The Anterior Capsule of the Hip Joint Is a Major Hindrance to the Surgical Treatment of Basicervical Intertrochanteric Fractures: A Single Center, Retrospective Observational Study

Background: The basicervical intertrochanteric fracture has recently received considerable attention as one of the major risk factors for failure of internal xation. We aimed to analyze the anatomical characteristics of basicervical intertrochanteric fractures related to their surgical outcomes using three-dimensional computed tomography (3D CT) and to determine the causes for the high failure rate of surgery. Methods: Between May 2015 and October 2019, 226 patients with intertrochanteric fracture of the femur including 58 basicervical types who were treated with femoral nail were evaluated. Following data were collected to nd out the risk factors for surgical failure: fracture patterns (unstable or basicervical type), tip apex distance, type of postoperative reduction (acceptable or unacceptable), and continuity of anterior capsule for logistic regression analysis to determine the risk factors for surgical failures Results: Union was achieved in 215 patients (95.1%), with 11 surgical failures (4.9%), including 10 non-unions (4.6%) and 1 osteonecrosis of the femoral head (0.4%). Seven surgical failures (8.7%) were identied in the unstable fracture group, 6 (10.3%) in the basicervical type group, and 10 (19.6%) in the unacceptable reduction group. Multivariate analysis showed that unstable fracture (OR 10.311, 95% CI 1.165 – 91.229, p = 0.036), basicervical type (OR 11.564, 95% CI 1.339 – 99.872, p = 0.026), and unacceptable reduction (OR 28.364, 95% CI 3.428 – 234.695, p = 0.002) were signicant predictors for surgical failure. Unacceptable reduction (OR 13.180, 95% CI 1.336 – 130.059, p = 0.027) was the only risk factor for surgical failure in the basicervical intertrochanteric fracture group. A chi-square test showed that an intact anterior capsule was statistically correlated with surgical failure (p = 0.043) and unacceptable reduction (p = 0.008). Conclusion: Because the intact anterior capsule of the hip joint may hinder achieving acceptable reduction, the evaluation of the anterior capsule should be included as an important preoperative assessment for the surgical treatment of basicervical intertrochanteric fracture. This single center observational study was conducted at a tertiary university hospital based on a retrospective review of prospectively collected clinical and radiologic data. The study was conducted in accordance with the tenets of the Declaration of Helsinki. The study protocol was approved by the Pusan National University Hospital Institutional Review Board (H-2008-013-094). Informed consent was obtained from all subjects. Inclusion criteria of this were patients intertrochanteric criteria


Introduction
Despite many efforts for the biomechanical understanding of proximal femoral fractures and newly designed intramedullary xation devices, orthopedic surgeons still experience may failures after internal xation in elderly patients with intertrochanteric fracture [1][2][3][4][5]. Speci cally, the basicervical type of intertrochanteric fracture has recently received considerable attention as one of the major risk factors for failure of internal xation [6][7][8][9]. Although osteosynthesis through internal xation is the standard surgical treatment for these fractures, studies have emphasized taking special precautions with respect to the high rate of surgical failure. Watson et al. [9] reported 6 failures from 11 patients with twopart basicervical fractures treated with cephalomedullary nailing, even though two of them showed anatomical reduction postoperatively. Based on these results, they asserted the chance of failure in two-part basicervical fractures was high.
Nonetheless, the speci c factors that contribute to the high failure rate of surgeries for basicervical intertrochanteric fractures have not yet been elucidated. The relationship between xation device and surgical outcomes has been suggested, but all surgical failures cannot be attributed to implant problems [6,10,11]. Several studies have indicated the inherent instability of basicervical intertrochanteric fractures, which is related to their anatomical characteristics, as the cause of the high rate of surgical failure; however, these studies have failed to provide speci c statistical data [9,10,12,13].
Thus, in this study, we aimed to analyze the anatomical characteristics of basicervical intertrochanteric fractures related to their surgical outcomes using three-dimensional computed tomography (3D CT) and to determine the causes for the high failure rate of surgery. We hypothesized that failure of anteromedial cortical buttressing would be a risk factor for surgical failure of basicervical intertrochanteric fractures and that the anterior capsule of the hip joint could be a main cause of the inherent instability of these fractures.

Research ethics and study population
This single center observational study was conducted at a tertiary university hospital based on a retrospective review of prospectively collected clinical and radiologic data. The study was conducted in accordance with the tenets of the Declaration of Helsinki. The study protocol was approved by the Pusan National University Hospital Institutional Review Board (H-2008-013-094). Informed consent was obtained from all subjects.
Between May 2015 and October 2019, 348 patients with intertrochanteric fracture of the femur were identi ed in our institution. Inclusion criteria of this study were patients with intertrochanteric fractures treated with an intramedullary nail. Exclusion criteria included patients who had 1) follow-up time of less than 6 months, 2) pathologic fracture, 3) open fracture, 4) surgery using extra-medullary device, 5) age younger than 65 years, and 6) multiple fractures in the affected limb. A total of 323 patients underwent surgical treatment, and 97 of them were excluded in this study (Fig. 1).

Surgical Principle
All surgeries were performed on a radiolucent fracture table for uoroscopic guidance with the patient in the supine position. An intramedullary device (proximal femoral nail anti-rotation and trochanteric femoral nail anti-rotation, Depuy Synthes GmbH, Oberdorf, Switzerland) was used for fracture xation. The neck shaft angle of the proximal femur on the coronal plane was restored using a traction device on the fracture table in reference to the normal side of the femur, and anteromedial cortical reduction was assessed through a trans-axial image. When posterior displacement of the proximal fragment (unacceptable reduction) was identi ed on the trans-axial image, indirect reduction was conducted using reduction tools to make a cortical buttress in the anteromedial part of the fracture (Fig. 2). We used a helical blade in all patients and made a tip apex distance of less than 25 mm, and Cleveland index ve or eight [14,15].

Radiologic Assessments
We classi ed an intertrochanteric fracture as stable or unstable based on Evans' classi cation [16]. An unstable fracture is an intertrochanteric fracture with a large posteromedial defect. A basicervical intertrochanteric fracture refers to more than 50% of identi ed fracture line proximal to the intertrochanteric crest where the anterior capsule of the hip joint is attached to. However, when all fracture lines were located more than 5 mm proximal to the intertrochanteric crest, the fracture was considered to be a femoral neck fracture. This concept is a modi ed de nition of the basicervical intertrochanteric fracture by Hu et al. [17], and this type of fracture can be either a partial or total an intracapsular fracture (Fig. 3).
Postoperative reduction was divided into three categories, extramedullary, anatomical, and intramedullary reductions based on the displacement of the anteromedial cortex of the proximal fragment on the trans-axial image [2] (Fig. 4). Extramedullary and anatomical reductions were considered acceptable reductions and intramedullary reduction was considered an unacceptable reduction. We assessed the status of the anterior joint capsule using 3D CT in all patients with basicervical intertrochanteric fractures. When the continuity of the anterior capsule was identi ed without detachment, tear, or avulsion from bony insertion, we considered it to be an intact anterior capsule. When this intact anterior capsule was located between the two fragments, we considered it to be an incarcerated anterior capsule (Fig. 5).
Radiologic follow-up was conducted 1, 3, 6, 9, and 12 months after surgery. We de ned union as when the cortical bridging was identi ed at least three cortices in the anteroposterior and lateral image of the hip, with a patient that could ambulate without pain [18]. Non-union was de ned as when the proximal or lateral migration of the helical blade, cut-out or cut-through of the helical blade, varus deformation of the proximal fragment, implant failure around the fracture site, and obvious fracture line with antalgic gait at 9 months follow-up X-ray or CT were identi ed [2]. All radiologic assessments were independently conducted by two orthopedic surgeons who had more than 10 years of experience and had never participated in patient treatment during this study. When they could not reach a consensus, the decision was based on a third orthopedic surgeon's assessment.

Statistical analysis
Data were analyzed using SPSS software (ver. 17.0 for Windows; IBM, Armonk, NY). Preoperative data including age, sex, bone mass density (BMD), body mass index (BMI), and pre-injury mobility score of enrolled patients were collected based on the review of medical records. Preoperative data were used as covariates with pre-and postoperative radiologic values, including fracture patterns (unstable or basicervical type), tip apex distance, type of postoperative reduction (acceptable or unacceptable), and continuity of anterior capsule for logistic regression analysis to determine the risk factors for surgical failures. After unadjusted analysis, signi cant variables (p value of 0.10) were included in the model for adjusted analysis. A multivariate logistic regression analysis was performed to examine the association between possible risk factors and surgical failure. We report odds ratios (ORs) and 95% con dence intervals (CIs) for all associations. A p value of < 0.05 was considered signi cant. The inter-observer reliability of all radiologic values was evaluated using the interclass correlation coe cient. The results were interpreted as follows: > 0.8 = almost perfect agreement, 0.7-0.8 = strong agreement, 0.5-0.6 = moderate agreement, 0.3-0.4 = fair agreement, and 0-0.2 = poor agreement. P < 0.05 was considered statistically signi cant. Inter-observer reliabilities for unstable fracture, basicervical fracture, postoperative reduction, and continuity of anterior capsule are shown in Table 1. A chi-square test was used to evaluate statistical correlation between continuity of anterior capsule and radiologic values in patients with a basicervical intertrochanteric fracture.   (Table 3). The details of the 6 patients with basicervical intertrochanteric fracture who showed surgical failure are presented in Table 4.    This study showed that a basicervical fracture type is one of the signi cant risk factors for surgical failure of an intertrochanteric fracture. Previous studies have suggested the high susceptibility of basicervical intertrochanteric fractures to surgical failure. Su et al. [12] suggested that basicervical fractures may have increased biomechanical instability and a higher tendency to collapse compared with other types of intertrochanteric fractures. Bojan et al. [19] reported 9% of screw cutout rate in patients with basicervical fracture compared with 1% of screw cutout rate overall in a large series of more than 3,000 fractures in the trochanteric regions. Watson et al. [9] suggested the inherent instability of basicervical proximal femoral fractures makes treatment more di cult based on the surgical result when treated with a cephalomedullary xation. Kwak et al. [10] mentioned that the inherent instability of basicervical proximal femoral fractures was associated with the anatomical vulnerability of this fracture, including the narrow cortical base of the proximal fragment and the subsequent narrow contact area at the main fracture site, along with insu cient cancellous interdigitation compared with other types of intertrochanteric fractures. Furthermore, the short proximal fragment of this fracture can cause rotational instability during bone healing regardless of xation devices. For these reasons, achieving anteromedial cortical buttressing around the fracture site of the basicervical intertrochanteric fracture postoperatively may be a vital aspect of surgery. Anteromedial cortical buttressing has been previously emphasized in unstable intertrochanteric fractures [2,20]. However, to the best of our knowledge, the importance of anterior medial cortical buttressing in basicervical intertrochanteric fracture has not yet been elucidated. We believe that the biomechanical understanding of the proximal femur from previous studies and the surgical results of our study might offer enough theoretical evidence to support the importance of anteromedial cortical buttressing in these fractures.
We focused on the fact that 4 out of 6 surgical failures in the basicervical intertrochanteric fracture group were associated with displaced two-part fracture corresponding to A12 fracture based on the AO classi cation. The incidence of unacceptable reduction in this group was 32.8% (19 out of 58) despite efforts to achieve acceptable reduction under uoroscopic guidance. In particular, among 35 A12 fractures, 13 patients (37.1%) showed unacceptable reduction, which was a higher incidence than that shown in all patients with intertrochanteric fractures enrolled in this study (22.6%). This result indicates that achieving an acceptable reduction in an A12 type of basicervical intertrochanteric fracture can be more di cult than in other types of intertrochanteric fractures.
High incidences of postoperative unacceptable reduction in A12 fractures may be attributed to the radiologic misunderstanding of uoroscopic trans-axial images during surgery. When there is distraction of fracture site, even though intraoperative reduction seems anatomical, it can become an unacceptable reduction during follow-up. This is because the sliding of the proximal fragment occurs from the anterior to the posterior, along the direction of the helical blade. In this study, four out of ve patients with non-union in the basicervical intertrochanteric group showed unacceptable reduction in the follow-up radiographs, even though they showed anatomical reduction on intraoperative uoroscopy and postoperative radiographs. Due to the distraction of the fracture site regarding excessive traction or intentional valgus reduction during the surgery, the intraoperative uoroscopic images and postoperative radiographs may be misconstrued as anatomical reduction. The fracture gap can lead to the failure of anteromedial cortical buttressing and intramedullary reduction during follow-up, because the sliding direction of the proximal fragment is consistent with the direction of the helical blade. Thus, slight over-reduction should be considered for the surgical treatment of basicervical intertrochanteric fracture when the distraction of fracture site is expected.
We attempted to prove our hypothesis that the anterior capsule of the hip joint is a potential inherent vulnerability that may make it di cult to achieve acceptable reduction. The anterior capsule of a basicervical intertrochanteric fracture can be intactly attached distally to the fracture site because this fracture shows the characteristics of a partially or totally intra-capsular fracture. If the fracture occurred without detachment of the anterior capsule from its distal insertion, it may be a considerable hindrance to the accurate reduction or proper sliding of the proximal fragment. We identi ed ve cases with an intact anterior capsule out of six surgical failures in the basicervical intertrochanteric fracture group. When we excluded one case with osteonecrosis of the femoral head, all patients with non-union had an intact anterior capsule, and two cases showed incarceration of the anterior capsule in the fracture site. Based on these results, we believe that the anterior capsule of the hip joint can be a major hindrance that negatively affects fracture reduction during surgery.
The present study has several limitations. First, this is a retrospective study with relatively few enrolled cases of surgical failures, which might have affected the reliability of the logistic regression analysis. Second, despite high inter-observer reliability achieved, radiologic evaluation for the anterior capsule using 3D CT has not been previously validated. Furthermore, although the anterior capsule of the hip joint and surgical failure of the basicervical intertrochanteric fracture may be correlated, studies offering clinical evidence on the biomechanical effect of the anterior capsule on the surgical outcome are needed. Third, although we de ned basicervical intertrochanteric fractures using a modi ed concept, this de nition might be ambiguous. In addition, although the inter-class correlation coe cient of this fracture was 0.856 in this study, which can be interpreted as almost perfect agreement, identifying the insertion site of the anterior capsule and setting the boundary between the femoral neck and the basicervical intertrochanteric fracture can be subjective. Finally, this study did not suggest a surgical solution to overcome the inherent vulnerability of basicervical intertrochanteric fractures. Based on the results of the present study, anterior capsulectomy might contribute to achieving proper anteromedial cortical buttressing. However, additional studies are need to determine if this surgical technique can improve surgical outcomes or if it may take a toll on the soft tissue around the hip joint with additional blood loss.

Conclusion
We determined that the basicervical fracture type is a signi cant predictor for surgical failure in intertrochanteric fractures. Due to its inherent instability, achieving anteromedial buttressing is crucial to improving the stability around the fracture site in basicervical intertrochanteric fractures. Because the intact anterior capsule of the hip joint may hinder achieving acceptable reduction, the evaluation of the anterior capsule should be included as an important preoperative assessment for the surgical treatment of basicervical intertrochanteric fracture. In addition, further studies detailing potential surgical solutions to achieve acceptable reduction in basicervical intertrochanteric fractures with an intact anterior capsule are also needed.

Declarations
Ethics approval and consent to participate The study protocol was approved by the Pusan National University Hospital Institutional Review Board (H-2008-013-094). Informed consent was obtained from all subjects.