R² = 0.979

Both curves reached an inflection point at the 15th case, indicating the transition from the exploratory learning phase to the proficiency phase. Notably, the biplane US curve showed a steeper descending trend after the inflection point and a higher R² value, reflecting more stable and efficient proficiency acquisition.

Phase-Specific Procedural Performance According to the Learning Curve

Based on the CUSUM-derived inflection point at case 15, procedures in each group were further divided into a learning phase and a proficiency phase (Table 4). In the single-plane US group, first-attempt success improved significantly from 0% in the learning phase to 45% in the proficiency phase (P = .004). In comparison, final success increased from 53.3% to 85% but did not reach statistical significance (P = .062). In the biplane US group, first-attempt success increased from 33.3% to 60% and final success from 86.7% to 95%, although neither change was statistically significant. Both techniques showed significant reductions in puncture time and needle adjustments after the learning phase.

Table 4.

Procedural Performance in the Learning and Proficiency Phases of Single-Plane and Real-Time Biplane Ultrasound-Guided Hip Joint Injection ^a

Variable	Within Single-Plane Group			Within Biplane Group			Between-Group Comparison
	SP-L N = 15	SP-P N = 20	P Value, SP	BP-L N = 15	BP-P N = 20	P Value BP	P Value, L	P Value, P
First-attempt success rate	0 (0)	9 (45)	004	5 (33.3)	12 (60)	.176	.042	.527
Final success rate	8 (53.3)	17 (85)	.062	13 (86.7)	19 (95)	.565	.109	.605
Number of needle adjustments	3 (2-3)	1 (0-2)	<.001	1 (0-2)	0 (0-1)	.029	.007	.182
Puncture time	259 (226-366)	100 (85-155)	<.001	185 (160-215)	87 (55-123)	<.001	.005	.020

^aData are presented as n (%) or median (IQR). BP-L, biplane learning phase; BP-P, biplane proficiency phase; P value (BP), comparison between BP-L and BP-P; P value (L), comparison between SP-L and BP-L; P value (P), comparison between SP-P and BP-P; P value (SP), comparison between SP-L and SP-P; SP-L, single-plane learning phase; SP-P, single-plane proficiency phase.

During the learning phase, biplane US showed significant advantages over single-plane US in first-attempt success rate, needle adjustments, and puncture time. During the proficiency phase, between-technique differences in first-attempt success, final success, and needle adjustments were no longer statistically significant. However, puncture time remained significantly shorter in the biplane group.

Discussion

This prospective study underscores the value of real-time biplane US guidance in hip joint injection, demonstrating superior procedural performance compared with the conventional single-plane technique, including higher first-attempt success rates, shorter puncture times, and greater operator confidence. A key challenge in hip joint injection is the steep learning curve faced by novice operators, owing to the anatomical complexity of the hip joint and the requirement for precise needle placement. Designed as an observational study with pragmatic inclusion criteria rather than the restrictive thresholds of randomized controlled trials, our study reflects real-world clinical practice. Notably, even when performed by a novice operator with basic interventional US experience but limited exposure to hip joint injection, biplane US guidance still yielded significant benefits, effectively mitigating learning-related barriers. The phase-specific analysis further indicated that these benefits were most evident during the learning phase, while the between-technique differences narrowed after proficiency was achieved except for puncture time. This may be partly explained by the dual-plane feedback provided by biplane imaging, which can facilitate earlier recognition of needle deviation and more timely trajectory correction for novice operators without repeated probe tilting or needle reidentification. However, these findings should be interpreted within the context of a single-operator, single-center study design using 1 standardized anterior approach. Collectively, our data support real-time biplane US guidance as a practical and effective alternative to the single-plane technique, with potential value for operator training and routine clinical practice.

Hip joint injection demands precise needle placement into the joint cavity while avoiding injury to adjacent neurovascular structures. Owing to the hip's deep anatomical location and complex articular architecture, US has emerged as the preferred imaging modality for guiding such procedures, enabling real-time visualization of the needle tract and surrounding soft tissues. ¹² For single-plane US guidance, maintaining alignment of the needle tip, US beam, and target structure within a single imaging plane is critical. An alternative strategy is for the operator to maintain in-plane needle visualization by directly monitoring probe orientation and comparing it with the advancing needle direction. Although this approach may be feasible for experienced operators, it can be technically demanding for novices and may increase the need for repeated probe tilting and needle reidentification. As a result, needle displacement or target loss frequently occurs during needle advancement, necessitating repeated probe tilting and needle repositioning. These challenges are particularly pronounced among novice operators, who may require multiple attempts to re-establish optimal visualization, thereby increasing procedural complexity, duration, and potential risk. ¹⁷

To address these limitations, several studies have explored magnetic navigation US systems for puncture guidance.^3,8,14 These systems overlay virtual needle trajectories and tip positions onto real-time images, partially overcoming the limitations of single-plane guidance and improving targeting accuracy. ¹⁹ Nevertheless, such technologies require specialized additional hardware (eg, magnetic field emitters, signal receivers), involve complex calibration protocols and data processing workflows, and often increase clinical operational complexity. In contrast, real-time biplane imaging with a matrix array probe resolves these issues without the need for supplementary equipment. By simultaneously displaying orthogonal longitudinal and transverse views of the needle path, its spatial relationship to the target, and adjacent anatomical structures, this technique may enhance procedural accuracy and efficiency while remaining straightforward to implement in routine clinical settings. ⁹

Our findings demonstrate that the real-time biplane group achieved significantly higher first-attempt (48.6% vs 25.7%) and final (91.4% vs 71.4%) success rates, fewer needle adjustments (median [IQR]: 1 [0-2] vs 2 [0-3]), and shorter puncture times (median, 120 [IQR, 64-182] vs 153 [90-234]) compared with the single-plane group. These improvements may be explained by the synergistic visualization provided by concurrent longitudinal and transverse planes: the longitudinal view enables continuous monitoring of the needle's axial path toward the target. In contrast, the transverse view clarifies the needle tip's cross-sectional position relative to adjacent neurovascular structures and the joint recess. When needle deviation is detected, operators can promptly correct the trajectory using real-time transverse plane feedback without repositioning the probe, thereby minimizing target loss and simplifying the adjustment process. This technical advantage likely underpins the higher success rates, reduced needle manipulations, and shorter procedural durations observed in the biplane group. Additionally, the higher operator confidence scores in the biplane group (median, 2 [IQR, 1-3] vs 1 [1-2]) may have further contributed to these favorable outcomes by reducing decision-making delays during needle advancement.

Previous studies have reported success rates of 96% to 100% for US-guided hip joint injection,^6,10 which are higher than those observed in our study. This variation is likely attributable to differences in operator experience: previous investigations often featured highly experienced interventional clinicians, whereas our operator had only 1 year of general US-guided intervention experience and no previous exposure to hip joint injection. Furthermore, the majority of our enrolled patients lacked hip joint effusion, resulting in a narrow anterior synovial recess that inherently increases targeting difficulty under single-plane guidance. Importantly, despite these challenging conditions, the real-time biplane group achieved a final success rate of 91.4%, a value comparable to those reported in studies with experienced operators. This suggests that biplane guidance can enable less experienced clinicians to achieve high procedural proficiency without an apparent increase in immediate complications, addressing a critical unmet need in clinical training and service delivery. Nevertheless, because our cohort was relatively young and had a relatively low mean body mass index (BMI), caution is warranted when extrapolating these findings to older or obese patients, in whom US-guided hip injection is often technically more challenging.

CUSUM modeling for learning curve analysis revealed that both techniques followed a cubic trend: single-plane group (y = −278.374 + 246.056x – 11.676x²+ 0.141x³; R² = 0.893; P < .05) and biplane group (y = 40.530 + 122.836x – 5.327x²+ 0.052x³; R² = 0.979; P < .05). Both curves reached an inflection point at the 15th case, indicating the transition from the exploratory learning phase to the proficiency phase. Notably, the biplane group's learning curve exhibited a gentler slope and higher R² value (0.979 vs 0.893), reflecting more consistent and rapid skill acquisition. This finding further supports that real-time biplane US guidance was associated with faster early skill acquisition for novice operators. Future studies with larger cohorts should evaluate learning curves using multiple technical-performance metrics, including procedural success and needle adjustment measures, in addition to puncture time.

The phase-specific analysis based on the CUSUM-defined inflection point provides additional insight into how procedural performance evolved. In the single-plane group, the significant improvement in first-attempt success, needle adjustments, and puncture time after the learning phase suggests that this technique is strongly dependent on accumulated hands-on experience, particularly for maintaining stable in-plane needle visualization and spatial orientation. In contrast, the biplane group already showed better performance during the learning phase, supporting the premise that orthogonal real-time visualization can reduce the technical burden during early skill acquisition. After proficiency was achieved, between-technique differences in success rates and needle adjustments were no longer significant, whereas puncture time remained shorter with biplane guidance. This suggests that biplane imaging may be most valuable for facilitating early training while continuing to streamline workflow after basic competence is reached. Although several phase-specific success measures improved numerically, not all differences reached statistical significance. This may partly reflect the limited sample size within each phase, particularly for categorical endpoints, and the relatively high baseline final success rate in the biplane learning phase, which left less room for further detectable improvement.

Limitations

This study had several limitations. First, this nonrandomized, single-center observational study was susceptible to selection bias, had limited external validity, and cannot fully distinguish technique-related effects from temporal learning effects. In addition, the modest sample size, particularly after division into learning and proficiency phases, may have limited the statistical power of some subgroup comparisons. Second, all procedures were performed by a single novice operator using 1 standardized anterior approach and a dedicated matrix-array transducer. Although this design reduced procedural variability and enabled a focused comparison between techniques, it limits generalizability across operators, injection approaches, and clinical settings where this technology is not routinely available. Third, the study population was relatively young and had a relatively low mean BMI, which may limit extrapolation of our findings to older or obese patients, in whom US-guided hip injection is often more technically challenging. Future multicenter studies involving multiple operators, alternative injection approaches, and more diverse patient populations are needed to validate these findings.

Conclusion

Real-time biplane US guidance was associated with higher first-attempt and final success rates, fewer needle adjustments, shorter puncture time, and faster early skill acquisition than conventional single-plane US for hip joint injection in a novice operator setting, with the most pronounced benefit observed during the learning phase. These findings suggest that biplane guidance may be a valuable tool for hip joint injection, particularly in training settings or for novice operators. However, further multicenter studies are needed to confirm its generalizability.