The Hueter-Volkmann law is used to describe the scoliosis mechanism. The typical thoracic spine has a physiologic curve that causes the distractive force on the dorsally placed vertebral column while a compressive force on the ventrally positioned vertebral column.
The rotation of vertebral bodies in the axial plane, which results in discrepant axial loading between the ventrally and dorsally positioned sections of the implicated vertebrae, is regarded to be the first step in the process that leads to improper spine curvature. The discrepancy shows itself as a shift in the orientation of the spinal curve, causing scoliosis. The ventrally positioned vertebral column now becomes the concave side and the dorsally located vertebral column becomes the convex side.
Therefore, growth slows when compression is applied to the vertebral growth plates at the predefined concave side of the curvature. Whereas growth accelerates when is traction on the growth plates at the predefined convex side of the curvature.
In layman’s terms, the Hueter Volkmann Law states that bending the spine increases growth on the convex side, resulting in the body posture deformity known as scoliosis. The law was propounded by two German orthopaedics, Carl Hueter and Richard von Volkmann. So, keep reading the article to understand various principles of the Hueter Volkmann law and its theory.
Growth in pre-adult muscoskeletal system
The pre-adult musculoskeletal system is like a tensegrity structure. The tensegrity is the skeletal structure’s ability to produce a rigid form by having continuous tension members and discontinuous compression ones. Bones and cartilage lengthen the child’s muscles, ligaments, and fascia, which causes remodelling and growth. Muscles must not only elongate but also strengthen to tolerate the growing body mass and lever arms. When this balance of skeletal and muscular growth gets disturbed, adolescent idiopathic scoliosis (AIS) begins, which is also known as the first stage of scoliosis.
The age of adolescence features a growth spurt in both physiological and psychological areas. However, the delay in muscular maturation due to reduced tensegrity can lead to posture deformities.
The Hueter-Volkmann law helps us understand the process of disturbance of the tensegrity system. The affected axial compression leads to the growth of plates causing scoliosis and also lowering the vertebral bone density. Under this situation, the tension on the longitudinal ligaments and annulus fibrosis increase, causing a limitation to the remodelling process of the ligaments. Finally, the increased pressure on annulus fibrosis and locked ligament remodelling give way to the process of differential growth of the human spine.
Differential growth in the human spine
Differential growth is a slow and gradual progression of permanent deformation of the spine. This mechanical phenomenon takes place due to the mismatch in elongation of two tissues connected to each other. Different experiments conducted by medical experts show that while the differential growth starts slow, it expands exponentially. An experiment conducted by Crijins and colleagues, emphasises the effect of mismatched connecting tissues and plate growth rather than Euler Buckling for differential growth leading to scoliosis.
A straight elastic rod compressed at its ends experiences an instantaneous mechanical instability known as Euler buckling. In contrast, AIS is a gradual spinal deformation process that can take months or even years to manifest. Second, a healthy vertebrate spine comprises a sequence of vertebrae. These are hinged by intervertebral discs rather than an elastic rod. The range of motion for each spinal segment includes a neutral zone of several degrees where it can bend without experiencing significant resistance.
Skeletal growth and hormones
Growth hormones like insulin and estradiol regulate the skeletal growth of the human body. But, growth can also be a mechanical force. The Hueter-Volkmann law revolves around the moderation of skeletal growth as a mechanical phenomenon. Increased compression acting on a growth plate slows bone formation; on the other hand, less compression or tension speeds it up. This phenomenon is evident in the case of AIS. So, it is believed that the wedge-shaped distortion of the vertebrae in the late stages of AIS is caused by the Hueter-Volkmann Law.
Adolescent idiopathic scoliosis patients are generally tall with low bone density and a low body mass index. Bone density plays a crucial role in skeletal growth; thus, it is connected to the Hueter Volkmann Law too. Genetic, endocrine, hormonal, and dietary factors have all been linked to bone density, but from a mechanical standpoint, the observation that AIS patients have lower bone density than age-matched controls shows that they face less mechanical loading.
Reduced muscle mass and strength, caused by altered hormone levels or other mechanisms, may be responsible for this. It’s noteworthy to observe that increased skeletal growth affects the arms as well as the spine; arm length, ulna length, and radius length.
As a result, muscle mass is typically lower or delayed in scoliosis patients. To put it another way, there is an imbalance between skeletal growth and muscle-ligamental maturation. According to the Hueter-Volkmann Law, diminished muscular strength causes a reduction in skeletal loading, which in turn speeds up bone and cartilage formation.
Mechanical regulation of growth plate activity
Genetic, vascular, hormonal, and biomechanical variables all play a part in the complicated regulation of growth. Different degrees of chondrocyte hypertrophy in growth plates account for the majority (40–50%) of the variability in growth rate, with matrix synthesis and rate of chondrocyte proliferation accounting for the majority of the rest of the variability. The rate of chondrocyte formation in the proliferative zone and the rate of chondrocyte expansion and matrix synthesis in the hypertrophic zone, therefore, seem to be the two main factors influencing growth velocity.
Growth is ultimately halted under the AIS conditions where strain produces stresses that are contained by the growth plate staples. Growth plates that have been firmly stapled experience considerable and ultimately irreparable disturbance. Therefore, it appears that the mechanical loading’s effect on growth correlates with the level of stress exerted.
Prevention techniques for differential growth in the spine
According to the differential growth hypothesis and Hueter Volkmann law, AIS is brought on and developed as a result of excessive intradiscal pressure. This can only be explored in clinical investigations because AIS only affects people and there are no suitable animal models. The release of osmotic pressure in severely inflated intervertebral discs would be the most immediate intervention.
Young teens can also relieve intradiscal pressure in other ways. One theory holds that inadequate muscle strength may be the catalyst for enhanced spinal development. Training your core stability could therefore be a beneficial AIS therapy strategy. Improved core stability would also make the spine more tensile and lower the chance of scoliotic abnormalities. Studies on core stability have been carried out, and they have been shown to be quite effective, at least in young female scoliosis patients.
Another approach would be to stretch the locked ligaments to lower intradiscal pressure. Several conservative treatments target the spine’s flexibility and range of motion, but none of them addresses the intervertebral disc’s health. However, exercises may be helpful for both loosening ligaments and dynamic loading of the intervertebral disc.
Limitation of the Hueter Volkmann law
Differential growth is the physical mechanism for the onset and progression of the early stage of scoliosis. The Hueter-Volkmann law identifies intradiscal pressure as the process’s primary determinant, together with the spine’s longitudinal ligaments’ limited expansion. While decreased muscle activity may contribute to intervertebral disc growth.
The underlying causes of muscular weakness, which may be relates to hormone levels, physical exercise, or late menarche, are not explained by the principles.
Additionally, the differential growth ignores AIS’s extraneous effects like vertebral wedging and muscular asymmetry. Furthermore, it is evident that scoliosis manifests itself in various curve patterns.
A spine with inflated intervertebral discs, might stiffen and take on the characteristics of a curved elastic rod. This offers a compelling alternative to the differential growth concept for understanding the diverse curve patterns.
Various groups of scientists and physiologists have studied the Heuter-Volkmann law for bone growth modulation. However, the debate remains inconclusive. This presents a correlation rather than a physical mechanism of scoliosis. Differential growth, skeletal growth, hormones, bone density, body mass index, nutrition, and disc height are other factors that also affect spine posture. Thus, to conclude, we may say that the Heuter-Volksmann law is an effective mechanobiological perspective that provides cues for the development of growth plates that cause scoliosis.
Hueter and Volkmann’s orthopaedic law argues that higher mechanical compression slows bone growth and lower loading accelerates it. This rule, on the surface, seems like a straightforward justification for the development of scoliosis. But, as stated in the limitations, the law does not help in explaining the diverse curve patterns of scoliosis. Hueter and Volkmann’s laws when juxtaposed with the recent studies and experiments can deliver a more holistic understanding of the growth plates.