This study is the first attempt to qualify the possible effects of the Hueter-Volkmann law in the immature shoulder joints. We tried to demonstrate and to qualify the Hueter-Volkmann law (stating that an increase of pressure leads to reduction of bone growth and decrease of pressure leads to stimulation of bone growth) in the development of scapular deformity in children with dorsal subluxation of the humeral head caused by OPBL. We assumed that dorsal subluxation of the humeral head in OPBL leads to increase of pressure at the dorsal side of the glenoid and decrease of pressure at the ventral side of the glenoid. According to the Hueter-Volkmann law, we hypothesized that this aberrant pressure would lead to decreased growth of the dorsal scapula and increased growth of the ventral scapula. However, in contrast to our hypothesis, we could not fully confirm this law in this study of shoulder deformities in children with OPBL. We showed that growth reduction was present both at the ventral and dorsal side of the scapula, with most of the reduction on the dorsal side. On the ventral side of the scapula, where glenohumeral pressure is reduced, there was no increase in growth.
We used a clinical neuromuscular model of subluxation of the shoulder caused by OPBL to qualify the Hueter-Volkmann law. However, we could not prove the Hueter-Volkmann law exactly in our neuromuscular model. It may be questioned why the Hueter-Volkmann law does not fully apply for children with subluxation resulting from OPBL. Hueter describes in his original study the idea that movements and forces are factors influencing joint form and illustrates this on the knee joint
. In an experimental setting, growth plates of various animals in various locations show the same reaction to altered mechanical compression and distraction forces
. Scapular growth is to some extent different from growth of long bones. It resembles growth of the acetabulum and iliac bone. In the scapula, two growth plates are present: a growth plate along the margo medialis and a growth plate at the glenoid
. These two growth plates contribute both to scapular growth but their relative contributions are unknown. This influences the coefficients in the equations on scapular length. It seems intuitively correct to assume that subluxation causes growth reduction primarily on the glenoid growth plate. This means that the coefficients in the equations are larger than given: if the glenoid side contributes 50% of total growth it would result in doubling of the coefficients and even stronger evidence of our findings. Moreover, forces in a neuromuscular disorder differ from physiological forces acting on the joints of long bones. Not only the direction but the nature of forces active is relevant: static compression decreases biosynthetic activity
, whereas cyclic hydrostatic pressure increases activity
. The combined effect of subluxation and abnormal movement caused by neuromuscular dysfunction results in reduced cyclic loading and more static compression on the glenoid, both leading to a reduced local growth and subsequent joint deformation.
Contrary to the Hueter-Volkmann law stating that a reduction of pressure leads to stimulation, we could not find a stimulating effect on growth of the ventral side following reduced ventral “pressure” in case of dorsal subluxation. Growth reduction is present on both the ventral and dorsal side of the scapula, but differs with most of the reduction on the dorsal side. This asymmetry in reduction affects joint development and leads to glenoid and humeral head deformation
. Glenoid form and orientation are determined by the combined growth of ventral and dorsal length. Subluxation of the humeral head leads to changes in this balance, leading to increased retroversion or glenoid deformation. In addition we showed that the VS/DS ratio, the balance between ventral and dorsal length of the scapula, is strongly related to the extent of joint subluxation (R2 = 0.33, p < 0.001).
The relation between stress and growth is complex: as mentioned in the introduction an increase in stress would first lead to increased growth and excessive stress would lead to retardation
[5–8]. However, with no or very little stress growth proceeds at a basal rate
[5, 7], which we think is the fact on the ventral side of the by OBPL affected scapula. In the normal scapula optimal pressure at both ventral and dorsal side results in a higher (and physiological) growth rate, resulting in optimal and symmetrical lengthening. On the dorsal side of the affected scapula excessive stress of the subluxated humeral head leads to retardation of growth, to the extend that it comes under the basal growth rate at the ventral side. To summarize: growth rate on ventral and dorsal side are reduced because of the paresis in OBPL, on the ventral side because of stress reduction, while on the dorsal side growth is even more severely reduced because of increased stress.
In an effort to demonstrate and qualify the Hueter-Volkmann law in children with OPBL, we encountered some strong points and some weak points. Strong points of our study are the homogeneous set of infants and children, the imaging technique suited for cartilaginous structures and for contralateral comparison. We did not measure growth intra-individually over time, so growth was estimated by the inter-individual differences in age. All measurements and ratio-calculations were intra-individually as we could compare affected scapulas to the normal contralateral scapulas. However, our study has some limitations. One limitation is that scapular growth is different from growth of long bones, as stated earlier. As explained, the presence of two growth plates could theoretically make our findings stronger. Another limitation is that measurements were done on transversal MRI projections that were standard used for imaging of shoulder deformity in OBPL
[9, 11]. However in OBPL both size and tilt of the affected scapula are different and this affects length measurements
. The affected scapula is tilted in several planes. In the coronal plane the inferior angle of the scapula is tilted medially-upward
. This tilt leads to a systematic overestimation of the lengths measured on the affected side. This means that growth reduction is larger than described in this study and our findings would even be stronger.