A variety of hip diseases are effectively treated by THA, which relieves hip pain and improves hip function. The cementless THA prothesis stem has good mid- and long-term survival [11–13]. Early and long-term stability after implantation depend on the achievement of a close fit between the stem and the proximal femoral medullary cavity; micromotion after implantation promotes fibrous tissue growth rather than osseointegration, and this leads to early postoperative failure [14, 15].
Hip replacement surgery was introduced in the early 19th century, and has since continued to develop. With the continuous development of prosthetic design and the use of testing methods such as finite element analysis and photoelastic coating analysis, it has been proved that metaphyseal fixed prostheses are biomechanically sound and minimize stress shielding and disadvantageous bone remodeling [16, 17]. Therefore, the use of short stems in primary THA is increasing, and has good clinical efficacy [18].
Radiographic measurements provide acceptable accuracy in measuring the shape of the proximal femoral medullary cavity, and radiography remains the most convenient method for assessing conventional THA. However, medullary measurements vary with leg rotation, which significantly alters the NSA and isthmus width [19]. In comparison with radiographic measurements, CT-based anatomical measurements have improved accuracy and reduced errors, as CT it is not affected by femoral rotation and the subjective determination of the bone threshold. Preoperative CT planning provides accurate anatomical information of the medullary cavity, and provides strong evidence for determining surgical goals and plans, especially in the preoperative design of custom prostheses [20].
In the process of CT scanning, the position of the patient cannot be completely controlled, even with the most rigid positioning, which often causes asymmetry of the pelvis and makes the calculations inaccurate. In addition, the traditional method of using the coronal or sagittal axis of the CT scan itself as the coordinate system does not coincide with the coordinate system corresponding to the implant placement during the operation. Therefore, we abandoned the use of the CT coordinate system as the sagittal and coronal planes. Furthermore, as the femur itself has an anterior arch and rotation, the axis of the femoral medullary canal is not straight, although the axis of the proximal femoral medullary cavity is usually approximated with a straight line.
During the establishment of the CT measurement coordinate system in the present study, we did not strictly use the coordinate system provided by the original CT scan and the "cortical-medullary cavity" for division and measurement. The following points were considered during the establishment of our methods of measurement.
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1. The plane passing through the center of the femoral head and the axis of the medullary cavity was used as the frontal plane to measure the shape of the medullary cavity and to guide the design of the prosthetic stem. This was done to restore the central position of the femoral head as much as possible.
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2. The femoral calcar is located in the posteromedial part of the junction between the femoral neck and the femoral shaft, deep in the lesser trochanter, and extends from the posterior side of the femoral neck to the longitudinal dense bone plate behind the lesser trochanter. CT imaging showed that the density of metaphyseal cancellous bone in the lesser trochanter and the proximal and posterior medullary cavity was lower than that of cortical bone, but higher than that of the medullary cavity, and the cancellous bone in this area was not removed by intraoperative reaming. The aim of analyzing the medullary cavity morphology was to aid in the design of femoral prostheses. The femoral stem is often press-fitted through the inside and outside, while its anterior-posterior diameter, especially the sagittal posterior diameter, does not contact the cortical bone. Therefore, to attain a better fitting stem design, this cancellous area should not be included in the medullary cavity measurements (yellow part in Fig. 2).
The measurement method used in the present study showed that there were still significant differences between the ML and LD and between the AP and VL (Table 9), but the degree of difference was not very large. Furthermore, the cross-sectional measurement shape tended to be elliptical, while threshold separation is strictly implemented in the measurement of the maximum diameter using Mimics software, which may be the reason for the significant differences between the ML and LD and between the AP and VL. In the present study, the medullary cavity cross-sectional parameters were measured, which avoids the measurement differences caused by the establishment of a different coordinate system, and is an effective supplement to the measurement of the medullary cavity coronal diameter and sagittal diameter [10].
Table 9
Differences between the coronal ML and LD and between the AP and VL
| ML [Median (Q1 - Q3)] | LD [Median (Q1 - Q3)] | P | AP [Median (Q1 - Q3)] | VL [Median (Q1 - Q3)] | P |
T-25 | 46.78(43.59, 52.01) | 50.08 (45.88, 54.44) | 0.000 | 31.58 (29.57,34.41 ) | 32.33 (28.65, 36.13) | 0.421 |
T-20 | 44.55 (40.51, 48.95) | 46.67(42.90, 51.74) | 0.000 | 31.85 (30.58, 34.35) | 30.50 (28.70,33.84) | 0.000 |
T-15 | 39.95 (35.99, 43.44) | 41.27 (37.89, 45.31) | 0.000 | 25.37 (22.74, 28.80) | 24.15 (22.21, 26.75) | 0.002 |
T-10 | 34.86 (32.01, 38.22) | 36.09 (32.48, 40.15) | 0.000 | 22.86 (20.71, 25.47) | 22.95 (20.12, 25.10) | 0.092 |
T-5 | 30.72 (28.36, 33.93) | 31.49 (28.15, 34.56) | 0.160 | 21.89 (20.26, 24.27) | 21.80 (20.22, 23.59) | 0.167 |
T-0 | 27.23 (24.74, 30.06) | 27.57 (4.47, 31.09) | 0.645 | 19.60 (17.72, 22.65) | 19.67 (17.75, 22.56) | 0.467 |
T + 5 | 24.65 (21.75, 27.40) | 24.71 (21.86, 27.84) | 0.337 | 18.81 (17.24, 21.77) | 18.46 (16.88, 20.53) | 0.002 |
T + 10 | 22.08 (19.72, 24.96) | 22.12 (19.71, 24.56) | 0.635 | 18.24 (16.43, 19.87) | 17.33 (15.86, 19.09) | 0.000 |
T + 15 | 19.99 (18.09, 22.89) | 20.19 (18.55, 22.93) | 0.556 | 17.54 (15.86, 19.46) | 16.98 (15.02, 19.00) | 0.001 |
T + 20 | 18.83 (16.61, 21.22) | 19.67 (18.29, 22.15) | 0.000 | 17.04 (15.38, 18.42) | 17.46 (15.65, 19.90) | 0.000 |
T + 30 | 16.53 (15.07, 18.59) | 16.79 (15.66, 19.63) | 0.000 | 15.31 (14.36, 17.02) | 14.58 (13.10, 16.48) | 0.000 |
T + 40 | 15.08 (13.53, 17.08) | 15.73 (14.55, 17.31) | 0.000 | 13.78 (12.51, 15.74) | 12.61 (11.83, 14.77) | 0.000 |
T + 50 | 13.99 (12.58, 15.70) | 15.37 (13.93, 16.64) | 0.000 | 13.02 (11.75, 14.61) | 11.98 (10.86, 13.47) | 0.000 |
T + 60 | 13.26 (12.33, 14.98) | 14.53 (13.32, 16.50) | 0.000 | 12.57 (11.50, 14.21) | 11.46 (10.31, 12.53) | 0.000 |
ML: coronal medial-lateral diameter of the medullary cavity; LD: longest diameter of the medullary cavity; AP: sagittal anterior-posterior diameter of the medullary cavity; VL: vertical line of the longest diameter of the medullary cavity
The present study focused on the femoral medullary cavity from the level 25 mm proximal to 60 mm distal to the lesser trochanteric line, which does meet the needs of the medullary cavity design of short-stem prostheses. At present, the design concept of the cementless prosthetic stem is the internal and external press-fit fixation of the medullary cavity. Although the study of the 3D morphology of the medullary cavity using CT has gradually increased, most studies have focused on the medial and lateral morphology of the medullary cavity rather than the anterior-posterior diameter [5, 21]. The present study found that the medial-lateral morphology of the medullary cavity of the proximal femur had the following characteristics. 1. The ML of the medullary cavity gradually narrowed from proximal to distal, and the trend of narrowing gradually decreased; this narrowing was less obvious at the levels of MLT + 5 or MLT + 10. 2. M was wider than L in the metaphysis of the medullary cavity; the distal constriction was particularly obvious proximal to the MT + 10. The present study also evaluated the AP morphology of the medullary cavity, which revealed the following characteristics. 1. The A of the medullary cavity gradually narrowed from the proximal to the distal end, and this narrowing was less obvious from AT + 5 or AT + 10. 2. The PT-25 was smaller than PT-20, and tended to be straight from the level of PT-10 to PT + 30.
Our study showed that the ML and AP were not symmetrical at the reaming point. M and A were longer than L and P, respectively, and the femoral osteotomy surface may have a smaller S than the distal end. Under such circumstances, if the medullary cavity was opened at the midpoint, this would often lead to the reaming point being placed anteriorly and medially; thus, the selected reaming point should be as lateral and posterior as possible. Current stem designs have minimal width in the anterior-posterior dimensions, which are not connected to cortical bone; however, as stem designs become shorter and have more filling geometries within the anterior-posterior direction (fit and fill designs), a 3D model may be required. It may be optimal to set the AP of the metaphyseal stem asymmetrically to better match the medullary cavity and guide the placement of the prosthetic stem, especially in cases with excessive bowing of the femur.
Previous studies have reported differences in femoral medullary cavity anatomy between the sexes. Compared with females, males reportedly have a wider diameter of the femoral head, ML and AP of the medullary cavity, but a smaller CFI, indicating that the isthmus of the medullary cavity is wider in males than in females [8]. Another study reported that the width of the metaphysis of the medullary cavity is similar in males and females, but is wider in males at the diaphysis [10]; furthermore, the CFI60 in the coronal and sagittal planes is smaller in males than in females in those aged < 80 years, but the CFI60 of women > 80 years old is smaller than that of men > 80 years old [10]. In our study, the average length of the femur was longer in men than women, while the mean medullary cavity was wider in women than men. However, there were no significant differences between sexes regarding offset, anteversion, NSA, and CFI60. This suggests that men and women can use the same brand of prosthesis, except that men require a larger stem. Previous research has shown that age affects the morphology of the femoral medullary cavity; with increasing age, the bone cortex becomes thinner, and the medullary cavity at the femoral diaphysis enlarges [10, 22, 23]. In our study, the ML of the CFI60 and L of the CFI60 decreased with age, suggesting a gradual transition of the medullary cavity morphology from a champagne glass shape to a chimney shape.
The morphology of the proximal end of the femoral medullary cavity is influenced by human ethnicity. One study found significant differences in medullary cavity morphology between Caucasian and Japanese and non-Japanese Asian women [5]; however, the femoral NSA was similar in all Asian races [5]. The proximal femoral medullary cavity of the present northern Chinese study population was similar to that described in a Caucasian population [24], but there were significant differences at the level of the lesser trochanteric line and distally, suggesting that the application of the same type of stem may cause distal implantation difficulties for Asians, and that Asians may require a finer distal stem than Caucasians. There were still differences between the present study population from northern China in comparison with a previous study population from southern China [8]. Therefore, even within the same race, the same implant may lead to unsatisfactory surgical results in the mismatched part of the population. Femoral medullary cavity morphology is determined by a series of genetic and environmental factors, such as ethnicity, sex, age, and lifestyle, which all contribute to the formation of a unique geometrical skeletal structure [9].
In the present study, the cross-sectional parameters of the medullary cavity were positively correlated with the femoral length and femoral head diameter, suggesting that the medullary cavity became wider as the height increased. Femoral head height was negatively correlated with the ML-CFI60, AP-CFI60, M of the CFI60, and A of the CFI60, and negatively correlated with the L-CFI60 and P of the CFI60. A negative correlation was also found between the NSA and the ML-CFI60 and AP-CFI60.
The present study had the following limitations. 1. The study only focused on the proximal femoral medullary cavity morphology of the northern Chinese population. Previous literature has demonstrated differences in medullary cavity morphology between Asian and Caucasian races [5]. However, the study has limited explanatory power regarding Caucasian medullary cavity morphology. 2. The sample size was only 94, and the oldest participant was only 68 years old. Thus, the medullary cavity morphology of the very elderly population was not evaluated. 3. The present study participants were all healthy, and so the present results cannot be generalized to patients with diseases such as developmental hip dysplasia, ischemic necrosis of the femoral head epiphysis, and intracapsular/extracapsular deformity of the hip. However, this was not the aim of the present study. For patients with femoral deformity, a disease-specific bone morphology database should be established to aid in the design of prosthesis engagement at specific points and in 3D placement.