The Journal of Bone & Joint Surgery

The Journal of Bone & Joint Surgery, Volume 89, Issue 6

Scientific Articles | June 01, 2007

Correlation of Body Mass Index and Radiographic Deformities in Children with Blount Disease

Sanjeev Sabharwal, MD¹; Caixia Zhao, MD¹; Emily McClemens, PA-C¹

¹ Department of Orthopedics, University of Medicine and Dentistry of New Jersey-New Jersey Medical School, Doctor's Office Center, 90 Bergen Street, Suite 7300, Newark, NJ 07103. E-mail address for S. Sabharwal: sabharsa@umdnj.edu

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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

Note: The authors acknowledge the assistance of Dr. James Lee Jr. in the planning phase of the study.

Investigation performed at the University of Medicine and Dentistry of New Jersey-New Jersey Medical School, Newark, New Jersey

The Journal of Bone and Joint Surgery, Incorporated

J Bone Joint Surg Am, 2007 Jun 01;89(6):1275-1283. doi: 10.2106/JBJS.F.01135

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Abstract

Background: Children with Blount disease tend to be heavier than their peers; however, the relationship between the magnitude of obesity and the severity of limb deformities in Blount disease has not been well studied.

Methods: A retrospective review of the preoperative medical records and radiographs of patients with previously untreated Blount disease was conducted. Demographic information including gender, ethnicity, the age when deformity was first noted, the age at the examination, and the body mass index was recorded. Frontal and sagittal plane deformities were analyzed by one examiner using full-length standing radiographs. The association of body mass index with various demographic and deformity parameters was then analyzed.

Results: Over an eight-year period, forty-five patients with sixty-five limbs affected by Blount disease were identified. Seventeen children (twenty-seven limbs) had early-onset Blount disease, and twenty-eight children (thirty-eight limbs) had late-onset disease. Fifteen of the children with early-onset disease and twenty-six of those with late-onset disease were overweight. There was no significant relationship between body mass index and gender, ethnicity, or laterality. The children with early-onset disease tended to have a lower body mass index but a greater magnitude of radiographic deformities compared with the children with late-onset disease. Greater varus malalignment (r = 0.74, p < 0.0001) and tibial procurvatum (r = -0.79, p = 0.002) were noted with an increasing body mass index in the early-onset, but not the late-onset, group of patients. Irrespective of the age at onset, the correlation of body mass index with frontal and sagittal plane deformities was stronger in extremely obese children (body mass index of =40).

Conclusions: There is a significant relationship between the magnitude of obesity and biplanar radiographic deformities in children with the early-onset form of Blount disease and in those with a body mass index of =40. These clinical findings are consistent with the literature concerning the effect of compressive forces on growth at the proximal tibial physis.

Level of Evidence: Prognostic Level II. See Instructions to Authors for a complete description of levels of evidence.

Figures in this Article

Blount disease is a developmental condition characterized by disordered endochondral ossification of the medial aspect of the proximal tibial physis, and it results in multiplanar deformities of the lower limb. As a result of asymmetric growth with relative inhibition of the growth of the posteromedial portion of the proximal tibial physis, a three-dimensional deformity of the tibia including varus, procurvatum, and internal tibial torsion develops^1-3 and can lead to progressive deformity with gait deviations, limb-length inequality, and premature arthritis of the knee^4-6. On the basis of the age at the onset of the deformity, two clinically distinct forms of Blount disease have been described: early-onset and late-onset⁷. Although there are key clinical and radiographic differences between these two categories, there are also several similarities, including an increased incidence in obese, black children and histologic changes at the proximal tibial physis^7,8. Some authors have speculated^1,9-11 that the Blount disease seen in obese, black children has a worse prognosis than that seen in other children and adolescents.

The prevalence of obesity in children is increasing worldwide and represents a major health challenge^12-15. On the basis of results published in 2006, it was estimated that 17% of children and adolescents between the ages of two and nineteen years old in the United States are overweight¹³. Childhood obesity has been linked to the development of numerous health problems, including hypertension, non-insulin-dependent diabetes, and cardiovascular diseases¹². Certain musculoskeletal disorders, such as slipped capital femoral epiphysis^16-18 and Blount disease^7,8, are also noted with increased frequency in overweight children. The etiology of Blount disease remains controversial, although most agree that it is related to abnormal and asymmetric compressive forces across the proximal tibial physis, causing local growth inhibition and osseous deformities^7,8,19,20.

While several authors have suggested a mechanical basis as the cause of Blount disease^19-22, none have correlated the magnitude of obesity with the severity of limb deformities in detail, to our knowledge. Our goal was to assess the relationship between obesity and biplanar radiographic deformities of the lower limb in patients with previously untreated Blount disease. The second purpose of the study was to establish whether there are differences in the relationship between body mass index and radiographic limb deformities between the early and late-onset forms of Blount disease⁷.

Materials and Methods

Materials and Methods | Results | Discussion | Appendix | References

After our institutional review board granted approval, the surgical database of the Division of Pediatric Orthopedics at our institution was searched for patients with a diagnosis of Blount disease who had been treated between 1997 and 2005. Clinical charts and radiographic records were reviewed for this cross-sectional retrospective study. Patients were excluded from the study if they had had previous surgical or orthotic treatment of the lower extremity, had a history of a lower-extremity fracture, or did not have adequate clinical and radiographic records. The diagnosis of Blount disease was made on the basis of radiographic evidence of persistent or worsening genu varum with changes consistent with Blount disease²³ and exclusion of any other etiology that would cause the observed physical or radiographic findings. The demographic and radiographic findings noted at the final preoperative visit were used for data analysis.

A retrospective review of outpatient medical records and preoperative radiographs was conducted. Demographic information, including gender, ethnicity, age when the deformity was noticed by the family, age at the time of the examination, body weight, and height, was recorded. Patients were classified as having early or late-onset Blount disease on the basis of whether the lower-limb deformity was noted before or after the age of four years⁷.

The body mass index is a number derived by dividing the patient's weight in kilograms by the square of the patient's height in meters (kg/m²). This index is calculated in the same way for both adults and children, but there is a difference in the interpretation of the body mass index of children as compared with that of adults. For those twenty years of age and older, body mass index is interpreted with use of standard categories, irrespective of age and gender. The Center for Disease Control and Prevention (CDC) defines "obesity" in adults as a body mass index of >30 and "extreme obesity" as a body mass index of =40¹³. For children and teenagers, the interpretation of body mass index is both age and gender-specific. The reason for this discrepancy is twofold: the amount of body fat differs between girls and boys, and it also changes with age. The body mass index-for-age growth charts based on CDC guidelines (see Appendix)^14,15 take into account these differences and allow conversion of the body mass index to a percentile, based on an individual child's age and gender. A body mass index equal to or greater than the 5th percentile and less than the 85th percentile is considered a "healthy weight," that greater than the 85th percentile and less than the 95th percentile is "at risk for overweight," and that at or above the 95th percentile is "overweight."^14,15

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Fig. 1: Standing full-length anteroposterior radiograph of a patient with right-sided, late-onset Blount disease. The frontal plane deformity parameters, including the mechanical axis deviation (MAD), lateral distal femoral angle (mechanical) (LDFA-m), and medial proximal tibial angle (MPTA), are illustrated.

Preoperative radiographic assessment included evaluation of a standing anteroposterior radiograph of both lower extremities with the patellae pointing forward and lateral radiographs of the entire lengths of both tibiae. Deformity analysis was performed with use of the methodology described by Paley et al.²⁴. Frontal plane analysis included measurement of the mechanical axis deviation, the lateral distal femoral angle (mechanical), and the medial proximal tibial angle (Fig. 1). A medial mechanical axis deviation denotes varus malalignment of the lower extremity, and a lateral deviation represents valgus malalignment of the lower extremity. The angle between the anatomic axes of the femur and tibia—i.e., the tibiofemoral angle—was also measured. Sagittal plane analysis included measurement of the posterior proximal tibial angle (Fig. 2). Intraobserver reliability was calculated with use of a randomized set of eighteen radiographs that were measured a few weeks apart by the same examiner (S.S.).

The mean height and weight of children with early and late-onset forms of Blount disease were compared with the 50th percentile values from age and gender-matched growth charts¹⁵. For statistical analyses, the proportion of overweight children with Blount disease was compared with the age-matched population prevalence^13,14 with use of the binomial test. The Fisher exact test was employed to examine whether there was a difference in the percentage of overweight patients based on gender, ethnicity, laterality of the extremity involved, or age at onset. The t test was utilized to evaluate the mean difference in body mass index on the basis of gender, ethnicity, laterality of the extremity involved, and early or late-onset Blount disease. The t test was also used to evaluate the difference in the mean values for various radiographic deformity parameters, including the mechanical axis deviation, lateral distal femoral angle (mechanical), medial proximal tibial angle, tibiofemoral angle, and posterior proximal tibial angle, between the early and late-onset groups of patients as well as between the patients with a body mass index of <40 and those with an index of =40. Correlation of body mass index with various limb-deformity parameters was calculated with use of the Pearson correlation coefficient (r) for the early and late-onset groups and for the patients with a body mass index of <40 and those with an index of =40. Possible values of r range from -1 to +1. An absolute r value (plus or minus) of 0.1 to 0.29 denotes weak correlation; 0.3 to 0.49, moderate correlation; 0.5 to 0.99, strong correlation; and 1, perfect correlation. Linear regression analysis was performed to determine the effect of the body mass index on the mechanical axis deviation on the basis of the age at onset and the severity of obesity. The level of significance was set at p < 0.05. All analyses were performed with SAS for Windows 9.1 software (SAS Institute, Cary, North Carolina).

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Fig. 2: Lateral radiograph of the patient in Figure 1, demonstrating the proximal posterior tibial angle (PPTA), a sagittal plane deformity parameter.

Results

Materials and Methods | Results | Discussion | Appendix | References

Over an eight-year period, forty-five patients with a total of sixty-five affected limbs met the inclusion criteria. Demographic details are listed in Table I. Seventeen children (twenty-seven limbs) had early-onset Blount disease, and twenty-eight children (thirty-eight limbs) had late-onset disease. There was a predominance of girls in the early-onset group and a predominance of boys in the late-onset group. There were thirty-eight black patients and seven Hispanic patients. Ten (59%) of the patients in the early-onset group and ten (36%) of those in the late-onset group had bilateral involvement. The average age at the onset of the deformity was 2.3 years in the early-onset group and 10.5 years in the late-onset group. The average age at the final preoperative visit, when clinical and radiographic examinations were performed, was 4.5 years in the early-onset group and 12.9 years in the late-onset group.

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TABLE I Demographic Information on Patients with Blount Disease

	Early Onset	Late Onset	Total
Patients (no. [%])	17 (38%)	28 (62%)	45
Limbs (no. [%])	27 (42%)	38 (58%)	65
Gender (no. [%])
Male	3 (18%)	23 (82%)	26
Female	14 (82%)	5 (18%)	19
Ethnicity (no. [%])
Black	11 (65%)	27 (96%)	38
Hispanic	6 (35%)	1 (4%)	7
Laterality (no. [%])
Unilateral	7 (41%)	18 (64%)	25
Bilateral	10 (59%)	10 (36%)	20
Mean age (95% confidential interval) (yr)
When deformity first noted	2.3 (1.9-2.6)	10.5 (9.5-11.4)	7.2 (5.9-8.6)
At time of examination	4.5 (3.4-5.5)	12.9 (12.2-13.5)	9.7 (8.4-11.1)

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TABLE I Demographic Information on Patients with Blount Disease

	Early Onset	Late Onset	Total
Patients (no. [%])	17 (38%)	28 (62%)	45
Limbs (no. [%])	27 (42%)	38 (58%)	65
Gender (no. [%])
Male	3 (18%)	23 (82%)	26
Female	14 (82%)	5 (18%)	19
Ethnicity (no. [%])
Black	11 (65%)	27 (96%)	38
Hispanic	6 (35%)	1 (4%)	7
Laterality (no. [%])
Unilateral	7 (41%)	18 (64%)	25
Bilateral	10 (59%)	10 (36%)	20
Mean age (95% confidential interval) (yr)
When deformity first noted	2.3 (1.9-2.6)	10.5 (9.5-11.4)	7.2 (5.9-8.6)
At time of examination	4.5 (3.4-5.5)	12.9 (12.2-13.5)	9.7 (8.4-11.1)

The mean height of the children with early-onset Blount disease was 108 cm, whereas that of the children with late-onset disease was 165 cm. The mean weight of the children with early-onset disease was 38 kg, whereas that of the children with late-onset disease was 106 kg. The mean heights of both the boys and the girls with Blount disease (early or late-onset) were very similar to the 50th percentiles of age and gender-matched population values (1.01 to 1.07 times those values)¹⁵. However, the mean weights of both the boys and the girls with Blount disease were substantially (2.2 to 2.7 times) greater than the 50th percentiles of age and gender-matched population values¹⁵.

Because height information was missing from the medical record, the body mass index could not be calculated for one black adolescent. The average body mass index for the remaining forty-four patients was 35.6. The average body mass index for the children with early-onset disease was 29.2, and that of the children with late-onset disease was 39.7 (p < 0.006). The average body mass index of both the boys and the girls with Blount disease (early or late-onset) was substantially (1.9 to 2.3 times) greater than the 50th percentiles of age and gender-matched population values^14,15. On the basis of CDC criteria for overweight children, fifteen (88%) of the seventeen children with early-onset Blount disease and twenty-six (96%) of the twenty-seven patients with late-onset Blount disease were classified as overweight (p = 0.55) (Table II). This prevalence of overweight children was significantly higher (p < 0.0001) than the 17% prevalence in the general pediatric population¹³. There was no significant difference in the body mass index based on gender, ethnicity, or laterality (Table II).

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TABLE II Body Mass Index and Number of Overweight Patients Based on Gender, Ethnicity, Laterality, and Age at Onset

		Body Mass Index^*			Overweight Patients†
	Total No. of Patients	Mean (95% Confidence Interval)	P Value	Number (%)	P Value
Gender
Male	25	38.2 (34.4-42.2)	0.07	23 (92%)	1
Female	19	32.1 (26.1-38.2)		18 (95%)
Ethnicity
Black	37	36.4 (32.9-39.8)	0.30	35 (95%)	0.41
Hispanic	7	31.6 (16.9-46.2)		6 (86%)
Laterality
Unilateral	25	34.7 (30.1-39.2)	0.53	23 (92%)	1
Bilateral	19	36.8 (31.3-42.4)		18 (95%)
Age at onset
Early	17	29.2 (22.7-35.7)	0.006	15 (88%)	0.55
Late	27	39.7 (36.5-42.8)		26 (96%)

Body mass index was calculated as the patient's weight in kilograms divided by the square of his or her height in metersThe determination of overweight was based on the age and gender-specific guidelines of the 2000 CDC growth charts¹³. Because of missing height information, the body mass index could not be calculated for one black adolescent male

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TABLE II Body Mass Index and Number of Overweight Patients Based on Gender, Ethnicity, Laterality, and Age at Onset

		Body Mass Index^*			Overweight Patients†
	Total No. of Patients	Mean (95% Confidence Interval)	P Value	Number (%)	P Value
Gender
Male	25	38.2 (34.4-42.2)	0.07	23 (92%)	1
Female	19	32.1 (26.1-38.2)		18 (95%)
Ethnicity
Black	37	36.4 (32.9-39.8)	0.30	35 (95%)	0.41
Hispanic	7	31.6 (16.9-46.2)		6 (86%)
Laterality
Unilateral	25	34.7 (30.1-39.2)	0.53	23 (92%)	1
Bilateral	19	36.8 (31.3-42.4)		18 (95%)
Age at onset
Early	17	29.2 (22.7-35.7)	0.006	15 (88%)	0.55
Late	27	39.7 (36.5-42.8)		26 (96%)

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Fig. 3: Scatterplot of the regression analysis, illustrating the relationship of body mass index (BMI) and mechanical axis deviation (MAD) in the patients with the early-onset form of Blount disease.

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Fig. 4: Scatterplot of the regression analysis, illustrating the relationship of body mass index (BMI) and mechanical axis deviation (MAD) in the patients with the late-onset form of Blount disease.

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Fig. 5: Scatterplot of the regression analysis, illustrating the relationship of body mass index (BMI) and mechanical axis deviation (MAD) in the patients with a body mass index of =40.

The intraobserver reliability for the measurements of all deformity parameters was good, with the intraclass correlation coefficients ranging from 0.87 to 0.89. The results of the analysis of frontal and sagittal plane deformities are detailed in Table III. As expected, the patients with early-onset Blount disease were significantly shorter than those with the late-onset form. However, with the numbers studied, the mechanical axis deviation of the affected extremities was similar in the two groups (p = 0.90). The remaining frontal plane deformities were more severe in the younger group of patients. The CDC defines adults as "extremely obese" if their body mass index is =40¹³. Thirty-one children (forty-four limbs) with Blount disease had a body mass index of <40, and the remaining thirteen children (nineteen limbs) had a body mass index of =40. The children with a body mass index of =40 had worse frontal and sagittal plane deformities, although only two of the five parameters reached significance (Table III).

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TABLE III Limb Deformity Parameters Based on Age at Onset and Severity of Obesity

Limb Deformity Variables	Early Onset^*	Late Onset^*	P Value	Body Mass Index <40^*	Body Mass Index =40^*	P Value
Medial mechanical axis deviation (mm)	69 (55-84)	70 (60-81)	0.90	63 (54-72)	88 (71-106)	0.004
Lateral distal femoral angle (mechanical) (deg)	99 (95-104)	94 (93-95)	0.02	96 (94-99)	96 (92-100)	0.98
Medical proximal tibial angle (deg)	68 (63-74)	75 (73-77)	0.04	73 (70-76)	70 (64-76)	0.28
Tibiofemoral angle (deg)	36 (29-42)	18 (12-25)	0.001	29 (21-36)	32 (22-41)	0.59
Posterior proximal tibial angle (deg)	64 (56-72)	70 (68-73)	0.13	72 (69-74)	65 (59-70)	0.02

The values are given as the mean with the 95% confidence interval in parentheses

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TABLE III Limb Deformity Parameters Based on Age at Onset and Severity of Obesity

Limb Deformity Variables	Early Onset^*	Late Onset^*	P Value	Body Mass Index <40^*	Body Mass Index =40^*	P Value
Medial mechanical axis deviation (mm)	69 (55-84)	70 (60-81)	0.90	63 (54-72)	88 (71-106)	0.004
Lateral distal femoral angle (mechanical) (deg)	99 (95-104)	94 (93-95)	0.02	96 (94-99)	96 (92-100)	0.98
Medical proximal tibial angle (deg)	68 (63-74)	75 (73-77)	0.04	73 (70-76)	70 (64-76)	0.28
Tibiofemoral angle (deg)	36 (29-42)	18 (12-25)	0.001	29 (21-36)	32 (22-41)	0.59
Posterior proximal tibial angle (deg)	64 (56-72)	70 (68-73)	0.13	72 (69-74)	65 (59-70)	0.02

The values are given as the mean with the 95% confidence interval in parentheses

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Fig. 6: Scatterplot of the regression analysis, illustrating the relationship of body mass index (BMI) and mechanical axis deviation (MAD) in the patients with a body mass index of <40.

The correlation of body mass index and biplanar deformity parameters based on age at onset and severity of obesity is shown in Table IV. A strong correlation between mechanical axis deviation and body mass index was noted in the early-onset group (r = 0.74, p < 0.0001) but, with the numbers studied, not in the late-onset group (r = 0.27, p = 0.12) (Table IV). With regard to the sagittal plane, there was a strong negative correlation between the body mass index and the posterior proximal tibial angle, suggesting increasing procurvatum deformity of the proximal part of the tibia with increasing body mass index, in the early-onset group (r = -0.79, p = 0.002) and a weak negative correlation in the late-onset group (r = -0.23, p = 0.26). The extremely obese children (body mass index of =40) demonstrated a stronger correlation of biplanar deformity parameters with changing body mass index compared with those with a body mass index of <40, although not all comparisons reached significance (Table IV).

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TABLE IV Correlation of Body Mass Index and Limb Deformity Parameters Based on Age at Onset and Severity of Obesity

	Early Onset			Late Onset			Body Mass Index <40		Body Mass Index =40
Limb Deformity Variables	Correlation Coefficient (R)	P Value	Correlation Coefficient (R)	P Value	Correlation Coefficient (R)	P Value	Correlation Coefficient (R)	P Value
Mechanical axis deviation	0.74	<0.0001	0.27	0.12	0.24	0.13	0.78	0.0002
Lateral distal femoral angle (mechanical)	0.03	0.88	0.09	0.59	—0.28	0.07	0.26	0.28
Medial proximal tibial angle	—0.34	0.09	—0.18	0.30	0.12	0.44	—0.52	0.02
Tibiofemoral angle	0.24	0.26	0.56	0.06	—0.41	0.05	0.91	<0.0001
Posterior proximal tibial angle	—0.79	0.002	—0.23	0.26	—0.28	0.23	—0.67	0.004

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TABLE IV Correlation of Body Mass Index and Limb Deformity Parameters Based on Age at Onset and Severity of Obesity

	Early Onset			Late Onset			Body Mass Index <40		Body Mass Index =40
Limb Deformity Variables	Correlation Coefficient (R)	P Value	Correlation Coefficient (R)	P Value	Correlation Coefficient (R)	P Value	Correlation Coefficient (R)	P Value
Mechanical axis deviation	0.74	<0.0001	0.27	0.12	0.24	0.13	0.78	0.0002
Lateral distal femoral angle (mechanical)	0.03	0.88	0.09	0.59	—0.28	0.07	0.26	0.28
Medial proximal tibial angle	—0.34	0.09	—0.18	0.30	0.12	0.44	—0.52	0.02
Tibiofemoral angle	0.24	0.26	0.56	0.06	—0.41	0.05	0.91	<0.0001
Posterior proximal tibial angle	—0.79	0.002	—0.23	0.26	—0.28	0.23	—0.67	0.004

Linear regression analysis revealed a significant positive relationship between mechanical axis deviation and body mass index in the early-onset group (Fig. 3), but there was no significant relationship in the late-onset group (Fig. 4). Moreover, there was a significant positive relationship between mechanical axis deviation and body mass index in the group with a body mass index of =40 (Fig. 5), but no significant relationship between those two parameters could be found in the group with a body mass index of <40 (Fig. 6).

Discussion

Materials and Methods | Results | Discussion | Appendix | References

It is well recognized that children with Blount disease tend to be heavier than their peers^7,8,10,22. The cause of genu varum in Blount disease has been thought to be based on the Heuter-Volkmann principle of increased compressive forces on the physis leading to asymmetric growth inhibition^19-22. Excessive pressure at the medial aspect of the proximal tibial cartilaginous epiphysis and physis leads to altered structure and function of the chondrocytes along with delayed ossification of the epiphyses²⁵. In a child with genu varum, obesity can substantially increase the compressive forces generated on the medial compartment of the knee joint^19,21,22. However, the "dose-effect" relationship between the magnitude of obesity and the severity of limb deformities in children with previously untreated Blount disease has not been studied in detail, to our knowledge. Dietz et al.²² examined the relationship of body weight with angular deformities in fifteen young children with Blount disease. Although their radiographic technique did not include use of full-length standing radiographs or analysis of the sagittal plane deformities, they observed a substantial correlation between body weight and the tibiofemoral shaft angle (r = 0.75). When only the nine obese children were considered, an even stronger relationship between body weight and frontal plane deformity was noted (r = 0.92). When we divided our patients on the basis of body mass index, those with an index of =40 had a similar correlation between body mass index and the tibiofemoral angle (r = 0.91). In addition, we observed an increased magnitude of the other frontal and sagittal plane deformities in extremely obese children.

Using finite element analysis, Cook et al.¹⁹ found that increasing varus deformity resulted in a considerable increase in the compressive stress at the proximal tibial physis in two-year-old and five-year-old children during simulated single-limb stance. In a five-year-old obese child, the calculated compressive forces generated with 10° of varus angulation exceeded those necessary to retard physeal growth. Using gait analysis, Gushue et al.²¹ studied the effect of childhood obesity on three-dimensional knee joint biomechanics. Compared with children of normal weight, overweight children showed a significantly higher peak internal knee abduction moment during early stance, with increased loading of the medial compartment of the knee joint. Davids et al.²⁰ also examined the gait deviations related to increased thigh girth associated with adolescent obesity. Normally, during the single-limb-stance phase of gait, individuals tend to minimize the horizontal displacement of their center of mass by positioning the foot centrally, close to the line of progression. An obese child with large thighs has difficulty adducting the hips adequately and may produce a so-called fat-thigh gait by generating a varus moment on the knee, thus increasing pressure at the medial aspect of the proximal tibial physis. This concept supports the observation that preexisting varus alignment of the knee is not necessary to initiate the pathologic changes seen in some patients with late-onset Blount disease²⁶. Recent studies suggest that childhood obesity reduces bone mineral content to levels below what would be predicted on the basis of body weight²⁷. This factor may further predispose obese children with Blount disease to the development of progressive deformities.

Wenger et al. ⁸ suggested that the proximal tibial growth plate responds differently at different ages, with the increased pliability of the unossified epiphyses in younger patients causing more growth inhibition compared with that seen in adolescents. This hypothesis is consistent with our observation of an increased magnitude of biplanar tibial deformities in patients with the early-onset form of Blount disease. The proximal tibial physis in the early-onset group of patients appears to be more susceptible to the compressive forces of obesity. In our study, despite having a lower body mass index, children with early-onset Blount disease had more severe varus and procurvatum deformities of the proximal part of the tibia than those with late-onset disease. The increased severity of deformities in the early-onset group may also be related to the delayed ossification of the affected medial aspect of the tibial epiphysis secondary to increased compressive forces²⁵. As established by basic-science studies^25,28-30 and validated clinically³¹, the inhibitory effect of compressive forces on the growth plate is not an all-or-none phenomenon. Only an extreme increase in compressive forces across a physis leads to permanent inhibition of growth. Moreover, this inhibitory effect on the growth plate appears to be reversible to a certain extent, if these forces are removed before skeletal maturity²⁸. Thus, if the medial compartment of the knee joint is unloaded by realignment of the axis deviation laterally, the medial aspect of the proximal tibial physis may respond by resuming normal growth. Several authors have suggested that the risk of recurrent deformity in patients with early-onset Blount disease is minimized if realignment osteotomies are performed prior to four years of age, before the onset of advanced pathologic changes in the proximal tibial physis^9,32-34. These changes include the development of initially cartilaginous and later osseous physeal bars, which will likely inhibit the longitudinal growth of the medial side of the proximal part of the tibia. The question of whether weight reduction can also affect progression of limb deformities requires further investigation.

Our review of the literature indicated that this is the first detailed report of the association of obesity with biplanar radiographic deformities in children with previously untreated Blount disease. However, our study had a few limitations. First, given the lack of complete ossification of the proximal tibial epiphyses, especially in the early-onset group, the magnitude of proximal tibial deformity may have been overestimated on the radiographs. Detailed evaluation of the proximal tibial deformity with imaging modalities such as arthrography or magnetic resonance imaging may have been useful, but these studies are currently performed with the patient supine. Second, while this may be one of the larger studies on Blount disease, given the multiple variables that were analyzed there is a possibility of a type-II error³⁵ in the interpretation of the results that did not achieve significance. Third, although we were able to establish a relationship between obesity and biplanar deformity in the young as well as the extremely obese children, causality cannot be established given the cross-sectional retrospective study design. Although increased body weight has a strong relationship with the incidence of Blount disease and probably influences the severity of limb deformities, other factors may influence the age at onset and the magnitude of deformity. Why only certain obese children exhibit classic features of Blount disease, what explains unilateral as well as asymmetric limb involvement, and whether weight reduction can decrease the incidence of Blount disease and influence the magnitude of limb deformities remain largely unknown. A larger, possibly multicenter study may be required to validate our findings and further illuminate the relationship of obesity and various demographic and deformity parameters.

In summary, children in the United States with Blount disease are more likely to be obese compared with the general pediatric population. There is a significant relationship between the magnitude of obesity and biplanar radiographic deformities in children with the early-onset form of Blount disease and those with a body mass index of =40. These clinical findings are consistent with the literature concerning the effect of increased compressive forces on the proximal tibial physis.

Appendix

Materials and Methods | Results | Discussion | Appendix | References

Body mass index-for-age percentile graphs for boys and girls are available with the electronic versions of this article, on our web site at (go to the article citation and click on "Supplementary Material") and on our quarterly CD-ROM (call our subscription department, at 781-449-9780, to order the CD-ROM). ?

References

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Topics

obesity ; body mass index procedure ; tibia ; limb deformity ; blount's disease ; overweight

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Sanjeev Sabharwal, Caixia Zhao, Emily McClemens; Correlation of Body Mass Index and Radiographic Deformities in Children with Blount Disease. The Journal of Bone & Joint Surgery. 2007 Jun;89(6):1275-1283.

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