Introduction

Total hip replacement, widely regarded as one of the most successful surgeries of the 21 st century, annually aids millions of patients by treating and repairing damaged hip joints due to many different causes. The most common indications for total hip replacement include hip fractures, osteoarthritis, and osteonecrosis of the femoral head.

Proper placement of the acetabular component in total hip replacement enhances surgical outcomes by increasing the stability of the artificial joint, reducing impingement, minimizing wear on the artificial joint, and avoiding the risk of dislocation. Most surgeons today tend to achieve the cup position within Lewinnek’s “safe zone”1 including the inclination from 30o to 50o, and the version from 5o to 25o. Anatomical referencing is a method of using the patient’s existing anatomical structures as a landmark to place the acetabulum component. There are many anatomical structures used such as the native acetabular, anterior and posterior horn of the acetabular notch, acetabular rim cartilage system, and most notably, the transverse acetabular ligament (TAL), which is recommended by many authors. Archbold found that when the acetabular component was placed parallel to the TAL, the postoperative dislocation rate was 0.6%2. Revanngowda’s research indicated that using the TAL as an anatomical landmark resulted in 96.15% of the version angles and 84.6% of the inclination angles falling within the Lewinnek’s safe zone3.

This study sought to demonstrate the directional correlation between TAL and the acetabulum based on preoperative multislice computed tomographic system with multiplanar reconstruction (CT-MPR) and MRI arthrography (MR-a). With preoperative planning using CT-MPR and MR-a, we can determine the orientation angles of the TAL and the acetabulum, compare with intraoperative parameters based on a handmade protractor to determine the linear correlation, and subsequently evaluate the role of the TAL as an anatomical landmark.

Materials and methods

Study design & patients

From January 2021 to December 2022, we prospectively analyzed a total of 122 acetabula in the patients who received indications for total hip arthroplasty at our institution (Department of Orthopaedic Surgery and Sports Medicine of the E Hospital, Hanoi, Vietnam). Inclusion criteria were: (1) adult patients (≥ 18 years) who underwent primary total hip arthroplasty (THA) at E Hospital between 1 January 2021 and 31 December 2022; (2) primary surgical indications of femoral neck fracture, primary osteoarthritis, or avascular necrosis of the femoral head; (3) availability of preoperative pelvic CT with multiplanar reconstruction (CT-MPR); (4) intraoperative measurement of TAL and acetabular orientation using the handmade protractor developed at our institution; and (5) written informed consent for study participation and for the imaging procedures. Exclusion criteria were: (1) prior ipsilateral hip surgery (including internal fixation, osteotomy, or previous arthroplasty); (2) active hip joint or systemic infection; (3) pelvic or proximal femoral tumor; (4) severe congenital or acquired acetabular deformity or marked bone loss that precluded reliable identification of the transverse acetabular ligament (TAL); (5) contraindication to MR-arthrography (for example, MRI-incompatible implant, severe renal impairment, allergy to contrast) or patient refusal to undergo MR-arthrography; and (6) incomplete preoperative imaging or inability to obtain intraoperative measurements because the TAL could not be identified. Each hip was analyzed as a separate unit; bilateral procedures were counted as separate hips. Note on MR-arthrography: MR-arthrography was planned for all patients as part of preoperative assessment; however, MR-a was not performed in cases with contraindications or patient refusal. Therefore, MR-a measurements are reported for the subset of hips for which MR-a was available (n = 109).

Preoperative characteristics of the patients were, in detail, described in Table 1. The study patients consisted of 84 males (68.85%) and 38 females (31.15%). The mean age was 59.25 ± 13.15 years (range: 24–87). The mean BMI was 22.17 ± 2.84 kg/m² (range: 11.66–31.64). Indications for surgery included hip fracture in 57 patients (46.72%), osteoarthritis in 25 patients (20.49%), and osteonecrosis in 40 patients (32.79%).

Table 1 Preoperative characteristics of the patients.
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Preoperative measurement

The 64 Slice Siemens SOMATOM Perspective CT Scanner with model DC045D from Germany for imaging was applied for preoperatively measuring orientation angles of the TAL and the acetabulum. Patients were scanned in a supine position and balanced on the scanner table, with their legs in a neutral position and their knees extended straight at 0 degrees. Imaging parameters: the field of view (FOV) width ranges from 300 to 400 mm; the voltage is between 110 and 140 kV; the current intensity is 100 mAs; the slice thickness is 0.6–1 mm; the reconstruction interval is 1–1.2 mm for the bone window and the software on the coronal, axial, and sagittal planes; and the table feed is 0.8–1.2 mm. Then the orientation angle of the acetabulum on the axial and coronal planes were measured by CT-MPR (Fig. 1). We also determined and measured the TAL orientation on CT-MPR (Fig. 2).

Fig. 1
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Preoperative CT-MPR-based measurement of acetabular anteversion angle and acetabular inclination angle4.

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Fig. 2
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Preoperative determination of TAL and measurement of TAL inclination angle by CT-MPR4.

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MR-a of the hip joint was performed using a 2-step procedure5. Step 1: Inject contrast. The patient Lies on a slightly elevated table with a pillow placed under the kneecap. The hip joint is rotated 15 degrees by securing the big toes together with adhesive tape. Inject 10 ml of magnetic contrast through a 20G lumbar puncture needle. Step 2: Perform MR-a with T1-weighted fat-saturation pulse sequence (TR = 600 ms/TE = 14 ms) on 3 planes. T2-weighted fat-saturation pulse sequence (TR = 3000ms/TE = 90ms) images are obtained in the coronal plane. All sequences on the diagonal axis are imaged with a 5 mm slice thickness, FOV 24, and a 320 × 224 image matrix. On the sagittal plane, the image matrix is 256 × 224 (Fig. 3).

Fig. 3
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Preoperative determination of TAL (A) and measurement of the angle of TAL anteversion angle (B) and TAL inclination angle (C).

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Intraoperative measurement

Intraoperatively, we used a handmade protractor with 2 alignment rods placed into the acetabulum and TAL to measure the orientation angle of these two structures (Fig. 4). This handmade protractor is developed by our institution (E Hospital) and is being introduced for the first time in this study. The protractor’s structure is created by combining a system of 3 planes and 2 alignment rods. The position of the two alignment rods can be adjusted to coincide with the plane to be measured. The entire protractor system is mounted on a handle for easy use during surgery (Fig. 5).

All surgeries were performed using the direct lateral approach (Hardinge approach), with patients positioned in the lateral decubitus position on a standard operating table.

Fig. 4
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Intraoperative determination of TAL and and acetabulum and measurement of orientation angle of TAL and acetabulum by handmade protractor.

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Fig. 5
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Structure of the handmade protractor developed by E Hospital for intraoperative measurement.

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Statistical analysis

Parameters were measured using RadiAnt DICOM Viewer software. The data obtained was entered into EpiData 3.1, and responses were coded appropriately before being exported to Stata® 15 (StataCorp LLC, College Station, TX, USA) for analysis. All data was first performed a visual inspection for coding errors, outliers, or funky distributions. We tabulated the proportion of patients with the baseline characteristic variables listed above for those received indications for total hip arthroplasty, using means and standard deviation for continuous variables and frequency and proportions for categorical variables. Preoperative and intraoperative anatomical parameters of acetabulum and TAL were presented using means and standard deviation. The orientation angles of the TAL and the acetabulum between two genders were compared using either the Mann-Whitney U test or the t-test. The Pearson and Spearman coefficients (r) were utilized to evaluate the correlation between orientation angles of the acetabulum and the TAL. p-value < 0.05 was considered to be statistically significant.

Results

The analysis included a total of 122 acetabula. As was shown in Table 2, there were no significant difference in the preoperative orientation angles of the TAL and the acetabulum on CT-MPR between men and women.

Table 2 Preoperative measurements for the orientation angles of the TAL and the acetabulum on CT-MPR.
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Preoperative correlations amongst the orientation angles of the TAL and the acetabulum on CT-MPR were shown in Table 3. Significant positive correlations were observed between the inclination of the TAL and the acetabulum (p < 0.05; r = 0.4207), and between the anteversion of the TAL and the acetabulum (p < 0.05; r = 0.4107).

Table 3 Preoperative correlations amongst the orientation angles of the TAL and the acetabulum on CT-MPR.
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Table 4 showed there were no significant difference in preoperative measurements for the orientation angles of TAL on MR-a between men and women.

Table 4 Preoperative measurements for the orientation angles of TAL on MR-a.
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Table 5 indicated there were significant positive correlation between cTIA and mTIA (p < 0.05; r = 0.8656), and between cTAA and mTAA (p < 0.05; r = 0.9555).

Table 5 Preoperative correlations amongst the orientation angles of the TAL on CT-MPR and MR-a.
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Table 6 showewd there was no significant difference between men and women according to the angles describing the orientation of the acetabulum and TAL intraoperative.

Table 6 Intraoperative measurements for the orientation angles of the TAL and the acetabulum.
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Table 7 indicated there were significant positive correlations of oAAA with cAAA(r = 0.9257) and cTAA (r = 0.5160), of oAIA with cAIA (r = 0.9830) and cTIA (r = 0.4079), of oTIA with cTIA (r = 0.9006) and cAIA (r = 0.4008), and of oTAA with cTAA (r = 0.9350) and cAAA (r = 0.4839).

Table 7 Correlations of orientation angles of acetabulum and TAL between preoperative CT-MPR and intraoperative measurement.
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As was shown in Table 8, there were significant correlations amongst angle orientation parameters of TAL between preoperative MR-a and intraoperative measurement.

Table 8 Correlations amongst angle orientation parameters of TAL between preoperative MR-a and intraoperative measurement.
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Discussion

Lewinnek’s safe zone is defined as an acetabular inclination angle between 30° and 50° and an acetabular anteversion angle between 5° and 25°1. In total hip replacement surgery, positioning the acetabular component within the Lewinnek’s safe zone significantly reduces the rates of postoperative dislocation, impingement, and tendinopathy. Many authors recommend using the TAL as an anatomical landmark, citing a high success rate in achieving the safe zone postoperatively3.

In Vietnam, options for aligning the acetabular cup orientation are currently Limited. The most commonly used tool is the traditional alignment instrument with fixed angles, lacking individual customization. This instrument typically has a fixed inclination angle of 45° and a fixed version angle of 20° (Fig. 6). Consequently, accurate axis alignment still heavily depends on the surgeon’s experience.

Fig. 6
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Traditional alignment instrument commonly used in Vietnam.

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To increase customization and allow intraoperative measurement of acetabular and TAL orientation angles, our institution have designed a protractor system. The basic principle involves aligning the structural plane to be measured within the surgical field with a parallel plane outside the surgical field. This allows the orientation angle of that plane to be measured using a protractor system that forms three planes perpendicular to the Ox, Oy, and Oz axes. Each plane has a protractor engraved for easy observation. This simple protractor can be easily applied in surgery to measure the orientation angles of the acetabulum and the TAL, and to guide the reaming angle of the acetabulum and the placement of the acetabular cup. However, the disadvantage of this protractor is that its accuracy is not as high as electronic protractors, and its numerous small components make cleaning and sterilization challenging. We will continue to optimize this system in subsequent studies. To address the limitations of the handmade protractor, such as its inability to account for intraoperative pelvic motion and its lower accuracy compared to electronic systems, integrating real-time pelvic tracking systems or optical navigation in future iterations of the device is a potential solution. These technologies could provide dynamic feedback on pelvic orientation during surgery, enhancing the precision of cup positioning and reducing variability. By combining these advancements with the simplicity and cost-effectiveness of our current protractor design, we aim to create a more robust and reliable tool for acetabular alignment.

Our study results showed that there was a positive correlation in the inclination angle and anteversion angle between TAL and the acetabulum when evaluated on CT-MPR and MR-a, as well as with the orientation angle measured intraoperatively. A series of previous studies evaluating the role of TAL in orient cup position were presented in Table 9. Archolbd whose first study results were reported in 20062. Regarding the role of TAL in orienting the anteversion of the cup component, 11 out of 19 studies recommended that TAL was a useful landmark for cup positioning in terms of anteversion. For the inclination angle, 8 out of 11 studies conclude that TAL can be used to determine cup inclination.

Table 9 Literature review on the role of TAL in cup Positioning.
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Our study demonstrates a positive correlation in the orientation of the TAL and acetabulum across CT-MPR, MR-a, and intraoperative measurements. Thereby, we recommend that the TAL be used as a patient-specific anatomical landmark to orient the acetabulum when positioning it parallel to the TAL. However, we acknowledge that the TAL may not always be well defined in all cases, which could affect its utility as an anatomical reference. In cases where the TAL is less visible or obscured due to anatomical variations or pathologies, it may be challenging to use it as a reliable landmark. Surgeons should consider alternative anatomical landmarks or imaging guidance in such situations.

Additionally, we recognize that preoperative CT assessment of acetabular anteversion could be influenced by osteophytes, which may obscure the natural acetabular contours, potentially altering the measurements. In this study, measurements were carefully evaluated by experienced radiologists to minimize this impact, but this does not completely eliminate inaccuracies. Furthermore, interobserver variability remains a valid concern despite the use of standardized protocols. Future research should include quantitative analyses of measurement errors and strategies to address these challenges, such as advanced imaging algorithms or automated measurement systems.

Additionally, while preoperative imaging, including CT and MRI, is not routinely used specifically to assess TAL orientation in current practice, it may be valuable for precise preoperative planning, especially in patients with challenging anatomy or those at risk for complications such as dislocation. In our study, the preoperative imaging techniques were applied specifically for research purposes, providing detailed insights into the correlation between TAL orientation and acetabular positioning. Although TAL alignment is not a standard preoperative procedure in routine clinical practice, incorporating it into the planning phase could enhance the accuracy of acetabular component placement, particularly in complex cases.

Also herein, this study presents certain limitations. First, standardized procedures for measuring hip joint and acetabular parameters in Vietnamese patients are lacking; thus, the techniques used in this study were developed and implemented for the first time by the research team at E Hospital. Second, the reliability and validity of the handmade protractor remain unverified, and its inability to account for intraoperative pelvic motion, a critical factor in acetabular alignment, limits its accuracy. Future iterations should incorporate technologies like real-time pelvic tracking to address this issue. Additionally, a quantitative analysis of measurement errors was not performed, and the protractor’s design, tailored to Vietnamese anatomy, may limit its applicability to other populations. Furthermore, future studies should include a preliminary validation of the protractor by comparing its measurements to those obtained using established electronic navigation systems, to further evaluate its accuracy and applicability in clinical practice. Lastly, while significant correlations were observed between intraoperative and imaging-based measurements, the study did not assess long-term clinical outcomes such as dislocation or impingement rates. Two cases of postoperative dislocation were observed during the follow-up period. Both patients had acetabular components positioned within Lewinnek’s “safe zone” as confirmed by postoperative imaging. One patient experienced a domestic slip-and-fall accident, while the other was involved in a motorbike accident. Both dislocations were managed with closed reduction under spinal anesthesia, and no further dislocations were observed during the 12-month follow-up period. These cases highlight the importance of external factors, such as trauma, in contributing to dislocation risk, even when optimal acetabular positioning is achieved. These limitations underscore the need for further research to validate the device and explore the clinical efficacy of TAL-guided cup positioning in diverse populations.

Conclusions

The TAL is a personal, visible, and useful landmark for anatomical alignment in acetabular cup positioning during total hip replacement. Utilizing this anatomical landmark can help position the cup within the Lewinnek’s safe zone and reduce the incidence of postoperative dislocation and impingement.