Background

Patients with Developmental dysplasia of the hip (DDH) undergoing total hip arthroplasty (THA) encounter three primary challenges: acetabular reconstruction, soft tissue balancing around the hip joint, and femoral canal preparation. Notably, on the acetabular side, patients classified as Crowe III or IV often exhibit inadequate coverage of the femoral head by the acetabulum, alongside a diminutive and shallow acetabulum with defects in the anterior and lateral walls. These conditions complicate the identification of the true acetabulum during surgery, potentially leading to incorrect placement of the acetabular prosthesis. Such misplacement may result in prosthesis loosening due to insufficient support [1].

The direct anterior approach (DAA) for THA is characterized by a steep learning curve and demands extensive skill in preoperative planning and intraoperative techniques from surgeons. As for surgical navigation, only a limited number of hospitals are equipped with robotic systems, leaving the majority of hip arthroplasties to depend on the surgeon's expertise and approximate measurements [2]. While seasoned surgeons are able to achieve satisfactory surgical outcomes using these methods, less experienced practitioners may struggle to obtain optimal results, potentially increasing the risk of postoperative complications [3].

According to reports, the incorporation of technologies such as robotics, navigation, and fluoroscopic-based systems has enhanced the precision of implant placement in THA. The benefits of PSI include its user-friendliness and the facilitation of more accurate outcomes [4]. Further, advancements in artificial intelligence and 3D printing technologies have shown promising potential in medical applications, notably in enhancing the efficiency of PSI production.

Therefore, this clinical study aims to develop an artificial intelligence-enhanced, 3D printed PSI to facilitate accurate placement of hip socket and femur implants via a direct anterior approach. It seeks to reduce surgical duration and assess whether PSI can yield superior clinical outcomes and implant placement precision.

Methods

Our facility received approval from the institutional review board. Between November 1, 2021, and May 31, 2022, we collected data from 60 patients undergoing THA treatment. Included patients met specific criteria: they had not undergone previous hip replacement surgery on either side; had not been subjected to shelf procedures, femoral osteotomies, or other hip surgeries; underwent 3D preoperative planning based on computerized tomography (CT) at our institution; received elective primary THA; were diagnosed with DDH; and provided written informed consent. Exclusion criteria were refusal to participate, the presence of a metal prosthesis or infection in the same hip joint. Patients were randomized based on bed number, with nurses managing care blinded to the study groups. Thirty patients received PSI-assisted THA, and thirty underwent conventional THA. No statistically significant differences were found between the two groups in terms of age, gender, height, weight, Body Mass Index (BMI), among other factors (Table 1). There were no instances of complications such as dislocation, periarticular infection, intraoperative fracture, or periprosthetic fracture in either group. Preoperative hemoglobin (HGB) levels showed no significant difference between the groups (P > 0.05).

Table 1 Demographic characteristics
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Preoperative planning

Patients in both the PSI and control groups underwent CT-based 3D preoperative planning using AIHIP software (Version 3.0, Longwood Valley Technology, China), facilitated by two orthopedic surgeons. In contrast to other image processing software, which often involves numerous parameters and complex usability [5], AIHIP was designed to automate preoperative evaluations and simulate postoperative results. Convolutional neural networks (CNNs) were trained to automatically segment images, distinguishing between the femur and pelvis, and to identify key anatomical landmarks. The software then adjusted the pelvic orientation to a neutral stance using the bilateral anterior superior iliac spines and the pubic symphysis plane as references.

Before surgery, a detailed dysmorphic evaluation using established biomarkers was conducted. This evaluation first determined the position and size of the acetabular prosthesis, making adjustments to the acetabular cup's abduction and anteversion angles, as well as its coverage. The process then specified the position and size of the femoral prosthesis and automatically established the level of femoral resection. Simulations to forecast post-operative outcomes were performed, illustrating potential differences in leg length and modifications in femoral offset.

Patient-specific instrument

The PSI's design was preoperatively planned as illustrated in Fig. 1, using a three-dimensional planning system to simulate the PSI’s size, position, and orientation. The objective was to maximize the contact area while reducing surgical exposure. The design for each PSI received validation from two orthopedic surgeons, followed by the production of 3D models and surgical templates for the acetabulum and proximal femur according to the preoperative plan. The PSI comprised two components: one for guiding horizontal and directional cuts of the proximal femur, and another for directing the acetabulum's cup reaming and implant placement. The average duration from CT data acquisition to PSI completion was under 12 h, including CT processing within 1 h, THA planning and PSI simulation within 1 h, and 3D printing within 8 h.

Fig. 1
figure 1

The design process for Patient-Specific Instruments (PSI) employs artificial intelligence for the automated processing of computed tomography (CT) scan images. This preoperative planning, grounded in the processed CT images, facilitates the determination of the PSI's size and position

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Radiographic outcomes

Postoperative evaluation for each case involved assessing anterior–posterior X-rays of the pelvis, executed by a skilled technician with more than 10 years of experience in musculoskeletal imaging. This technician adjusted the tilt, rotation, and magnification to secure standard hip joint views. All postoperative imaging results were anonymized. Measurements were carried out by an experienced orthopedic resident with over 4 years of clinical experience. Before the study commenced, the resident underwent training in standardized radiographic measurements to ensure accuracy and consistency. The average of the resident's two measurements was utilized for statistical analysis. Follow-up X-ray examinations were conducted on the patients at least 4 weeks post-surgery.

The Lewinnek method [6, 7] was employed to measure the acetabular abduction angle and the acetabular anteversion angle. The abduction angle is defined by the angle between the line connecting the bilateral ischial tuberosities and the line through the center of the acetabular cup. The formula for the acetabular anteversion angle is given by: acetabular anteversion angle = ARC sine(a/b). The safe zone for implant placement, as defined by Meermans and Abdel et al. [8,9,10] and supported by previous research, is an abduction angle of 40 ± 10° and an anteversion angle of 15 ± 10°. Leg length discrepancy (LLD) was identified as the variance in vertical distance from the inferior margin of the lesser trochanter to the bilateral teardrop lines. The difference in femoral offset (FO) was calculated as the discrepancy in vertical distance between the centers of the two femoral head prostheses and the proximal center axis of the femur [11].

Functional outcomes

Functional outcomes were assessed using the Harris hip score and the visual analog scale (VAS) for pain score preoperatively, at 3 days, and at 1, 3, and 12 months postoperatively.

Other outcomes

Blood loss data were extracted from surgical records. The number of bone cuts was determined by the frequency of oscillating saw use for femur segmentation. The number of acetabular reaming iterations performed before finalizing implant selection was recorded. Hemoglobin (HGB) levels were measured preoperatively (7 days before surgery) and 3 days postoperatively. A decrease in HGB levels was identified as the difference between preoperative and postoperative measurements. The agreement rate between planned and actual implant placement was calculated, considering a variance of one implant model as acceptable.

Surgical technique and perioperative management

All THA procedures were conducted by an experienced surgical team employing a uniform anterior approach technique, which was novel to the team in the context of anterior THA PSI guide usage. The standard direct anterior approach was adopted. Technical specifics included positioning patient’s supine under general anesthesia, with standard preparation and dra** of the surgical site. Using the direct anterior route, the femoral neck was sequentially exposed. In the PSI group, the cutting guide was positioned between the femoral neck and anterior bony structures as determined by preoperative planning, guiding the femoral neck cut. Following femoral head removal and acetabular exposure, the superior joint capsule and labrum were excised, and the bone was delicately detached. The round ligament was severed, and the obturator foramen identified. Prosthetic design matched the femoral head and original acetabulum sizes. Reaming guide placement and assembly followed the "Lego principle," which involves completing the assembly and utilization of the guide structure within the patient's incision.. Guide pins, fixed in parallel, determined the anterior tilt angle for acetabular reaming based on their angle with the horizontal plane and the abduction angle from their orientation to the body axis. PSI application is depicted in Fig. 2 and 3. Subsequently, femoral preparation included proximal opening and medullary cavity broaching, with femoral implant anterior tilt angle aligned to the distal femoral canal line direction. The control group similarly prepared the acetabulum, maintaining a 40° abduction angle and a 15° anterior tilt angle during reaming, before implant insertion. Post-implantation, limb length was measured, and hip stability assessed, followed by layered incision closure. C-arm fluoroscopy verified implant positioning for both groups. The protheses used were Pinnacle Cup (DePuy Orthopaedics, Warsaw, IN), Corail Stem (DePuy Orthopaedics), routine procedure involved screws, excluding femoral head allografts, augments, or cement. The lead surgeon documented bone cuts and reaming instances. Standard perioperative care and patient education were uniformly provided to all patients.

Fig. 2
figure 2

Utilization of 3 dimensional-printed PSI, A. Acetabular parts of PSI; B. Insertion of the fitting guide to locate the designed position and direction, with additional parts assembled following the Lego principle, and two pins secured through designated holes; C. Removal of the fitting guide, with the reaming direction aligned parallel to the pins, as indicated by the yellow arrow; D. Femoral components of PSI; E, F. The resection guide was anchored with 2 pins, and both the level and direction of resection were established by the resection guide, denoted by the yellow line

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Fig. 3
figure 3

Intraoperative application of 3-dimentional printed PSI: A. The resection guide was anchored with 2 pins; B. The resection guide determined the level and direction of resection; C. The two anchored pins served as guiding pins, positioned in parallel; D. The angle between the guiding pins and the body axis defined the direction of the acetabular reaming abduction angle

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Data analyses

Data analysis was conducted using SPSS version 22.0. Continuous data were represented as mean ± standard deviation. The independent t-test was used for comparisons between groups, while the paired t-test facilitated pre- and post-operative comparisons within groups. For categorical data, rates were provided, with the chi-square test or Fisher's exact test applied to intergroup comparisons. A significance level of α = 0.05 was established.

Results

The AIPSI group showed a significant reduction in both neck cuts and reams. Specifically, the mean absolute error for neck cut frequency in the AIPSI group was 1.1 ± 0.4 mm, with 87% (26/30) of cases requiring only one cut, compared to the Control group at 2.1 ± 0.7 mm (P < 0.001) and 20% (6/30) necessitating just one cut. The AIPSI group had an average of 2.9 ± 0.9 reams, versus the Control group's 4.4 ± 1.1 reams (P < 0.001) (Fig. 4A).

Fig.4
figure 4

Postoperative outcomes. A Number of intraoperative neck cuts and reams; B. Discrepancy between preoperative and postoperative cup inclination and anteversion; C. Postoperative femoral offset discrepancy; D. Postoperative leg length discrepancy; E. Accuracy in predicting cup and stem placement for AIPSI and Control methods

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In terms of achieving the target inclination angle within a 5° margin of error, the Control group saw an increase in the number of cases failing to meet this criterion, with 17 cases, as opposed to only 5 in the PSI group (P = 0.003). For a 10° margin of error, no cases in the PSI group failed, compared to 2 in the control group (P = 0.492). The PSI group also had significantly fewer cases failing to achieve the target anteversion angle within a 5° margin of error, with only 3 cases, against 19 in the Control group (P < 0.001). Within a 10° margin, no cases in the PSI group failed, whereas there were 5 in the Control group (P = 0.052) (Fig. 4B).

The mean absolute error for cup inclination was 2.9 ± 1.8°in the PSI group versus 7.4 ± 4.3°in the free-hand group (P < 0.001). For cup anteversion, the mean absolute error was 3.7 ± 2.7°in the PSI group versus 5.6 ± 3.5°in the free-hand group (P = 0.002) (Fig. 4B).

The AIPSI group exhibited significantly better performance in achieving the desired femoral offset (FO), with 83.3% (25/30) achieving an offset within a ± 5 mm margin of error, compared to only 56.7% (13/30) in the control group (P = 0.047) (Fig. 4C).

The mean postoperative LLD was 4.5 ± 2.0 mm in the PSI group versus 6.4 ± 2.9 mm in the free-hand group (P < 0.001)( Fig. 4D). LLD exceeded 10 mm in three cases within the control group, while no cases in the PSI group experienced LLD beyond this threshold (P = 0.052)(Table 2).

Table 2 Absolute error between preoperative planning and postoperative outcomes
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Operation time and blood loss

The average operation duration was 79.7 ± 20.8 min for the AIPSI group compared to 87.0 ± 26.6 min for the Control group (P = 0.235). The mean blood loss amounted to 405.7 ± 176.6 mL in the PSI group and 397.0 ± 188.3 mL in the control group (P = 0.855) (Table 3).

Table 3 Operation time and blood loss
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Functional outcome

To assess postoperative functional outcomes, the Harris score and the Visual Analog Scale (VAS) pain score were evaluated preoperatively, 3 days postoperatively, and at 1, 3, and 12 months postoperatively (Table 4). There were no significant differences in preoperative Harris scores or VAS pain scores between the two groups. Three days postoperatively, the AIPSI group had an average Harris score of 61.3 ± 8.7, while the Control group scored 62.4 ± 12.2 (P = 0.668). At 12 months postoperatively, the AIPSI group's average Harris score was 95.5 ± 3.1, compared to 96.0 ± 2.0 for the Control group (P = 0.431). No significant differences were observed in VAS pain scores from 3 days to 12 months postoperatively.

Table 4 Functional Outcome
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Discussion

Our findings demonstrate significant reductions in the number of bone cuts and reams in the PSI group, attributed to the precise localization of the femoral osteotomy position by the PSI guide, which leads to fewer repeat operations and potentially shorter surgery times. Currently, no significant differences in surgery duration and blood loss exist between the two groups. However, with increased surgical experience and improved operator proficiency, it is anticipated that surgery times may decrease. This study had some limitations; we utilized 3D preoperative planning based on CT data, whereas postoperative measurements were conducted using radiographs, which may affect the accuracy of the results.

This research represents the inaugural application of PSI in the DAA to THA. Uniquely, this study introduces metrics such as the number of bone cuts and reams, which are directly correlated with surgery duration, to assess the reliability of preoperative planning. According to **ement during activities of daily living after total hip arthroplasty. J Arthroplasty. 2010;25(8):1275-81.e1. https://doi.org/10.1016/j.arth.2009.10.007 ." href="/article/10.1186/s12891-024-07449-3#ref-CR26" id="ref-link-section-d161323350e1839">26] suggested that accurately restoring the femoral offset positively influences the range of motion (ROM), by mitigating im**ement and enhancing abductor lever efficiency.

Conclusion

Our research findings demonstrate that the use of PSI significantly aids surgeons in enhancing the precision of prosthetic implant placement, which helps in restoring the patient's leg length, and may offer additional insights and guidance for managing more complex anatomical variations or cases.