Fig. 23.1
(a) Scheme of successive stages of treatment of patients with achondroplasia. (b) Chart of the simultaneous removal of the tibia and femur
Fig. 23.2
Dependence of step length on lower limb length of normal subjects and patients with achondroplasia before and after lengthening performance
Bone injury is an indispensable starting moment for bone regeneration. The time constant of regenerative process start in soft tissues adapted to the action of axial mechanical loads is far greater. In addition, an orthopedist must have a clear idea of the fact, what’s the difference between natural longitudinal growth of children’s limbs and growth of limbs in case of their surgical lengthening; what biomechanical conditions contribute to such a growth, and how much the limbs lengthened are functionally proper.
We analyzed the contemporary records of treatment of 235 patients with achondroplasia at different age when the patients were admitted for treatment within the period of 1975–1990 and when other institutions had no sufficient experience of treating such patients, while in the clinic of our scientific center optimal conditions for treatment were being searched for the purpose of achieving maximal lengthening of limbs. Age-related distribution of patients’ sample can be considered as a quite objective value. Most often limb lengthening started at the age of 8–16 years (Fig. 23.3).
Fig. 23.3
Age-related distribution among 235 patients with achondroplasia admitted for treatment
The analysis of the dependence of maximally obtained values of leg lengthening (L, см) on the age of treatment start (in patients above 10 years) has revealed that these values decrease every year (t): L = 20- 0.36 * t, r = −0.950 (p < 0.001). As far as the age of treatment start in patients 1 year increases, the possibility of relative leg lengthening is 2 % limited [2].
Average daily rate of leg lengthening at two levels simultaneously also decreased as far as patients’ age 0.9 % increased. Fixation period duration of the leg being lengthened reflects bone tissue regenerative potential and increases as far as patients’ age increases (Fig. 23.4).
Fig. 23.4
Age-related dynamics of relative duration of fixation period
The facts revealed became a stimulating reason for studying the age-related dynamics of biomechanical properties of different leg tissues, determined by changes in structure and volume of soft-tissue elements.
In the process of leg lengthening the amount of distraction forces is known to reach from 170 to 400 N depending on tissue mass [3, 4]. Preferable character of «lengthening-strength» dependence consists in continuous increase of tissue resistance that conforms to their elastic deformity preservation. Time periods of treatment for such patients are shorter. The fall of strengths occurs in case of tissue transfer from elastic deformity state to plastic one. In this case cavities appear in regenerated bone, and muscle contractility after treatment remains substantially decreased.
Techniques to Study Elasticity of Skin Integuments and Arterial Walls
Study of skin elastic properties was performed with a device worked out by us. The technique of biomechanical testing proposed allows to measure the state of leg skin integuments in the process of patients’ treatment. The principle of operation of the device for studying biomechanical properties of human common integuments is based on measuring degree of skin rotation, produced by application of tangentially directed rotational moment of force. In the simplest design of elastometer the moment of force is applied to skin surface through a supporting disk precovered with adhesive material (Fig. 23.5).
Fig. 23.5
The principle of the device to study the elastic properties of human skin
The device consists of a rod, being an axis, at one end of which the supporting disk is attached in a stationary way, and a metal plate is attached at its other end. A scale is marked in degrees on the sector. A pointer serves to read the angle of superimposed disk rotation under the influence of the force moment applied. A flat spring is connected with the rod free of motion. The scale and another pointer are intended for controlling the rotational moment required. The disk diameter is 20 mm, the amount of working rotational moment – 6 N*cm.
During testing of skin integuments using moments of force over the range of 5–10 N*cm the connection between the moments of force, the superimposed disk area and the angle of skin rotation can be described to an accuracy of ±5 % using the following approximate dependence:
where Е – indicator of skin elasticity, М – moment of force, а – angle of skin rotation, S – area of supporting disk.
In the limits of acting forces the angle of skin rotation is proportional to the moment of force applied and inversely proportional to the area of superimposed disk and the elastic properties of skin integuments. Indicator dimensions are expressed in N/(cm*degrees). E indicator is inversely proportional to skin rotation angle provided that both moment of force and superimposed disk area are constant.
Elastic properties of leg back surface skin integument were studied under the condition of physical rest in supine position of the subject examined.
It has been revealed that the value of skin integument elasticity in normal children below 10 years increases in proportion to leg length increase. In patients with achondroplasia this value is below normal one and it increases after limb length surgical gain, reaching estimated values in the long-term periods (above 1 year) after treatment (Fig. 23.6).
Fig. 23.6
Elasticity values of leg skin integuments normally and in patients before treatment, in the immediate and long-term periods after leg lengthening
Elasticity of leg arterial walls was judged by the value of arterial pulse wave velocity (APWV) in the region from popliteal fossa to foot. In children below 6 years the value traces changes in leg length. Subsequently its increase rate is determined by the number of life years elapsed. In patients with achondroplasia APWV increases after operative changes in longitudinal leg size (Fig. 23.7). During distraction, unlike biomechanical parameters of other tissues, APWV value shouldn’t increase substantially, contributing to maintain the lumens of blood carrying arteries.
Fig. 23.7
The dependence of the velocity of arterial pulse wave from the length of the legs
Dynamics of Tissue Biomechanical Properties in the Process of Natural Growth and Under Surgical Leg Lengthening
The increase of limb longitudinal sizes leads to quantitative and qualitative changes in developing muscles. Specifically, the values of contractility increase, as well as transverse hardness of muscles [5]. Unlike skin integuments and arterial walls the muscle biomechanical properties of lower limbs are determined by their longitudinal sizes up to the end of natural longitudinal growth period (to 18 years) and in case of leg length increase up to 60 cm.
Myotonometry
Measurements of the transverse hardness of muscles, which is caused by their tone and muscle structure itself, were performed using the simplest but highly sensitive mechanical myotonometer (Fig. 23.8) made on the basis of transfer indicator of dial type with 0.01-mm least graduation, and on its stem a support cylinder with 20-mm outside diameter is fastened, inside of which a movable rod is set with a supporting heel of 5-mm diameter. The depth of the rod descending is 4.35 mm.
Fig. 23.8
A system illustrating the procedure for determining the hardness of transverse gastrocnemius
The myotonometer was lowered strictly vertically on the leg part examined (the belly of m. gastrocnemius lateral head). The myotonometer’s own mass (250 g) was traded off for the constant value of tissue squeezing cup pressure. The transverse hardness of muscles was estimated in arbitrary units under the condition of physical rest in supine position of the subject examined. In normal women at the age of 20–40 years this value amounts to 75–100 arbitrary units, in men of the same age – 100–150 arbitrary units.
Besides, there is correlation between the value of m. gastrocnemius transverse hardness and the value of intramuscular pressure determined by modified Henderson method [6] (Fig. 23.9), which increases as far as leg longitudinal sizes increase. 50 mmHg is a critical value of intramuscular pressure at which blood flow in arterioles is blocked. During leg lengthening in the first 20 days of distraction elasticity value of the posterior group of leg muscles 346 % increased in comparison with initial level. In addition, leg circumference became 3 cm more. Probably, elasticity increase is associated with change in soft tissue hydration. Further increase of segment length led to the fact, that the increment of elasticity value reached 400 % in comparison with initial level.
Fig. 23.9
Correlation between the values of m. gastroсnemius transverse hardness and those of intramuscular pressure in human subjects of different age normally and for leg lengthening
After the end of leg lengthening throughout fixation period the value of muscle elasticity decreased, being preserved substantially higher than initial values (183 ± 8 arbitrary units) due to remaining increased hydration of tissues. Subsequently, in the long-term periods (above 1 year) after fixator removal the value approaches the level conforming to that of normal subjects of the same age with the analogous sizes of the lower limbs (Fig. 23.10). In the process of limb natural longitudinal growth in children not only bone length increases spontaneously, but the contractile part of muscles as well [7].
Fig. 23.10
Dependence of transverse hardness value on leg longitudinal sizes in normal subjects examined and in patients before and after leg lengthening
That is why the value of their transverse hardness remains comparatively low. In adolescents the tendinous part of muscles increases under the influence of their traction by actively growing bone. Moreover, tension stress of tissues increases. The same is observed for surgical limb lengthening as well, but the rate of size increase in this case is about 30-fold higher, and tension stress is two to three-fold more, respectively. Plastic potentials of soft-tissue elements are significantly lower, that is why the complete compensation of the increased values of muscle elasticity at the expense of longitudinal growth of muscle tendinous part occurs in the immediate months after the end of distraction period and manifests itself in increasing the change range of muscle contractile part length. In this situation it’s important to avoid the increase of intramuscular pressure during distraction up to the values of arteriolar bed blocking, that leads to ischemia and irreversible damage of tissues.
There are some ways to prevent ischemic involvement of muscles: creation of the initial reserve of muscle free motion during the fixator application and insertion of wires beyond the belly of the most important muscles, pharmacological increase of the value of muscle tension stress; high-divisional distraction (1 mm for 60 times per 24 h) throughout 24 h; control of the state of the muscles being lengthened, that of limb blood supply, as well as patient’s general condition and his or her arterial pressure level.
If these measures are observed, contractility of muscles after treatment should be recovered completely. When lengthening gain is more than 10 % of the initial length of limb, its reduction should not exceed 25 % of the initial level. Muscle contractility should be controlled using dynamometric stands.
Effect of Muscle Biomechanical Properties on Blood Supply of the Leg Being Lengthened
Volumetric rate of leg circulation has been studied by the technique of occlusion plethysmography. After osteotomy and the Ilizarov fixator application this value two-fold increases in comparison with initial level and remains at increased level all over the period of distraction, being normalized not at once after lengthening cessation in the period of fixation (Table 23.1). At the end of lengthening period the values of peak blood flow decreases evidencing the reduction of vascular bed reserve potentials. The blood flow at rest decreases up to 1.53 ± 0.19 ml/min.*100 cm3 in the immediate months after the fixator removal.
Table 23.1
Dynamics of blood supply of the leg being lengthened in patients with achondroplasia (М ± m)
Stage of treatment | Number of observa-tions | Muscle elasticity (arbitr. units) | Volumetric blood flow (ml/min.*100 cm3) | |
|---|---|---|---|---|
VBF at rest | Peak VBF | |||
Before treatment | 29 | 16 ± 2 | 1.24 ± 0.09 | 4.9 ± 0.4 |
Distraction: 0.2 month | 8 | 53 ± 8 | 2.42 ± 0.49 | 4.3 ± 0.6 |
0.5 month | 6 | 84 ± 1 | 2.42 ± 0.26
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