Abstract
Background
In total hip arthroplasty, stem alignment has been proposed to occur in the neutral and flexed positions. Nonetheless, there is a paucity of research on the relationship between the shape of the medullary cavity cross-section and its compatibility with the stem shape.
Methods
A total of 216 female (Asian) patients with no history of fracture or surgery on the contralateral side with complete computed tomography (CT) data were included in the study. The plane of the medullary cavity in the neutral (N group) and flexion (F group) alignments was defined. The ratio of the plane of medullary cavity widths at 80 mm and 130 mm distal from the center of the femoral head in these two medullary cavity planes was defined as the modified canal flare index (mCFI). The mCFI was used to assess the compatibility between the configuration of the medullary canal plane and that of the stem.
Results
All patients in the N group formed a plane, whereas 58 patients in the F group (26.9%) lacked a clear calcar femorale. Fifteen patients in the F group (9.4%) were excluded because of the inability to insert the stem, leaving 143 patients in the F group. The mean mCFI was significantly lower in the F group (0.692 ± 0.128) than in the N group (0.807 ± 0.073). The mCFI values of frequently used stems were all close to the mCFI of flexion alignment.
Conclusions
The medullary cavity shape differed in the neutral and flexion alignments. Preoperative three-dimensional planning is helpful because the flexion alignment may not allow for the insertion of an optimally sized stem.
Supplementary Information
The online version contains supplementary material available at 10.1186/s12891-025-09466-2.
Keywords: Stem alignment, Neutral alignment, Flexion alignment, Image analysis study
Background
Total hip arthroplasty (THA) improves pain and function in hip joint osteoarthritis [1]. The posterior approach has been used in THA for many years. However, the concept of minimally invasive surgery has been introduced, and the number of anterior approaches is increasing annually [2].
Elevating the proximal femur with the anterior approach is reportedly difficult, and the stem tends to be inserted in flexion alignment with the proximal femur anterior to the distal femur [3]. Flexible alignment is considered advantageous because it increases the impingement-free range of motion of the hip joint [4]. Conversely, in neutral alignment, the contact area between the cortical bone and the stem has been thought augmented, and the initial fixation is more stable. Numerous studies have demonstrated that inadequate stem alignment negatively affects the postoperative outcomes of cementless THA [5, 6]. Inadequate stem insertion causes localized stress concentration, which also leads to bone resorption and loosening [7]. For cemented stems, neutral stem alignment is recommended to prevent complications such as fracture, loosening, and stem subsidence due to an uneven cement mantle [8, 9]. Therefore, the ideal or optimal stem alignment is controversial.
Additionally, high medullary cavity occupancy of the femoral component has been reported to be important for postoperative stability and long-term prognosis [10]. Low medullary cavity occupancy of the stem can also cause stem subsidence and loosening [11]. Nevertheless, no studies have investigated the relationship between the stem alignment and fit. Therefore, the current study aimed to investigate the medullary cavity shape in neutral and flexion alignments, examine its compatibility with the stem, and clarify the differences in medullary cavity shape in neutral and flexion alignments and its compatibility with the modern stem.
Methods.
Study design and patient
The protocol of this retrospective study was approved by the Ethical Review Committee of our hospital (approval no. R6-12). Between January 2022 and May 2024, our hospital treated 354 patients with proximal femoral fractures. The exclusion criteria were as follows: male sex, history of contralateral fracture, surgical history of the contralateral femur, and incomplete computed tomography (CT) data. A total of 216 patients (Age:84.2 ± 9.7) were included in this study (Fig. 1). Due to the variation in femur shape between males and females, the exclusion of males was deemed necessary to enhance the homogeneity of the analysis. Image analysis software (Synapse VINCENT®, Fujifilm, Tokyo, Japan) was utilized for image processing and measurement. All data were obtained from the contralateral side to the subject. Thus, no additional imaging was required for this research.
Fig. 1.
Flowchart of patient selection
Definition of the plane of medullary cavity
The medullary planes for neutral and flexion alignments of the stem were defined as follows: Each plane of the medullary cavity was defined as a cross-section consisting of the following three points. The plane of the medullary cavity in neutral alignment was defined as a section consisting of three points: the center of the femoral head, and 80 mm and 130 mm distal centers of the medullary cavity from the center of the femoral head parallel to the proximal femoral axis (Fig. 2a). The two distances (80 and 130 mm) were set with reference to the total length and position of the lower edge of the porous surface of stem models frequently used worldwide (Fig. 2b).
Fig. 2.
A The plane of medullary cavity of neutral alignment.①The center of the femoral head, ② 130 mm distal from ①, ③ 80 mm distal from ①. B The plane of medullary cavity and the stem. Figure 2b shows Figure 2a withthe stem superimposed. C Second point in flexion alignment: the midpoint between the calcar femoraleand anterior cortex in the axial section is perpendicular to the proximal femoral axis,where the calcar femorale is most clearly visible. D Third point in flexion alignment: the midpoint between the most distal pointof the calcar femorale and the anterior cortex in the sagittal plane is perpendicularto the proximal femoral axis. E The plane of medullary cavity in flexion alignment
Regarding the definition of flexion alignment, insertion of the stem along the calcar femorale has been reported to constitute flexion alignment [12]. Therefore, in this study, we defined the following three points of flexion alignment with reference to the calcar femorale: The first point was centered on the femoral head. The second point was the midpoint between the calcar femorale and anterior cortex in the axial section perpendicular to the proximal femoral axis, where the calcar femorale was most clearly visible (Fig. 2c). The third point was the midpoint between the most distal point of the calcar femorale and the anterior cortex in the sagittal plane, perpendicular to the proximal femoral axis (Fig. 2d). The plane of the medullary cavity of flexion alignment was the plane passing through the three points (Fig. 2e).
Definition of the modified canal flare index (mCFI)
Nobel et al. proposed the canal flare index (CFI), which measures the ratio of the medullary cavity diameter 20 mm proximal to the lesser trochanter to the narrowest part of the medullary cavity, as an index for evaluating the medullary cavity shape in the proximal femur [13]. However, in the case of CFI, the evaluation of the medullary cavity shape is based on the position of the lesser trochanter, and it is impossible to measure the CFI using the plane of medullary cavity in this study. Therefore, modified CFI (mCFI) was created. The mCFI was measured by drawing a line parallel to the femoral axis at the center of the femoral head and measuring the medullary cavity diameter 80 mm and 130 mm distal to the center of the femoral head. The diameter of the proximal medullary cavity (80 mm distal to the center of the femoral head) was defined as A mm (proximal part A), and that of the distal medullary cavity (130 mm distal to the center of the femoral head) as B mm (distal part B) (Fig. 3). We defined mCFI as B/A. The smaller the mCFI value, the more the medullary cavity shape is of the champagne flute type, and the larger the value, the more the medullary cavity shape is of the stove pipe type.
Fig. 3.
Modified canel flare index (mCFI = B/A)
Assessment of conformity of the medullary cavity geometry
In all 216 patients, a plane of the medullary cavity was created the neutral and flexion alignments of the stem (N and F groups). The mCFI was measured in each complete plane of the medullary cavity. The mean, maximum, and minimum values of the mCFI were calculated for both groups. In addition, the correlation between the mCFI in both groups and the distribution of the mCFI in both groups were examined. Compatibility of the plane of the medullary cavity and stem was examined in both groups. For the three stem models used in this study (Accolade II 127°, Stryker; Avenir Complete 135° standard offset, Zimmer Biomet; and Insignia high offset, Stryker), the stem width in the mediolateral direction was investigated at 80 mm (proximal part A) and 130 mm (distal part B) distal from the center of the femoral head. Distributions of stem size and medullary cavity width in both groups were graphically compared. In addition, the average value of the optimal stem size was calculated for the distal part B in both groups. The compatibility of the stem and medullary cavity geometries between both groups was evaluated by comparing the values of the distal part B of the stem. Figure 4 shows a typical case of neutral and flexion alignment of the stem.
Fig. 4.
The difference of the shape of medullary cavity between neutral and flexion alignment
Statistical analysis
All statistical analyses were performed using EZR (Saitama Medical Center, Jichi Medical University, Saitama, Japan), which is a graphical user interface for R (The R Foundation for Statistical Computing, Vienna, Austria) [14]. The mean mCFI was calculated for the paired case groups in the N and F groups, and differences between the groups were evaluated using a t-test. Statistical significance was set at P < 0.05.
An a priori power analysis was also conducted on the number of cases using G*Power software (version 3.1) [15]. Based on a two-tailed t-test at a significance level of 0.05, power of 0.80, and expected effect size of Cohen’s d = 1.03, the required sample size was estimated to be 23 participants per group.
Results
All 216 patients in the N group were able to produce the stem inserted planes. In the F group, 58 cases (58/216, 26.9%) did not demonstrate a clear calcar femorale, and the formation of a plane at the three points was not feasible. Consequently, we were able to generate a cross-section for 158 cases. Fifteen cases (15/158, 9.4%) could not be measured for mCFI because there was no medullary cavity in the distal part B. The F group comprised 143 patients.
Table 1 summarizes the number of cases and mean mCFI in the N and F groups. The mean mCFI was 0.807 ± 0.073 in the N group and 0.692 ± 0.128 in the F group. The mean mCFI of the F group paired with that of the N group was significantly different, with the F group having a significantly smaller mCFI .
Table 1.
Results for the average of mCFI in the N group, the N group paired up with the F group, and the F group
| N-group | N-group pair up with F-group | F-group | P Value | |
|---|---|---|---|---|
| Number of patients | 216 | 143 | 143 | |
| Age | 84.21 ± 9.77 | 83.7 ± 10.02 | 0.61 | |
| BMI | 19.55 ± 3.51 | 19.42 ± 3.19 | 0.71 | |
| Average of mCFI | 0.807 ± 0.073 | 0.800 ± 0.073 | 0.692 ± 0.128 | < 0.05 * |
| Maximum of mCFI | 1.0 | 0.947 | 0.932 | - |
| Minimum of mCFI | 0.561 | 0.561 | 0.277 | - |
body mass index
* N-group pair up with F-group vs. F-group
In all cases, the mCFI values were greater than or equal to those in the F group (Fig. 5).
Fig. 5.
The correlation of mCFI between the N and F groups
Most of the cases in F group had a smaller distal medullary cavity width (part B) compared to N group (Fig. 6). Similar evaluations were conducted for the other two stems, but the compatibility results did not change significantly. Therefore, the fit of the stems was better in Group F for all three models. Further detailed data are provided in Supplemental data.
Fig. 6.
This graph shows the distribution of medullary cavity width in proximal part A and distal part B in both groups based on the width of the proximal part. The abscissa denoted the number of cases and size of particular stem, whereas the ordinate represented the width of the medullary cavity and stem. In the gray box marked #N, the two gray squares at the left end indicated that the each stem width was 80 mm and 130 mm from the center of the stem head in #N. In both groups, the optimal stem was determined based on proximal part A. Cases in the gray boxes marked with #N were indicated as the optimal size of stems for #N. A The medullary cavity width and stem (Accolade II 127°, Stryker) compatibility at proximal part A and distal part B in neutral alignment. B The medullary cavity width and stem (Accolade II 127°, Stryker) compatibility at proximal part A and distal part B in flexion alignment
Discussion
In this study, we evaluated the difference in the shape of the medullary cavity cross-section between neutral stem insertion and flex-positioned insertion. The mismatch between the medullary cavity and stem geometry in each alignment was examined. Clinical complications arising from incongruity between the medullary cavity cross-section and stem geometry include poor initial fixation, stem instability, stress shielding around the stem, stem loosening, and the necessity for revision, which have deleterious effects on long-term outcomes [10]. Inadequate initial fixation of the stem is associated with an insufficient contact area between the stem and bone, resulting in micromotion [16]. With regard to stress shielding around the stem, when load transfer is nonuniform owing to a mismatch, excessive stress is concentrated at certain sites [17]. Furthermore, if the load is not transferred to the bone in a satisfactory manner, the result will be bone resorption occurs, which in turn increases the risk of stem loosening [17]. The combination of these problems may result in premature stem laxity necessitating revision. In the case such as the stem distal shape is much smaller than the medullary cavity, insufficient distal contact of the stem reduces rotational resistance, making it susceptible to micro-rotation when walking or experiencing rotational loading [10, 16, 17]. However, it is not necessary for the distal end of the stem to match the shape of the medullary cavity. In the Tapered Wedge and Full HA stems examined here, proximal bone contact is the primary source of fixation. Therefore, bone contact within the proximal medullary cavity is crucial. The design of the stem should match the shape of the patient’s medullary cavity.
With regard to stem shape, the use of short stems has increased in recent years, and tapered-wedge types and full HA have been reported to have good clinical results [18]. However, a definitive answer regarding the optimal cementless stem type remains unclear. Conversely, the use of standard-sized stems has declined because of cumbersome insertion through the intermuscular approach, challenging removal, and positioning as a revision. Therefore, this study was conducted on the modern Tapered Wedge types and Full HA, which have been used frequently in recent years.
When evaluating the shape of the medullary cavity cross-section, previous studies reported that the ratio was measured based on the lesser trochanter [19]. However, the medullary cavity ratio differs in each case based on the lesser trochanter, and the location of the lesser trochanter varies anatomically. In this study, the medullary cavity ratio was calculated in cross-sections created based on CT data. The mCFI measurement criterion was adopted as it is a more reproducible method. If distal part B is used as the denominator, the denominator becomes zero in some cases, making calculation impossible; as a result, mCFI was defined as B/A.
The results of the mCFI correlation showed that the mCFI of the neutral alignment was higher than or equal to that of the flexion alignment in all cases. This result indicates that the medullary cavity width narrowed more distally with flexion alignment. The mCFI of the examined stems is summarized in supplemental data. These stems demonstrated values close to flexion alignment. Additionally, one previous report indicated that the smaller the medullary cavity width in the distal mediolateral direction, the less the mismatch between the stem and medullary cavity [19]. Based on the results of this study, it is possible that flexion alignment may be a better fit for the medullary cross-section and stem in many cases.
Regarding the comparison of the shape of medullary cavity and stem, the optimal size of stem was determined based on the width of the medullary cavity of the proximal femur in this study. The mean value at the distal part B in the flexion alignment was closer to the mean value at the distal part B of the stem of optimum size. This result showed that the medullary cavity shape in the flexion alignment resembled that of an optimally sized stem. To the best of our knowledge, there are no reports showing that the medullary cavity shape during flexion alignment is similar to that of the stem. Flexion alignment may reduce the mismatch between the medullary cavity cross-section and the stem. Reducing the mismatch with the medullary cavity cross-section may improve long-term results.
In 9.4% of the cases, the distal part of the flexion section interfered with the cortex. In fact, there has been a report of a case in which the posterior cortex penetrated when the stem was inserted in flexion alignment [20]. We also showed that, in some cases, the distal part of the stem hit the cortex and could not be inserted. In flexion alignment, the optimal stem size cannot be inserted, and the stem may be undersized. It is helpful to create a three-dimensional planning before surgery to confirm that a stem of the expected size can be inserted.
Our study has some limitations. First, this was a cohort of female (Asian) patients with proximal femoral fractures. Male were excluded due to the small number of cases. The present study examined only elderly women, and we believe that further investigations, including the differences of gender and race and cases in which THA was performed, are needed. Furthermore, this study did not examine patient factors (such as age, sex, Body Mass Index, osteoporosis, and ethnic and racial differences)that affect the mCFI. We were not able to examine in which patients the discrepancy in the mCFI between the neutral and flexion alignments was greater. Second, only one cross-section was evaluated. Therefore, a three-dimensional study is necessary to investigate the detailed medullary cavity shape. Third, the consideration was given only some of the stems were considered. The tapered-wedge and full HA types, which have been used more frequently in recent years, were investigated. Because there is a high possibility that stem models used in different facilities may differ, it is necessary to consider different types. This study was a simulation study based entirely on preoperative CT data. In order to elucidate the relationship with the mid- to long-term outcomes of implants, a more detailed examination of actual cases is necessary.
The strength of the present study is that, to the best of our knowledge, this is the first study to clarify the difference in the medullary cavity cross-section between neutral and flexion alignments. Because the medullary cavity shape differs between neutral and flexion alignments, it is necessary to determine which insertion method to use and to draw a three-dimensional plan before surgery. Based on these results, we believe that this study will be useful in future research on mismatches with stems. Further comparative evaluations that consider patient backgrounds are required.
Conclusions
The medullary cavity shape was different in the neutral and flexed alignments. The compatibility of the medullary cavity shape with the stem shape may differ between the neutral and flexed alignments. The flexion alignment might be performed using an anterior approach. The flexion alignment provided a better match than the neutral alignment between the stems investigated in this study and the shape of medullary cavity. Creating a three-dimensional drawing before surgery is necessary because it may not be possible to insert an appropriately sized stem into the flexion alignment.
Supplementary Information
Below is the link to the electronic supplementary material.
Acknowledgments
Not applicable.
Abbreviations
- CFI
Canal flare index
- CT
Computed tomography
- mCFI
Modified canal flare index
- THA
Total hip arthroplasty
Authors’ contributions
Y.Yoshimoto: Writinge review & editing, Writinge original draft. Y.Kajino: Writinge review & editing, Conceptualization. T.kabata: Writinge review & editing. D. Inoue: Writinge review & editing. All authors read and approved the final manuscript.
Funding
Not applicable.
Data availability
The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.
Declarations
Ethics approval and consent to participate
The study protocol was approved by the Ethical Review Committee of Kaga medical center (approval no. R6-12). All procedures were performed in accordance with the Declaration of Helsinki. All patients provided informed consent prior to their inclusion in this study.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher’s Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.