Factors influencing the outcome of hip preservation in femoral head necrosis treated with rotational osteotomy via the base of the femoral neck, combined with finite element analysis

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Factors influencing the outcome of hip preservation in femoral head necrosis treated with rotational osteotomy via the base of the femoral neck, combined with finite element analysis

https://doi.org/10.21203/rs.3.rs-3687446/v1

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Background :The tip-apex distance (TAD), calcar referenced tip-apex distance (Cal-TAD), and neck-shaft angle (NSA) demonstrate reliability in predicting outcomes following internal fixation of femoral neck fractures. However, the potential influence of TAD, Cal-TAD, and NSA on predicting hip preservation outcomes through trans-trochanteric rotational osteotomy at the base of the femoral neck for treating femoral head necrosis remains unexplored.

Purposes: To assess the prognostic value of TAD, Cal-TAD, and NSA for hip preservation outcomes following rotational osteotomy at the base of the femoral neck in conjunction with finite element analysis, we compared the outcomes of different patients who underwent this procedure for femoral head necrosis.

Methods: The clinical data of 33 patients (33 hips) who underwent rotational osteotomy at the base of the femoral neck between January 2017 and December 2021 in our hospital were retrospectively analyzed. TAD, Cal-TAD, and NSA were measured on immediate postoperative radiographs, and the patients were categorized into Alive and Dead groups based on hip preservation outcomes for statistical analysis. Additionally, three-dimensional models with varying neck shaft angles were constructed to examine changes in maximum displacement of the femoral head and stress on the osteotomy surface.

Results:During a mean follow-up period of approximately 40 months, 26 patients survived while 7 patients experienced hip preservation failure. The differences in age, TAD, and Cal-TAD between the two groups were not statistically significant (P>0.05), whereas NSA exhibited statistical significance (P<0.05). Biomechanical analysis revealed a close association between NSA and postoperative biomechanical changes.

Conclusion: When performing rotational osteotomy at the base of the femoral neck for the treatment of femoral head necrosis, both TAD and Cal-TAD exhibit negligible impact on hip preservation outcomes, while the size of NSA significantly influences surgical success rates. Therefore, meticulous attention should be given to precise measurement and adjustment of NSA during surgery in order to achieve optimal surgical outcomes.

Osteonecrosis of the femoral head

Rotational osteotomy through the base of the femoral neck

TAD

Cal-TAD

Neck-shaft Angle

Osteonecrosis of the femoral head (ONFH) is a debilitating arthropathy with high morbidity, commonly affecting young and middle-aged populations. Without proper diagnosis and treatment, approximately 80% of ONFH patients will experience collapse, significantly impairing their quality of life and necessitating artificial total hip arthroplasty. However, due to limited prosthesis lifespan and increasing complications, revision becomes inevitable for young and middle-aged patients. Therefore, enhancing the success rate of hip preservation treatments remains a crucial focus in the clinical research on ONFH. [1-3].

The rotational osteotomy of the base of the femoral neck is a novel technique derived from the modification of Sugioka's rotational intertrochanteric osteotomy proposed in 1976 [4]. This approach involves artificially truncating the femoral neck, relocating the necrotic area away from weight-bearing regions, and securing the osteotomy using internal fixation.

The TAD, a conventional technique for determining screw position in the femoral head, has gained widespread acceptance among clinicians. The TAD range for centrally placed tension screws is 10 to 25 mm, with an optimal value of 20 mm; moreover, a threshold of 25 mm can be utilized to predict potential cut-out. In the case of centrally placed screw inserts, the TAD varies from 20 to 30 mm [5]. However, further biomechanical and clinical investigations are required to validate whether the concept of Cal-TAD is more suitable for off-center screw placement [6]. Notably, studies have demonstrated that TAD, Cal-TAD, and NSA all impact the success rate following internal fixation of fractures.

Therefore, this retrospective study analyzed clinical data from 33 patients (33 hips) who underwent rotational osteotomy at our institution between January 2017 and December 2021. The aim was to evaluate the effects of TAD, Cal-TAD, and NSA on hip preservation outcomes while assessing postoperative biomechanical changes using different NSAs in combination with finite element analysis.

1.1 Patient selection criteria

Inclusion criteria: (1) patients who can be consistently followed up post-surgery; (2) individuals without a history of hip preservation failure within 6 months after the surgical procedure; (3) participants who have not undergone any additional treatments during the follow-up period; (4) subjects who have undergone pre- and post-operative CT examinations, as well as periodic imaging assessments throughout the follow-up duration.

Exclusion criteria: (1) patients who did not comply with the follow-up; (2) patients experiencing severe adverse reactions, allergies, or accidents; (3) individuals with incomplete imaging data.

This study included a total of 33 patients (33 hips) who underwent rotational osteotomy of the base of the femoral neck at our hospital between January 2017 and December 2021.

Ethical approval was obtained from the Medical Ethics Committee of the Affiliated Hospital of Nanjing University of Traditional Chinese Medicine (2023NL-058-02).After providing all the enrolled patients with detailed information about the surgical procedure, their written informed consent was obtained. All procedures are carried out in accordance with the “Helsinki Declaration” of the World Medical Association.

1.2 General information.

The study included a total of 33 participants, comprising 30 males (representing 30 hips) and 3 females (representing 3 hips), with an age range of 18-48 years (mean age: 31.8). Among them, there were 13 cases of left hip involvement, 12 cases of right hip involvement, and 4 cases of bipartite hips. The types of osteonecrosis of the femoral head (ONFH) observed in this study were as follows: idiopathic ONFH accounted for 15 cases (15 hips), hormonal ONFH accounted for 9 cases (9 hips), alcoholic ONFH accounted for7 cases(7hips), and traumatic ONFH accounted for2cases(2hips).

1.3 Surgical methods

After preoperative planning and administration of general anesthesia, the patient is positioned laterally for surgery. A lateral incision is made on the affected hip joint to expose and protect the insertion points of gluteus medius muscle and rectus femoris muscle. Osteotomy is performed at the greater trochanter with careful preservation of both upper and lower attachments. The anterior side of the femoral neck joint capsule is dissected to assess any damage to the glenoid labrum. Subsequently, dislocation of the femoral head allows observation of its anterior and posterior collapse status. To evaluate blood supply, a hole is drilled using a Cricket's pin in order to investigate perfusion within the femoral head. On the posterior side of greater trochanter, soft tissue surrounding external rotator muscle group is meticulously peeled off as a flap.

1.4 Observation index and follow-up

The TAD, Cal-TAD, and NSA measurements were conducted on immediate postoperative frontal and lateral radiographs by two independent observers who were attending orthopaedic surgeons. Both observers received a detailed briefing regarding the TAD, Cal-TAD, and NSA measurements[ 11-12 ].

1.5 Finite element analysis: an example of FNS

The CT data of a male patient, aged 30 and weighing 80kg, with left-sided alcoholic femoral head necrosis ARCO type III was collected. The patient had a history of heavy drinking for 5 years.

CT scan parameters included a layer thickness of 1mm and scanning range from the upper edge of the pelvis to 15cm below the lesser trochanter. The CT data was saved in DICOM format, along with an original data copy.

1.5.1 Main software

Mimics 21.0 software, Geomagic Studio 2017 software, Solidworks 2021 software, ANSYS 17.0 software.

1.5.2 Modelling process

The CT image in Mimics Research 21.0 was segmented based on a specific threshold to obtain an initial model of the femoral head. Subsequently, the initial model was imported into Geomagic 2017 for establishing a preliminary geometric representation. Various processes including noise reduction, removal of regional features, and frosting were performed to optimize the femoral head model. After surface fitting, the optimized model was further assembled using SolidWorks 2021 software to establish the final preoperative geometric representation (Figure 1). This representation was then imported into ANASYS 17.0 software for material assignment, body mesh and surface mesh generation, as well as finite element analysis.

1.5.3 Mesh Division

The meshing settings of the model: the number of cells and nodes after meshing are shown in Table 1.

Table 1: Number of nodes and cells in the preoperative finite element model

Sports event Rotational osteotomy model of the base of the femoral neck

Number of nodes 160456

Number of units 89005

1.5.4 Establishment of different NSA and FNS models after rotational osteotomy via the base of the femoral neck

The femur model was imported into the 3D engineering software SolidWorks 2021 as a STEP file. A cross-section was created at the base of the femoral neck, with the osteotomy surface perpendicular to the axis of the femoral neck. The necrotic area was repositioned outside of the weight-bearing region by rotating the osteotomy block forward along the axis of the femoral neck, resulting in merging of both proximal and distal osteotomy surfaces. Six distinct postoperative NSA models were established based on variations in osteotomy surfaces.

The internal fixation device model was developed using Johnson & Johnson's FNS data, comprising a femoral neck splice plate, a femoral neck power bar, and a femoral neck anti-rotation screw. The FNS model was assembled with the femoral osteotomy model and positioned according to clinical fixation methods to create the final model (Figure 2). Subsequently, ANASYS17.0 software was employed for material assignment, body mesh formation, surface mesh generation, and finite element analysis.

1.5.5 Material property assignment

The material parameters for cortical bone, cancellous bone, necrotic zone, and internal fixation were inputted into ANSYS 17.0（Detailed information is presented in Table 2）, with corresponding material properties assigned to each component of the analysis. The FNS internal fixation device was fabricated using titanium alloy material [7-8].

1.5.6 Loading load and setting constraints

According to relevant biomechanical studies, the unilateral loading pressure is calculated as 2.5 times the product of body weight (M), gravity coefficient (G), and a constant value of 0.5N. In this study, loads of 920N, 1500N, and 2000N were applied to the weight-bearing area above the femoral head to investigate the biomechanical changes following rotational osteotomy under different stress conditions. The weight-bearing area was defined as an arc spanning 40° in both internal and external directions from the center of the femoral head, as well as an arc spanning 80° in the anterior-posterior direction relative to its center (Figure 3) [9]. The distal end of the femur was immobilized as a fixed interface.

1.5.7 Evaluation index

Examine the variations in maximum displacement values and stress concentration phenomena at the osteotomy surface across different models of rotational osteotomy via the base of the femoral neck.

1.6 Statistical methods

The data were statistically analyzed using SPSS 27.0 software and GraphPad Prism 8. The normality of the distribution of continuous variables was assessed using the Shapiro-Wilk test. For variables with a symmetrical distribution, mean (mean) and standard deviation (SD) were reported, while median and interquartile range (IQR) were used for non-normal distributions. Categorical data were presented as absolute numbers and percentages.

Statistical comparisons of continuous variables were assessed using the Student's t-test for normally distributed variables and the Mann-Whitney U test for non-normally distributed variables. Binary logistic regression analyses were conducted with TAD, Cal-TAD, and NSA as independent variables, while the occurrence of total hip arthroplasty for failed hip preservation was considered as the dependent variable. Intraclass correlation coefficients (ICC) along with 95% confidence intervals (CI) were calculated for interval data. The results of the logistic regression analysis were visualized using forest plots and Nomogram plots. The Nomogram plots were utilized to develop a scale based on the magnitude of regression coefficients from all independent variables, assigning scores to each level of every value for each independent variable. Subsequently, a total score was computed for each patient, followed by calculating the probability of outcome occurrence at that specific time through a transformation function between score and outcome probability.

The area under the ROC curve (AUC) was calculated to assess the performance of TAD, Cal-TAD, and NSA models and determine the optimal threshold predictive value. AUC values were interpreted as follows: AUC = 0.5 indicated no discrimination; 0.5 < AUC ≤ 0.7 represented low precision; 0.7 < AUC ≤ 0.9 indicated moderate precision; AUC > 0.9 suggested high precision; and an AUC of 1 denoted perfect discrimination. The optimal thresholds for TAD, Cal-TAD, and NSA were determined based on Youden's J statistic by identifying the point on the ROC curve with maximum distance from the diagonal line.

LASSO regression effectively mitigated overfitting by compressing the coefficients of variables in the regression model, leading to the generation of a Risk Score. Utilizing risk factor analysis, we examined the association between the Risk Score and both survival status and survival time, subsequently generating a heatmap illustrating the relationship between Risk Group and various risk factors.

The present study retrospectively analyzed a total of 40 patients who underwent rotational osteotomy of the base of the femoral neck for femoral head necrosis. Four patients were excluded due to loss to postoperative visits, and among them, three had poor quality postoperative x-rays that hindered proper visualization of the femoral head. Consequently, our study included a final sample size of 33 patients (33 hips) with femoral head necrosis.

In our study, there were 26 male and 7 female participants, among whom 19 patients underwent fracture fixation using hollow screws, 8 patients received Dynamic Hip Screw (DHS) treatment, and 6 patients underwent Femoral Neck System (FNS) intervention. A total of 7 patients experienced failure in hip preservation: 4 patients exhibited further collapse of the femoral head, 1 patient encountered dislodgement of the screws, and 2 patients suffered from non-healing of the osteotomy surfaces and subsequently required Total Hip Arthroplasty (THA). Based on the outcomes of hip preservation, we categorized the cohort into two groups consisting of a total of 33 patients. Detailed information is presented in Table 3.

The average transverse acetabular diameter (TAD) of the surviving group was 3.960±1.652 cm, while it was 4.329±1.321 cm in the failing group. Additionally, the average calibrated TAD (Cal-TAD) of the surviving group was 4.197±1.235 cm, compared to 4.263±1.306 cm in the failing group.Patient distribution as shown in Figure 4; however, gender, age, TAD, and Cal-TAD did not exhibit any significant impact on hip preservation outcome (p > 0.05). Conversely, there was a significant association between neck-shaft angle (NSA) and hip preservation outcome with mean NSA being 133.625°±11.039° in the survival group and 121.121°±5.189° in the failure group (p=0 .023).

The forest plot presents a summary of the Cox regression analysis（Figure 5）, indicating significant findings for TAD values (OR = 0.608 [95% CI, 0.093-3.985]; P = 0.604). Notably, cal-TAD values also demonstrate significance (OR = 0.458 [95% CI, 0.28-5.362]; P = 0.458). Furthermore, nsa values exhibit a positive correlation with hip preservation success (OR = 1.229 [95% CI, 1.005-1.503]; P = 0.044) and serve as a reliable independent prognostic predictor in this study's context.Nomogram plots（Figure 6） depict the logistic regression results incorporating TAD, Cal-TAD, and NSA as independent variables.

To estimate a threshold for predicting hip preservation outcomes, we employed the receiver operating characteristic (ROC) curve analysis（Figure 7）. The sensitivity and specificity of TAD were determined to be 87.5% and 85.7%, respectively. The area under the curve (AUC) was calculated as 0.571, with an optimal threshold value of 4.06 being identified. In terms of Cal-TAD, its specificity was found to be 71.4%. The AUC was computed as 0.514, with an optimal cut-off value of 3.993 established accordingly.Furthermore, NSA exhibited a specificity rate of 69.2%. Its AUC reached a value of 0.857, while the optimal cut-off value was determined to be at 127.5°

To further elucidate the optimal influencing factors, we conducted a LASSO Cox regression analysis to construct a prognostic model. The distribution and expression patterns of the three influential factors, namely risk score, survival time, and survival status, are depicted in Figure 8. As anticipated, NSA exhibited significantly elevated expression levels across all low-risk groups.

2.3 Finite element analysis: difference in forces for different NSAs

Revised sentence: "Based on the displacement cloud of the femoral head in different NSA models following rotational osteotomy via the base of the femoral neck (the transparent model representing pre-compression and the solid model representing post-compression displacement), it is observed that the largest displacement of the femoral head occurs in the weight-bearing region, gradually decreasing layer by layer downwards. The maximum postoperative displacement after rotational osteotomy via the base of the femoral neck decreases with increasing NSA, but significantly increases when NSA exceeds 160°. These findings suggest that increasing NSA leads to decreased femoral head displacement and increased stability after rotational osteotomy.

The stress distribution maps of the femoral head following rotational osteotomy via the base of the femoral neck reveal a high-stress concentration at its junction with the femoral neck. The peak stress is observed on the surface where osteotomy was performed during this procedure. Notably, for an NSA value of 117° compared to other five models, stress concentration primarily occurs at one specific location near their junction, which intensifies under higher forces applied. Conversely, multiple stress concentrations are observed in other five models.Detailed information is presented in Figure 9-10.

These results emphasize that different NSAs significantly influence both displacement and stress distribution patterns within the femoral head following rotational osteotomy via its base at a neck level. Therefore, these factors should be thoroughly considered while selecting surgical approaches and designing treatment plans to optimize surgical outcomes and minimize potential risks.

In recent decades, the prediction of success rates following internal fixation of fractures has been a primary focus for orthopaedic surgeons [10]. Numerous studies have aimed to identify a reliable predictor or model to enhance clinical decision-making. In 1995, Baumgaertner et al [11] introduced the concept of TAD, proposing that a TAD <25 mm could effectively prevent upward cutting of the tension screw from the femoral head. Subsequently, this idea gained widespread acceptance and was well validated in clinical practice. Later on, TAD was applied to dynamic hip screw (DHS) treatment for intertrochanteric fracture and it was observed that when the TAD > 30 mm, there was a 27% chance of screw cut out; when the TAD ranged between 25-30 mm, there was a 2% chance; and when the TAD <25 mm, there was no chance of screw cut out. However, measuring TAD solely based on the central apex of the femoral head does not consider three-dimensional spatial positioning of screws and fails to explain cases where screws are biased downward resulting in a TAD >25 mm but with improved stability. In light of this limitation, Kuzyk et al [12] modified conventional TAD by introducing Cal-TAD which corrects for measurement errors in orthopantomographs while maintaining identical lateral slice measurements as conventional TAD. Apart from considering factors such as TAD and Cal-Tad, several other variables may influence femoral screw stability.

In this study, we conducted an analysis on the factors influencing the postoperative outcomes of trans-base rotational osteotomy for hip preservation. A total of 33 patients who underwent this procedure had a mean TAD (tip-apex distance) of 4.04±1.618 cm and a Cal-TAD (calcar referenced tip-apex distance) of 4.211±1.234 cm. Statistical analysis revealed that gender, age, as well as TAD and Cal-TAD measurements did not significantly impact the success rate of the operation. Therefore, despite being an artificially induced femoral neck fracture, trans-base rotational osteotomy exhibited distinct prognostic effects in comparison to conventional internal fixation methods for femoral neck fractures due to minimal bone loss at the base of the neck and preserved blood supply to the femoral head, necessitating only internal fixation at the osteotomy site.

Our findings demonstrate that among all the measurements, the angle of NSA emerges as the sole reliable indicator. The magnitude of NSA exhibits a significant correlation with postoperative efficacy and the success rate of hip preservation procedures. Maintaining NSA within a normal or even larger range yields superior outcomes, whereas deviating from the reasonable range for hip preservation success, either excessively large or small, results in suboptimal postoperative efficacy or even failure of hip preservation.

Based on the aforementioned studies, we further investigated the effects of different neck-shaft angles (NSAs) on femoral head displacement and stress distribution after rotational osteotomy via the base of the femoral neck. Finite element analysis revealed that smaller NSAs were more likely to cause stress concentration on the osteotomy surface, which could affect healing and even lead to loosening of internal fixation or protrusion of cortical bone in the femoral head. In this study, screw dislodgement occurred in one case and two cases showed non-healing at the osteotomy site; all had NSAs smaller than 120°. Therefore, NSA size is closely related to intraoperative rotation of the femoral head and angle of osteotomy. During surgery, operators should pay attention to appropriate rotation angle selection, ensure alignment of osteotomy plane, and consider NSA size. Additionally, ROC curve analysis indicated an optimal threshold for NSA at 127.5° may be beneficial; biomechanical results demonstrated maximum displacement within 120°-160° decreased with increasing NSA size. Further patient evaluation will be conducted to validate these observations.

However, this study still has some limitations. Firstly, the sample size of patients who underwent this procedure was small due to the challenges associated with rotational osteotomy through the base of the femoral neck, and a significant number of patients were lost during incomplete follow-up. Moreover, since there were only 7 cases of hip preservation failure, our estimated confidence intervals were wide, indicating a low level of precision. Therefore, future steps should involve including a larger number of patients in order to enhance statistical power and generalizability. Additionally, it is important to acknowledge that certain errors exist within the data presented in this paper; specifically regarding postoperative X-rays where variations in location and angle may have introduced inaccuracies into our findings. Furthermore, while we recognize that our femoral model material assignments did not account for factors such as ligaments and other forces acting on the femur itself; we aimed to minimize confounding variables in order to highlight meaningful differences between preoperative and postoperative states. It is also worth noting that our simulation solely focused on mechanical aspects without considering potential impacts related to blood supply or healing processes following truncated bone block procedures. Lastly, further investigation will be conducted to determine whether disparities exist between simulated postoperative forces derived from models and actual postoperative forces experienced by patients.

In summary, our findings demonstrate the efficacy of NSA as a reliable predictor for hip preservation outcomes in patients with femoral head necrosis. Furthermore, utilizing the postoperative NSA angle enables more precise prognostication of patient survival status and duration, thereby facilitating the development of surgical strategies. In conclusion, while TAD and Cal-TAD are commonly employed indicators for assessing fracture internal fixation success, their applicability to rotational osteotomies via the base of the femoral neck may not be universally applicable as initially anticipated. Biomechanical results underscore that different NSAs exert significant influence on femoral head displacement and stress distribution following rotational osteotomy through the base of the femoral neck.

Availability of data and materials

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

Authors’ contributions

Sun Jiahao made significant contributions to this article. He conducted data analysis and interpretation, as well as drafted the manuscript. Xia Tianwei was responsible for radiological examinations, including X-ray and CT scans. Jiahao Sun and Chen Zhiyuan collected the data and performed statistical analyses. Jiahao Sun also created the figures. The surgery was performed by Jirong Shen (operator), Tianwei Xia (assistant), and Zhang Zhiguang (assistant). Jirong Shen and Tianwei Xia designed the protocol and revised the draft.

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No competing interests reported.

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Factors influencing the outcome of hip preservation in femoral head necrosis treated with rotational osteotomy via the base of the femoral neck, combined with finite element analysis

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