Abstract
Background
It has been reported that trochlear dysplasia occurs very early in development, and environmental factors like swaddling may cause developmental dysplasia of the hip, which is associated with a shallower trochlear groove. However, to our knowledge, there are no definitive studies about the relationship between trochlear dysplasia and traditional straight-leg swaddling.
Questions/purposes
Using a rat model of femoral trochlear dysplasia, we asked: Does straight-leg swaddling for 1 and 2 weeks in newborn Wistar rats alter the femoral trochlea with respect to (1) gross morphology, (2) histologic appearance, as well as (3) trochlear sulcus angle, width, and depth?
Methods
Eighty-four newborn Wistar rats (44 females and 40 males) were divided into two equal groups: 42 in the unswaddled group and 42 in the swaddled group; each group was comprised of 22 females and 20 males. In the swaddled group, the rats were wrapped in surgical tape to maintain hip and knee extension to simulate traditional human straight-leg swaddling. To determine whether longer periods of swaddling were associated with more severe trochlear dysplasia, 21 rats in each group were euthanized at 1 and 2 weeks, respectively, and the gross morphology of the femoral trochlea was observed by one observer blinded to condition. Then hematoxylin and eosin staining of the femoral trochlea was performed and the distribution and number of the chondrocytes of the trochlear groove were viewed through a microscope. The trochlear sulcus angles, depth, and width were measured by an experienced technician blinded to condition.
Results
By observing the gross morphology, we found that the trochlear groove in the swaddled group became qualitatively flatter compared with the unswaddled group at 1 week, and at 2 weeks, the trochlear groove became much shallower. At 1 and 2 weeks, histologic examinations showed obvious qualitative changes in the distribution and number of chondrocytes of the trochlear groove in the swaddled than in the unswaddled groups. In the swaddled group, trochlear dysplasia was more common at 2 weeks, occurring in 62% (26 of 42 [16 of 22 females and 10 of 22 males]) versus 33% (14 of 42 [8 of 22 females and 6 of 20 males]) at 1 week. At 1 week, the swaddled group showed more trochlear dysplasia compared with the unswaddled group as measured by angle of the trochlear groove (137° ± 6° versus 132°± 3.6°, mean difference 5° [95% confidence interval 2.9° to 7.2°]; p < 0.001), depth of the trochlear grove (0.28 ± 0.04 mm versus 0.31 ± 0.02 mm, mean difference 0.03 mm [95% CI 0.01 to 0.04]; p < 0.001). At 2 weeks, the swaddled group showed more severe trochlear dysplasia than at 1 week compared with the unswaddled group as measured by the angle of the trochlear groove (135° ± 6.0° versus 128° ± 4.8°, mean difference 7° [95% CI 5.7° to 10.4°]; p < 0.001), depth of the trochlear grove (0.32 ± 0.04 mm versus 0.36 ± 0.02 mm, mean difference 0.04 mm [95% CI 0.03 to 0.06]; p < 0.001). There was no difference in the width of the trochlear sulcus between the swaddled and the unswaddled groups at 1 week (1.29 ± 0.14 mm versus 1.30 ± 0.12 mm, mean difference 0.01 mm [95% CI -0.05 to 0.07]; p = 0.73) and 2 weeks (1.55 ± 0.12 mm versus 1.56 ± 0.12 mm, mean difference 0.01 mm [95% CI -0.05 to 0.07]; p = 0.70).
Conclusion
Our results indicate that traditional straight-leg swaddling could induce trochlear dysplasia in this model of newborn rats. With an increased swaddling time of 2 weeks, more severe trochlear dysplasia appeared in the swaddled group.
Clinical Relevance
Our findings suggest that traditional straight-leg swaddling may impair trochlear development in the human neonate and lead to trochlear dysplasia in infants. We believe our animal model will be useful in future work to observe and study the change of cartilage and subchondral bone in each stage of the development of trochlear dysplasia and the change of mechanotransduction-associated proteins (such as, TRPV4/ Piezo1 and CollagenⅡ) in cartilage and subchondral osteocytes. It will also be helpful to further investigate the mechanism of developmental femoral trochlea dysplasia caused by biomechanical changes.
Introduction
The patellofemoral joint is a complex articulation with high functional and biomechanical requirements. The geometry of the trochlear groove plays an especially important role in the stability of the patellofemoral joint, although it is influenced by a complex interplay of dynamic stability and static stabilizers. Trochlear dysplasia is an important geometric anomaly of the trochlea in patients with this condition [4]. Previous studies have shown that femoral trochlea dysplasia is a major predisposing factor for instability of the patellofemoral joint, and up to 96% of patients who have patellofemoral instability have trochlear dysplasia [4, 6, 19]. Studies have also shown that the treatment of patellar instability combined with femoral trochlea dysplasia is difficult and complex, and the rate of the recurrence is high [14, 16, 20]. Lewallen et al. [20] reported that skeletally immature patients with trochlear dysplasia had only a 31% success rate with nonoperative management and nearly half of patients with recurrent instability required surgical intervention to gain stability. Fones et al. [7] identified that there was an association between trochlear dysplasia and the incidence of osteochondral damage in the patients with instability of the patellofemoral joint.
Currently, the etiology of trochlear dysplasia is not clear. Animal experiments have shown that early patellar dislocation may cause femoral trochlear dysplasia [15, 17, 22]. The surface morphology of the femoral trochlea has been reported to develop and take shape very early during growth [8, 9, 38]. Recently, Kohlhof et al. [18] and Øye et al. [27] reported that trochlear dysplasia occurs very early in development, perhaps even in utero. The breech position, especially the frank breech position, in which the knee of the fetus remains in the extended position, has been reported to be a major risk factor for the development of trochlear dysplasia [26].
Swaddling is a traditional practice that began in ancient times. Because of the positive physiologic effects of swaddling, especially protection against hypothermia and sudden infant death syndrome in neonates, swaddling has become very popular in Western countries in recent years [36]. Babies may sleep better and cry less when they are swaddled [10, 23, 35]. However, the support for swaddling is not uniform. Studies have shown that more swaddling may cause a higher rate of developmental dysplasia of the hip [8, 30], and, importantly, that there is a close relationship between hip dysplasia and femoral trochlear dysplasia. Li et al. [21] reported that patients with hip dysplasia have smaller femoral condyles and exhibit greater medial and lateral condylar asymmetry than people with normal hips, whereas patients with hip dislocation have a shallower trochlear groove, leading us to question whether swaddling may also be a predisposing factor for trochlear dysplasia.
The purpose of this study was to develop an animal model to study femoral trochlear dysplasia. Therefore, we asked: Does straight-leg swaddling for 1 and 2 weeks in newborn Wistar rats alter the femoral trochlea with respect to (1) gross morphology, (2) histologic appearance, as well as (3) trochlear sulcus angle, width, and depth?
Materials and Methods
Study Design Overview
In our study, newborn Wistar rats were swaddled with surgical tape to maintain hip and knee extension to simulate the position of traditional human straight-leg swaddling. Although rat and human anatomies are different, when the leg of the rat is swaddled to maintain knee extension and hip extension and adduction, the biomechanical changes between the patella and femoral trochlea are like the human. When the newborn is swaddled, the relative motion between the patella and the femoral trochlea is reduced. To determine whether longer periods of swaddling are associated with trochlear dysplasia, rats in each group were euthanized at 1 week (human equivalent: about 6 months) and 2 weeks (human equivalent: toddlers) [1], respectively, and the gross morphology of the femoral trochlea was examined by one observer (XBC) blinded to condition. Then, hematoxylin and eosin staining of the femoral trochlea was performed and the distribution and number of the chondrocytes of the trochlear groove were viewed through a microscope. The trochlear sulcus angles, depth and width were measured by an experienced technician (XBC) blinded to condition.
Experimental Procedures
Eighty-four newborn Wistar rats (44 females and 40 males) from eight litters provided by the laboratory animal center of our university were used in this study. The rats of each litter were paired randomly and divided into an unswaddled group (42 rats; 22 were female and 20 were male) and a swaddled group (42 rats; 22 were female and 20 were male). In the swaddled group, the rats were wrapped with surgical tape (3M Durapore) to maintain hip and knee extension to simulate traditional straight-leg swaddling in humans (Fig. 1). This swaddling permitted minor hip and knee movement (knee ROM less than 30°), and the rats were released from swaddling for approximately 30 minutes per day. The rats in the unswaddled group were not swaddled and were permitted unrestricted motion in their cage. All rats were fed in a specific pathogen–free animal room; the ambient temperature was 24° C, and the humidity was 50%. Each litter of rats were fed in separate plastic cages with wood chip bedding material. All rats were fed by their mothers, and the mothers of the rats were cared for by professional keepers to ensure their normal intake. All the rats in the two groups developed normally, and they qualitatively were similar in weight at each stage of development.
Fig. 1.
A-B These photographs show the straight-leg swaddling model in newborn Wistar rats: (A) the dorsal view of the model, and (B) the lateral view of the model.
Adverse Events
In the preliminary experiment, four rats died because of too-tight swaddling. To reduce this adverse event, we permitted minor hip and knee movement (ROM of knee less than 30°), and the rats were released from swaddling for approximately 30 minutes per day.
Gross Observation
Twenty-one rats in each group were euthanized by CO2 inhalation consistent with the American Veterinary Medical Association Guidelines on Euthanasia at 1 and 2 weeks after the experiment, and the femoral trochlea was isolated. One observer (XBC) who was blinded to the condition visually assessed the size and the gross morphology of the femoral trochlea. We obtained a cross section of the femoral trochlea. The angle, depth, and width of the femoral trochlea were visually qualitatively evaluated and compared between the two groups. At the same time, whether there was cartilage degeneration of the surface of the femoral trochlea was also evaluated for the two groups.
Hematoxylin and Eosin Staining and Histologic Analysis
The femoral trochleas of the rats were isolated at 1 and 2 weeks. After gross observation, hematoxylin and eosin staining of the femoral trochlea was performed. The femoral trochleas were carefully separated from the femur. The specimens were soaked in 4% paraformaldehyde (pH = 7.40) overnight at 4° C and then immersed in 10% ethylenediaminetetraacetic acid at 4° C for decalcification for approximately 30 days. Subsequently, a concentration gradient of alcohol and xylenes was used for dehydration, and the specimens were embedded in paraffin for subsequent tissue staining. Next, three 5-μm sections were cut along the femoral axis to obtain transverse images of the trochlear sulcus and stained with hematoxylin and eosin to show the cartilage and subchondral bone [2, 28, 29, 31]. The distribution and number of the cartilage cells was observed through a microscope, and the surface of the articular cartilage was evaluated. The sites for the sections were selected based on published studies [15, 22, 37]. The measurements of the middle trochlear sulcus angle, depth, and width were taken from the section through the intersection of the lower femoral physis line along with its posterior cortex. This work was performed by one experienced author (XBC) blinded to condition. The measurements of the proximal and distal trochlear sulcus angles, depth, and width were taken from a section 0.5-mm proximal and distal from the middle trochlear sulcus level (Fig. 2). The trochlear sulcus angles, depth, and width were measured at the three sections of each specimen, respectively. The mean value from the three sections of each rat was then used to calculate the mean and SD of each group.
Fig. 2.
A-D (A) This figure is a sagittal view of the femoral trochlea. (B-D) These photographs show sections of the trochlea that correspond to the red lines in Fig. 2A that are labeled B, C and D. (B) This figure shows the distal trochlear sulcus. (C) This figure shows the middle trochlear sulcus. (D) This figure shows the proximal trochlear sulcus. A color image accompanies the online version of this article.
Measurement of the Trochlear Sulcus
Pathologic sections were scanned via Pannoramic Digital Slide Scanners (Pannoramic MIDI II, 3DHISTECH Ltd), and an image containing the entire femoral trochlea of the pathologic sections was generated. The cartilaginous sulcus angle of the trochlear groove was defined as the angle of the deepest point of the trochlea connecting with the lateral trochlear cartilaginous surface and the medial trochlear cartilaginous surface, based on the measurements of depth and width (Fig. 3). The trochlear groove at different growth stages (1 and 2 weeks) was measured on the images of tissue sections. An experienced technician (XBC) with no knowledge of the groups independently calculated the femoral trochlear angle, and the calculations were repeated over an interval of 1 week. The reliability of the measurements was assessed using intraclass correlation coefficients. Agreement between the first measurements and the retest was evaluated used Cohen’s kappa (κ). Measurement of the sulcus angle had an accuracy of 1°. Because the femoral trochlea of the rat was too small at 1 and 2 weeks, the measurement of the depth and the width were accomplished on the pathological section, which had the smallest scale 50 µm. Thus, we used 0.01 mm as the measuring accuracy of depth and width.
Fig. 3.
This is a histological section of the middle trochlear sulcus relative to Fig. 2C. The angle ABC is defined as the trochlear groove, in which the distance from A to C is the width, and the distance from B to D is the depth.
Primary and Secondary Study Outcomes
Our primary study goal was to observe the change of gross morphology of the femoral trochlea after the rats were swaddled for 1 and 2 weeks. To achieve this, rats in each group were euthanized at 1 and 2 weeks, and the femoral trochlea was isolated. One blinded observer (XBC) visually observed the size and the gross morphology of the femoral trochlea. A cross section of the femoral trochlea was obtained, and the angle, depth, and width of the femoral trochlea were qualitatively evaluated.
Our secondary study goals were to evaluate the change of the histologic appearance and measure the trochlear sulcus angles, depth, and width. After gross observation, hematoxylin and eosin staining of the femoral trochlea was performed. The distribution and number of the chondrocytes of the trochlear groove were viewed through a microscope. And the trochlear sulcus angles, depth, and width were measured by an experienced technician (XBC) blinded to condition.
Trochlear dysplasia in the swaddled rats was defined by a trochlear sulcus angle 10° greater than the average degree in the unswaddled group [4, 5].
All measurements were demonstrated to be reliable (intraclass correlation coefficient > 0.89). The agreement between the first measurements and the retest was good (the Cohen unweighted κ = 0.90).
Ethical Approval
All experimental protocols in this study were reviewed and approved by our animal research committee.
Statistical Analysis
For the estimation of sample size, the trochlear groove served as the primary variable with a difference of 4° according to the previous experience in our institution. Based on a confidence level of 95% (α = 0.05) and a power (1–β) of 80%, a sample size of 18 rats per group was required. Thus, a sample size of 21 rats of each group for each comparison was deemed sufficient, and a total of 84 rats were included in our experiment. All the descriptive statistical data from both femoral trochlea of each rat are expressed as the mean ± SD. We used the statistical software SPSS version 16.0 for data analysis. We used the Shapiro-Wilk test to evaluate the normality of the distribution of values for each variable, and we used the Levene test to evaluate the homogeneity of variance. Independent-samples t tests were used to analyze parametric data. We used a chi-square test to analyze differences between sexes and between groups. A p value < 0.05 was considered statistically significant.
Results
Gross Observation
One week after swaddling, the size of the femoral trochleas in the two groups were similar, and the surface of the articular cartilage was smooth in both the swaddled and unswaddled groups. However, in some specimens from the swaddled group, the trochlear grooves were visually shallower than those in the unswaddled group. At 2 weeks, no qualitative changes were found in the size of the femoral trochleas and surface of the articular cartilage between the swaddled and unswaddled groups. The trochlear groove of the swaddled group became qualitatively flatter compared with the unswaddled group at 1 week and the flattening of the trochlear groove became more severe at 2 weeks (Fig. 4).
Fig. 4.
A-D These photographs show the gross anatomy of the femoral trochlea at (A) 1 week in the unswaddled group, (B) 1 week in the swaddled group, (C) 2 weeks in the unswaddled group, and (D) 2 weeks in the swaddled group.
Histologic Analysis
Examination of the pathological sections demonstrated that the distribution and arrangement of articular cartilage cells was orderly, and the surface of the articular cartilage was smooth in the swaddled and unswaddled groups at 1 and 2 weeks after swaddling. The trochlear grooves were qualitatively shallower in some specimens in the swaddled group than those in the unswaddled group at 1 week. At 2 weeks, the number of the shallower trochlear grooves increased in the swaddled group and the shallowness of the trochlear groove became more severe than at 1 week (Fig. 5).
Fig. 5.
A-D These are histological sections of the femoral trochlear were taken at (A) 1 week in the unswaddled group, (B) 1 week in the swaddled group, (C) 2 weeks in the unswaddled group, and (D) 2 weeks in the swaddled group (hematoxylin and eosin staining). The distribution of cells are uniform, the number of the cells are similar in the swaddled group and unswaddled group, and the surface of the articular cartilage was normal and smooth in the swaddled group and unswaddled group at 1 and 2 weeks. The trochlear grooves in swaddled group were shallower compared with the unswaddled group.
Trochlear Sulcus Angle, Depth, and Width at 1 and 2 Weeks
After 1 Week
After 1 week, 14 trochleas (from 8 of 22 females and 6 of 20 males) in the swaddled group exhibited dysplasia (a difference in angle exceeding 10° compared with the unswaddled group), whereas in the unswaddled group, no trochleas exhibited dysplasia (p < 0.001) (Table 1). The incidence rate of the trochlear dysplasia in female and male rats in the swaddled group was similar (8 of 22 versus 6 of 20; p = 0.66) (Table 1). The trochlear sulcus angle was larger in the swaddled group (137° ± 6° [95% confidence interval 135.0° to 138.7°]) than in the unswaddled group (132° ± 3.6° [95% CI 130.7° to 133°], mean difference 5° [95% CI 2.9° to 7.2°]; p < 0.001) (Table 2). The depth of the trochlea was shallower in the swaddled group (0.28 ± 0.04 mm [95% CI 0.27 to 0.29]) than in the unswaddled group (0.31 ± 0.02 mm [95% CI 0.30 to 0.31], mean difference 0.03 mm [95% CI 0.01 to 0.04]; p < 0.001) (Table 3). There was no difference in the width of the trochlear sulcus between the swaddled group (1.29 ± 0.14 mm [95% CI 1.25 to 1.33]) and the unswaddled group (1.30 ± 0.12 mm [95% CI 1.26 to 1.34], mean difference 0.01 mm [95% CI -0.05 to 0.07]; p = 0.73) (Table 4).
Table 1.
The incidence rates of trochlear dysplasia between the unswaddled group and the swaddled group and the sexual difference
| One week | Two weeks | |||||||
| Unswaddled group (n = 42) | Swaddled group (n = 42) | Chi-square (group difference) | p value | Unswaddled group (n = 42) | Swaddled group (n = 42) | Chi-square (group difference) | p value | |
| Female | 0 of 22 | 8 of 22 | 9.78 | 0.004 | 2 of 22 | 16 of 22 | 18.43 | < 0.001 |
| Male | 0 of 20 | 6 of 20 | 7.06 | 0.02 | 0 of 20 | 10 of 20 | 13.33 | < 0.001 |
| Total | 0 of 42 | 14 of 42 | 16.80 | < 0.001 | 2 of 42 | 26 of 42 | 30.86 | < 0.001 |
| Chi-square (sexual difference) | 0.19 | 1.91 | 2.29 | |||||
| p value | ＞ 0.99 | 0.66 | 0.49 | 0.13 | ||||
Table 2.
Measurements of the trochlear sulcus angle of the unswaddled and swaddled groups
| Parameter | Unswaddled group (n = 42) (95% CI) |
Swaddled group (n = 42) (95% CI) |
Mean difference (95% CI) | p value |
| Trochlear sulcus angle in ° at 1 week | 132 ± 3.6 (130.7-133) | 137 ± 6.0 (135-138.7) | 5 (2.9-7.2) | < 0.001 |
| Trochlear sulcus angle in ° at 2 weeks | 128 ± 4.8 (126-129) | 135 ± 6.0 (133.7-137.4) | 7 (5.7-10.4) | < 0.001 |
Data presented as mean ± SD.
Table 3.
Measurements of the trochlear sulcus depth of the unswaddled and swaddled groups
| Parameter | Unswaddled group (n = 42) (95% CI) |
Swaddled group (n = 42) (95% CI) |
Mean difference (95% CI) | p value |
| Trochlear sulcus depth in mm at 1 week | 0.31 ± 0.02 (0.30-0.31) | 0.28 ± 0.04 (0.27-0.29) | 0.03 (0.01-0.04) | < 0.001 |
| Trochlear sulcus depth in mm at 2 weeks | 0.36 ± 0.02 (0.35-0.37) | 0.32 ± 0.04 (0.30-0.33) | 0.04 (0.03-0.06) | < 0.001 |
Data presented as mean ± SD; the measurement of the depth was performed on the pathological section as shown in Fig. 3, which had the smallest scale of 50 µm.
Table 4.
Measurements of the trochlear sulcus width of the unswaddled and swaddled groups
| Parameter | Unswaddled group (n = 42) (95% CI) |
Swaddled group (n = 42) (95% CI) |
Mean difference (95% CI) | p value |
| Trochlear sulcus width in mm at 1 week | 1.30 ± 0.12 (1.26 to 1.34) | 1.29 ± 0.14 (1.25 to 1.33) | 0.01 (-0.05 to 0.07) | 0.73 |
| Trochlear sulcus width in mm at 2 weeks | 1.56 ± 0.12 (1.52 to 1.59) | 1.55 ± 0.12 (1.52 to 1.59) | 0.01 (-0.05 to 0.07) | 0.70 |
Data presented as mean ± SD; the measurement of the width was performed on the pathological section as shown in Fig. 3, which had the smallest scale of 50 µm.
After 2 Weeks
After 2 weeks, 26 trochleas (16 of 22 females and 10 of 20 males) in the swaddled group exhibited dysplasia (a difference in angle exceeding 10° compared with the unswaddled group) and in the unswaddled group only two trochleas exhibited dysplasia (p < 0.001) (Table 1). There was no difference between the sexes in femoral trochlear dysplasia: 16 of 22 in females versus 10 of 20 in males (p = 0.13) (Table 1). The trochlear sulcus angle was much larger in the swaddled group (135° ± 6° [95% CI 133.7° to 137.4°]) than the unswaddled group (128° ± 4.8° [95% CI 126° to 129°], mean difference 7° [95% CI 5.7° to 10.4°]; p < 0.001) (Table 2). The depth of the trochlea was much shallower in the swaddled group (0.32 ± 0.04 mm [95% CI 0.30 to 0.33]) than the unswaddled group (0.36 ± 0.02 mm [95% CI 0.35 to 0.37], mean difference 0.04 mm [95% CI 0.03 to 0.06]; p < 0.001) (Table 3). There was no difference in the width of the trochlear sulcus between the swaddled group (1.55 ± 0.12 mm [95% CI 1.52 to 1.59]) and the unswaddled group (1.56 ± 0.12 mm [95% CI 1.52 to 1.59], mean difference 0.01 mm [95% CI -0.05 to 0.07]; p = 0.70) (Table 4).
One Week Versus 2 Weeks After Swaddling
More trochlear dysplasia occurred in the swaddled group at 2 weeks (62% [26 of 42]) than at 1 week (33% [14 of 42]). The change of the trochlear angle and depth in the swaddled group compared with the unswaddled group was a mean difference of 7° (95% CI 5.7° to 10.4°) and a mean difference of 0.04 mm (95% CI 0.03 to 0.06) at 2 weeks, respectively, which was more severe than the change at 1 week (mean difference 5° [95% CI 2.9° to 7.2°] and mean difference 0.03 mm [95% CI 0.01 to 0.04]).
Discussion
An abnormal trochlear groove morphology is defined as trochlear dysplasia, which is a common phenomenon among young children with recurrent patellar dislocation [6]. In children with patellar dislocation, the incidence of trochlear dysplasia has increased in recent years [25, 32], and the cause of trochlear dysplasia varies. However, to our knowledge, there is no study of the risk factors of trochlear dysplasia in infants. Studies have shown that more swaddling may cause a higher rate of developmental dysplasia of the hip [8, 30]. Swaddling may also be a predisposing factor for trochlear dysplasia in infants. The most important findings of our study were that swaddling can cause femoral trochlear dysplasia in newborn Wistar rats, and that a longer swaddling time corresponded to more severe femoral trochlear dysplasia.
Limitations
This study has some limitations. First, the most important limitation was that we used an animal model. However, because the leg of the rat was swaddled to maintain knee extension and hip extension, we believe the resulting geometrical changes between the patella and femoral trochlea were similar to what would be expected to occur in human neonates. Therefore, we believe that it was feasible to simulate the human swaddling position in an animal model. Second, we used the rat rather than a larger animal, such as pig or sheep, in this animal model. Compared with the rat, the anatomical structure of the pig patellofemoral joint may be much closer to human anatomy. However, it was easier to get more newborn rats than pigs or sheep, the swaddling of the rats was easier, and their growth cycle was shorter. Therefore, we believed that rats were more suitable for this animal model. Third, although the changes of the depth and the angle of the trochlear groove were obvious after 1 and 2 weeks of swaddling application, the range of the angle of the trochlear groove for both groups overlapped to a considerable extent. Swaddling only increased the risk of femoral trochlea dysplasia, but not all trochlea demonstrated dysplasia after swaddling.
Discussion of Key Findings
In the current study, we established a straight-leg swaddling model in newborn Wistar rats using surgical tape. During the study, we found that traditional straight-leg swaddling can affect the development of newborn rats and lead to trochlear dysplasia. The trochlear grooves were qualitatively shallower in some specimens in the swaddled group than those in the unswaddled group at 1 week. At 2 weeks, the number of rats with a shallower trochlear groove increased in the swaddled group and the shallowness of the trochlear groove became more severe than at 1 week. However, there was no difference in the width of the trochlear groove between the unswaddled group and swaddled group after 1 or 2 weeks of swaddling. We believe the main reason for these results was that trochlear development in neonatal rats requires normal stress stimuli. Because of swaddling, knee movement was restricted, leading to weakening of the biomechanical interplay between the patellar and femoral trochlea. Therefore, trochlear dysplasia was evident in the neonatal rats in the swaddled group, and as the swaddling time increased, the number of rats with trochlear dysplasia increased and trochlear dysplasia became more severe. This animal model seems to emulate trochlear dysplasia in human neonates that could arise from swaddling, and we believed the gross and histologic findings in these rats were similar to humans because if a newborn were swaddled too tight, the movement between the patella and the femoral trochlea may be restricted, too.
Articular cartilage and subchondral bone development is a complex process. Currently, there is no consensus on the exact cause of trochlear dysplasia. The cartilaginous trochlear angle is known to have already formed at birth [24, 38]. Genetic factors, maltracking, and unbalanced forces play important roles in trochlear remodeling in infancy. However, the bony trochlear angle develops during adolescence, and all of the aforementioned components can influence remodeling of the trochlear groove [24, 38]. Studies have shown that mechanical stress is one of the most important factors affecting cartilage and bone development [11-13]. Tardieu and Dupont [34] showed that femoral obliquity develops through learning to walk. If the biomechanical interplay between the patellar and femoral trochlea is absent during growth, a flat trochlea may form [24, 38]. The development of the femoral trochlear groove is also influenced by genetic factors and mechanical stress on the patella [8]. During knee bending, the patella enters the femoral trochlear groove track, and loads are produced between the femoral trochlea and patella. Mechanical loading from the patella is transmitted from the trochlear cartilage to subchondral bone and then to cancellous and cortical bone. This mechanical stress stimulates bone growth and remodeling of the femoral trochlea and patella [12, 13]. Therefore, a normal trajectory of patellar movement is necessary for the development of the femoral trochlear groove and patella. On the other hand, a previous study [39] reported that abnormal stress loads might lead to elevated osteoclast activity and turnover rates, influencing bone remodeling of the trochlear groove. Thus, appropriate stress stimulation of the patella on the femoral trochlea plays an important role in facilitating trochlear development. In a previous study, we discovered that early patellar subluxation or dislocation in growing rabbits may lead to femoral trochlear dysplasia or flattening, and that early reduction may minimize femoral trochlear dysplasia [37]. This mechanism is also supported by clinical findings. Acetabular dysplasia may be a secondary pathologic change after hip dislocation [33]. Simultaneously, bone marrow stimulation may also induce chondrogenesis of the trochlear groove [3]. Taken together, these studies support that the normal development of the trochlear groove is dependent on the normal biomechanical interaction between the patella and the femoral trochlea. However, there are few studies on developmental dysplasia of femoral trochlea of the infant when the normal biomechanical interplay between the patellar and femoral trochlea is decreased or even absent, and the mechanism of developmental femoral trochlea dysplasia caused by these biomechanical changes are not clear.
We believe our animal model will be useful in future work to observe and study the change of cartilage and subchondral bone in each stage of the development of trochlear dysplasia and the change of mechanotransduction-associated proteins (such as, TRPV4/ Piezo1 and CollagenⅡ) in cartilage and subchondral osteocytes. It will also be helpful to further investigate the mechanism of developmental femoral trochlear dysplasia caused by biomechanical changes.
Conclusion
Traditional straight-leg swaddling has a deleterious influence on the development of the neonatal trochlea and may induce trochlear dysplasia in this in vivo newborn rat model. In terms of clinical relevance, our findings imply that traditional straight-leg swaddling may impair trochlear development in human neonates, and that traditional straight-leg swaddling may lead to trochlear dysplasia in infants. To our knowledge, the relationship between swaddling of newborns and trochlear dysplasia has yet to be explored. Efforts should be devoted to researching the epidemiology of trochlear dysplasia in populations in which traditional straight-leg swaddling is popular.
Acknowledgments
We thank Tao Li and all colleagues of the laboratory animal center of the Third Affiliated Hospital of Hebei Medical University for helping us to feed and care for the animals. We also thank Mei Li for statistical counseling and assisting us with statistical analysis.
Footnotes
The institution of one or more of the authors (FW) has received, during the study period, funding from the National Natural Science Foundation of China (81873983).
Each author certifies that there are no funding or commercial associations (consultancies, stock ownership, equity interest, patent/licensing arrangements, etc.) that might pose a conflict of interest in connection with the submitted article related to the author or any immediate family members.
All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research ® editors and board members are on file with the publication and can be viewed on request.
Ethical approval for this study was obtained from Hebei Medical University Third Hospital, Hebei, China.
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