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Introduction

As the global population ages, the number of primary total hip arthroplasty (THA) procedures and that of THA revision procedures is expected to increase markedly (1,2). A systematic literature review showed that hip prosthesis survival rates over 10 years ranged from 91.0-99.4% among 1,385 patients with THA in the Danish National Patient Registry from January 2004 to December 2017 (3-5), and the volume of hip revision arthroplasty in the United States increased to 10-15% of the total joint arthroplasty (6,7). The common causes of THA revision include wear of prosthetic components, mechanical loosening, hip instability and infection, among which acetabular bone defects are present in >50% cases and are difficult to locate and quantify (8). Reconstruction of acetabular bone defects is the most critical factor for successful replacement and is the most daunting challenge for THA revision (9-11).

Computer-aided technology has been successfully applied in orthopedic instrument design, materials and surgical simulation with mature technology applications (12-14). Previous studies have (15,16) used Mimics software to simulate the reconstruction of a high dislocated acetabulum, with an individualized and precise preoperative design to obtain satisfactory acetabular cup position and coverage and to avoid problems such as iliopsoas impingement. However, to the best of our knowledge, no studies have been carried out yet on computer-assisted simulation in acetabular revision. Thus, to improve the accuracy of acetabular prosthesis implantation and surgical efficiency during revision surgery, the present study admitted 10 patients with loose acetabular prosthesis revision From January 2017 to June 2021, and applied Mimics 17.0 software to preoperatively simulate the release and acetabular prosthesis implantation and determine the ideal acetabular center, actual acetabular center and acetabular prosthesis size, and obtained satisfactory acetabular prosthesis position and limb length, which are reported below.

Materials and methods

Patient information

This study included 10 patients who underwent hip revision at Nanchang and Yingtan hospitals of The 908th Hospital of the Joint Logistic Support Force (Great Wall Hospital Affiliated to Nanchang University) from January 2017 to June 2021. The ethical approval number was approval no. Hospital Medical Service (2017)13. Each patient included in this study provided written informed consent. The present study included 10 hips of 10 patients [mean age, 31-61 (44.2±10.1) years; seven men (seven hips) and three women (three hips)]. All patients had loose or broken acetabular prostheses, two had initial replacement femoral neck fracture and eight had osteonecrosis of the femoral head. The time between initial replacement and revision surgery ranged from 1 to 15 (5.2±4.3) years. Preoperative shortening of the affected limb was evaluated by orthogonal pelvis radiography and computed tomography (CT) from 15 to 35 (25.1±6.3) mm. Patients underwent preoperative radiography and 3D CT to clarify the fusion of the acetabulum and acetabular component and the acetabular bone volume and quality. Magnetic resonance imaging (MRI) was repeated to rule out infection in patients with a history of previously infected fusion. However, because of significant artifacts associated with outdated models of prosthetics, not all cases could undergo MRI to rule out infection. For patients in whom infection could not be excluded by MRI, the present study primarily relied on the following methods for infection assessment: i) Preoperative levels of C-reactive protein and erythrocyte sedimentation rate should both not exceed 40; and ii) if infection was suspected during the operation, frozen section was taken for pathological examination (neutrophil <5 per high-power field) to rule out infection.

Simulated surgery and intraoperative and perioperative management. Imaging data collection

Hardware: 64-row spiral CT (LightSpeed VCT 64) from GE Healthcare Life Sciences; Thinkpad T460 (I5; 16G; 1TB; NVIDIA GeForce940/2GB; Lenovo Group). Software: Win10 64-bit Professional (Microsoft Corporation); Mimics 17.0 (Materialise NV); Solidworks 2011 SP0.0 (Dassault Systèmes SE). Patients underwent pelvic scanning (including the hip and upper femur) with the following parameters: Layer thickness, 0.625 mm; 120 kV; and 240 mA.

Pre-operative 3D CT reconstruction

The preoperative CT image data were saved in Digital Imaging and Communications in Medicine format and imported into Mimics software. The thresholding function button in Mimics software was used to create a bone window mask and generate a 3D model of the pelvis. The ‘Cut with Polyplane’ function was used to create a Polyplane (CP1) at the anterior and posterior edges of the initial acetabulum. The osteophytes of the femur were removed using the Split function to simulate the femoral prosthesis dislocation and to establish the acetabular model. Subsequently, 2-3 ‘spheres’ of different diameters were created in Medcad to simulate the reaming of the acetabulum and to confirm that they reached the cancellous bone surface at all three levels, sagittal, coronal and transverse. After confirming the ‘spheres’ diameter, the Boolean calculation was performed to obtain the model of the acetabulum after reaming. The corresponding diameter of the spherical acetabular model was established in Solidworks according to the hole made in the acetabulum. The model was saved in the stereolithography format and imported into Mimics. It was placed into the osteotomy acetabulum through movement and rotation to retain the appropriate size of the acetabulum. A coverage rate of >75% was considered eligible (17). There are two concepts involved in the operation, namely the ‘ideal acetabular center’ and ‘actual acetabular center’. The ideal acetabular center is calculated by Sphere simulation based on the reference of the contralateral acetabular center, while the actual acetabular center is the position that needs to be reamed to the cancellous bone surface when the original acetabular component is removed. The ideal acetabular center combined with bone graft or tantalum implant was used. If the height difference was <10 mm, the actual acetabular center combined with lengthening of the femoral ball head was used (Fig. 1).

Figure 1 (A) Simulated implantation of an acetabular prosthesis: 3D models generated from bone values under thresholding. (B) Simulation of the decomposition model through the Cut function CP1. (C) After dislocation of the femoral prosthesis. (D) Importing acetabular model; (E) Confirming the center of the acetabular cup through the healthy acetabulum in three images in sagittal, coronal and transverse views. (F) Confirming the height difference between the actual acetabular cup and the ideal acetabular cup center. (G) Selecting the appropriate sphere for the Boolean operation to derive the acetabular model and place the corresponding size acetabular cup. (H) Confirmation of the anteversion angle, abduction angle and bone coverage of the acetabular cup. (I) Final simulated orthopantomogram is obtained.

Surgical method

The patient was placed in the lateral decubitus position and general anesthesia was induced. Via a posterolateral approach to the hip joint, the femoral neck, large trochanter and minor trochanter and the femoral prosthesis were exposed, making sure to protect the sciatic nerve. Upon fully loosening the acetabulum and large trochanter and minor trochanter, the acetabulum and the femoral prosthesis were exposed. It was repeatedly confirmed that the femoral stem was still loose; if not, we continued to loosen and dislocate the lateral femoral prosthesis to the top of the outer acetabulum to provide sufficient space for the removal of the acetabular prosthesis. After loosening the edge of the acetabular prosthesis, the scar on the edge of the acetabular prosthesis was removed to expose the bony border, subsequently we tapped with a special curved osteotome close to the edge of the acetabular prosthesis to separate it from the acetabular bone. This separation step was performed gently to avoid removing too much cancellous bone or causing an acetabular fracture. After removing the prosthesis and referring to the size of the preoperative simulated acetabular cup, the acetabulum was reamed to 2-3 sizes smaller than the preoperative simulated size, using the ‘wall-holding’ method when reaming the acetabulum, starting from the position of the transverse ligament. After the acetabular scar was basically removed, the reverse acetabulum reamer was used when it was close to the size of the preoperative simulation. The maximum size of the acetabulum reamer did not exceed the maximum size of the preoperative simulation. Finally, the appropriate acetabular cup was placed, and C-arm fluoroscopy was used to confirm that the position of the prosthesis was correct and then sutured layer by layer.

Perioperative management

Cefazolin sodium (1.0 g) were administered routinely 30 min preoperatively and no more than 48 h postoperatively. Preoperative intravenous drip of 0.5 of injectable tranexamic acid, with 20 ml saline dissolved in 0.5 g tranexamic acid was injected into the drainage tube after closing the incision, and the drainage tube was clamped shut for 2 h and then released. The drainage tube was removed within 48 h postoperatively. Postoperative low molecular-weight heparin sodium anticoagulation was performed. Muscle strength training of hip abductors and other muscles was started after awakening from anesthesia, and standing was assisted by a walker 48 h postoperatively.

Observation items

The operation time, intraoperative bleeding and Harris hip score (18) at 3 months and 12 months after surgery were recorded, and the postoperative acetabular anteversion and abduction angles were measured according to the Pradhan method (19).

Statistical analysis

The statistical analysis was conducted using SPSS 25.0 software (IBM Corp.), and the measurement data are presented as mean ± standard deviation. To assess the normal distribution of the data, we performed the Shapiro-Wilk test and examined the associated P-values for all test items, which indicated a normal distribution. Repeated measures analysis of variance (ANOVA) was employed to evaluate Harris scores at three consecutive time points post-enrollment, with within-subject effect testing applied when Mauchly's sphericity assumption was met. Post hoc tests for Harris scores before operation, at 3 months, and at 12 months after operation were conducted using Bonferroni correction. P<0.05 was considered to indicate statistically significant differences, and a two-tailed hypothesis test with a 95% confidence interval was utilized.

Results

All patients were followed-up for 12-48 (23.5±11.8) months. No postoperative dislocation or infection of the prosthesis occurred, and one case had numbness of the lateral calf skin due to incomplete injury of the sciatic nerve caused by intraoperative pulling, which was treated with mecobalamin (0.5 g three times a day) and the patient recovered 3 months after surgery.

Postoperative acetabular cup abduction angle (39.7±3.4˚), anteversion angle (15.8±2.8˚), abduction angle and anteversion angle were within Lewinnek's safety (20) range in all cases. Preoperative limb shortening was 25.1±6.3 mm, intraoperative ideal acetabular center and actual acetabular center difference was 13.3±6.3 mm and postoperative limb shortening was 1.6±4.9 mm. Postoperative acetabular cup bone coverage was 85.2±8.2%; the preoperative Harris hip score points were 20.9±4.3, at 3 months post-operation it was 47.2±7.7 points and at 12 months post-operation it improved to 85.4±5.3 points; bleeding volume was 332.5±61.3 ml; and operative time was 79.0±7.0 min (Table I) The typical cases are illustrated in Fig. 2. There was a statistically significant overall difference in preoperative, 3 months postoperative and 12 months postoperative Harris hip scores (repeated measures ANOVA; F=245.23; P<0.001). There were statistically significant differences in hip Harris scores before and after operation, 3 months and 6 months after operation (P<0.001; Table II).

Figure 2 (A) Typical case: The patient was a 61-year-old woman who presented with a loose flip of the acetabular lateral prosthesis 5 years after total hip arthroplasty for a femoral neck fracture (first surgery with a Stryker all-ceramic prosthesis). At 5 years after femoral neck fracture surgery, radiography and computerized tomography both suggested loosening of the lateral acetabular prosthesis and acetabular flip to the anterior acetabulum, which was assessed as a Paprosky type I bone defect with preoperative measurement of 28-mm shortening of the affected limb. (B) According to the methodology of the present paper, Mimics 17.0 software was used to preoperatively simulate the surgery, and the maximum outer diameter of the acetabular prosthesis could not exceed 52 mm (coverage rate, 91.7%), and the ideal acetabular center was 6-mm lower compared with the actual reamed acetabular cup. Moreover, the actual reamed acetabular center was planned to be selected intraoperatively, while the long ceramic ball head of the same brand was replaced to avoid shortening of the affected limb. (C) During the operation, the acetabular side prosthesis was completely removed with very little bone destruction. The acetabular prosthesis of 52-mm diameter was replaced according to the preoperative plan, and a large ceramic ball head of plus 7 mm was selected. (D) On the postoperative review radiograph, the affected limb was shortened by ~1 mm, and the acetabular prosthesis was well positioned with an abduction angle of 39˚ and an anteversion angle of 16˚. (E) Radiographs were repeated 12 months after surgery and were in good position.

Table I Pre-operative and post-operative follow-up data for each case.

Table I

Pre-operative and post-operative follow-up data for each case.

															Harris score
Case	Sex	Age, years	Medical history, years	Type of primary acetabular component	Shortening (pre-op), mm	Acetabular center difference, mm	Paprosky type	Shortening (Post-op), mma	Intraoperative bleeding, ml	Operation duration, min	Follow-up time, months	Abduction angle, ˚	Anteversion angle, ˚	Acetabular coverage, %	Pre-op	3 months (Post-op)	12 months (Post-op)
1	M	32	1	Zweymuller biocon cup	25	11	I	5	450	90	48	40	10	88.9	15	45	90
2	M	46	2	Beijing AiKang APT A-Cup	18	15	I	2	275	80	37	41	15	94.4	16	42	87
3	M	44	3	Beijing Montagne KG	20	8	I	2	350	70	24	39	13	77.8	22	56	85
4	M	48	4	Zweymuller biocon cup	22	12	I	5	275	85	21	40	18	70.0	28	61	80
5	M	49	2	Beijing AiKang APT A-Cup	31	15	II	7	400	80	16	38	20	80.6	23	43	81
6	F	42	8	Beijing Montagne KG	15	20	II	-5	350	80	13	32	17	83.3	19	39	90
7	M	56	15	Zweymuller biocon cup	27	25	III	8	300	85	15	45	18	86.1	16	54	80
8	F	61	5	Strykle Osteonics Crossfire	28	6	I	1	250	70	12	39	16	91.7	25	47	78
9	M	31	3	Zweymuller biocon cup	30	16	II	-3	350	80	18	43	15	97.2	21	48	91
10	F	33	9	Beijing AiKang APT A-Cup	35	5	I	-6	325	70	31	40	16	82.2	24	37	92

[i] ^aThe negative sign in this column indicates that the affected limb is longer compared with the healthy side.

Table II Comparison of preoperative (n=10), 3 months postoperative (n=10) and 12 months postoperative (n=10) Harris scores of patients.

Table II

Comparison of preoperative (n=10), 3 months postoperative (n=10) and 12 months postoperative (n=10) Harris scores of patients.

		Repeated measures analysis of variance
Group	Harris score	F	P-value
Preoperative	20.9±4.3	245.23	<0.001
3 months postoperative	47.2±7.7a
12 months postoperative	85.4±5.3a,b

[i] ^aP<0.001 compared with preoperative;

[ii] ^bP<0.001 compared with 3 months postoperative.

Discussion

Loosening of the prosthesis after hip arthroplasty often causes pain, resulting in limited walking and thus disuse osteoporosis (21-23). Revision hip arthroplasty has high risks and a number of variables, making it a difficult operation in joint surgery (24). Inadequate preoperative and tools preparation can easily lead to catastrophic intraoperative consequences, such as massive bone loss, acetabular fracture and acetabular prosthesis sinking into the pelvis (25,26). Understandably, acetabular reconstruction is one of the major difficulties of this operation. The aim of acetabular reconstruction is to restore the anatomical center of the hip joint and construct a peri-acetabular support band to bring the new acetabular prosthesis into full contact with the acetabular bone and maintain long-term stability and durability, and to minimize the bone defects formed in the replacement (27-30). However, the risk of a poor acetabular cup mounting position due to unclear anatomical landmarks and inaccurate positioning during hip revision is high, and the use of the traditional reaming acetabular positioning method requires a high level of surgical skill and clinical experience (31). Moreover, it is relatively difficult to accumulate surgical experience and hone skills given the small number of cases, too many intraoperative variables and a long learning curve (25-27).

Preoperative 3D analysis simulates the surgery to help preoperatively fully understand the morphological characteristics of the acetabulum, assess the degree of bone loss and select the appropriate size acetabular prosthesis (32-34). Zeng et al (35) have performed preoperative 3D simulated surgery in Mimics for patients with congenital hip dislocation with high dislocation, but it requires a high level of operating skills. Sugano et al (36) report the use of an intraoperative acetabular Computer-Aided Design (CAD) device to improve the accuracy of acetabular cup implantation, but its diffusion was affected by the expensive device and relatively cumbersome intraoperative operation. Zhang et al (37) used a positioning-guidance assisted technique for intraoperative acetabular reaming in patients with congenital hip dislocation. This technique is accurate in positioning and easy to operate intraoperatively, but it is relatively difficult to produce and design, because the design of the guide template requires CAD software operating skills and experience. Wu et al (38) designed a self-developed CAD/rapid prototyping/intraoperative positioner system to simulate acetabular cup prosthesis implantation, and used self-developed digital light processing 3D printer-light-curing surface-forming technology to match and position the acetabular positioner claw tip with the external orifice edge of the model to guide the direction of acetabular reaming. However, because the directional rod operation occupies a certain space, the intraoperative operation requires a large surgical area to be exposed and is not suitable for surgery to preserve the femoral prosthesis.

Our group previously used the Mimics software for simulations to assess acetabular prosthesis size, bone defects and acetabular prosthesis coverage in hip dysplasia cases, but in acetabular prosthesis revision, the bone defects resulting from acetabular prosthesis removal need to be considered when reconstructing the simulated surgery owing to the influence of the original acetabular prosthesis (39,40). Therefore, the present research will help to judge the size of the bone defect after prosthesis implantation based on the difference between the ideal acetabular center and the actual acetabular center, to choose whether to add a pad or to solve the problem of limb shortening by replacing the ceramic ball head, which is relatively simple and easy to operate and promote the use of. The present study also used the method provided by previous studies (17,27) to assess the bone coverage of the new acetabular prosthesis after removal of the loosened acetabular prosthesis. Since the femoral stem prosthesis was not removed in any of the cases in this group, the present study replaced the plus-sized ball heads or padded blocks as appropriate according to the height difference between the ideal acetabular center and the actual acetabular center, and ground the acetabulum to match the large ball head of 32 mm or ≥38 mm in diameter as much as possible to compensate for the limb shortening owing to the bone defect. All patients in this group had intraoperative loosening of the acetabular lateral prosthesis. Because of the gentle intraoperative operation, the degree of intraoperative acetabular lateral bone defect due to surgical technique was mild, so none of the pads were used. In addition, the postoperative shortening was reduced to within 0.5 cm, and all patients were satisfied with the postoperative results. Due to the limited number of patients in this category, the present study collected a relatively small number of cases. Currently, the present study is just a preliminary experimental report, so a control group has not been established. More data will be gathered in the future to enhance the study.

In conclusion, the use of digital simulation assistance can improve the accuracy of hip revision acetabular prosthesis implantation and reduce postoperative shortening of the affected limb. Especially for surgeons with relatively little experience in hip revision surgery, it provides a reference for the reasonable placement of the prosthesis and greatly reduces complications such as hip dislocation because of poor postoperative prosthesis position. However, because of the small number of cases in the present study and the absence of cases with bone defects of >3 cm, the effectiveness of the method could not be verified in such cases.

Acknowledgements

Not applicable.

Funding

Funding: The present study was supported by The PLA's High-level Talents Independently Projects [grant no. (2022)17-02] and Subject of Health Commission of Jiangxi Province (grant no. 20203826).

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Authors' contributions

JJZ, HDL, DL, BC, FS, HL, SLL and LL analyzed the data and edited the manuscript. JJZ, HDL, DL and BC helped to perform the follow-up and to collect patients' data. JJZ, DL and FS confirm the authenticity of all the raw data. All authors read and approved the final manuscript.

Ethics approval and consent to participate

Ethics committee approval for use of individual participant data was granted by the ethics committee of the 908th Hospital of Joint Logistic Support Force of PLA to this study [approval no. Hospital Medical Service (2017)13]. Signed written informed consent was obtained from the patients and/or guardians.

Patient consent for publication

Consent for publication of the patients' data and images was obtained from patients or their relatives.

Competing interests

The authors declare that they have no competing interests.

References