During pregnancy, reduced dimensions of long bones in relation to gestational age found in routine ultrasound examinations allow for diagnosing developmental defects and skeletal dysplasias, as well as for observing abnormal morphological features and bone mineralization, and the presence of fractures . The assessment of the crural bones is easier than the antebrachial bones, because the tibia and fibula are more stabilized and both begin and end at the same level. Contrariwise, at the elbow joint the ulna both starts and ends more proximally, when compared to the radius .
Basing on an ultrasound study of 663 fetuses aged 12 to 42 weeks, Chitty and Altman  measured the length of the fibula for the 50th percentile, which increased from 6.8 mm at 12 weeks to 65.8 mm at 42 weeks. The fibula elongated in accordance with the function: y = 13,697/age2−2458.0/age + 116.51 (SD = 0.053841 × age + 1.0451). Brons et al.  ultrasonically measured the fibular length in 63 fetuses aged 12 to 40 weeks, and found its increase for the 50th percentile from 0.3 cm at 12 weeks to 6.3 cm at 40 weeks. Of note, the authors observed an increase in length of the fibular shaft, according to a natural logarithmic model. With the use of ultrasound, Zorzoli et al.  measured lengths of long bones, including crural bones, in 179 fetuses aged 64 to 108 days from the last menstrual period. Regrettably, these authors did not distinguish the tibia and fibula in their measurements, and reported aggregate findings. The length of the leg bones increased in a directly proportionate manner to fetal age, following the function: y = − 19.633 + 0.31473 × age. In addition, Exacoustos et al.  ultrasonically measured lengths of long bones, including the fibula, in 1951 fetuses aged 13–40 weeks. Only for the femur and humerus, measurements were done from week 13, while lengths of other bones were measured from week 15. The mean length of the fibula for the 50th percentile increased from 15.0 mm at 15 weeks to 56.0 mm at 40 weeks. The fibular growth increased following the quadratic function: y = 36.563 + 3.963 × age−0.037 × age2, (SD = 1.697). An increase in length was 2.43 ± 1.56 mm between weeks 13 and 28, and 1.42 ± 1.02 mm between weeks 29 and 40. With the use of anatomical methods, Bareggi et al.  measured total lengths and lengths of ossified parts of limbs long bones, including the fibula in a group of 58 autopsied, immersed in 95% ethanol fetuses with a CRL between 38 and 116 mm, and so aged 8 to 14 weeks of gestation. The authors did not find any bilateral or sex differences. At week 8, the total lengths of the fibula were 5.5 ± 1.84 and 5.5 ± 1.85 mm on the right and left sides, respectively. Correspondingly, at week 14 these parameters reached values of 22.1 ± 0.70 and 22.0 ± 0.64 mm. Furthermore, at week 8 lengths of the ossified parts were 3.6 ± 1.56 mm on the right, and 3.6 ± 1.57 mm on the left, while at week 14–19.8 ± 0.59 and 19.8 ± 0.53 mm, respectively. Since our study involved somewhat older fetuses, i.e. week 17 onwards, the length of the ossification center at that starting time was 13.72 ± 0.80 mm. The difference between results by Bareggi et al.  and ours might result from different measurement methods used, our findings, however, were based on CT and digital image analysis, thus allowing a more precise determination of the ossified structures.
Engaging X-rays to examine 379 autopsied fetuses aged 21 to 42 weeks, Pryse-Davies et al.  found a faster development of ossification centers in female fetuses, and also demonstrated that in fetuses with lethal malformations, the development of ossification centers was either significantly retarded or accelerated. A clearly slower development of ossification centers was observed in fetuses with low birth weight associated with D- and E-trisomy, lethal dysplasia, as well as primary developmental defect of long bones. Contrariwise, an accelerated development of ossification centers occurred in fetuses with anencephaly. In our study, the investigated fibular shaft ossification center demonstrated neither sex nor laterality differences, which clearly corresponded with our previous CT findings concerning femoral  and iliac  primary ossification centers in human fetuses.
According to our knowledge, this paper is the first report to describe morphometric parameters of the fibular shaft ossification center in human fetuses using computed tomography and mathematical growth models. The mean length, proximal transverse diameter, projection surface area and volume of the fibular ossification center were directly proportionate to fetal age, following the consecutive linear functions: y = − 13.241 + 1.567 × age ± 1.556, y = − 0.091 + 0.063 × age ± 0.073, y = − 69.038 + 4.699 × age ± 4.055 and y = − 126.374 + 9.462 × age ± 8.845, respectively. In turn, the middle and distal transverse diameters increased logarithmically, as follows: y = − 1.201 + 0.717 × ln(age) ± 0.054 and y = − 2.956 + 1.532 × ln(age) ± 0.090, respectively. It should be noted that in our previous study dedicated to the femur, the growth dynamics of the femoral ossification center transverse diameter increased in a directly proportionate manner to fetal age expressed in weeks, as follows: y = − 3.579 + 0.368 × age ± 0.529 for proximal diameter; y = − 1.105 + 0.187 × age ± 0.309 for middle diameter, and y = − 2.321 + 0.323 × age ± 0.558 for distal diameter. The volume of the femoral ossification center increased following the cubic function: y = − 91.458 + 0.390 × age3 ± 92.146 .
We failed to find any reports in the medical literature concerning dimensions of the fibular shaft ossification center, thus precluding a more comprehensive discussion on this topic.
The dimensions of the fibular shaft ossification center obtained in the present study may be critically useful in diagnosing skeletal dysplasias that are often characterized by a disrupted or restricted growth of fetuses. Developmental defects of the fibula include femur–fibula–ulna complex, fibular hemimelia without or with foot deformation.
Femur–fibula–ulna complex is a congenital defect characterized by an asymmetric shortening of the femur, fibula and ulna, which may concur with finger defects. This deformation can affect from one to all four limbs . Basing on 491 cases, Lenz et al.  found the deformation to occur more often unilaterally than bilaterally, especially in the upper limb, on the right side, and in males. The most common associated deformations are disturbances in the development of the fibula and foot bones, the femur and ulna, the fibula and ulna, as well as the femur, fibula and ulna. The most common hemimelia refers to the fibula. The fibula can be shortened or not formed at all, and concurrently, uneven length of the limbs can be observed along with foot and knee deformations. Fibular hemimelia leads to a difference in the length of the limbs, as on the affected side, the tibia grows more slowly than that on the normal side. One of the most serious problems accompanying fibular hemimelia is foot deformation, associated with both abnormal and incomplete structures of the talocrural joint. Patients with fibular hemimelia usually have a deformed knee. This deformation can be associated with the distal end of femur or the proximal end of tibia, or both. In most cases, the defect occurs separately [18, 20].
If skeletal dysplasia is suspected, using only ultrasound is not sufficient to make a comprehensive diagnosis. In such cases, the following four methods should be employed: radiographic examination , ultrasound imaging , CT [3, 4] and MRI . Van Zalen-Sprock et al.  compared the sensitivity of imaging methods in detecting ossification centers in the fetal skeleton. They compared X-rays, as well as abdominal and transvaginal ultrasound examinations. The earliest ossification center could be observed using X-ray imaging, while transvaginal ultrasound examination allowed for the observation of ossification centers at the same time, or a week later. In turn, abdominal ultrasound allowed observation of ossification centers 1–2 weeks later, when compared to transvaginal ultrasound. In skeletal dysplasias, a greater diagnostic precision was demonstrated using 3D–CT compared to 2D–US [6, 23]. A big advantage of the CT technique is the possibility of observing the examined structure in every plane and at any time without sacrificing image detail after the examination [3, 4]. Compared to 2D X-ray, computed tomography eliminates the overlap of anatomical structures and allows easy distinction between different body tissues. A currently limiting factor for CT examinations is the lack of numerical data describing the fetal skeletal system at the defined weeks of pregnancy in comparison with ultrasound examinations. Magnetic resonance imaging has become a clinical complement for ultrasound and is currently the best diagnostic tool used to assess fetal anatomy in both prenatal and post-mortem examinations. The use of MRI in fetal anatomy examinations is critical in the 2nd and 3rd trimesters of pregnancy, when ultrasound imaging offers results that are either ambiguous or limited by small volume of the amniotic fluid (oligohydramnios) . In view of the progress in fetal surgery, the use of fetal MRI refers mainly to congenital defects of the central nervous system and the skeletal system, as well as congenital defects of thoracic and abdominal organs . The newly developed cine-MRI techniques provide an innovative insight into the movements of the entire fetus in the three-dimensional environment of the uterus during pregnancy . Unfortunately, the safety of this method has not yet been established, therefore, it is advisable to exercise particular caution when using MRI in women in the first trimester of pregnancy due to the potential risk of teratogenic effect. Moreover, the noise generated by the MRI scanner coil can potentially cause hearing loss in the fetus .
The main limitation of the present study was a relatively narrow fetal age group, ranging from the 17th to the 30th week of pregnancy, and a somewhat small number of individuals, including 47 human fetuses.